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Recently developed temperature proxies based on hydroxylated isoprenoid Glycerol Dialkyl Glycerol Tetraethers (OH-isoGDGTs), such as % O H , R I - O H , R I - O H ′ and O H C , have shown potential for reconstructing past temperature changes. However, progress has been limited by the lack of a global core-top calibration with ample geographical coverage. Here, we compile an extensive global surface sediment dataset of OH-isoGDGTs as well as regular isoprenoid GDGTs (isoGDGTs), with both data generated at NIOZ (n = 575) and previously published data from other laboratories (n = 297). We find interlaboratory differences for proxy indices that incorporate both OH-isoGDGTs and regular isoGDGTs, indicating that care must be taken in compiling large GDGT datasets from multiple laboratories. Our results confirm a strong temperature signal in the isoGDGT distribution, especially for OH-isoGDGT-0 and non-hydroxylated isoGDGTs, but also reveal that water depth might have an impact on the distribution of OH-isoGDGTs with 1 and 2 cyclopentane moieties. This will affect the R I - O H and R I - O H ′ indices, particularly in tropical regions, where OH-isoGDGT-0 occurs at low abundance. We explore new proxy indices that combine the temperature dependence of both isoGDGT and OH-isoGDGT distributions and propose the use of T E X 86 O H , which includes OH-isoGDGT-0 in the denominator of the T E X 86 index. This modification leads to a much higher temperature sensitivity of the index, especially in regions with annual mean sea surface temperatures between 5 and 15 °C. Application of this novel paleothermometer to a polar sediment core suggests that this proxy is likely to result in more reliable temperature reconstructions in polar regions where OH-isoGDGTs are abundant. [ABSTRACT FROM AUTHOR]

Hydroxylated isoprenoid GDGTs (OH‐GDGTs) have emerged as a novel tool for reconstructing sea surface temperatures. However, when using marine OH‐GDGT calibration in lacustrine settings, it leads to a significant overestimation of temperatures, emphasizing the necessity for a thorough examination of OH‐GDGTs in lakes. Here, we investigated OH‐GDGT distributions in surface sediments from 65 lakes in West China and compiled published Asian lake and global marine OH‐GDGT data sets. Among all GDGT‐based indices, RI‐OH showed the strongest correlation with temperature across Asian lakes. The RI‐OH value was higher in lakes than in marine sediments, likely due to differences in the composition of Group 1.1a thaumarchaeotal species between the two settings. The first RI‐OH temperature calibration for lakes was developed and it addressed the issue of temperature overestimation when applied to both water column and sediment core, highlighting the potential of OH‐GDGTs as a new terrestrial paleothermometer. Plain Language Summary: Reconstruction of past terrestrial temperatures plays a crucial role in understanding modern global warming and projecting future climate patterns, yet reliable terrestrial temperature proxies remain limited. In this study, we present compelling evidence that the ring index of OH‐GDGTs, expressed as RI‐OH, is predominantly influenced by temperature in lacustrine settings, regardless of the wide range of physical and chemical characteristics of the lakes. We developed the first lacustrine RI‐OH‐temperature calibration and successfully applied it to a water column and a sediment core. These findings suggest that RI‐OH can be used as a novel paleothermometer in lacustrine settings. Key Points: OH‐GDGTs were first investigated in lacustrine settings along a wide temperature gradientTemperature was the most important factor affecting OH‐GDGT distributions in lacustrine settingsThe first calibration in lacustrine settings was established and successfully applied to a water column and a sediment core [ABSTRACT FROM AUTHOR]

The TetraEther indeX of 86 carbon atoms (TEX 86) is widely used as a proxy to reconstruct past sea surface temperatures. Most current applications of TEX 86 are primarily based on analyzing the composition of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) that comprise TEX 86 in sediments, with the assumption that the sedimentary isoGDGTs are mainly derived from the surface mixed layer. Here we report on the variations in the isoGDGT distribution, archaeal abundance and community through the water column of the Western Pacific Ocean, directly testing the export depth of isoGDGTs and constraining the temperature records of TEX 86. Our data show that maximum isoGDGT concentrations occurred in subsurface waters (150–200 m) with maximum archaeal abundances. The ratio between isoGDGTs bearing 2 vs. 3 cyclopentane moieties, i.e. [2/3] ratio, increased with depth, which is likely related to the shift of the archaeal community from Ca. Nitrosopelagicus-dominance to norank_f__Nitrosopumilaceae -dominance. Models based on the [2/3] ratios in the water column predicted an average export depth of isoGDGTs to sediments of around 150–200 m, consistent with the robust relationship between the compiled sedimentary TEX 86 and the annual mean subsurface temperature. Taken together, our findings support that TEX 86 records subsurface rather than surface temperatures in the open ocean. [ABSTRACT FROM AUTHOR]

Abstract Hydroxylated isoprenoid GDGTs (OH‐GDGTs) have emerged as a novel tool for reconstructing sea surface temperatures. However, when using marine OH‐GDGT calibration in lacustrine settings, it leads to a significant overestimation of temperatures, emphasizing the necessity for a thorough examination of OH‐GDGTs in lakes. Here, we investigated OH‐GDGT distributions in surface sediments from 65 lakes in West China and compiled published Asian lake and global marine OH‐GDGT data sets. Among all GDGT‐based indices, RI‐OH showed the strongest correlation with temperature across Asian lakes. The RI‐OH value was higher in lakes than in marine sediments, likely due to differences in the composition of Group 1.1a thaumarchaeotal species between the two settings. The first RI‐OH temperature calibration for lakes was developed and it addressed the issue of temperature overestimation when applied to both water column and sediment core, highlighting the potential of OH‐GDGTs as a new terrestrial paleothermometer.

Nitrososphaeria in the phylum Crenarchaeota, is a widespread archaeal class in the oceanic realm, playing an important role in the marine carbon and nitrogen cycle. Nitrososphaeria-derived membrane lipids, i.e., isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), are commonly employed to reconstruct past water temperatures using the TetraEther indeX of 86 carbon atoms (TEX86). This index is of particular importance for the brackish Baltic Sea as to date it appears to be the only applicable organic temperature proxy. In this study, we investigated the distribution of intact and core GDGTs and their potential source organisms in the water column of three deep basins located in the central Baltic Sea to evaluate the application of TEX86. A lipidomic approach on suspended particulate matter was combined with the molecular techniques 16S rRNA gene amplicon sequencing and CARD-FISH. The archaeal community was dominated by Nitrosopumilus (~83-100% of the total archaeal sequences). As other detected taxa known to produce GDGTs each represented less than 2% of the total archaeal sequences, Nitrosopumilus is likely the most dominant GDGT producer in the central Baltic Sea. However, the occurrence of phosphohexose (PH), instead of hexose-phosphohexose (HPH) headgroups, suggested that Nitrosopumilus in the Baltic Sea may differ physiologically from representatives of marine settings and other marginal seas, such as the Black Sea. In the Baltic Sea, Nitrosopumilus is most abundant in the suboxic zone, where intact cells peak according to both CARD-FISH data and intact polar lipid concentrations. The presented data therefore suggest that TEX86 reflects subsurface rather than surface temperature in the central Baltic Sea. [ABSTRACT FROM AUTHOR]

The possibility of a collapse (or marked weakening) of the Atlantic Meridional Overturning Circulation (AMOC) within a few centuries boosted the interest of the scientific community on the responses of different compartments of the climate system to past AMOC collapses (or marked weakening). The most recent examples of such periods are Heinrich Stadial 1 (HS1,18-15 ka BP; near-collapsed AMOC) and the Younger Dryas (YD, 12.9-11.7 ka BP; markedly weakened AMOC). Here, we present data from elemental ratios, spectral reflectance, organic biomarkers, sea surface temperatures and planktonic foraminiferal assemblage from two marine sediment cores recovered from the western tropical South Atlantic Ocean spanning the last ca. 20 kyr. The recovered sediments record the responses of eastern tropical South American hydroclimate, western tropical South Atlantic productivity and sea surface temperatures to changes in AMOC strength associated with HS1 and the YD. Our data show that both periods were characterized by wetter conditions over eastern tropical South America associated with increased continental runoff compared to the Last Glacial Maximum. High sea surface temperatures during HS1 and the YD are in line with marked AMOC decreases that led to heat accumulation in the surface layer of the western South Atlantic Ocean, facilitating increased precipitation over eastern tropical South America. Our data demonstrate for the first time that intense runoff during HS1, and hence nutrient transfer to the ocean, affected the pelagic ecosystem, likely increasing primary productivity in the otherwise oligotrophic western tropical South Atlantic Ocean. Our study demonstrates a tight coupling between marine and terrestrial processes in the eastern South American realm during periods of strong climate instability. [ABSTRACT FROM AUTHOR]

The reconstruction of accurate sea‐surface temperatures (SST) is of utmost importance due to the ocean's central role in the global climate system. Yet, a number of environmental processes might bias reliable SST estimations. Here, we investigate the fidelity of SST reconstructions for the western tropical South Atlantic (WTSA) for the interval covered by Marine Isotope Stages (MIS) 6–5, utilizing a core collected off eastern Brazil at ∼20°S. This interval was selected as previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a peculiar pooling of warm surface waters in the WTSA during MIS 6 despite glacial boundary conditions. To ground‐truth the Mg/Ca‐based SST data we generated SST reconstructions on the same core material using the alkenone and TEX86 paleothermometers. Comparison with alkenone‐based temperature estimates corroborate the previous Mg/Ca‐based SST reconstructions, supporting the suggestion of a warm‐water anomaly during MIS 6. In contrast, TEX86‐derived temperatures, albeit representing annual mean SST in recent core top samples, are up to 6°C colder than Mg/Ca‐ and alkenone‐based SST reconstructions. We interpret the periods of anomalously cold TEX86‐temperatures as a result of a vertical migration of the TEX86 producers (heterotrophic marine Thaumarchaeota) toward greater water depths, following food availability during phases of enhanced fluvial suspension input. Likewise, the data suggest that alkenone‐based SST are, albeit to a minor degree when compared to TEX86, affected by river run‐off and/or a seasonal bias in the growth season of haptophyte algae. Plain Language Summary: In this study, we investigate the accuracy of sea‐surface temperature (SST) reconstructions for the western tropical South Atlantic (WTSA) for the interval covered by Marine Isotope Stages (MIS) 6–5—the penultimate glacial‐interglacial cycle (ca. 190,000 to 70,00 years before present). This time interval was selected because previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a pooling of warm surface waters in the WTSA during late MIS 6 despite the cold glacial conditions. To verify the Mg/Ca‐based SST data, we generated temperature reconstructions from a core located off Eastern Brazil using two common paleothermometers that based on lipid biomarkers: alkenone and TEX86. The alkenone‐based temperature estimates agree with previous Mg/Ca‐based SST reconstructions, supporting the existence of a warm‐water anomaly in the WTSA during MIS 6. On the other hand, TEX86‐derived temperatures were up to 6°C colder than Mg/Ca‐ and alkenone‐based SST reconstructions. This discrepancy might be a result of a vertical migration of the TEX86 producers toward greater water depths where they feed on particles of organic matter. These migrations into deeper waters occurred during phases of increased river run‐off fluvial suspension input which enhanced surface primary productivity and facilitated vertical particle flux through the water column. Key Points: Parallel Mg/Ca (Globigerinoides ruber), alkenone and TEX86‐based temperature estimates in the western tropical South Atlantic (WTSA) across Marine Isotope Stages (MIS) 6–5Mg/Ca and alkenones represent annual mean sea surface temperatures, but most TEX86‐based temperatures deviate to colder valuesAnomalous sea surface warming in the WTSA during late MIS 6 appears as a robust signal [ABSTRACT FROM AUTHOR]

The TEX86 paleothermometer has been extensively used to reconstruct past sea water temperatures, but it remains unclear which export depths the proxy represents. Here we used a novel approach to better constrain the proxy recording depths by investigating paleotemperature proxies (TEX86, U37K′ ${\mathrm{U}}_{37}^{{\mathrm{K}}^{\prime }}$, RI−OH and RI−OH′) from two pairs of proximal (<12 km apart) cores from Chilean and Angola margins, respectively. These cores are from steep continental slopes and lower shelves, which leads to a substantial difference in water depth between them despite being closely located. Surprisingly, the deep and the shallow U37K′ ${\mathrm{U}}_{37}^{{\mathrm{K}}^{\prime }}$ records at the Chilean margin show dissimilarities, in contrast to the similar records from the Angola margin, which may be due to post‐depositional alteration at the former sites. In contrast, the TEX86 records were statistically indistinguishable between the sites at both the locations, even though the GDGT [2]/[3] ratio suggests GDGTs derived from potentially different archaeal communities residing at different depths. A short‐lived difference between the TEX86 records is observed during the last glacial period at the Angola margin, possibly due to a contribution of Antarctic Intermediate Waters to the deep site. Modelling suggests that the TEX86 source signal at our core sites reaches its peak abundance at water depths shallower than 350 m. The RI−OH and RI−OH′ records show similar variability as the TEX86 records, although regional differences in their absolute temperature estimates exist. Our approach using proximal sediment cores at steep slopes appears useful to constrain the export depth of organic proxy signals for paleo‐reconstructions. Key Points: Source signal of TEX86 proxy is likely derived from depths shallower than 350 m at Chilean and Angola marginsOH‐GDGT based proxies show similar variability as TEX86 records, but show regional differences in their absolute temperature estimatesU37K′ ${\mathrm{U}}_{37}^{{\mathrm{K}}^{\prime }}$ proxy signal may be impacted by post‐depositional changes or lateral transport even for closely located cores [ABSTRACT FROM AUTHOR]

Understanding long-term climate change is particularly relevant in Australasia, where precipitation exhibits marked variability on annual and decadal timescales. Although numerous studies have investigated hydroclimate change during the Common Era in Australasia, high-quality records covering the late Holocene remain scarce in northern Australia. Here we present an isoprenoid glycerol dialkyl glycerol tetraethers (GDGT) record from Lake Barrine, to evaluate the environmental significance of archaeal lipid-based proxies in the lake system and possible drivers of changes during the late Holocene. The fractional composition of isoprenoid GDGTs in the sediments from Lake Barrine is dominated by GDGT-0 and crenarchaeol. The ratio of crenarchaeol and its regio-isomer (cren/cren'), GDGT-0/crenarchaeol ratios and Methane Index suggest that a dominant contribution of archaeal lipids in Lake Barrine derived from Group Ⅰ.1a Thaumarchaeota, with a minor contribution from Group Ⅰ.1b Thaumarchaeota and methanogenic archaea. Centennial-scale variation in cren/cren' ratios correlates well with other hydroclimate records in the tropical Australia, due to a preference of Group Ⅰ.1a Thaumarchaeota for deeper lakes, and humid climates typically do not offer a large niche for soil Group Ⅰ.1b Thaumarchaeota. Variability of hydroclimate in northern Australia is linked to the mean position and strength of the intertropical convergence zone. Furthermore, the TEX 86 proxy can effectively record lake surface temperature during the past 0.8 kyr, however, a slight increase in the contribution of non-Thaumarchaeota in deeper sediments lead to underestimate of the paleotemperature. [ABSTRACT FROM AUTHOR]

Nitrososphaeria in the phylum Crenarchaeota, is a widespread archaeal class in the oceanic realm, playing an important role in the marine carbon and nitrogen cycle. Nitrososphaeria-derived membrane lipids, i.e., isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), are commonly employed to reconstruct past water temperatures using the TetraEther indeX of 86 carbon atoms (TEX86). This index is of particular importance for the brackish Baltic Sea as to date it appears to be the only applicable organic temperature proxy. In this study, we investigated the distribution of intact and core GDGTs and their potential source organisms in the water column of three deep basins located in the central Baltic Sea to evaluate the application of TEX86. A lipidomic approach on suspended particulate matter was combined with the molecular techniques 16S rRNA gene amplicon sequencing and CARD-FISH. The archaeal community was dominated by Nitrosopumilus (~83–100% of the total archaeal sequences). As other detected taxa known to produce GDGTs each represented less than 2% of the total archaeal sequences, Nitrosopumilus is likely the most dominant GDGT producer in the central Baltic Sea. However, the occurrence of phosphohexose (PH), instead of hexose-phosphohexose (HPH) headgroups, suggested that Nitrosopumilus in the Baltic Sea may differ physiologically from representatives of marine settings and other marginal seas, such as the Black Sea. In the Baltic Sea, Nitrosopumilus is most abundant in the suboxic zone, where intact cells peak according to both CARD-FISH data and intact polar lipid concentrations. The presented data therefore suggest that TEX86 reflects subsurface rather than surface temperature in the central Baltic Sea.

Glycerol dialkyl glycerol tetraethers (GDGTs), which are derived from microbial membranes, occur widely in soils, peats, lake sediments, marine sediments, hot springs, and other environments. GDGTs respond sensitively to environmental changes and have become an important tool in paleoenvironmental reconstruction. However, their application in speleothems, one of the most important archives of the study of past climates, has been quite limited. In this paper, we review the recent progress in the study of GDGTs in speleothems, from their sources to their distribution characteristics and paleoenvironmental applications. GDGTs in speleothems have been shown to come from dripping water, and cave systems. The distribution of GDGTs in speleothems shows that archaeal isoprenoid GDGTs (isoGDGTs) dominate over bacterial branched GDGTs (brGDGTs). The proxies based on isoGDGTs (TEX86) and brGDGTs (MBT/CBT) are correlated with temperature, which may offer promising potential methods for reconstructing terrestrial palaeotemperature. Ultimately, we discuss the difficulties and problems to be solved in studying speleothem GDGTs and the possible future directions for this field.

Indian Ocean sea surface temperatures impact precipitation across the basin through coupled ocean‐atmosphere responses to changes in climate. To understand the hydroclimate response over the western Indian Ocean and equatorial east Africa to different forcing mechanisms, we present four new proxy reconstructions from core VM19‐193 (2.98°N, 51.47°E) that span the last 250 ky. Sub‐surface water temperatures (Sub‐T; TEX86) show strong precessional (23 ky) variability that is primarily influenced by maximum incoming solar radiation (insolation) during the Northern Hemisphere spring season, likely indicating that local insolation dominates the upper water column at this tropical location over time. Leaf waxes, on the other hand, reflect two different precipitation signals: δ13Cwax (in phase with boreal fall insolation) is likely reflecting vegetation changes in response to local rainfall over east Africa, whereas δDprecip (primarily driven by boreal summer insolation) represents changes in regional circulation associated with the summer monsoon. Glacial‐interglacial changes in ocean temperatures support glacial shelf exposure over the Maritime Continent in the eastern Indian Ocean and the subsequent weakening of the Indian Walker Circulation as a mechanism driving 100 ky climate variability across the tropical Indo‐Pacific. Additionally, the 100 ky spectral power in δDprecip supports a basin‐wide weakening of summer monsoon circulation in response to glacial climates. Overall, the proxy records from VM19‐193 indicate that both precession and glacial‐interglacial cycles exert control over hydroclimate at this tropical location. Key Points: New multi‐proxy reconstructions from the tropical western Indian Ocean show both precession and glacial‐interglacial variabilityA new precipitation isotope record likely reflects large‐scale Indian summer monsoon circulation changes over timeGlacial‐scale changes in upper water column structure across the Indian Ocean support shelf exposure as a primary hydroclimate influence [ABSTRACT FROM AUTHOR]

Early Eocene (∼56-48 Ma) climates are useful to investigate polar climate dynamics in the absence of ice. We explore early Eocene orbital variability of Arctic climate using sediments recovered by the Arctic Coring Expedition (ACEX). High resolution records of lipid biomarkers (GDGTs; 2-kyr) and palynological assemblages (5-kyr) in the ∼4 m interval below Eocene Thermal Maximum 2 (∼54 Ma) show cyclic signals related to ∼20-kyr precession, ∼40-kyr obliquity, and ∼100-kyr eccentricity. Biomarkers indicate obliquity and precession variability representative of sea surface temperature (SST) variations up to ∼1.4 and ∼0.5°C, respectively. Peak SSTs coincide with an elevated supply of pollen and spores and increased marine productivity. This implies an orbital control on precipitation and terrestrial nutrient supply to the Arctic Basin. Assuming that SST maxima correspond to Arctic insolation maxima (precession minima/obliquity maxima), precipitation maxima also correspond to insolation maxima, implying regional hydrological processes as a forcing rather than variations in meridional water transport, contrasting Pleistocene Arctic hydrology. The relative amplitudes of precession and obliquity in the SST record match that of local insolation between spring and fall, corroborating a seasonal GDGT bias. The reconstructed complete orbital imprint refutes a bias to one end of the orbital variability. Eccentricity-related SST variability was ∼0.8°C, ∼2-3 times higher than synchronous variability in the deep ocean, and 3-4 times higher than similar variations in the tropics. This confirms eccentricity-forced global temperature variability and that this had pronounced polar amplification, despite the absence of ice-albedo feedbacks., (© 2024 The Author(s).)

• GDGTs were investigated in soils along an aridity-driven chemistry gradient. • 6-methyl brGDGT concentrations increase at high cation exchange capacity. • With temperature constant MBT′ 5ME variations respond to cation exchangeable capacity. • Neutral pH and arid soils have large range in MBT′ 5ME values. Branched and isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs, and isoGDGTs) are two families of membrane lipids commonly used to reconstruct paleo-environmental parameters. Their use as a quantitative proxy for past temperatures has been hindered by the discovery of other environmental controls on their distribution in soils, such as changes in bacterial community composition, chemistry and aridity. To test for the impact of aridity-driven soil chemistry changes, GDGT concentrations and derived proxies were measured in 43 soils along a chemical gradient in the Okavango Delta. All brGDGT concentrations increase with decreasing pH. Alkalinity-promoted (6-methyl and cyclopentane-containing) brGDGTs show a secondary concentration increase in arid soils, characterized by a high pH>8 and cation exchange capacity (CEC>30 cmolc kg−1). The concentration of 5-methyl brGDGTs increases faster that of 6-methyl brGDGTs in arid compared with non-arid soils. Although limited variability in temperature is present (∼2 °C), significant variation in MBT′ 5ME values is observed (0.63–0.96) likely driven by the variation in CEC. IsoGDGTs are present in lower concentrations than brGDGTs, and Ri/b values, a potential proxy for paleohydrological reconstruction, correlating with soil water content (r = 0.45, p < 0.01). TEX 86 values (0.57–0.97) correlate with pH across the aridity transect. In this region, where accurate proxies and quantitative paleostudies are scarce, the impact of aridity-driven chemistry changes on GDGT-proxies is shown, i.e., MBT′ 5ME is overall controlled by CEC, but correlates negatively with pH in non-arid soils and with IR 6ME in arid alkaline soils. Furthermore, we propose GDGT-based proxies for concentration in exchangeable calcium, past hydrological changes and soil pH. [ABSTRACT FROM AUTHOR]

• OH-isoGDGT abundances analyzed in the Mediterranean Sea and Red Sea. • OH-isoGDGT abundances, %OH and T E X 86 O H strongly correlate with water depth. • RI-OH and RI-OH′ show no trend with water depth. • Low abundances of OH-isoGDGTs (<1 %) observed in the Red Sea. • %OH, RI-OH and RI-OH′ do not form a distinct 'Red Sea cluster', unlike TEX 86. Hydroxylated Isoprenoidal Glycerol Dialkyl Glycerol Tetraethers (OH-isoGDGTs) have recently been utilized as paleothermometers in the marine environment. However, their ability to reconstruct temperature in the Mediterranean and Red Sea has not been adequately investigated. Previous research has shown that archaeal communities inhabiting different water depths in these basins exert a substantial influence on the regular isoGDGT distributions and associated proxies such as the T E X 86. However, the impact of these archaea on OH-isoGDGTs and their corresponding proxies remains unclear. In this study, we examined the distribution of OH-isoGDGTs and their associated proxies (% O H , RI-OH, RI-OH′ and T E X 86 O H) in surface sediments of the Mediterranean and the Red Sea. We observe strong correlations between the fractional abundances of OH-isoGDGTs, relative to all isoGDGTs and OH-isoGDGTs, and water depth which suggests that deep-water archaeal communities have a lower OH-isoGDGT abundance compared to the shallow communities. As a result, % O H and T E X 86 O H are strongly correlated with water depth, particularly at depths <500 m in the Mediterranean Sea. Interestingly, RI-OH and RI-OH′ show no correlation with water depth in the Mediterranean Sea. Instead, they correlate more strongly with satellite-derived sea surface temperature compared to other isoGDGT-based proxies, indicating their potential as paleothermometers. Finally, unlike T E X 86 and T E X 86 O H , the % O H , RI-OH and RI-OH′ do not exhibit distinct 'Red Sea cluster' and display comparable values to sediments from other tropical oceans. Further research on sedimentary OH-isoGDGT distributions with broader geographical coverage within these basins and enrichment cultures of deep-water archaea are needed to confirm these observations. [ABSTRACT FROM AUTHOR]

Archaeal membrane lipids known as glycerol dibiphytanyl glycerol tetraethers (GDGTs) are the basis of the TEX86 paleotemperature proxy. Because GDGTs preserved in marine sediments are thought to originate mainly from planktonic, ammonia-oxidizing Thaumarchaeota, the basis of the correlation between TEX86 and sea surface temperature (SST) remains unresolved: How does TEX86 predict surface temperatures, when maximum thaumarchaeal activity occurs below the surface mixed layer and TEX86 does not covary with in situ growth temperatures? Here we used isothermal studies of the model thaumarchaeon Nitrosopumilus maritimus SCM1 to investigate how GDGT composition changes in response to ammonia oxidation rate. We used continuous culture methods to avoid potential confounding variables that can be associated with experiments in batch cultures. The results show that the ring index scales inversely (R(2) = 0.82) with ammonia oxidation rate (ϕ), indicating that GDGT cyclization depends on available reducing power. Correspondingly, the TEX86 ratio decreases by an equivalent of 5.4 °C of calculated temperature over a 5.5 fmol·cell(-1)·d(-1) increase in ϕ. This finding reconciles other recent experiments that have identified growth stage and oxygen availability as variables affecting TEX86 Depth profiles from the marine water column show minimum TEX86 values at the depth of maximum nitrification rates, consistent with our chemostat results. Our findings suggest that the TEX86 signal exported from the water column is influenced by the dynamics of ammonia oxidation. Thus, the global TEX86-SST calibration potentially represents a composite of regional correlations based on nutrient dynamics and global correlations based on archaeal community composition and temperature.

RI‐OH′ and RI‐OH (ring index of hydroxylated tetraethers) are two novel organic paleothermometers which could either complement or replace more established paleothermometric proxies, such as UK′37 (C37 ketone unsaturation ratio) and TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms). Despite a few promising attempts, the paleothermometric potential of RI‐OH′ and RI‐OH is not fully constrained. Here we present new high‐resolution temperature records over the 160‐45 ka BP (before present = year 1950 CE) period using four organic proxies (RI‐OH′, RI‐OH, TEX86, and UK′37) from three deep sea sediment cores located in a north‐south transect along the Iberian Margin. We analyzed all organic proxies from a single set of lipid extracts to optimize proxy‐proxy comparisons and phase relationship studies. RI‐OH′ responds to Dansgaard‐Oeschger and Heinrich events, better resembles UK′37 than TEX86, and better records the influence of (sub)polar waters during Heinrich events than does RI‐OH. While RI‐OH′ gives realistic sea surface temperatures and latitudinal gradients coherent with those from independent paleothermometers, a more extensive RI‐OH′‐temperature calibration for the North Atlantic is clearly needed. However, the absence of a significant warm bias in RI‐OH′‐based temperatures compared to a shallow sea site suggests that endemic, deep‐dwelling archeal communities affect TEX86 but not RI‐OH′ in the Iberian Margin. TEX86 leads RI‐OH′ and UK′37 during four Heinrich‐like events, potentially due to background fluxes from deep waters for nonhydroxylated tetraethers summed with primary productivity dependent fluxes from surface waters for all investigated lipid classes. Relationships with Greenland temperatures further support RI‐OH′‐based paleothermometry. Key Points: RI‐OH′ not only responds to Dansgaard‐Oeschger and Heinrich events but also better resembles UK′37 than TEX86RI‐OH′ gives realistic sea surface temperatures and latitudinal gradients consistent with those from independent paleothermometersRelationships with Greenland temperatures further support RI‐OH′‐based paleothermometry [ABSTRACT FROM AUTHOR]

Climatic changes in the western Pacific marginal seas are influenced by global forcing and regional processes, including monsoons, and ocean circulation. To better understand the process of hydrographic and temperature changes, we applied the UK′37 as our index of Sea Surface Temperature (SST) and TEX86 as the index of Subsurface Water Temperature (SWT) for the last 8400 years using the sediment core MZ01 from the continental shelf of the East China Sea (ECS). To focus on centennial and millennial variabilities, the original SST and SWT are filtered with the Ensemble Empirical Mode Decomposition (EEMD) of the Hilbert-Huang Transform (HHT), with the confidence defined by a new method, the Continuity Superposition Error Calculation Method (CSECM). The SST and SWT both have a quasi-period of 1000–2000 years, exhibiting some teleconnection with the north Atlantic climatic changes. The SWT decreased during approximately 6–4 ka and then increased by ∼4°C to the late Holocene, almost anti-phase with the SST. The stronger Asian winter monsoon and China Coastal Current (CCC), are very likely responsible for the decreased SST in the late Holocene. In contrast, the increased SWT may imply that the stronger CCC has brought more Changjiang Diluted Water (CDW) southward and formed a thicker barrier layer in the ECS, which dampened bottom water heat loss that was transported from the Taiwan Warm Current (TWC), and Western Kuroshio Branch Current (WKBC). This process is tested by the hosing experiment that supports stronger stratification when the north Atlantic cooled. The combined results by UK′37 and TEX86 provide a new insight into the interaction mechanism among the winter monsoon, precipitation and the Kuroshio Current, and also raises caution to take more regional factors into account in the application of TEX86.

Falling atmospheric CO2 levels led to cooling through the Eocene and the expansion of Antarctic ice sheets close to their modern size near the beginning of the Oligocene, a period of poorly documented climate. Here, we present a record of climate evolution across the entire Oligocene (33.9 to 23.0 Ma) based on TEX86 sea surface temperature (SST) estimates from southwestern Atlantic Deep Sea Drilling Project Site 516 (paleolatitude ~36°S) and western equatorial Atlantic Ocean Drilling Project Site 929 (paleolatitude ~0°), combined with a compilation of existing SST records and climate modeling. In this relatively low CO2 Oligocene world (~300 to 700 ppm), warm climates similar to those of the late Eocene continued with only brief interruptions, while the Antarctic ice sheet waxed and waned. SSTs are spatially heterogenous, but generally support late Oligocene warming coincident with declining atmospheric CO2. This Oligocene warmth, especially at high latitudes, belies a simple relationship between climate and atmospheric CO2 and/or ocean gateways, and is only partially explained by current climate models. Although the dominant climate drivers of this enigmatic Oligocene world remain unclear, our results help fill a gap in understanding past Cenozoic climates and the way long-term climate sensitivity responded to varying background climate states. [ABSTRACT FROM AUTHOR]

Ocean gateways facilitate circulation between ocean basins, thereby impacting global climate. The Indonesian Gateway transports water from the Pacific to the Indian Ocean via the Indonesian Throughflow (ITF) and drives the strength and intensity of the modern Leeuwin Current, which carries warm equatorial waters along the western coast of Australia to higher latitudes. Therefore, ITF dynamics are a vital component of global thermohaline circulation. Plio‐Pleistocene changes in ITF behavior and Leeuwin Current intensity remain poorly constrained due to a lack of sedimentary records from regions under its influence. Here, organic geochemical proxies are used to reconstruct sea surface temperatures on the northwest Australian shelf at IODP Site U1463, downstream of the ITF outlet and under the influence of the Leeuwin Current. Our records, based on TEX86 and the long‐chain diol index, provide insight into past ITF variability (3.5–1.5 Ma) and confirm that sea surface temperature exerted a control on Australian continental hydroclimate. A significant TEX86 cooling of ~5°C occurs within the mid‐Pliocene Warm Period (3.3–3.1 Ma) suggesting that this interval was characterized by SST fluctuations at Site U1463. A major feature of both the TEX86 and long‐chain diol index records is a strong cooling from ~1.7 to 1.5 Ma. We suggest that this event reflects a reduction in Leeuwin Current intensity due to a major step in ongoing ITF constriction, accompanied by a switch from South to North Pacific source waters entering the ITF inlet. Our new data suggest that an additional ITF constriction event may have occurred in the Pleistocene. Plain Language Summary: The Indonesian Throughflow (ITF) represents warm water masses flowing from the western Pacific into the Indian Ocean. The ITF flows through the narrow marine passages of the Indonesian Archipelago. This Indonesian Gateway in turn limits the amount of water moving from the Pacific into the Indian Ocean. The depth and width of the Indonesian Gateway has decreased gradually over the past 5 million years due to tectonic movement, which has caused Indonesian islands such as New Guinea and Halmahera to grow and block water entering the Indonesian Archipelago from the warm South Pacific. As a result, most ITF water now derives from the cooler North Pacific, which impacts Indian Ocean temperatures and broader global ocean circulation and heat distribution. The timing of this shallowing of the Indonesian Gateway and transition from warmer South Pacific to cooler North Pacific ITF source waters is not yet fully understood. Here, we present a new sea surface temperature record from near the ITF outlet that shows intense cooling just after 1.7 million years ago. We suggest that this cooling occurred in response to a significant step in the ongoing tectonic constriction and shallowing of the Indonesian Gateway. Key Points: We provide new sea surface temperature (SST) reconstructions of the Eastern Indian Ocean near the Indonesian Throughflow (ITF) outletSea surface temperatures near the ITF outlet across the mid‐Pliocene Warm Period are up to ~2°C warmer than modern core top valuesThe ITF likely underwent restriction that caused a net decrease in Leeuwin Current intensity at ~1.7 Ma [ABSTRACT FROM AUTHOR]

Archaea play an important role in marine biogeochemical cycle; however, their phylogenetic distribution and lipid composition in the hadal zone (6–11 km water depth) are poorly known. Here, we analyzed archaeal membrane lipids and 16S rRNA gene sequences in sediments from Mariana Trench (MT), Massau Trench (MS), and New Britain Trench (NBT), varying from 1,560 to 10,840 m depth. Forty‐two intact polar lipids (IPLs) were identified, including glycerol dialkyl glycerol tetraethers (GDGTs), OH‐GDGTs, glycerol dialkyl diethers (GDDs), and archaeol (AR) with polar headgroups of monohexose (1G), dihexose (2G), trihexose (3G), and hexose‐phosphohexose (HPH). Compositional and spatial distribution patterns of archaeal lipids suggest benthic Thaumarchaeota as a major source for IPLs, consistent with the predominance of Thaumarchaeota genes (>80%). The redundancy analysis (RDA) based on lipid and 16S rRNA data separates samples into three groups: extremely deep water (MT), significant terrestrial influence (NBT 1, 4, and 6), and predominant marine influence (MS, NBT 2, 3, 7, and 10). 1G‐GDDs and 1G‐AR positively correlate with water depth, likely reflecting the adaptation of benthic archaea to elevated hydrostatic pressure or variation of archaeal community in trench sediments. Bathyarchaeota are more abundant in sediments receiving terrestrial input; this pattern was attributed to their capability of utilizing terrestrial organic matter as an energy source. Our study highlights important environmental influences (e.g., pressure and organic matter quality and quantity) on benthic archaeal community and archaeal IPL compositions, which should be considered when IPLs and core lipids are applied as chemotaxonomic markers and paleo‐proxies. Key Points: Benthic archaea are a major source for IPLs in trench sedimentsSome IPLs positively correlate with water depth, reflecting adaption of archaea to hydrostatic pressure or changes of archaeal communityBathyarchaeota are more abundant in sediments receiving terrestrial input, attributed to utilization of terrestrial organic matter Plain Language Summary: This work is the first report on intact polar lipids (IPLs) of archaea in the hadal trenches, the deepest ocean realm with water depth exceeding 6,000 m. We have investigated in IPL biomarker 16S rRNA gene sequences and biogeochemical parameters in 19 sediments from the Mariana Trench, Massau Trench, and New Britain Trench. These samples are characterized by different water depth (1,560 to 10,840 m), net primary productivity (48 to 123 C m−2 year−1), and terrestrial influence. By comparing biomarker, gene data, and bulk geochemical parameters, we have achieved three key findings: (1) Benthic Thaumarchaeota are a major source for IPLs in hadal sediments; (2) Some IPLs positively correlate with water depth, likely reflecting an adaption mechanism of benthic archaea to elevated hydrostatic pressure or variation of archaeal community in different environments; and (3) Bathyarchaeota are more abundant in sediments receiving terrestrial input (New Britain Trench); this pattern is attributed to their capability of utilizing terrestrial organic matter as an energy source. Our findings provide important information toward understanding of the distribution and adaptation of hadal microbes to extremely high pressure, dark, and food‐depleted environments. [ABSTRACT FROM AUTHOR]

Key features of late Neogene climate remain uncertain due to conflicting records derived from different sea surface temperature (SST) proxies. To understand scenarios in which proxy‐derived temperature estimates can be used interchangeably or are instead measuring different aspects of the same system, it is necessary to explore both the consistencies and differences between specific paleothermometers. Here, we report orbital‐scale climate records from ODP Site 846 in the eastern equatorial Pacific (EEP) for the interval from ~5–6 Ma using alkenone and archaeal lipid paleothermometers. Results from both proxies are similar in their secular trends and magnitude of long‐term temperature change, and spectral analysis demonstrates that the records are coherent and in‐phase in both the obliquity and precession bands. However, we find that the temperatures reconstructed by TEX86 are consistently offset toward colder values by ~2 °C relative to Uk′37‐derived temperatures in global calibrations, or by ~0.8 °C in Bayesian‐based calibrations. All combinations of calibrations also yield approximately twice the amplitude of orbital‐scale variation in TEX86 relative to Uk′37‐derived temperature fluctuations. Both temperature proxies are negatively correlated with sedimentary alkenone concentrations, which we use as an indicator of increased export productivity. Removing this productivity contribution from TEX86 results in an adjusted TEX86 record with temperature sensitivity identical to Uk′37. In future applications, this signal may be decoupled using additional sedimentary indicators of paleoproductivity, which likely will be most important for upwelling zone environments. There remain other nonexplained factors that contribute to differences between TEX86 and Uk′37 that warrant additional investigation. Plain Language Summary: The remains of organisms found in ocean sediments contain information about the climate at the time the organisms were alive. Some of these remains enable the reconstruction of past temperature and are used as "paleothermometers." Many different types of paleothermometers exist; however, none work across all time periods or geographic locations. Thus, data from different paleothermometers are commonly used interchangeably to study Earth's past climate. However, little previous work has been done to study the limitations of this approach. Two of the most widely used paleothermometers are based on lipids of marine microorganisms. Here, we generated high‐resolution temperature reconstructions using both techniques. This allowed us to compare the timing, direction, and magnitude of the reconstructed temperature changes over a time interval of nearly 1 million years. Encouragingly, both paleothermometers show the same general patterns through time, although they yield modestly different results. These differences are greater during cold climate conditions and appear to be in part due to the influence of the amount of local biological productivity on one of the paleothermometers. We propose some ways that the contribution of factors other than temperature, including not only productivity but also community composition, might be explored in the future. Key Points: Orbital‐resolution Uk′37 and TEX86 paleotemperature records are coherent and in‐phase in the late Neogene eastern equatorial PacificRegardless of calibration choice, archaeal lipid records yield colder temperatures with larger‐amplitude variations than alkenone recordsTemperature and productivity explain half the variance between Uk′37 and TEX86 records; other unconstrained factors warrant exploration [ABSTRACT FROM AUTHOR]

We reconstruct sea surface temperatures (SSTs) at Deep Sea Drilling Project Site 608 (42.836°N, 23.087°), north of the Azores Front, and Ocean Drilling Program Site 982 (57.516°N, 15.866°), under the North Atlantic Current, in order to track Miocene (23.1–5.3 Ma) development of North Atlantic surface waters. Mean annual SSTs from TEX86 and UK′37 proxy estimates at both sites were 10–15 °C higher than modern through the Miocene Climatic Optimum (17–14.5 Ma). During the global cooling of the Middle Miocene Climate Transition (~14.5–12.5 Ma), SSTs at midlatitude Site 608 cooled by ~6 °C, whereas high‐latitude Site 982 cooled by only ~2 °C, resulting in an ~4 Myr collapse of the SST gradient between the two sites. This regional pattern is inconsistent with an increased latitudinal surface temperature gradient, as generally associated with global cooling episodes linked to decreasing pCO2 levels. Instead, the pattern is best explained by enhanced ocean heat transport into the high‐latitude North Atlantic superimposed on the global cooling trend, probably due to enhanced Atlantic meridional overturning circulation and/or a stronger North Atlantic Current. During global late Miocene cooling (~8–7 Ma), surface waters cooled by ~6 °C at Site 982 while minimal change occurred at Site 608, reestablishing the North Atlantic SST gradient. The collapse and reemergence of the SST gradient between the middle‐ and high‐latitude North Atlantic suggests that interaction between changes in regional ocean circulation and the global response to changes in greenhouse gas concentration was important in Miocene climate evolution. Key Points: Biomarker estimated North Atlantic sea surface temperature was 10–15 °C warmer than modern during Miocene Climatic Optimum (17–14.5 Ma)Surface temperature gradient from middle‐ to high‐latitude North Atlantic collapsed for ~4 Myr following global cooling starting at ~14.5 MaOcean heat transport to the high‐latitude North Atlantic increased at ~14 Ma, likely due to oceanic gateway or ocean circulation changes [ABSTRACT FROM AUTHOR]

• High diversity of the archaeal community in the Mediterranean Sea water column. • Dominance of Euryarchaeota Group II and III in upper epipelagic waters (0–100 mbsl). • Shift in archaeal community composition coincided with a change in IPL distribution. • Ratio of GDGT-2/GDGT-3 increased with water depth. • Thaumarchaeota were abundant in the deeper part of the water column. Thaumarchaeota are one of the most abundant groups of Archaea in the marine water column. Their membrane consists of isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) which are applied in the widely used TEX 86 proxy to reconstruct past sea surface temperatures (SSTs). However, in some specific marine systems, such as the Mediterranean Sea, core-top TEX 86 -derived temperatures do not seem to reflect annual mean SSTs. This has been attributed to contributions of deep-water dwelling Thaumarchaeota. Here, we investigate the potential causes of this bias by studying both the archaeal diversity as well as the intact polar lipid (IPL) GDGT composition in the Mediterranean water column by a combined 16S rRNA gene amplicon sequencing and a lipidomic approach on suspended particulate matter (SPM) at different water depths. The archaeal distribution showed a dominance of archaea other than Thaumarchaeota, i.e. Marine Euryarchaeota group II and III in the upper epipelagic waters (0–100 m deep), while Thaumarchaeota (Marine group I; MGI) dominated the subsurface and the deeper waters. This shift in the archaeal community composition coincided with a decrease in IPL GDGT-0 and increase of IPL crenarchaeol. The ratio of GDGT-2/GDGT-3 increased with water depth, but values were lower than observed in deep marine waters of some other regions. The increase of the GDGT-2/GDGT-3 ratio coincided with the high relative abundance of deep-water MGI, which may be linked to the high temperature and salinity found in specific water masses of the Mediterranean Sea. We conclude that these particularities of the Mediterranean Sea are responsible for the overestimated SST based on TEX 86. [ABSTRACT FROM AUTHOR]

Millennial-scale oscillations are known to be important in the climatic evolution of the Atlantic basin, but which internal processes originates these oscillations are still uncertain. In this study, we investigated how the Greenland and Antarctic climates affect the SW Atlantic through basin-wide oceanographic features (such as the NADW formation and the Agulhas leakage). We reconstructed sea surface and subsurface temperatures (SST and subT) using three lipid-based biomarker proxies (UK' 37 , TEX 86 and LDI indexes) from a sediment core (NAP 63-1) retrieved from the SW Atlantic slope (24.8°S, 44.3°W). This location allowed us to evaluate the temperature oscillations of the Brazil Current without any terrigenous or upwelling-derived biases. Both TEX 86 -based and LDI-based estimates represent the mean annual SST, while the UK' 37 -based estimates represent the subT (around 30 m water depth). The periods with the most well-mixed water column were observed during intervals of cooling orbital trends due to the time required to transfer the surface cooling to the subsurface. The temperature reconstructions showed a general colder MIS 3 when compared to the MIS 4. They also showed evidence of a late response to deglaciation, with its onset in the SW Atlantic occurring in the middle of the Last Glacial Maximum. Based on these reconstructions, the NAP 63-1 SST orbital-scale trend seems to be linked to the Antarctic climate, influenced by local insolation changes. These temperature records also presented a clear millennial periodicity around 8 kyr. On this timescale, the millennial oscillations in the SW Atlantic's SST are likely linked to the NADW formation. • Distinct proxies represented different water depths based on the local biota. • A more mixed water column (0–30 m) was observed during cooling periods. • The SST presented a millennial-scale oscillation with an 8 kyr periodicity. • NADW formation may have caused SST millennial-oscillations in the SW Atlantic. [ABSTRACT FROM AUTHOR]

• LDI-reconstructed sea water temperature is impacted by seasonality. • F C32 1,15 is influenced by sea-level changes in the Sea of Okhotsk. • 1,14-diols and opal suggest that primary productivity is controlled by sea-ice cover. • TEXL 86 -temperatures shows a 100-kyr cyclicity after 1100 ka. Long-chain diols have been detected in a wide range of environments and have been used to reconstruct past environmental changes, however only a few long-term records exist to date. Here we reconstructed past environmental changes in the central Sea of Okhotsk over the last 1.5 million years, covering the Mid-Pleistocene Transition (MPT). Sea surface temperatures (SST) reconstructed using the Long-Chain Diol Index (LDI) reflects glacial/interglacial changes. However, when compared with other organic paleothermometers (Uk′ 37 and TEXL 86) the LDI-SST is lower during interglacials and similar or higher during glacials, possibly suggesting a shift of diol production season during interglacials. The LDI-SST does not change in periodicity around the MPT as observed for the TEXL 86 , likely due to this seasonal shift. Diatom productivity, as recorded by 1,14-diols and biogenic opal content, increased during the main deglaciations with a succession from Proboscia diatoms to diatoms with a more heavily silicified shell, confirming that primary productivity in the central Sea of Okhotsk is driven by sea-ice progress and retreat. In contrast to the LDI-SST, the 1,14-diols record shows a change in periodicity around the MPT from 41- to 100-kyr cycle, suggesting an influence of orbital parameters on diatom productivity. In the central Sea of Okhotsk, the relative amount of C 32 1,15-diol (F C32 1,15), a proxy for riverine input, correlates with sea-level change with more riverine-derived material reaching the core site when the Amur River mouth is closer at lower sea-levels. In agreement, F C32 1,15 shows a change in periodicity during the MPT, with the appearance of a 100-kyr cycle. Our results show that the long chain diols can provide important paleoceanographic information in subpolar environments over long time scales, but that temperature reconstructions can be severely impacted by changes in seasonality. [ABSTRACT FROM AUTHOR]

• Suspended particular matter from Lake Malawi was analyzed for isoprenoid GDGTs. • HPH-crenarchaeol concentrations were greatest just below the chlorophyll maximum. • Maximum CL concentrations likely reflect degradation of recently produced IPL GDGTs. • A second, deeper-dwelling Thaumarchaeotal population exists near the chemocline. Several long paleoclimate records generated from Lake Malawi sediments rely on an assumption that the TEX 86 paleothermometer reflects annual mean lake surface temperatures. Thaumarchaeota, the producers of the isoprenoid glycerol dialkyl glycerol tetraether (iGDGT) lipids that are the basis of the TEX 86 proxy, can occupy a wide range of habitats in the upper water column of lacustrine systems, so it is crucial to specifically constrain the ecology of Thaumarchaeota in Lake Malawi to properly interpret its sedimentary TEX 86 record. To investigate the spatial and vertical distribution of Thaumarchaeotal iGDGT production in Lake Malawi, suspended particulate matter (SPM) was collected from the upper water column (>300 m) at three sites spanning the north, central, and south basins of the lake and analyzed for intact polar (IPL) and core (CL) iGDGT lipid abundances. Samples were collected in January during the austral summer when the lake is strongly stratified. Concentrations of the most labile IPL, hexose-phosphohexose (HPH)-crenarchaeol, were greatest just below the deep chlorophyll maximum at ∼50 m water depth in the deeper north and central basins and ∼30 m in the shallow south basin. Maximum CL concentrations occur below the maximum HPH-crenarchaeol concentrations and therefore possibly reflect the accumulation of recently produced IPL GDGT degradation products. If the export of CLs to the sediments is dominated by this CL pool, sedimentary TEX 86 would reflect Thaumarchaeota living within the thermocline during the stratified season and therefore may have a cool bias rather than reflecting true surface water temperatures. An increase in abundances of GDGT-2, crenarchaeol isomer, and monohexose (MH)-crenarchaeol at ∼150–200 m suggests that a secondary Thaumarchaeotal population, likely Group I.1b Thaumarchaeota, inhabits the subsurface water column near the anoxic-suboxic boundary. Total production of iGDGTs by this group appears to be much lower than the surface-dwelling clade, but its imprint on sedimentary TEX 86 is unknown. An analysis of iGDGT production in the water column throughout the annual cycle is needed to resolve the timing and magnitude of export of CLs to the sediments from these two Thaumarchaeotal populations. [ABSTRACT FROM AUTHOR]

RI‐OH (ring index of hydroxylated tetraethers) has recently been proposed to reconstruct paleotemperatures in middle‐ to low‐latitude marginal seas. However, RI‐OH has barely been tested in marginal seas under substantial terrigenous inputs. Here we analyze tetraether lipids in two adjacent marine cores from the Gulf of Lions. We then test for the first time the RI‐OH paleothermometer from 160 to 9 ka BP in the western Mediterranean Sea. While terrigenous inputs prevent TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) from behaving as a paleothermometer, RI‐OH is generally consistent with other paleothermometric proxies. RI‐OH also responds systematically and coherently to glacial‐interglacial transitions as well as to abrupt climatic events. The average difference between RI‐OH temperatures and November–May UK′37 (C37 ketone unsaturation ratio) temperatures is −2.0 °C with a standard error of 0.4 °C based on 249 RI‐OH‐UK′37 comparisons. This systematic difference suggests that hydroxylated tetraethers and alkenones record different temperatures, for instance, winter and/or subsurface temperatures for RI‐OH. Another source of bias could be linked to the available RI‐OH‐temperature calibration, which clearly needs more work at the global and regional scales, notably for semienclosed basins such as the Mediterranean Sea. Nevertheless, our RI‐OH‐based interglacial‐glacial anomalies are of 10 °C, a value within the high end of anomalies from previously published temperature records in the western Mediterranean Sea (from 3 to 13 °C). The RI‐OH‐based temperature anomalies also confirm the regional differences and seasonal contrasts in interglacial‐glacial anomalies produced by models. Key Points: RI‐OH consistently records glacial‐interglacial transitions and abrupt climatic events despite substantial terrigenous inputsRI‐OH could reflect winter and/or subsurface temperatures rather than annual sea surface temperatures in the Gulf of LionsRI‐OH‐based interglacial‐glacial anomalies are high but plausible compared to reported anomalies in the western Mediterranean Sea [ABSTRACT FROM AUTHOR]

The Late Cretaceous was a greenhouse world, characterized by elevated temperatures and high atmospheric pCO2. Even in the context of an extreme greenhouse climate, existing planktic foraminiferal δ18O data from the Falkland Plateau (paleolatitude of ~55°S) suggest anomalous warmth, with sea‐surface temperatures (SSTs) >30 °C for much of the Late Cretaceous, followed by sudden cooling in the Campanian. Over the last two decades, there has been discussion as to whether these high δ18O‐based SSTs reflect a genuine temperature signal and, if so, whether there was a local temperature anomaly in the South Atlantic or whether the data are representative of zonal paleotemperatures at 55°S. To provide new insights into the degree of ocean warming in the southern high latitudes during the Late Cretaceous (Cenomanian to Campanian), new SST records from the Falkland and Kerguelen Plateaus are presented here using the organic geochemical paleothermometer TetraEther indeX of 86 carbon atoms (TEX86). Overall, the TEX86 data support the δ18O data, indicating extreme and widespread warmth in the middle to high southern latitudes in the Late Cretaceous, with SSTs from 27 to 37 °C. Crucially, the TEX86 data show slow, steady cooling from the Turonian to the Campanian and suggest that temperature gradients during the Campanian did not become as steep as suggested by some planktic foraminiferal data. Key Points: New TEX86 data show extreme warmth in the southern high latitudes during the Late CretaceousThese data indicate slow, steady cooling from the Turonian–mid–Campanian [ABSTRACT FROM AUTHOR]

Highlights • Exceptionally high abundance of GDGT-0 in the volcanic lakes of northeastern China. • iGDGT production increases with lake water depth. • TEX 86 values in water column SPMs are correlated with in situ water temperature. • Sedimentary TEX 86 infers a colder temperature in deeper lakes. • TEX 86 has potential for lake level reconstructions with temperature constraints. Abstract The TEX 86 index, based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), has been widely applied for reconstructing past sea surface temperature (SST). However, its successful application in lacustrine environments has been limited to certain large lakes in which aquatic Thaumarchaeota are thought be the dominant iGDGT producers. In small lakes, multiple factors, such as iGDGTs from soil and non-thaumarchaeotal sources, could complicate the use of TEX 86 as a temperature proxy (Powers et al., 2010). Here we investigate iGDGTs from surface sediments and suspended particulate matter (SPM) in a series of small volcanic lakes (areas: ∼0.1–8.2 km2) from three main volcanic fields of northeastern China. The iGDGT distribution patterns from surface sediments of these lakes differ significantly from those of surrounding soils, global marine sediments, as well as cultured Group I.1a and Group I.1b Thaumarchaeota, suggesting a substantial contribution of autochthonous non-thaumarchaeotal sources. Interestingly, however, the correlation between TEX 86 values in the SPM samples and in situ water temperatures remain strong and display temperature sensitivity similar to previously published linear calibrations from global marine and lake sediments. Our results suggest that the in situ TEX 86 -temperature relationship is little affected by the non-thaumarchaeotal iGDGT sources in these lakes. However, the TEX 86 -inferred temperatures from surface sediments show increasingly colder offsets relative to air temperatures with increasing lake depth, which can be explained by the observed increase in iGDGTs production and decrease in water temperatures in water column as depth increases. Our results thus suggest that TEX 86 values in the sediments of these volcanic lakes are affected by both temperature and lake depth, and when temperature can be constrained independently or remains constant over a given period of time, have potential application as a lake level proxy in deep volcanic lakes. [ABSTRACT FROM AUTHOR]

Highlights • Dominant di-unsaturated C 40 alkenone observed in Late Cretaceous sediments. • Cretaceous GDGT distributions similar to modern-day confirming TEX 86 consistency. • Diagenetic conversion of GDGTs into the GDDs has no apparent effect on TEX 86. • Unusual long-chain diol distributions observed before Miocene, compromising LDI. • Long-chain keto-ols have same distributions as diols suggesting diagenetic link. Abstract We evaluated changes in the distributions of long-chain alkenones, long-chain diols and GDGTs, lipids commonly used for paleothermometry, over the last 90 Myrs for sediments deposited on the New Jersey shelf (the Bass River site) and assessed potential effects of different ancestral producers and diagenesis on their distributions and their impact on the associated temperature proxies. As reported before, the Paleogene distributions of alkenones are generally similar to those in modern haptophytes, but unusual alkenone distributions, characterized by a dominant di-unsaturated C 40 alkenone, are observed for Late Cretaceous sediments, suggesting different ancestral source organisms for alkenones in this interval. The isoprenoid GDGT distributions remained comparable to modern-day distributions, suggesting that TEX 86 can be applied up to ca. 90 Ma. The Miocene long-chain diol distributions are similar to modern-day distributions, but the older sediments reveal unusual distributions, dominated by the C 28 1,12- and C 26 1,13-diols, suggesting different source organisms before ∼30 Ma. Accordingly, the LDI does not match other paleotemperature proxies, suggesting its applicability might be compromised for sediments older than the Miocene. Our results indicate that of the three proxies, the TEX 86 seems to be the most applicable for deep time temperature reconstructions. [ABSTRACT FROM AUTHOR]

Highlights: • Mass spectrometry imaging (MSI) is enabled for a suite of molecular biomarkers. • Silver- or carbon-based matrices are more effective than conventional MALDI matrices. • Tested sediments provide an adequate natural matrix for most compounds. • Dedicated MS methods and narrow m / z ranges are required for each biomarker class. • Use of MSI for multiproxy ultra-high resolution molecular stratigraphy is explored. Abstract Mass spectrometry imaging (MSI) collects mass spectra of organic compounds from individual micrometer-sized spots and yields high-resolution images of the spatial distribution of target analytes on sample surfaces. MSI can potentially open a new avenue to ultra-high resolution molecular stratigraphy by resolving the fine-scale distribution of molecular biomarkers in geological records. However, ionization of organic molecules within their sediment matrix remains a critical challenge. Building on the recent introduction of MSI of archaeal tetraether lipids, we have extended the analytical spectrum to additional biomarkers and provide guidelines for the generation of multiproxy, ultra-high resolution paleoenvironmental records. We evaluated the addition of artificial matrices to promote ionization, defined the most suitable MS settings, and increased analytical complexity from pure compounds to their investigation in sediment. Most compounds relevant to molecular stratigraphy are not properly ionized in the presence of conventional matrices, but require atypical ones, such as those based on carbon or silver. Sediments serve as a natural matrix, directly allowing detection of many of these compounds. However, the sediment matrix also inhibits some reactions that otherwise promote detection, such as derivatization of alkenones or silver-mediated ionization of n -alkanes. The robustness of MSI-based molecular stratigraphy is enhanced by analysis of target compounds in narrow m / z ranges, and by summation of mass spectra from several coeval measurement spots. We present an initial inventory of compounds readily detected in the sediment tested, including long chain alkenones and diols, sterols, and pigments, and provide an outlook into the use of MSI in multiproxy studies. [ABSTRACT FROM AUTHOR]

Abstract Ammonia-oxidizing archaea (AOA) are among the most abundant microbes in the oceans and are one of the major sources of glycerol dibiphytanyl glycerol tetraethers (GDGTs) in the water column and underlying sediments. However, little is known about the mechanistic steps during biosynthesis of GDGTs that form the basis of the TEX 86 paleothermometer. Recent results showed that, apart from temperature, physiological factors such as growth stage and variations of the ammonium oxidation rate may affect the TEX 86 temperature proxy. We performed a short term incubation experiment with radiolabeled 14C-bicarbonate to accurately trace the effect of high and low ammonium (NH 4 +) supply, on the production of individual membrane lipids by the AOA model organism Nitrosopumilus maritimus. The 14C incorporation during growth was monitored at five time intervals by liquid chromatography coupled to flow-through scintillation counting of the hydrolyzed membrane lipid extract, allowing a straight forward and sensitive on-line detection of 14C incorporation into archaeal lipids on time-scales lower than a single cell cycle. The experiments showed that low NH 4 + supply results in higher cyclization of GDGTs with a preferential synthesis of crenarchaeol, whereas excess NH 4 + led to predominant production of GDGT-0. Consequently, the cultures with a high NH 4 + supply resulted in up to 10 °C lower estimated incubation temperatures than the cultures with low quantities of available NH 4 + using the TEX 86 L calibration. Interestingly, a high relative production of archaeol was observed at the beginning of all experiments (up to 27%), independent of the NH 4 + supply; likewise the degree of cyclization was initially lowest indicating delayed production of cycloalkylated derivatives. This pattern is consistent with N. maritimus synthesizing GDGTs by head-to-head condensation of two archaeol molecules and subsequent cyclization of the resulting acyclic tetraether. This study provides robust information on the biosynthesis of GDGTs in N. maritimus and advances our understanding of the influence of NH 4 + supply on the TEX 86 proxy. [ABSTRACT FROM AUTHOR]

The TEX86 proxy, based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) from planktonic Thaumarchaeota, is widely used to reconstruct sea surface temperature (SST). Recent observations of species‐specific and regionally dependent TEX86‐SST relationships in cultures and the modern ocean raise the question of whether nonthermal factors may have impacted TEX86 paleorecords. Here we evaluate the effects of ecological changes on TEX86 using one Pliocene and two Pleistocene sapropels from the Mediterranean Sea. We find that TEX86‐derived SSTs deviate from U37K'‐derived SSTs before, during, and after each sapropel event. U37K'‐derived SSTs vary by less than 6 °C, while TEX86‐derived SSTs vary by up to 15 °C within a single record. Compound‐specific carbon isotope compositions indicate minimal confounding influence on TEX86 from exogenous sources. Some of the variation can be accounted for by changes in nitrogen cycling intensity affecting thaumarchaeal iGDGT biosynthesis, as demonstrated by an inverse relationship between TEX86 and δ15NTN. TEX86‐derived SSTs also consistently show warm anomalies in the Pleistocene, while the Pliocene samples exhibit both warmer and cooler relative offsets. These anomalies result from systematic differences between Plio‐Pleistocene iGDGT distributions and both modern Mediterranean and modern, globally distributed core top samples. Through characteristic GDGT distributions, we suggest the existence of three distinct endemic populations of Thaumarchaeota in the Pliocene, Pleistocene, and modern Mediterranean Sea, respectively. Importantly, these communities prevailed during both sapropel and oligotrophic conditions. Our results demonstrate that ecological and community‐specific effects must be considered when applying the TEX86 proxy to paleorecords. Key Points: Mediterranean Pliocene‐Pleistocene sapropels were investigated for TEX86, archaeal lipid carbon isotopic compositions, and UK'37Nontemperature effects on TEX86 ocean temperature reconstructions are observed in all sapropels and surrounding marlsDistinct archaeal lipid distributions could be indicative of changing thaumarchaeal communities through time [ABSTRACT FROM AUTHOR]

Abstract The origins of intact polar lipid glycerol dialkyl glycerol tetraethers (IPL-GDGTs) with glycosidic headgroups that are present in deep-sea sediments are still unclear. Here, we tried to clarify this issue by comparing variations in TEX 86 temperatures and compositions of IPL (dihexose (DH) and monohexose (MH) head groups) and core lipid (CL) GDGTs in a sediment core covering a period since the last glacial time in the northern South China Sea (SCS). Haptophyte-derived alkenones were also analyzed to provide an independent paleo-sea surface temperature (SST) record. The sedimentary DH-GDGTs were composed mainly of DH-GDGT-1, -2, -3, and the crenarchaeol isomer. In contrast, both the sedimentary MH- and CL-GDGTs were composed mainly of crenarchaeol and GDGT-0. However, in each pool of GDGTs, crenarchaeol correlated significantly with other GDGT compounds, suggesting a shared origin. The DH-TEX 86 -derived temperature values were systematically higher than, but well correlated with CL-TEX 86 and UK' 37 -derived SST values, showing an expected glacial-interglacial increase. This finding suggests that the DH-GDGTs could have originated mainly from the upper water column and may have been well preserved in sediments for a long time. However, the MH-TEX 86 -derived temperature did not display such a climatic pattern but exhibited unexpected higher values during the last glacial period. We propose that the elevated glacial MH-TEX 86 -derived temperature was likely associated with the ammonia limitation and subsequently reduced ammonia oxidation rate in seawater during the last glacial period. This study indicates that the sedimentary glycosidic GDGTs could settle mainly from the water column rather than being produced in situ. [ABSTRACT FROM AUTHOR]

Abstract Seasonal variations in fluxes of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and the estimated temperatures based on TEX 86 were examined in sinking particles collected using moored sediment traps in the eastern Atlantic upwelling regions. In the equatorial Guinea Basin, GDGT fluxes show a correlation with opal fluxes, implying that GDGTs are mainly transported via aggregation with diatoms. The flux-weighted T E X 86 H temperatures derived from particles collected both at 853 m and 3921 m depth correspond to the water temperature (24.1 °C) of ca. 50 m depth, where nitrate concentration starts to increase. This suggests that nutrient concentrations may affect the depth habitat of Thaumarchaeota, thereby influencing the TEX 86 -derived water temperatures. In the coastal upwelling off Namibia, T E X 86 H temperatures are similar to satellite-derived sea surface temperature (SST) during the warm season, but the record derived from the trap is delayed relative to the SST by approximately 26 days. Warm biases, however, occur during the cold season. Higher TEX 86 values have been found within the oxygen minimum zones (OMZs) in the water column of coastal upwelling regions. Thus, contributions from GDGTs produced in OMZs might explain the warmer temperature estimates during the cold season in regions where OMZs are pronounced. This scenario could explain the observed warm bias off Namibia. We, therefore, suggest that in the eastern Atlantic upwelling systems, nutrient depth distribution and GDGTs produced in OMZs can be potential environmental factors influencing TEX 86 in sinking particles. In paleoenvironmental records of TEX 86 , non-thermal signals have to be considered on regional scales. [ABSTRACT FROM AUTHOR]

Highlights • Crenarchaeol and its isomer were isolated from marine surface sediments. • Structures were studied by ether cleavage and GC–MS using mild ionization. • A tricyclic biphytane stereoisomer is formed from the crenarchaeol isomer. • 13C NMR data suggest the cis stereochemistry of one of the cyclopentane moieties. • This difference likely affects the packing of lipid membranes of Thaumarchaeota. Abstract Isolation of crenarchaeol and its isomer from marine surface sediments, followed by ether cleavage and GC–MS characterization using supersonic molecular beam (SMB) ionization of the biphytanes formed, revealed that the crenarchaeol isomer comprises a tricyclic biphytane that is stereochemically different from the tricyclic biphytane of crenarchaeol. This isomeric tricyclic biphytane was also released from the crenarchaeol isomer in extant Thaumarchaeotal biomass. Reinterpretation of previously obtained 13C NMR data of the crenarchaeol isomer suggested that the cyclopentane moiety adjacent to the cyclohexyl moiety of the tricyclic biphytane of the crenarcheaol isomers possesses the unusual cis stereochemistry in comparison to the trans stereochemistry of all cyclopentane moieties in crenarchaeol. This stereochemical difference likely affects the packing of lipid membranes of Thaumarchaeota and therefore provides a biophysical explanation for the role of the crenarchaeol isomer in the TEX 86 palaeothermometer based on fossilized Thaumarcheotal lipids. [ABSTRACT FROM AUTHOR]

Archaea are widespread and abundant in aquatic and terrestrial habitats and play fundamental roles in global biogeochemical cycles. Archaeal lipids, such as isoprenoid glycerol diakyl glycerol tetraethers (iGDGTs), are important biomarkers tracing changes in archaeal community structure and biogeochemical processes in nature. However, the linkage between the archaeal populations and the GDGT distribution in the natural environment is poorly examined, which hindered the application and interpretation of GDGT-based climate or environmental proxies. We addressed this question by investigating changes in archaeal lipid composition and community structure in the context of environmental variables along the subtropical Jiulong River Watershed (JRW) and Jiulong River Estuary (JRE) in southern China. The results showed that both the archaeal cells and the polar GDGTs (P-GDGTs) in the JRW and JRE were mostly autochthonous rather than exogenous input from surrounding soils. We further found that only five ( Methanobacteriales, Ca. Bathyarchaeota, Marine Benthic Groups A (MBGA), Marine Benthic Groups B (MBGB), and Marine Benthic Groups D (MBGD)) out of sixteen lineages showed significant impacts on the composition of P-GDGTs, suggesting the significant contribution of those archaea to the changes of P-GDGT compositions. Salinity and total phosphorus (TP) showed significant impact on the distribution of both genetic and P-GDGTs compositions of archaea; whereas, sand and silt contents only had significant impact on the P-GDGTs. MBGD archaea, which occur widely in marine sediments, showed positive correlations with P-TEX in the JRW and JRE, suggesting that uncultivated MBGD might also contribute to the variations in TEX signals in marine sediments. This study provided insight into the sources of P-GDGTs and the factors controlling their distributions in river-dominated continental margins, which has relevance to applications of GDGT-based proxies in paleoclimate studies. [ABSTRACT FROM AUTHOR]

Wetter climatic conditions prevailed across northern and Equatorial Africa during the terminal Pleistocene until the middle Holocene, a climate event known as the 'African Humid Period' (AHP). Although hydrologic changes during this period are well-known, very few records of temperature are available for evaluating the mechanisms and dynamics of climate change during the AHP across tropical Africa. To quantify changes in temperature during the onset and termination of the AHP, the TEX temperature proxy was used to generate a 14,000 year record of the surface temperature of Lake Turkana. This biomarker and related measures have been used to reconstruct regional and high-latitude paleotemperatures from the oceans and other continental systems, including other large African lakes. Although Lake Turkana's climate and hydrology are very different from other large African lakes, our reconstruction exhibits a temperature history that shares a number of climatic trends with previous reconstructions. The TEX temperatures from Lake Turkana from 14 to 0.4 ka range from 24.3 to 28.6 °C, with a gradual decrease in temperature from early to late Holocene. This cooling roughly follows northern hemisphere summer insolation, similar to the trend observed in Lake Victoria and other regional records. However, the record from Turkana contains many abrupt temperature shifts not seen in other large lakes in the region. Multi-century-scale fluctuations persist through most of the record, and can be attributed to periods of lake mixing. Larger temperature perturbations are likely associated with changes in evaporation and cloud cover. [ABSTRACT FROM AUTHOR]

TEX86 [TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms] has been widely applied to reconstruct (paleo-) sea surface temperature. Marine Group I (MG-I) Thaumarchaeota were thought to be the primary source of GDGTs constituting the TEX86 formula; however, recent research has suggested that Marine Group II (MG-II) Euryarchaeota may also contribute significantly to the GDGT pool in the ocean. Little is known regarding the potential impact of MG-II Euryarchaeota-derived GDGTs on TEX86 values recorded in marine sediments. In this study, we assessed the relationship between distributions of GDGTs and MG-II Euryarchaeota and evaluated its potential effect on the TEX86 proxy. Lipid and DNA analyses were performed on suspended particulate matter and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (SCS, seawater). TEX86-derived temperatures from the water column and surface sediments were significantly correlated and both were lower than satellite-based temperatures. The ring index (RI) values in these environments were higher than predicted from the calculated TEX86-RI correlation, indicating that the GDGT pool in the water column of the PR estuary and coastal SCS comprises relatively more cyclopentane rings, which thereby altered TEX86 values. Furthermore, the abundance of MG-II Euryarchaeota 16S rRNA gene in the mixing water was two to three orders of magnitude higher than those observed in the river or seawater. Significant linear correlations were observed between the gene abundance ratio of MG-II Euryarchaeota to total archaea and the fractional abundance of GDGTs with cyclopentane rings. Collectively, these results suggest that MG-II Euryarchaeota likely produce a large proportion of GDGTs with 1–4 cyclopentane moieties, which may bias TEX86 values in the water column and sediments. As such, valid interpretation of TEX86 values in the sediment record, particularly in coastal oceans, should consider the contribution from MG-II Euryarchaeota.

TEX86 [TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms] has been widely applied to reconstruct (paleo-) sea surface temperature. Marine Group I (MG-I) Thaumarchaeota were thought to be the primary source of GDGTs constituting the TEX86 formula; however, recent research has suggested that Marine Group II (MG-II) Euryarchaeota may also contribute significantly to the GDGT pool in the ocean. Little is known regarding the potential impact of MG-II Euryarchaeota-derived GDGTs on TEX86 values recorded in marine sediments. In this study, we assessed the relationship between distributions of GDGTs and MG-II Euryarchaeota and evaluated its potential effect on the TEX86 proxy. Lipid and DNA analyses were performed on suspended particulate matter and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (SCS, seawater). TEX86-derived temperatures from the water column and surface sediments were significantly correlated and both were lower than satellite-based temperatures. The ring index (RI) values in these environments were higher than predicted from the calculated TEX86-RI correlation, indicating that the GDGT pool in the water column of the PR estuary and coastal SCS comprises relatively more cyclopentane rings, which thereby altered TEX86 values. Furthermore, the abundance of MG-II Euryarchaeota 16S rRNA gene in the mixing water was two to three orders of magnitude higher than those observed in the river or seawater. Significant linear correlations were observed between the gene abundance ratio of MG-II Euryarchaeota to total archaea and the fractional abundance of GDGTs with cyclopentane rings. Collectively, these results suggest that MG-II Euryarchaeota likely produce a large proportion of GDGTs with 1-4 cyclopentanemoieties, whichmay bias TEX86 values in the water column and sediments. As such, valid interpretation of TEX86 values in the sediment record, particularly in coastal oceans, should consider the contribution from MG-II Euryarchaeota. [ABSTRACT FROM AUTHOR]

A variety of biomarkers were examined from Ocean Drilling Program Core 1202B to reconstruct temperature and phytoplankton community structures in the southern Okinawa Trough since 20 ka. Two molecular temperature proxies (U37K' and TEX86) show 5°C to ~6°C warming during the glacial-interglacial transition. Prior to the Holocene, The U37K' -derived temperature was generally 1°C to 4°C higher than TEX86-derived temperature. This difference, however, was reduced to <1°C in the Holocene. Correspondingly, the phytoplankton biomarkers (e.g., C37:2 alkenone, brassicasterol, C30 1,15 diol, and dinosterol) indicate a shift of planktonic community structures, with coccolithophorids becoming more abundant in the Holocene at the expense of diatoms/dinoflagellates. This shift is related to the variability of nutrients, temperature, and salinity in the Okinawa Trough, likely controlled by the sea level and the intensity of the Kuroshio Current. The phytoplankton community change may have had profound implications for atmospheric CO2 fluctuations during glacial-interglacial cycles since diatoms and dinoflagellates have a higher efficiency of the biological pump than coccolithophorids. [ABSTRACT FROM AUTHOR]

Early Jurassic marine palaeotemperatures have been typically quantified by oxygen-isotope palaeothermometry of benthic and nektonic carbonate and phosphatic macrofossils. However, records of Early Jurassic sea-surface temperatures that can be directly compared with general circulation model simulations of past climates are currently unavailable. The TEX86 sea-surface temperature proxy is based upon the relative abundance of glycerol dialkyl glycerol tetraethers preserved in organic-carbon-bearing sediments. This proxy has been used extensively on Cretaceous and Cenozoic materials and, in one study, on Middle and Upper Jurassic sediments. Here, TEX86 is applied, for the first time, to Lower Jurassic (Sinemurian-Pliensbachian) sediments cored at Deep Sea Drilling Project Site 547 in the North Atlantic. The abundance of glycerol dialkyl glycerol tetraethers in these sediments is very low, despite biomarker and Rock-Eval data suggesting that thermal maturity is, generally, low. Sea floor oxygenation and a high input of reworked terrestrially sourced organic matter may explain the low concentrations. For samples from which it was possible to quantify the relative abundance of glycerol dialkyl glycerol tetraethers, TEX86 values range from 0·78 to 0·88, equating to sea-surface temperatures in excess of >28°C. These temperatures are broadly comparable with new general circulation model simulations of the Sinemurian and Pliensbachian stages and support the general view of a predominantly warm climate. The new proxy data suggest that, under favourable geological conditions, it is possible to extend the record of TEX86-based sea-surface temperatures back into the Early Jurassic. [ABSTRACT FROM AUTHOR]