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Highlights • New chromatographic method resolves new bacteriohopanepolyol (BHP) isomers. • BHP isomer distribution varies in marine sediments from different environments. • Method improves BHP quantification. • Method is promising for future environmental proxy development and isotope analyses. Abstract Bacteriohopanepolyols (BHPs) are characterized by a large structural diversity, although methodological constraints have limited investigations of the occurrence of isomers among composite BHPs in environmental samples. Here, we describe a novel chromatography method that uses three serial Phenomenex Kinetex C 18 columns to successfully resolve new structural isomers of BHPs commonly observed in marine sediment samples. The investigated samples consistently contain a high diversity of BHP isomers, but their relative abundances differ significantly between samples. These differences in relative abundance have potential to reflect different environmental conditions such as depositional setting or redox conditions. The improved BHP resolution and baseline separation of the new method is promising for accurate quantification and future environmental proxy development and compound-specific isotope work. [ABSTRACT FROM AUTHOR]

We present a novel method to measure the 13 C/ 12 C isotope ratio (reported as δ 13 C) of individual, intact bacteriohopanepolyols (BHPs) using semi-preparative ultrahigh pressure liquid chromatography (UPLC) followed by high temperature gas chromatography–isotope ratio mass spectrometry (HT-GC–IRMS). The method is reproducible, as indicated by the precision of δ 13 C values for bacteriohopanetetrol (BHT) extracted from R. palustris biomass and analyzed across an order-of-magnitude range of IRMS peak areas (δ 13 C = −33.4 ± 0.6‰ VPDB, n  = 94, ±1σ). To show that this method successfully separates individual BHPs from complex environmental matrices, we report δ 13 C values for BHT and the BHT stereoisomer (BHT-II) from a ca. 2.9 Ma, organic-rich Mediterranean Sea sediment sample. These analyses suggest that intact BHP δ 13 C measurements can greatly improve the interpretation of environmental signals by alleviating the need for side-chain cleavage which reduces BHP source-specificity. [ABSTRACT FROM AUTHOR]

Understanding the preservation and deposition history of organic molecules is crucial for the understanding of paleoenvironmental information contained in their abundance ratios such as U k′ 37 and TEX 86 used as proxies for sea surface temperature (SST). Based on their relatively high refractivity, alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) can survive postdepositional processes like lateral transport, potentially causing inferred SSTs to be misleading. Likewise, selective preservation of alkenones and GDGTs may cause biases of the SST proxies themselves and can lead to decoupling of both proxy records . Here we report compound-specific radiocarbon data of marine biomarkers including alkenones, GDGTs, and low molecular weight (LMW) n -fatty acids from Black Sea sediments deposited under different redox regimes to evaluate the potentially differential preservation of both biomarker classes and its effect on the SST indices U k′ 37 and TEX 86 . The decadal Δ 14 C values of alkenones, GDGTs, and LMW n -fatty acids indicate similar preservation under oxic, suboxic, and anoxic redox regimes and no contribution of pre-aged compounds, e.g., by lateral supply. Moreover, similar 14 C concentrations of crenarchaeol, alkenones, and LMW n -fatty acids imply that the thaumarchaeotal GDGTs preserved in these sediments are produced in the euphotic zone rather than in subsurface/thermocline waters. However, we observe biomarker-based SSTs that strongly deviate (ΔSST up to 8.4 °C) from in situ measured mean annual SSTs in the Black Sea. This is not due to redox-dependent differential biomarker preservation as implied by their Δ 14 C values and spatial SST pattern. Since contributions from different sources can largely be excluded, the deviation of the U k′ 37 and TEX 86 proxy-derived SSTs from in situ SSTs requires further study of phylogenetic and other yet unknown environmental controls on alkenone and GDGT lipid distributions in the Black Sea. [ABSTRACT FROM AUTHOR]

• Bacteriohopanepolyol (BHP) water column profiles trace Black Sea stratification. • Highest BHP production in the suboxic zone and sulfidic waters. • BHP-inferred metabolisms are dominated by anammox and aerobic methane oxidation. • Some BHPs assigned to aerobic bacteria have a distinct sulfidic niche. • Sediments underestimate the contribution of methylated BHPs in the water column. Redox gradients in oceanic oxygen minimum zones are hotspots of diverse bacterial populations and metabolisms. Here, we test whether bacteriohopanepolyols (BHPs), a suite of membrane-regulating pentacyclic triterpenoid lipids, are suitable tracers of major bacterial metabolisms and whether their structural diversity (BHP lipidomics) allows tracing the bacterial ecology across the Black Sea chemocline. We analyzed suspended particulate matter (SPM) sampled along depth-profiles across the redox/chemocline at two sites in the eastern and western gyres and underlying core-top sediments at the western gyre site. Our results show that BHPs abundances reflect the stratification of the Black Sea and capture ecological niches across the chemocline, being highest in the lower suboxic zone and in upper to deep sulfidic waters. BHP-inferred bacterial metabolisms in the lower suboxic zone are dominated by anammox and aerobic methane oxidizers (primarily by Type I/gammaproteobacteria). Bacterial biomass production in the upper to deep sulfidic zone is mirrored by a range of BHPs that are normally associated with aerobic metabolisms, most likely explained by either survival in oxygenated micro-niches or production by anaerobic bacteria (which could possibly include sulfate reducers and nitrite-dependent methane oxidizers). Most surprisingly, this ecological niche harbors seemingly unknown anaerobic bacteria which produce abundant C-2 and C-3 methylated BHPs. Core-top sediments reflect the BHP structural diversity found in SPM, but with notable compositional differences. Most strikingly, the export dynamics seem to result in near-absence of C-2 methylated BHPs in surface sediments. Sedimentary BHP distributions therefore underestimate the importance of C-2 methylated BHPs in the water column, which has implications for the geological record. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bacteriohopanepolyols are potent tracers of diverse bacterial metabolisms/processes. • Recent analytical advances reveal greater structural diversity than previously known. • The advent of BHP lipidomics paves the way towards new proxy discoveries/applications. • Frontiers: improve quantification, new isotope methods, paired -omics approaches. In this review we look back on 30+ years of bacteriohopanepolyol (BHP) research within the field of organic geochemistry. BHPs are ubiquitous, intact polar lipids in modern environments. They have been found in lacustrine, marine, riverine and soil and peat environments, and they are noteworthy lipids in biological symbiont studies. BHPs are the precursors of hopanoids, which are the most abundant fossil lipids found in the geological record. BHPs are synthesized by members of various bacterial taxa, and their distributions are often used to help to identify bacterial communities, in studies of both modern and past environments. However, less than 10% of known bacterial species are genetically capable of synthesizing BHPs, and many BHPs are not specific to particular bacterial sources. Nonetheless, a range of BHPs with specific side chain configurations and/or A-ring modifications have proven very useful for tracing bacterial metabolism and for identifying ecological niches in various environments (e.g., aerobic methanotrophy, possibly nitrite-dependent intra-aerobic methanotrophy, and anaerobic ammonium oxidation) or for tracing environmental processes (e.g., soil inputs into aquatic settings). Moreover, BHPs (with previously unknown terminal groups and side chain configurations) are continuously being discovered, thanks to recent methodological and instrumental advances. These highlight the advent of a new era of BHP lipidomics which awaits full exploitation in organic geochemistry. Here, we provide a summary of the state-of-the-art of BHP knowledge, analytical frontiers, and suggest directions for future research. [ABSTRACT FROM AUTHOR]

Bacteriohopanepolyols (BHPs) are biomarkers providing taxonomically and environmentally diagnostic information. BHPs may help to unravel the composition of bacterial communities residing in recent as well as ancient permafrost soils and sediments and also provide information on associated environmental conditions. However, detailed data on their distribution in the heterogeneous Arctic environment are scarce. The distribution and structural diversity of BHPs were studied in the annually thawing (active) layer of three different sites in the polygonal tundra of the Lena Delta in the Siberian Arctic. Variations between permafrost structures and soil horizons caused by differences in the physical and chemical soil properties were observed. C and N content is significantly correlated with the BHP composition so that the highest BHP concentrations and greatest structural diversity occur in the uppermost organic soil horizons, which consist mainly of fresh or little degraded plant material. Furthermore, statistical analyses reveal that higher abundances of adenosylhopane-type soil marker BHPs are linked to higher soil pH values. Small scale environmental controls on BHP distributions are reflected by amine-functionalised BHPs from methanotrophic bacteria only occurring in the water-saturated, oxygen-depleted polygon centres and by soil marker BHPs, which are significantly more abundant in the well aerated polygon rims than in the centres. In contrast, C-2 methylated BHPs, putative indicators of plant-bacterial interactions, are present in all soil horizons and permafrost structures and their relative distribution is not systematically linked to soil properties. Overall, lipid-based results agree with published 16S rRNA based community structure assessments highlighting the usefulness of BHPs to represent bacterial populations in recent and ancient permafrost soils. [ABSTRACT FROM AUTHOR]

Abstract: Transfer of organic carbon (OC) from the terrestrial to the oceanic carbon pool is largely driven by riverine and aeolian transport. Before transport, however, terrigenous organic matter can be retained in intermediate terrestrial reservoirs such as soils. Using compound-specific radiocarbon analysis of terrigenous biomarkers their average terrestrial residence time can be evaluated. Here we show compound-specific radiocarbon (14C) ages of terrigenous biomarkers and bulk 14C ages accompanied by geochemical proxy data from core top samples collected along transects in front of several river mouths in the Black Sea. 14C ages of long chain n-alkanes, long chain n-fatty acids and total organic carbon (TOC) are highest in front of the river mouths, correlating well with BIT (branched and isoprenoid tetraether) indices, which indicates contribution of pre-aged, soil-derived terrigenous organic matter. The radiocarbon ages decrease further offshore towards locations where organic matter is dominated by marine production and aeolian input potentially contributes terrigenous organic matter. Average terrestrial residence times of vascular plant biomarkers deduced from n-C29+31 alkanes and n-C28+30 fatty acids ages from stations directly in front of the river mouths range from 900±70years to 4400±170years. These average residence times correlate with size and topography in climatically similar catchments, whereas the climatic regime appears to control continental carbon turnover times in morphologically similar drainage areas of the Black Sea catchment. Along-transect data imply petrogenic contribution of n-C29+31 alkanes and input via different terrigenous biomarker transport modes, i.e., riverine and aeolian, resulting in aged biomarkers at offshore core locations. Because n-C29+31 alkanes show contributions from petrogenic sources, n-C28+30 fatty acids likely provide better estimates of average terrestrial residence times of vascular plant biomarkers. Moreover, sedimentary n-C28 and n-C30 fatty acids appear clearly much less influenced by autochthonous sources than n-C24 and n-C26 fatty acids as indicated by increasing radiocarbon ages with increasing chain-length and are, thus, more representative as vascular plant biomarkers. [Copyright &y& Elsevier]

Abstract: Paired radiocarbon measurements on haptophyte biomarkers (alkenones) and on co-occurring tests of planktic foraminifera (Neogloboquadrina dutertrei and Globogerinoides sacculifer) from late glacial to Holocene sediments at core locations ME0005-24JC, Y69-71P, and MC16 from the south-western and central Panama Basin indicate no significant addition of pre-aged alkenones by lateral advection. The strong temporal correspondence between alkenones, foraminifera and total organic carbon (TOC) also implies negligible contributions of aged terrigenous material. Considering controversial evidence for sediment redistribution in previous studies of these sites, our data imply that the laterally supplied material cannot stem from remobilization of substantially aged sediments. Transport, if any, requires syn-depositional nepheloid layer transport and redistribution of low-density or fine-grained components within decades of particle formation. Such rapid and local transport minimizes the potential for temporal decoupling of proxies residing in different grain-size fractions and thus facilitates comparison of various proxies for paleoceanographic reconstructions in this study area. Anomalously old foraminiferal tests from a glacial depth interval of core Y69-71P may result from episodic spillover of fast bottom currents across the Carnegie Ridge transporting foraminiferal sands towards the north. [Copyright &y& Elsevier]

• Global compilation of compound-specific n -alkyl lipid 14C data. • Intrinsic and extrinsic processes determine the plant wax 14C composition. • Plant wax mean age primarily controlled by temperature and precipitation. Sedimentary high molecular weight (HMW) n -alkyl lipids derived from the waxes of terrestrial plants are common target compounds in biogeochemical and paleoenvironmental research. These plant waxes derive predominantly from the epicuticular cover of vascular plant leaves and their relative and absolute abundances and stable isotopic composition can be used as proxies to decipher, e.g., continental climate and land-ocean carbon transfer processes. In marine sediments, however, compound-specific radiocarbon analysis has revealed that plant waxes are often not syn -depositional, but instead are substantially 14C-depleted ('pre-aged') upon deposition. This 14C-depletion can be caused by various processes that either promote retention of plant waxes during transport from source to sink such as storage in soils or entrainment in deposition-resuspension loops in rivers and on continental shelves or, alternatively, by processes that add HMW n -alkyl lipids from other sources (e.g., petrogenic inputs). Here, we review the intrinsic and extrinsic processes affecting the sedimentary plant wax 14C composition (ranging from chemical processes to continental-scale environmental conditions), how plant wax 14C compositions translate into mean ages, and which processes control plant wax mean ages in marine sediments. Finally, we use a compilation of available and new compound-specific plant wax 14C data to provide a synthesis and evaluate the major controls on plant wax mean ages in marine sediments at the global scale. [ABSTRACT FROM AUTHOR]

• "Soil marker" bacteriohopanepolyols detected in all investigated ODZ settings. • Water column distribution and fluxes consistent with in situ chemotrophic production. • Individual "soil marker" BHPs have different and/or shared biogeochemical niches. • Presence of "soil marker" BHPs alone is not diagnostic for soil input. • Effect on R soil index is small. Bacteriohopanepolyols (BHPs) that are commonly associated with soil and peat environments ("soil marker" BHPs), were analyzed along redox gradients in marine oxygen deficient zones (ODZs). The latter included suspended particulate organic matter (POM) from two fjords on Vancouver Island (Canada) and the Black Sea, as well as sinking POM collected in sediment trap time-series deployed in the Cariaco Basin. Our results show that the so-called "soil marker" BHPs occur in all ODZ settings. They have distributions that are best explained by in situ chemotrophic production within the chemocline and in anoxic/euxinic waters, rather than exclusive supply from soils. In particular, adenosylhopane, adenosylhopane-type 2 and 2Me-adenosylhopane-type 2, appear to be produced within a common biogeochemical niche in the water column, irrespective of setting. On the other hand, 2Me-adenosylhopane, adenosylhopane-type 3 and 2Me-adenosylhopane-type 3 may be supplied partly from the continent, as they are also abundant in surface water suspended POM on Vancouver Island and accumulate almost exclusively in the shallower (upper chemocline) trap in the Cariaco Basin. However, their water column suspended POM distributions and time-series sinking POM fluxes also indicate additional chemotrophic sources that have spatially and temporally diverse origins. Our study suggests that caution is warranted when using "soil marker" BHPs and their associated soil input proxies in ODZ settings, as the diverse microbial community in these settings produces BHP signatures similar to those found in soils. [ABSTRACT FROM AUTHOR]

• GDGT sources, distribution and environmental controls examined in 129 permafrost samples. • Isoprenoid GDGTs not controlled by physicochemical parameters. • Branched GDGTs show (anti-)correlations with %TOC, %TN, and soil pH. • Branched GDGT distributions yield a permafrost-specific soil pH calibration. • Deposit types show distinct GDGT fingerprints, which may bias sedimentary proxy records. Archaeal and bacterial glycerol dialkyl glycerol tetraethers (GDGTs) are globally abundant in soils under various climatic conditions, but little is known about their sources, relative distribution, and environmental controls on their diversity in high latitude permafrost deposits. Thus, constraints on GDGT-based proxies, such as methylation of branched GDGTs (MBT) or cyclization of branched GDGTs (CBT) used to infer mean annual temperature or soil pH, are also sparse. We investigated the GDGT diversity in typical North Siberian permafrost deposits including the active layer of polygonal tundra soils (seasonally frozen ground), fluvial terrace/floodplain sediments, Holocene and Pleistocene thermokarst sediments, and late Pleistocene Ice Complex (Yedoma). Our data show that isoprenoid GDGTs are produced by both methanotrophic and methanogenic Euryarchaeota, as well as Thaumarchaeota, but their abundance does not seem to be controlled by the investigated physicochemical parameters including %TOC, %TN, and soil pH. Branched GDGTs (brGDGTs) show similar distributional changes to those observed in other high latitude soil samples, i.e., a dominance of pentamethylated and hexamethylated brGDGTs, likely reflecting the adaptation to low temperatures and a positive correlation of the degree of cyclization with soil pH. Specifically, brGDGT-IIIa correlates positively with %TOC and %TN and negatively with soil pH, while brGDGT-Ib and brGDGT-IIb correlate negatively with %TOC and %TN and positively with pH. Moreover, we observe a negative correlation between 5-methyl and 6-methyl brGDGTs without cyclopentane moieties (except brGDGT-IIIa), but this anticorrelation does not seem to be related to the investigated physicochemical parameters. The observed brGDGT distribution yields a permafrost-specific soil pH calibration, p H ' PF = 1.8451 × C B T ' PF + 8.5396 (r2 = 0.60, RMSE = 0.074; n = 109). The different investigated deposit types show some distinct GDGT distributional changes and appear to be distinguishable based on the relative abundance of crenarchaeol, GDGT-0/(crenarchaeol + GDGT-0) ratios, and CBT' PF values, although we also observe strong heterogeneity for each deposit type. In particular, Yedoma and the active layer of polygonal tundra soils represent distinct endmembers, which differ from each other, as well as from fluvial terrace/floodplain sediments and thermokarst sediments, while the latter two deposit types have similar GDGT fingerprints that are not easily distinguishable. Yet, the observed GDGT distributional differences have implications for GDGT proxies analyzed in aquatic suspended matter and sediments. Quantitative estimates of permafrost erosion, as well as soil pH inferred using BIT indices or CBT' PF , respectively, may be biased by changing relative contributions of different deposit types (carrying their respective GDGT signals) to the exported permafrost OC, particularly from Yedoma and the active layer of polygonal tundra soils. [ABSTRACT FROM AUTHOR]

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]

Thaumarchaeota, the major sources of marine glycerol dibiphytanyl glycerol tetraether lipids (GDGTs), are believed to fix the majority of their carbon directly from dissolved inorganic carbon (DIC). The δ 13 C values of GDGTs ( δ 13 C GDGT ) may be powerful tools for reconstructing variations in the ocean carbon cycle, including paleoproductivity and water mass circulation, if they can be related to values of δ 13 C DIC . To date, isotope measurements primarily are made on the C 40 biphytane skeletons of GDGTs, rather than on complete tetraether structures. This approach erases information revealed by the isotopic heterogeneity of GDGTs within a sample and may impart an isotopic fractionation associated with the ether cleavage. To circumvent these issues, we present δ 13 C values for GDGTs from twelve recent sediments representing ten continental margin locations. Samples are purified by orthogonal dimensions of HPLC, followed by measurement of δ 13 C values by Spooling Wire Microcombustion (SWiM)-isotope ratio mass spectrometry (IRMS) with 1 σ precision and accuracy of ±0.25‰. Using this approach, we confirm that GDGTs, generally around −19‰, are isotopically “heavy” compared to other marine lipids. However, measured δ 13 C GDGT values are inconsistent with predicted values based on the 13 C content of DIC in the overlying water column and the previously-published biosynthetic isotope fractionation for a pure culture of an autotrophic marine thaumarchaeon. In some sediments, the isotopic composition of individual GDGTs differs, indicating multiple source inputs. The data appear to confirm that crenarchaeol primarily is a biomarker for Thaumarchaeota, but its δ 13 C values still cannot be explained solely by autotrophic carbon fixation. Overall the complexity of the results suggests that both organic carbon assimilation ( ca. 25% of total carbon) and multiple source(s) of exogenous GDGTs (contributing generally <30% of input to sediments) are necessary to explain the observed δ 13 C GDGT values. The results suggest caution when interpreting the total inputs of GDGTs to sedimentary records. Biogenic or open-slope sediments, rather than clastic basinal or shallow shelf sediments, are preferred locations for generating minimally-biased GDGT proxy records. [ABSTRACT FROM AUTHOR]

• δ 13 C and F14C of sedimentary fatty acids in a mudbelt off the Pearl River mouth. • C 28+30 fatty acids are more reliable to trace soil OC than C 24 and C 26 fatty acids. • A plant wax mean age of 3060 ± 90 yr is determined in the Pearl River basin. • Lower burial efficiency of soil OC (∼50%) than fossil OC (∼85%) in the mudbelt. Tropical and subtropical rivers deliver large quantities of terrestrial organic carbon (OC terr) to the ocean, acting as a crucial part of the global carbon cycle, but little is known about the timescale and efficiency of its transport to and in the adjacent coastal sea. Here we examined source-specific biomarker (fatty acids, FAs) contents and isotope compositions in surface sediments in an alongshore transect southwestward from the Pearl River mouth. The C 28+30 , rather than other long-chain saturated FAs, were found to be the most representative for OC terr , and a plant wax mean age of 3060 ± 90 yr (resulting from protracted storage) was estimated in the Pearl River watershed from the 14C age of C 28+30 FA in a river mouth sample. A compilation of plant wax mean ages in global (sub)tropical river systems including this study suggests that regional differences in climate and morphology may have a limited impact on plant wax mean ages in (sub)tropical regions. A four-source mixing model based on bulk OC and biomarker isotope compositions demonstrated that surface sediments in the Pearl River-derived mudbelt consist of 0.15–0.36 wt.% marine OC, 0.03–0.13 wt.% riverine primary production-derived OC, 0.18–0.49 wt.% soil OC, and 0.07–0.16 wt.% fossil OC. The mean burial efficiency of fossil and soil OC is ∼85% and 49%, respectively, indicating the refractory nature of fossil OC but a significant loss of soil OC due to remineralization during transport in the marine environment before final burial. Over longer timescales, the OC terr loss experienced during transport may, thus, to some extent reduces the capacity of terrestrial ecosystems (particularly soils) as CO 2 sinks. [ABSTRACT FROM AUTHOR]

• New Early Cretaceous South Atlantic TEX 86 samples and climate modelling. • Systematic differences in Cretaceous isoGDGT abundances. • Regional similarities to modern Mediterranean and Red Sea sediments. Regional TEX 86 -temperature calibration enables model-data congruence for OAE 1a. The Cretaceous Period (145-66 Ma) provides an opportunity to obtain insights into the adaptation of the climate system to increased atmospheric greenhouse gas concentrations. The organic paleothermometer TEX 86 is one of the few proxies available for reconstructing quantitative estimates of upper ocean temperatures of this time period. Here we show that the sedimentary TEX 86 signal in the Early Cretaceous North and South Atlantic shows systematic differences to other Cretaceous samples. In particular, the relative increase in the fractional abundances of the crenarchaeol isomer compared to crenarchaeol exhibits similarities with surface sediments from the modern Mediterranean and Red Sea. Dedicated climate model simulations suggest that the formation of warm and saline deep waters in the restricted North and South Atlantic may have influenced TEX 86 export dynamics leading to a warm bias in reconstructed upper ocean temperatures. Applying a regional calibration from the modern Mediterranean and Red Sea to corresponding TEX 86 data significantly improves the model-data fit for the Aptian Oceanic Anoxic Event 1a and the overall comparison with other temperature proxies for the Early Cretaceous. Our results demonstrate the need to consider regional and temporal changes of the TEX 86 -temperature relation for the reconstruction of deep-time ocean temperatures. [ABSTRACT FROM AUTHOR]