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The Neogene (23.04–2.58 Ma) is characterised by progressive buildup of ice volume and climate cooling in the Antarctic and the Northern Hemisphere. Heat and moisture delivery to Antarctica is, to a large extent, regulated by the strength of meridional temperature gradients. However, the evolution of the Southern Ocean frontal systems remains scarcely studied in the Neogene. Here, we present the first long-term continuous sea surface temperature (SST) record of the subtropical front area in the Southern Ocean at Ocean Drilling Program (ODP) Site 1168 off western Tasmania. This site is, at present, located near the subtropical front (STF), as it was during the Neogene, despite a 10∘ northward tectonic drift of Tasmania. We analysed glycerol dialkyl glycerol tetraethers (GDGTs – on 433 samples) and alkenones (on 163 samples) and reconstructed the paleotemperature evolution using TEX86 and U37k′ as two independent quantitative proxies. Both proxies indicate that Site 1168 experienced a temperate ∼ 25 ∘C during the early Miocene (23–17 Ma), reaching ∼ 29 ∘C during the mid-Miocene climatic optimum. The stepwise ∼ 10 ∘C cooling (20–10 ∘C) in the mid-to-late Miocene (12.5–5.0 Ma) is larger than that observed in records from lower and higher latitudes. From the Pliocene to modern (5.3–0 Ma), STF SST first plateaus at ∼ 15 ∘C (3 Ma), then decreases to ∼ 6 ∘C (1.3 Ma), and eventually increases to the modern levels around ∼ 16 ∘C (0 Ma), with a higher variability of 5∘ compared to the Miocene. Our results imply that the latitudinal temperature gradient between the Pacific Equator and the STF during late Miocene cooling increased from 4 to 14 ∘C. Meanwhile, the SST gradient between the STF and the Antarctic margin decreased due to amplified STF cooling compared to the Antarctic margin. This implies a narrowing SST gradient in the Neogene, with contraction of warm SSTs and northward expansion of subpolar conditions.

Sediment cores from the Mid-Norwegian Margin, IODP Expedition 396 recovered several glendonite (calcite) pseudomorphs after cold-water ikaite, CaCO3•6H2O, within sediments deposited during the Paleocene-Eocene Thermal Maximum hyperthermal and the early Eocene greenhouse. This presents an apparent climate paradox, since during this time interval, deep sea bottom water temperatures are not believed to have been lower than c. 10 °C anywhere on Earth, mostly much warmer (Meckler et al., 2022 and references therein), far above temperatures typical for natural ikaite formation (Vickers et al., 2022 and references therein). The glendonites are found in close association with ash horizons from the nearby North Atlantic large igneous province (NAIP), with some actually in the infill of a hydrothermal. This, coupled with the presence of glendonites in sediments of the same age from Svalbard and Denmark (Spielhagen and Tripati, 2009; Vickers et al., 2020), may point to volcanically-driven climate and environmental changes in this region, perhaps on temporal and spatial scales hitherto unresolved by global-scale datasets. Here, we present reconstructed ikaite crystallisation temperatures from clumped isotope palaeothermometry and biomarker sea surface and air temperature reconstructions from glendonite-bearing horizons in ash-rich sediments at four IODP Expedition 396 sites. We find that the glendonites indicate bottom water temperatures considerably lower than the majority of other localities so far studied for this time interval (0 – 10 °C). The biomarker signals are harder to interpret, but may indicate sea surface and air temperatures significantly lower than many other studies across this time interval. We discuss possible causes and mechanisms for this cooling, and the conditions driving ikaite growth, focussing on whether these both may be linked to NAIP volcanism.

River-dominated continental margins receive large inputs of terrestrial organic matter (TerrOM). This TerrOM can potentially form a long-term sink for atmospheric CO2 upon burial in continental margin sediments, thereby forming a key component of the global carbon cycle. However, only part of the TerrOM that is delivered to the coastal zone is preserved on the seafloor, as its sequestration efficiency depends on its properties and composition. The composition and quality of TerrOM on the seafloor is usually not determined, hampering estimates of its contribution to carbon sequestration on the continental margin. Moreover, TerrOM is thought to form associations with mineral surfaces that protect it from degradation, but it is not fully known to which extent these associations persist in the marine realm and if different types of TerrOM preferentially bind to certain minerals. Here, we investigated the TerrOM composition in different grain size fractions (>250, 250–125, 125–63, 63–30, 30–10 and

Glycerol dialkyl glycerol tetraethers (GDGTs) are ubiquitous membrane-spanning lipids with a wide environmental distribution. In soils, branched GDGTs are produced by a possibly large diversity of bacteria. The relative abundance of methyl groups attached to the central alkyl chains is at the basis of the paleotemperature proxy MBT’5ME. However, MBT’5ME values in soils can also be directly influenced by pH (De Jonge et al., 2021). A second group of compounds, the isoprenoid GDGTs, are produced by archaea. They have been used only sparsely as environmental proxies in soils, although they are at the base of the marine paleotemperature proxy TEX86. In soils, a recent compilation by Yang et al. (2016) illustrates that the temperature dependency of TEX86 is sometimes present, but potentially influenced by other soil (chemistry) parameters.In addition to temperature, other soil parameters are expected to vary with time, even on a Holocene timescale. For instance, soil mineral fertility (specifically, the concentration of exchangeable cations) will vary following climate or land use changes. As soil mineral fertility will impact the soil nutrient status for vegetation, and impact the soil capacity to store organic carbon (von Fromm et al., 2021), it is a relevant parameter to reconstruct over time. However, as soil fertility of surface soils will decrease during eroision or burial, this parameter can currently not be reconstructed quantitatively.To investigate the potential of GDGTs as soil fertility proxies, branched and isoprenoid GDGTs were measured in soils from 5 elevation transects (Austria, Bolivia, China, Indonesia and Tanzania, n=74) that cover a large gradient in mean annual temperature (0-28 ℃), seasonality, and soil chemical parameters. Supplemented with climate (temperature and precipitation) data, we evaluate both changes in absolute concentration and relative distribution of the GDGTs. Of the chemical parameters, exchangeable calcium and exchangeable iron are shown to correlate with the absolute abundance of several branched (6 methyl brGDGTs) and isoprenoid (crenarchaeol isomer) GDGT compounds. Based on these relations we have developed ratios to quantify calcium (and summed bases) and iron (and summed metals) [r2=0.61-0.68, p

The Miocene Climatic Optimum (MCO, ~17-15 Ma) was a relatively warm interval that interrupted the Cenozoic cooling trend and bears analogies with projected end-of-the-century climate. Proxy data and model simulations indicate temperatures were on average ~7 – 8°C higher than present day and atmospheric pCO2 values were 500-600 ppmV. At high latitudes, upper ocean temperatures were ~10 – 15°C warmer than present day, but available tropical temperature records are limited. Importantly, to be able to use Miocene paleoclimate records to quantify key climate parameters like the polar amplification of climate change, accurate reconstructions of tropical surface oceans are required.We present high resolution Early to Middle Miocene (~15 – 18 Ma) records of tropical sea surface temperature (SST) variability based on the lipid biomarker paleothermometer TEX86 at Ocean Drilling Program (ODP) Site 959 in the eastern equatorial Atlantic Ocean and at ODP Site 1007 at Bahama Bank in the western tropical Atlantic Ocean. The age model for both sites is based on chemo- and biostratigraphy. Moreover, analyses of bulk carbonate oxygen- (δ18O) and stable carbon (δ13C) isotope ratios at ~2 – 4 kyr resolution at Site 959 facilitated orbital tuning to eccentricity, obliquity and precession. Bulk elemental composition records, total organic carbon concentrations and dinoflagellate cyst assemblages were also generated to assess paleoenvironmental change.At both sites, warming associated with the onset of the MCO (~17 Ma) was identified as an average SST increase of ~2°C (using the TEX86-H calibration). At ~16.8 Ma, bulk carbonate δ13C gradually increased by ~1‰ at both sites, indicating the onset of the Monterey carbon isotope excursion. Combined with available temperature information from the high latitudes and deep ocean, we assess meridional temperature gradients across the MCO.At ODP Site 959, sediments are characterized by alternations of biogenic silica, carbonates, and terrigenous material (i.e., clays). Following the onset of the MCO (~17 – 16.5 Ma), high variability in the oceanographical setting is reflected in striking Babio peaks, indicating productivity changes. These peaks coincide with lowest SSTs (~28°C) and increased dust supply (increased Fe and Ti) on precession and obliquity timescales. In the same interval, (inner-) neritic dinoflagellate species indicate increased water column stratification. Heterotrophic dinocyst groups vary on timescales coherent with the other geochemical records, relative to comparably stable background abundances of oceanic genera throughout the MCO. Combined, this suggests that a highly dynamic monsoon-driven upwelling regime was present at Site 959 during the MCO. Combined, these patterns imply a highly dynamic African monsoon-driven upwelling regime that intensified at the onset of the MCO.

Dust deposition can trigger phytoplankton growth in oligotrophic low nutrient low chlorophyl (LNLC) regions by providing essential nutrients to the surface ocean. As LNLC regions comprise 60% of the global ocean, dust fertilisation and potential subsequent increased downward carbon export could affect the strength of the biological carbon pump considerably. Additionally, ballasting effects of large dust particles could enhance downward carbon export even further, independent from fertilisation effects. However, compared to high nutrient low chlorophyl (HNLC) regions, the biogeochemical effect of dust deposition and its sensitivity to future climate change is less well understood for LNLC regions. For the LNLC Mediterranean Sea mesocosm experiments and satellite data suggest that some, but not all, dust events lead to increased primary production. However, the exact relationship between dust deposition, productivity and carbon export remains unresolved. Here, we aim to identify and quantify the relationships between Saharan dust deposition (deposition mode, dust source), phytoplankton response (changes in community composition, phytoplankton vs heterotrophic bacterial growth) and carbon export in the eastern Mediterranean Sea by studying an exceptional high-resolution, 30-year sediment-trap time series of settling Saharan dust particles and phytoplankton remains (partly at 500m, 1500m, and 2500m water depth), combining sedimentological, biogeochemical, and remote sensing techniques. We here present a combined record of dust and organic matter fluxes for one full year of the time series (April 2017 to May 2018, 2200m water depth). Furthermore, the response of specific phytoplankton groups to dust input as well as the input of terrestrial plant material associated with desert dust is determined based on the presence and distribution of lipid biomarkers in the trap material.Dust fluxes vary substantially over this one-year period, but peaks occur during spring 2017 and 2018, summer 2017, as well as some smaller, less pronounced peaks during autumn 2017. Some of these dust events indeed correspond to increased fluxes of lipid biomarkers, suggesting a relationship between dust input and enhanced sinking of organic material. However, due to the depth of trap deployment, the record does not allow to differentiate between the influence of dust input as fertiliser or as ballasting effect. This will later be assessed by comparing biomarker records from sediment traps from different depths representing the surface and deep ocean. Nevertheless, the lipid biomarkers representing different phytoplankton groups (e.g., long-chain alkenones for coccolithophores, 23,24-dimethylcholesta-5,22E-dien-β-ol for diatoms, dinosterol for dinoflagellates, long-chain diols for eustigmatophytes) do not show a uniform response to dust input, indicating that the response of these phytoplankton groups depends on different conditions. Moreover, some dust events do not seem to trigger any phytoplankton response at all as they do not coincide with enhanced biomarker fluxes. This indicates that other factors such as dust source, deposition mode and/or trophic state of the surface ocean determine whether dust input triggers enhanced export of organic material or not. Differences in grain-size distribution and terrestrial plant content (indicated by terrestrial plant biomarkers) indeed suggest that the observed contrasting response might be due to differences in dust source and composition.

Fossil ikaite, preserved as the pseudomorph glendonite, occurs in vast amounts in Jurassic and Cretaceous successions in the high Arctic. Thermodynamics predict that ikaite is only stable at near-freezing temperatures and glendonite is thus widely used as a paleo-indicator of cold climate conditions, conflicting with traditional views of a very warm and equable Mesozoic greenhouse. Here, we show based on a multi-proxy investigation of Jurassic and Cretaceous glendonites from Siberia and Svalbard that this one-dimensional view detracts from their exceedingly complex biogeochemistry. NanoSIMS analyses of a Jurassic glendonite from Siberia produced large C-isotope gradients (> 40‰) over micrometer distances hinting at strong kinetic fractionation that is coupled to the formation of various precipitates, including an inclusion-rich primordial phase with C-isotope values as low as -38‰ that records methane oxidation. In line with previous results from Siberia, all investigated glendonites from the Cretaceous of Svalbard contain methane gas (700 - 2500 ppb/g) with enriched δ13C-CH4 signatures (-44 to -50‰ V-PDB), depleted δ2H--CH4 (-285 to -245‰ V-SMOW), and relatively large proportions of C2-C5 gas. Such values are potentially indicative of thermogenic methane gas sourced from structure II gas hydrates. Organic geochemistry of glendonites from Svalbard shows the presence of abundant hopanes, including bisnorhopanes with a CSIA signature of -41‰, suggesting activity of sulfide oxidizing bacteria possibly also linked to the inclusion of oil droplets. Moreover, exceptionally heavy bulk δ34Scas values of +46.2‰ clearly link marine ikaite formation in deep time to sulfate-driven anaerobic methane oxidation. Marine ikaite formation and preservation is thus a highly complex process, driven by temperature and (bio)chemical processes in the sea floor, complicating its use as a simple paleoclimate proxy. Regardless, glendonite episodically trapped large amounts of greenhouse gases and stored those for hundreds of millions of years, making this authigenic mineral a potential recorder of past carbon cycle perturbations.

The Oligocene (33.9 – 23.03 Ma ago) and early Miocene (23.03 – 19 Ma) experienced large climate and ice sheet fluctuations in a most likely unipolar icehouse, with glaciation limited to the Southern Hemisphere. This time interval provides a useful test case for studying polar amplification patterns under atmospheric greenhouse gas concentrations similar to those projected for the future. Large-amplitude climate variability has been recorded close to Antarctica during this time interval, but climatic and environmental conditions and variability in the tropical band are poorly knownpaleoceanographic conditions and sea (sub)surface temperatures (SSTs) will deliver an insight into the climate variability and sensitivity of the lower latitudes under unipolar conditions.Reconstructed surface oceanographic conditions and variations in the depositional environment are based on bulk carbonate stable isotope ratios (δ18O, δ13C), weight% carbonate, magnetic susceptibility and dinoflagellate cyst assemblages at Ocean Drilling Program Site 959, offshore Ghana.. In addition, long term SSTs reconstructions for the Oligocene and early Miocene equatorial Atlantic are derived from lipid biomarker-based paleothermometry (TEX86). Lastly, long term (~31 Ma – 19Ma) atmospheric CO2 concentrations based on stable carbon isotopic fractionation of marine organic carbon and alkenones were generated.The organic carbon isotopic fractionation showshat pCO2 ranged from 280 – 570ppm between ~19 – 33 Ma and shows no signs of large variability, which is consistent with previous findings. The TEX86 indicates that the average SSTs during the Oligocene was ~27℃ which is ~1 – 4℃ colder than in the west Equatorial Atlantic (e.g., ODP Site 929, Ceara Rise), using the same transfer function. Dinoflagellate cyst assemblages indicate upwelling alternated with strong stratification events on ~50 to 100 kyr timescale. Monsoonal upwelling could explain the lower SSTs at Site 959 which is consistent with modern east-west gradients. Subsequent comparison of our equatorial SST record with general circulation modelling studies and SST records from high latitudes should reveal the polar amplification of warming and climate sensitivity on long and short (orbital) timescales during the Oligocene.

Rapid climatic and carbon cycle upheavals in the early Eocene have been of strong interest for the past decades. Multiple of these events represent global warming events (i.e. hyperthermals), including the Paleocene Eocene Thermal Maximum (PETM; 56 Ma), but global data coverage is still too limited to fully resolve the spatial patterns of climate, ecosystems and hydrology for the other hyperthermals. This is particularly due to the lack of continuous continental margin sequences suitable for high-resolution paleoclimate reconstructions.We present high-resolution, multi-proxy records of climatic and environmental changes across multiple hyperthermals from Core RH-323, Northern Negev Desert, Israel. Lower Eocene sediments, dominantly orbitally paced alternations of marls and chalk, were deposited in the Tethys Ocean at a latitude of ~15º N, on the northward dipping slope of the presumably dry northern bound of the African continent. They provide a unique opportunity to capture tropical climate variability and continental-hydrological response to hyperthermals. We reconstruct regional variability of (sub)surface temperature, plankton ecology and continental hydrology by TEX86 paleothermometry, bulk carbonate isotope ratios, magnetic susceptibility, bulk-XRF and dinoflagellate cyst (dinocyst) assemblages.We identify various hyperthermal events, including the PETM, ETM2 and ETM3, based on combined chemo- and biostratigraphy. During the PETM, TEX86-based temperatures indicate a warming of 3­–4 ºC, coinciding with a high abundance of representatives of the classic warm water dinocyst genus Apectodinium. The PETM is marked by a thin marl-rich interval at the onset, followed by a carbonate-rich interval during the body, suggesting very different hydrological forcing of siliciclastic input than recorded at mid-latitude sites. We interpret this to reflect strong seasonality (possibly monsoon like) with periods of intense precipitation followed by prolonged drought. Interestingly, subsequent smaller hyperthermals seem to predominantly coincide with increased siliciclastic content, thus representing episodes of increased (seasonal) precipitation.

At present, a strong latitudinal sea surface temperature (SST) gradient of ~16 °C exists across the Southern Ocean, maintained by the Antarctic Circumpolar Current (ACC) and a set of complex frontal systems. Together with the Antarctic ice masses, this system has formed one of the most important global climate regulators. The timing of the onset of the ACC-system, its development towards modern-day strength, and the consequences for e.g., the latitudinal SST gradient around the southern Atlantic Ocean, are still uncertain. Here we present new TEX86-biomarker records, calibrated to SST, from two sites located east of Drake Passage (southern South Atlantic) to assist in better understanding two critical time intervals of prominent climate transitions during the Cenozoic: The Late Eocene–Early Oligocene (ODP Site 696) and Middle–Late Miocene (IODP Site U1536) transitions. Our results overall show rather temperate conditions (20–11 °C) during the Late Eocene to Early Oligocene interval, with a weaker latitudinal SST gradient (~8 °C) across the Atlantic sector of the Southern Ocean compared to present day (~16 °C). We ascribe the regional similarity in SSTs across the Late Eocene–Early Oligocene South Atlantic to a persistent, strong Subpolar Gyre circulation, connecting all sites, which can only exist in absence of a strong throughflow across the Drake Passage. Surprisingly, the southern South Atlantic records show comparable SSTs (~12–14 °C) during both the Earliest Oligocene Oxygen Isotope Step (EOIS, ~33.65 Ma) and the Miocene Climate Optimum (MCO, ~16.5 Ma). Apparently, maximum Oligocene Antarctic ice volume could coexist with warm ice-proximal surface ocean conditions, while at similar ocean temperatures, the Middle Miocene Antarctic ice sheet was strongly reduced. Southern South Atlantic SSTs cooled to ~5 °C at the onset of the Middle Miocene Climate Transition (MMCT, 14 Ma), making it the coldest oceanic region recorded around Antarctica and the likely main location for deep water formation. The already cold southern South Atlantic conditions at MMCT meant it experienced little cooling during the latter part of the Miocene, which contrasts the profound cooling due to northward expansion of the Southern Ocean frontal systems in the lower latitudes and other sectors of the Southern Ocean.

Glycerol dialkyl glycerol tetraethers (GDGTs), including both the archaeal isoprenoid GDGTs (isoGDGTs) and the bacterial branched GDGTs (brGDGTs), have been used in paleoclimate studies to reconstruct temperature in marine and terrestrial archives. However, GDGTs are present in many different types of environments, with relative abundances that strongly depend on the depositional setting. This suggests that GDGT distributions can be used more broadly to infer paleoenvironments in the geological past. In this study, we analyzed 1153 samples from a variety of modern sedimentary settings for both isoGDGT and brGDGTs. We used machine learning on the GDGT relative abundances from this dataset to relate the lipid distributions to the physical and chemical characteristics of the depositional settings. We observe a robust relationship between the depositional environment and the lipid distribution profiles of our samples. This dataset was used to train and test the Branched and Isoprenoid GDGT Machine learning Classification algorithm (BIGMaC), which identifies the environment a sample comes from based on the distribution of GDGTs with high accuracy. We tested the model on the sedimentary record from the Giraffe kimberlite pipe, an Eocene maar in subantarctic Canada, and found that the BIGMaC reconstruction agrees with independent stratigraphic information, provides new information about the paleoenvironment of this site, and helps improve paleotemperature reconstruction. In cases where paleoenvironments are unknown or are changing, BIGMaC can be applied in concert with other proxies to generate more refined paleoclimatic records.

The Neogene (23.04–2.58 Ma) is characterized by progressive buildup of Antarctic and Northern Hemisphere ice volume and climate cooling. Heat/moisture delivery to Antarctica is to a large extent regulated by the strength of meridional temperature gradients. However, the evolution of the Southern Ocean frontal systems remains scarcely studied in the Neogene. Here we present the first long-term continuous sea (sub)surface temperature (SST) record of the subtropical front area in the Southern Ocean at Ocean Drilling Program (ODP) Site 1168 off western Tasmania. This site is at present located near the subtropical front (STF), as it was during the Neogene, despite a 10 degree northward tectonic drift of Tasmania during the Neogene. We analyzed glycerol dialkyl glycerol tetraethers (GDGTs, on 433 samples) and alkenones (on 163 samples) and reconstructed the paleotemperature evolution using TEX86 and Uk’37 as two independent quantitative proxies. Both proxies indicate that Site 1168 experienced a temperate ~25 °C during early Miocene (23–17 Ma), reaching ~29 °C during the mid-Miocene Climatic Optimum. The stepwise ~10 °C cooling (20–10 °C) in the mid-to-late Miocene (12.5–5.0 Ma) is larger than observed in records from lower and higher latitudes. From the Pliocene to modern (5.3–0 Ma), STF SST first plateaus at ~15 °C (3 Ma), then decreases to ~6 °C (1.3 Ma), and eventually increases to the modern levels around ~16 °C (0 Ma), with a higher variability of 5 degrees compared to the Miocene. Our results imply that the latitudinal temperature gradient between the Pacific equator and STF during late Miocene cooling increased from 4 °C to 14 °C. Meanwhile, the SST gradient between the STF and the Antarctic margin decreased due to amplified STF cooling compared to the Antarctic Margin. This implies a narrowing SST gradient in the Neogene, with contraction of warm SSTs and northward expansion of subpolar conditions.

Monsoonal rivers play an important role in the land-to-sea transport of soil-derived organic carbon (OC). However, spatial and temporal variation in the concentration, composition, and fate of OC in these rivers remains poorly understood. We investigate soil-to-sea transport of OC by the Godavari River in India using branched glycerol dialkyl glycerol tetraether (brGDGT) lipids in soils, river suspended particulate matter (SPM), riverbed sediments, and in a marine sediment core from the Bay of Bengal. The abundance and composition of brGDGTs in SPM and sediments in the Godavari River differs between the dry and wet season. In the dry season, 6-methyl brGDGTs dominate SPM and riverbed sediments in the whole basin. Currently, mobilisation and transport of soils from the upper basin is limited due to deficient rainfall and damming. This promotes aquatic brGDGT production in this part of the basin, which is reflected by a high relative abundance of 6-methyl brGDGTs in both seasons. In the wet season, brGDGT distributions in SPM from the lower basin closely resemble those in soils, mostly from the North and East Tributaries, corresponding to precipitation patterns. The brGDGT composition in SPM and sediments from the delta suggests that soil OC is only effectively transported to the Bay of Bengal in the wet season, when the river plume extends beyond the river mouth. The sediment geochemistry indicates that also the mineral particles exported by the Godavari River primarily originate from the lower basin, similar to the brGDGTs. River depth profiles of brGDGTs in the downstream Godavari reveal no hydrodynamic sorting effect in either season, indicating that brGDGTs are not associated with certain minerals. The similarity of brGDGT distributions in bulk and fine-grained sediments (≤63 μm) further confirms the absence of selective transport mechanisms. Nevertheless, the composition of brGDGTs in a Holocene, marine sediment core near the river mouth appears substantially different from that in the modern Godavari basin, suggesting that terrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay of Bengal and/or overprinted by marine in situ production. The change in brGDGT distributions at the river-sea transition implies that this zone is key in the effective transfer of soil OC, as well as for the interpretation of paleorecords based on brGDGTs in coastal marine sediment archives.

Large Oligocene Antarctic ice sheets co-existed with warm adjacent ocean waters. To provide a broad Southern Ocean perspective to such warmth, we reconstruct the strength and variability of the Oligocene Australian-Antarctic latitudinal sea surface temperature (SST) gradient. Our Oligocene TEX86-based SST record from offshore southern Australia shows temperate (20–29°C) conditions throughout, despite northward tectonic drift. A persistent SST gradient (~5–10°C) exists between Australia and Antarctica, which becomes larger during glacial maxima. The SST gradient increases from ~26 Ma onwards, due to decreasing Antarctic-proximal SSTs. Meanwhile, benthic foraminiferal oxygen isotope decline indicates ice loss/deep-sea warming. These contrasting patterns are difficult to explain by greenhouse gas forcing alone. Timing of the SST cooling coincides with deepening of Drake Passage and fits well with results of ocean model experiments, suggesting Antarctic-proximal cooling. We conclude that Drake Passage deepening cooled Antarctic coastlines which enhanced thermal isolation of the Antarctic ice sheet.

A range of future climate scenarios are projected for high atmospheric CO2 concentrations, given uncertainties over future human actions as well as potential environmental and climatic feedbacks. The geological record offers an opportunity to understand climate system response to a range of forcings and feedbacks which operate over multiple temporal and spatial scales. Here, we examine a single interglacial during the late Pliocene (KM5c, ca. 3.205±0.01 Ma) when atmospheric CO2 exceeded pre-industrial concentrations, but were similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were almost identical to today, we are able to focus on equilibrium climate system response to modern and near-future CO2. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial values, by ∼2.3 ∘C for the combined proxy data (foraminifera Mg∕Ca and alkenones), or by ∼3.2–3.4 ∘C (alkenones only). Compared to the pre-industrial period, reduced meridional gradients and enhanced warming in the North Atlantic are consistently reconstructed. There is broad agreement between data and models at the global scale, with regional differences reflecting ocean circulation and/or proxy signals. An uneven distribution of proxy data in time and space does, however, add uncertainty to our anomaly calculations. The reconstructed global mean sea-surface temperature anomaly for KM5c is warmer than all but three of the PlioMIP2 model outputs, and the reconstructed North Atlantic data tend to align with the warmest KM5c model values. Our results demonstrate that even under low-CO2 emission scenarios, surface ocean warming may be expected to exceed model projections and will be accentuated in the higher latitudes.

The large difference in the fractionation of stable carbon isotopes between C3 and C4 plants is widely used in vegetation reconstructions, where the predominance of C3 plants suggests wetter and that of C4 plants drier conditions. The isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the carbon isotopic signature of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the carbon isotopic signature of C3 and C4 plants (δ13C) and of organic carbon (δ13Corg) in soils, river Suspended Particulate Matter (SPM) and riverbed sediments, to gain insight in the control of precipitation on C3 and C4 plant δ13C values and to assess changes in δ13Corg values along the plant–soil–river continuum. This information allows us to elucidate the implications of different δ13C end-members on C3/C4 vegetation reconstructions. Our analysis was performed in the Godavari River basin, which has mixed C3 and C4 vegetation and is situated in the Core Monsoon Zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The Godavari C3 and C4 plants revealed more negative δ13C values than global average vegetation values, suggesting region-specific plant δ13C signatures. Godavari C3 plants confirmed a strong control by Mean Annual Precipitation (MAP) on their δ13C values, with an isotopic enrichment of ~2.2 ‰ for the interval between ~500 and 1500 mm y-1. Tracing δ13Corg values from plant to soils and rivers revealed that soils and riverbed sediments reflected the transition from mixed C3 and C4 vegetation in the dry upper basin to more C3 vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered the plant-derived δ13C signal. Phytoplankton dominated the δ13Corg signal carried by SPM in the dry season and year-round in the upper basin. Our analysis revealed that the reconstructed C3/C4 vegetation composition was sensitive to the plant δ13C end-members used as mixing model input. The %C4 plants in the different subbasins was ~10–19 % higher using Godavari-specific end-members than using global averages, and including a correction for drought enrichment in Godavari C3 plants resulted in a 2–10 % lower estimated C4 plant cover. Hence, incorporating region-specific plant δ13C end-members and drought correction of the C3 end-member in mixing models need to be considered to determine C3 and C4 distributions of modern- and paleo-vegetation in monsoonal regions.

TEX86 is a paleothermometer based on Thaumarcheotal glycerol dialkyl glycerol tetraether (GDGT) lipids and is one of the most frequently used proxies for sea-surface temperature (SST) in warmer-than-present climates. However, the calibration of TEX86 to SST is controversial because its correlation to SST is not significantly stronger than that to depth-integrated surface to subsurface temperatures. Because GDGTs are not exclusively produced in and exported from the surface ocean, sedimentary GDGTs may contain a depth-integrated signal that is sensitive to local subsurface temperature variability, which can only be proved in downcore studies. Here, we present a 15 Myr TEX86 record from ODP Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX86 variability. Relatively high GDGT[2/3] ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (> 200 m) waters. Moreover, late Pliocene TEX86 variability is highly sensitive to glacial-interglacial cyclicity, as is also recorded by benthic δ18O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (Uk’37 and Mg/Ca), is not primarily explained by glacial-interglacial cyclicity. Combined, these observations are best explained by TEX86 sensitivity to sub-thermocline temperature variability. We conclude that the TEX86 record represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible the present-day depth distribution of Thaumarchaeota and with the GDGT[2/3] distribution in core tops. The depth-integrated TEX86 record can potentially be used to infer SST variability, because subsurface temperature variability is generally tightly linked to SST variability. Using a subsurface calibration with peak calibration weight between 100–350 m, we estimate that east equatorial Atlantic SST cooled by ~4.5 °C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX86 record as an Antarctic Intermediate Water (AAIW) proxy and evaluate climatological leads and lags around the Pliocene M2 glacial (~3.3 Ma). Our record, combined with published information, suggests that the M2 glacial was marked by AAIW cooling during an austral summer insolation minimum, and that decreasing CO2 levels were a feedback, not the initiator, of glacial expansion.

We here estimate past temperature and hydroclimate variations in eastern Anatolia for marine isotope stages 4 to 1 (MIS4 to MIS1) respectively using branched glycerol dialkyl glycerol tetraethers (brGDGTs) and the hydrogen isotopic composition of n-C29 leaf-wax n-alkanes (δ2Hwax) stored in the sedimentary record of Lake Van (Turkey). Our millennial-scale lipid biomarker records reflect warm and wet conditions during interstadials/interglacials and colder and dryer periods with increased evapotranspiration and aridity during stadials/glacials. The degree of methylation of the 5-methyl brGDGTs (MBT´5me) indicates increasing mean annual air temperatures (MAT) during stadial/interstadial transitions, that characterize Dansgaard-Oeschger events, and during the last glacial termination. Based on the effects of changes in precipitation amount on apparent enrichment factors between the δ2H of the C29 n-alkanes and δ2H of precipitation, a total increase in annual precipitation of about 200 mm during transitions from stadials to interstadials, and of 300–350 mm during glacial-interglacial transitions can be determined, in line with previous paleoclimate reconstructions for the Eastern Mediterranean. High sterol concentrations in sediments deposited during warm and humid interstadials reflect increases in lake level, vegetation density and algal blooms, whereas lower values of the branched versus isoprenoid tetraethers ratio (BIT) likely reflects the increased niche of Thaumarchaeota resulting from enhanced windiness and mixing of the water column during colder periods. This quantitative hydroclimate reconstruction from Lake Van (Turkey), as it is optimally situated to respond to North Atlantic climate change via changes in the large-scale wind fields shed light into millennial-scale global climate variability., Palaeogeography, Palaeoclimatology, Palaeoecology, 577, ISSN:0031-0182, ISSN:1872-616X

Specific soil bacterial membrane lipids, branched glycerol dialkyl glycerol tetraethers (brGDGTs), are used as an empirical proxy for past continental temperatures. Their response to temperature change is assumed to be linear, caused by physiological plasticity of their, still unknown, source organisms. A well-studied set of geothermally warmed soils (Iceland) shows that the brGDGT fingerprint only changes when the soil mean annual temperature is warmer than 14 degrees C. This sudden change in brGDGT distribution coincides with an abrupt shift in the bacterial community composition in the same soils. Determining which bacterial OTUs are indicative for each soil cluster shows that Acidobacteria are possible brGDGT producers, together with representatives from the Actinobacteria, Chloroflexi, Gemmationadetes and Proteobacteria. Projecting the lipid fingerprint of the cold and warm bacterial communities onto the global soil calibration dataset creates two distinct soil clusters, in which brGDGTs respond differently to temperature and, especially, soil pH. We show that, on a local scale, a community effect rather than physiological plasticity can be the primary driver of the brGDGT-based paleothermometer along large temperature gradients. This threshold response needs to be taken into account when interpreting brGDGT-based paleo-records. (C) 2019 Elsevier Ltd. All rights reserved.

Sea surface temperature (SST) reconstructions based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions from the Eocene southwest (sw) Pacific Ocean are unequivocally warmer than can be reconciled with state-of-the-art fully coupled climate models. However, the SST signal preserved in sedimentary archives can be affected by contributions of additional isoGDGT sources. Methods now exist to identify and possibly correct for overprinting effects on the isoGDGT distribution in marine sediments. We here use the current proxy insights to assess the reliability of the isoGDGT-based SST signal in 69 newly analysed and 242 re-analysed sediments ODP Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques, in context of paleo-environmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ~130 kyr-resolution Maastrichtian-Oligocene TEX86-based SST record confirms previous conclusions of anomalous warmth in the early Eocene sw Pacific and remarkably cool conditions during the mid-Paleocene. Dinocyst diversity and assemblages show a strong response to the local SST evolution, supporting the robustness of the TEX86 record. Soil-derived branched GDGTs stored in the same sediments are used to reconstruct mean annual air temperature (MAAT) of the nearby land using the MBT'5me proxy. MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen. General trends in SST and MAAT are similar, except for: 1) an enigmatic absence of MAAT rise during the Paleocene-Eocene Thermal Maximum and Middle Eocene Climatic Optimum, and 2) a subdued middle–late Eocene MAAT cooling relative to SST. Both dinocysts and GDGT signals suggest a mid-shelf depositional environment with strong river-runoff during the Paleocene-early Eocene, progressively becoming more marine thereafter. This trend reflects gradual drying and more pronounced wet/dry seasons in the northward drifting Australian hinterland, which may also explain the subdued middle Eocene MAAT cooling relative to that of SST. The overall correlation between dinocyst assemblages, marine biodiversity and SST changes suggests that temperature exerted a strong influence on the surface-water ecosystem, probably in part through sea level changes caused by steric effects. Finally, we find support for a potential temperature control on compositional changes of branched glycerol monoalkyl glycerol tetraethers (brGMGTs) in marine sediments. It is encouraging that a critical evaluation of the GDGT signals confirms the vast majority of the generated data is reliable. However, this also implies the high TEX86-based SSTs for the Eocene sw Pacific, and the systematic offset between absolute TEX86-based SST and MBT'5me-based MAAT estimates remain unexplained.

Ocean currents can transport sinking particles hundreds of kilometers from their origin at the ocean surface to their burial location, resulting in an offset between sea surface temperatures (SSTs) above the burial site and the particle’s origin. Quantifying this offset in particles carrying molecules used in SST proxies can reduce uncertainty in paleoclimate reconstructions. In the Mediterranean Sea, where δ18Oforaminifera, UK’37- and TEX86-based SSTs can exhibit large offsets from surface conditions, understanding the possible contribution of lateral transport to proxy bias can provide additional insight when interpreting paleoclimate records.In this study, Lagrangian particle tracking experiments are performed using the NEMO flow field to simulate transport and allow for a quantitative estimate of transport bias. The model determines the ocean surface origin locations of foraminifera and sedimentary particles that carry alkenones or GDGTs to compare with surface sediment datasets for δ18Oforaminifera, UK’37 and TEX86, respectively. A range of sinking speeds appropriate for the export of organic matter (6, 12, 25, 50, 100, 250, and 500 m/d) is used in the model to represent different export modes (i.e., individual coccoliths, coccospheres, aggregates), where the three fastest sinking speeds can also represent sinking foraminifera. Results show that lateral transport bias is generally small within the Mediterranean Sea and cannot explain the large offsets in proxy-based SST reconstructions in this basin.

Sea surface temperature (SST) reconstructions based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions from the Eocene southwest (sw) Pacific Ocean are unequivocally warmer than can be reconciled with state-of-the-art fully coupled climate models. However, the SST signal preserved in sedimentary archives can be obscured by contributions of additional isoGDGT sources. We here use current proxy insights to assess the reliability of the isoGDGT-based SST signal in 69 newly analysed and 242 re-analysed samples covering the Maastrichtian to Oligocene from ODP Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques. We then reinterpret the record in context of paleo-environmental and paleoclimatologic reconstructions based on dinoflagellate cysts. Our ~130 kyr-resolution SST record reaffirms previous reconstructions of anomalous warmth in the early Eocene sw Pacific and remarkably cool conditions during the mid-Paleocene. Dinocyst diversity and temperature-sensitive taxa show a strong response to the local SST evolution, supporting the robustness of the marine biomarker record. In addition, the long-term isoGDGT and dinocyst records provide further support for an apparent temperature control on compositional changes of branched glycerol monoalkyl glycerol tetraethers (brGMGTs), recorded in the same samples.Soil-derived branched GDGTs (brGDGTs) stored in the same sediments are used to reconstruct mean annual air temperature (MAAT) of the nearby land through the MBT’5me proxy. General trends in SST and MAAT are similar, except for 1) an enigmatic absence of MAAT rise during the Paleocene-Eocene Thermal Maximum and Middle Eocene Climatic Optimum, and 2) a subdued middle–late Eocene MAAT cooling relative to SST. Both dinocyst assemblages and brGDGT indices (the isomerization index) suggest a mid-shelf depositional environment with strong river-runoff during the Paleocene-early Eocene, becoming more open marine thereafter. This trend reflects gradual drying and more seasonal precipitation regime in the northward drifting Australian hinterland. The overall correlation between dinocyst assemblages, biodiversity and SST changes suggests that temperature and associated environmental changes exert a strong influence on the surface-water ecosystem.

Summary In order to improve our understanding of Earth’s climate history, proxies that accurately reconstruct past temperatures are needed. The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), the membrane lipids of certain bacteria, is correlated to temperature in modern settings, and therefore is at the basis of several paleothermometers. Here we apply several brGDGT based temperature calibrations developed specifically for lakes to the sediment sequence from Lake Chala, in equatorial East African, to generate temperature records which cover the last 170 kyr in unprecedented high temporal resolution (210 years on average). Surprisingly, application of the recently developed East African lake calibration leads to ambiguous results. By contrast applying a calibration created before the discovery of 5- and 6-Me brGDGT isomers produces a temperature record strongly reminiscent of established climate records such as the isotope records from Antarctica and contains periodicities relating to orbital precession and obliquity. Therefore it appears that 6-Me brGDGTs are key to the temperature signal archived in Lake Chala. In Lake Chala today, 5- and 6-Me brGDGTs generally occupy different parts of the water column, and thus their relative abundance may be indirectly related to temperature through changes in lake depth or seasonal mixing.

Summary Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are a group of molecules that have been successfully used for paleoclimate reconstructions in a wide range of environments. GDGTs have shown to be useful biomarkers as they are widely available, and their structure is relatively resistant to diagenetic alteration. Specific GDGTs are associated with particular environments, which suggests that they could potentially be used to classify ancient depositional conditions in cases where the exact paleoenvironment is not known. In this work we present a comprehensive branched and iGDGT dataset from published and novel analyses, with more than 1000 modern samples from marine, lake and river sediments as well as soils and peat samples. Unsupervised and supervised machine learning techniques were applied to this dataset to generate GDGT and environmentally based clusters and train a classification algorithm. The trained algorithm has the potential to be applied to samples formed in unclear or transitional depositional environments, allowing us to identify the potential environment in which they were formed. Furthermore, the classification model can help determine which environmental calibration is most appropriate for the setting, thereby improving paleoenvironmental reconstructions.

A range of future climate scenarios are projected for high atmospheric CO2 concentrations, given uncertainties over future human actions as well as potential environmental and climatic feedbacks. The geological record offers an opportunity to understand climate system response to a range of forcings and feedbacks which operate over multiple temporal and spatial scales. Here, we examine a single interglacial during the late Pliocene (KM5c, ca. 3.205 +/- 0.01 Ma) when atmospheric CO2 concentrations were higher than pre-industrial, but similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were almost identical to today, we are able to focus on equilibrium climate system response to modern and near-future CO2. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial, by ~2.3 ºC for the combined proxy data (foraminifera Mg/Ca and alkenones), or by ~3.2ºC (alkenones only). Compared to the pre-industrial, reduced meridional gradients and enhanced warming in the North Atlantic are consistently reconstructed. There is broad agreement between data and models at the global scale, with regional differences reflecting ocean circulation and/or proxy signals. An uneven distribution of proxy data in time and space does, however, add uncertainty to our anomaly calculations. The reconstructed global mean sea-surface temperature anomaly for KM5c is warmer than all but three of the PlioMIP2 model outputs, and the reconstructed North Atlantic data tend to align with the warmest KM5c model values. Our results demonstrate that even under low CO2 emission scenarios, surface ocean warming may be expected to exceed model projections, and will be accentuated in the higher latitudes.