Paleotemperature Reconstructions Research Articles

A solution for constraining past marine Polar Amplification

Most climate proxies of sea surface temperatures suffer from severe limitations when applied to cold temperatures that characterize Arctic environments. These limitations prevent us from constraining uncertainties for some of the most sensitive climate tipping points that can trigger rapid and dramatic global climate change such as Arctic/Polar Amplification, the disruption of the Atlantic Meridional Overturning Circulation, sea ice loss, and permafrost melting. Here, we present an approach to reconstructing sea surface temperatures globally using paired Mg/Ca - δ18Oc recorded in tests of the polar to subpolar planktonic foraminifera Neogloboquadrina pachyderma. We show that the fidelity of Mg/Ca-based paleoclimate reconstructions is compromised by variations in seawater carbonate chemistry which can be successfully quantified and isolated from paleotemperature reconstructions using a multiproxy approach. By applying the calibration to the last glacial maximum, we show that marine polar amplification has been underestimated by up to 3.0 ± 1.0 °C in model-based estimates.

Environmental DNA sheds new insight on molecular adaptation of foraminifera to temperature from laboratory-controlled culture experiment.

Foraminifera is the most important temperature proxy of the ocean on long time scales. However, the absence of temperature-controlled experiments at different water depths hinders the advancement of paleotemperature reconstruction with foraminifera from the continental shelf. For the first time, this study investigated the response of benthic foraminifera to temperature change using microcosm culture and metabarcoding. Foraminiferal communities from three continental stations at varying water depths (6.0, 9.2, and 26.0 m) were cultured under five temperature gradients (6, 12, 18, 24, and 30°C), with each treatment performed in triplicate. The foraminifera were fed with microalgae every 4 days, and the filtered seawater (through 0.22 μm pores), acting as a medium, was changed accordingly. The experiment lasted for 80 days, and 47 DNA samples were obtained and analyzed, including three insitu samples. The results showed that foraminifera adjusted its growth rate within the low-temperature range and adopted an r-strategy to cope with high-temperature stress. In addition, the foraminifera from deeper water stations exhibited a pronounced vulnerability to diminishing read counts. The read counts, operational taxonomic units (OTU) counts and Margalef index of foraminifera and the read counts of Rotaliida exhibited a remarkably positive correlation with temperature. The recommended relationships were described as read counts = 1314.75*T + 44754.51; OTU counts = 1.13*T + 44.26; Margalef index =1.13*T + 44.26. This study established the first quantitative relationship between temperature and foraminifera molecular parameters that holds significant implications for long-time paleotemperature calibration in climate change.

High-resolution UK’37 sea surface temperature reconstruction in the southwestern Atlantic Ocean during the Holocene

Modern sea surface temperatures (SST) were assessed using the alkenone calibration index (UK′37; SST-UK′37) values from twenty-one core-top sediment samples collected along the Southwestern Atlantic Ocean and validated by comparison with satellite-derived SSTs. The SST-UK′37 values revealed an annual signal between 28 and 30°S and a winter signal (83% of the region samples) between 30 and 34° S, with deviations falling within the UK′37 calibration error range. The paleotemperature (SST-UK′37) reconstruction through the Holocene was performed using two high-resolution marine sediment cores from 33 to 34°S latitude band. Millennial-scale variations dominate most of the SST-UK′37 results with changes up to ∼6 °C. Considering the modern ocean circulation dynamics, the SST-UK’37 in our records likely stems from the combined influence of the Subtropical Shelf Front (STSF) position and the interaction between the warm, salty, and nutrient-poor Subtropical Shelf Water (STSW) and the cold, fresh, and nutrient-rich Subantarctic Shelf Water (SASW). Our SST-UK’37 results also suggest that the northernmost position limit of the STSF reached around 33°S during the mid-to-late Holocene, while after ∼5500 cal yr BP the Plata Plume Water became the dominant influence on the regional SST-UK’37 signal. Spectral analyses revealed statistically significant oscillations of ∼8600 and ∼ 2250 years in record MDBT 557, and ∼ 5300 and ∼ 2600 years in record MDBT 561 which suggests the influence of Southern Westerly Winds (SWW) on the SST-UK’37, and therefore, on the STSF dynamic through the Holocene.

Late Miocene cooling and uplift recorded by bacterial H-GDGTs in the Xining Basin, northeastern Tibetan Plateau

The uplift history of the Tibetan Plateau (TP) remains uncertain despite many scientific studies. The development of a novel paleoaltimeter may provide a crucial tool for addressing this issue. H-shaped glycerol dialkyl glycerol tetraethers (H-GDGTs) are analogs of GDGTs and considered to have potential in paleotemperature reconstructions, becoming, in turn, a potential indicator of paleoaltitude. This study analyzed the H-GDGTs in Late Miocene sediments from the Mojiazhuang (MJZ) Section in the Xining Basin (XB) and evaluated the potential of these less-explored biomarkers in paleotemperature and paleoaltitude reconstructions. Only branched H-GDGTs (H-brGDGTs) were detected; their relative abundance in relation to brGDGTs (%H-brGDGTs) exhibited a strong correlation with proxy-based palaeotemperature. However, their exceptionally elevated values necessarily impose limitations on the feasibility of quantitatively reconstructing paleotemperatures based on %H-brGDGTs, rendering them suitable solely as a qualitative temperature indicator. The methylation index of H-brGDGTs (H-MBTacyclic) can be effectively used to quantitatively reconstruct paleotemperatures. The challenge H-MBTacyclic poses is its potentially limited use for lower temperatures, as it may exhibit incomplete detection of H-brGDGT components. These provisos being the case, both %H-brGDGTs and H-MBTacyclic were nonetheless effectively used to document the cooling and TP uplift event in the XB during the ∼10.5–8 Ma period, aligning with the reconstruction based on brGDGTs. The quantitative paleoaltitude reconstruction using H-MBTacyclic values indicated an uplift amplitude of ∼1 km. As an independent proxy of paleotemperature and paleoaltitude, H-brGDGTs once again would appear to underscore the presence and significance of this Late Miocene uplift event. Given the challenges encountered when using H-brGDGT-based proxies, further investigations into modern samples and evaluations of deep-time samples are imperative for any future application.

ISOSCAPES AND PALEOISOTHERMS OF THE HOLOCENE MEAN JANUARY AIR TEMPERATURE IN THE NORTHWESTERN SIBERIA (BASED ON STABLE OXYGEN ISOTOPE COMPOSITION OF ICE WEDGES)

Holocene ice wedges at 16 sites in the Northwestern Siberia were studied, the age of ice wedges was determined, and stable isotope data ( 18O and 2Н values) for all studied ice wedges have been summarized. The δ2H-δ18O ratio and the dexc values for ice wedge ice indicate good preservation of the isotope signal of winter precipitation in ice, which allows to use the obtained δ18O values for paleotemperature reconstructions. The isoscapes (lines of equal 18O values) created for the Holocene ice wedges and modern ice veinlets are generally submeridional; the 18O values decrease from west to east of the study area. Taking into account a new Holocene tripartite division scheme it was shown that the mean January air temperature (TmJ) in the Northwestern Siberia ranged approximately from -21 to -30°С during Greenlandian and the first half of the Northgrippian stage of the Holocene (11,4 to 6 cal ka BP) and approximately from -24 to -27°С from the end of the Northgrippian - to the Meghalayan stage of the Holocene (5,2 to 0,9 cal ka BP). The Holocene isotherms are close to the modern sub-meridional position and show an eastward decrease in TmJ values. Winter climatic conditions in the Northwestern Siberia were generally stable during the Holocene, meanwhile TmJ was on average 1-2°C lower than modern ones.

Structural insights linking H-bridging of archaeal GDGTs to high temperature

Isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), characteristic membrane lipids of archaea, are widely used in ecological and geochemical studies, especially for paleoenvironmental reconstruction. Glycerol monoalkyl glycerol tetraethers (GMGTs, also known as H-GDGTs), a unique variant of GDGTs, have covalent bonds linking the two alkyl chains. Despite some studies suggesting a link between GMGTs and high temperatures, the reliability and mechanisms remain unclear. Using molecular dynamics simulations, we elucidated the mechanism connecting GMGTs to high temperatures. Our findings show that H-bridging linkages reduce the distance between alkyl chains, leading to thicker and denser membranes with lower fluidity and permeability. The diffusion coefficient of GMGTs decreased by approximately 35 % compared to GDGTs, indicating their role as a archaeal high-temperature adaptation. This study provides a mechanistic basis for using archaeal GMGTs in geochemical studies and enhances confidence in their use for paleotemperature reconstruction.

Development of a quantum cascade laser absorption spectrometer for simultaneous measurement of 13C-18O and 18O-18O clumping in CO2.

Dual clumped isotope paleothermometry determines carbonate formation temperatures by measuring the frequency of 13C-18O (∆638) and 18O-18O (∆828) pairs in carbonates. It resolves isotopic kinetic biases and thus enables more accurate paleotemperature reconstructions. However, high-precision measurements of 18O-18O clumping using current techniques requires large sample sizes and long acquisition times. We developed a mid-infrared isotope ratio laser spectrometer (IRLS) for simultaneous measurement of the isotopologue ratios ∆638 and ∆828 in gas-phase carbon dioxide (CO2) at room temperature. Our IRLS uses a single laser scanning from 2290.7 to 2291.1cm-1 and a 31 m pathlength optical cell, and it simultaneously measures the five isotopologues required for calculating ∆638 and ∆828: 16O12C16O, 16O13C16O, 16O12C18O, 16O13C18O, and 18O12C18O. In addition, our IRLS can measure 16O12C17O, enabling ∆17O analysis. At ~20°C and a CO2 pressure of ~2Torr, our IRLS system achieved precisions of 0.128‰ and 0.140‰ within 20 s for abundances of the clumped isotopologues 16O13C18O and 18O12C18O, respectively, and precisions of 0.267‰, 0.245‰, and 0.128‰ for 16O12C16O, 16O13C16O, and 16O12C18O. This yielded precisions of 0.348‰ (∆638) and 0.302‰ (∆828) within 25 s. Simulated sample-reference switching highlights the potential of our system and the need for further development. We demonstrated simultaneous measurements of ∆638 and ∆828 in CO2 to precisions of <0.35‰ within 25 s using a room-temperature, single-laser IRLS. Future developments on better resolving 16O12C16O and 16O13C16O peaks and system temperature control could further improve the measurement precision.

Highly efficient purification of long-chain alkenones using silver dimercaptotriazine flash chromatography

Long-chain alkenones (LCAs) are important tools for paleotemperature reconstructions in lacustrine and marine environments. However, some neutral lipid compounds that co-elute with LCAs often occur in natural sediment samples, which seriously interferes with the identification and quantification of the LCAs. In this study, we report a new flash column chromatography based on silver(I)-dimercaptotriazine (Ag-DMT) functionalized silica material (≡Si-DMT-Ag) for purifying and isolating the individual LCAs from complex lake and marine sediment samples. Compared to silver(I)-mercaptopropyl (Ag-MP) functionalized (≡Si-(CH2)3-S-Ag) and silver nitrate (SiO2 + AgNO3) impregnated silica materials used by previous studies, the Ag-DMT stationary phase displays the best efficiency in removing co-eluting compounds (including wax ester, hopanoid, hopanoic acid, sterane, alkanoic acid, and some unknown compounds) from the LCAs in our study lake and marine sediment samples. The Ag-DMT material also shows its high retention capacity for the LCAs, probably due to a strong interaction of more positively charged silver sites on Ag-DMT with double bonds. Employing the Ag-DMT chromatography, we develop an optimal solvent elution scheme to efficiently isolate the individual LCAs with the same chain length but different numbers of unsaturation for their isotopic analysis in the future. Our study provides a highly effective method for eliminating co-eluting compounds and isolating individual LCAs for paleoclimate studies.

Distributions of bacterial branched tetraether lipids associated with bacterial communities in hummock and hollow soils from Southern Tibetan peatland

Branched glycerol dialkyl glycerol tetraethers (brGDGTs), special membrane-spanning lipids derived from some bacteria, are widely used as molecular tools in paleoenvironmental studies to reconstruct terrestrial temperature and pH conditions. However, the impact of microtopography discrepancies on the accuracy and applicability of brGDGT-based proxies remains unclear. Here we investigated the distributional patterns of brGDGTs and bacterial communities within the hummock-hollow microtopography in the mid-southern Tibetan peatlands. Our results show that the methylation index of C5-methyl brGDGTs (MBT′5ME) values are significantly correlated with the local mean annual air temperature (MAAT). However, the peat-specific temperature calibration exhibits an underestimation of MAATpeat compared to the present MAAT. We find the MBT′5ME values are significantly lower in the moist hollows than that in the drier hummocks. This discrepancy could be primarily attributed to the shifts in bacterial communities and the physiological adaptation of brGDGT-producing bacteria driven by the high soil water content. These findings highlight the remarkable variability of MBT′5ME values at small spatial scales, which holds profound implications for paleotemperature reconstruction in alpine peatlands.

Distribution of branched glycerol dialkyl glycerol tetraether (brGDGT) lipids from soils and sediments from the same watershed are distinct regionally (central Chile) but not globally

Quantitative reconstructions of past continental climates are vital for understanding contemporary and past climate change. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are unique bacterial lipids that have been proposed as universal paleothermometers due to their correlation with temperature in modern settings. Thus, brGDGTs may serve as a crucial paleotemperature proxy for understanding past climate variations and improving regional climate projections, especially in critical but under constrained regions. That said, complications can arise in their application due to varying source contributions (e.g., soils vs. peats vs. lacustrine). As such, this study investigates brGDGT distributions in Chilean lake surface sediments and corresponding watershed soils to determine the source of brGDGTs to lake sediments. Global datasets of brGDGTs in lake sediments and soils were additionally compiled for comparison. Distinct brGDGT distributions in Chilean lakes and soils indicate minimal bias from soil inputs to the lacustrine sediments as well as in situ lacustrine production of brGDGTs, which supports the use of brGDGTs in lake sediments as reliable paleotemperature proxies in the region. The ΣIIIa/ΣIIa ratio, initially promising as a brGDGT source indicator in marine settings, shows global complexities in lacustrine settings, challenging the establishment of universal thresholds for source apportionment. That said, we show that the ratio can be successfully applied in Chilean lake surface sediments. Direct comparisons with watershed soils and further research are crucial for discerning brGDGT sources in lake sediments and improving paleotemperature reconstructions on regional and global scales moving forward. Overall, this study contributes valuable insights into brGDGT variability, essential for accurate paleoreconstructions.

EARLY HOLOCENE OXYGEN ISOTOPE CHRONOLOGIES (11,267–6420 CAL BP) FROM ICE WEDGE AT CHARA, TRANSBAIKALIA

ABSTRACTAccelerator mass spectrometry radiocarbon (AMS 14C) dating was used for determining the age of wedge ice. It has been found that between 11,270 and 6420 cal BP, or the Greenlandian and Northgrippian stages of the Holocene, ice wedges grew syngenetically in sandy deposits with gravel in the Chara River valley. The variations of δ18O values in the ice wedges are about 8‰, from –25.5‰ to –18.8‰. Based on the stable isotope composition of ice wedges, paleotemperature reconstructions revealed that the mean January temperature was as low as –38°C during the coldest periods of the early half of the Holocene and as high as –28°C during the warmer periods.

Archaeal Hydroxylated Isoprenoid GDGTs in Asian Lake Sediments: A New Tool for Terrestrial Paleotemperature Reconstructions

AbstractHydroxylated 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.

BrGDGT-based seasonal paleotemperature reconstruction for the last 15 000 years from a shallow lake on the eastern Tibetan Plateau

Abstract. Understanding Holocene temperature changes is vital for resolving discrepancies between proxy reconstructions and climate models. The intricate temperature variations across the Tibetan Plateau (TP) add complexity to studying continental climate change during this period. Discrepancies between model-based and proxy-based reconstructions might stem from seasonal biases and environmental uncertainties in the proxies. Employing multiple proxies from a single sediment core for quantitative temperature reconstructions offers an effective method for cross-validation in terrestrial environments. Here, we present an ice-free-season temperature record for the past 15 kyr from a shallow, freshwater lake on the eastern TP, based on brGDGTs (branched glycerol dialkyl glycerol tetraethers). This record shows that the Holocene Thermal Maximum lags the pollen-based July temperature recorded in the same sediment core. We conclude that the mismatch between the brGDGT-based and pollen-based temperatures is primarily the result of seasonal variations in solar irradiance. The overall pattern of temperature changes is supported by other summer temperature records, and the Younger Dryas cold event and the Bølling–Allerød warm period are also detected. A generally warm period occurred during 8–3.5 ka, followed by a cooling trend in the late Holocene. Our findings have implications for understanding the seasonal signal of brGDGTs in shallow lakes and provide critical data for confirming the occurrence of seasonal biases in different proxies from high-elevation lakes. To further investigate the significance of the brGDGTs and temperature patterns on the TP, we examined existing brGDGT-based Holocene temperature records, which interpret these compounds as indicators of mean annual or growing season temperatures. The existing and available temperature records show complicated patterns of variation, some with general warming trends throughout the Holocene, some with cooling trends, and some with a warm middle Holocene. We analyzed the possible reasons for the diverse brGDGTs records on the TP and emphasize the importance of considering lake conditions and modern investigations of brGDGTs in lacustrine systems when using brGDGTs to reconstruct paleoenvironmental conditions.

Control of the temperature signal in Antarctic proxies by snowfall dynamics

Abstract. Antarctica, the coldest and driest continent, is home to the largest ice sheet, whose mass is predominantly recharged by snowfall. A common feature of polar regions is the warming associated with snowfall, as moist oceanic air and cloud cover increase the surface temperature. Consequently, snow that accumulates on the ice sheet is deposited under unusually warm conditions. Here we use a polar-oriented regional atmospheric model to study the statistical difference between average and snowfall-weighted temperatures. During snowfall, the warm anomaly scales with snowfall amount, with the strongest sensitivity occurring at low-accumulation sites. Heavier snowfall in winter helps to decrease the annual snowfall-weighted temperature, but this effect is overwritten by the event-scale warming associated with precipitating atmospheric systems, which particularly contrast with the extremely cold conditions that occur in winter. Consequently, the seasonal range of snowfall-weighted temperature is reduced by 20 %. On the other hand, the annual snowfall-weighted temperature shows 80 % more interannual variability than the annual temperature due to the irregularity of snowfall occurrence and its associated temperature anomaly. Disturbances of the apparent annual temperature cycle and interannual variability have important consequences for the interpretation of water isotopes in precipitation, which are deposited with snowfall and commonly used for paleotemperature reconstructions from ice cores.

Impact of saponification and silver-nitrate purification on lacustrine alkenone distributions and alkenone-based indices

A growing interest in lacustrine alkenones as a proxy for continental paleotemperature reconstructions accompanied important methodological improvements over the past two decades. New gas chromatography (GC) columns were used for alkenone analysis, that drastically improved alkenone separation, especially for freshwater lakes. However, these recent advances are sometimes not sufficient in separating compounds that interfere with alkenones in the resulting chromatograms and concurrently, new chemical procedures were implemented to further clean up the samples. Here we investigate the impact of two clean-up procedures, saponification and silver-nitrate purification, on alkenone distribution, alkenone quantification, and C37 alkenone-based indices, including the U37K index. The silver-nitrate purification modified the C37 alkenone distribution and thus the C37 alkenone-based indices, especially the U37K index, in 6 out of the 9 studied samples by further retaining alkenones with more double bonds. These changes would result on an average error of 3 °C in reconstructed temperatures. Saponification also influenced the C37 alkenone distribution mainly by removing co-eluting compounds, thereby improving the quality of the results. Both saponification and purification resulted in the reduction of the C37 alkenone concentration by almost half. Clean-up steps should thus be used carefully, paying particular attention to any change in alkenone distribution and concentration. Limiting the use of additional clean-up steps reduces the risk of modifying the alkenone distribution.

Late Holocene brGDGTs-based quantitative paleotemperature reconstruction from lacustrine sediments on the western Tibetan Plateau

Late Holocene brGDGTs-based quantitative paleotemperature reconstruction from lacustrine sediments on the western Tibetan Plateau

Investigation of Seasonal Settlement and Clam Harvest Pressure in the Sechelt Inlet System, British Columbia, Canada, Through Sclerochronology and Stable Oxygen Isotope Analyses

ABSTRACT The Sechelt inlet system (SIS), situated on the inner Sunshine Coast of British Columbia, lies within the territory of the shíshálh Nation. This study focuses on ∼1000 years of shíshálh shellfish harvesting from 930-0 cal. BP. Specifically, we investigated the seasonal timing of shellfish collection, relative shellfish harvesting pressure and paleo-temperature reconstruction at four archaeological sites. Thirty archaeological butter clam shells (Saxidomus gigantea) from the SIS underwent high-resolution shell oxygen isotope (δ18O) analysis and 662 individual archaeological shell fragments were analysed for growth stage determination. The δ18O results showed a pattern of year-round collection with an emphasis on spring collection at village sites. Results from shell growth stage analysis suggest an intensive pattern of shellfish harvesting in the region, regardless of the site type. Archaeological shells show a larger δ18Oshell range, possibly reflecting a greater sea surface temperature amplitude than modern shells (i.e. a difference of ∼6.5°C; modern range: ∼3.7–20.4°C; archaeological range: ∼ −0.8–22.4°C). We contextualise our results with previous studies of seasonal timing and intensity of shellfish harvesting from the Pacific Northwest Coast, while interpreting these new data in the context of shíshálh occupation and landscape use.

Temperature and hydrological variations during the late-glacial in the Central Mediterranean: Application of the novel ostracod-clumped isotope thermometer

This study shows, for the first time, the absence of a vital effect in the clumped isotope carbonate (Δ47) fossil ostracod signal and confirms the ability of the novel ostracod-Δ47 thermometer to reconstruct past temperatures and hydrological conditions in complex lacustrine systems. Furthermore, the application of Δ47 analyses on the ostracod species Candona angulata and Cyprideis torosa from Lake Trasimeno record (central Italy), which today precipitate their shells during the cold and the warm season respectively provides evidence that by combining biological (i.e., ostracod shell precipitation timing), paleontological (i.e., identification of ostracod species) and geochemical (i.e., Δ47) approaches, the ostracod-Δ47 thermometer accurately reconstructs past seasonality. Despite the absence of a vital effect, not all species can be combined for Δ47 analyses in environments with seasonal temperature variations; only those that precipitate their shells during the same season should be considered. The application of the ostracod-Δ47 thermometer on the Trasimeno lacustrine record gives rise to the first continental warm season paleotemperature reconstruction of the last 43 ky in central Mediterranean area. The combination of Δ47 and oxygen isotope composition (δ18Oost) measured on ostracod shells provides the isotopic composition of the water from which the carbonate precipitated (δ18Ow) and thereby, changes in the evaporation/precipitation balance in this area. Before the Last Glacial Maximum (LGM), equivalent to the Marine Isotopic Stage 3 (MIS3, from 43 to 29ky), warm season temperatures ranged from 15 ± 1.6 °C to 22 ± 2.3 °C, equivalent to 2 to 6 °C colder than today. Hydrological conditions during this period are similar to the present-day ones, characterized by a permanent lake and a high evaporation/precipitation ratio (E/P). The drastic decrease of the warm season temperatures (ranging from 10 ± 2.9 °C to 17 ± 3.1 °C) and of the E/P ratio during LGM and Late-glacial (MIS2, from 29 to 11.6 ky) correspond to the global climate cooling and low summer insolation, suggesting an amplifying role, of the latter, in the effects of the millennial scale climatic variations. At the Pleistocene/Holocene transition, both warm season temperature (25 ± 2 °C) and the E/P ratio increased in conjunction with the summer insolation. During the early Holocene, warm season temperature (23 ± 2 °C) closely resembles present-day values. However, cold season temperature (12 ± 2 °C) is approximately 4 °CC warmer than today. Notably, no hydrological differences are identified between the warm and the cold season underlying a lower seasonality contrast compared to the present, along with enhanced warm season precipitation. The good agreement between the Δ47 temperatures reconstructed for the last 1 ky and the temperatures presently recorded at Lake Trasimeno (8 °C cold and 22 °C for warm season), confirms the accuracy of the analyses and the applicability of the ostracod-Δ47 thermometer to reconstruct seasonal temperature changes.

BrGDGT-based quantitative reconstructions of paleotemperature in lakes: Regional vs. site-specific calibrations

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that have been widely used as tools for the quantitative reconstruction of past terrestrial temperatures in lakes. Two approaches, i.e., regional/global and site-specific calibrations, have been previously employed in the reconstruction. However, their strengths and weaknesses have not been properly evaluated yet, making it challenging to choose a suitable approach to paleoreconstruction. Here, we investigated the distribution of brGDGTs in surface sediments of 65 lakes along a mean annual lake water temperature (MLWT) transect in West China and developed a regional MBT'5ME (methylation degree of 5-methylated brGDGTs) calibration for MLWT reconstruction. Meanwhile, a site-specific brGDGT calibration for Lake Yangzonghai (YZH), located within the West China transect, was generated based on the relationship between the instrumental temperature record over the past 100 years and individual brGDGT abundances in a short sediment core taken from the lake. These two calibrations were applied to global lake surface sediments, as well as a long sediment core spanning the Holocene from Lake YZH. The linear correlations between MBT'5ME and temperature are highly consistent among lakes from different regions, indicating the near-universal response of brGDGTs in global lakes to temperature. However, MLWT inferred from the site-specific calibration shows a weak relationship with actual MLWT in global lakes, indicating that the site-specific calibration might be only a result of overfitting rather than reflecting the response of brGDGT distribution to ambient temperature. Applications to a Holocene sediment core in Lake YZH suggest that the regional MBT'5ME calibration is more suitable for reconstructing paleotemperature than the site-specific calibration. The regional MBT'5ME calibration derived temperature record shows a cold Younger Dryas (YD), followed by a rapid warming of ∼3 °C during the last deglaciation, and a long-term cooling of ∼1 °C from 9.9 to ∼0.2 cal ka BP. The principal component analysis shows that temperature is the dominant factor affecting brGDGT distributions in the long sediment core whereas the site-specific calibration is strongly affected by human activities rather than reflecting the ambient temperature. We highlight the importance of using appropriate approaches to developing calibrations for paleotemperature reconstruction and assessing the reliability of the temperature records.

Branched GDGT source shift identification allows improved reconstruction of an 8,000-year warming trend on Sumatra

Branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) in sedimentary archives are increasingly used for paleotemperature reconstructions due to their strong correlation with mean annual air temperature. However, environmental factors can influence the brGDGT producing bacterial community, potentially affecting the brGDGT-temperature relationship and introducing fundamental errors in reconstructions. Here we assess the reliability of the methylation index of brGDGTs (MBT′5ME) in sediments as a paleotemperature proxy by tracking provenance differences based on brGDGT fractional abundances in a short lake core, two peat cores and surface soils on Sumatra (n = 333 in total). Then, we attempt to reconstruct the Holocene paleotemperatures on Sumatra using the two peat cores. Our results indicate distinct brGDGT and H-shaped brGDGT (H-GDGT) compositions in soil, lake and peat environments, suggesting production by different bacterial communities. We introduce a new index, the isomerization of H-GDGTs (IRH) that can distinguish between these environments. In an 11,000-year long peat core from Diatas, we find that brGDGT composition changes are dominated by bacterial community shifts rather than temperature changes. In contrast, a core from the nearby Padang peatland can be robustly used for a brGDGT-based paleotemperature reconstruction since there are no signs of past environmental or brGDGT source shifts. The results from Padang indicate a gradual warming trend over the past 8,000 years, consistent with climate model simulations and nearby sea surface temperature reconstructions. However, current MBT′5ME calibrations yield larger warming trends compared to simulations and other proxy studies, suggesting the need for tropical and/or peat-specific brGDGT temperature calibrations. Our findings demonstrate the importance of assessing environmental shifts and bacterial source community changes when employing brGDGT paleothermometry. The methodological framework outlined in this study can be used in future research for reliable down-core brGDGT temperature reconstructions. Our proxy reconstruction over the past 8,000 years offers novel insights into the Holocene temperature evolution from a region with low climate seasonality.