Past Temperature Reconstructions Research Articles

Replicability of paleotemperature records in the northern Okinawa Trough and its implications for paleoceanographic reconstructions

Geochemical proxies are frequently utilized in the reconstruction of past ocean temperatures. Due to resource constraints, these reconstructions typically rely on a single sediment core, raising questions about the local and regional representativeness of paleotemperature records. To address this, we analyzed four sediment cores located within a 10-km radius in the northern Okinawa Trough (OT), which share the same climatic forcing and thus should reflect similar climate variations. We compiled published data and generated new paleotemperature estimates based on three widely used geochemical proxies (foraminiferal Mg/Ca, U37K′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{U}}_{37}^{{{\ ext{K}}^{\\prime}}}$$\\end{document}, TEX86\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{TEX}}_{86}$$\\end{document}). Analysis of the mean absolute deviations for nearby records based on the same proxy revealed that U37K′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{U}}_{37}^{{{\ ext{K}}^{\\prime}}}$$\\end{document} has the highest reproducibility, followed by Mg/Ca and TEX86\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{TEX}}_{86}$$\\end{document}. However, inconsistencies in inter-proxy offsets among nearby sites suggest the presence of noise in the proxy records, likely stemming from instrumental errors and sediment heterogeneity. Furthermore, the Mg/Ca and U37K′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{U}}_{37}^{{{\ ext{K}}^{\\prime}}}$$\\end{document} paleotemperature records agree within uncertainty when accounting for inter-site variability and calibration uncertainties, challenging previous interpretations of temperature signals from different seasons. All proxies indicate similar glacial-interglacial trends, albeit with varying magnitudes of temperature change. Both Mg/Ca and U37K′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{U}}_{37}^{{{\ ext{K}}^{\\prime}}}$$\\end{document} records suggest a glacial cooling of ~ 3 °C, whereas TEX86\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{TEX}}_{86}$$\\end{document} sea surface temperature (SST) data indicate a stronger glacial cooling of approximately ~ 6–8 °C. Modern observations indicate a subsurface TEX86\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{TEX}}_{86}$$\\end{document} recording depth of 50–100 m, coinciding with the thermocline. However, the TEX86\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{TEX}}_{86}$$\\end{document} subsurface temperature (subT) record does not resemble the Mg/Ca records of thermocline-dwelling foraminifera species. Instead, there is a better agreement with benthic foraminiferal Mg/Ca records of Uvigerina spp. (~ 700 m) and the intermediate temperature record derived from radiolarian assemblages (~ 500 m), pointing to a TEX86\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{TEX}}_{86}$$\\end{document} recording depth that is deeper than the thermocline. In summary, our findings show that proxy noise can impact inter-proxy comparisons of paleotemperature records, but not the direction of glacial-interglacial shifts. Future research should prioritize constraining the recording depth of paleotemperature proxies and reducing calibration uncertainty for more precise and reliable quantitative paleotemperature reconstruction.

Sea surface temperature reconstruction in the Pacific Ocean using multi-elemental proxy in Porites and Diploastrea corals: Application to Palau Archipelago

Massive reef-building Porites corals are commonly studied to obtain long-term reconstructions of past sea surface temperature (SST) using temperature-sensitive elemental proxies, such as Sr/Ca or Li/Mg ratios. Most recently, a multi-proxy approach combining these two ratios (e.g. D'Olivo et al., 2018) and the SrU method (DeCarlo et al., 2016) have proved to be more robust to reconstruct paleo-temperatures. To date, no study has been carried out on the application of these new approaches on the Diploastrea heliopora coral, another massive reef-building genus that can potentially provide longer temperature records. Moreover, only a few studies have assessed coral SST calibrations at the scale of the Indo-Pacific basin and compared SST reconstructions obtained from two massive coral genera from the same site.In this study, we investigated the elemental composition of the topmost portion of 34 modern tropical Porites and 6 Diploastrea colonies collected during the Tara Pacific expedition (2016–2018) from various hydrological contexts in the Pacific Ocean. We derived and discussed annual Sr/Ca, Li/Mg, combined Sr/Ca-Li/Mg and Sr/Ca-Li/Ca-Mg/Ca and SrU vs. SST calibrations as well as potential intra-colonial and genus specific effects and evaluated the use of these basin-wide calibration equations. Overall, multi-ratio and multi-genera SST calibrations perform better than single-ratio calibrations and seem to improve temperature reconstructions.These new SST calibrations were applied to two colonies of Porites and Diploastrea collected from the same site in the North-West of the Palau archipelago located in the western Pacific Ocean to evaluate the applicability of universal calibrations based on different proxies and their combination. Coral-based SST records spanning the last 141 years show decadal changes and recent warming episodes that are related to major El Niño Southern Oscillation events. However, differences in reconstruction remain between both genera in the long-term trends, amplitudes, and absolute temperatures, depending on which genus or temperature proxy is considered.

Climate change is rapidly deteriorating the climatic signal in Svalbard glaciers

Abstract. The Svalbard archipelago is particularly sensitive to climate change due to the relatively low altitude of its main ice fields and its geographical location in the higher North Atlantic, where the effect of Arctic amplification is more significant. The largest temperature increases have been observed during winter, but increasing summer temperatures, above the melting point, have led to increased glacier melt. Here, we evaluate the impact of this increased melt on the preservation of the oxygen isotope (δ18O) signal in firn records. δ18O is commonly used as a proxy for past atmospheric temperature reconstructions, and, when preserved, it is a crucial parameter to date and align ice cores. By comparing four different firn cores collected in 2012, 2015, 2017 and 2019 at the top of the Holtedahlfonna ice field (1100 m a.s.l.), we show a progressive deterioration of the isotope signal, and we link its degradation to the increased occurrence and intensity of melt events. Our findings indicate that, starting from 2015, there has been an escalation in melting and percolation resulting from changes in the overall atmospheric conditions. This has led to the deterioration of the climate signal preserved within the firn or ice. Our observations correspond with the model's calculations, demonstrating an increase in water percolation since 2014, potentially reaching deeper layers of the firn. Although the δ18O signal still reflects the interannual temperature trend, more frequent melting events may in the future affect the interpretation of the isotopic signal, compromising the use of Svalbard ice cores. Our findings highlight the impact and the speed at which Arctic amplification is affecting Svalbard's cryosphere.

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.

Disentangling multiproxy temperature reconstructions from the subtropical North Atlantic

Reliable reconstruction of past sea surface temperature (SST) is of prime importance for understanding the Earth’s sensitivity to external forcing. Yet, it remains a major challenge in paleoceanography because comparison between SST estimates from different proxies reveals mismatches and raise the question as to what the contrasting proxies actually record. A better understanding of these mismatches in the light of seasonal occurrence of the proxy bearing organisms (archives) and water mass changes help to assess climate models. Here, we analyze data from the last deglaciation using a sediment core site situated at the northern boundary of the North Atlantic subtropical gyre influenced by fast latitudinal migrations of the subtropical Azores Front (AF) and resulting changes in water masses that may affect the SST records. Differences between the SST estimates from different deglacial SST reconstructions obtained from (1) Mg/Ca in planktic foraminifer tests, (2) alkenone UK′37, and (3) planktic foraminifer assemblages (SIMMAX), are assumed to result from the ecology of the proxy bearing organisms, and are assessed for the impact on different SST reconstructions from local seawater δ18O (δ18Ow) reconstructions. The general trends of SSTs from all four proxies confirm the well-known deglacial succession of warm and cold events. Mismatches between amplitudes of temperature changes are explained by differences in the phenology of the proxy-bearing organisms and local changes in hydrography. The combination of δ18O SST from the three different archives of δ18Ow reconstructions may cause offsets that exceed the climate driven signals.

Tree rings reveal changes in the temperature pattern in eastern China before and during the Anthropocene

A comprehensive understanding of the spatial characteristics of warming trends and temperature variability is important given global warming. Paleoclimate reconstruction has played an important role in evaluating warming and extreme climactic events in recent decades. Using the ring width of Pinus tabulaeformis, we reconstructed the ground surface temperature changes at Mengshan Mountain, in the central part of eastern China (EC), from 1667 to 2019. There were 3 extremely low-temperature years, 42 low-temperature years, 256 normal years, 37 high-temperature years and 15 extremely high-temperature years over the past 353 years. High and extremely high temperatures mainly occurred in the first half of the 19th century, the end of the 20th century and the beginning of the 21st century; low and extremely low temperatures occurred in almost all periods except for the last 50 years. By combining the past temperature reconstructions from the Great Xing’an Mountains in northern EC and Taiwan in southern EC, we also found that after the start of the Anthropocene, there were strong positive correlations among the temperatures of the northern, central and southern parts of EC. However, before the Anthropocene, these correlations were weak, and there was even a significant negative correlation during some periods. Additionally, the temperature in EC exhibited different change patterns before and during the Anthropocene. Before the Anthropocene, the temperature in southern EC showed an upward trend; in central EC, the temperature first rose and then began to decline in the 1820s; there was no obvious trend in the temperature changes in northern EC. The spatial consistency of temperature changes during the Anthropocene might be related to the fact that greenhouse gases emitted by human activities diffuse evenly withatmospheric circulation and absorb longwave radiation to directly heat the atmosphere.

Evaluation of the distributions of hydroxylated glycerol dibiphytanyl glycerol tetraethers (GDGTs) in Holocene Baltic Sea sediments for reconstruction of sea surface temperature: the effect of changing salinity

Abstract. Hydroxylated glycerol dibiphytanyl glycerol tetraethers (OH-GDGTs) produced by both marine and freshwater thaumarchaea are increasingly used for the reconstruction of past sea surface temperature (SST). They occur throughout the modern Baltic Sea, but it is unknown if OH-GDGTs can be used for assessing past SST in this area, where salinity has changed considerably over the Holocene. Three commonly applied OH-GDGT proxies for SST reconstruction, i.e., the OH-GDGT%, RI-OH, and RI-OH′ indices, were tested using a thaumarchaeotal culture enriched from the Baltic Sea grown at 4 and 22 ∘C, as well as 12 surface sediments from the Baltic Sea and the adjacent Skagerrak. In the culture experiments all three proxies showed the expected response with the rise in temperature, but their absolute values were not always in line with existing marine core-top calibrations, especially for the OH-GDGT% index. Of the two proxies based on the distribution of OH-GDGTs, the RI-OH index shows no increase with increasing mean annual SST, whilst the RI-OH′ index shows only a linear correlation with SST when the sediments from the Bothnian Sea and Bothnian Bay area, with a reduced salinity and increased lateral sediment influx, are omitted from the dataset. Two sedimentary Holocene records from the Arkona and Gotland basins were studied, the latter in high resolution. In the brackish phase of the Baltic Sea (the Littorina Sea stage), the RI-OH′ index shows a good correlation with the TEX86L, an established temperature proxy in the Baltic Sea, and can be used to identify important climatic events. However, during the preceding Ancylus Lake phase the RI-OH′ (and RI-OH) index records values that are far too high, resulting in anomalously high SST estimates. This is probably because freshwater thaumarchaea adjust their OH-GDGTs differently, as has been shown for thaumarchaea in Lake Lugano's water column. In the Littorina Sea, Ancylus Lake, and Yoldia Lake phases of the Baltic basin, the record of the RI-OH′ index thus most likely reflects changes in both temperature and salinity. Overall, our study indicates that a reduced salinity increases the values of the RI-OH′ (and RI-OH) indices substantially, and this should be considered when applying these proxies in other settings.

Tree-ring anatomy of Pinus cembra trees opens new avenues for climate reconstructions in the European Alps

Tree rings form the backbone of high-resolution palaeoclimatology and represent one of the most frequently used proxy to reconstruct climate variability of the Common Era. In the European Alps, reconstructions were often based on tree-ring width (TRW) and maximum latewood density (MXD) series, with a focus on European larch. By contrast, only a very limited number of dendroclimatic studies exists for long-lived, multi-centennial Pinus cembra, despite the widespread occurrence of the species at treeline sites across the European Alps. This lack of reconstructions can be ascribed to the difficulties encountered in past studies in extracting a robust climate signal from TRW and MXD chronologies. In this study, we tested various wood anatomical parameters from P. cembra as proxies for the reconstruction of past air temperatures. To this end, we measured anatomical cell parameters and TRW of old-growth trees from the God da Tamangur forest stand, known for being the highest pure, and continuous P. cembra forest in Europe. We demonstrate that several wood anatomical parameters allow robust reconstruction of past temperature variability at annual to multidecadal timescales. Best results are obtained with maximum latewood radial cell wall thickness (CWTrad) measured at 40 μm radial band width. Over the 1920–2017 period, the CWTrad chronology explains 62 % and >80 % of interannual and decadal variability of air temperatures during a time window corresponding roughly with the growing season. These values exceed those found in past work on P. cembra and even exceed the values reported for MXD chronologies built with L. decidua and hitherto considered the gold standard for dendroclimatic reconstructions in the European Alps. The wood anatomical analysis of P. cembra records therefore unveils a dormant potential and opens new avenues for a species that has been considered unsuitable for climate reconstructions so far.

Evolution of tropical land temperature across the last glacial termination

The tropical West Pacific hosts the warmest part of the surface ocean and has a considerable impact on the global climate system. Reconstructions of past temperature in this region can elucidate climate connections between the tropics and poles and the sensitivity of tropical temperature to greenhouse forcing. However, existing data are equivocal and reliable information from terrestrial archives is particularly sparse. Here we constrain the magnitude and timing of land temperature change in the tropical West Pacific across the last deglaciation using an exceptionally precise paleothermometer applied to a well-dated stalagmite from Northern Borneo. We show that the cave temperature increased by 4.4 ± 0.3 °C (2 SEM) from the Last Glacial Maximum to the Holocene, amounting to 3.6 ± 0.3 °C (2 SEM) when correcting for sea-level induced cave altitude change. The warming closely follows atmospheric CO2 and Southern Hemisphere warming. This contrasts with hydroclimate, as reflected by drip water δ18O, which responds to Northern Hemisphere cooling events in the form of prominent drying, while temperature was rising. Our results thus show a close response of tropical temperature to greenhouse forcing, independent of shifts in the tropical circulation patterns.

The cold-water coral Solenosmilia variabilis as a paleoceanographic archive for the reconstruction of intermediate water mass temperature variability on the Brazilian continental margin

Recent oceanographic observations have identified significant changes of intermediate water masses characterized by increased temperatures, lowered pH and deoxygenation. In order to improve our understanding as to how these changes may impact deep-sea ecosystems one important strategy is to reconstruct past oceanic conditions. Here we examine the applicability of the scleractinian cold-water coral Solenosmilia variabilis as a marine archive for the reconstructions of past intermediate water mass temperatures by using Lithium (Li)/Magnesium (Mg) ratios. In particular, our study addresses 1) the calibration of Li/Mg ratios against in-situ temperature data, 2) the reconstruction of past intermediate water mass temperatures using scleractinian coral fossil samples from the Brazilian continental margin and 3) the identification of intraspecies variability within the coral microstructure. Results showed that Li/Mg ratios measured in the skeletons of S. variabilis fit into existing Li/Mg-T calibrations of other cold-water scleractinian. Furthermore, the coral microstructure exhibits interspecies variability of Li/Ca and Mg/Ca ratios were also similar to what has been observed in other cold-water scleractinian corals, suggesting a similar biomineralization control on the incorporation of Li and Mg into the skeleton. However, the Li/Mg based temperature reconstruction using fossil samples resulted in unexpectedly high variations >10°C, which might not be solely related to temperature variations of the intermediate water mass over the last 160 ka on the Brazilian continental margin. We speculate that such temperature variability may be caused by vertical movements of the aragonite saturation horizon and the associated seawater pH changes, which in turn influence the incorporation of Li and Mg into the coral skeleton. Based on these results it is recommended that future studies investigating past oceanic conditions need to consider the carbonate system parameters and how they might impact the mechanisms of Li and Mg being incorporated into skeletons of cold-water coral species such as S. variabilis.

Alongshore upwelling modulates the intensity of marine heatwaves in a temperate coastal sea

Analyses of long-term temperature records based on satellite data have revealed an increase in the frequency and intensity of marine heatwaves (MHWs) in the world oceans, a trend directly associated with global change according to climate model simulations. However, these analyses often target open ocean pelagic systems and rarely include local scale, field temperature records that are more adequate to assess the impact of MHWs close to the land-sea interface. Here, we compared the incidence and characteristics of open ocean MHWs detected by satellites with those observed in the field over two decades (1998–2019) at two temperate intertidal locations in the central Cantabrian Sea, southern Bay of Biscay. Satellite retrievals tended to smooth out cooling events associated with intermittent, alongshore upwelling, especially during summer. These biases propagated to the characterization of MHWs and resulted in an overestimation of their incidence and duration close to the coast. To reconcile satellite and field records, we developed a downscaling approach based on regression modeling that enabled the reconstruction of past temperatures and analyze MHW trends. Despite the cooling effect due to upwelling, the temperature reconstructions revealed a six-fold increase in the incidence of MHWs in the Cantabrian Sea over the last four decades. A comparison between static (no trend) vs. dynamic (featuring a linear warming trend) MHW detection thresholds allowed us to attribute over half of the increase in MHW incidence to the ocean warming trend. Our results highlight the importance of local processes to fully characterize the complexity and impacts of MHWs on marine coastal ecosystems and call for the conservation of climate refugia associated with coastal upwelling to counter the impacts of climate warming.

Interrogating glacier mass balance response to climatic change since the Little Ice Age: reconstructions for the Jotunheimen region, southern Norway

Developing a long‐term understanding of the cryosphere is important in the study of past climatic change. Here we used a nested approach combining diverse instrumental (monthly meteorological data from four weather stations, as well as gridded data) and proxy data (based on blue intensity measurements from local tree ring records) to create a reconstruction of past summer temperature for the central Jotunheimen area in southern Norway. This record was then used to reconstruct annual glacier mass balance from 1962, the start of the yearly measurements, back to 1722, immediately prior to the regional Little Ice Age maximum. Our reconstruction of the ‘average’ Jotunheimen cumulative glacier mass balance is based on three representative glaciers (Storbreen, Hellstugubreen and Gråsubreen) that were synthesized into one composite record which we term ‘Gjennomsnittsbreen’ (‘mean glacier’ in Norwegian) to filter out localized controls on the behaviour of individual glaciers. While not ignoring the role of precipitation on glacier mass balance, our reconstruction demonstrates that glaciers in this region exhibit a strong summer temperature control and appear to have been declining more or less continuously since the mid‐18th century. However, it also shows that this long‐term trend of overall retreat in Jotunheimen is punctuated by relatively short‐lived periods of neutral or occasionally positive glacier mass balance, signifying periods of stillstand or small‐scale glacier advance. These periods or ‘events’ in our reconstruction were compared with an independent record of 12 moraine‐building events developed using lichenometry. A minimum of 10 of the moraine‐building events identifiable in our reconstruction were also identifiable in the lichenometric data which affords confidence in the performance of our interrogative model. A critical implication of this successful glacier mass balance reconstruction based on just summer temperature is that for Jotunheimen – in contrast to Norwegian maritime glaciers further to the west – there is no need (as was proposed in some previous studies) to invoke large, prolonged increases in winter snowfall to explain glacier advances, not even for events such as the Little Ice Age.

Experimental determination of the temperature dependence of oxygen-isotope fractionation between water and chitinous head capsules of chironomid larvae

Oxygen-isotope values of invertebrate cuticle preserved in lake sediments have been used in palaeoenvironmental reconstructions, generally with the assumption that fractionation of oxygen isotopes between cuticle and water (upalpha_{text{cuticle}-text{H}_{2}text{O}}) is independent of temperature. We cultured chironomid larvae in the laboratory with labelled oxygen-isotope water and across a range of closely controlled temperatures from 5 to 25 °C in order to test the hypothesis that fractionation of oxygen isotopes between chironomid head capsules and water (upalpha_{text{chironomid}-text{H}_{2}text{O}}) is independent of temperature. Results indicate that the hypothesis can be rejected, and that upalpha_{text{chironomid}-text{H}_{2}text{O}} decreases with increasing temperature. The scatter in the data suggests that further experiments are needed to verify the relationship. However, these results indicate that temperature-dependence of upalpha_{text{chironomid}-text{H}_{2}text{O}} should be considered when chironomid δ18O is used as a paleoenvironmental proxy, especially in cases where data from chironomids are combined with oxygen-isotope values from other materials for which fractionation is temperature dependent, such as calcite, in order to derive reconstructions of past water temperature.

A comprehensive calibrated phytolith based climatic index from the Himalaya and its application in palaeotemperature reconstruction

Past climate reconstructions from palaeoecological records require an understanding of relationships between modern vegetation and climate. Phytoliths are being used widely to reconstruct variations in C3/C4 grasses in the past vegetation and corresponding climate. However, little understanding is available on their relationships with the climate driver(s). Even though, the driver(s) regulating C3/C4 grass distributions vary regionally, while reconstructing the past distributions, a grass phytolith-based climatic index (Ic) has often been found to be used globally without assessing its regional consistency. In the Himalaya, the working potential of Ic has proven to be unsatisfactory when compared to other regions of the globe. To improve the efficacy of Ic, we have identified the redundant grass phytolith morphs and revised it by including four exclusive C3-grass indicator morphotypes (bilobate trapezoidal, bilobate scooped, saddle tall and saddle plateaued) to the existing Ic calculation. Thus, a new climatic index, revised Ic (rIc) is proposed in this article. We have compared the rIc with modern climate variables and a relationship with mean annual temperature (MAT) is established with statistical validation. To assess the working potential of the proposed calibration function in the past temperature reconstructions, we have estimated the late Holocene MAT variations in the Himalaya using rIc. We infer that in the mountainous regions like the Himalaya, even with irregular precipitation distribution, variability in C3/C4 grass distributions and their phytolith spectra seem to be a primary function of temperature. Further, we recommend that rIc can be satisfactorily used to reconstruct past temperature variations in the Himalaya and similar mountainous regions where soil water availability is not a limiting factor.

Soil and Air Temperature Calibrations Using Branched GDGTs for the Tropical Andes of Colombia: Toward a Pan‐Tropical Calibration

AbstractBranched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial cell membrane lipids that, when preserved in sedimentary archives, can be used to infer continental paleotemperatures. Although commonly used global calibrations capture a relationship between the distribution of brGDGTs and temperature, they underestimate temperatures for tropical regions as much as ~16°C. Furthermore, some global calibrations reach saturation at around 24–25°C, and, in general, they have root‐mean‐squared errors (RMSEs ≈ ~4°C) that are too large for them to resolve small variations in paleoclimate variability in tropical regions. We present an in situ regional calibration of soil brGDGTs along altitudinal transects on both flanks of the Eastern Cordillera of Colombia in the northern tropical Andes that spans ~3,200 m in elevation and 17°C and 19°C in mean annual soil and air temperatures, respectively. These new soil and air regional calibrations yield RMSEs of 1.5°C and 1.9°C, respectively. When combined with existing data from elsewhere in the tropics, the integrated data (n = 175) not only fit a linear calibration with a RMSE of 2.7°C but also fit a nonlinear calibration with a RMSE of 2.2°C. These calibrations allow for a more precise and reliable reconstruction of past temperatures in the tropics than global calibrations.

Greenland temperature and precipitation over the last 20 000 years using data assimilation

Abstract. Reconstructions of past temperature and precipitation are fundamental to modeling the Greenland Ice Sheet and assessing its sensitivity to climate. Paleoclimate information is sourced from proxy records and climate-model simulations; however, the former are spatially incomplete while the latter are sensitive to model dynamics and boundary conditions. Efforts to combine these sources of information to reconstruct spatial patterns of Greenland climate over glacial–interglacial cycles have been limited by assumptions of fixed spatial patterns and a restricted use of proxy data. We avoid these limitations by using paleoclimate data assimilation to create independent reconstructions of mean-annual temperature and precipitation for the last 20 000 years. Our method uses oxygen isotope ratios of ice and accumulation rates from long ice-core records and extends this information to all locations across Greenland using spatial relationships derived from a transient climate-model simulation. Standard evaluation metrics for this method show that our results capture climate at locations without ice-core records. Our results differ from previous work in the reconstructed spatial pattern of temperature change during abrupt climate transitions; this indicates a need for additional proxy data and additional transient climate-model simulations. We investigate the relationship between precipitation and temperature, finding that it is frequency dependent and spatially variable, suggesting that thermodynamic scaling methods commonly used in ice-sheet modeling are overly simplistic. Our results demonstrate that paleoclimate data assimilation is a useful tool for reconstructing the spatial and temporal patterns of past climate on timescales relevant to ice sheets.

Delta blue intensity vs. maximum density: A case study using Pinus uncinata in the Pyrenees

We explore the potential of the delta blue intensity (DBI) parameter as a proxy of past summer temperatures using a well replicated (85 trees) chronology of Pinus uncinata from upper treeline in the Spanish Pyrenees. Principal component analysis, correlation response function analysis and Superposed Epoch Analysis show definitively that the DBI data are indistinguishable to other MXD datasets in the region and that DBI expresses a similarly “pure” time-stable climate signal as MXD when compared to their RW counterparts. Calibration r2 values > 0.5 are attainable depending on period used. The signal strength of DBI data is weaker than MXD and behave more like RW data with ca. 19 trees being needed to attain an EPS value > 0.85. However, as the generation of DBI data is cheaper than MXD, this limitation is not deemed to be a serious issue. This pilot study suggests that robust reconstructions of past summer temperatures could be gained using DBI data at a much-reduced cost than relying on MXD. Future dendroclimatic efforts in the region therefore should focus on the measurement of this parameter and the expansion of the pinus ring-density network.

Lipidomics in archaeal membrane adaptation to environmental stresses and growth conditions: A review of culture-based physiological studies

Membrane lipids are thought to be a crucial part of the homeoviscous adaptation of archaea to extreme conditions. This article reviews the recent lipidomic studies of physiological membrane adaptations of archaea, assesses the biomolecular basis of an organic paleothermometer, TEX86, and contemplates the future directions of archaeal lipidomics. The studies of extremophilic archaea have revealed that at least three different molecular mechanisms are involved in membrane adaptation of archaea: (1) regulation of the number of cyclopentane rings of caldarchaeol, (2) alteration of the diether-to-tetraether lipid ratio, and (3) variation of the proportion of saturated and unsaturated lipids. However, most of the studies have focused on a limited number of archaeal ether-linked lipids, such as glycerol dialkyl glycerol tetraethers (GDGTs), which only represent a fraction of the entire lipidome. Environmental factors such as growth temperature and pH have been most frequently reported, but biotic factors, including growth phases, nutrition, and enzymatic activities affecting the membrane lipid composition are often overlooked. Membrane lipids of mesophilic ammonia-oxidizing marine Thaumarchaeota have been applied in the reconstruction of past sea surface temperatures. However, recent culture-based physiological studies have demonstrated that non-thermal biotic factors, including dissolved oxygen, ammonia oxidation rate and the growth rate, are the main drivers of GDGT cyclization in Nitrosopumilus maritimus . Moreover, other related strains or ecotypes exhibit a markedly different set of stress adaptations. A trend is now developing to examine the whole lipid profile (lipidome) for studies of archaeal physiology and biochemistry related to lipid biosynthesis (lipidomics) to gain a better understanding of the biological mechanisms underpinning the applications of membrane lipid-based proxies in biogeochemical or ecological research.

The potential to strengthen temperature reconstructions in ecoregions with limited tree line using a multispecies approach

AbstractTree-ring reconstructions of temperature often target trees at altitudinal or latitudinal tree line where annual growth is broadly expected to be limited by and respond to temperature variability. Based on this principal, regions with sparse tree line would seem to be restricted in their potential to reconstruct past temperatures. In the northeastern United States, there are only two published temperature reconstructions. Previous work in the region reconstructing moisture availability, however, has shown that using a greater diversity of species can improve reconstruction model skill. Here, we use a network of 228 tree-ring records composed of 29 species to test the hypothesis that an increase in species diversity among the pool of predictors improves reconstructions of past temperatures. Chamaecyparis thyoides alone explained 31% of the variability in observed cool-season minimum temperatures, but a multispecies model increased the explained variance to 44%. Liriodendron tulipifera, a species not previously used for temperature reconstructions, explained a similar amount of variance as Chamaecyparis thyoides (12.9% and 20.8%, respectively). Increasing the species diversity of tree proxies has the potential for improving reconstruction of paleotemperatures in regions lacking latitudinal or elevational tree lines provided that long-lived hardwood records can be located.

Is the Noble Gas‐Based Rate of Ocean Warming During the Younger Dryas Overestimated?

AbstractNoble gases in ice cores enable reconstructions of past mean ocean temperature. A recent result from the clathrate‐containing WAIS Divide Ice Core showed tight covariation between ocean and Antarctic temperatures throughout the last deglaciation, except for the Younger Dryas interval. In the beginning of this interval, oceans warmed at 2.5 °C/kyr—three times greater than estimates of modern warming. If valid, this challenges our understanding of the mechanisms controlling ocean heat uptake. Here we reconstruct mean ocean temperature with clathrate‐free ice samples from Taylor Glacier to test these findings. The two records agree in net temperature change over the Younger Dryas, but the Taylor Glacier record suggests sustained warming at the more modest rate of 1.1 ± 0.2°C/kyr. We explore mechanisms to explain differences between records and suggest that the noble gas content for the Younger Dryas interval of WAIS Divide may have been altered by a decimeter‐scale fractionation during bubble‐clathrate transformation.