Palaeotemperature Reconstructions Research Articles

Relation between soil glycerol dialkyl glycerol tetraethers and temperature in different climatic regions in China

AbstractTemperature variation in the past periods can provide valuable insights into the characteristics and driving mechanisms of current global warming and future changes. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are commonly used to as a temperature proxy to quantitatively reconstruct past temperature changes. However, it still remains unclear whether the relation between brGDGTs and temperature is influenced by other environmental factors, thereby affecting the accuracy of quantitative reconstructions. A comprehensive analysis of the relation between brGDGTs and temperature across various climate regions can greatly enhance the accuracy of reconstructing past temperature changes. Here, we investigate the distribution patterns of soil brGDGTs using 28 new topsoils collected from altitudes ranging from 2100 to 4000 m.a.s.l. in the Hengduan Mountains in the southeastern margin of the Tibetan Plateau (TP), compiled previously published soil brGDGT data in China. Our results reveal distinct distribution patterns of soil brGDGTs between humid and arid regions. Furthermore, we further investigated the correlation between brGDGT indices and various environmental factors from both altitudinal and regional scales. At the altitudinal scale, brGDGT indices exhibit a stronger correlation with mean annual air temperature (MAAT) in alkaline soils (pH > 7) than that in acidic soils (pH < 7). At the regional scale, this correlation is higher in alkaline soils than that in acidic soils as well and is stronger in humid soils (mean annual precipitation [MAP] > 800 mm) than that in semi‐humid and semi‐arid soil (400 < MAP < 800 mm) and arid soil (MAP < 400 mm). In addition, brGDGT‐inferred temperature is more affected by MAP in humid alkaline soils but less affected in humid acidic soils. Therefore, caution is needed to use suitable brGDGT index for palaeotemperature reconstructions in various climate regions.

Late Cretaceous (Cenomanian-Turonian) temperature evolution and biotic response in the Adriatic Carbonate Platform region of Friuli, northeast Italy

Climate modelling and palaeotemperature reconstructions derived from several proxies (TEX86, δ18O) suggest that the Late Cretaceous was one of the warmest intervals in Earth history. The greenhouse climate reached its acme near the Cenomanian-Turonian boundary (CTB) interval with sea surface temperatures over 35 °C in low and mid- palaeolatitudes, coinciding with the highest sea level stands of the Phanerozoic. Following this warming phase, a general cooling trend punctuated by short-term hot snaps characterized the rest of the Cretaceous. Palaeotemperature and palaeoenvironmental changes are well documented in deep-water carbonate deposits based on variations in geochemical proxies and distribution patterns of macro- and microfauna. However, such climatic and environmental fluctuations severely impacted carbonate platforms, sedimentary environments that are particularly sensitive to external perturbations. Palaeontological data show major changes within the main carbonate platform producers (rudists and benthic foraminifera) during the early Late Cretaceous. Both groups experienced major taxonomic turnovers at the CTB, followed by a further re-radiation in the middle/late Turonian. The lack of detailed palaeoclimatic reconstructions in shallow water carbonate succession hampers the possibility to investigate a possible causal relationship between temperature changes and faunal evolution. In this paper, we report integrated geochemical, sedimentological, and palaeontological data from the Adriatic Carbonate Platform (Friuli, north-east Italy). Benthic foraminifera biostratigraphy and isotope stratigraphy (δ13C and 87Sr/86Sr) were used to precisely constrain the stratigraphy of the late Cenomanian-middle Turonian interval. δ18O analysis on both well-preserved rudist shells and bulk rock samples enabled the construction of high-resolution palaeotemperature curves. The resulting temperature trends are comparable with those from the deep-water record across the late Cenomanian-Turonian interval. The warmest phase was in the Early Turonian with sea surface palaeotemperatures as high as 41–45 °C recorded in rudist shells. Data show temperature fluctuation from the late Cenomanian to the Early Turonian with a magnitude >7 °C. Benthic foraminifera and rudist evolution responded to these palaeotemperature fluctuations, and their decline in the late Cenomanian and full recovery in the Turonian suggest a strong link with temperature changes.

Seasonal Mg/Ca-inferred temperatures of brackish water ostracods

The common brackish water ostracods Cyprideis torosa and Loxoconcha elliptica frequently occur together in high abundances in marginal marine environments. Seasonality of calcification differs between species and can have important implications for palaeotemperature reconstructions. There are existing palaeotemperature calibrations for both genera. However, the Loxoconcha spp. Mg/Ca temperature calibration has not thus far been applied to L. elliptica. The equation for Loxoconcha spp. does not rely on a known Mg/Cawater value, unlike the calibration for C. torosa, suggesting it may be possible to reconstruct temperatures without an estimation of Mg/Cawater, which is potentially particularly beneficial in environments that have highly dynamic Mg/Cawater. However, the calibration has only been applied in environments with marine-like Mg/Cawater. Demonstrating the applicability of the equation in marginal marine environments (with Mg/Cawater 3–5 mol/mol) and tracking the seasonal calcification of L. elliptica alongside C. torosa, therefore, has the potential to improve uncertainty in seasonal palaeotemperature reconstructions. Here, we compare previous monitoring of C. torosa with L. elliptica from the same collections. We demonstrate that the Mg/Ca temperature calibration for Loxoconcha spp. is appropriate to use with L. elliptica. Mg/Ca-inferred temperatures broadly track spring temperatures and suggest spring calcification. Cyprideis torosa Mg/Ca-inferred temperatures record the range of expected temperatures between spring and autumn. When analysing multiple single valves of L. elliptica and C. torosa simultaneously, the maximum Mg/CaC.torosa can, therefore, be used to reconstruct maximum summer temperatures, the minimum Mg/CaC.torosa to reconstruct autumn temperatures, and the Mg/CaL.elliptica to reconstruct the range in spring temperatures.

The Cretaceous (early Albian to early Campanian) biostratigraphy and palaeotemperature reconstruction of the eastern Tethys: Calcareous nannofossil evidence from southern Tibet, China

Studies of geochemical proxies, such as carbonate δ18O and TEX86, show that the Earth's climate has changed significantly during the Cretaceous Period. However, knowledge about the sea surface temperature (SST) history of the eastern Tethys is limited. Based on the calcareous nannofossil record of the Nirang section, Tethys Himalayas, this paper attempts to establish the biostratigraphic framework and SST evolution of the Cretaceous Period in the southern Tibet. A total of 131 species were identified from the studied succession. Twelve bioevents were recognized, allowing for early Albian to the early Campanian (CC8 to CC18 biozones) in the section. Meanwhile, a disconformity within 81–71 Ma was also identified. The SST evolution was analysed by the calcareous nannofossils modified temperature index (mTI), and the results reveal that the long-term SST changes in the Tethys Himalayas of southern Tibet experienced progressive warming from the early Albian to the early Turonian and the warm interval persisted until the early Campanian. Short-term warming and cooling events, e.g., rapid warming in the early and late Albian, subsequent cooling in the latest Albian and peak warmth at the Cenomanian–Turonian transition, are also recorded in the study area. The SST history of the Albian-Cenomanian in the eastern Tethys can be well correlated to the western Tethys (central Italy). However, the compilation of TEX86 data indicates substantial global cooling during the Coniacian to Campanian. This study suggests that the northward drift of the Indian continent towards the equator is likely to be responsible for the sustained warmth in the Tethys Himalayas from the middle Turonian to the early Campanian.

A terrestrial record of climate variation during MIS 11 through multiproxy palaeotemperature reconstructions from Hoxne, UK

AbstractA terrestrial (lacustrine and fluvial) palaeoclimate record from Hoxne (Suffolk, UK) shows two temperate phases separated by a cold episode, correlated with MIS 11 subdivisions corresponding to isotopic events 11.3 (Hoxnian interglacial period), 11.24 (Stratum C cold interval), and 11.23 (warm interval with evidence of human presence). A robust, reproducible multiproxy consensus approach validates and combines quantitative palaeotemperature reconstructions from three invertebrate groups (beetles, chironomids, and ostracods) and plant indicator taxa with qualitative implications of molluscs and small vertebrates. Compared with the present, interglacial mean monthly air temperatures were similar or up to 4.0°C higher in summer, but similar or as much as 3.0°C lower in winter; the Stratum C cold interval, following prolonged nondeposition or erosion of the lake bed, experienced summers 2.5°C cooler and winters between 5°C and 10°C cooler than at present. Possible reworking of fossils into Stratum C from underlying interglacial assemblages is taken into account. Oxygen and carbon isotopes from ostracod shells indicate evaporatively enriched lake water during Stratum C deposition. Comparative evaluation shows that proxy-based palaeoclimate reconstruction methods are best tested against each other and, if validated, can be used to generate more refined and robust results through multiproxy consensus.

Development of global temperature and pH calibrations based on bacterial 3-hydroxy fatty acids in soils

Abstract. Gram-negative bacteria produce specific membrane lipids, i.e. 3-hydroxy fatty acids with 10 to 18 C atoms. They have been recently proposed as temperature and pH proxies in terrestrial settings. Nevertheless, the existing correlations between pH or temperature and indices derived from 3-OH FA distribution are based on a small soil dataset (ca. 70 samples) and only applicable regionally. The aim of this study was to investigate the applicability of 3-OH FAs as mean annual air temperature (MAAT) and pH proxies at the global level. This was achieved using an extended soil dataset of 168 topsoils distributed worldwide, covering a wide range of temperatures (5 to 30 ∘C) and pH (3 to 8). The response of 3-OH FAs to temperature and pH was compared to that of established branched glycerol dialkyl glycerol tetraether (GDGT)-based proxies (MBT'5Me/CBT). Strong linear relationships between 3-OH-FA-derived indices (RAN15, RAN17 and RIAN) and MAAT or pH could only be obtained locally for some of the individual transects. This suggests that these indices cannot be used as palaeoproxies at the global scale using simple linear regression models, in contrast with the MBT'5Me and CBT. However, strong global correlations between 3-OH FA relative abundances and MAAT or pH were shown by using other algorithms (multiple linear regression, k-NN and random forest models). The applicability of the three aforementioned models for palaeotemperature reconstruction was tested and compared with the MAAT record from a Chinese speleothem. The calibration based on the random forest model appeared to be the most robust. It generally showed similar trends with previously available records and highlighted known climatic events poorly visible when using local 3-OH FA calibrations. Altogether, these results demonstrate the potential of 3-OH FAs as palaeoproxies in terrestrial settings.

Reef-building Pacific oysters record seasonal variations in water mass-properties of tidal basins from the Central Wadden Sea (North Sea)

High-resolution proxy records are needed to evaluate models simulating past, present and future climatic conditions. Before such records can be reliably employed, calibration studies have to be conducted to assess the confidence intervals of the proxies. Here, we use shells of the fast growing Pacific oyster Magallana gigas from the Central Wadden Sea, North Sea, a temperate barrier island-backbarrier tidal flat-salt marsh system with large seasonal changes of water mass-properties, for the calibration of geochemical proxies. M. gigas represents a non-native invasive species that rapidly develops oyster reefs. Calcite shells of two specimens from the intertidal and subtidal zones were sampled in high resolution yielding sub-monthly data sets. The time period represented in the shell, based on δ18O age modeling, was estimated at 8–10 years and the growth of the shells was restricted from (late) spring to (early) autumn of each year. Mg/Ca, Mn/Ca and Sr/Ca ratios of the intertidal and subtidal specimens show similar seasonal patterns. Mg/Ca and Sr/Ca ratios are investigated as high-resolution sea surface temperature (SST) proxies. Important ontogenetic effects (i.e., increasing time-averaging with increasing age) as well as inter-specimen variability are discussed as limiting factors for the proxy development. Intertidal Mg/Ca ratios show only a significant correlation to the high-resolution SST record of the Central Wadden Sea when the early ontogenetic stage is considered. Sr/Ca ratios were comparable in terms of absolute values and amplitudes to those of M. gigas in the Northern Wadden Sea, but amplitudes were decreasing with increasing age. These findings seriously hamper the application of Mg/Ca and Sr/Ca for reliable palaeotemperature reconstructions regardless of ontogenetic stage. The Mn/Ca ratios were investigated as proxy for Mn cycling in tidal basins, where it is interrelated with seasonal changes in primary production. In addition to the generally observed seasonal variability of the Mn/Ca oyster records, the subtidal Mn/Ca is significantly elevated compared to intertidal Mn/Ca. The subtidal Mn/Ca offset likely reflects differences in Mn cycling in tidal settings and could, therefore, serve in the palaeorecord as indicator to differentiate inter- and subtidal habitats in the same embedding sedimentary facies. This habitat effect has to be considered as an important factor besides environmental change when interpreting the high-resolution proxy record of fossil oysters.

Age-estimate evidence for a complex Middle to Late Pleistocene fluvial terrace aggradation spanning more than a 100-kyr interglacial–glacial cycle at Sutton Cross, eastern England

At Sutton Cross, eastern England, an undated River Nene 2nd Terrace was known to record a complex sequence of late Middle to Upper Pleistocene deposits. Amino acid geochronology was carried out for samples from four separate facies associations in order to provide a chronostratigraphic framework for this complex deposit. Each sample comprised three specimens of Bithynia tentaculata opercula. The intracrystalline protein decomposition results confirmed the sedimentary stratigraphic interpretation previously proposed for the site, i.e., that fossiliferous sediments yielding fully temperate Mollusca and Ostracoda faunas are equivalent in age to marine isotope stage (MIS) 7. Palaeotemperature reconstructions based on the Ostracoda fauna (July: +14 to +26 °C; January: −4 to +5/+3 °C) embrace those of today at this location (July: +17 °C; January +3 °C). The sedimentary succession at Sutton Cross comprises vertically aggraded packages with a probable maximum age of MIS 8 and minimum age of MIS 2. This aggradation pattern (and age range) is also common to downstream sedimentary successions at Whittlesey, and is counterintuitive to the traditional terrace stratigraphy approach that expects laterally aggrading downstepping packages. The preservation of sediments of MIS 7 age and older at Sutton Cross with no evidence of glacial overriding places constraints on recently proposed late Middle Pleistocene regional ice limits.

Timing and characterization of multiple fluid flow events in the Beibuwan Basin, northern South China Sea: Implications for hydrocarbon maturation

The history of thermal and fluid flow events is of crucial importance to understand the mechanism of hydrocarbon maturation. The northern South China Sea (SCS) contains abundant hydrocarbon resources with high heat flow anomalies, but the mechanisms responsible for modification of thermal regimes are poorly understood. Here we conducted the first high-precision Rb-Sr isochron dating and geochemical analyses of authigenic illites separated from the Palaeogene sandstones in the Beibuwan Basin, northern SCS. Eight high-precision Rb-Sr isochron ages reveal three major thermal and fluid flow events in the Beibuwan Basin, northern SCS. The 34.5 ± 0.9 Ma and 23.6 ± 0.8 Ma events occurred in the western area are probably related to the Eocene-Oligocene transition characterised by a significant regional unconformity resulted from intensive uplift of Hainan Uplift and Oligocene-Miocene transition marked by a post-rift unconformity, respectively. By contrast, the 31.2 ± 0.6 Ma event affecting the eastern area appears to be closely associated with the igneous intrusion dated by zircon U/Pb to 32.3 ± 0.7 Ma. REE and stable isotope characteristics indicate that the illites from the western area formed within a meteoric-hydrothermal system, whilst those from the eastern area were likely precipitated from fluids heated by the large-scale igneous intrusion at higher water/rock ratios. Moreover, our data in conjunction with palaeo-temperature reconstructions show that the fluid flow events had profound effects on the rapid organic maturation in the northern SCS.

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.

Centennial-Scale Temperature Change During the Common Era Revealed by Quantitative Temperature Reconstructions on the Tibetan Plateau

Quantitative palaeotemperature reconstruction is crucial for understanding the evolution of Earth’s climate and reducing uncertainty in future climate predictions. Clarifying the temperature change over the Tibetan Plateau (TP) during the Common Era is critical because it plays a vital role in the prediction of cryosphere changes in such regions under a future warming climate. In this paper, we report a comprehensive synthesis of currently available quantitative temperature records to refine the temperature history of the TP during the Common Era. To date, Common Era quantitative temperature reconstructions are sparse and mainly concentrated in the northeastern TP. Considering seasonal bias of the available quantitative temperature reconstructions, three different composite temperature records for TP were derived, namely the “Standardization” composite, the “Mean annual air temperature anomaly” composite, and the the “Mean summer temperature anomaly” composite individually. All the integrated temperature series reveal the Medieval Climate Anomaly and the Little Ice Age, but the start and end timings of these multi-centennial-scale periods and their temperature amplitudes differ. There is strong seasonality in temperature variations on this high plateau, and the 20th century warming was characterized by rapid winter temperature increases, while summer temperatures displayed weak variations. Spatial analysis suggests a relatively consistent signal marking a warm Tibetan Plateau during 600-1400 CE and a cold plateau during 1400-1900 CE. Large-scale trends in temperature history for the TP resemble those for China and the Northern Hemisphere. Many factors, such as seasonality of temperature proxies, might lead to uncertainty in the reconstructed series. The results highlight that it is of crucial importance to develop more seasonal temperature reconstructions to improve the reliability of quantitative palaeoclimatic reconstructions based on geological records across the TP.

Dual clumped isotope thermometry resolves kinetic biases in carbonate formation temperatures

Surface temperature is a fundamental parameter of Earth’s climate. Its evolution through time is commonly reconstructed using the oxygen isotope and the clumped isotope compositions of carbonate archives. However, reaction kinetics involved in the precipitation of carbonates can introduce inaccuracies in the derived temperatures. Here, we show that dual clumped isotope analyses, i.e., simultaneous ∆47 and ∆48 measurements on the single carbonate phase, can identify the origin and quantify the extent of these kinetic biases. Our results verify theoretical predictions and evidence that the isotopic disequilibrium commonly observed in speleothems and scleractinian coral skeletons is inherited from the dissolved inorganic carbon pool of their parent solutions. Further, we show that dual clumped isotope thermometry can achieve reliable palaeotemperature reconstructions, devoid of kinetic bias. Analysis of a belemnite rostrum implies that it precipitated near isotopic equilibrium and confirms the warmer-than-present temperatures during the Early Cretaceous at southern high latitudes.

Summer temperature development 18,000–14,000 cal. BP recorded by a new chironomid record from Burgäschisee, Swiss Plateau

The termination of the Last Ice Age after the Last Glacial maximum (LGM) represents a dynamic period in the history of the circum-north Atlantic region. So far, there are few reliably dated climatic reconstructions covering the Lateglacial period prior to 14,700 cal. BP in Central and Northern Europe. We present a new chironomid record for the period 18,000–14,000 cal. BP, from Burgäschisee, Switzerland. Chironomid assemblages immediately following glacier retreat were dominated by taxa indicative of cold, oligotrophic conditions such as Sergentia coracina -type and Micropsectra radialis -type. A gradual transition to assemblages with moderate abundances of taxa indicative of warmer climatic conditions such as Dicrotendipes nevosus -type and Tanytarsus glabrescens -type started after ca. 16,100 cal. BP. This initial and gradual chironomid assemblage shift culminated in a more pronounced and rapid inferred temperature change at the Oldest Dryas/Bølling transition at ca. 14,700 cal. BP, where further types indicative of warm conditions such as Tanytarsus lactescens -type first occurred and replaced chironomids indicative of colder conditions such as Paracladius and Protanypus. We estimated past July air temperature changes from the chironomid assemblages by applying to the record a chironomid-temperature transfer function that is based on chironomid distribution data from 274 lakes in Switzerland and Norway. The resulting reconstruction, which features a sample-specific root mean square error of prediction of 1.36–1.46 °C, indicates temperatures around 9 °C at the beginning of the record. An initial gradual warming phase initiating at ca. 16,100 cal. BP is recorded reaching values around 10 °C for the period 16,100–15,500 cal. BP. Temperatures continue to increase reaching values around 12 °C for the period preceding the Bølling warming, when temperatures rose rapidly to values around 15 °C. The early temperature rise to values of 10–12 °C prior to the Bølling warming agrees with widespread vegetation changes recently reported for this region based on palaeobotanical analyses, which indicate a shift from herbaceous tundra to shrub tundra with low density tree birch stands with open canopies. Together, these results suggest an earlier Lateglacial temperature increase in southwest Central Europe than expected based on earlier palaeobotanical reconstructions, although with a less pronounced warming than has been reported for ca. 16,000 cal. BP from south of the Alps. This early Lateglacial warming agrees with glacier reconstructions which suggest several step-wise reductions of glacier extent in this period as well as with evidence from other palaeotemperature reconstructions and suggests that not only Southern Europe but significant parts of Europe north of the Alps may have been characterized by early Lateglacial warming well before the rapid warming at ca. 14,700 cal. BP.

Chitinous aquatic invertebrate assemblages in Quaternary lake sediments as indicators of past deepwater oxygen concentration

Chitinous aquatic invertebrate remains are abundant in late Quaternary lake sediment sequences. However, for standard analysis of invertebrate indicator groups usually only the remains of one particular invertebrate group (e.g. water fleas, midges) are systematically isolated from the sediments and analysed numerically. In a calibration study focusing on surface sediment samples from 36 small Swiss lakes, we show that the abundances of other chitinous aquatic invertebrate remains in samples prepared for chironomid analysis provide important information on in-lake oxygen conditions when assessed relative to and together with the overall number of chironomid remains. Our results show that particularly high abundances of chironomid and chaoborid remains are indicative for lakes with high and low deepwater oxygen concentrations, respectively, but that also the remains of other invertebrate groups such as Ceriodaphnia ephippia and Plumatella statoblasts tend to be more abundant in warm, stably stratified and anoxic lakes. Direct gradient analyses of the isolated invertebrate assemblages indicate that deepwater oxygen concentration during late summer is the strongest predictor for assemblage changes in our dataset, followed by variables representing temperature conditions (summer surface water temperature, altitude), summer stratification stability (temperature difference between surface and deepwater layers), water depth and the relative volume of the late summer hypoxic water layer. We show, based on invertebrate assemblages in a Lateglacial sediment sequence from Rotsee, Switzerland, that our new calibration data can provide information on past changes in deepwater oxygen availability from late Quaternary lake sediment sequences, when fossil invertebrate assemblages are added as passive samples to ordinations of the surface sediment calibration data. This provides a new approach for developing assessments of past changes in deepwater oxygen concentrations from late Quaternary sediment samples prepared for chironomid analysis. Such assessments can provide important supplementary information for e.g. interpreting palaeotemperature reconstructions based on chironomid remains.

Tracking the seasonal calcification of Cyprideis torosa (Crustacea, Ostracoda) using Mg/Ca-inferred temperatures, and its implications for palaeotemperature reconstruction

Abstract Ostracod shell chemistry data are widely used for palaeoenvironmental reconstruction. Despite this, there has been little systematic research into the implications of the timing of calcification or the duration of each moult stage. Consequently, it is unclear whether palaeoenvironmental reconstructions are recording restricted (inter-seasonal) time periods or reflect the mean annual conditions. The seasonality of shell formation can therefore have implications for palaeoenvironmental reconstructions based on geochemical signatures, especially palaeotemperature, particularly in environments that show large inter-annual variations in water conditions. Cyprideis torosa is a geographically widespread and eurytopic species that has great potential for a range of palaeoenvironmental reconstructions, but inhabits environments with large seasonal and inter-annual variation. Using hourly water and air temperature data, ostracod shell and surface water chemistry from a shallow coastal pond in SE UK, we improve knowledge of the timing of Cyprideis torosa calcification, and thus our understanding of the potential seasonality of signals in palaeotemperature datasets. We suggest seasonal calcification in spring and autumn, with persistence at the adult life stage for up to 12–18 months. Sr/Ca values of C. torosa appear to reflect a Sr/Cawater control on calcification timing and have no temperature dependence. For Mg/Ca, we show a minimum temperature control on calcification of 7 °C, with C. torosa Mg/Ca-inferred temperatures broadly tracking spring and autumn temperatures.

Constraining multiple controls on planktic foraminifera Mg/Ca

The Mg/Ca of planktic foraminifera is widely used to determine past surface ocean temperatures but temperature is not the only factor that controls test Mg/Ca. Here we quantify the combined influence of seawater temperature, carbon chemistry, and cation chemistry on Orbulina universa Mg/Ca, based on experimental cultures where these factors were varied independently and simultaneously. We fit a new empirical multi-parameter model that quantifies the effects of each of these variables on the Mg/Ca of O. universa:Mg/CaO.universa=Mg/CaswA·DICswB·expC·Casw+D·T+EWe extend our approach to published Mg/Ca from cultured and sediment trap collected Globigerinoides ruber, and show that a similar equation can be used to describe test Mg/Ca, using [CO32−]sw or pH in place of DIC. The cause of this difference is unknown, but the sensitivity of these two ecophysiologically similar species to different carbon chemistry parameters highlights the uncertainty inherent in applying modern Mg/Ca calibrations to uncalibrated and/or extinct species.In both O. universa and G. ruber, Mg/Casw, [Ca]sw and carbon chemistry (DIC, [CO32−]sw or pH) each modulate the sensitivity of Mg/Ca to temperature. These variables are not constant in the ocean over time-scales relevant to palaeotemperature reconstructions, and must be accounted for when deriving temperature from foraminiferal Mg/Ca. Over time-scales longer than the residence times of Mg and Ca, secular changes in Mg/Casw and [Ca]sw will exert a primary influence on foraminiferal Mg/Ca and its sensitivity to temperature. On shorter timescales, co-variance between temperature and seawater carbon chemistry will influence Mg/Ca-derived temperature estimates. This is particularly relevant to transient hyperthermal events, where the Mg/Ca palaeothermometer is used to evaluate the relationship between atmospheric CO2 and ocean temperature. We explore the potential impact of these effects in an illustrative re-interpretation of Mg/Ca records across the Paleocene-Eocene Thermal Maximum.

High-resolution record of Holocene climate change dynamics from southern Africa's temperate-tropical boundary, Baviaanskloof, South Africa

South Africa's southern Cape is a highly dynamic climatic region that is influenced by changes in both temperate and tropical atmospheric and oceanic circulation dynamics. Recent research initiatives suggest that the major elements of the regional climate system have acted both independently and in combination to establish a mosaic of distinct climate regions and potentially steep climate response gradients across the southern Cape. To consider this further, we present new high resolution δ13C and δ15N data from a rock hyrax (Procavia capensis) midden from Baviaanskloof, in the Cape Fold Mountains of South Africa's Eastern Cape Province. Spanning the last 7200 years, these data provide detailed information regarding environmental changes in the mid- and late Holocene, allowing us to assess the spatio-temporal nature of climate change anomalies across the wider region. Throughout the full duration of the record, a negative correlation between humidity and palaeotemperature reconstructions from nearby Cango Cave is observed. In conjunction with correlations with Southern Ocean sea-surface temperatures and sea-ice presence, we infer a dominant influence of the southern westerlies in determining multi-millennial scale hydroclimate variability. At shorter, multi-centennial to millennial timescales, the Baviaanskloof data indicate a clearer expression of tropical influences, highlighting a delicate balance between the mechanisms driving regional climate dynamics across timescales, and the sensitivity of the region to changes in global boundary conditions.

Evaluating the efficacy of the mollusc Tridacna crocea for reconstructing ancient sea‐surface temperatures in the Rock Islands of Palau, Micronesia

ABSTRACTPrevious stable isotope studies for shellfish collected in waters surrounding the Palauan archipelago (north‐western tropical Pacific) have demonstrated that various local shellfish taxa may be reliable recorders of their ambient water temperatures from modern and archaeological contexts. In this study, we analysed stable oxygen isotope (δ18O) signatures of Tridacna crocea from the archaeological site of Metuker ra Bisech (B:IR‐2:24) to reconstruct average nearshore sea‐surface temperatures (SST) from two components dated to c.560 and 120 calBP. Modern shellfish samples and environmental data were collected from intertidal zones on nearby Orrak Island to provide a modern analogue. These shells were tested using X‐ray diffraction to ensure no secondary recrystallisation of the aragonitic structure. Uncontaminated samples were measured for δ18O to pair with recorded SST. The most appropriate oxygen isotope‐to‐SST conversion formula was then applied to δ18O signatures from archaeological T. crocea to estimate SST averages. The results of this study indicate that T. crocea accurately records ambient SST and is a good proxy for palaeotemperature reconstruction. Our results were compared to previous isotopic studies on Palauan taxa collected to identify regional changes to SST through time, thus allowing for comparisons between human subsistence practices, resource availability and past palaeoenvironments.

The first data on the n-isotopic composition of the Permian and Triassic of North-Eastern Russia and their significance for palaeotemperature reconstructions

It is proposed that oscillating temperature conditions in the late Wuchiapingian and early Changhsingian (Late Permian) followed in the Boreal Superrealm to less variable climatic conditions in the late Changhsingian and early Induan (the time of trap formation of the Siberian Platform), with stable trend of increasing temperature in the Early Triassic. The Problem of the absence of signs of mass extinction of marine organisms at the Permian-Triassic boundary in the Boreal Superrealm is discussed.

The last forests on Antarctica: Reconstructing flora and temperature from the Neogene Sirius Group, Transantarctic Mountains

Fossil-bearing deposits in the Transantarctic Mountains, Antarctica indicate that, despite the cold nature of the continent’s climate, a tundra ecosystem grew during periods of ice sheet retreat in the mid to late Neogene (17–2.5 Ma), 480 km from the South Pole. To date, palaeotemperature reconstruction has been based only on biological ranges, thereby calling for a geochemical approach to understanding continental climate and environment. There is contradictory evidence in the fossil record as to whether this flora was mixed angiosperm-conifer vegetation, or whether by this point conifers had disappeared from the continent. In order to address these questions, we have analysed, for the first time in sediments of this age, plant and bacterial biomarkers in terrestrial sediments from the Transantarctic Mountains to reconstruct past temperature and vegetation during a period of East Antarctic Ice Sheet retreat. From tetraether lipids (MBT′/CBT palaeothermometer), we conclude that the mean continental summer temperature was ca. 5 °C, in agreement with previous reconstructions. This was warm enough to have allowed woody vegetation to survive and reproduce even during the austral winter. Biomarkers from vascular plants indicate a low diversity and spatially variable flora consisting of higher plants, moss and algal mats growing in microenvironments in a glacial outwash system. Abietane-type compounds were abundant in some samples, indicating that conifers, most likely Podocarpaceae, grew on the Antarctic continent well into the Neogene. This is supported by the palynological record, but not the macrofossil record for the continent, and has implications for the evolution of vegetation on Antarctica.