Penultimate Deglaciation Research Articles

Sedimentary ancient DNA sequences reveal marine ecosystem shifts and indicator taxa for glacial-interglacial sea ice conditions

Sedimentary ancient DNA (sedaDNA) analysis is a promising new approach for reconstructing the impact of past climate and environmental changes on marine paleobiodiversity. By recovering, amplifying, and sequencing taxonomically informative sedaDNA fragments preserved in sediments, it is possible to assess the response of a broad range of eukaryote taxa, including non-fossilizing lineages, to past climate change. Here we present a unique marine derived sedaDNA metabarcoding record, spanning the penultimate glacial-interglacial transition across Marine Isotope Stages 6 to 5d (>135–107 ka) from an Eirik Drift core-site in the Labrador Sea. We identified a range of marine groups including dinoflagellates, diatoms, coccolithophores, chlorophytes, and copepods. There were representatives of primary/secondary producers, grazers, and parasites, which may represent remnants of complex ecosystems and ancient food webs. There were significant biodiversity shifts following the penultimate deglaciation and changing sea ice conditions throughout the Last Interglacial. These shifts reflected the striking increase in community richness during periods of seasonal sea ice and reduction under extensive perennial sea ice cover and open ocean. We identify two potential sedaDNA indicator taxa sequences associated with past seasonal sea ice which are most likely pico-eukaryote representatives of Micromonas and Pyramimonas, both green algae with known sea ice associations in modern ecosystems. Our work demonstrates the importance of high resolution marine sedaDNA metabarcoding for unravelling climate-ecosystem linkages and strengthens the potential of sedaDNA signals for past sea ice reconstructions through indicator sequences.

Asian monsoonal temperature and environmental changes since the penultimate deglaciation as revealed by alkenones and alkanes in sediments from the northern South China Sea

A new alkenone sea surface temperature (SST) record from the South China Sea (SCS) over the last 170 ka has been reported here. Our analysis of magnetic susceptibility and AMS 14C dating on the top portion of the core suggests that this record dates to the Marine Isotope Stage (MIS) 6. As indicated in Termination I and II, SST changes are correlated with clear glacial/interglacial cycles and likely dominated by atmospheric CO2 and Western Pacific Warm Pool temperature changes. But the two Terminations were different in magnitude. In Termination I, the minimum and maximum SSTs are 24.5 °C and 27.5 °C, respectively, showing a smaller rise (3 °C) than in Termination II (24–28.3 °C). The n-alkane indices, such as average chain length (ACL) and carbon preference index (CPI), exhibited high/low values during glacial/interglacial periods, and possible more arid climate during MIS 6 than MIS 2. During Termination II, the maximum surface warming in the SCS is accompanied by a progressive lag of approximately 7000 years in relation to maximum summer insolation until Termination I, when they are nearly synchronous. According to the study, SSTs are continuously changing and are influenced by solar insolation, global ice volume changes, and ocean–atmosphere interactions across hemispheres.

Towards spatio-temporal comparison of simulated and reconstructed sea surface temperatures for the last deglaciation

Abstract. An increasing number of climate model simulations is becoming available for the transition from the Last Glacial Maximum to the Holocene. Assessing the simulations' reliability requires benchmarking against environmental proxy records. To date, no established method exists to compare these two data sources in space and time over a period with changing background conditions. Here, we develop a new algorithm to rank simulations according to their deviation from reconstructed magnitudes and temporal patterns of orbital and millennial-scale temperature variations. The use of proxy forward modeling allows for accounting for non-climatic processes that affect the temperature reconstructions. It further avoids the need to reconstruct gridded fields or regional mean temperature time series from sparse and uncertain proxy data. First, we test the reliability and robustness of our algorithm in idealized experiments with prescribed deglacial temperature histories. We quantify the influence of limited temporal resolution, chronological uncertainties, and non-climatic processes by constructing noisy pseudo-proxies. While model–data comparison results become less reliable with increasing uncertainties, we find that the algorithm discriminates well between simulations under realistic non-climatic noise levels. To obtain reliable and robust rankings, we advise spatial averaging of the results for individual proxy records. Second, we demonstrate our method by quantifying the deviations between an ensemble of transient deglacial simulations and a global compilation of sea surface temperature reconstructions. The ranking of the simulations differs substantially between the considered regions and timescales, which suggests that optimizing for agreement with the temporal patterns of a small set of proxies might be insufficient for capturing the spatial structure of the deglacial temperature variability. We attribute the diversity in the rankings to more regionally confined temperature variations in reconstructions than in simulations, which could be the result of uncertainties in boundary conditions, shortcomings in models, or regionally varying characteristics of reconstructions such as recording seasons and depths. Future work towards disentangling these potential reasons can leverage the flexible design of our algorithm and its demonstrated ability to identify varying levels of model–data agreement. Additionally, the algorithm can be applied to variables like oxygen isotopes and climate transitions such as the penultimate deglaciation and the last glacial inception.

Loess chronology and composition as interpreted through the lens of fossiliferous lake sediments in central Illinois, USA

Silty lacustrine and paludal records spanning from the penultimate deglaciation (late Illinois Episode) to the present are preserved in kettles formed during marine isotope stage (MIS) 6 that lie adjacent to the MIS 2 Manito terrace along the valleys of the Sangamon and Illinois Rivers. Geochemical, mineralogical, particle-size, and chronological records from two basins, Smith Lake and Lake Ben, provide a unique opportunity to assess loess age and provenance of the south-central Laurentide ice sheet. Here, I focused on deposits of redeposited loess of the last glaciation (Wisconsin Episode). Other than subtle laminations and fossils of emergent and aquatic plants and animals, the silty texture, chemical composition, and mineralogy of the lake sediment closely resemble Peoria Silt (loess). Sediment-core records sampled from Smith Lake and Lake Ben in Mason County, Illinois, confirm rapid deposition of proximal loess derived from the nearby outwash plain of the Illinois River from ca. 30.9−18.0 calibrated (cal) k.y. B.P. From 18.0−14.7 cal k.y. B.P., deposition of proximal loess was gradually replaced by far-traveled (distal) loess chiefly derived from Superior lobe provenance and global dust. The succession is capped by a cumulic gleysol that was truncated by recent plowing. Proximal loess deposits in lakes Smith and Ben are composed primarily of very fine sand, coarse silt, and medium silt (125−16 µm) compared to the distal facies (fine silt and finer sediment: <16 µm). Proximal loess that originated from outwash of the Lake Michigan lobe contains abundant Ca, Zr, Sr, illite, and magnetic minerals. The distal facies contains relatively abundant Rb, Al, K, Si, Cu, and Pb, and it is enriched in smectite (relative to illite) and depleted in magnetic minerals. Radiocarbon ages on terrestrial gastropod aragonite in loess from the nearby New Cottonwood School and Thomas Quarry sites indicate that Peoria Silt deposition ended between 18 cal k.y. B.P. and 16 cal k.y. B.P. At these localities, the upper part of the record is complicated by the modern soil, which has altered soil organic matter and leached carbonate. At Lake Ben, the radiocarbon record of emergent aquatic vegetation indicates that sediment accumulation rates and the ratio of proximal to distal loess decreased at the correlative interval that yielded the youngest snail shell ages at Cottonwood School. The upward increase in element concentrations such as Al and decrease in coarse/fine silt ratios in the modern soil are prominent features in the Lake Ben record dating from 18−14 cal k.y. B.P. This correlation strongly suggests that many of the features of the modern soil were not developed into uniform parent material but, rather, were inherited from incremental addition of distal loess to proximal loess, collectively identified as Peoria Silt. The paucity of key elements (Cd, Hg, Se, Ag, Au) in this transition zone suggests limited contributions from western lobes (i.e., Des Moines and James lobes), but abundant Ti and Cu suggest a contribution from the Superior lobe. A new wrinkle in this interpretation is that the rise in elements such as Al, Rb, and Si suggests significant additions from generic dust from unknown sources.

Precisely constrained 134-ka strong monsoon event in the penultimate deglaciation by an annually laminated speleothem from the Asian monsoon domain

AbstractThe penultimate deglaciation was characterized by a sub-millennial-scale warm event in the Heinrich Stadial 11(HS11), termed the 134-ka event. However, its precise timing and structure remain poorly constrained due to the lack of high-resolution and precisely dated records. We present an oxygen isotope record of a speleothem with well-developed annual lamina from Zhangjia Cave, located on the north margin of the Sichuan Basin, characterizing Asian summer monsoon (ASM) changes in the 134-ka event, which included an increase excursion of ca. 149 years and decrease excursion of ca. 200 years, inferred from 3.3‰ δ18O variations. This event also divided the weak ASM interval-II (WMI-II), corresponding to HS11, into two stages, the WMI-IIa 132.8–134.1 ka and WMI-IIb 134.4–136.4 ka. With a comparable climatic pattern globally, the 134-ka event is essentially similar to the millennial-scale events in last glacial–deglacial period. Particularly, the observed weak-strong-weak ASM sequence (138.8–132.8 ka) is largely controlled by changes in the Atlantic Meridional Overturning Circulation (AMOC) forced by the meltwater of northern high-latitude ice sheets. Moreover, our results underpin that AMOC, rather than the global ice volume, is more critical to ASM variations during the last two deglaciations.

Was there a nonglacial episode in the western Hudson Bay Lowland during Marine Isotope Stage 3?

AbstractEstablishing the timing of glacial and nonglacial intervals in the core regions of the Laurentide Ice Sheet (LIS) is essential to constrain ice-sheet configuration at times of globally reduced ice volume, such as during Marine Isotope Stage (MIS) 3 (~57–29 ka). Hudson Bay Lowland (HBL) deglaciation, at the centre of the LIS, has been inferred at MIS 3 based on near-infinite wood radiocarbon ages and limited luminescence ages. To better constrain the age of the penultimate deglaciation of the western HBL, this study initially identified the youngest intertill nonglacial sediments, based on extensive fieldwork and till characterization. Next, vetted radiocarbon ages were combined with revised stratigraphy to show that five previously identified “MIS 3” sites were likely deposited during an earlier ice-free period. Finally, new optical ages targeting the youngest intertill nonglacial bed at three localities yielded ages ranging from 166 to 146 ka; all older than MIS 3. These ages indicate that the penultimate deglaciation of the western HBL likely occurred during MIS 5e. This interpretation better explains accompanying paleobotanic data sets that indicate vegetation similar to vegetation existing under present interglacial conditions. Currently no firm evidence exists in the terrestrial stratigraphic record for the deglaciation of the western HBL during MIS 3.

Sedimentary deformation relating to episodic seepage in the last 1.2 million years: a multi-scale seismic study from the Vestnesa Ridge, eastern Fram Strait

Seafloor hydrocarbon seepage is a natural fluid release process that occurs worldwide on continental shelves, slopes, and in deep oceanic basins. The Vestnesa sedimentary ridge in the eastern Fram Strait hosts a deep-water gas hydrate system that became charged with hydrocarbons ∼2.7 Ma and has experienced episodic seepage along the entire ridge until a few thousand years ago, when seepage activity apparently ceased in the west but persisted in the east. Although it has been documented that faults and fractures play a key role in feeding the seeps with thermogenic gases, the mechanisms controlling seepage periodicity remain poorly understood. Here we integrate high-resolution P-cable 3D seismic and Chirp data to investigate the spatial and temporal evolution of high-resolution fractures and fluid flow features in the west of the Vestnesa Ridge. We characterize sediment deformation using a fracture density seismic attribute workflow revealing two highly deformed stratigraphic intervals and associated small-scale pockmarks (<20 m diameter). Chronostratigraphic constraints from the region show that these two highly deformed intervals are influenced by at least three major climatic and oceanic events during the last 1.2 million years: the Mid-Pleistocene Transition (∼1.25–0.7 Ma), the penultimate deglaciation (∼130 ka) and the last deglaciation (Heinrich Stadial 1: ∼16 ka). These periods of deformation appear associated with seismic anomalies potentially correlated with buried methane-derived authigenic carbonate and have been sensitive to shifts in the boundary of the free gas-gas hydrate interface. Our results show shifts (up to ∼30 m) in the depth of the base of the gas hydrate stability zone (GHSZ) associated with major changes in ocean bottom water temperatures. This ocean-driven effect on the base of the GHSZ since the Last Glacial Maximum coincides with the already highly deformed Mid-Pleistocene Transition sedimentary interval and likely enhanced deformation and gas leakage along the ridge. Our results have implications for understanding how glacial cycles impact fracture formation and associated seepage activity.

Micropaleontological and Isotopic Perspective of Surface Water Mass Variability in the NE Atlantic from MIS 6 to 5e (188 to 115 ka)

The penultimate glaciation (marine isotope stage (MIS) 6) is considered regionally extreme compared to the last glacial maximum, in which the European ice sheets had a vast areal extent. In contrast to the last deglaciation (19–7 ka), the penultimate deglaciation (140–130 ka) hosts one of the most rapid oceanographic changes of the late Pleistocene. In this study, we reconstructed changes in the near-surface and thermocline in the central to northeast Atlantic by analyzing sediments from two Integrated Ocean Drilling Program Expedition 306 sites. Sites U1313 (41°00.6′ N, 32°57.4′ W) and U1314 (56°21.9′ N, 27°53.3′ W) were drilled on the eastern flank of the mid-Atlantic ridge and Gardar Drift of the eastern subpolar North Atlantic, respectively. We analyzed planktonic foraminiferal assemblages, ice-rafted debris (IRD), and oxygen isotopes in two planktonic foraminifers, Globigerina bulloides, and Globorotalia inflata, from MIS 6 to 5e (185–115 ka). Warmer and colder sea-surface conditions were marked by a change in the relative abundance of polar, subpolar, and transitional planktonic foraminifers. Oxygen isotopes in G. bulloides and G. inflata suggest that the thermocline deepened at the subtropical Site U1313 during MIS 6. The lack of Globorotalia inflata prevented us from profiling the mixed layer and thermocline at the subpolar Site U1314. In contrast to MIS 6, the mixed layer and thermocline were re-stratified during the last interglacial. The lack of major IRD events at both sites suggests the stability of the Laurentide ice sheet during MIS 6 compared to the subsequent glaciation. The presence of Heinrich event 11 indicates the discharge of freshwater that freshened the sea surface, resulting in mixing between the mixed layer and thermocline. Our results were placed into a broader context using published data that shed light on the sensitivity of freshwater discharge to the North Atlantic and the following changes with a transition from a penultimate glacial to an interglacial period in surface circulation.

Rapid northern hemisphere ice sheet melting during the penultimate deglaciation

The rate and consequences of future high latitude ice sheet retreat remain a major concern given ongoing anthropogenic warming. Here, new precisely dated stalagmite data from NW Iberia provide the first direct, high-resolution records of periods of rapid melting of Northern Hemisphere ice sheets during the penultimate deglaciation. These records reveal the penultimate deglaciation initiated with rapid century-scale meltwater pulses which subsequently trigger abrupt coolings of air temperature in NW Iberia consistent with freshwater-induced AMOC slowdowns. The first of these AMOC slowdowns, 600-year duration, was shorter than Heinrich 1 of the last deglaciation. Although similar insolation forcing initiated the last two deglaciations, the more rapid and sustained rate of freshening in the eastern North Atlantic penultimate deglaciation likely reflects a larger volume of ice stored in the marine-based Eurasian Ice sheet during the penultimate glacial in contrast to the land-based ice sheet on North America as during the last glacial.

Late Pleistocene glacial advances, equilibrium-line altitude changes and paleoclimate in the Jakupica Mts (North Macedonia)

In the Jakupica Mts a plateau glacier was reconstructed (max. area ∼45 km2, max thickness: ∼260 m). The study area comprises six formerly glaciated valleys, five of which were fed by the plateau glacier and one had an independent cirque when local glaciation reached its maximum ice extent (MIE). The equilibrium line altitude (ELA) of the most extended glacial phase was at 2075+37/-25 m asl. The 10Be cosmic ray exposure (CRE) age of this phase was estimated at 19.3+1.7/-1.3 ka, conformable with the Last Glacial Maximum (LGM). CRE ages from the next moraine generation placed the first phase of deglaciation to 18.2+1.0/-3.0 ka. The samples from the moraine of the penultimate deglaciation phase provided CRE ages with large scatter and biased towards old ages, which is probably the result of inherited cosmogenic nuclide concentrations within the rock.Glacio-climatological modelling was performed for the MIE, which has a well-established LGM age. The degree-day model was used to calculate the amount of accumulation required to sustain the glaciological equilibrium assuming a certain temperature drop at the ELA. The degree-day model constrained by the pollen-based July paleo-temperature reconstructions yielded an annual total melt at the LGM ELA comparable to or slightly higher than the current mean annual precipitation at the same elevation. These wetter LGM conditions inferred from the paleo-glaciological evidence in Jakupica Mts suggest an enhanced moisture advection in the region.

Massive Natural Gas Hydrate Dissociation During the Penultimate Deglaciation (~130 ka) in the South China Sea

There is widespread and growing scientific interest in the impact of massive gas hydrate dissociation on the global environment and climate in geological history. Based on that a global negative excursion of carbon isotopic compositions in marine (foraminifera) and terrestrial (the organic carbon and calcite) sedimentary records occurred during the penultimate deglaciation (~130 ka), we believe that methane released by hydrate dissociation may play a role in accelerating the initial increase of atmospheric methane. In order to prove that massive natural gas hydrate dissociation occurred in this period, we aim to seek for evidence of gas hydrate dissociation from seep carbonate. Here, X-ray diffraction, carbon and oxygen isotopic compositions, trace elements, and U-Th dating analyses were conducted on the deeply-buried authigenic carbonate obtained by drilling in the northern continental slope of the South China Sea. Authigenic carbonate formed at ~130 ka showed obvious characteristics of negative excursion of carbon isotope, positive excursion of oxygen isotope, and enrichment of redox sensitive elements such as Mo, U and As. These results, in particular the high oxygen isotopic compositions of carbonate, point to massive gas hydrate dissociation in the northern continental slope of the South China Sea during the penultimate deglaciation. It is further speculated that massive gas hydrate dissociation might have also occurred on a global scale, contributing to the increase of atmospheric carbon dioxide and methane concentrations during the penultimate deglaciation, and may eventually cause global carbon isotope negative excursion.

Abrupt climate changes in the last two deglaciations simulated with different Northern ice sheet discharge and insolation

There were significant differences between the last two deglaciations, particularly in Atlantic Meridional Overturning Circulation (AMOC) and Antarctic warming in the deglaciations and the following interglacials. Here, we present transient simulations of deglaciation using a coupled atmosphere–ocean general circulation model for the last two deglaciations focusing on the impact of ice sheet discharge on climate changes associated with the AMOC in the first part, and the sensitivity studies using a Northern Hemisphere ice sheet model in the second part. We show that a set of abrupt climate changes of the last deglaciation, including Bolling–Allerod warming, the Younger Dryas, and onset of the Holocene were simulated with gradual changes of both ice sheet discharge and radiative forcing. On the other hand, penultimate deglaciation, with the abrupt climate change only at the beginning of the last interglacial was simulated when the ice sheet discharge was greater than in the last deglaciation by a factor of 1.5. The results, together with Northern Hemisphere ice sheet model experiments suggest the importance of the transient climate and AMOC responses to the different orbital forcing conditions of the last two deglaciations, through the mechanisms of mass loss of the Northern Hemisphere ice sheet and meltwater influx to the ocean.

Penultimate deglaciation Asian monsoon response to North Atlantic circulation collapse

During glacial terminations, massive iceberg discharges and meltwater pulses in the North Atlantic triggered a shutdown of the Atlantic Meridional Overturning Circulation (AMOC). Speleothem calcium carbonate oxygen isotope records (δ18OCc) indicate that the collapse of the AMOC caused dramatic changes in the distribution and variability of the East Asian and Indian monsoon rainfall. However, the mechanisms linking changes in the intensity of the AMOC and Asian monsoon δ18OCc are not fully understood. Part of the challenge arises from the fact that speleothem δ18OCc depends on not only the δ18O of precipitation but also temperature and kinetic isotope effects. Here we quantitatively deconvolve these parameters affecting δ18OCc by applying three geochemical techniques in speleothems covering the penultimate glacial termination. Our data suggest that the weakening of the AMOC during meltwater pulse 2A caused substantial cooling in East Asia and a shortening of the summer monsoon season, whereas the collapse of the AMOC during meltwater pulse 2B (133,000 years ago) also caused a dramatic decrease in the intensity of the Indian summer monsoon. These results reveal that the different modes of the AMOC produced distinct impacts on the monsoon system.

Drivers of the evolution and amplitude of African Humid Periods

During orbital precession minima, the Sahara was humid and more vegetated, providing potential corridors for Hominins migration. Uncertainties remain over the climatic processes controlling the initiation, demise and amplitude of these African Humid Periods. Here we study these processes using a series of transient simulations of the penultimate deglaciation and Last Interglacial period, and compare the results with a transient simulation of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation at the end of deglacial millennial-scale events exerts a dominant control on the abrupt initiation of African Humid Periods as the Atlantic Meridional Overturning Circulation modulates the position of the Intertropical Convergence Zone. In addition, residual Northern Hemispheric ice-sheets can delay the peak of the African Humid Period. Through its impact on Northern Hemispheric ice-sheets disintegration and thus Atlantic Meridional Overturning Circulation, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt onset of the African Humid Period during the Last Interglacial period. Finally, we show that the mean climate state modulates precipitation variability, with higher variability under wetter background conditions.

CH<sub>4</sub> and N<sub>2</sub>O fluctuations during the penultimate deglaciation

Abstract. Deglaciations are characterized by the largest natural changes in methane (CH4) and nitrous oxide (N2O) concentrations of the past 800 000 years. Reconstructions of millennial- to centennial-scale variability within these periods are mostly restricted to the last deglaciation. In this study, we present composite records of CH4 and N2O concentrations from the EPICA Dome C ice core covering the penultimate deglaciation at temporal resolutions of ∼100 years. Our data permit the identification of centennial-scale fluctuations during the transition from glacial to interglacial levels. At ∼134 000 and ∼129 000 years before present (hereafter ka), both CH4 and N2O increased on centennial timescales. These abrupt rises are similar to the fluctuations associated with the Dansgaard–Oeschger events identified in the last glacial period. In addition, gradually rising N2O levels at ∼130 ka resemble a pattern of increasing N2O concentrations on millennial timescales characterizing the later part of Heinrich stadials. Overall, the events in CH4 and N2O during the penultimate deglaciation exhibit modes of variability that are also found during the last deglaciation and glacial cycle, suggesting that the processes leading to changes in emission during the transitions were similar but their timing differed.

Sea-Surface Characteristics of the Newfoundland Basin of the Northwest Atlantic Ocean during the Last 145,000 Years: A Study Based on the Sedimentological and Paleontological Proxies

Dramatic changes occur in the sea-surface characteristics (i.e., temperature and salinity) and freshwater input due to the interaction of cold and fresh Labrador Current and warm and salty North Atlantic Current (NAC) on the southeast Grand Banks. As a result, the biological productivity and seasonal stratification of the upper water masses are intensified. Such changes must have been more dramatic during the glacial times due to the penetration of the Polar and Arctic fronts and southward migration of the Gulf Stream/NAC. However, the extent to which such changes impacted the sea-surface characteristics in the Newfoundland Basin is poorly known. We report changes in the sea-surface characteristics using a piston core (Hu9007-08) collected from the Milne seamount during the last 145,000 years. Heinrich layers H1, H2, H4, and H5 and H11 within the MIS3 and at the penultimate deglaciation were identified by the ice-rafted detritus (IRD) and Neogloboquadrina pachyderma peaks and lighter oxygen isotopes. Rapid turnover by the foraminiferal species with distinct depth habitats and ecological niches in the mixed-layer and thermocline suggests an interplay between the polar and subpolar water masses during the Heinrich and non-Heinrich periods. Only two North Atlantic-wide cooling events, C24 and C21, in which the latter event linked to the minor IRD event during the marine isotope stage (MIS) 5 in Hu90-08, compared to the eight events in the eastern subpolar gyre (e.g., ODP site 984). Millennial-scale N. pachyderma variability in the western subpolar gyre appears to be absent in the eastern subpolar gyre during the MIS3 suggesting the occasional presence of salty and warm water by the NAC inflow, implying a different climate state between the western and eastern subpolar gyre. Although T. quinqueloba data are fragmentary, there are differences between the western and eastern subpolar gyre in addition to the differences within the western subpolar gyre during MIS5 that might imply a variable influence by the subpolar water. This finding suggests that the influence by the NAC outweighs the impact of cold and fresh polar water in the northern northwest Atlantic during the MIS5.

Speleothem‐Based Hydroclimate Reconstructions During the Penultimate Deglaciation in Northern China

AbstractTrace elements in a speleothem have been considered to be effective proxies to characterize hydroclimatic changes. In this study, we present speleothem trace element records of the penultimate glacial‐interglacial transition from 138 to 125.8 ka BP from the Shangxiaofeng cave in northeastern China. We used two analytical methods, namely, inductively coupled plasma optical emission spectrometry (ICP‐OES) and the Avaatech high‐resolution X‐ray fluorescence (XRF) core scanner. Both methods showed good consistency for Sr/Ca but not for Mg/Ca because of the high Mg detection limit of XRF. By comparing the trace element ratios (Sr/Ca and Mg/Ca) with δ13C and by analyzing the slope of the regression line of ln(Sr/Ca) versus ln(Mg/Ca), we found that trace element variations were dominated by hydroclimate‐related prior calcite precipitation (PCP) and water‐rock interaction (WRI). A gradually decreasing trend of Sr/Ca, Mg/Ca, and δ13C records during the glacial‐interglacial transition indicated that the hydroclimate moved toward a moist condition with an intensified East Asian summer monsoon (EASM) and transgression of the coastline. The transgression shortened the distance between the cave site and the coastline, possibly contributing to persistent rainfall enhancement at the study site.

Recent advances in Quaternary paleoclimate research using Antarctic blue ice

<p indent="0mm">Remarkable scientific discoveries have been made from the studies of ice cores drilled in polar ice sheets and mountain ice caps. These samples serve as the most accurate archives of the past atmospheric composition thus far. Two milestone achievements of ice cores include a continuous record of atmospheric greenhouse gas concentrations over the past eight glacial cycles (800 ka) and the discovery of abrupt climate change events on millennial timescale, both made possible by deep drilling in polar ice sheets. Yet, this conventional approach of deep drilling is faced with challenges from prohibitive logistical and operational cost and limited sample supply, particularly with greater depth. To address these challenges, shallow coring in blue ice areas has emerged as an appealing alternative. Two significant advances have been recently achieved from blue ice studies. Ice older than <sc>800 ka</sc> from the Allan Hills Blue Ice Area was discovered and the trapped gases inside the ice allow, for the first time, for the direct observation of atmospheric carbon dioxide concentrations (pCO₂) before <sc>800 ka.</sc> The data show no long-term decline in atmospheric pCO₂, and consequently, the Mid-Pleistocene Transition is not the result of cooling resulting from a decreasing pCO₂. Moreover, large-volume samples retrieved by a large-diameter ice drill (“Blue Ice Drill”) in the Taylor Glacier Blue Ice Area allow for novel geochemical analyses such as the radiocarbon (¹⁴C) content of methane (CH₄). The ¹⁴C of CH₄ trapped in the Taylor Glacier ice suggests that old carbon reservoirs were not responsible for the deglacial rise of CH₄ in the last glacial cycle. Another example of new geochemical methods enabled by the unprecedented sample availability offered by the blue ice areas is the elemental ratios of xenon, krypton, and nitrogen which are sensitive to the mean ocean temperature (MOT). Taylor Glacier ice samples dating back to the penultimate glacial period record <sc>1.1±0.3°C</sc> warmer-than-modern MOT values at <sc>129 ka</sc>, near the end of the penultimate deglaciation. Despite these advances, several challenges and limits remain. First, to accurately establish a chronology for the blue ice samples requires methods of absolute dating, which have considerable uncertainties at present. A precise chronology can be established via timescale synchronization with existing deep ice cores only when the blue ice is younger than <sc>800 ka.</sc> Second, the original deposition site of the blue ice field is not always known, complicating the interpretations of climate proxies that are sensitive to depositional environments such as the stable water isotopes of the ice. Third, glacial flows disrupt the chronology of the ice and could lead to the incomplete and biased preservation of climate signals. In light of these issues and shortcomings, stratigraphically continuous ice cores cannot be fully replaced by temporally discrete samples from the blue ice areas, although blue ice records hold the potentials to provide snapshots of the critical intervals of the Quaternary climate. Future progress in blue ice research may be made in three directions: (1) The prospecting of new blue ice fields and identification of blue ice areas containing the climate records of key understudied intervals; (2) numerical simulations of glacial dynamics to better the interpretation of blue ice records; and (3) the development of novel analytical techniques that reveal new climate information.

An end to the Last Interglacial highstand before 120 ka: Relative sea-level evidence from Infreschi Cave (Southern Italy)

The timing, duration and evolution of sea level during the Marine Isotope Stage 5e (MIS 5e) highstand is a subject of intense debate. A major problem in resolving this debate is the difficulty of chronologically constraining the sea level fall that followed the peak of the highstand. This was mainly controlled by ice-sheet dynamics, the understanding of which is relevant for assessing future sea-level behavior due to global warming. Here we use stratigraphical and geochoronological (U/Th dating and tephra fingerprinting) evidence from the Infreschi archaeological cave (Marina di Camerota, Southern Italy) to constrain relative sea level (RSL) evolution during the MIS 5e highstand and younger stages. Uranium-thorium dating of speleothem deposition phases places the maximum highstand RSL at 8.90 ± 0.6 m a.s.l., as indicated by the near-horizontal upper limit of Lithophaga boreholes measured for along a ∼3.5 km coastal cliff section. Geochronological data show that RSL fell more than 6 m before ∼120 ka, suggesting a duration of the Last Interglacial highstand significantly shorter than proposed in some previous studies. Modelling shows that the RSL trend predicted by the ICE-5G and ICE-6G ice-sheet simulations is consistent with our data, but requires an additional significant reduction of both Greenland and Antarctic ice sheets to match the height of the local maximum highstand if no correction for tectonics is applied. Reconciling field data and models requires an earlier and likely shorter duration of the MIS 5e highstand. This suggests that our new data can constrain global ice-volume variations during the penultimate deglaciation, as well as glacial inception at the end of the Last Interglacial. According to our chronology, there is no local evidence of higher-than-present-day sea levels after 120 ka.

Evidence of Stable Foraminifera Biomineralization during the Last Two Climate Cycles in the Tropical Atlantic Ocean

Planktonic foraminiferal biomineralization intensity, reflected by the weight of their shell calcite mass, affects global carbonate deposition and is known to follow climatic cycles by being increased during glacial stages and decreased during interglacial stages. Here, we measure the dissolution state and the mass of the shells of the planktonic foraminifera species Globigerina bulloides from a Tropical Eastern North Atlantic site over the last two glacial–interglacial climatic transitions, and we report no major changes in plankton calcite production with the atmospheric pCO2 variations. We attribute this consistency in foraminifera calcification to the climatic and hydrological stability of the tropical regions. However, we recorded increased shell masses midway through the penultimate deglaciation (Termination II). In order to elucidate the cause of the increased shell weights, we performed δ18O, Mg/Ca, and μCT measurements on the same shells from a number of samples surrounding this event. Compared with the lighter ones, we find that the foraminifera of increased weight are internally contaminated by sediment infilling and that their shell masses respond to local surface seawater density changes.