Changes In Global Ice Volume Research Articles

Diverse manifestations of the mid-Pleistocene climate transition

The mid-Pleistocene transition (MPT) is widely recognized as a shift in paleoclimatic periodicity from 41- to 100-kyr cycles, which largely reflects integrated changes in global ice volume, sea level, and ocean temperature from the marine realm. However, much less is known about monsoon-induced terrestrial vegetation change across the MPT. Here, on the basis of a 1.7-million-year δ13C record of loess carbonates from the Chinese Loess Plateau, we document a unique MPT reflecting terrestrial vegetation changes from a dominant 23-kyr periodicity before 1.2 Ma to combined 100, 41, and 23-kyr cycles after 0.7 Ma, very different from the conventional MPT characteristics. Model simulations further reveal that the MPT transition likely reflects decreased sensitivity of monsoonal hydroclimate to insolation forcing as the Northern Hemisphere became increasingly glaciated through the MPT. Our proxy-model comparison suggests varied responses of temperature and precipitation to astronomical forcing under different ice/CO2 boundary conditions, which greatly improves our understanding of monsoon variability and dynamics from the natural past to the anthropogenic future.

Demographic Expansion and Contraction in a Neotropical Fish during the Late Pleistocene-Holocene

Demographic changes during the late Pleistocene-Holocene left signatures in the DNA of contemporary populations. These signatures reveal demographic phenomena like the increase or decrease in effective population size. In this paper we searched for signatures of demographic change in the DNA of the Neotropical freshwater fish Poecilia vivipara. Also, we investigated whether demographic changes are correlated with palaeoclimatic events of the late Pleistocene-Holocene, in particular, if changes in effective population size are correlated with expansion and contraction of available habitats, induced by global ice-volume changes and sea-level fluctuations. We used Bayesian Skyline Plot (BSP) analysis with sequences from the mitochondrial gene cytochrome b to estimate the ancestral demography of the Neotropical freshwater fish P. vivipara. To test the assumptions of neutrality and absence of population structure we used Tajima’s D and Spatial Analysis of Molecular Variance (SAMOVA), respectively. Effective population size of P. vivipara remained stable until 75,000 years ago, increased by 10-fold reaching a maximum at approximately 25,000 years ago, then suddenly declined at the Pleistocene-Holocene boundary. Variation in effective population size in P. vivipara correlates with expansion and contraction of habitats induced by sea-level fluctuations, caused by the advance and retreat of ice sheets during the Last Glacial Maximum (LGM).

A below-the-present late Holocene relative sea level and the glacial isostatic adjustment during the Holocene in the Malay Peninsula

This study has reconstructed the Holocene relative sea-level history in the relatively tectonically stable Malay Peninsula with two specific objectives: quantifying the sea-level indicative meanings of mangrove sediments and producing new high-quality sea-level index points. In this study, a wetland on the east coast of the Peninsula was extensively surveyed; the elevational relationship between habitat zones and their pollen and diatom assemblages was established; and a number of indicative meanings were precisely quantified for this wetland. This quantified relationship is extrapolated to other coastal sites of the Peninsula for re-calibration of previously published mangrove-based sea-level data. The reconstructed sea-level history from the new and re-calibrated data demonstrates a period of rapid sea-level rise at a rate of 15.8 ± 3.9 mm/a between 10,500 cal a BP at −35.0 ± 4.3 m and 9000 cal a BP at −14.2 ± 1.6 m. The rate of rising sea level was gradually reduced to zero from 9000 to 5000 cal a BP, and by the end of this period the relative sea level reached a highstand of c. 3.3 ± 0.2 m. Over the past 5000 years relative sea level fell to its lowest point at −0.6 ± 0.1 m by 800 cal a BP. This far-field sea-level history has been driven by the global ice-volume change and glacial isostatic adjustment, and it matches well with the modelled ice-volume equivalent sea level.

Sandy Loess Records of Precipitation Changes and Monsoon Migrations in the Hunshandake Sandy Land Since the Last Glacial Maximum

AbstractThe Hunshandake Sandy Land (Hunshandake) in central Inner Mongolia, China, is located within the fringe of the East Asian summer monsoon (EASM) domains. Precipitation records in Hunshandake are essential to studying the migrations of the EASM. This work reproduced the precipitation changes since the Last Glacial Maximum (LGM) by measuring the magnetic susceptibility and carbonate content through optically stimulated luminescence dating of two sandy loess profiles within the sandy land to obtain a high‐resolution sedimentary chronological sequence. In addition, eolian topsoil samples were collected along the modern precipitation gradient and their magnetic susceptibility values were compared among the monsoon region, the monsoon marginal region, and the nonmonsoon region. The results demonstrate the following: (1) from 25 to 15 ka, precipitation was low in Hunshandake and exhibited no substantial variability or trend; from 15 to 6 ka, precipitation increased gradually and reached its maximum at approximately 6 ka; and after 6 ka, precipitation decreased again; (2) precipitation changes in Hunshandake since the LGM were mainly determined by the migration of the EASM; and (3) changes in the monsoon migrations in Hunshandake since the LGM are primarily driven by the changes in global ice volume and subordinately by solar radiation.

Changes in the distribution of Uvigerinidae species over the past 775 kyr: Implications for the paleoceanographic evolution of the Japan Sea

The distribution of benthic foraminifera of the Uvigerinidae family, which are abundant in middle to upper Pleistocene samples from the Yamoto Basin in the Japan Sea, have been analyzed quantitatively and used for paleoceanographic interpretations. Twenty-three Uvigerinidae species are reported from IODP Site U1426, 6 of which are present in at least three or more samples with 10% or more abundance and considered as dominant species. The abundance and ecological preferences of dominant Uvigerinidae species, comprised of Uvigerina mediterranea, Uvigerina yabei, Uvigerina sp.1, Uvigerina peregrina suggest dysoxic conditions, with high influx of organic matter between 775 and 475 ka in response to the enhanced flow of the Tsushima Warm Current. The end of MIS 13 marked the onset of more extreme glacial interglacial climate cycles in the Japan Sea. Glacial interglacial alternations of Uvigerina peregrina and Trifarina angulosa from 475 ka to the Holocene indicate major fluctuations in circulation and productivity within this semi-enclosed basin. Both U. peregrina and T. angulosa respond to 100 kyr variability, reflecting the paleoclimatic shift and global ice volume changes associated with the Middle Pleistocene Transition.

Orbital time scale records of Asian eolian dust from the Sea of Japan since the early Pliocene

A high-resolution potassium content record of sediments from Integrated Ocean Drilling Program (IODP) Site U1422 drilled in the northern Sea of Japan was employed to yield an astronomical timescale for this ∼205 m thick sedimentary archive. The K content was quantified using the natural gamma radiation (NGR) data routinely measured on DV JOIDES Resolution. The U1422 K (wt.%) series shows an increasing trend with time, which parallels the growth of North Hemisphere ice sheets since ∼4 Ma, as revealed by the global benthic foraminifer's oxygen isotope stack (LR04). We propose that K content variations reflect changes in the relative contributions of Asian eolian dust and volcanic weathering products, in response to changes in global ice volume. Using the shipboard age model, constrained by palaeomagnetism and nanofossils datums, we tuned the U1422 K content variations to the LR04 stack. Our tuned age model for site U1422 goes back to 3.9 Ma. The studied record indicates that Asian dust generation is enhanced during periods of global cooling. The synchronous variations between the U1422 K record and the LR04 stack also suggest that global cooling played a dominant role in promoting the central Asian aridification since ∼3.9 Ma, while the uplift of Tibetan Plateau plays a secondary role, considering many disagreements still exists between the timing and amplitude of the Qinghai-Tibetan uplift and the evolution of central Asian aridity since the early Pliocene.

Penultimate deglacial warming across the Mediterranean Sea revealed by clumped isotopes in foraminifera

The variability of seawater temperature through time is a critical measure of climate change, yet its reconstruction remains problematic in many regions. Mg/Ca and oxygen isotope (δ18OC) measurements in foraminiferal carbonate shells can be combined to reconstruct seawater temperature and δ18O (δ18OSW). The latter is a measure of changes in local hydrology (e.g., precipitation/evaporation, freshwater inputs) and global ice volume. But diagenetic processes may affect foraminiferal Mg/Ca. This restricts its potential in many places, including the Mediterranean Sea, a strategic region for deciphering global climate and sea-level changes. High alkalinity/salinity conditions especially bias Mg/Ca temperatures in the eastern Mediterranean (eMed). Here we advance the understanding of both western Mediterranean (wMed) and eMed hydrographic variability through the penultimate glacial termination (TII) and last interglacial, by applying the clumped isotope (Δ47) paleothermometer to planktic foraminifera with a novel data-processing approach. Results suggest that North Atlantic cooling during Heinrich stadial 11 (HS11) affected surface-water temperatures much more in the wMed (during winter/spring) than in the eMed (during summer). The method’s paired Δ47 and δ18OC data also portray δ18OSW. These records reveal a clear HS11 freshwater signal, which attenuated toward the eMed, and also that last interglacial surface warming in the eMed was strongly amplified by water-column stratification during the deposition of the organic-rich (sapropel) interval known as S5.

Loess deposits since early Pleistocene in northeast China and implications for desert evolution in east China

Loess deposits and deserts are regarded as coupled geological systems and loess deposits on the periphery of deserts can often be used to reconstruct desert evolution. Previous studies of desert evolution in Asia are mainly concentrated in northwest China and the China Loess Plateau, and little is known about long-term desert evolution in east China. In this study, we selected the Sishijiazi loess section in the Chifeng area in northeast China to study the long-term evolution of the desert in east China. A high-resolution magnetostratigraphy combined with optically stimulated luminescence dating indicated that the age of the section base is approximately 1.02Ma. The Brunhes-Matuyama boundary is at the depth of 39.8m in loess unit L8, and the upper boundary of the Jaramillo Subchron is at the depth of 60.8m in paleosol S10. The results of grain-size analysis indicate a coarsening grain-size trend in the past 1.0Ma. In addition, based on grain-size variations, the desert evolution in east China since ∼1.0Ma can be divided into three stages: stability from 1.0 to 0.8Ma, desert recession from 0.8 to 0.5Ma, and gradual expansion since 0.5Ma. Our results further indicate that the evolution of desert in east China was mainly controlled by changes in global ice volume, and that the uplift of the Tibetan Plateau may have had an additional effect.

Snow thickness profiling on Antarctic sea ice with GPR—Rapid and accurate measurements with the potential to upscale needles to a haystack

AbstractSnow thickness on sea ice is a largely undersampled parameter yet of importance for the sea ice mass balance and for satellite‐based sea ice thickness estimates and thus our general understanding of global ice volume change. Traditional direct thickness measurements with meter sticks can provide accurate but only spot information, referred to as “needles” due to their pinpoint focus and information, while airborne and satellite remote sensing snow products, referred to as “the haystack,” have large uncertainties due to their scale. We demonstrate the remarkable accuracy and applicability of ground‐penetrating radar (GPR) snow thickness measurements by comparing them with in situ meter stick data from two field campaigns to Antarctica in late winter/early spring. The efficiency and millimeter‐to‐centimeter accuracy of GPR enables practitioners to acquire extensive, semiregional data with the potential to upscale needles to the haystack and to potentially calibrate satellite remote sensing products that we confirm to derive roughly 30% of the in situ thickness. We find the radar wave propagation velocity in snow to be rather constant (± 6%), encouraging regional snow thickness surveys. Snow thinner than 10 cm is under the detection limit with the off‐the‐shelf GPR setup utilized in our study.

Mediterranean Outflow Water dynamics during the past ~570 kyr: Regional and global implications

AbstractThe Gulf of Cadiz constitutes a prime area to study teleconnections between the North Atlantic Ocean and climate change in the Mediterranean realm. In particular, the highly saline Mediterranean Outflow Water (MOW) is an important modulator of the North Atlantic salt budget on intermediate water levels. However, our understanding of its paleoceanographic evolution is poorly constrained due to the lack of high‐resolution proxy records that predate the last glacial cycle. Here we present the first continuous and high‐resolution (~ 1 kyr) benthic δ18O and δ13C as well as grain size records from Integrated Ocean Drilling Program Site U1386 representing the last ~570 kyr. We find three distinct phases of MOW variability throughout the Late to Middle Pleistocene at Site U1386 associated with prominent shifts in its composition and flow strength. We attribute this long‐term variability to changes in water mass sourcing of the MOW. Superimposed on the long‐term change in water mass sourcing is the occurrence of distinct and precession paced δ18O enrichment events, which contrast the pattern of global ice volume change as inferred from the global mean δ18O signal (i.e., LR04) but mimics that of the adjacent Mediterranean Sea. We attribute these enrichment events to a profound temperature reduction and salinity increases of the MOW, aligning with similar changes in the Mediterranean source region. These events might further signify ice volume increases as inferred from significant sea level drops recorded in the Red Sea and/or increased influence of North Atlantic intermediate water masses when MOW influence was absent at Site U1386.

Shifting Quaternary migration patterns in the Bahamian archipelago: Evidence from the Zamia pumila complex at the northern limits of the Caribbean island biodiversity hotspot.

The Bahamas archipelago is formed by young, tectonically stable carbonate banks that harbor direct geological evidence of global ice-volume changes. We sought to detect signatures of major changes on gene flow patterns and reconstruct the phylogeographic history of the monophyletic Zamia pumila complex across the Bahamas. Nuclear molecular markers with both high and low mutation rates were used to capture two different time scale signatures and test several gene flow and demographic hypotheses. Single-copy nuclear genes unveiled apparent ancestral admixture on Andros, suggesting a significant role of this island as main hub of diversity of the archipelago. We detected demographic and spatial expansion of the Zamia pumila complex on both paleo-provinces around the Piacenzian (Pliocene)/Gelasian (Pleistocene). Populations evidenced signatures of different migration models that have occurred at two different times. Populations on Long Island (Z. lucayana) may either represent a secondary colonization of the Bahamas by Zamia or a rapid and early-divergence event of at least one population on the Bahamas. Despite changes in migration patterns with global climate, expected heterozygosity with both marker systems remains within the range reported for cycads, but with significant levels of increased inbreeding detected by the microsatellites. This finding is likely associated with reduced gene flow between and within paleo-provinces, accompanied by genetic drift, as rising seas enforced isolation. Our study highlights the importance of the maintenance of the predominant direction of genetic exchange and the role of overseas dispersion among the islands during climate oscillations.

Holocene temperature and precipitation variability on the central Tibetan Plateau revealed by multiple palaeo-climatic proxy records from an alpine wetland sequence

This study has reconstructed the Holocene environmental history of central Tibetan Plateau, a transitional area between monsoon-dominated Asia and Westerlies-dominated Asia, to examine the interplay between the two circulation systems and effects of other potential mechanisms on the temporal variability of Holocene climatic conditions in the study area. Specifically, this study has produced a twin record of palaeo-hydrology and palaeo-temperature from an alpine wetland sediment sequence using two sets of lipid-based biomarker indices: n-alkanes and branched glycerol dialkyl glycerol tetraethers (bGDGTs). The results show that (1) the Asian summer monsoon has supplied moisture that helped initiate the wetland habitat around 8000 cal. yr BP, about 2000 years later than locations along the southeastern edge of the plateau; (2) a significant decline in moisture supply is evident since the Medieval Warm period; (3) the Westerlies have transmitted variability of the North Atlantic climate to the study area, resulting in periodic increases in evaporation and causing short-term expansion or contraction of the wetland and (4) the palaeo-air temperature change follows closely that of global sea level and sea surface temperature, implying a link to global ice volume change throughout the Holocene. This study concludes that the Holocene climate history of central Tibetan Plateau is a result of monsoonal moisture supply interacted with the Westerlies variability, while the palaeo-temperature has followed changes in global ice volume during the present warm interglacial.

Continued obliquity pacing of East Asian summer precipitation after the mid-Pleistocene transition

Records from natural archives show that the strength of the East Asian summer monsoon (EASM) strongly depends on the orbital configuration of the Earth. However, the dominant orbital cycles driving EASM have been found to be spatially different. Speleothem stable oxygen isotopic records from southern China, which are believed to reflect large-scale changes in the Asian monsoon system, are dominated by climatic precession cycles. Further north, on the Chinese Loess Plateau (CLP), loess-and-paleosol sequences, which are argued to be controlled by monsoon intensity, are in pace with global ice volume changes dominated by obliquity, and after the mid-Pleistocene transition by 100-kyr cycles. To understand these critical discrepancies, here we apply a novel proxy based on the trace metal compositions of pedogenic carbonate in the eolian deposits on the CLP to reconstruct summer precipitation over the last 1.5 million years. Our reconstructions show that summer precipitation on the CLP is dominantly forced by obliquity not in pace with the ice-volume-imprinted loess-paleosol sequences before and after the mid-Pleistocene transition or with the precession-paced speleothem oxygen isotopic records. Coupled with climate model results, we suggest that the obliquity-driven variations of summer precipitation may originate from the gradient of boreal insolation that modulates the thermal contrast between the Asian continent and surrounding oceans.

Episodic speleothem deposition tracks the terrestrial impact of millennial-scale last glacial climate variability in SW Ireland

Abstract Eighty four new U-Th ages are presented for twenty randomly selected broken, displaced and reworked calcite speleothems retrieved from clastic sedimentary fill and from isolated bedding-plane shelves in Crag cave (SW Ireland). The dated pre-Holocene samples span much of the last glacial, ranging in age from 85.15 ± 0.60 to 23.45 ± 0.17 ka. Speleothem deposition requires the presence of liquid water, and because Crag cave is a shallow system, deposition is considered likely only when mean annual air temperatures (MAAT) exceed the freezing point of water. Deposition at this mid-latitude ocean-marginal site occurred episodically during MIS5a through to MIS2, synchronously within dating uncertainties, with the timing of Greenland Interstadials (GI). In the latter part of Marine Isotope Stage 3 (MIS3), deposition was particularly intense, consistent with regional scale climate amelioration inferred previously from radiocarbon ages for sparse MIS3 organic and freshwater surficial deposits in N. Ireland. A brief episode of speleothem deposition at c.23.40 ± 0.22 ka coincides with GI-2, demonstrating the sensitivity of the site to brief climate amelioration episodes in Greenland during MIS2. Conditions favourable for speleothem deposition occurred periodically during the last glacial, indicating temperature changes of at least 10 °C between stadials and interstadials at this mid-latitude site. Deposition ceased during Greenland Stadials (GS), including during periods of ice-rafting in the adjacent N. Atlantic Ocean (Heinrich events). Oxygen and carbon isotope ratios of the last glacial speleothems are generally elevated, reflecting non-equilibrium isotope fractionation effects. However, establishment of low δ13C values often occurred within a few decades of climate amelioration, indicating that biogenic CO2 production resumed rapidly at this site, particularly during MIS3. Speleothem δ18O variability was driven largely by long-term changes in the δ18O value of the adjacent North Atlantic surface water, in turn largely reflecting changes in global ice volume. In common with published speleothem datasets, warming episodes in Ireland associated with GI events typically pre-date their timing in the GICC05 and GICC05modelext time scales, but lie comfortably within the maximum counting uncertainties of these ice core age models.

Influence of external forcings on abrupt millennial-scale climate changes: a statistical modelling study

The last glacial period was punctuated by a series of abrupt climate shifts, the so-called Dansgaard-Oeschger (DO) events. The frequency of DO events varied in time, supposedly because of changes in background climate conditions. Here, the influence of external forcings on DO events is investigated with statistical modelling. We assume two types of simple stochastic dynamical systems models (double-well potential-type and oscillator-type), forced by the northern hemisphere summer insolation change and/or the global ice volume change. The model parameters are estimated by using the maximum likelihood method with the NGRIP Ca$^{2+}$ record. The stochastic oscillator model with at least the ice volume forcing reproduces well the sample autocorrelation function of the record and the frequency changes of warming transitions in the last glacial period across MISs 2, 3, and 4. The model performance is improved with the additional insolation forcing. The BIC scores also suggest that the ice volume forcing is relatively more important than the insolation forcing, though the strength of evidence depends on the model assumption. Finally, we simulate the average number of warming transitions in the past four glacial periods, assuming the model can be extended beyond the last glacial, and compare the result with an Iberian margin sea-surface temperature (SST) record (Martrat et al., Science, vol. 317, p. 502, 2007). The simulation result supports the previous observation that abrupt millennial-scale climate changes in the penultimate glacial (MIS 6) are less frequent than in the last glacial (MISs 2-4). On the other hand, it suggests that the number of abrupt millennial-scale climate changes in older glacial periods (MISs 6, 8, and 10) might be larger than inferred from the SST record.

Subduction of seafloor asperities revealed by a detailed stratigraphic analysis of the active margin shelf sediments of Central Ecuador

The uplift of the coastal cordillera of central Ecuador is a likely consequence of the subduction of the Carnegie Ridge, a 400-km-long and 2-km-high topographic asperity. This study aims at analyzing the impact of the subduction of the Carnegie Ridge on the continental shelf sedimentation during the Quaternary. We interpret high-resolution (50–450Hz frequency) multi-channel (72 channels) and single-channel (Sparker) seismic data, piston cores, sediment profiles (3.5kHz), and high-resolution multibeam bathymetry acquired during the ATACAMES cruise with the RV L’Atalante in 2012. In the La Plata Island–Cabo San Lorenzo Peninsula region, the results show a full and detailed record of the last ten Quaternary sedimentary sequences deposited in intra-shelf basins, at and -just seaward of the shelf break. These 100-ka-scale sequences are tied to piston core data, to well-dated Quaternary onshore exposures and to a flight of coastal marine terraces on Cabo San Lorenzo Peninsula and La Plata Island. These sequences correlate with the global ice volume and deep marine temperature changes (d18O) for the last 0.7Ma, which points to a strict climatic control on depositional sequence development and preservation. However, the subsidence of the continental shelf acoustic basement, estimated by the stepwise backstripping of the sedimentary record, exhibits a complex deformation pattern with uplifting and subsiding regions. Deep marine seismic data, currently under processing, show evidence for a subducted seamount beneath La Plata Island and GPS data indicate an important interplate coupling in the same area with the potential to generate a Mw 7–7.5 earthquake. The pattern of the continental shelf deformation is consistent with the shape of this seamount. We tested the hypothesis of a link between the margin deformation and the subduction of the seamount, by comparing a stepwise subduction of the seamount to the palinspastic restoration of the deformation of the continental shelf for the last 1Ma. This comparison shows that the collision probably started c.500ka ago, together with the syntectonic sedimentation, and drastically slowed down by c.50ka, with the sealing of most of the deformation on the shelf.

Neogene ice volume and ocean temperatures: Insights from infaunal foraminiferal Mg/Ca paleothermometry

AbstractAntarctic continental‐scale glaciation is generally assumed to have initiated at the Eocene‐Oligocene Transition, yet its subsequent evolution is poorly constrained. We reconstruct changes in bottom water temperature and global ice volume from 0 to 17 Ma using δ18O in conjunction with Mg/Ca records of the infaunal benthic foraminifer, O. umbonatus from Ocean Drilling Program (ODP) Site 806 (equatorial Pacific; ~2500 m). Considering uncertainties in core top calibrations and sensitivity to seawater Mg/Ca (Mg/Ca)sw, we produce a range of Mg/Ca‐temperature‐Mg/Casw calibrations. Our favored exponential temperature calibration is Mg/Ca = 0.66 ± 0.08 × Mg/Casw0.27±0.06 × e(0.114±0.02 × BWT) and our favored linear temperature calibration is Mg/Ca = (1.21 ± 0.04 + 0.12 ± 0.004 × BWT (bottom water temperature)) × (Mg/Casw−0.003±0.02) (stated errors are 2 s.e.). The equations are obtained by comparing O. umbonatus Mg/Ca for a Paleocene‐Eocene section from Ocean Drilling Program (ODP) Site 690 (Weddell Sea) to δ18O temperatures, calculated assuming ice‐free conditions during this peak warmth period of the Cenozoic. This procedure suggests negligible effect of Mg/Casw on the Mg distribution coefficient (DMg). Application of the new equations to the Site 806 record leads to the suggestion that global ice volume was greater than today after the Middle Miocene Climate Transition (~14 Ma). ODP Site 806 bottom waters cooled and freshened as the Pacific zonal sea surface temperature gradient increased, and climate cooled through the Pliocene, prior to the Plio‐Pleistocene glaciation of the Northern Hemisphere. The records indicate a decoupling of deep water temperatures and global ice volume, demonstrating the importance of thresholds in the evolution of the Antarctic ice sheet.

Early-Holocene warming in Beringia and its mediation by sea-level and vegetation changes

Abstract. Arctic land-cover changes induced by recent global climate change (e.g., expansion of woody vegetation into tundra and effects of permafrost degradation) are expected to generate further feedbacks to the climate system. Past changes can be used to assess our understanding of feedback mechanisms through a combination of process modeling and paleo-observations. The subcontinental region of Beringia (northeastern Siberia, Alaska, and northwestern Canada) was largely ice-free at the peak of deglacial warming and experienced both major vegetation change and loss of permafrost when many arctic regions were still ice covered. The evolution of Beringian climate at this time was largely driven by global features, such as the amplified seasonal cycle of Northern Hemisphere insolation and changes in global ice volume and atmospheric composition, but changes in regional land-surface controls, such as the widespread development of thaw lakes, the replacement of tundra by deciduous forest or woodland, and the flooding of the Bering–Chukchi land bridge, were probably also important. We examined the sensitivity of Beringia's early Holocene climate to these regional-scale controls using a regional climate model (RegCM). Lateral and oceanic boundary conditions were provided by global climate simulations conducted using the GENESIS V2.01 atmospheric general circulation model (AGCM) with a mixed-layer ocean. We carried out two present-day simulations of regional climate – one with modern and one with 11 ka geography – plus another simulation for 6 ka. In addition, we performed five ~ 11 ka climate simulations, each driven by the same global AGCM boundary conditions: (i) 11 ka Control, which represents conditions just prior to the major transitions (exposed land bridge, no thaw lakes or wetlands, widespread tundra vegetation), (ii) sea-level rise, which employed present-day continental outlines, (iii) vegetation change, with deciduous needleleaf and deciduous broadleaf boreal vegetation types distributed as suggested by the paleoecological record, (iv) thaw lakes, which used the present-day distribution of lakes and wetlands, and (v) post-11 ka All, incorporating all boundary conditions changed in experiments (ii)–(iv). We find that regional-scale controls strongly mediate the climate responses to changes in the large-scale controls, amplifying them in some cases, damping them in others, and, overall, generating considerable spatial heterogeneity in the simulated climate changes. The change from tundra to deciduous woodland produces additional widespread warming in spring and early summer over that induced by the 11 ka insolation regime alone, and lakes and wetlands produce modest and localized cooling in summer and warming in winter. The greatest effect is the flooding of the land bridge and shelves, which produces generally cooler conditions in summer but warmer conditions in winter and is most clearly manifest on the flooded shelves and in eastern Beringia. By 6 ka continued amplification of the seasonal cycle of insolation and loss of the Laurentide ice sheet produce temperatures similar to or higher than those at 11 ka, plus a longer growing season.

Patterns of vegetation and climate change in the northern South China Sea during the last glaciation inferred from marine palynological records

High-resolution marine palynological records from the northern South China Sea (SCS) provide new information concerning the regional history of vegetation and climate conditions during the last glaciation (~70–19cal ka BP). The preservation of rich pollen from tropical and subtropical broadleaf trees (largely composed of evergreen Quercus, Castanopsis, and Euphorbiaceae) suggests that a southern subtropical climate prevailed during the last glaciation, including the period of the Last Glacial Maximum. Relatively higher percentages of such pollen taxa occurred during Marine Isotope Stage (MIS) 3, when Phyllocladus pollen from the south also increased, indicating a warmer terrestrial climate and a strengthened East Asian summer monsoon comparing to MIS 4 and 2. Moreover, the maximum percentages of such pollen occurred during the early MIS 3, implying that an optimal climate prevailed in this period. The reconstructed terrestrial climate corresponds well to marine evidence such as δ18O values from planktonic foraminifer shell and records of sea surface temperature. This study confirmed that the regional terrestrial climate could be linked closely to changes in both global ice volume and the East Asian monsoon attributable to variations of the earth’s orbital parameters. Modern palynological investigations and statistical analyses of the pollen percentages in the profiles indicate that Pinus pollen and spores are easily transported by water flow in the northern SCS. Therefore, their relative abundance peaks suggest that several wet intervals occurred during the last glaciation.

Southern Laurentide ice-sheet retreat synchronous with rising boreal summer insolation

Establishing the precise timing for the onset of ice-sheet retreat at the end of the Last Glacial Maximum (LGM) is critical for delineating mechanisms that drive deglaciations. Uncertainties in the timing of ice-margin retreat and global ice-volume change allow a variety of plausible deglaciation triggers. Using boulder 10 Be surface exposure ages, we date initial southern Laurentide ice-sheet (LIS) retreat from LGM moraines in Wisconsin (USA) to 23.0 ± 0.6 ka, coincident with retreat elsewhere along the southern LIS and synchronous with the initial rise in boreal summer insolation 24–23 ka. We show with climate-surface mass balance simulations that this small increase in boreal summer insolation alone is potentially sufficient to drive enhanced southern LIS surface ablation. We also date increased southern LIS retreat after ca. 20.5 ka likely driven by an acceleration in rising isolation. This near-instantaneous southern LIS response to boreal summer insolation before any rise in atmospheric CO 2 supports the Milankovic hypothesis of orbital forcing of deglaciations.