Younger Dryas Cold Period Research Articles

Western visitors at the Blätterhöhle (city of Hagen, southern Westphalia) during the Younger Dryas? A new final palaeolithic assemblage type in western Germany.

Until now, it was considered certain that the last reindeer hunters of the Ahrensburgian (tanged point groups) existed exclusively in northwestern Central Europe during the Younger Dryas Cold Period (~ Greenland Stadial 1). The excavations carried out since 2006 on the forecourt (Vorplatz) of the small Blätterhöhle in Hagen on the northern edge of the Sauerland uplands of southern Westphalia (North Rhine-Westphalia, western Germany) have now changed this view. Beneath a surprisingly extensive sequence of Mesolithic find horizons, Pleistocene sediments could be reached whose excavations yielded a Final Palaeolithic lithic ensemble of the Younger Dryas, unusual for the region and beyond. It is characterised by numerous backed lithic projectile points of high variability. Comparisons suggest a typological-technological connection with the Western European Laborian / Late Laborian. Neither in the nearer nor in the wider surroundings has a comparable lithic find ensemble been found so far. In addition, there is a lack of clear evidence for the reindeer in the fauna. Surprisingly, the vast majority of radiocarbon dates of bones and charcoals from the investigated archaeological horizon of the Final Pleistocene proved to be significantly older than expected from their stratigraphic position. This phenomenon has not yet been clarified.

Lateglacial paleoglacier and paleoclimate reconstructions in the north-western Italian Alps

Lateglacial (19.0–11.7 ka ago) paleo-temperature records in the Western European Alps document several short-term cooling episodes within the general post-Last Glacial Maximum warming trend, in phase with North Hemisphere climatic oscillations. Alpine paleo-precipitation reconstructions are instead rare, and further constraints are needed in order to assess whether Lateglacial cold periods were associated with a modified atmospheric circulation pattern over the European Alps. The Alpine paleoglacial record offers a quantitative framework to investigate Lateglacial paleoclimatic conditions and glacier sensitivity in response to both temperature and precipitation changes. Through the combination of 10Be surface-exposure dating of glacial landforms and deposits constraining ice front and surface, together with numerical glacier simulations (iSOSIA), our study aims to reconstruct in space and time the main Lateglacial ice stages and associated paleo-climatic conditions in three tributary valleys (Valpelline, Valsavarenche and Val di Cogne) located within the Dora Baltea catchment (north-western Italian Alps). Our dating-modelling approach reveals in all the three investigated sectors two distinct paleoglacier stages at ca. 13 and 11 ka, documenting respectively the ice configurations at the transition between the Oldest Dryas cold period and the Bølling-Allerød interstadial, and between the Younger Dryas cold period and the early Holocene warming. Numerical ice-simulation outcomes suggest a similar-to-today precipitation pattern during the two ice stages, with either same absolute or homogeneously decreased precipitation values over the Dora Baltea catchment, although a unique quantitative solution of paleo-precipitation magnitudes could not be constrained from our dataset. Using present-day precipitation pattern and magnitude, our results provide paleo-temperature offsets from present-day between −3.3 and −3.8 °C for the older glacial stage, and −2.7 to −3.2 °C for the younger glacial stage, coinciding with the upper range of paleo-temperature reconstructions from other paleoclimatic proxies. Finally, Alpine glaciers’ sensitivity to climate fluctuations differ significantly between the investigated catchments, with a much higher glacier sensitivity to changes in the Equilibrium Line Altitude in the northern (Valpelline) compared to the southern (Valsavarenche and Val di Cogne) tributaries, reflecting different topographic and/or climatic conditions between the three valleys.

Arctic drainage of Laurentide Ice Sheet meltwater throughout the past 14,700 years

During the last deglaciation substantial volumes of meltwater from the decaying Laurentide Ice Sheet were supplied to the Arctic, Gulf of Mexico and North Atlantic along different drainage routes, sometimes as catastrophic flood events. These events are suggested to have impacted global climate, for example initiating the Younger Dryas cold period. Here we analyze the authigenic Pb isotopic composition of sediments in front of the Arctic Mackenzie Delta, a sensitive tracer for elevated freshwater runoff of the retreating Laurentide Ice Sheet. Our data reveal continuous meltwater supply to the Arctic along the Mackenzie River since the onset of the Bølling–Allerød. The strongest Lake Agassiz outflow event is observed at the end of the Bølling–Allerød close to the onset of the Younger Dryas. In context of deglacial North American runoff records from the southern and eastern outlets, our findings provide a detailed reconstruction of the deglacial drainage chronology of the disintegrating Laurentide Ice Sheet.

Revised chronology of northwest Laurentide ice-sheet deglaciation from 10Be exposure ages on boulder erratics

We present new 10Be surface exposure ages from boulders on bedrock to directly date northwest Laurentide ice-sheet deglaciation through a wide swath of the western Canadian Shield that had no previous reliable temporal constraints on ice-margin retreat. Uplift-corrected boulder 10Be surface exposure ages are 13.9 ± 0.2 ka (n = 6) at a site on the western edge of the Slave Craton and 12.4 ± 0.2 ka (n = 5, 1 outlier) at a second site ∼110 km up-ice to the east. These direct 10Be ages for ice-margin retreat are ∼2.4 kyr and ∼1.6 kyr older, respectively, than the canonical deglacial chronology for the northwest Laurentide ice sheet that is based on minimum-limiting 14C dates. We infer an ice-margin retreat rate of 60–100 m yr−1 between the two sites over an interval spanning the transition from the Allerød warm period into the Younger Dryas cold period. This is significantly slower than the rapid >800 m yr−1 retreat rate for the northwest Laurentide ice sheet inferred from earlier deglacial chronologies, which has been hypothesized as a potential source of meltwater forcing for the Younger Dryas cold period. These first direct ages on spatio-temporal patterns of deglaciation in this data-poor region suggest that additional refinement of the deglacial chronology is needed to test hypotheses on the relation between ice-sheet retreat, associated meltwater discharge, abrupt climate change, and rapid sea-level rise.

High-resolution geochemical (μXRF) and palynological analyses for climatic and environmental changes in lake sediments from Sultansazlığı Marsh (Central Anatolia) during the last 14.5 kyr

Multi-proxy analyses of Sultansazlığı Marsh sedimentary sequence covering the last ∼14.5 kyr documents climatic and environmental changes in Central Anatolia (Turkey) and can be correlated with Holocene Rapid Climate Changes (RCCs). Well-correlated geochemical compositions (μXRF), pollen and lithological analyses, together with an age-depth model, were used to determine the time intervals of cold, dry, warm, and wet conditions in Sultansazlığı. Geochemical and pollen data including Younger Dryas (YD) revealed 9 arid periods RCCs; 11.1 ka, 10.3 ka, 9.3 ka, 8.2 ka, 7.3 ka (for this study), 6.5 ka (for this study), 5.9 ka, 4.2 ka, and 2.8 ka. The analysis of glacial advance during the Lateglacial suggests that the climate was cold and wet. YD and two more hiatuses have been accurately determined from the μXRF geochemical proxies. However, YD cold period does not show severe conditions like in other areas in the world. Moreover, the other two hiatuses were clearly determined between Early- Middle and inside the Middle Holocene. In general, the period after 9.8 ka indicates warming conditions towards the recent with some fluctuations compared to an overall trend of the climate.

Biotic response of benthic foraminifera to OMZ variations in the northwestern Pacific since the last deglaciation

Recent declines in oxygen concentrations in the global oceans have been particularly acute in oxygen minimum zones (OMZ) at intermediate depths. Fossil benthic foraminifera have the potential to provide useful information on the long-term biotic responses of marine organisms to past variations in the OMZ. During the last deglaciation, the OMZ at 1 km water depth in the northwestern Pacific Ocean intensified. However, temporal variations in benthic foraminifera in the upper layer of the OMZ since the last deglaciation are poorly understood. Thus, we studied temporal variations in benthic foraminifera assemblages in a sediment core taken from a water depth of 777 m off of Hokkaido, North Japan, to determine their biotic responses to environmental changes in the upper layer of the OMZ from the last deglaciation to the late Holocene. The results suggested that dysoxic species such as Bolivina seminuda and Buliminella tenuata increased rapidly during the Bolling/Allerod warming event, which was associated with severe oxygen depletion and expansion of the OMZ. Similar dysoxic assemblages were recorded at 1 km water depth in this region. On the other hand, the dominance of the oxic species Nonionellina labradorica during the Younger Dryas cold period may indicate that oxygenation events were induced by intensification of intermediate water ventilation. Suboxic assemblages composed of Uvigerina akitaensis, Bolivina spissa, and Elphidium batialis appeared in the late Holocene. The increase in this late Holocene assemblage, which is similar to the modern assemblage in this region, may coincide with gradual cooling around the Japanese Islands.

Rapid retreat of a Scandinavian marine outlet glacier in response to warming at the last glacial termination

Marine outlet glaciers on Greenland are retreating, yet it is unclear if the recent fast retreat will persist, and how atmosphere and ocean warming will impact future retreat. We show how a marine outlet glacier in Hardangerfjorden retreated rapidly in response to the abrupt warming following the Younger Dryas cold period (approximately 11,600 years before present). This almost 1000 m deep fjord, with several sills at 300–500 m depth, hosted a 175 km long outlet glacier at the western rim of the Scandinavian Ice Sheet. We use a dynamic ice-flow model constrained by well-dated terminal and lateral moraines to simulate the reconstructed 500-year retreat of Hardangerfjorden glacier. The model includes an idealized oceanic and atmospheric forcing based on reconstructions, but excludes the surface mass balance-elevation feedback. Our simulations show a highly episodic retreat driven by surface melt and warming fjord waters, paced by the fjord bathymetry. Warming air and ocean temperatures by 4–5 °C during the period of retreat result in a 125-km retreat of Hardangerfjorden glacier in 500 years. Retreat rates throughout the deglaciation vary by an order of magnitude from 50 to 2500 m a−1, generally close to 200 m a−1, punctuated by brief events of swift retreat exceeding 500 m a−1, each event lasting a few decades. We show that the fastest retreat rates occur in regions of the bed with the largest retrograde slopes; ice shelf length and fjord water depth is less important. Our results have implications for modern glacial fjord settings similar to Hardangerfjorden, where high retreat rates have been observed. Our findings imply that increasing air temperatures and warming subsurface waters in Greenland fjords will continue to drive extensive retreat of marine outlet glaciers. However, the recent high retreat rates are not expected to be sustained for longer than a few decades due to constraints by the fjord bathymetry.

Rapid range shifts and megafaunal extinctions associated with late Pleistocene climate change

Large-scale changes in global climate at the end of the Pleistocene significantly impacted ecosystems across North America. However, the pace and scale of biotic turnover in response to both the Younger Dryas cold period and subsequent Holocene rapid warming have been challenging to assess because of the scarcity of well dated fossil and pollen records that covers this period. Here we present an ancient DNA record from Hall’s Cave, Texas, that documents 100 vertebrate and 45 plant taxa from bulk fossils and sediment. We show that local plant and animal diversity dropped markedly during Younger Dryas cooling, but while plant diversity recovered in the early Holocene, animal diversity did not. Instead, five extant and nine extinct large bodied animals disappeared from the region at the end of the Pleistocene. Our findings suggest that climate change affected the local ecosystem in Texas over the Pleistocene-Holocene boundary, but climate change on its own may not explain the disappearance of the megafauna at the end of the Pleistocene.

Comment on: Deglaciation of the Greenland and Laurentide ice sheets interrupted by glacier advance during abrupt coolings

We present new 10Be surface exposure ages from boulders on bedrock to directly date northwest Laurentide ice-sheet deglaciation through a wide swath of the western Canadian Shield that had no previous reliable temporal constraints on ice-margin retreat. Uplift-corrected boulder 10Be surface exposure ages are 13.9 ± 0.2 ka (n = 6) at a site on the western edge of the Slave Craton and 12.4 ± 0.2 ka (n = 5, 1 outlier) at a second site ∼110 km up-ice to the east. These direct 10Be ages for ice-margin retreat are ∼2.4 kyr and ∼1.6 kyr older, respectively, than the canonical deglacial chronology for the northwest Laurentide ice sheet that is based on minimum-limiting 14C dates. We infer an ice-margin retreat rate of 60–100 m yr−1 between the two sites over an interval spanning the transition from the Allerød warm period into the Younger Dryas cold period. This is significantly slower than the rapid >800 m yr−1 retreat rate for the northwest Laurentide ice sheet inferred from earlier deglacial chronologies, which has been hypothesized as a potential source of meltwater forcing for the Younger Dryas cold period. These first direct ages on spatio-temporal patterns of deglaciation in this data-poor region suggest that additional refinement of the deglacial chronology is needed to test hypotheses on the relation between ice-sheet retreat, associated meltwater discharge, abrupt climate change, and rapid sea-level rise.

A multiphasic Younger Dryas cold period recorded in sediments of Lake Steisslingen, SW-Germany: A biomarker perspective

The Younger Dryas (12,680–11,590 yr BP) was the last millennial scale cold event of the Quaternary, when a mean annual temperature decline of 3–5 °C impacted severely on the environment. Whereas pollen distributions document changes in land vegetation, details of limnic ecosystem responses to the cooling event and to its recovery are still not well known. Here, we studied the laminated sediments of Lake Steisslingen (SW-Germany) deposited between 13,500 and 11,100 yr BP in order to investigate the evolution of an aquatic ecosystem. Bulk sediment and organic matter composition mark onset and termination of the Younger Dryas, but remain monotonous through this interval, requiring application of organic and isotope geochemical techniques to follow past ecosystem dynamics. The Younger Dryas is characterized by both an abrupt appearance and demise of long-chain alkenones indicative for haptophyte algae. Based on algal and bacterial biomarker distribution, the Younger Dryas can be divided into three stages, each characterized by the dominance of a microorganism group. The first phase was dominated by nC17 alkanes and hopanoids indicative of cyanophytes and heterotrophic bacteria. The second phase shows a proliferation of cold-adapted haptophyte algae, while in the latest phase highly-branched isoprenoids (HBI) reveal an aquatic community shift towards diatoms. Heterocyte glycolipids document a moderate change in N2-fixing cyanobacterial community structure over the Allerød/Younger Dryas transition but show a marked collapse of the cyanobacteria association at the Younger Dryas/Preboreal climate recovery. Terrigenic wax lipids indicate significant changes in vegetation between the Allerød, Younger Dryas and Preboreal chronozones but remain homogeneous within the Younger Dryas.

Formation of the Yangtze Shoal in response to the post-glacial transgression of the paleo-Yangtze (Changjiang) estuary, China

The Yangtze shoal lies east of the Yangtze (Changjiang) River estuary, China, at water depths of ~25–55 m and is one of the largest subaqueous, sheet-like sand banks in the world. Previous studies have not reached a consensus regarding its origin and timing of formation, mainly due to the scarcity of coring and high-resolution seismic profiling in the shoal. We recovered a 70.20-m-long sediment core (CRE-1402) through the sand shoal and acquired 1586 km of high-resolution shallow seismic profiles across the estuary and shoal. We conducted systematic analyses of the core, including its sedimentary characteristics, down-core changes in benthic foraminiferal assemblages, and 14C and optically stimulated luminescence (OSL) dating. We also analyzed the seismic stratigraphy and integrated these results with data from other previously well-studied cores onshore and offshore of the Yangtze River mouth. Our results indicate that the stratigraphic architecture in the paleo-Yangtze estuary (i.e., the area from the present-day delta plain to the subaqueous delta and Yangtze Shoal) was controlled mainly by changes in sea level during the post-glacial transgression. The post-14 ka sedimentary successions in the paleo-estuary rest unconformably on fluvial or coastal deposits that formed mainly before MIS 1, and are separated by three distinctive bounding surfaces corresponding to MWP-1A, MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka shortly after the Younger Dryas cold period, and the 8.2 ka event, respectively, and the maximum marine flooding surface (MFS). The depositional unit of the Yangtze Shoal ranges in thickness from 6 to 32 m. The shoal exhibits large-scale, SW-dipping (generally 0.5°–2°) foresets with local cross-cutting of foresets, which are composed mainly of very fine- to medium-grained sands. The shoal is underlain by muddy intertidal to subtidal flat deposits that formed during the Younger Dryas, with an erosional surface corresponding to MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka between these and the shoal deposits. The Yangtze Shoal came into being mainly between ca. 11.5 and 7.0 ka in the form of tidal sand ridges actively moving southwestwards under intense and reciprocal tidal currents on a tidal shelf. The tidal sand ridges have evolved into sheet-like tidal sand banks with relatively low relief as a result of tidal remobilization under rotary tidal currents since ca. 7.0 ka when the MFS formed. Therefore, the Yangtze Shoal can be considered to comprise quasi-active tidal sand banks.

Widespread early Holocene deglaciation, Washington Land, northwest Greenland

Constraining the Greenland Ice Sheet (GIS) response to past climate change is imperative for understanding modern ice sheet sensitivity to rising temperatures. In this study, we investigate the response of the GIS to warming during the last deglaciation across the currently ice-free area of Washington Land in northwest Greenland. We apply 10Be cosmogenic surface exposure dating to glacially-transported boulders deposited across the landscape, the interpretation of the dates offering insight into GIS retreat after the Last Glacial Maximum. We find that the northwestern sector of the GIS sustained full ice cover across Washington Land until ∼11 ka, then began thinning, with widespread ice retreat culminating by 8.5 ± 1.1 ka. The record of our new exposure ages indicates that deglaciation occurred 2–3 ka after abrupt warming at the end of the Younger Dryas cold period, which suggests a delayed response of the GIS to this regional warming.

Source-bordering aeolian dune formation along the Scheldt River (southern Netherlands – northern Belgium) was caused by Younger Dryas cooling, high river gradient and southwesterly summer winds

Abstract The Younger Dryas cold period caused major changes in vegetation and depositional environments. This study focuses on the aeolian river-connected dunes along the former, Weichselian Late Glacial, course of the Scheldt River in the southern Netherlands. Aeolian dunes along the Scheldt have received little attention, as they are partly covered by Holocene peat and marine deposits. The spatial distribution of the dunes is reconstructed by digital elevation model analysis and coring transects. Dunes are present on the high eastern bank of the Scheldt and in the subsurface of the polder area west of the Brabantse Wal escarpment. A reach-specific higher channel gradient probably caused a channel pattern change from meandering to braiding during the Younger Dryas. This enabled deflation from the braid plain and accumulation in source-bordering river dunes east of the incised and terraced, subsurface Late Pleistocene Scheldt valley. The age of the dune formation is established by pollen analysis and radiocarbon dating of underlying and overlying peat beds. The peat layer below the dune at Zomerbaan is attributed to the Allerød and early Younger Dryas periods. Dune formation occurred predominantly during the second part of the Younger Dryas stadial, both on and in front (west) of the Brabantse Wal escarpment. Wind direction was reconstructed by geomorphic analysis and sedimentary structures on lacquer peels. A southwesterly wind direction is demonstrated by the parabolic dune morphology. For the first time, Younger Dryas wind direction is reconstructed based on adhesion ripple cross-laminated sets on lacquer peels. Sand-transporting south-southwesterly winds were dominant during the Younger Dryas, most likely during summer.

Multiple meltwater discharge and ice rafting events recorded in the deglacial sediments along the Beaufort Margin, Arctic Ocean

The first regional model for the deglacial history of the Beaufort margin slope, in the western Arctic, is presented. The conceptual model was developed using new high-resolution CHIRP seismic reflection, multibeam bathymetry, and sediment core data acquired along the margin. This synthesis provides important constraints on sediment source and dispersal patterns for the last deglaciation. The continental slope from Barrow Canyon to the Mackenzie Trough is characterized by thick Holocene sediments mostly sourced from Barrow Canyon and continental discharge. This acoustically transparent, fine-grained unit overlies acoustically laminated sediments sourced from the Mackenzie River and ice rafting. The deglacial history of the margin from Mackenzie Trough to the Amundsen Gulf is characterized by many ice rafting and meltwater discharge events. Ice rafted debris layers were deposited around ∼14.6 ka and ∼14.1 ka and likely document enhanced ice shedding events from the Amundsen and M'Clure ice streams as they retreated. There are three inferred meltwater discharge events, all sourced from the Mackenzie region. The oldest discharge event had two phases, one sometime between ∼14.5 ka and ∼14.2 ka and another between ∼14.0 ka and ∼13.0 ka. This discharge deposited finely laminated sediments more than 7 m thick sourced from proglacial lakes in the area. Following this was a flood event starting at ∼12.94 ka, which generated high amplitude reflectors, deposited coarse debris, and caused a lowering in the δ18O record. This is possibly a major outburst flood from glacial Lake Agassiz with an additional component of Agassiz meltwater that was diverted from its southern drainage pathway down the Mississippi to the Gulf of Mexico. Finally, a third discharge event was initiated by ∼11.3 ka, which deposited coarse laminated sediments focused in the Mackenzie Trough. Timing of these second and third events correlates with the onset of the Younger Dryas cold period and the preBoreal oscillation event, respectively, suggesting that fresh water discharge may have reached a tipping point, leading to those climate events.

Opening of glacial Lake Agassiz’s eastern outlets by the start of the Younger Dryas cold period

Opening of glacial Lake Agassiz’s eastern outlets by the start of the Younger Dryas cold period

Younger Dryas ice margin retreat triggered by ocean surface warming in central-eastern Baffin Bay

The transition from the last ice age to the present-day interglacial was interrupted by the Younger Dryas (YD) cold period. While many studies exist on this climate event, only few include high-resolution marine records that span the YD. In order to better understand the interactions between ocean, atmosphere and ice sheet stability during the YD, more high-resolution proxy records from the Arctic, located proximal to ice sheet outlet glaciers, are required. Here we present the first diatom-based high-resolution quantitative reconstruction of sea surface conditions from central-eastern Baffin Bay, covering the period 14.0–10.2 kyr BP. Our record reveals warmer sea surface conditions and strong interactions between the ocean and the West Greenland ice margin during the YD. These warmer conditions were caused by increased Atlantic-sourced water inflow combined with amplified seasonality. Our results emphasize the importance of the ocean for ice sheet stability under the current changing climate.

Wood14C Dating with AixMICADAS: Methods and Application to Tree-Ring Sequences from the Younger Dryas Event in the Southern French Alps

AbstractThe AixMICADAS facility is in part dedicated to research on radiocarbon (14C) calibration by means of various archives. For this purpose, we are improving upon the capacity to accurately date subfossil wood. In the current study, nine chemical pretreatment protocols are tested on six wood samples of known ages. The optimization based on14C ages,13C/12C ratios, carbon % and overall mass yield % leads us to favor the acid-base-acid-bleaching pretreatment (ABA-B). This efficient method is shown to provide a residue of holocellulose with optimal blanks equivalent to an age of 51,30014C BP with a standard deviation of 1500 yr based on 25 analyses. The seven wood samples from the Sixth International Radiocarbon Intercomparison (SIRI) are then analyzed as a further verification of the accuracy of our method. As a first scientific contribution, we studied two tree-ring sequences from subfossil pines (Barb12 and Barb17) collected in the southern French Alps. New14C analyses were performed at high resolution (every third year) and are shown to agree well with results obtained previously by high precision β-counting on CO2from large samples at lower resolution for Barb17 and accelerator mass spectrometry (AMS) data for Barb12. The new14C series are then matched to the Kauri and YDB chronologies: the new sequence of Barb12-17 tentatively corresponds to the interval between 12,836 and 12,594 cal BP within the Younger Dryas cold period. The14C comparison between the Barb12-17 sequence from France and the Kauri sequence from New Zealand allows calculating the14C Inter-Hemispheric Gradient (IHG), with an average value of ca. 57 yr. The IHG stayed relatively high throughout the studied period. Interestingly, the IHG exhibits a transient maximum value (ca. 100 yr) during the period of rapid Δ14C rise (12,750–12,720 cal BP), a behavior that could be due to a delayed response of the Southern Hemisphere.

A dual-biomarker approach for quantification of changes in relative humidity from sedimentary lipid <i>D</i>∕<i>H</i> ratios

Abstract. Past climatic change can be reconstructed from sedimentary archives by a number of proxies. However, few methods exist to directly estimate hydrological changes and even fewer result in quantitative data, impeding our understanding of the timing, magnitude and mechanisms of hydrological changes. Here we present a novel approach based on δ2H values of sedimentary lipid biomarkers in combination with plant physiological modeling to extract quantitative information on past changes in relative humidity. Our initial application to an annually laminated lacustrine sediment sequence from western Europe deposited during the Younger Dryas cold period revealed relative humidity changes of up to 15 % over sub-centennial timescales, leading to major ecosystem changes, in agreement with palynological data from the region. We show that by combining organic geochemical methods and mechanistic plant physiological models on well characterized lacustrine archives it is possible to extract quantitative ecohydrological parameters from sedimentary lipid biomarker δ2H data.

Younger-Dryas cooling and sea-ice feedbacks were prominent features of the Pleistocene-Holocene transition in Arctic Alaska

Declining sea-ice extent is currently amplifying climate warming in the Arctic. Instrumental records at high latitudes are too short-term to provide sufficient historical context for these trends, so paleoclimate archives are needed to better understand the functioning of the sea ice-albedo feedback. Here we use the oxygen isotope values of wood cellulose in living and sub-fossil willow shrubs (δ18Owc) (Salix spp.) that have been radiocarbon-dated (14C) to produce a multi-millennial record of climatic change on Alaska's North Slope during the Pleistocene-Holocene transition (13,500–7500 calibrated 14C years before present; 13.5–7.5 ka). We first analyzed the spatial and temporal patterns of δ18Owc in living willows growing at upland sites and found that over the last 30 years δ18Owc values in individual growth rings correlate with local summer temperature and inter-annual variations in summer sea-ice extent. Deglacial δ18Owc values from 145 samples of subfossil willows clearly record the Allerød warm period (∼13.2 ka), the Younger Dryas cold period (12.9–11.7 ka), and the Holocene Thermal Maximum (11.7–9.0 ka). The magnitudes of isotopic changes over these rapid climate oscillations were ∼4.5‰, which is about 60% of the differences in δ18Owc between those willows growing during the last glacial period and today. Modeling of isotope-precipitation relationships based on Rayleigh distillation processes suggests that during the Younger Dryas these large shifts in δ18Owc values were caused by interactions between local temperature and changes in evaporative moisture sources, the latter controlled by sea ice extent in the Arctic Ocean and Bering Sea. Based on these results and on the effects that sea-ice have on climate today, we infer that ocean-derived feedbacks amplified temperature changes and enhanced precipitation in coastal regions of Arctic Alaska during warm times in the past. Today, isotope values in willows on the North Slope of Alaska are similar to those growing during the warmest times of the Pleistocene-Holocene transition, which were times of widespread permafrost thaw and striking ecological changes.

Organic geochemical investigations of the Dali Lake sediments in northern China: Implications for environment and climate changes of the last deglaciation in the East Asian summer monsoon margin

Millennial-scale environment and climate changes in the East Asian summer monsoon margin during the last deglaciation are reconstructed by systematic studies on the characteristic of sedimentary organic matter from Dali Lake in northern China. Concurrent increases in the TOC and TN concentrations indicate increases in terrestrial organic matter and nutrient inputs to the lake and a development of terrestrial vegetation and phytoplankton productivity related to increases in regional temperature and precipitation. C/N ratios reflect changes in the proportions of terrestrial and aquatic organic matter. Decreases in both δ13Corg and δ15N values indicate increases in the isotopically lighter, terrestrial carbon and nitrogen inputs to the lake, due to increases in surface runoffs; while a sharp decrease in the δ15N value implies a significant weakening in the biological activities of nitrifying and amonifying bacteria, due to abrupt decrease in the water temperature. The geochemical data indicate that regional temperature and precipitation exhibited increasing trends from 15,000 to 12,350calyrBP; temperature decreased abruptly at 12,350calyrBP and then maintained a low level from 12,350 to 11,400calyrBP, precipitation decreased to a relatively low level from 12,350 to 11,400calyrBP; and both temperature and precipitation returned to increase after 11,400calyrBP. The climate change in the Dali Lake region during the last deglaciation corresponds, within age uncertainties, to the Bølling-Allerød (BA) warm phase and Younger Dryas (YD) cold reversal occurring over northern high latitudes. However, the gradual and mild increasing trends of regional temperature and precipitation during the BA warm period contrasts with the general cooling trend in northern high latitude temperature, implying a dominant influence from increases in the Northern Hemisphere summer insolation; while the slight decreases in regional precipitation relative to the rapid and significant decreases in northern high latitude temperature during the YD cold period may have resulted from local moisture recycling or from an insensitive response of hydrology and ecology to the regional precipitation change.