Younger Dryas Period Research Articles

Onset of the Younger Dryas Recorded with 14C at Annual Resolution in French Subfossil Trees

ABSTRACTSubfossil trees with their annual rings constitute the most accurate and precise archive to calibrate the radiocarbon (14C) method. The Holocene part of the IntCal curve is based on tree-ring chronologies, absolutely dated by dendrochronological matching. For the Northern Hemisphere, the absolute curve starts at 12,325 cal BP. For the early part of the Younger Dryas (YD) climatic event (≈ 12,850–11,650 cal BP), there are only a few floating dendrochronological sequences, mainly from Switzerland and France. We present new 14C results from subfossil trees (Pinus sylvestris L.) collected from the Barbiers site (southeast French Alps). The dendrochronological series covers 416 years, corresponding to the onset of the YD period. In order to date our sequence, we matched it with the 14C record based on kauri trees from New Zealand. The Barbiers data were first averaged at the same decadal resolution as the kauri record. Statistical comparison of the different averaging options and matching techniques enables dating the Barbiers sequence to 13,008–12,594 ±10 cal BP, which thus includes the boundary between the Allerød and YD events. The new Barbiers record allows to calculate the 14C inter-hemispheric gradient (14C-IHG) during the period overlapping the kauri sequence. For the optimal dating option, the mean 14C-IHG is 37 yr with a standard deviation (SD) of 21 yr based on 43 decadal estimations (−6‰ with SD of 2‰). The 14C-IHG record exhibits minimal values, down to zero, between 12,960–12,840 cal BP. Excluding these minima leads to an average 14C-IHG of 45 yr with a SD of 14 yr based on 33 decadal values, in agreement with observations for the last two millennia. The Barbiers record suggests a 14C-IHG increase between the end of the Allerød period (IHG of 37 yr with SD of 14 yr) and the early part of the YD (IHG of 48 yr with SD of 14 yr), which is compatible with previously reported drop of deep-water convection in the North-Atlantic and the associated increase in wind-driven upwelling in the Southern Ocean.

Guatemala's Late Pleistocene (Rancholabrean) fauna: Revision and interpretation

We present a revision, dating and interpretation of the Late Pleistocene megafauna of Guatemala based on paleontological material located in collections in the country and other fossils housed in the American Museum of Natural History, New York. Assemblages are dominated by proboscideans (Cuvieronius) and xenarthrans (Eremotherium, Glyptotherium), while co-occurring Equus and Mixotoxodon are significantly less frequent, and Holmesina, Palaeolama, Tapirus, Neocherus, Mammuthus and other ground sloth taxa (Paramylodon; Megalonyx) are rare. Contrary to published records the faunal assemblage is dominated by North American faunal elements. The underrepresentation of typical South American fauna therefore suggests a more southernly located biogeographic juncture between the two subcontinents, at least for the Late Pleistocene. The biogeographical barrier was either formed by the high mountain chains, or alternating periods of low and high precipitation that triggered the intermittent expansion of either grass- or woodland, thus leading to an alternating filter for either grazers or browsers. The presence of an oak-dominated forest vegetation with Mixotoxodon, Eremotherium and Cuvieronius supports high precipitation rates during MIS 3 and 2, followed by drought during the Late Pleistocene deglaciation. The expansion of grassland during the Younger Dryas period favored the migration of Mammuthus along the Mesoamerican Corridor, which is otherwise absent in Guatemala. Our data also suggest a survival of Cuvieronius into the early Holocene in the southeastern lowlands of Guatemala along the Motagua river. Our review is important as Guatemala is key to understanding migrations along the Mesoamerican Corridor that acted as a bridge but also as a filter of faunal interchange between North- and South America.

Influence of climate on the late Pleistocene depositional history of the Gulf of Gemlik (Sea of Marmara)

The late Pleistocene climate-induced water level fluctuations in the Gulf of Gemlik have been first documented in detail by analyses of high-resolution seismic reflection profiles that are tied to radiocarbon-dated sediment cores. In this study, synthetic seismogram for seismic-to-core comparison has enabled a precise correlation between seismic units and core sediments, on which basis it was possible to establish an accurate chronology of the sedimentary successions deposited in the gulf. In the seismic profiles, pre-Holocene depositional units reflect distinctive hydrological and sedimentological processes due to the fluctuating water levels in response to high climate variability. Progressive drop in water level, due to a dry climate phase in the Last Glacial Maximum, is marked by a progradation of a lacustrine delta on the northern shelf of the gulf. Post-glacial warming prior to ca. 18 cal ka BP gave rise to meltwater-driven, high-energy currents on the shelf slopes, producing erosional gullies. The later transgressive freshwater stage of the Gemlik “Lake” began as a result of the meltwater discharge to the gulf basin, which is associated with deposition of the retrograded clinoforms on the shelf margin and infilling the gullies along the slopes. The final post-glacial water level rise in the gulf was mediated by a regressive phase, inferred by seaward-dipping clinoforms and deltaic deposition on the shelf margin. Such a climatic shift to a drier period may coincide with the Heinrich event 1. In the Gulf of Gemlik, transition from the Bolling/Allerod to Younger Dryas periods is recognized by deposition of small clinoform packages on the shelf margin, marking a decrease in water level due to low precipitations on the catchment area. The lake level decrease in the gulf during the YD was modulated by a stillstand, producing a – 65-m erosional terrace and a beach-berm elsewhere along the gulf shelves.

Weathering and erosion in central Vietnam over the Holocene and Younger Dryas: Clay mineralogy and elemental geochemistry from the Vietnam Shelf, western South China Sea

Clay mineralogy and elemental geochemistry of Core SO18401 on the Vietnam Shelf are used to reconstruct the history of weathering and erosion in central Vietnam over the past 13.2 ka. The variability of the clay species across the 13.2 ka BP together with the age model allows a subdivision into four temporal stages. Provenance analysis based on clay mineralogy indicates that small mountainous rivers in southern-central Vietnam are the major sources of terrigenous sediments on the central Vietnam Shelf. Smectite and kaolinite originated from the chemical weathering of Mesozoic and Neogene-Quaternary basaltic rocks and Paleo-Mesozoic felsic intrusive rocks, respectively, under warm and humid climate conditions; whereas illite and chlorite were mainly derived from physical weathering of Paleo-Mesozoic felsic intrusive rocks and Precambrian metamorphic rocks in central Vietnam. Accordingly, smectite/(illite + chlorite) ratio, together with elemental ratios of Al2O3/K2O, TiO2/Al2O3, and SiO2/Al2O3, can be utilized to reflect the relative importance between chemical weathering and physical weathering/erosion of the parent rocks in southern-central Vietnam. Higher smectite/(illite + chlorite) and Al2O3/K2O ratios indicate a more important role of chemical weathering, and higher TiO2/Al2O3 and SiO2/Al2O3 ratios suggest enhanced contribution of physical weathering and erosion. A combination of these mineralogical and elemental proxies allows the distinction of four stages of weathering and erosion evolution in southern-central Vietnam over the Holocene and Younger Dryas periods. Enhanced physical weathering and erosion took place during the periods ∼13.2–11.0 ka BP (Stage I) and ∼3.9–2.0 ka BP (Stage III), associated with relatively arid conditions. By contrast, strengthened chemical weathering occurred during the periods ∼11.0–3.9 ka BP (Stage II) and 2.0 ka BP to present (Stage IV), corresponding to more warm and humid climatic conditions. Our study suggests that the weathering and erosion history of southern-central Vietnam over the Holocene is mainly controlled by the East Asian monsoon climate.

How dry was the Younger Dryas? Evidence from a coupled <i>δ</i><sup>2</sup>H–<i>δ</i><sup>18</sup>O biomarker paleohygrometer applied to the Gemündener Maar sediments, Western Eifel, Germany

Abstract. Causes of the Late Glacial to Early Holocene transition phase and particularly the Younger Dryas period, i.e. the major last cold spell in central Europe during the Late Glacial, are considered to be keys for understanding rapid natural climate change in the past. The sediments from maar lakes in the Eifel, Germany, have turned out to be valuable archives for recording such paleoenvironmental changes. For this study, we investigated a Late Glacial to Early Holocene sediment core that was retrieved from the Gemündener Maar in the Western Eifel, Germany. We analysed the hydrogen (δ2H) and oxygen (δ18O) stable isotope composition of leaf-wax-derived lipid biomarkers (n-alkanes C27 and C29) and a hemicellulose-derived sugar biomarker (arabinose), respectively. Both δ2Hn-alkane and δ18Osugar are suggested to reflect mainly leaf water of vegetation growing in the catchment of the Gemündener Maar. Leaf water reflects δ2H and δ18O of precipitation (primarily temperature-dependent) modified by evapotranspirative enrichment of leaf water due to transpiration. Based on the notion that the evapotranspirative enrichment depends primarily on relative humidity (RH), we apply a previously introduced “coupled δ2Hn-alkane–δ18Osugar paleohygrometer approach” to reconstruct the deuterium excess of leaf water and in turn Late Glacial–Early Holocene RH changes from our Gemündener Maar record. Our results do not provide evidence for overall markedly dry climatic conditions having prevailed during the Younger Dryas. Rather, a two-phasing of the Younger Dryas is supported, with moderate wet conditions at the Allerød level during the first half and drier conditions during the second half of the Younger Dryas. Moreover, our results suggest that the amplitude of RH changes during the Early Holocene was more pronounced than during the Younger Dryas. This included the occurrence of a “Preboreal Humid Phase”. One possible explanation for this unexpected finding could be that solar activity is a hitherto underestimated driver of central European RH changes in the past.

Ion concentrations in ice wedges: An innovative approach to reconstruct past climate variability

For atmospheric gases and aerosols, snow and ice provide a useful archive of paleoenvironmental history. In the northern hemisphere, this archive has been largely limited to Greenland ice, but promising pioneering work has been done recently using chronology of an ice wedge in Barrow, northern Alaska. Here, we investigate past aerosols at the same ice-wedge sampling site, reconstructing the sea-ice fluctuations in the adjacent Beaufort Sea during the Bølling/Allerød (BA) and Younger Dryas (YD) periods. We confirm the integrity of methanesulfonate ion (MS−) concentrations in the BIWS as marine proxies, and then find that the ice wedge has a high MS− concentration through the beginning (coldest) YD periods. The high MS− concentration indicates that even during the coldest YD periods (12,900–12,700 yrBP), the near-shore region in the Alaskan Beaufort Sea near Barrow may not have been completely filled by permanent sea ice.

High-resolution isotopic evidence for a potential Saharan provenance of Greenland glacial dust

Dust concentrations in Greenland ice show pronounced glacial/interglacial variations with almost two orders of magnitude increase during the Last Glacial Maximum. Greenland glacial dust was previously sourced to two East Asian deserts: the Taklimakan and Gobi deserts. Here we report the first high-resolution Pb and Sr isotopic evidence for a significant Saharan dust influence in Greenland during the last glacial period, back to ~31 kyr ago, from the Greenland NEEM ice core. We find that during Greenland Stadials 3–5.1 (~31 to 23 kyr ago), the primary dust provenance was East Asia, as previously proposed. Subsequently, the Saharan isotopic signals emerge during Greenland Stadials 2.1a–2.1c (~22.6 to 14.7 kyr ago) and from the late Bølling-Allerød to the Younger Dryas periods (~13.6 to 12 kyr ago), coincident with increased aridity in the Sahara and efficient northward transport of dust during these cold periods. A mixing isotopic model proposes the Sahara as an important source, accounting for contribution to Greenland glacial dust of up to 50%, particularly during Greenland Stadial 2.1b and the late Bølling-Allerød to the Younger Dryas periods. Our findings provide new insights into climate-related dust provenance changes and essential paleoclimatic constraints on dust-climate feedbacks in northern high latitudes.

Animals and humans in the European Russian Arctic towards the end of the last Ice Age and during the mid‐Holocene time

Here, we present and discuss results from geo‐archaeological and palaeo‐zoological investigations at the Palaeolithic site Pymva Shor, in the Russian Arctic. As many as 3324 vertebrate fauna remains were recovered during two excavations. This includes bones of mammals, birds and fish. Radiocarbon dates were obtained from 26 specimens. The results show ages in the range 30–3 cal. ka BP. Hare and reindeer are the best represented amongst the identified mammalian species, whilst ptarmigan and various wader species dominate the avian bones. The Pleistocene assemblage includes herbivorous herd animals such as horse, bison and musk ox. These species are typical of the treeless tundra‐steppe landscape that existed during the Lateglacial. Of particular interest is a cave lion specimen that has been radiocarbon dated to approximately 15.5 cal. ka BP. According to our knowledge, this is one of the latest dated examples of this species in Eurasia. The faunal composition in the Holocene assemblage is strikingly different and includes distinct forest taxa such as beaver and pine marten. The avifauna also supports a forested environment with the presence of black grouse. A few stone artefacts were found within the strata, and have been radiocarbon dated to 16–15 cal. ka BP, suggesting that there were humans in the Pymva Shore area at that time. We identified impact notches and cut marks on some radiocarbon‐dated reindeer and bison bones, showing that humans were present twice during the Younger Dryas period. A fourth occupation phase is identified during the mid‐Holocene (6–5 cal. ka BP). We also investigated river terraces and obtained a series of luminescence dates. These have been used to reconstruct the geological history and the relationship to the find‐bearing strata.

High-and low-latitude forcing of the East African climate since the LGM: Inferred from the elemental composition of marine sediments off Tanzania

We present X-Ray Fluorescence (XRF) Scanner measurements from a 6 m long sediment core (GeoB12624-1) on the upper slope of Tanzania to reconstruct the climatic evolution in East Africa since the Last Glacial Maximum (LGM). Log-ratios of Fe/Ca and Ti/Ca are indicative for sediment discharge of the Rufiji River, which is controlled by climatic conditions in the Rufiji catchment area. The most significant changes in major elemental composition occurred at 15.1 and 7.4 ka highlighted by the regime shift index values. The data set records distinct precipitation peaks during the early Holocene. This corresponds a maximum in the Northern Hemisphere (NH) summer insolation and results in a transition from the arid LGM to the humid early Holocene. Our geochemical record also indicates that this climatic transition was interrupted by two severe droughts that occurred during NH cold intervals: the Heinrich stadial 1 (HS1) and the Younger Dryas (YD). Through a comparison with other nearby paleoclimatic records, we suggest that arid climatic conditions only occurred in East Africa north of 8–10°S, whereas in southern East Africa around 15–20°S increased humidity during the HS1 and YD prevailed. We thus conclude that these two drought events were caused by a southward migration of the Intertropical Convergence Zone (ITCZ) which was fostered by the NH cooling during the HS1 and YD. Hence, our new geochemical record clearly documents that the East African climate not only responded to low-latitude insolation forcing on sub-orbital time scales, but also, was strongly influenced by high-latitude cooling during the HS1 and YD periods.

A New Graphitization System for Radiocarbon Dating with AMS in the Dendrochronological Laboratory at AGH-UST Kraków

AbstractA new vacuum system for the preparation of graphite samples for radiocarbon (14C) measurement using an accelerator mass spectrometer (AMS) was constructed at the Dendrochronological Laboratory in AGH-UST Kraków. The central part of the system is a manual vacuum line for the production of graphite from carbon dioxide for subsequent AMS measurements. The graphitization system can handle up to five samples simultaneously, and the process lasts for approximately 1 hour. The graphitization line was built to support the preparation of wood samples for a project dedicated to dating a subfossil tree from the Younger Dryas period. For this purpose, the chemical preparation procedure for wood samples was optimized to obtain more reliable results. This includes the extraction of α-cellulose to increase the precision of the age determination. The performance of the system was tested with NIST OxII, IAEA standards (IAEA C3, C5, C6, and C8), and background samples. The results of the 13 samples of subfossil wood were tested and are presented. The methodology gives good reproducibility of results obtained for the samples prepared using this system.

Mean global ocean temperatures during the last glacial transition

Little is known about the ocean temperature's long-term response to climate perturbations owing to limited observations and a lack of robust reconstructions. Although most of the anthropogenic heat added to the climate system has been taken up by the ocean up until now, its role in a century and beyond is uncertain. Here, using noble gases trapped in ice cores, we show that the mean global ocean temperature increased by 2.57 ± 0.24 degrees Celsius over the last glacial transition (20,000 to 10,000 years ago). Our reconstruction provides unprecedented precision and temporal resolution for the integrated global ocean, in contrast to the depth-, region-, organism- and season-specific estimates provided by other methods. We find that the mean global ocean temperature is closely correlated with Antarctic temperature and has no lead or lag with atmospheric CO2, thereby confirming the important role of Southern Hemisphere climate in global climate trends. We also reveal an enigmatic 700-year warming during the early Younger Dryas period (about 12,000 years ago) that surpasses estimates of modern ocean heat uptake.

Late Pleistocene to Holocene paleoenvironmental evolution of Lake Hazar, Eastern Anatolia, Turkey

High-resolution seismic reflection profiles and core analyses in Lake Hazar provide a detailed record of the lake level fluctuations and the robust chronology of paleoclimatic events of the Eastern Anatolia during the late Pleistocene to Holocene. The earlier period of Marine Isotope Stage 3 (MIS-3) prior to 48 cal ka BP was accompanied by considerable lake level drop below −95 m, whereas the lake level increase during the later period (ca. 48-29 cal ka BP) gave rise to deposition of a transgressive unit with typical of aggradational architecture in the seismic reflection profiles. High climate variability with the Greenland interstadials and stadials (Dansgard-Oeschger and Heinrich events) in Lake Hazar are sensitively recorded in the core sediments by using multi-proxy analyses. Adjustment of seismic units correlating with the radiocarbon-dated chronostratigraphic units in the studied cores implies that the early Marine Isotope Stage 2 (MIS-2) is marked by an another lowstand lake level existed at ca. −90 m during 29–23 cal ka BP. In comparision to MIS-3 stage, the multi-proxy analyses reveal a general dry evaporative condition during most of the Last Glacial Maximum. In Lake Hazar, the maximum humidity in the late glacial period existed during 14.9–13.5 cal ka BP. The existence of a hiatus in the sedimentary record is documented in the seismic data that coincides with the cold and dry Younger Dryas period, implying a remarkable lake level drop. The multi-proxy records of the Holocene sediments reveal that a maximum precipitation in the early Holocene period prevailed during 10.1–9.3 cal ka BP, leading water level rise in the lake. In Lake Hazar, the middle Holocene until 4.9 cal ka BP is represented by highly climate variations, indicating a series of shorter wet and longer dry climate periods. The late Holocene is accompanied by lake level rises under a general wet climate condition that was interrupted by short dry climate intervals during 3.7–3.3 cal ka BP, 2.8–2.6 cal ka BP and 2.1–1.8 cal ka BP.

Evidence for a Younger Dryas deglaciation in the Galicica Mountains (FYROM) from cosmogenic 36Cl

This study presents the first cosmogenic 36Cl surface exposure data from a moraine in the Former Yugoslav Republic of Macedonia (FYROM). Five limestone boulders from an end moraine in the Galicica Mountains (40.94°N, 20.83°E, 2050 m a.s.l.) were used for cosmogenic 36Cl surface exposure dating. The 36Cl concentrations from the five boulders are identical within their measurement uncertainties ruling out major effects of inheritance, erosion, or snow cover. The calculated ages are very consistent ranging from 11.3 ± 1.3 to 12.8 ± 1.4 ka (mean 12 ± 0.6 ka) after applying a Ca-spallation production rate of 56 at g−1 a−1 (LSD scaling) and correction for 5 mm ka−1 carbonate weathering and 2% snow shielding. The applied corrections for weathering and snow shielding cause a shift to older ages in the order of magnitude of ca. 5% on average, making the production rate the main impact on exposure ages. The ages point to a moraine formation during the Younger Dryas period, consistent with the timing of the last deglaciation in the Galicica Mountains derived from previous geomorphological studies in the area. The formation of a glacier was likely favoured by several topoclimatic factors, accounting for additional snow input. This interpretation is in line with regional studies on glaciation chronologies from Šara Range (FYROM/Republic of Kosovo), Retezat Mountains (Romania), Mt. Chelmos (Greece), Mount Orjen (Montenegro) and Durmitor (Montenegro). Lake sediment analyses of Prespa (Republic of Albania/FYROM/Greece), Maliq (Republic of Albania) and Dojran lakes (FYROM/Greece) indicate that cold conditions promoted the formation of a local cirque glacier. However, studies of sediment records of the adjacent lakes Ohrid (Republic of Albania/FYROM) and Prespa do not indicate the presence of a proximal glaciation. An explanation might be a combination of the small size of the cirque glacier, generating only small amounts of debris, and the karstic bedrock, which hampers fluvial transport and acts by its aquifer system as a natural sediment trap, as the fluvial transport of the sediments to the lakes is absorbed by the karst system.

The last glacial termination on the eastern flank of the central Patagonian Andes (47 ° S)

Abstract. Few studies have examined in detail the sequence of events during the last glacial termination (T1) in the core sector of the Patagonian Ice Sheet (PIS), the largest ice mass in the Southern Hemisphere outside of Antarctica. Here we report results from Lago Edita (47°8′ S, 72°25′ W, 570 m a.s.l.), a small closed-basin lake located in a valley overridden by eastward-flowing Andean glaciers during the Last Glacial Maximum (LGM). The Lago Edita record shows glaciolacustrine sedimentation until 19 400 yr BP, followed by organic sedimentation in a closed-basin lake and a mosaic of cold-resistant hygrophilous conifers and rainforest trees, along with alpine herbs between 19 400 and 11 000 yr BP. Our data suggest that the PIS retreated at least ∼ 90 km from its LGM limit between ∼ 21 000 and 19 400 yr BP and that scattered, low-density populations of cold-resistant hygrophilous conifers, rainforest trees, high-Andean and steppe herbs thrived east of the Andes during the LGM and T1, implying high precipitation levels and southern westerly wind (SWW) influence at 47° S. The conifer Podocarpus nubigena increased between 14 500 and 13 000 yr BP, suggesting even stronger SWW influence during the Antarctic Cold Reversal, after which it declined and persisted until 11 000 yr BP. Large increases in arboreal pollen at ∼ 13 000 and ∼ 11 000 yr BP led to the establishment of forests near Lago Edita between 10 000 and 9000 yr BP, suggesting a rise in the regional tree line along the eastern Andean slopes driven by warming pulses at ∼ 13 000 and ∼ 11 000 yr BP and a subsequent decline in SWW influence at ∼ 11 000 yr BP. We propose that the PIS imposed a regional cooling signal along its eastern, downwind margin through T1 that lasted until the separation of the northern and southern Patagonian ice fields along the Andes during the Younger Dryas period. We posit that the withdrawal of glacial and associated glaciolacustrine environments through T1 provided a route for the dispersal of hygrophilous trees and herbs from the eastern flank of the central Patagonian Andes, contributing to the afforestation of the western Andean slopes and pacific coasts of central Patagonia during T1.

Polygenetic soil formation on Late Glacial Loess on the Susak Island reflects paleo-environmental changes in the Northern Adriatic area

A polygenetic soil formed on Late Glacial Loess on the Island of Susak was investigated by means of high-resolution depth approach to give pedosedimentary and palaeoenvironmental reconstructions of the Late Glacial/Holocene pedosequence. The complete polygenetic profile (A-2C1-2C2-3Btb-4Ck), defined as Eutric Leptic Regosol (Geoabruptic, Siltic, Ruptic) over Argic horizon over loess deposit, is a result of the interaction of the sediments (loess) with environmental conditions from the Late Glacial throughout the Holocene. Both normal pedogenesis and erosion/sedimentation-driven pedogenesis influenced its development. Therefore, this profile bears valuable imprints of climate, vegetation, land surface, and land use changes in this part of the Adriatic area for the time period covering the past 16 ka. The 4Ck horizon is a remnant of an A-C soil profile, whose incipient formation corresponds to a steppe-like palaeo-vegetation and a semi-humid to semi-arid palaeo-climate. The 3Btb horizon has blocky structure; common clay coatings and infillings that are related to the walls of channel macrovoids; increased CIA, Al/Si and Ba/Sr ratios due to the increase of the Al and clay contents; highest Fet, Fed, Mnt and Mnd values due to abundant redox concentrations; and specific clay mineralogy. Since the obtained age of the loess deposit (4Ck horizon) underlying the 3Btb horizon is 16.1 ± 1.3 ka (can be correlated to the Late Glacial Maximum, i.e., OIS-2), we find that the period of the Bølling-Allerød warming might have enabled the formation of the highly weathered 3Btb horizon within a Luvisol-like soil. After the period of surface stability, a period of forest decay and surface instability took place, resulting with the erosion of the Luvisol. The 3Btb horizon was then covered with a homogeneous loess material (whose deposition can presumably be related to the Younger Dryas period), inside of which the horizons 2C1 and 2C2 are recognized. Both these horizons are negligibly affected by pedogenesis and contain artefacts. The incipient soil formation in the upper part of the profile, along with the irregular lower boundary of the A horizon and the lithic discontinuity between the A horizon and the 2C1 horizon, points to the possibility of a very recent and minor aeolian deposition. However, one should not overlook the potential impacts of human activities and/or colluvial depositions on the properties of the A, 2C1, and 2C2 horizons.

Pore fluids in Dead Sea sediment core reveal linear response of lake chemistry to global climate changes

Abstract Pore fluids extracted from a 456 m sediment core, recovered within the framework of a multinational and International Continental Scientific Drilling Program (ICDP) co-sponsored effort at the bottom of the terminal Dead Sea, recorded the chemical variations in the deep lake over the past 220 k.y. Mg2+ and Br– were shown to be conservative in the pore fluids, increasing in concentration during interglacial periods, diluting during glacials, and providing excellent proxies for deep lake net water balance changes. Furthermore, the Na/Cl ratio recorded the process of halite precipitation and dissolution induced by these hydrological changes. Mg2+ and Br– records follow a glacial-interglacial pattern, such as observed in atmospheric CO2 concentrations and global sea-surface temperatures, albeit with a phase offset. At the end of the last interglacial (ca. 116 ka), there is a delay in onset of dilution of the deep lake, most likely due to the limnological transition from holomictic to meromictic conditions. The increase in deep lake concentrations at Last Glacial Termination I is delayed as a result of freshwater input into the deep lake during the cooler Younger Dryas period. There is a persistent relationship between precipitation in the watershed and North Atlantic sea-surface temperatures, similar to conditions observed over the past instrumental record. Deviations from the long-term trends occurred during interglacial periods, Marine Isotope Stages MIS 5e and MIS 1, when the deep Dead Sea was significantly diluted, and coincided with Mediterranean sapropel layers S5 and S1.

Environmental and climate reconstruction of the late-glacial-Holocene transition from a lake sediment sequence in Aubrac, French Massif Central: Chironomid and diatom evidence

The analysis of fossil chironomid and diatom assemblages from a sedimentary record from Les Roustières peat bog (Massif Central, France, 1196m asl) allows the reconstruction of past environmental and climate changes during the late-glacial and early Holocene. Chironomid assemblages showed that the infilling of the palaeolake had commenced during the Oldest Dryas (GS-2b) as suggested by the rapid decrease in chironomid species associated with the cold and deep zone of lakes and by their replacement by littoral and eutrophic taxa. Quantitative July temperature reconstructions, based on the chironomid data, suggest that mean July air temperature (Tjul) ranged between 6°C and 11°C at the termination of the Oldest Dryas period (GS-2b). Climate began to warm at the start of the Bølling period (GI-1e), between 15,000 and 14,800calyr BP, with a rise in Tjul of about 4°C. This climate warming is contemporaneous with lake eutrophication as suggested by diatoms and chironomids. Maximum temperatures of 13–14°C were reached around 13,600calyr BP during the Allerød period (GI-1c–GI-1a). The Younger Dryas period (GS-1) is marked by a return to cold conditions with Tjul of about 10°C during a first phase, then 13°C in its terminal part. A probable increase in the duration of the ice-cover may have favoured arctic and alpine diatom species. The early-Holocene climate improvement is marked by a rise in Tjul of about 3°C.

Climate change since the last glacial period in Lebanon and the persistence of Mediterranean species

Abstract In this study, we quantified the mean January temperature (Tjan) and both winter (Pw) and summer (Ps) precipitation from three fossil pollen records from Lebanon. Tjan showed a strong correlation with the global temperature changes retrieved in the NGRIP Greenland ice core. The amplitude of ca. 8 °C between the Younger Dryas (YD) period and the Holocene is coherent with climate reconstructions from the Eastern Mediterranean. The overall amount of precipitation was also lower during the YD than during the Holocene but the contrast between Pw and Ps was much more reduced (less than 2 times) during the YD than during the Holocene (up to 8 times). Such different seasonal contrast compare to the present day is coherent with some climate proxies from the Levant that tend to indicate the presence of moisture during the last glacial period. In effect, the low Pw during the YD reflects the replacement of the forest ecosystem by a more shrubby or herbaceous vegetation. Concomitantly, the occurrence of an amount of precipitation higher than the current one during the summer season, along with a reduced evaporation, due to lower temperature, may have contributed to some local observed high lake levels in the area. During the last glacial period, Lebanon was not under a typical Mediterranean climate such as the one we know today, i.e. with a strong precipitation and temperature contrast between summer and winter seasons, but rather under a less contrasted climate. Mediterranean species persisted in this area due to the low amplitude of temperature change between the last glacial period and the Holocene as well as to an availability of moisture throughout the year instead of an occurrence mainly during the winter season as is the case today.

Sea surface temperature variation during the last deglaciation in the southern Okinawa Trough: Modulation of high latitude teleconnections and the Kuroshio Current

Three paleotemperature records (foraminiferal Mg/Ca, TEX86 and UK′37) were generated for the past 17.3ka in the southern Okinawa Trough (OT) using a sediment core (OKT-3) taken at a water depth of 1792m. Temperature estimates based on the inorganic and organic indices for the OKT-3 core top sample approximated modern sea surface temperature (SST) and extended to the mixed layer (<50m) in warmer seasons. Reconstructed SSTs from OKT-3 gradually increased towards the top of the core with lower values occurring during Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). These values coincided with the glacial–interglacial cycles established by the Globigerinoides ruber δ18O curve. Notably, SSTs decreased during the BØlling-AllerØd (B-A) but warming occurred before and after the B-A, from approximately 16 to 14ka and 12.5 to 10ka. The SST cooling during the B-A corresponded to the timing of the Antarctic Cold Reversal (ACR), which is thought to be connected with melt water pulse 1a (MWP1a). The cooling was also concurrent with the spread of millennial-scale cold signals to the OT during the HS1 and YD periods, when widespread melt and collapse of Northern Hemisphere ice sheets occurred along with reduced Atlantic Meridional Overturning Circulation (AMOC) and a southward shift of the Intertropical Convergence Zone (ITCZ). The last deglaciation, which began approximately 16–15ka BP in the OT, might mark the time when the Kuroshio Current (KC) began to strengthen.

Evidence for meltwater drainage via the St. Lawrence River Valley in marine cores from the Laurentian Channel at the time of the Younger Dryas

Debate is ongoing about the source(s) and paths of meltwater that drained into the North Atlantic Ocean at the time of the Younger Dryas (YD), especially the eastern route from glacial Lake Agassiz and predecessor lakes of the Laurentian Great Lakes located along the southeastern edge of the Laurentide Ice Sheet. Here, evidence is presented for meltwater drainage via the St. Lawrence eastern route from five new sediment cores from Cabot Strait, Laurentian Channel and Scotian Shelf at the time of the YD. Palynological analyses are used to reconstruct sea surface conditions based on dinoflagellate cyst records, and pollen data are used for additional correlation. The reconstructions show distinct drops in salinity and temperature and increased sea ice cover duration within the YD period. In addition to these new records, we present a re-examination of original data and paleoceanographic interpretation of surface waters based on a new analysis of dinoflagellate cyst zonation in combination with an updated chronology supported by new radiocarbon dates and refined calibrations. The results clearly define the YD core intervals which contain strong evidence of lowered salinity, thereby re-establishing the St. Lawrence drainage system as a significant route for inflow of YD meltwater to the North Atlantic. This inflow does not exclude the possibility for another source of freshwater, as suggested by geographical differences in the duration of cold, low salinity conditions west and east of Laurentian Channel associated with the YD climatic event.