Cosmogenic ¹°Be chronology for the last deglaciation of the southern Scandinavian Ice Sheet

The goal of this dissertation is to develop a chronology of the retreat of the southern margin of the Scandinavian Ice Sheet (SIS) during the late Pleistocene using surface exposure dating with cosmogenic 10Be. A sequence of seven prominent moraines in northeastern Europe (the Leszno Moraine, the Pomeranian Moraine, the Middle Lithuanian Moraine, the North Lithuanian Moraine, the P andivere Moraine, the Palivere Moraine, and the Salpausselka I Moraine) indicates a potential millennialscale record of climate variability. However, there are no direct dating constraints on the timing of this ice sheet retreat. Surface exposure dating using cosmogenic nuclide elements produced by secondary high-energy particles in the upper part of the lithosphere allows for direct dating of moraines deposited by the SIS. Cosmogenic 10Be concentrations were measured in quartz sampled from erratic boulders deposited on top of the moraines. The SIS margin withdrew from its maximum extent at 17,9 ± 2,4 10Be ka, which is correlative to the end of the Last Glacial Maximum period (23,0 to 19,0 ka). Following a recessional period, the ice readvanced to the Pomeranian Moraine. Deglaciation from this moraine is dated at 14,1 ± 1,8 10Be ka, and subsequent deglaciation is marked by the Middle Lithuanian Moraine (13,1 ± 1,7 "Be ka), the North Lithuanian Moraine (13,1 ± 1,7 10Be ka), the Pandivere Moraine (13,0 ± 1,1 10Be ka), the Palivere Moraine (10.0 ± 1,3 10Be ka), and the Salpausselka I Moraine (12,5 ± 1,5 10Be ka). This new chronology is compared to abrupt climate variability recorded in the North Atlantic region during the late Pleistocene. Paleo air temperatures recorded from the GISP2 ice core and paleo sea surface temperatures recorded from the North Atlantic region display warmings at the end of the Last Glacial Maximum period, during the Balling-Allerod interstadial, and at the end of the Younger Dryas stadial. These records also display abrupt coolings during the Oldest Dryas and the Younger Dryas stadials. Based on the 10Be chronology, the margin of the SIS responded to climate changes that occurred in the North Atlantic region. This new chronology contributes the first direct evidence of climate forcing on the southern margin of the SIS

A multiple dating-method approach applied to the Sanabria Lake moraine complex (NW Iberian Peninsula, SW Europe)

New evidence in the NW region of the Iberian Peninsula (c. 42º N 6 ºW) of a glacial advance coeval with the global Last Glacial Maximum (LGM) of the Marine Isotope Stage 2 has been identified through a dataset of exposure ages based on 23 10Be concentration measurements carried out on boulder samples taken from a set of latero-frontal moraines. Results span the interval 19.2e15.4 10Be ka, matching the last deglaciation period when Iberia experienced the coldest and driest conditions of the last 25 ka, and are consistent with Lateglacial chronologies established in other mountain regions from SW Europe. The extent of the LGM stade identified in this work is similar to the local maximum ice extent stade recorded and dated as prior to 33 ka using radiocarbon and optically stimulated luminescence. This work showcases how multiple-dating approaches and detailed geomorphological mapping are required to reconstruct realistic palaeoglacier evolution models

Implications of 36Cl exposure ages from Skye, northwest Scotland for the timing of ice stream deglaciation and deglacial ice dynamics

The French national AMS facility ASTER (CEREGE, Aix en Provence) is supported by the INSU/CNRS, the ANR through the "Projets thématiques d’excellence" program for the "Equipements d’excellence" ASTER-CEREGE action, IRD and CEA. The authors would like to thank Shasta Marrero for helpful and informative discussion on the CRONUScalc online calculator. DS was supported by a SAGES studentship and fieldwork by funds from the QRA and BSG.Geochronological constraints on the deglaciation of former marine based ice streams provide information on the rates and modes by which marine based ice sheets have responded to external forcing factors such as climate change. This paper presents new 36Cl cosmic ray exposure dating from boulders located on two moraines (Glen Brittle and Loch Scavaig) in southern Skye, northwest Scotland. Ages from the Glen Brittle moraines constrain deglaciation of a major marine terminating ice stream, the Barra-Donegal Ice Stream that drained the former British-Irish Ice Sheet, depending on choice of production method and scaling model this occurred 19.9 ± 1.5–17.6 ± 1.3 ka ago. We compare this timing of deglaciation to existing geochronological data and changes in a variety of potential forcing factors constrained through proxy records and numerical models to determine what deglaciation age is most consistent with existing evidence. Another small section of moraine, the Scavaig moraine, is traced offshore through multibeam swath-bathymetry and interpreted as delimiting a later stillstand/readvance stage following ice stream deglaciation. Additional cosmic ray exposure dating from the onshore portion of this moraine indicate that it was deposited 16.3 ± 1.3–15.2 ± 0.9 ka ago. When calculated using the most up-to-date scaling scheme this time of deposition is, within uncertainty, the same as the timing of a widely identified readvance, the Wester Ross Readvance, observed elsewhere in northwest Scotland. This extends the area over which this readvance has potentially occurred, reinforcing the view that it was climatically forced.PostprintPeer reviewe

Atmospheric drying as the main driver of dramatic glacier wastage in the southern Indian Ocean

This study was funded by IPEV-1048 GLACIOCLIM-KESAACO and LEFE-INSU KCRuMBLE programs, and the Agence Nationale de la Recherche through contract ANR-14-CE01-0001-01 (ASUMA).The ongoing retreat of glaciers at southern sub-polar latitudes is particularly rapid and widespread. Akin to northern sub-polar latitudes, this retreat is generally assumed to be linked to warming. However, no long-term and well-constrained glacier modeling has ever been performed to confirm this hypothesis. Here, we model the Cook Ice Cap mass balance on the Kerguelen Islands (Southern Indian Ocean, 49°S) since the 1850s. We show that glacier wastage during the 2000s in the Kerguelen was among the most dramatic on Earth. We attribute 77% of the increasingly negative mass balance since the 1960s to atmospheric drying associated with a poleward shift of the mid-latitude storm track. Because precipitation modeling is very challenging for the current generation of climate models over the study area, models incorrectly simulate the climate drivers behind the recent glacier wastage in the Kerguelen. This suggests that future glacier wastage projections should be considered cautiously where changes in atmospheric circulation are expected.Publisher PDFPeer reviewe

The Ponto-Caspian basin as a final trap for southeastern Scandinavian Ice-Sheet meltwater

This paper provides new data on the evolution of the Caspian Sea and Black Sea from the Last Glacial Maximum until ca. 12 cal kyr BP. We present new analyses (clay mineralogy, grain-size, Nd isotopes and pollen) applied to sediments from the river terraces in the lower Volga, from the middle Caspian Sea and from the western part of the Black Sea. The results show that during the last deglaciation, the Ponto-Caspian basin collected meltwater and fine-grained sediment from the southern margin of the Scandinavian Ice Sheet (SIS) via the Dniepr and Volga Rivers. It induced the deposition of characteristic red-brownish/chocolate-coloured illite-rich sediments (Red Layers in the Black Sea and Chocolate Clays in the Caspian Sea) that originated from the Baltic Shield area according to Nd data. This general evolution, common to both seas was nevertheless differentiated over time due to the specificities of their catchment areas and due to the movement of the southern margin of the SIS. Our results indicate that in the eastern part of the East European Plain, the meltwater from the SIS margin supplied the Caspian Sea during the deglaciation until ∼13.8 cal kyr BP, and possibly from the LGM. That led to the Early Khvalynian transgressive stage(s) and Chocolate Clays deposition in the now-emerged northern flat part of the Caspian Sea (river terraces in the modern lower Volga) and in its middle basin. In the western part of the East European Plain, our results confirm the release of meltwater from the SIS margin into the Black Sea that occurred between 17.2 and 15.7 cal kyr BP, as previously proposed. Indeed, recent findings concerning the evolution of the southern margin of the SIS and the Black Sea, show that during the last deglaciation, occurred a westward release of meltwater into the North Atlantic (between ca. 20 and 16.7 cal kyr BP), and a southward one into the Black Sea (between 17.2 and 15.7 cal kyr BP). After the Red Layers/Chocolate Clays deposition in both seas and until 12 cal kyr BP, smectite became the dominant clay mineral. The East European Plain is clearly identified as the source for smectite in the Caspian Sea sediments. In the Black Sea, smectite originated either from the East European Plain or from the Danube River catchment. Previous studies consider smectite as being only of Anatolian origin. However, our results highlight both, the European source for smectite and the impact of this source on the depositional environment of the Black Sea during considered period

Moraine crest or slope: An analysis of the effects of boulder position on cosmogenic exposure age

Terrestrial cosmogenic nuclide dating of ice-marginal moraines can provide unique insights into Quaternary glacial history. However, pre- and post-depositional exposure histories of moraine boulders can introduce geologic uncertainty to numerical landform ages. To avoid geologic outliers, boulders are typically selected based on their depositional context and individual characteristics but while these criteria have good qualitative reasoning, many have not been tested quantitatively. Of these, boulder location is critical, as boulders located on moraine crests are prioritised, while those on moraine slopes are typically rejected. This study provides the first quantitative assessment of the relative utility of moraine crest and moraine slope sampling using new and published 10Be and 36Cl ages (n = 19) and Schmidt hammer sampling (SH; n = 635 moraine boulders, ∼19,050 SH R-values) in the northern and southern Pyrenees. These data show that for many of the studied moraines, the spatial distribution of “good” boulders is effectively random, with no consistent clustering on moraine crests, ice-proximal or -distal slopes. In turn, and in contrast to prior work, there is no clear penalty to either moraine crest or moraine slope sampling. Instead, we argue that landform stability exerts a greater influence on exposure age distributions than the characteristics of individual boulders. For the studied landforms, post-depositional stability is strongly influenced by sedimentology, with prolonged degradation of matrix-rich unconsolidated moraines while boulder-rich, matrix-poor moraines stabilised rapidly after deposition. While this pattern is unlikely to hold true in all settings, these data indicate that differences between landforms can be more significant than differences at the intra-landform scale. As ad hoc assessment of landform stability is extremely challenging based on geomorphological evidence alone, preliminary SH sampling, as utilised here, is a useful method to assess the temporal distribution of boulder exposure ages and to prioritise individual boulders for subsequent analysis

Ice-margin and meltwater dynamics during the mid-Holocene in the Kangerlussuaq area of west Greenland

Land-terminating parts of the west Greenland ice sheet have exhibited highly dynamic meltwater regimes over the last few decades including episodes of extremely intense runoff driven by ice surface ablation, ponding of meltwater in an increasing number and size of lakes, and sudden outburst floods, or 'jökulhlaups', from these lakes. However, whether this meltwater runoff regime is unusual in a Holocene context has not been questioned. This study assembled high-resolution topographical data, geological and landcover data, and produced a glacial geomorphological map covering ~1200 km2. Digital analysis of the landforms reveals a mid-Holocene land-terminating ice margin that was predominantly cold-based. This ice margin underwent sustained active retreat but with multiple minor advances. Over c. 1000 years meltwater runoff became impounded within numerous and extensive proglacial lakes and there were temporary connections between some of these lakes via spillways. The ice-dams of some of these lakes had several quasi-stable thicknesses. Meltwater was apparently predominantly from supraglacial sources although some distributary palaeochannel networks and some larger bedrock palaeochannels most likely relate to mid-Holocene subglacial hydrology. In comparison to the geomorphological record at other Northern Hemisphere ice-sheet margins the depositional landforms in this study area are few in number and variety and small in scale, most likely due to a restricted sediment supply. They include perched fans and deltas and perched braidplain terraces. Overall, meltwater sourcing, routing and the proglacial runoff regime during the mid-Holocene in this land-terminating part of the ice sheet was spatiotemporally variable, but in a manner very similar to that of the present day

Abrupt changes of temperature and water chemistry in the late Pleistocene and early Holocene Black Sea

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q01004, doi:10.1029/2007GC001683.New Mg/Ca, Sr/Ca, and published stable oxygen isotope and 87Sr/86Sr data obtained on ostracods from gravity cores located on the northwestern Black Sea slope were used to infer changes in the Black Sea hydrology and water chemistry for the period between 30 to 8 ka B.P. (calibrated radiocarbon years). The period prior to 16.5 ka B.P. was characterized by stable conditions in all records until a distinct drop in δ 18O values combined with a sharp increase in 87Sr/86Sr occurred between 16.5 and 14.8 ka B.P. This event is attributed to an increased runoff from the northern drainage area of the Black Sea between Heinrich Event 1 and the onset of the Bølling warm period. While the Mg/Ca and Sr/Ca records remained rather unaffected by this inflow; they show an abrupt rise with the onset of the Bølling/Allerød warm period. This rise was caused by calcite precipitation in the surface water, which led to a sudden increase of the Sr/Ca and Mg/Ca ratios of the Black Sea water. The stable oxygen isotopes also start to increase around 15 ka B.P., although in a more gradual manner, due to isotopically enriched meteoric precipitation. While Sr/Ca remains constant during the following interval of the Younger Dryas cold period, a decrease in the Mg/Ca ratio implies that the intermediate water masses of the Black Sea temporarily cooled by 1–2°C during the Younger Dryas. The 87Sr/86Sr values drop after the cessation of the water inflow at 15 ka B.P. to a lower level until the Younger Dryas, where they reach values similar to those observed during the Last Glacial Maximum. This might point to a potential outflow to the Mediterranean Sea via the Sea of Marmara during this period. The inflow of Mediterranean water started around 9.3 ka B.P., which is clearly detectable in the abruptly increasing Mg/Ca, Sr/Ca, and 87Sr/86Sr values. The accompanying increase in the δ 18O record is less pronounced and would fit to an inflow lasting ∼100 a.This research was funded by the DFG grants LA 1273/2-1, LA 1273/2, and WE 992/47-3. RCOM 0517

Late Glacial deglaciation of the Zackenberg area, NE Greenland

The Greenland Ice Sheet (GrIS) is a key component of the global climate system. However, our current understanding of the spatio-temporal oscillations and landscape transformation of the GrIS margins since the last glacial cycle is still incomplete. The objective of this work is to study the deglaciation of the Zackenberg Valley (74°N, 20°E), NE Greenland, and the origin of the derived landforms. Based on extensive fieldwork and high-detail geomorphological mapping we identified the different types of landforms, from which those of glacial and paraglacial origin were used to understand the processes driving regional environmental evolution. We applied cosmic-ray exposure (CRE) dating to 32 samples taken from erosive and depositional glacial landforms distributed across the valley. Geomorphological evidence shows that >800-m-thick Late Quaternary glacier filled the valleys and fjords and covered mountain summits. In subsequent phases, as ice thickness decreased, the glacier was limited to the interior of the valley, leaving several lateral moraines. The deglaciation of the Zackenberg Valley that started by ~13.7–12.5 ka also accelerated slope paraglacial processes. Many blocks from lateral moraines were remobilized and fell, reaching the valley floor where they covered the thinning glacier tongue; transforming it into a debris-covered glacier that subsequently melted gradually. By ca. 10.5 ka, the last remnants of glacial ice disappeared from the Zackenberg Valley floor, a chronology of deglaciation that is similar to that observed in other sites across NE Greenland. The results of this work must be considered in similar studies, reinforcing the need to support CRE ages of the different geomorphological phases with paleoclimatic data from other sedimentary records