Application of in situ cosmogenic nuclides in dating glacial till : research potential, limitations and prospects

Determination of numerical age for glacial till is one of the most intriguing issues in glacial geology and geomorphology. Till layers are unequivocal evidence of ice sheets advances and retreats, and therefore they are very important for Quaternary palaeoclimatic reconstructions. Herein, we present a review dedicated to the methodology of glacial till dating with in situ cosmogenic nuclides. This relatively novel approach enables direct dating of deposition of till and its exposure after deglaciation. Getting reliable numerical age (in years) of till opens a new prospect for reconstructions of past glaciations and palaeogeographic studies. Despite some limitations of applying in situ cosmogenic exposure and burial dating, we argue that this technique may be successfully employed in studies of glacial deposits in Poland

Erratics selection for cosmogenic nuclide exposure dating – an optimization approach

International audienceThe paper presents a method for the selection of large erratics to be sampled for terrestrial cosmogenic nuclide exposure dating (TCNED) in areas previously covered by Pleistocene ice sheets. Our approach is based on (1) a GIS analysis of an extensive dataset of erratics, (2) field inspection of pre-selected boulders and (3) Schmidt hammer (SH) testing of erratics selected for sampling. An initial database of 491 erratic boulders in NW Poland was filtered using a GIS software, based on their characteristics, digital elevation and surface geology. The secondary data set of pre-selected erratics consisted of 135 boulders \textendash i.e. proper targets for field inspection. Ground-truthing in the field resulted in the final selection of 63 boulders suitable for sampling for TCNED. These erratics are located on moraine plateaux and hills formed during the Saalian glaciation (Marine Isotope Stage 6) as well as Leszno/Brandenburg, Poznań/Frankfurt and Pomeranian Phase ice marginal belts from the Weichselian glaciation (Marine Isotope Stage 2). The GIS desk-based analysis of erratics properties resulted in a 73% reduction of the initial dataset, which demonstrates the added value of this selection technique. The field inspection of pre-selected boulders resulted in a 53% reduction of the number of boulders suitable for TCNED. SH testing of the sampled erratics provided a quantitative proxy of their surface hardness. This allowed the quantification of their weathering degree and identification of erratics potentially affected by postglacial erosion. Our systematic approach to selecting erratics and their SH testing could be a useful tool for other researchers facing the problem of choosing appropriate erratics for TCNED in areas of continental Pleistocene glaciations

Wielkie głazy narzutowe północno-zachodniej Polski i ich przydatność do datowania recesji ostatniego lądolodu skandynawskiego metodą izotopów kosmogenicznych

Północno-zachodnia (NW) Polska jest obszarem kluczowym dla uzupełnienia chronologii recesji ostatniego lądolodu skandynawskiego uzyskanych w rezultacie datownia metodą izotopów kosmogenicznych, które są obecnie dostępne w obszarze położonym na południe od Morza Bałtyckiego. Nowy projekt datowania kosmogenicznego (http://www.daterr.ug.edu.pl) w regionie perybałtyckim ma na celu doprowadzić do wypełnienia luki istnięjącej pomiędzy zapisem dostępnym w zachodniej (Dania, Niemcy) oraz we wschodniej (północno-wschodnia Polska, Litwa, Białoruś) części południowego sektora zasiegu ? ostatniego lądolodu skandynawskiego. Wiarygodne datowanie recesji dawnych lądolodów metodą nuklidów kosmogenicznych wymaga poboru próbek z przekształconych glacjalnie wychodni skalnych lub wielkich głazów narzutowych położonych in-situ na powierzchni wysoczyzn morenowych bądź moren czołowych. W obszarze zlokalizowanym na południe od Morza Bałtyckiego, gdzie na powierzchni terenu dominuje pokrywa osadowa, pochodzące z tarczy fennoskandzkiej wielkie narzutniaki są najbardziej odpowiednimi obiektami, z których można pobierać próbki do datowania kosmogenicznego. Prezentujemy wyniki inwentaryzacji wielkich głazów narzutowych Polski północno- zachodniej oraz kameralnej i terenowej weryfikacji ich przydatności do datowania metodą izotopu kosmogenicznego Be

Analysis of Pomeranian erratics suitability for the study of the last Scandinavian Ice Sheet recession with cosmogenic nuclides

Wielkie eratyki położone in situ na powierzchni form morenowych mogą być obiektami datowania wieku ekspozycji metodą ziemskich izotopów kosmogenicznych (ang. terrestrial cosmogenic nuclides – TCN). Środkowe oraz wschodnie Pomorze jest regionem kluczowym z punktu widzenia uzupełnienia kosmogenicznej chronologii recesji ostatniego lądolodu skandynawskiego w północnej Polsce. Artykuł prezentuje metodę oraz wyniki selekcji głazów narzutowych na tym obszarze, pod kątem ich przydatności do datowania wieku ekspozycji metodą TCN. Proces selekcji wielkich eratyków przebiegał dwuetapowo. W pierwszej kolejności, z wykorzystaniem wszelkich dostępnych informacji, skonstruowano bazę GIS wielkich eratyków (254 obiekty), a następnie na podstawie rozmiarów i lokalizacji na cyfrowym modelu terenu oraz mapach geologicznych wytypowano przydatne głazy. W drugim etapie dokonano weryfikacji terenowej wybranych obiektów i 27 eratyków zakwalifikowano do datowania wieku ekspozycji metodą TCN. Znaczna redukcja głazów na poszczególnych etapach selekcji pokazuje wagę zaproponowanego w artykule systematycznego procesu selekcji. Rozmieszczenie wytypowanych głazów (skupienie na powierzchni wysoczyzn morenowych lub akumulacyjnych form marginalnych) oraz ich cechy indywidualne dobrze rokują dla wyników datowania wieku ekspozycji metodą TCN.Large erratic boulders located in situ on moraines may be a target for surface exposure age dating with TCN (Terrestrial Cosmogenic Nuclides ). This paper presents the method and the results of the selection of erratics in Pomerania for this type of dating. Middle and eastern Pomerania are key regions to complete the cosmogenic chronologies of the geomorphology left by the last Scandinavian Ice Sheet retreat in the northern Polish landscape. Our selection of massive erratics consists of two stages. First, a GIS database (254 erratics) was constructed based on all available information about large boulders, and erratics were selected based on their dimensions and distribution against a digital elevation model and geologic maps. Second, field inspection of preliminarily selected boulders was conducted and 27 erratics were finally selected as suitable for surface exposure age dating with TCN. The significant reduction of the number of boulders at particular stages of qualification shows the importance of the proposed systematic selection. Spatial distribution of the selected boulders (location on moraine plateaux and paleo ice-marginal belts) and their individual features show their high usefulness for surface exposure age dating with TCN

Asynchronous dynamics of the last Scandinavian Ice Sheet along the Pomeranian ice-marginal belt: A new scenario inferred from surface exposure 10Be dating

International audienceWe present a new set of 25 10 Be surface exposure ages of boulders located on the Pomeranian moraines and of erratic boulders located directly upstream of the Pomeranian moraines in northern Poland. Together with recalculated 10 Be surface exposure ages along the Pomeranian ice-marginal belt from Denmark to the west to Lithuania and Belarus to the east, the full data set (n ¼ 86) enabled us to constrain the timing of the ice front standstill and its subsequent retreat. The investigated area consists of geomorphological record along ~2000 km of the ice margin associated with the ice sheet limit correlated so far with the Late Weichselian Pomeranian Phase (Baelthav in Denmark, Baltija in Lithuania and Braslav in Belarus). We constrained deposition of the Pomeranian ice-marginal formations to discrete intervals: (1) ~20 e19 ka in the area of the Baltic Ice Stream, (2) ~19e18 ka in the area of the Odra Ice Stream, (3) ~20e19 ka in the interstream zone between the Odra and the Vistula Ice Streams, (4) ~19e18 ka in the area of the Vistula Ice Stream, (5) ~18e17 ka in the area of the Mazury Ice Stream, (6) ~17e16 ka in the area of the Riga Ice Stream, and (7) ~16e15 ka in the area of the Novgorod Ice Stream. Our best age estimates are based on: (1) a minimum age of the ice margin retreat inferred from new and recalculated 10 Be ages of boulders as well as interpretation of available radiocarbon ages from organic deposits and OSL ages from sediments overlying tills, and (2) a maximum age of the ice margin stillstand and retreat inferred from interpretation of available OSL ages from sandur sediments deposited in front of the ice sheet. The asynchrony of the ice margin positions along the Pomeranian ice-marginal belt shows about 3e5 ka difference between the Baelthav ice margin in Denmark and the Braslav ice margin in Belarus. We propose a new scenario of the ice sheet evolution and a time-slice reconstruction of the last Scandinavian Ice Sheet's southern fringe for the period ~20e15 ka. We suggest that the term "Pomeranian Phase" should not be used but rather that the terms "Pomeranian moraines" or "Pomeranian ice-marginal belt" are adequate to describe geomorphological record of the palaeo-ice margin positions, which occurred during various phases of the last deglaciation

Fluvial response to environmental changes during MIS 4-3: a sedimentary record at the Brześnica site, central-western Poland

Fluvial deposits subjected to this study are exposed at the Brześnica site, in the south-western part of the Wielkopolska region in Poland, which was close to the ice-sheet limit during the Last Glacial Maximum (LGM). Sedimentological analyses, including lithofacies descriptions, palaeocurrent measurements, grain size and rounding analyses and heavy mineral compositions indicate that the fluvial deposits at Brześnica were deposited in braided river environment. The following fluvial sedimentary processes were inferred: (1) shallow, rapid flow alternating with waning flow, (2) infilling of erosional channels with fluvial sediments during flood events, (3) changes from supercritical to subcritical flows, and (4) shallow sheet floods. The results of OSL dating indicated sediment deposition ~65.2 ±1.5 ka, i.e. in MIS 4, that was here characterized by cold environmental conditions and a general shift from meandering to braided fluvial sedimentation style. This finding contrasts with accumulation/erosion phases interpreted previously in this region for that time interval; however, it is consistent with recent studies of fluvial systems functioning during MIS 5-2 and of factors responsible for sedimentation style

Northern Central Europe: glaciallandforms from the Younger DryasStadial

International audienc