U–Th and 10 Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia

Antoine Cogez , Frédéric Herman (1) , Eric Pelt (2) , Thierry Reuschlé (3) , Gilles Morvan (4) , Christopher Darvill , Kevin Norton , Marcus Christl (5) , Lena Märki , François Chabaux (2)

1 Institute of Earth Sciences
2 LHyGeS - Laboratoire d'Hydrologie et de Géochimie de Strasbourg
3 IPGS-GE - Géophysique expérimentale (IPGS)
4 CGS - Centre de géochimie de la surface
5 ETH Zürich - Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich]

The estimation of sediment transfer times remains a challenge to our understanding of sediment budgets and the relationships between erosion and climate. Uranium (U) and thorium (Th) isotope disequilibria offer a means of more robustly constraining sediment transfer times. Here, we present new uranium and tho-rium disequilibrium data for a series of nested moraines around Lago Buenos Aires in Argentine Patagonia. The glacial chronology for the area is constrained using in situ cosmogenic 10 Be analysis of glacial outwash. Sediment transfer times within the periglacial domain were estimated by comparing the deposition ages of moraines to the theoretical age of sediment production, i.e., the comminution age inferred from U disequilibrium data and recoil loss factor estimates. Our data show first that the classical comminution age approach must include weathering processes accounted for by measuring Th disequilibrium. Second, our combined data suggest that the pre-deposition history of the moraine sediments is not negligible, as evidenced by the large disequilibrium of the youngest moraines despite the equilibrium of the corresponding glacial flour. Monte Carlo simulations suggest that weathering was more intense before the deposition of the moraines and that the transfer time of the fine sediments to the moraines was on the order of 100-200 kyr. Long transfer times could result from a combination of long sediment residence times in the proglacial lake (recurrence time of a glacial cycle) and the remobilization of sediments from moraines deposited during previous glacial cycles. 10 Be data suggest that some glacial cycles are absent from the preserved moraine record (seemingly every second cycle), supporting a model of reworking moraines and/or fluctuations in the extent of glacial advances. The chronological pattern is consistent with the U-Th disequilibrium data and the 100-200 kyr transfer time. This long transfer time raises the question of the proportion of freshly eroded sediments that escape (or not) the proglacial environments during glacial periods.

Domaines: Géophysique [physics.geo-ph]

Antoine Cogez, Frédéric Herman, Eric Pelt, Thierry Reuschlé, Gilles Morvan, et al.. U–Th and 10 Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia. Earth Surface Dynamics, 2018, 6 (1), pp.121-140. ⟨10.5194/esurf-6-121-2018⟩. ⟨hal-02376396⟩