U-Th and <sup>10</sup>Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia

Abstract

Abstract. 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 Thorium disequilibrium data for a series of nested moraines around Lago Buenos Aires in Argentine Patagonia, as well as a refined glacial chronology for the area using in situ cosmogenic 10Be analysis from 5 corresponding 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 10 is not negligible, as evidenced by the large disequilibrium of the youngest moraines despite the equilibrium of 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 ka. Long transfer times could result from a combination of long sediment residence times 1 Earth Surf. Dynam. Discuss., https://doi

The Influence of Climate on the Dynamics of Mountain Building Within the Northern Patagonian Andes

“…With the exception of prior exposure to cosmic rays, which results in an overestimate of the true age of the sampled landform, geological factors like erosion generally reduce cosmogenic nuclide concentrations in rock surfaces, resulting in apparent surface exposure ages that underestimate the true depositional age of the sampled landform. In this regard, the oldest sample (Sample CA14: 1,313 ± 37 ka [±144 ka external]) is considered closest to the age of deposition, because nuclide inheritance is expected to be low in outwash sediment of Patagonia (Cogez et al., 2018; Hein et al., 2009), and because of field evidence for deflation, which could potentially reduce the exposure age recorded by the sample. If the arithmetic mean of the two oldest samples is considered as the best approximation to the timing of deposition, then the formation of the Caleufu outwash dates to 1,199 ± 161 ka.…”

Extensive mountain glaciation in central Patagonia during Marine Isotope Stage 5

“…Furthermore, the outwash cobbles rooted in the terraces are subject to less wind erosion, which is a significant source of boulder erosion in Patagonia. The outwash method has proved effective at dating pre-LGM glacial advances as old as one million years in this region (Hein et al, 2009(Hein et al, , 2011(Hein et al, , 2017Darvill et al, 2015;Cogez et al, 2018). To develop our chronology, we measured cosmogenic 10 Be in both moraine boulders and glacio-fluvial outwash cobbles.…”

The evolution of the Patagonian Ice Sheet from 35 ka to the Present Day (PATICE)

“…The best evidence for Mid-Pleistocene and pre-LGM glaciation in Lago GCBA is derived from the lateral moraines; terminal moraines here are heavily eroded by outwash and meltwater systems, which were funnelled along the Moreno scarp and down the Deseado River. During the last glacial cycle, the Local LGM ice extent was reached at ca 35 to 30 ka, well inside the Pleistocene maximum extent (Hein et al, 2009(Hein et al, , 2010Cogez et al, 2018). The LGM terminal moraines for Lago GCBA lie at 500 to 600 m asl.…”

“…Large tracts of icescoured and sometimes streamlined bedrock are present along major ice discharge routes to the east of the contemporary icefields, as well as in and around the fjord areas of Patagonia (Glasser and Ghiglione, 2009;. Glacial outwash plains (sandur) and meltwater channels are widespread in Patagonia, especially east of the Andes where they are associated with the large terminal moraine complexes (Cogez et al, 2018;Bendle et al, 2017b;Hein et al, 2009Hein et al, , 2011Smedley et al, 2016) (Fig. 7).…”

“…Casassa et al, 1997). More recently, the application of innovative forms of cosmogenic nuclide exposure-age dating (Ackert et al, 2008;Douglass et al, 2006;García et al, 2012;Hein et al, 2011;Kaplan et al, 2005Kaplan et al, , 2011Cogez et al, 2018), cosmogenic nuclide depth profiles (Darvill et al, 2015b;Hein et al, 2009), optically stimulated luminescence dating (Blomdin et al, 2012;Smedley et al, 2016;García et al, 2019), tephrochronology (Kilian et al, 2003;Stern, 2008;Stern et al, 2016;Weller et al, 2015) and varve-age dating (Bendle et al, 2017a(Bendle et al, , 2019 has resulted in new insights into the timing and dynamics of both ice-sheet behaviour and palaeolake development.…”