Comparing modern ice extents of three southern Patagonian glaciers relative to their Holocene variations using the 14C-10Be geochronometer
Description
Here I quantify the duration that ice extent was bigger or smaller relative to today of three glaciers of southernmost Patagonia (50-55°S) based on measurements of in situ carbon-14 and beryllium-10 from recently exposed proglacial bedrock. I crowdsourced samples collected along the modern ice margins of Upsala Glacier (~50°S), a large valley glacier of the Southern Patagonia Icefield, Torres Glacier (~51°S), a small alpine glacier situated against the Torres del Paine massif, and Chato Glacier (~55°S), a small alpine glacier in Tierra del Fuego. Proglacial bedrock at Upsala Glacier yielded 10Be concentrations that indicate 3.8 ± 1.6 kyr of exposure, while the 14C concentrations suggest 3.1 ± 0.77 kyr of exposure; corresponding 14C-10Be ratios suggest at least one period of burial. At the Torres site, low 10Be concentrations suggest <0.3 kyr of exposure, while the 14C concentrations suggest 0.46 ± 0.25 kyr of exposure. The Chato site yielded mean 10Be and 14C exposure durations of 0.31 ± 0.12 kyr and 0.6-1.3 kyr, respectively. Elevated 14C-10Be ratios from the Torres (4-17) and Chato (5-37) ice margin exceed values that would result from continuous exposure. The elevated ratios at Torres and Chato could be the result of the following histories: 1) Holocene exposure at some time as suggested by Upsala samples, but a recent erosional event exposing deep muon produced nuclides, or 2) continuous ice cover, nuclide production under that cover, and then, recent exposure. I explore the scenarios and look at climatic reasons for the observed results. I suggest that Upsala Glacier was previously smaller than at present during the Holocene, while Torres and Chato Glacier have receded past their minimum Holocene extent. The inferred exposure ages of the sites are tightly correlated with the elevation of our sample transects. The difference in exposure and burial histories may indicate that smaller mountain glaciers at higher elevations of southern Patagonia have reached a minimum Holocene length due to modern climate change, while the extents of larger outlet glaciers at lower elevations fluctuated more dynamically to climate change within the Holocene.