Timescale-Dependent Glacially-Derived Sediment Yields and Erosion Rates in Lago Argentino (Patagonia, Argentina)

Abstract:

Tectonically-active and glaciated mountain ranges experience a complex geomorphic response to the interaction of glaciers with tectonic and isostatic stresses. Quantifying the components of these interactions – rates of tectonic uplift, fluctuations in ice mass, and volumes of glacial bedrock erosion and sediment redistribution – on a variety of timescales is critical to understanding the role of glaciers in landscape evolution. In this study, we address first-order constraints on sediment yields and glacial erosion rates averaged over 10¹-10³ yr timescales by examining the subaqueous basins of Lago Argentino, a proglacial lake east of the Southern Patagonian Icefield (SPI) that has been accumulating sediment from eight alpine lake glaciers since ice retreated from its Last Glacial Maximum extent ~21 kyrs BP. We present erosion rates derived near present-day glacier frontal positions (i.e., short term <10² yr estimates), as well as those from distal sub-basins encompassing longer term (>10³ yr) sediment yields and erosion rates. Effective erosion rates are estimated from annual sediment yields, which in turn are obtained from analysis of glaciolacustrine sediment stratigraphy and sedimentary facies in proglacial lakes. For this study, we use 350 km of newly acquired single-channel seismic CHIRP data imaging soft sediment with decimeter-scale vertical resolution to a depth of 50 m. Data were frequency filtered, depth converted, and gain corrected. We interpret four seismic facies that represent different stages of the glacial evolution of the basin. Within the glaciolacustrine sediment, we observe distinctive reflectors that correlate ubiquitously across systems of moraines and are datable by gravity cores retrieved as part of the project, or by cross relationship with known features (i.e., moraines). The shallowest of these reflectors is the top of a high amplitude double reflector (i.e., a “doublet”) that shows consistent 1 m thickness and 4-7 m depth below lake floor. We interpret this marker as the older of a sequence of tephra deposits preliminarily dated ~1300 yrs BP based on varve stratigraphy in cores. This analysis represents the first step in determining the magnitude and timing of bedrock load removed from the Andes by glacial activity in the SPI.