OGS Core Logging Lab - logging sediment cores in Lago Argentino (Brazo Sur), Argentina

The spectacular water outburst occurring semi-periodically when the ice-dam formed by the external front of the Perito Moreno glacier collapses, is one of the most attracting events in the UNESCO ‘Parque Nacional Los Glaciares’ of southern Patagonia. These occurrences have been documented since 1936. Instead, evidence of previous events has been only indirectly provided by dendrochronology analysis. Four sediments cores have been collected on coastal soil in 2017, analysed by X rays, HR photography and Magnetic Susceptibility. The radiographies of these cores allowed to identify lake floodings deposits due to glacier readvance over the coastal soil related to the collapse of the Perito Moreno ice-dam. In November 2018, 10 undisturbed sediment gravity cores were collected within a small inlet of Brazo Sur, that is, the southern arm of Lago Argentino, at water depths ranging from 10 to 6 m using a 4.5 cm diameter gravity corer ‘KC Kajak Sediment Sampler’ Model 13.030.

The length of these cores varies from 45 to 65 cm. X rays, HR photography and magnetic susceptibility provide the first evidence of an abrupt change in the stratigraphic record found at variable depths of 14–18 cm from the top of the cores, marked by a hiatus spanning ca. 3200 years, separating planar-laminated sediments below from an alternation of erosional and depositional events above it, indicating recurring high-energy conditions generated by the emptying of the lake basin, as well as ash layers observed in the longest cores. Radio carbon data collected on three of these cores record ice-daming in the Little Ice age, at 324-266 cal yrs BP. These well-preserved stratigraphic records highlight the key role of glaciolacustine deposits in reconstructing the glacial dynamics and palaeoclimate evolution of a glaciated region.