High-resolution chronology for deglaciation of the Patagonian Ice Sheet at Lago Buenos Aires (46.5°S) revealed through varve chronology and Bayesian age modelling. (1st December 2017)

• Record Type: Journal Article
• Authors: Bendle, Jacob M.; Palmer, Adrian P.; Thorndycraft, Varyl R.; Matthews, Ian P.
• Is Part Of: Quaternary science reviews. Volume 177(2017)
• Page Start: 314
• Page End: 339
• Publication Date: 2017-12-01
• Subjects: Deglaciation -- Palaeoglaciology -- Patagonia -- Varve chronology -- Tephrochronology -- Bayesian age-modelling; Geology, Stratigraphic -- Quaternary -- Periodicals; Stratigraphie -- Quaternaire -- Périodiques; 551.79
• DOI: 10.1016/j.quascirev.2017.10.013
• Languages: English
• ISSNs: 0277-3791

Abstract

Abstract: Glaciolacustrine varves offer the potential to construct continuous, annually-resolved chronologies for ice-sheet deglaciation, and improved understanding of glacier retreat dynamics. This paper investigates laminated glaciolacustrine sediments deposited around the waning margins of the Patagonian Ice Sheet, following the local Last Glacial Maximum (LGM). Detailed macro- and microfacies analyses confirm an annual (varve) structure within these sediments. The correlation of annual layers (varves) across five sites in eastern Lago Buenos Aires yields a 994 ± 36 varve-year (vyr) chronology and thickness record. The floating chronology has been anchored to the calendar-year timescale through identification of the Ho tephra (17, 378 ± 118 cal a BP) in the varve sequences. Using a Bayesian age model to integrate the new varve chronology with published moraine ages, the onset of deglaciation at 46.5°S is dated to 18, 086 ± 214 cal a BP. New age estimates for deglacial events are combined with high-resolution analysis of varve thickness trends, and new lithostratigraphic data on ice-margin position(s), to reconstruct ice-margin retreat rates for the earliest ca. 1000 years of ice-sheet demise. Glacier retreat rates were moderate (5.3–10.3 m yr −1 ) until 17, 322 ± 115 cal a BP, but subsequently accelerated (15.4–18.0 m yr −1 ). Sustained influxes of ice-rafted debris (IRD) after 17, 145 ± 122 cal a BP suggest retreat rates were enhanced by calving after ice contracted into deeper lake waters. Ice persisted in eastern Lago Buenos Aires until at least 16, 934 ± 116 cal a BP, after which the glacier started to retreat towards the Patagonian mountains. Highlights: Glaciolacustrine varves were deposited around the waning Patagonian Ice Sheet at Lago Buenos Aires (46.5°S). Varve counting and tephrochronology yields a calendar-year chronology for the onset of Patagonian Ice Sheet deglaciation. At 46.5°S, ice sheet deglaciation after the local Last Glacial Maximum (LGM) commenced at 18, 086 ± 214 cal a BP. Ice sheet retreat rates were modest until 17, 322 ± 115 cal a BP, but then accelerated, in part influenced by calving. Ice persisted close to its local LGM limit until at least 16, 934 ± 116 cal a BP.