EP029-0003
Composition and Seasonal Structure of Glaciolacustrine Varves in Wind-Driven Lago Argentino
Thursday, 10 December 2020
Poster
Emi Ito, University of Minnesota Twin Cities, Earth and Environmental Sciences and Limnological Research Center, Minneapolis, MN, United States, Mark Shapley, University of Minnesota, Earth and Environmental Sciences, and the Continental Scientific Drilling and Coring facility, Minneapolis, MN, United States, Maximillian S. Van Wyk de Vries, University of Minnesota, Department of Earth and Environmental Sciences and Limnological Research Center, Minneapolis, MN, United States and Guido Brignone, Universidad Nacional de Córdoba, Centro de Investigaciones en Ciencias de la Tierra (CICTERRA-CONICET), Córdoba, Argentina
Abstract:
A mechanistic understanding of annual sediment composition and grain size change is key to support varve-sequence chronology. We use X-ray fluorescence (XRF) and laser-diffraction particle size analysis (LD-PSA) to analyze sub-mm scale sediment variability in Lago Argentino, a 1415 km
2 ultra-oligotrophic proglacial lake located in Southern Patagonia. We selected 24 intervals from 18 characteristic sites for high-resolution (200-500 mm) XRF and coupled lamina-scale LD-PSA from 47 lake cores distributed throughout the lake basin. High-resolution XRF intervals were selected from well-preserved cores representing the major lake depositional settings. Where core structures are interpreted as varves, high-resolution scans encompass 15-30 annual dark/light couplets. XRF data sets are nested within core-length scans at lower spatial resolution (5-10 mm), providing broader down-core compositional context. XRF and LD-PSA are further supported by petrographic smear slide observations.
Annual couplets are exhibit systematically varying grain size and Ca:K. Light-colored laminae are finer grained and carry a relatively high-K signal attributed to K-bearing clays. Darker laminae are coarser grained and carry a relatively Ca-rich signal attributable to feldspars and possibly Ca-bearing pyroxenes. Color phasing of Lago Argentino laminae is inverted relative to the light-summer, dark-winter norm for glacio-lacustrine varves. With the exception of shoreline embayments, Lago Argentino lacks seasonal ice cover, and experiences a vigorous wind regime with a pronounced summertime (Nov-Jan) maximum. We therefore interpret dark-phase laminae to represent summertime deposition under conditions of both maximum glacial melt and strong wind-driven mixing. Light-phase winter laminae reflect both seasonally reduced sediment supply, and less-energetic suspension and in-lake transport. While differing from the clastic varve norm, this distinct annual change in sediment properties enables varve-sequence chronology of the Lago Argentino cores, and the dependent analyses of sediment mass-transfer rates and crustal loading sought by the GUANACO project (Van Wyk De Vries et al (EP010); Fedotova and Magnani, (G004); Magnani et al (G004)).
