Timing of Late Quaternary Climatic Events in Subtropical South America, Determined by U-series Dating of Evaporites

MIKE KAPLAN; YAEL KIRO; KELCEY LOGAN; EDUARDO PIOVANO

Congreso; AGU Fall Meeting 2019; 2019

Laguna Mar Chiquita is an endorheic salt lake located in subtropical northern Argentina. The site is located in the path of the South American Monsoon System (SAMS), and affected in general by the easterly derived moisture. Thus, changes in lake levels, lithology, and the sediment geochemistry reflect changes in these associated climate systems. Cores obtained from the lake are expected to cover the past ~45,000 years based on previous studies in the region. The sediments in the core alternate between organic rich material and evaporites (carbonate, gypsum, and halite), reflecting changes in the lake levels. This project aims to date the evaporite minerals using U-Th disequilibrium to complement 14C dates in order to generate a more accurate timeline, which will allow a better understanding of the climatic events represented in this core.Specifically, evaporites from dry periods were collected between 120cm and 382cm in core TMC-02 from Laguna Mar Chiquita; these will be dated using an isochron and total dissolution method. Preliminary results show relatively high (~40 ppb) uranium concentrations in the present lake, U concentration range between 1.1 and 23.5 ppm in the lake evaporite minerals, and relatively low concentrations of Th in the evaporite minerals. The organic fraction in the lake core varies between 3% and 77% according to Nb concentrations (which is insoluble). These data are crucial for obtaining U-series dates successfully and suggest that correction for dissolved 230Th in the lake is negligible, whereas a correction for detrital 230Th is necessary. The U-series dating effort will help us to better understand the development and progression of the SAMS in the late Quaternary.