The grain size of Pampean loess as a proxy of southern South American atmospheric circulation during the Last Glacial Maximum

COPPO, R.; NOGUÉS, V.; MONTECINO JARA, D.; TORRE, G.; COSENTINO, N. J.; TUR, V.M.; OLIVEIRA-SAWAKUCHI, A.; KOESTER, EDINEI; GAIERO, D. M.

Roma

Congreso; XXI INQUA Congress; 2023

INQUA

The Pampean loess is the most extensive terrestrial paleo-dust deposit in the Southern Hemisphere and a distinctive geological archive that records changes in past atmospheric circulation. Recent studies reported luminescence ages, mass accumulation rates (MAR), and Nd and Sr isotopic composition on three late Pleistocene- early Holocene Pampean loess sections (Lozada, Tortugas, and Gorina). These studies found a very mixed isotopic signal for the <44- and <5-µm fractions, dominated by a Puna-Altiplano Plateau (PAP) signal. Additionally, past investigations have reported a multi-modal grain size distribution on these loess sections.To disentangle the origin of the Pampean loess multi-modal distribution, we report here a grain size unmixing analysis of bulk fraction, along with new Nd-Sr data for the >44-µm grain size fraction, on the late Pleistocene- early Holocene 2-m profiles of the Lozada and Tortugas sections. Unmixing methods allow differentiating three grain-size subpopulations/end members (EM) in the Pampean loess. EM1, only present in Tortugas, is composed of very fine silt and clay (mode = 5.7 µm) and has been associated with background dust transported from distant sources such as the PAP (~1300 km). EM2 is composed of fine silt (Lozada mode = 10.1 µm, Tortugas mode = 14.6 µm) and has also been previously associated with the PAP. EM3 is composed of a coarse-to-very-coarse silt fraction (Lozada mode = 48.8 µm, Tortugas mode = 49.5 µm) previously interpreted as a short-range-transported fraction associated with proximal sources (i.e., South-Central Western Argentina and/or Northern Patagonia). In line with these interpretations, Nd-Sr isotopic data indicate Northern Patagonia and/or South-Central Western Argentina as the principal sources contributing to the >44-µm fraction (i.e., EM3) and the PAP as the dominant source contributing to the <44- and <5-µm fractions (i.e., EM2 and EM1, respectively). Our results indicate that the multi-modal grain-size distribution observed in Lozada and Tortugas loess sections originated at least partially from the contribution of multiple dust sources. Further investigations should concentrate on understanding the temporal variability of the different source contributions and EM abundances, and their implications on the atmospheric circulation of southern South America.