Late Pleistocene and Holocene Vegetation Changes in the arid andean piedmont of central Argentina revelead by sedimentary stable carbon isotes and C/N ratios

ROJO, LEANDRO D.; MEHL, ADRIANA; ZÁRATE, MARCELO A.; GARCÍA, ADRIANA; CHIVAS, ALLAN R.

San Rafael

Taller; II Taller de arqueología e isótopos estables en el sur de Sudamérica; 2017

Laboratorio de Isótopos Estables en Cs. Ambientales, IANIGLA-CONICET y Museo de Historia Natural de San Rafael

Stable carbon isotopes (δ13C) and C/N ratios are used as indicators of C3 and C4 vege-tation functional types from bulk sedimentary organic matter at the ca. 50 ka cal. yr BP domi-nantly alluvial sequence of Arroyo La Estacada (33°29?15??S/69°01?30??W), arid eastern Andean piedmont of central Argentina. Although this sedimentary sequence has the potential to con-tribute to the knowledge of past vegetation dynamics throughout its fossil pollen content, previous palynological studies have provided scant information constrained to a few temporal windows due to poor pollen preservation. The δ 13C results obtained from three lithostrati-graphic sections spanning the past 40 ka BP show shifts in the isotopic signal associated with C3 and C4 plant types suggesting this proxy is able to detect changes in these photosynthetic path-ways, representative of Patagonian (and/or stream-margin macrophytes) and Monte phytoge-ographic units, respectively. Climate and local factors, such as water availability in the basin, have been inferred as the main drivers of past vegetation dynamics. The C/N ratios show that at some intervals the organic matter sources are from algae and/or phytoplankton and not solely from C3 or C4 vascular plants. The δ13C record from Arroyo La Estacada is an example of how isotope geochemistry circumvented difficulties in pollen analysis and provided a reliable tool to investigate past vegetation changes in regions where C3 and C4 are differentially distrib-uted. Isotopic signal for Marine Isotopic Stage 3 (MIS3) and for the middle Holocene is attributed to C3 local floodplain plants with a little or null contribution of regional terrestrial (Patagonian) vegetation. Particularly, the late Pleistocene-Holocene transition palaeosols has a clear C4 signal of terrestrial dominance and is representative of Monte plants contribution. This signal was particularly useful to constrained the end of MIS2 and the beginning of MIS1 stages. A mixture of C3- C4 signals for the last 3-4 ka documents the beginning of present-day environ-mental conditions settling.