Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution

Abstract

Calcareous dust is often closely associated with periglacial features in the Pleistocene deposits of extra-Andean Argentina. Conventionally interpreted as resulting from continental pedogenic weathering and precipitation during interglacial episodes, little has yet been demonstrated about its origin. Strontium, oxygen and carbon isotopic compositions and REE distributions were determined for such calcareous dust from a wide geographic area including sites in Patagonia, the southern Pampa and the Mendoza Pre-Cordillera. The δ13C values between −1.9 and −7.2‰ (V-PDB) and δ18O values between −2.6 and −6.0‰ (V-SMOW) confirm a glaciogenic origin and not the generally advocated pedogenic genesis. The narrow 87Sr/86Sr ratio from 0.706463±0.000008 to 0.707477±0.000006 (2σ) and the constant REE distribution with a specific negative Ce anomaly both require crystallization in a large and homogeneous reservoir with some seawater supply during successive glacial episodes.The homogeneous Sr isotopic and REE data suggest that the Ca, Sr and REEs released from Andean basement rocks during glacial episodes: (1) were transported by rivers to the eastern continental shelf of which up to ~1million of km2 was probably emerged during the successive glacial episodes, (2) were mixed there in a semi-marine lacustrian meander-rich environment with seawater supply where they dissolved and re-precipitated, before (3) being eroded and dispersed by southeasterly winds over most of Argentina.