Paleolimnological Records in the Northern Pampa Plain

Abstract

The Northern Pampa, located between 30º and 34ºS and bounded by the Sierras Pampeanas and the Río Paraná, is characterized by gentle slopes and low tectonic activity. Nevertheless, Quaternary deformation by faulting reactivation of pre-existing structures around 67,500 years BP, resulted in topographic inversions, forming morphostructural highs and depressions occupied subsequently by closed lakes such as Laguna Mar Chiquita (LMCh, 30º S/62º W) and Laguna Melincué (LMe, 33ºS/61ºW). Given their tectonic origin, these lakes provide long-lasting paleolimnological records. As in most of the lakes of the Pampa region, their hydrological balance is strongly linked to the activity of the South American Monsoon, responding to oscillations of the precipitation with different frequencies (i.e. intra-annual, multidecadal, centennial, millennial). The most significant paleolimnological record is represented by the sediments of LMCh, which is the largest saline lake in South America. The studied temporal frame of the LMCh record ranges from the Late Pleistocene to the present, although the complete lacustrine sequence is expected to be even longer. Towards the south of the Northern Pampas, the paleolimnological record from LMe comprises paleoenvironmental evidence since, at least, the Late Holocene, which is concurrent with the uppermost record from LMCh. The paleoenvironmental evolution of both lake systems has been reconstructed through numerous and detailed studies with a multi-proxy approach, including analyses of stable isotopes, pigments, diatoms, geochemistry, paleoenvironmental magnetism, microfacies, among others. The temporal scale of the records was established through radiocarbon, 210Pb and 137Cs isotopes, and further constrained by stratigraphic markers. In this chapter, the available information about the paleoenvironmental reconstructions of both sites is synthesized, along with the analysis of the main methodologies developed during the last twenty years of research. Furthermore, the main reconstructions are compared by examining their regional paleoclimate implications. Particularly, the abrupt shifts of lake levels and the associated deposits are explored, emphasizing the transformation of ephemeral saline lakes and playa-lakes to sub-saline/saline/hypersaline perennial systems. The identification of multi-decadal changes revealed by geochemical proxies during the current warm period is additionally revised. A special focus is set on disentangling the overlapping signal of anthropogenic activity to the natural hydroclimate oscillations imprinted in the sedimentary record of LMCh over the last fifty years. The revised studies finally show that the lacustrine deposits of the Northern Pampas are ideal for inspecting inter-regional correlation with different natural paleoclimate records subject to the South American Monsoon control.