Early Diagenesis of Organic Matter in two Lacustrine Evaporitic Environments in Brazil

Abstract

The coastal region of the Rio de Janeiro state (Brazil) is characterized by a semi-arid microclimate associated with the upwelling coastal system of nearby Cabo Frio, which affects the hydrological and biogeochemical cycles in the region. Lakes and lagoons are natural laboratories to study biogeochemical signals that occur over geological times because they generally have higher sedimentation rates than the oceans. Lagoa Vermelha (LV) and Brejo do Espinho (LBE) beside amplify signals can register fluctuation in the local climate which is related to ocean circulation and biomineralization. These lagoons represent one of the few places in the world where modern precipitation of dolomite occurs. This study uses a multiproxy approach to characterize the deposition of carbonate sediments at these evaporitic environments. Sedimentary cores from LBE and LV, 1.6 and 6.1 cal kyr BP respectively, demonstrated mixed organic matter source reaching the lagoons with large input of terrestrial components. The dolomite-rich layers deposited ~2.1 cal kyr BP displayed enriched |18O and depleted |13C suggesting intense microbial activity and dryness. Using an approach combining organic and inorganic geochemical proxies has led to the recognition of dryness as an important regional climatic characteristic on the carbonate sedimentation in these hypersaline coastal lagoons.