Magnetic Mineralogy of Neoproterozoic Cap Carbonates from Brazil: Magnetization, Re- magnetization and Environmental Significance

Neoproterozoic glacial deposits are ubiquitously capped by carbonates with unusual sedimentary structures and negative carbon isotopic signature. These cap carbonates correspond to the transgressive sequence deposited after ice thaw, and include a basal dolomitic unit covered by a limestone unit. They record the relative influence of several factors such as ocean mixing, atmosphere and ocean warming, gas exchange, and continental and plataformal weathering. Three carbonate units from the Amazon and Sao Francisco cratons were studied for their paleomagnetism and magnetic mineralogy. Cap dolostones present a primary signal recording several reversals. In contrast, the cap limestones show evidence of pervasive re-magnetization. Hysteresis, IRM-acquisition curves and MPMS data, as well as SEM observations and other magnetic tests (e.g., Lowrie and Cisowski tests) show that the cap dolostones typically contain hematite (detrital) as the main magnetic carrier, whereas cap limestones contain pyrrhotite and magnetite (usually as authigenic framboids). The mechanisms of acquisition and resetting of magnetization in these carbonates will be discussed as well as the implications for the paleoenvironmental models proposed for the end of the Neoproterozoic.