The influence of basement fluid upwelling and diagenesis on CaCO_3 stability in sediments from the eastern flank of the Juan de Fuca ridge

摘要

Basement fluid discharge in zones associated with basaltic outcrops and seamounts of the eastern flank of the Juan de Fuca ridge (NE Pacific) has been investigated through heat flow measurements and coring during the Retroflux cruise (September 2000). Porewaters of the 93 collected cores have been analyzed onboard for chlorinity, Ca, Mg, pH, alkalinity, and nutrients. Upwelling basement fluids have an alkalinity lower than that of seawater. In contrast, porewaters from cores without porewater flow have alkalinities that increase with depth resulting from the oxidation of organic matter and the reduction of sulfate during diagenesis. Thus alkalinity-depth profiles reflect the competition between alkalinity production from diagenetic reactions and the transport of an alkalinity-depleted basement fluid. Thermodynamic calculations reveal that 1) cores that lack porewater flow are increasingly supersaturated with respect to calcite and aragonite as a function of depth in response to the increase in alkalinity, 2) upwelling of basement fluid at speeds of up to several cm/yr results in a porewater carbonate system that is in equilibrium with aragonite, and 3) extremely rapid flow (10 cm/yr) for core PC-GC28 taken at Wuzza Bare induces the transition from equilibrium with aragonite to equilibrium with calcite.