Kilometre-scale thermohaline overturn of pore waters in the Louisiana Gulf Coast

Abstract

Convincing evidence exists for the large-scale convection of sea water through the upper oceanic crust, even at considerable distances from mid-ocean ridges^1,2. There has been considerable interest in verifying the existence of convective overturn of pore fluid in large sedimentary basins, a mass-transport process which could profoundly influence the transport of heat, hydrocarbons, metals and diagenetically reactive dissolved constituents^3,4. The existence of fluid convection in sediments of the United States Gulf Coast has been inferred on the basis of the large volumes of fluid necessary to account for the observed diagenetic alteration⁵ and the existence of thermal anomalies that cannot be explained by simple conductive transport of heat^3,6. Theoretical Rayleigh–Darcy calculations^6,7 support the notion that thermally driven convection is possible. Here I present geochemical and physical evidence for the existence of density inversions in Gulf Coast pore fluids sufficient to drive large-scale convective fluid flow at rates conceivably as high as metres per year. The density inversions are caused in part by the dissolution of salt diapirs and the formation of dense, saline brines at shallow depths.