Numerical analysis of seawater circulation in carbonate platforms: I. Geothermal convection

Differences in fluid density between cold ocean water and warm ground water can drive the circulation of seawater through carbonate platforms. circula ting water can be the major source of dissolved constituents for diagenetic reactions such as dolomitization, This study was undertaken to investigate the conditions under which such circulation can occur and to determine whi ch factors control both the nux and the patterns of fluid circulation and t emperature distribution, given the expected ranges of those factors in natu re. Results indicate that the magnitude and distribution of permeability wi thin a carbonate platform are the most important parameters. Depending on t he values of horizontal and vertical permeability, heat transport within a platform can occur by one of three mechanisms: conduction, forced convectio n, or free convection, Depth-dependent relations for porosity and permeabil ity in carbonate platforms suggest circulation may decrease rapidly with de pth, The fluid properties of density and viscosity are controlled primarily by their dependency on temperature. The bulk thermal conductivity of the r ocks within the platform affects the conductive regime to some extent, espe cially if evaporite minerals are present within the section. Platform geome try has only a second-order effect on circulation. The relative position of sealevel can create surface conditions that range from exposed (with a fre sh-water lens present) to shallow water (with hypersaline conditions create d by evaporation in constricted now conditions) to submerged or drowned (wi th free surface water circulation), but these boundary conditions and assoc iated ocean temperature profiles have only a second-order effect on fluid c irculation. Deep, convective circulation can be caused by horizontal temper ature gradients and can occur even at depths below the ocean bottom. Temper ature data from deep holes in the Florida and Bahama platforms suggest that geothermal circulation is actively occurring today to depths as great as s everal kilometers.