Fluid flow through carbonate platforms: constraints from U-234/U-238 and Cl- in Bahamas pore-waters

The geometry, timing, and rate of fluid-flow through carbonate margins and platforms is not well constrained. In this study, we use U concentrations a nd isotope ratios measured on small volumes of pore-water from Bahamas slop e sediment, coupled with existing chlorinity data, to place constraints on the fluid-flow in this region and, by implication, other carbonate platform s. These data also allow an assessment of the behaviour of U isotopes in an unusually well constrained water-rock system. We report pore-water U conce ntrations which are controlled by dissolution of high-U organic material at shallow depths in the sediment and by reduction of U to its insoluble 4+ s tate at greater depths. The dominant process influencing pore-water (U-234/ U-238) is alpha recoil. In Holocene sediments, the increase of pore-water ( U-234/U-238) due to recoil provides an estimate of the horizontal flow rate of 11 cm/year, but with considerable uncertainty. At depths in the sedimen t where conditions are reducing, features in the U concentration and (U-234 /U-238) profiles are offset from one another which constrains the effective diffusivity for U in these sediments to be approximate to 1-2 x 10(-8) cm( 2) s(-1). At depths between the Holocene and these reducing sediments, pore -water (U-234/U-238) values are unusually low due to a recent increase in t he dissolution rate of grain surfaces. This suggests a strengthening of flu id flow, probably due to the flooding of the banks at the last deglaciation and the re-initiation of thermally-driven venting of fluid on the bank top and accompanying recharge on the slopes. Interpretation of existing chlori nity data, in the light of this change in flow rate, constrain the recent h orizontal flow rate to be 10.6 (+/-3.4) cm/year. Estimates of flow rate fro m (U-234/U-238) and Cl- are therefore in agreement and suggest flow rates c lose to those predicted by thermally-driven models of fluid flow. This agre ement supports the idea that flow within the Bahamas Banks is mostly therma lly driven and suggests that flow rates on the order of 10 cm/year are typi cal for carbonate platforms where such flow occurs. (C) 1999 Elsevier Scien ce B.V. All rights reserved.