Applied Geochemistry, Vol, 6, pp. 97-103, 1991 0883-2927/91 $3.00+ .00 Printed in Great Britain ~) 1990 Pergamon Press plc Geochemistry and isotope systematics of sulphur in the mixing zone of Bahamian blue holes SIMON H . BOTTRELL Department of Earth Sciences, Leeds University, Leeds LS2 9JT, U.K. and PETER L. SMARTa n d FIONA WHITAKER Department of Geography, Bristol University, Bristol BS8 1SS, U.K. and ROB RAISWELL Department of Earth Sciences, Leeds University, Leeds LS2 9JT, U.K. (Received 11 October 1989; accepted in revised form 2 August 1990) Abstraet--A profile of aqueous S species concentrations and stable isotope compositions is presented for the mixingzone of Cousteau's Blue Hole, North Andros, Bahamas, together with similar, but less detailed results from two other blue holes. These data show that S o and S 2- are produced by bacterially mediated sulphate reduction near the base of the mixing zone and that these species are reoxidized at shallower levels. Acidity generated by oxidation can contribute to corrosiOnof limestone walbrock. Estimated rates of such corrosion range up to 1200 mm wall-rock recession per 10 ka at Cousteau's Blue Hole (equivalent to 16% porosity generation per l0 ka) and are comparable with rates of dissolution caused by inorganic mixing corrosion. INTRODUCTION BLUE holes are natural, water-filled caverns devel- oped in the Bahamas carbonate platform. Those exposed on the islands contain a freshwater lens at their surface but pass through a stratified mixing zone to saline water at depth. These blue holes can be explored by the use of specialized cave-diving tech- niques (PALMER, ~85) and therefore provide a rela- tively accessible route by which the mixing zone may be sampled and chemical processes acting here may be studied. Analogous processes may be acting throughout the mixing zone in the aquifer, hence studies in blue holes c~fl provide a valuable window into the hydrogeochemistry of the freshwater-saline interface in the aquifer as a whole. Pore-water undersaturation with respect to calcite and aragonite can be generated by mixing of fresh and saline groundwaters (BOGLI, 1964; PLUMMER, 1975; WIGLEY and PLUMMER, 1976) and may be responsible for the development of porosity in mixing zones within carbonate aquifers (HANSHAW and BACK, 1980; SANFORD and KONIKOW, 1989). How- ever, SMART et al. (1988) proposed that bacterially mediated processes may further enhance undersatu- ration (and consequently increase corrosion) in the mixing zones of Bahamian blue holes. Here, we report the results of a chemical and isotopic investi- gation of the systematics of S species in the water column of blue holes and assess the role of acidity generated by bacterially mediated S redox reactions in corroding the limestone wall-rocks. Karstic and mixing zone corrosion of carbonate rocks provides secondary porosity important in the formation of many oil reservoirs. Mixing zone poro- sity in carbonate rocks is characterized by an essen- tially horizontal zone of well connected matrix and cave porosity which ideally rims the flanks of topo- graphic highs. Such features can be recognized in seismic sections (e.g. WIGLEY et al., 1988) and form important components of porosity in many carbonate oil reservoirs, e.g. North Bridgeport Field, Illinois (CHOQUETTE and STEINMAN, 1985); San Andres For- mation (CRAm, 1988); Amposta Marino Field (WtG- LEY et al., !988 ). In this study we document the occurrence of a bacterially mediated process which may significantly enhance rates of formation of mix- ing zone porosity and which should be considered in the evaluation of porosity development in ancient carbonate rocks with reservoir potential. SAMPLING AND ANALYTICAL PROCEDURES Sampling Water samplgs were collec[ed at four blue holes on North Andros (Fig. 1). A complete profile of water samples was taken through the mixing zone in Cousteau's Blue Hole and 97 AG6-I-D