Interstitial Water Geochemistry and Carbonate Diagenesis in the Sub-Surface of a Holocene Mud Island in Florida Bay

Diagenetic reactions occurring in the sub-surface of an exposed Holocene island in Florida Bay, were investigated in a core taken on Crane Key. Pore waters squeezed from the sediments at 10-cm intervals were filtered and analyzed for major, trace and stable isotopic (C and O) compositions. These data were compared with the mineralogy, strontium concentration and carbon and oxygen isotopes of the bulk sample and isolated dolomite. Salinities throughout the core were in excess of 80 g⋅kg^–1. To eliminate effects of evaporation pore water data were normalized to the concentration of chloride. These data indicate that while Ca²⁺/Cl^–, Mg²⁺/Cl^– and Sr²⁺/Cl^– ratios all increase downcore, Mg²⁺/Ca²⁺ and Sr²⁺/Ca²⁺ ratios exhibit a decrease in certain intervals which we believe is attributable to the dissolution of high mag- nesium calcite and perhaps the precipitation of dolomite. These findings are supported by the discovery of dolomite with a ¹⁴C age some 2,000 years younger than the host sediment. The flux of magnesium, calculated from the present day gradient, indicates that more than enough Mg²⁺ has been available from the dissolution of high magnesium calcite to account for the observed dolomite. The dolomites which were analyzed in this investigation did not form contemporaneously with deposition, but rather at some time between 2,600 B. P. and the present day. These conclusions are supported by stable carbon and oxygen isotopic data and calculation of the saturation state of relevant carbonate minerals.