Local Sulphide Oxidation Drives Carbonate Dissolution in Modern Shallow Marine Carbonates: Evidence from Oxygen and Sulphur Isotope Composition of Pore Water Sulphate

Abstract

We have investigated sulphur cycling in modem carbonate sediments from the Florida Platform, U.S.A. Relations among pore water chemistry (SO24 , ZCO2, Ca2+/C1 ) and oxygen and sulphur stable isotope composition of SO 2require a direct coupling between sulphur redox cycling and syndepositional carbonate dissolution. Importantly, despite rapid rates of sulphate reduction, organic-rich carbonate sediment pore waters typically maintain SO2-/C1 ratios within 5% of seawater values, suggesting local reoxidation of H2S. Shallow shelf carbonates comprise 93% of the entire Phanerozoic carbonate rock record and about 38% of the modem oceanic carbonate accumulation, so quantifying sulphur and carbon coupling during the early marine diagenesis is vital in deciphering biogeochemical cycling (e.g. Ronov, 1980; Milliman, 1993). In low-Fe carbonates, H2S-O2 reactions can lower pH values and pore water saturation states promoting carbonate dissolution (e.g. Boudreau, 1991). Given that platform carbonate dissolution fluxes are roughly half of gross biogenic carbonate production, sulphur cycling may greatly affect carbonate preservation and accumulation (Walter and Burton, 1990; Walter et al., 1993). Pore water sulphate ~180 and 834S values were markedly shifted from the established seawater values of +9.5%o (SMOW) and +21.0%o (CDT), respectively. Chemical evolution was least in environments where the sediment-pore water system was relatively 'open' to the overlying seawater. The greatest chemical evolution was observed in bioturbated sediments colonized by Thalassia grass. Here, there was a ~lSOso 4 shift of 5%0, var iab le ~348SO 4 values (+17.7 to +23.3%o), and exceptionally high Ca2+/C1 ratios. These shifts in pore water sulphate isotopic compositions exist despite the apparent lack of net sulphate reduction (Fig. 1). The natural system isotope trend is in marked contrast to the closed system isotope evolution observed in sediment incubation experiments (Fig. 1). Importantly, sulphide oxidation is required as an acid source because pore water Ca+2/C1 and ZCO2 values exceed those attributable to the utilization of dissolved oxygen and the small amount of net sulphate reduction. The observed degree of carbonate dissolution (up to 2.3 mM excess Ca 2+) and sulphate isotopic values are best explained by a redox cycle where bacterial sulphate reduction is followed by efficient H2S oxidation resulting in relatively invariant SO24-/C1 ratios. The 02 required to locally oxidize pore water H2S may come from oxygenated seawater advection or from marine grass rhizome systems which enhance oxygen supply rates. A mass balance model demonstrates that the observed 81SOso4 values are consistent with measured sulphate reduction rates and the short pore water residence times. Pore water geochemistry and sediment incubation experiments confirm that sulphur cycling has a profound effect on the early marine diagenesis of shallow platform carbonates. The important diagenetic processes are oxic respiration, bacterial sulphate reduction, sulphide oxidation, and carbonate dissolution. In most sediments, pore water SO2-/C1 ratios are near overlying ocean values, yet in many cases SO24 has been recycled from HzS. The regularity of pore water evolution and sediment environment becomes more apparent when the oxygen and sulphur isotope compositions of SO 2are examined. Diagenetic effects are most