DISSOLUTION AND RECRYSTALLIZATION IN MODERN SHELF CARBONATES - EVIDENCE FROM PORE-WATER AND SOLID-PHASE CHEMISTRY

We present an overview of geochemical data from pore waters and solid phases that clarify earliest diagenetic processes affecting modern, sh allow marine carbonate sediments. Acids produced by organic matter dec omposition react rapidly with metastable carbonate minerals in pore wa ters to produce extensive syndepositional dissolution and recrystalliz ation. Stoichiometric relations among pore water solutes suggest that dissolution is related to oxidation of H2S which can accumulate in the se low-Fe sediments. Sulphide oxidation likely occurs by enhanced diff usion of O2 mediated by sulphide-oxidizing bacteria which colonize oxi c/anoxic interfaces invaginating these intensely bioturbated sediments . Buffering of pore water stable isotopic compositions towards values of bulk sediment and rapid Ca-45 exchange rates during sediment incuba tions demonstrate that carbonate recrystallization is a significant pr ocess. Comparison of average biogenic carbonate production rates with estimated rates of dissolution and recrystallization suggests that ove r half the gross production is dissolved and/or recrystallized. Thus i sotopic and elemental composition of carbonate minerals can experience significant alteration during earliest burial driven by chemical exch ange among carbonate minerals and decomposing organic matter. Temporal shifts in palaeo-ocean carbon isotope composition inferred from bulk- rocks may be seriously compromised by facies-dependent differences in dissolution and recrystallization rates.