The Effect of Early Marine Diagenesis on Stable Isotope Signatures in Carbonates—Implications for the Interpretation of Stable Isotope Signatures in Potential Reservoir Environments

Mueller, Anne¹, H. McGregor², M. K. Gagan³, J. M. Lough⁴ (1) The University of Queensland, Brisbane, Australia (2) Bremen University, Bremen, Germany (3) Australian National University, Canberra, Australia (4) Australian Institute of Marine Science, Townsville, Australia

Early marine aragonite cements are commonly precipitated from pore waters at the basal portions of coral skeletons. Inorganic calcite may also be added to the coral skeleton during early diagenesis. The progressive addition of early diagenetic inorganic aragonite and calcite toward the base of massive corals in Western Australia and Papua New Guinea produces an apparent increase in density and an increase in ‰¹³C. Both the diagenetic aragonite and calcite are enriched in ¹³C relative to coral aragonite. A comparison of the change in densi­ty and the change in ‰¹³C values in our coral leads to sound interpretations of the relation between the nature and degree of diagenesis and their effect on the ‰¹³C values. Additional consideration of the relationship between ‰¹³C and ‰¹⁸O signatures confirmed the above interpretations. Calcite has almost the same density as aragonite (2.71 cf 2.93 g/cm³), and thus the addition of similar amounts of secondary calcite in the Papua New Guinea coral as for the altered Ningaloo Reef coral would produce the same artificial densi­ty increase. An apparent increase in density by about 25% due to the infill of the pores of the coral skeleton would be accompanied by a 1.6? decrease in ‰¹³C in the coral affected by the precipitation of secondary inorganic aragonite as the latter is enriched in ¹³C, rela­tive to coral aragonite. Inorganic calcite precipitated in equilibrium with seawater is also enriched in ¹³C relative to pristine coral aragonite (O’Neil et al. 1969), though not to the same extent as inorganic aragonite. Despite this, ‰¹³C would still show a decrease of ~0.7‰ towards the present. Based on our results we suggest new means of identifying the degree and nature of diagenesis in carbonates and offer suggestions of how to interpret geo­chemical indicators of diagenetic processes in potential reservoir environments.