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.