Quantitative Paleobathymetry Using Oxygen Isotopes and Shape Changes in Benthic Foraminifera

Anthony C. Gary, Douglas F. Williams, Nancy Healy-Williams

Accurate estimates of paleodepth are of critical importance to oil exploration in determining environment of deposition and geologic history. Models based on the test shape and the ¹⁸O/¹⁶O ratio in benthic foraminifera from the northwestern Gulf of Mexico indicate that a resolution of ±75 ft can be achieved in paleobathymetric reconstructions. The proportion of ¹⁸O and ¹⁶O incorporated into the tests of benthic foraminifera varies with bottom water temperature in a predictable manner. This depth/temperature relationship is the result of the temperature dependence of oxygen isotopic fractionation between seawater and calcium carbonate, and it allows the tests of benthic foraminifera to be used as indicators of paleotemperatu e. Since subbottom water temperatures on the outer shelf and slope decrease systematically with increasing water depth, these paleotemperatures can be used to reconstruct paleobathymetric trends. Paleobathymetric interpretations can also be independently inferred from Fourier shape analysis of benthic foraminiferal species. Combining the oxygen isotope and shape relationships relative to water depth increases the resolution of paleobathymetric reconstructions and provides an independent check on interpretations based on faunal assemblages and sedimentological data. These paleodepth models should allow extinct taxa to be used for paleobathymetric reconstructions as well.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.