CALCITE-SIDERITE OXYGEN ISOTOPE-BASED PALEOTEMPERATURE ESTIMATES, EXAMPLES FROM MID-UPPER ALBIAN AND MIDDLE TURONIAN DEPOSITS ALONG THE MARGIN OF THE WESTERN INTERIOR BASIN
P. L. Phillips, L. A. Gonzalez, G. A. Ludvigson, B. J. Witzke, and R. L. Brenner
Differences in the temperature-dependent oxygen isotope fractionations between co-precipitated calcite and siderite cements are sufficient to allow paleotemperature estimates of the pore fluids from which they formed. One primary obstacle is establishing co-precipitation of calcite and siderite from the same groundwater system. Petrographic techniques, cathodoluminescence, and evaluation of oxygen and carbon isotopes can be used to document approximate coeval precipitation. Moreover, early precipitation of cements preserves an open framework of grains with minus cement porosity like that of unconsolidated surficial deposits even after burial (i.e. high minus-cement porosity).
Carbon and oxygen isotopic analyses of early, approximately coeval calcite and siderite cements formed in low-lying coastal areas typically produce a trend reflecting mixing of marine with meteoric pore fluids. The lightest end-member d18O composition is interpreted as having precipitated from predominantly meteoric pore fluids. Thirty or more isotope analyses of an individual cement are required to accurately determine the meteoric end-member d18O composition, to establish an isotopic representation of pore fluid evolution, as well as to estimate temperature at the time of precipitation. Temperature estimates are sensitive to small shifts in d18O differences between calcite and siderite, a further requisite for multiple analyses. Calcite-siderite paleotemperature estimates have a minimum analytical error = ± 1.7°C.
Calcite and siderite cements analyzed from the Late Albian Swan River Formation, Manitoba yield a temperature of formation of 13.7°C. Cements analyzed from the Mid-Turonian Codell Sandstone, Carlile Formation, South Dakota yield a temperature of 10.7°C. These estimates are in accord with mid-Cretaceous climate model simulations and suggest that this technique can be used to determine earth surface temperatures.