Burial Diagenesis Effects on Clumped Isotope Signatures of Coexisting Dolomites and Calcites

Abstract:

Carbonate clumped isotope paleothermometry is a promising method for diagenetic and deep time paleoclimate studies, but original clumped isotope (Δ₄₇) compositions can be altered by fluid and thermal diagenetic processes. Previous work shows Δ₄₇ resetting of calcite occurs at temperatures exceeding 100°C over time periods of millions of years, but such thermally-driven effects have not been considered for dolomite. Differences between calcite and dolomite temperature calibrations are also largely unquantified, and the effect of burial diagenesis on dolomite Δ₄₇ has not been measured.

Coexisting calcites and dolomites in a ~4500 meter core from Andros Island, Bahamas, offer a unique opportunity to address these questions. These dolomites and calcites formed over a time span from the Cretaceous to Pleistocene under near-surface temperature conditions. Clumped isotope analysis of this material reveals that where these carbonate phases are buried to depths greater than ~3000 meters, realistic surface temperatures (~25 °C) are not preserved. Moreover, these phases do not record reasonable geothermal conditions (> 80 °C), but rather record temperatures between 40 and 60°C.

Here we evaluate whether this Δ₄₇ “error” is due to solid-state resetting of clumped isotopes, emplacement of minor burial cements, fabric retentive recrystallization, or some combination thereof. Our results show that clumped isotope compositions of both calcite and dolomite respond similarly to diagenetic resetting of primary values under conditions of burial. These data further emphasize the need to constrain the diagenetic history of samples used for clumped isotope work. The similar Δ₄₇ temperatures recorded by each carbonate type suggest that dolomites and calcites are equally viable temperature proxy sources under shallow burial conditions, yet both seem equally susceptible to “resetting” of their primary clumped isotope abundances.