Disequilibrium in Clumped Isotopes Caused by Diagenesis in Tertiary Carbonates

Abstract:

This work examines the clumped isotopes and carbonate associated sulfate (CAS) within a system which is being altered from aragonite to calcite and being subjected to partial dolomitization within the marine realm. Samples were collected from Clino, a ≈670m long core which represents slope carbonates composed of varying percentages of aragonite, low-magnesium calcite (LMC), and dolomite. The concentrations of these endmembers differ dramatically over short distances and are associated with varying degrees of marine diagenesis. In the deeper water portion of the core, previous work has shown no evidence of exposure throughout nor is there any evidence for hydrothermal fluids existing in the Bahamas.

 Bulk samples were collected from the portions of the core in which dolomite was most prominent. Samples were treated and measured for CAS and for their clumped isotope value. They were then subjected to a series of buffered acetic acid leaches to remove the aragonite and LMC portion of the sample. There were up to three treatments per sample with the resulting sediment measured on XRD to determine its % dolomite composition. These treatments were then also measured for clumped isotopes.

The δ³⁴S of the sediments yielded values of up to 10‰ more positive than contemporaneous sweater and implicate bacterial sulfate reduction in the formation of these dolomites. Clumped isotope results of the separates allowed for the calculation of end-member formation temperatures for the LMC and dolomite, whilst using a mixing model to account for non-linearity in ∆₄₇ between end-member combinations and varying ∆₄₇-temperature equations. In contrast to other dolomites in the Bahamas proposed to have formed by massive flow of normal seawater, the Clino temperatures values were significantly elevated compared to the presumed equilibrium values. These data suggest that BSR may result in carbonates with clumped isotopic values significant elevated to equilibrium.