Resolving the dependence of Δ47 thermometers on acid digestion temperature

Abstract:

Clumped isotope paleothermometry on carbonate minerals has emerged as a leading tool in paleoclimate and paleoaltimetry studies. The utility of Δ₄₇ measurements is especially pronounced when estimating formation temperature of carbonates that precipitated from solutions with unknown water oxygen isotope composition. However, the interpretation of Δ₄₇ values has been hindered by discrepancies among the many published thermometric calibrations. As the number of calibrations grows, it now appears that the slope of the Δ₄₇ vs. inverse temperature relationship clusters into two distinct groupings based on whether the carbonate minerals were digested at low temperature (25°C) vs. high temperature (generally 70 - 90°C).

We model the effect that small amounts of oxygen exchange between CO₂ liberated to the phosphoric acid solution and trace water in that solution can have on measured Δ₄₇. This can occur with very little shift in δ₄₇ and δ¹⁸O of the CO₂. The extent and effect of oxygen exchange following CaCO₃ dissolution is a complex function of the phosphoric acid’s temperature, water content, and viscosity. Conventional approaches to preparation of phosphoric acid (targeting a defined density range at room temperature) are likely inadequate for resolving the details of these dependencies. This stems in part from changes that can occur to the water content and density of phosphoric acid during heating of the acid under vacuum. Secondary effects may also arise that relate to the grain and crystallite size of the carbonate minerals, intercalation of carbonate minerals with siliciclastic matrix in sediments or paleosols, and possibly the reactivity of hydrous components of a siliciclastic rich sample.