Mass-47 clumped isotope thermal history reordering: Example from the Greater Green River basin

Abstract:

During the last several years, many studies have tried to reconstruct the paleoelevations of sedimentary basins and paleosol sequences using the mass-47 clumped isotope thermometer. Ideally, this technique directly preserves the temperature of carbonate formation, avoiding any speculation on the composition of surface or water from which the carbonate precipitated. Recently, however, concerns about post-depositional alteration of the mass-47 isotope signature, due to the effects of burial, O and C volume diffusion, and/or diagenetic alteration have arisen, potentially complicating the application of the clumped thermometer for determining paleo-surface conditions.

Here we investigate the effect of burial depth on mass-47 bond reordering. To this purpose we collected samples, from the surface and from drill cores, in two different areas of the Greater Green River basin: the Washakie Basin near Rock Springs, Wyoming and Green River basin near Pinedale, Wyoming. Both basins are filled with a thick Eocene lacustrine series that include numerous limestone beds. The thermal histories of the basins are well documented from petroleum prospecting studies. The Δ47 composition of lacustrine limestones with peak burial depths ranging from 1 to 6 km have been measured and compared to values derived from temperature history reordering models (THRMs). These results show that the THRMs does not predict the observed clumped isotope composition, suggesting than parameters other than temperature are controlling the Δ47 reordering. In order to refine the predictive model, we propose to independently model the best k₀ factor of each analyzed sample starting from their final measured Δ47 values and implementing the thermal history from current depth to the period of deposition. Resulting k₀ values are surprisingly well correlated with depth, suggesting that pressure and/or depth have a strong influence on the k₀factor, and consequently on Δ47 bond reordering.

These results suggest that temperature may not be the sole factor controlling Δ47 reordering, and that other depth-dependent parameters, such as pressure or XCO₂, especially in organic rich basins, could also influence the rate of solid-state Δ47 reordering and thus may need to be considered in alternate THRMs.