Alteration of carbonate clumped isotope composition by burial heating in foreland sediments of the Himalaya

Abstract

Clumped isotope-derived temperatures of various carbonate phases can be powerful tools for reconstructing past environmental signals such as surface and animal body temperatures, as well as calculating the oxygen isotope composition of ancient waters. However, solid-state isotope reordering can alter clumped isotope ordering and calculated temperatures without affecting the bulk carbon and oxygen isotopic composition of the samples. Such alteration can, in theory, be predicted using theoretical models when a thermal history is known. Here, we utilize a new infrared laser spectroscopic system for clumped isotope analysis to produce a large dataset (236 samples) of multiple carbonate phases from Himalayan foreland deposits in Nepal and Pakistan with well-known thermal histories. We use these to 1) establish the conditions required to alter primary clumped isotope composition in these phases, and 2) test whether the two latest theoretical models accurately predict the magnitude of reordering in these phases. Our results indicate that paleosol carbonate temperatures were altered by burial to 80–110 °C in less than 5Myr. Aragonitic shells show a similar extent of alteration to that observed in calcitic soil carbonates even though they were not converted to calcite. Ostrich eggshell and mammal tusk behaved similarly to each other and to soil carbonates. A large and diverse sample of mammalian tooth enamel from Pakistan did not consistently produce realistic body temperatures regardless of depth, species, or section. Carbonate-cemented sandstone and calcite veins either formed at higher-than-surface temperatures or underwent open system recrystallization. Based on soil carbonate samples from the section with the best-constrained thermal history (Surai Khola, Nepal), the two latest models reproduce the general alteration trend in the data, although some discrepancies are evident between the models. These results confirm the results of previous studies based on much smaller and narrower sample sets: the serious challenge of reconstructing primary temperatures using clumped isotope values from some commonly used non-marine carbonates. They also show that preservation of carbon and oxygen isotopic composition does not guarantee preservation of clumped isotope temperatures.