A Decrease to Low Carbonate Clumped Isotope Temperatures in Cryogenian Strata

Abstract

Preglacial and synglacial low-latitude carbonate sediments of the Elbobreen Formation, NE Svalbard, preserve facies changes associated with low-latitude glacial advance in Cryogenian “Snowball Earth” episodes (717–635 Ma). We present the first application of carbonate clumped (Δ47) isotope thermometry on synglacial Snowball Earth carbonates and combine results with sedimentologic and petrographic observations and stable isotope (δ13C and δ18O) geochemistry to assess Neoproterozoic environmental change. We find elevated calcite Δ47 temperatures, which likely reflect solid-state reordering during burial. Dolomites, however, record lower temperatures that vary with facies and stratigraphy. Preglacial dolomite Δ47 temperatures range from 48–77°C, with a reconstructed fluid δ18OVSMOW value of +0.6‰ in the coldest sample. Glacial diamictites and dolomicrites comprise (1) reworked detrital clasts similar to preglacial strata in stable isotope composition and petrographic textures and (2) autochthonous dolomicrite with more positive δ18O values than those of preglacial dolomites or cooccurring detrital clasts. Mean glacial autochthonous dolomicrite Δ47 temperatures are 26 ± 10°C (95% CL) cooler than preglacial strata, with four samples <25°C. All dolomite Δ47 temperatures reflect diagenesis associated with lithification, yet observed stratigraphic and textural Δ47 temperature differences indicate that this occurred early and only contributes to part of the preserved temperature signal. Alteration trends within populations are consistent with low water/rock ratio diagenesis or partial solid-state reordering; either possibility supports the likelihood of preserved δ18O trends. We postulate that the preserved temperature and δ18O differences between low-latitude preglacial Tonian and synglacial Cryogenian dolomites are an imperfect reflection of primary temperature change and ice sheet expansion.