A Decrease to Low Carbonate Clumped Isotope Temperatures in Cryogenian Strata
Tyler Mackey, A. B. Jost, Jessica R. Creveling, Kristin Bergmann
Abstract
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 (Δ 47 ) isotope thermometry on synglacial Snowball Earth carbonates and combine results with sedimentologic and petrographic observations and stable isotope ( δ 13 C and δ 18 O) geochemistry to assess Neoproterozoic environmental change. We find elevated calcite Δ 47 temperatures, which likely reflect solid‐state reordering during burial. Dolomites, however, record lower temperatures that vary with facies and stratigraphy. Preglacial dolomite Δ 47 temperatures range from 48–77°C, with a reconstructed fluid δ 18 O VSMOW 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 δ 18 O values than those of preglacial dolomites or cooccurring detrital clasts. Mean glacial autochthonous dolomicrite Δ 47 temperatures are 26 ± 10°C (95% CL) cooler than preglacial strata, with four samples <25°C. All dolomite Δ 47 temperatures reflect diagenesis associated with lithification, yet observed stratigraphic and textural Δ 47 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 δ 18 O trends. We postulate that the preserved temperature and δ 18 O differences between low‐latitude preglacial Tonian and synglacial Cryogenian dolomites are an imperfect reflection of primary temperature change and ice sheet expansion.