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 (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. Plain Language Summary Glaciations, termed "Snowball Earth" episodes, with evidence for low-latitude ice took place in the Cryogenian (717-635 Ma), and these extreme glaciations were likely a critical part of the transition between the Precambrian and the Phanerozoic (541 Ma to present). In this study, we analyzed strata from Svalbard that were originally deposited near the equator before and during one of these Snowball Earth events and present the first application of carbonate clumped isotope (Δ 47) thermometry to Cryogenian synglacial deposits. We combine Δ 47 results with both 18 O/ 16 O stable isotope composition of the carbonates and their microscopic character to assess evidence for environmental changes associated with this glaciation. Carbonates maintain a record of significantly (20-30°C) lower Δ 47 temperatures in glacial versus preglacial deposits. Carbonate 18 O/ 16 O enrichment in synglacial deposits is consistent with ice sheet expansion in the Cryogenian. If the observed drop in temperature from preglacial to glacial strata reflects a difference in primary or shallow pore fluid temperatures, it f...

Deep‐Time Paleoclimate Proxies

“…These rocks are one of the best available targets for extending the Δ 47 paleothermometer into the Proterozoic because Svalbard preserves low‐grade pre‐glacial carbonate and glacial deposits with carbonate mud (micrite) and cements, and we expect a large temperature difference between pre‐glacial and syn‐Snowball environments (Hoffman et al, 2017). Mackey et al (2020) see such a signal, reporting mean glacial Δ 47 temperatures that are 26 ± 10°C cooler than pre‐glacial strata with four samples <25°C. Although these temperatures are relatively warm (Figure 1) and may reflect early diagenetic temperatures, alteration and solid state reordering during subsequent burial result in higher paleotemperatures, so these data should be interpreted as maximum constraints.…”

“…However, extracting temperature and CO 2 estimates from ancient rocks has proven challenging. Mackey et al (2020) take on this challenge and present new clumped isotope thermometry from carbonate rocks that span the onset of the most extreme episode of climate change in the geological record, the ca. 717–660 Myr Sturtian Snowball Earth glaciation.…”

“…Mackey et al (2020) present Δ 47 results from Neoproterozoic carbonate deposited before and during the Sturtian glaciation at tropical latitudes in Svalbard. These rocks are one of the best available targets for extending the Δ 47 paleothermometer into the Proterozoic because Svalbard preserves low‐grade pre‐glacial carbonate and glacial deposits with carbonate mud (micrite) and cements, and we expect a large temperature difference between pre‐glacial and syn‐Snowball environments (Hoffman et al, 2017).…”

“…The interpretation is also reliant on the assumption that the difference in primary Δ 47 values resulting from initial formation temperature is maintained during subsequent processes such as solid‐state reordering during burial. Importantly, through careful petrography and sample selection, Mackey et al (2020) deliver a recipe for extracting the lowest Δ 47 temperatures from diagenetically altered, ancient micritic carbonate, and cements. Additionally, the extracted glacial δ 18 O fluid values are within error of hematite δ 18 O values from Sturtian glacial deposits, consistent with both cooling and formation of a large volume of ice during Snowball Earth (Galili et al, 2019).…”

“…And yet, we still do not know if these episodes of geological climate change are the result of changes in volcanic outgassing, changes in paleogeography and global weatherability, evolutionary changes in biogeochemical cycles, or some combination of these factors. Although Δ 47 on carbonate cements and micrite may never be able to achieve the accuracy of surface temperatures retrieved from fossil shells due to the propensity of resetting, the contribution by Mackey et al (2020) offers a roadmap for reconstructing maximum temperatures from ancient carbonate platforms, which constitute our greatest biogeochemical deep‐time data repository. As both the timing and magnitude of long‐term climate change are further refined, we will have new opportunities to test hypotheses for geological and biogeochemical drivers of long‐term climate change.…”