EXPLORING THE ASSEMBLY OF CRYOGENIAN CAP CARBONATES – WHAT DO COMPOSITE VS. LAMINAR SCALE INVESTIGATIONS REVEAL?

Mid-late Neoproterozoic cap carbonate (CC) successions are widely regarded as chemical sediments rapidly deposited during climatic recoveries from some of Earth’s most extreme icehouse episodes, each a response to an alkalinity imbalance in the ocean-atmosphere system. CCs often initiate as laterally continuous finely layered microcrystalline dolostone with negative δ¹³C compositions that become more positive higher within the cap succession. Lamina formation has been explained by allodapic deposition, inorganic chemical precipitation, microbial-mediated precipitation in the benthic realm or at the interface between anoxic deepwater and the surface ocean. Considering that initial fine layering is one of the unifying characteristics of CC successions, fine-scale geochemical variations may offer further insight to cap formation. Toward this end, CCs collected above glacigenic strata (Rasthof and Keilberg Formations, N. Namibia; Noonday Dolomite and Scout Mountain Member of Pocatello Formation, W. Laurentia), conventionally binned as “Sturtian” and “Marinoan”, were studied by ICP-MS of targeted microdrilled powders and LA-ICP-MS of corresponding intervals, for major and trace elements (including REEs), over lamina series with couplets ranging from 0.1 to 1 mm in scale. The most common PAAS-normalized REE patterns involve moderate LREE depletion, minor MREE enrichment, and slight HREE depletion. Middle Rasthof Fm examples exhibit flattest patterns and lowest REE concentrations, with one example preserving a positive Eu anomaly. Among the Marinoan examples, the Noonday Dolomite also displays a flat pattern, but has among the highest REE concentrations. REE abundances and patterns are similar between weak acetic acid and strong nitric acid leachates (medians of 26 and 29 ppm, respectively), possibly indicating depositional or diagenetic homogenization of REEs among detrital and hydrogenous sedimentary components. LA-ICPMS scans normal to bedding demonstrate that couplets can be geochemically resolved, with relative enrichments of Ba, Rb, Si, and Al highly correlated. The cause and timing of laminar variations is unresolved, but the extreme regularity and lateral continuity could be consistent with seasonal variations in upwelling and runoff during post-glacial transitions.