GUTTULATIC MICROFABRICS WITHIN THE BECK SPRING DOLOMITE

Neoproterozoic carbonate strata host disproportionately abundant occurrences of giant ooids, concentrically coated carbonate grains with diameters larger than 2000 µm. Phanerozoic ooids, commonly sand-sized, are predominantly found in strata formed in shallow, warm, low-latitude seas. While most Neoproterozoic strata containing giant ooids were also deposited at low latitudes, they commonly closely underlie Snowball Earth glacial diamictites—a surprising juxtaposition if they formed in warm, tropical conditions. Recent work hypothesized that the conditions needed for the growth of giant ooids included elevated seawater alkalinity and temperatures much warmer (40°C) or much colder (0°C) than Phanerozoic ooid-forming environments. Given the stratigraphic relationship of many giant ooid units to Snowball Earth glacial rocks, these hypothesized extreme temperatures challenge either Tonian climate models before the onset of the Sturtian Glaciation (~717 Ma) or conceptual models about standard modes and environments of carbonate sediment formation and deposition.

To test the hypothesis that Neoproterozoic giant ooids formed at extreme temperatures, we collected and analyzed a suite of samples from the late Tonian Beck Spring Dolomite (~780-730 Ma) in the Death Valley area (CA). Here, we present observations from light microscopy, Raman microspectroscopy, and electron microprobe element mapping that characterize guttulatic microfabrics. Guttulatic microfabric is a petrographic fingerprint of the neomorphic stabilization of ikaite, a hydrated calcium carbonate mineral that only forms in near-freezing conditions in natural environments. Fabrics within the dolomite exhibit pseudo-hexagonal cores with hexagonal and ellipsoidal zoned syntaxial overgrowths, a character consistent with ikaite stabilization.

Identifying guttulatic microfabrics within the Beck Spring Dolomite provides evidence that the low latitude, shallow marine environment where the giant ooids formed was cold. Our observations suggest that Earth was precariously balanced near the edge of global glaciation for millions of years and that vase-shaped microfossils and the development of adaptations resulting in complex multicellularity must be considered in the context of a cold ecosystem.