Glendonites in Neoproterozoic low-latitude, interglacial, sedimentary rocks, northwest Canada: Insights into the Cryogenian ocean and Precambrian cold-water carbonates

Abstract

Stellate crystals of ferroan dolomite in neritic siliciclastic and carbonate sedimentary rocks between Sturtian and Marinoan glaciations in the Mackenzie Mountains are interpreted as replaced glendonites. These pseudomorphs after ikaite indicate that shallow seawater at that time was near freezing. Stromatolites verify that paleoenvironments were in the photic zone and physical sedimentary structures such as hummocky cross-bedding confirm that the seafloor was repeatedly disturbed by storms. Glendonites within these low-latitude, continental shelf to coastal sedimentary deposits imply that global ocean water during much of Cryogenian time was likely very cold. Such an ocean would easily have cooled to yield widespread sea ice and, through positive feedback, growth of lowlatitude continental glaciers. In this situation gas hydrates could have formed in shallowwater, cold shelf sediment, but would have been particularly sensitive to destabilization as a result of sea-level change. Co-occurrence of pisolites and glendonites in these rocks additionally implies that some ooids and pisoids might have been, unlike Phanerozoic equivalents, characteristic of cold-water sediments.

Methane seeps, methane hydrate destabilization, and the late Neoproterozoic postglacial cap carbonates

“…However, seeking geochemical evidence such as the sulfur and iron isotopes of pyrites [149] from the glaciogenic diamictite may give insight into the redox condition of the seawater before cap carbonate deposition. In addition, searching specific cold-water minerals such as glendonite [150] in cap carbonates and their underlying strata would help to clarify whether cap carbonates started to precipitate from cold seawater. Also unresolved is the triggering mechanism for methane hydrate destabilization.…”

Neoproterozoic glaciation in the Earth System

“…Thick carbonate platform successions imply high rates of carbonate production to keep pace with thermal subsidence and so, in principle, are favoured in the tropical to sub-tropical belts, where rates of evaporation are highest. Although no definitive latitudinal or temperature limits on carbonate development can be given (carbonate sediments are common at all latitudes in the modern world), certain structures such as ooids (but not pisoids, James et al 2005), and thick displacive cement crusts associated with tepees in peritidal sediments, are particularly suggestive of rapidly evaporating, warm conditions (Fairchild & Hambrey 1984;Knoll & Swett 1990). Carbonate platform deposits with such characteristics can be found immediately beneath glacigenic units ( Fig.…”

Biomarker Evidence for Photosynthesis During Neoproterozoic Glaciation

“…Dropstones in the black shale are almost exclusively composed of organic-poor carbonate, indicating that the terrain eroded by the glaciers was not rich in black shale. Ikaite (the precursor to observed glendonite) forms in cold, anoxic sediments where rapid microbial remineralization of organic matter dramatically boosts porewater concentrations of both alkalinity and orthophosphate, a calcite inhibitor (18). Finely disseminated pyrite is also abundant in the matrix of the black shales, presumably resulting from extensive remineralization of organic matter by sulfate-reducing bacteria.…”

“…Methane, the primary absorber, is calculated from lineby-line techniques, using parameters from the HITRAN database (16) and assuming a Voigt line profile and the Voyager temperature profile (17). We also include pressureinduced absorption (18) due to H 2 and N 2 . We assume a methane relative humidity of 50% (0.088 mixing ratio) at the surface and a constant mixing ratio (0.017) in the stratosphere (19,20).…”