GEOCHEMICAL EVIDENCE FOR A STRATIFIED OCEAN AFTER A NEOPROTEROZOIC (MARINOAN) SNOWBALL GLACIATION

The Ravensthroat-Hayhook-Sheepbed cap-carbonate sequence to the Stelfox glacial deposits in northwest Canada shares sedimentological, mineralogical, and geochemical features with other Marionan (ca 635 Ma) cap carbonates. δ¹³C (~ -3‰) and δ¹⁸O (~ -7‰) are nearly constant through the Ravensthroat cap dolostone and decline abruptly at the transition with the overlying Hayhook limestone. Seafloor-encrusting barite cements at occur precisely at the transition from dolomite to limestone. The barite cements pass upward into formerly aragonitic, seafloor cements (crystal fans), which are draped and in-filled with calcimicrite. Each carbonate component has a distinctive geochemical signature. The seafloor cements represent the best available proxy for the composition of glacial seawater (uncorrected ⁸⁷Sr/⁸⁶Sr=0.7072). The fine-grained calcimicrite was more susceptible to requilibration with diagenetic fluids, but samples with Sr > 1000 ppm are significantly more radiogenic (> 0.7077) and ¹⁸O-depleted than crystal fan samples with equal or lower Sr concentrations. ⁸⁷Sr/⁸⁶Sr of the barite cements (0.7075) falls in between the fan and micrite compositions. These geochemical data corroborate sedimentological evidence for a stratified ocean following Marinoan glaciation during which a surface ocean dominated by meltwater and warm temperatures capped cool, anoxic deep waters. We infer that the barite cements record the mixing of these two distinct water masses in contact with the seafloor, whereas the calcimicrite and crystal fans were precipitated from surface and deep waters, respectively, as the mixed layer rose relative to the seabed during the ensuing rise in sea level.