Formerly-aragonite seafloor fans from Neoproterozoic strata, Death Valley and southeastern Idaho, United States: Implications for "cap carbonate" formation and snowball Earth

Seafloor-precipitated calcium carbonate fans, exceedingly rare in post-Paleoproterozoic time, make a dramatic reappearance in the post-glacial cap carbonates associated with Neoproterozoic low latitude glaciation (snowball Earth). Their presence is commonly interpreted to indicate elevated seawater alkalinity; the source of the anomalous alkalinity has been a critical, much-debated component in competing "snowball Earth" hypotheses. Two new Neoproterozoic seafloor fan occurrences have been reported recently in the Western United States (Death Valley and southeastern Idaho). Each were deposited during transgression and record negative delta(13)C values as do all known cap carbonates, but they lack a known underlying glacial deposit and do not necessarily rest on the transgressive surface. It is possible, but not likely, that the units represent post-glacial cap carbonates without a preserved/discovered underlying glaciogenic unit. More likely, processes independent of glaciation may cause negative delta(13)C excursions and cap carbonate-like facies and caution must be exercised when interpreting the meaning of seafloor precipitates in association with snowball Earth events.