Three-stage Formation of Cap Carbonates after Marinoan Snowball Glaciation Consistent with Depositional Timescales and Geochemistry

TL;DR

This study proposes a three-stage global geochemical model explaining the formation of cap carbonates after Marinoan Snowball glaciation, aligning with depositional timescales and geochemistry, and resolving previous hypotheses' limitations.

Contribution

It introduces a comprehensive three-stage process model for cap carbonate formation consistent with geochemical and depositional evidence.

Findings

Cap carbonate formation involves three distinct stages.

Post-glacial alkalinity from weathering drives carbonate precipitation.

Mixing of meltwater and deep-sea alkalinity extends deposition period.

Abstract

At least two global "Snowball Earth" glaciations occurred during the Neoproterozoic Era (1000-538.8 million years ago). Post-glacial surface environments during this time are recorded in cap carbonates: layers of limestone or dolostone that directly overlie glacial deposits. Postulated environmental conditions that created the cap carbonates lack consensus largely because single hypotheses fail to explain the cap carbonates' global mass, depositional timescales, and geochemistry of parent waters. Here, we present a global geologic carbon cycle model before, during, and after the second glaciation (i.e. the Marinoan) that explains cap carbonate characteristics. We find a three-stage process for cap carbonate formation: (1) low-temperature seafloor weathering during glaciation generates deep-sea alkalinity; (2) vigorous post-glacial continental weathering supplies alkalinity to a carbonate-saturated freshwater layer, rapidly precipitating cap carbonates; (3) mixing of post-glacial meltwater with deep-sea alkalinity prolongs cap carbonate deposition. We suggest how future geochemical data and modeling refinements could further assess our hypothesis.