δ44/40 Ca-δ88/86Sr Multi-proxy Constrains Primary Origin of Marinoan Cap Carbonates

By Keith Cowing
April 6, 2022
Filed under
δ44/40 Ca-δ88/86Sr Multi-proxy Constrains Primary Origin of Marinoan Cap Carbonates
Geologic map of Namibia (A and B), locations of studied sections6 (shown in red circles), and stratigraphic column of Otavi Group (C).

The Neoproterozoic Earth experienced at least two global-scale glaciations termed Snowball Earth events. ‘Cap carbonates’ were widely deposited after the events, but controversy surrounds their origin.

Here, we apply the novel δ44/40Ca-δ88/86Sr multi-proxy to two Marinoan (ca. 635 Ma) cap carbonate sequences from Namibia and show that the rocks archive primary environmental signals deriving from a combination of seawater-glacial meltwater mixing and kinetic isotope effects. In an outer platform section, dolostone δ44/40Ca and δ88/86Sr values define a line predicted for kinetic mass-dependent isotope fractionation. This dolostone mostly precipitated from meltwater.

Moreover, stratigraphically higher samples exhibiting the fastest precipitation rates correlate with elevated 87Sr/86Sr ratios, consistent with long-held expectations that a rapid deglacial weathering pulse forced cap carbonate formation. An inner-platform dolostone shows greater effects from water-mass mixing but still reveals that precipitation rates increased up-section. Overlying limestones show the greatest Ca and Sr contributions from seawater.

Amplification of local coastal processes during global ice sheet collapse offers a simple but sufficient proposition to explain the Ca isotope heterogeneity of cap carbonates. Detection of kinetic isotope effects in the rock record provides a basis for developing the δ44/40Ca-δ88/86Sr multi-proxy as an indicator of saturation state and pCO2.

Jiuyuan Wang, Andrew D. Jacobson, Bradley B. Sageman, Matthew T. Hurtgen

Subjects: Chemical Physics (physics.chem-ph); Earth and Planetary Astrophysics (astro-ph.EP); Geophysics (physics.geo-ph)
Cite as: arXiv:2204.02563 [physics.chem-ph] (or arXiv:2204.02563v1 [physics.chem-ph] for this version)
Submission history
From: Jiuyuan Wang
[v1] Wed, 6 Apr 2022 03:26:43 UTC (1,743 KB)

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