Glacial meltdowns and origins of Neoproterozoic 'cap dolostones'
Abstract
Shallow-water dolostones deposited world-wide in the aftermath of the terminal Cryogenian glaciation record large negative excursions in C isotopes attributed to the post-glacial high pCO2 hothouse. To better understand the origin of the 'cap dolostone' and associated geochemical records we measured Mg isotopes through a paleo shelf-slope transect of the Keilberg cap carbonate in Namibia. Cap dolostones are enriched in heavy Mg isotopes compared to modern examples, with δ26Mg values ranging from -0.85 to -1.88‰. In addition, δ26Mg values vary systematically both within and between sections; heavier and more variable δ26Mg values are observed on the inner bank whereas lighter and more homogeneous δ26Mg values are found towards the outer bank and on the slope. We interpret both the mean and variability in the Mg isotopic composition of Keilberg cap dolostones as reflecting early dolomization in a mixing zone between a global post-glacial freshwater lens and glacial brines. We explore implications of this model for geochemical records of the aftermath of a snowball Earth as recorded in cap carbonates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B11G..02H
- Keywords:
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- 0460 BIOGEOSCIENCES / Marine systems;
- 0473 BIOGEOSCIENCES / Paleoclimatology and paleoceanography;
- 1030 GEOCHEMISTRY / Geochemical cycles;
- 4870 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Stable isotopes