Shifting locus of carbonate sedimentation and the trajectory of Paleozoic pCO2
Abstract
The burial of calcium carbonate is a determinant of planetary habitability, dictated by CO2 input to the surface environment and rates of chemical weathering. An important source of CO2 is the metamorphism of carbon-bearing sediments, which is responsive to the locus of sedimentation. For example, deep sea sediments are prone to recycling as sea floor is consumed at convergent margins; by contrast, sediments deposited on continental crust can be stable for billions of years.The predominant feature in the empirical sedimentary rock record, as measured by Macrostrat (https://macrostrat.org) and global geological syntheses, is a step-wise increase in continental sedimentation at the Neoproterozoic-Paleozoic transition. Although early Paleozoic carbonate volumes are sufficient to account for a CO2 flux 5x greater than present, Proterozoic continental burial fluxes were likely below the modern estimate. This observation implies that most carbonate sedimentation in the Proterozoic took place on the deep sea floor. The establishment of persistent, widespread continental flooding during the Paleozoic shifted the locus of carbonate sedimentation to continental interiors. A major implication of this shift is that CO2 flux declined during the Paleozoic as carbonate-laden Precambrian seafloor was metamorphosed and recycled. This prediction is consistent with independent proxy records and our model for Phanerozoic carbonate burial. An important corollary is that as carbonate-rich Precambrian seafloor was progressively destroyed, the carbonate content of deep sea sediments decreased concordantly because Paleozoic continents effectively captured global alkalinity fluxes. This process culminated near the Permian/Triassic, with metamorphic CO2 flux at a Phanerozoic minimum and the global ocean uniquely unbuffered against acidification. Such a condition could enhance the environmental effects of transient CO2 injections. Because the mid-Mesozoic appearance of pelagic calcifiers and Cenozoic fall in continental flooding reestablished a deep sea carbonate sink, the Paleozoic was a unique time in Earth history when the continents were the only major, quantitatively important locus of carbonate burial.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP22B..05H
- Keywords:
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- 0325 Evolution of the atmosphere;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 1030 Geochemical cycles;
- GEOCHEMISTRYDE: 5220 Hydrothermal systems and weathering on other planets;
- PLANETARY SCIENCES: ASTROBIOLOGY