Sulphide oxidation and carbonate dissolution as a source of CO.sub.2 over geological timescales

Sulphide oxidation coupled to carbonate dissolution can provide a transient source of carbon dioxide to Earth's atmosphere and so balance the Cenozoic increase in carbon dioxide consumption by silicate weathering, reconciling this increase with the need for mass balance in the long-term carbon cycle. Balancing the Cenozoic carbon budget It is thought that mountain uplift stimulated carbon dioxide consumption by silicate mineral weathering during the Cenozoic era, roughly spanning the past 66 million years, but there are no signs of a corresponding increase in volcanic carbon dioxide emissions that could balance the carbon budget. This paper suggests that some of the missing carbon dioxide could have been produced by coupled sulphide oxidation and carbonate dissolution, a process that may also have been accelerated in response to uplift. This hypothesis is consistent with isotopic records and may help explain the interactions between the long-term carbon cycle, tectonics, and Earth's climate. The observed stability of Earth's climate over millions of years is thought to depend on the rate of carbon dioxide (CO.sub.2) release from the solid Earth being balanced by the rate of CO.sub.2 consumption by silicate weathering.sup.1. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium.sup.2,3,4 suggest that extensive uplift of mountain ranges may have stimulated CO.sub.2 consumption by silicate weathering.sup.5, but reconstructions of sea-floor spreading.sup.6 do not indicate a corresponding increase in CO.sub.2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO.sub.2 within a few million years.sup.7. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO.sub.2 to Earth's atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO.sub.2 during the Cenozoic. A variety of other hypotheses.sup.8,9,10 have been put forward to explain the 'Cenozoic isotope-weathering paradox', and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes.sup.2,3, atmospheric CO.sub.2 partial pressure.sup.11,12 and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO.sub.2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.
Author(s): Mark A. Torres [sup.1] , A. Joshua West [sup.1] , Gaojun Li [sup.2] Author Affiliations: (1) Department of Earth Sciences, University of Southern California, Los Angeles, USA (2) Department [...]