THE UPTAKE OF CARBON DURING ALTERATION OF OCEAN CRUST - MINERALOGY AND CHEMISTRY

Abstract

The distributions and abundances of alteration types and carbonate minerals in sections of upper ocean crust have been measured in order to determine the carbon budget in altered ocean crust. Our results show that the ocean crust is a sink for carbon, with an annual storage rate of 3.4 x 1012 mol C y-1, in close agreement with a previous estimate by Staudigel et al. (1989). This surpasses the total production rate of carbon in new oceanic crust and, besides accounting for uptake of all CO2 lost via degassing at MOR, results in a net sink in the oceanic crust of 1.5-2.4 x 1012 mol C y-1. This sink is significant for global carbon budgets, and subduction of altered ocean crust may be an important source of CO2 to the atmosphere and/or recycling into the mantle. The bulk CO2 content of the crust decreases with depth, with most of the carbon taken up in the permeable upper few hundred meters of the volcanic section at low temperatures (0-60°C) during aging of crust away from spreading ridges. The abundances of carbonate veins and the bulk CO2 contents of the upper crust are greater in older (110-165 Ma) than younger (6Ma) crust, suggesting progressive uptake of carbon by the upper ocean crust. Precipitation of carbonates within the crust is essentially complete within 100 Ma, and perhaps as soon as a few tens of million years after formation of the crust.