DISTRIBUTION AND EVOLUTION OF CARBON AND NITROGEN IN EARTH

Abstract

In this paper we discuss the distribution, geochemical cycle, and evolution of CO2 and N2 in Earth's degassed mantle, atmosphere, and continental crust. We estimate the present distribution of CO2 and N2 in Earth's degassed mantle based on amounts of He and Ar in the degassed mantle and observed outgassing ratios of CO2/He and N2/Ar at midocean ridges. Estimated CO2 in present degassed mantle is (18 +9 or -6) x 10 exp 22 mol, representing (72 +/- 10) percent of total degassable CO2, an amount far higher than fractions previously inferred for noble gases. This strongly suggests that most CO2 has been recycled from Earth's surface into the degassed mantle through subduction. For N2, the estimated amount in the present mantle is about 2.5 x 10 exp 19 mol, representing about 12 percent of total degassable N2. Recycling of N2 back to the mantle is also inferred, but on a much smaller scale. A simple model for the outgassing and recycling of CO2 and N2 in Earth is presented. Model results tentatively point to a higher CO2 content on Earth's surface in the Archean and Proterozoic than at present.