DISTRIBUTION AND EVOLUTION OF CARBON AND NITROGEN IN EARTH

In this paper we discuss the distribution, geochemical cycle, and evol ution of CO2 and N2 in Earth's degassed mantle, and atmosphere plus co ntinental 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 mid-ocea n ridges. Estimated CO2 in present degassed mantle is (1.8(-6)+9) X 10 (22) mol, representing (72 +/- 10)% of total degassable CO2, an amount far higher than fractions previously inferred for noble gases. This s trongly suggests that most CO2 has been recycled from Earth's surface into the degassed mantle through subduction, which is consistent with many recent discussions. For N2, the estimated amount in the present m antle is approximately 2.5 X 10(19) mol, representing approximately 12 % of total degassable N2. Recycling of N2 back to the mantle is also i nferred, but on a much smaller scale. A simple model for the outgassin g and recycling of CO2 and N2 in Earth is presented. Outgassing is ass umed to be via melt-vapor partitioning of volatiles. Recycling back in to the mantle via subduction is assumed to be proportional to the mass of the volatile component in the crust. This simple model is consiste nt with all currently available constraints. Difficulties arise from t he dependence of the recycling constant on time. Hence, no single evol ution history can be obtained for CO2 based on the available data. Mod el results tentatively point to a higher CO2 content on Earth's surfac e in the Archean and Proterozoic than at present. Important future con straints may come from records in sedimentary rocks, improved understa nding of carbonate production, and better modeling of the recycling pr ocess.