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, and atmosphere plus 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 mid-ocean ridges. Estimated CO2 in present degassed mantle is (1.8−6+9) × 1022 mol, representing (72 ± 10)% 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, which is consistent with many recent discussions. For N2, the estimated amount in the present mantle is ∼ 2.5 × 1019 mol, representing ∼ 12% 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. Outgassing is assumed to be via melt-vapor partitioning of volatiles. Recycling back into the mantle via subduction is assumed to be proportional to the mass of the volatile component in the crust. This simple model is consistent with all currently available constraints. Difficulties arise from the dependence of the recycling constant on time. Hence, no single evolution history can be obtained for CO2 based on the available data. Model results tentatively point to a higher CO2 content on Earth's surface in the Archean and Proterozoic than at present. Important future constraints may come from records in sedimentary rocks, improved understanding of carbonate production, and better modeling of the recycling process.