Carbon Isotopes in the Ocean Model of the Community Earth System Model (CESM1)

Abstract

Carbon isotopes in the ocean are frequently used as paleo climate proxies and aspresent-day geochemical ocean tracers. In order to allow a more direct comparisonof climate model results with this large and currently underutilized dataset, we added a carbon isotope module to the ocean model of the Community Earth System Model5(CESM), containing the cycling of the stable isotope13C and the radioactive isotope14C. We implemented the14C tracer in two ways: in the “abiotic” case, the14C tracer isonly subject to air–sea gas exchange, physical transport, and radioactive decay, whilein the “biotic” version, the14C additionally follows the13C tracer through all biogeo-chemical and ecological processes. Thus, the abiotic14C tracer can be run without10the ecosystem module, requiring significantly less computational resources. The car-bon isotope module calculates the carbon isotopic fractionation during gas exchange,photosynthesis, and calcium carbonate formation, while any subsequent biological pro-cess such as remineralization as well as any external inputs are assumed to occurwithout fractionation. Given the uncertainty associated with the biological fractionation15during photosynthesis, we implemented and tested three parameterizations of differ-ent complexity. Compared to present-day observations, the model is able to simulatethe oceanic14C bomb uptake and the13C Suess effect reasonably well compared toobservations and other model studies. At the same time, the carbon isotopes revealbiases in the physical model, for example a too sluggish ventilation of the deep Pacific Ocean.