Abstract
Abstract. Carbon isotopes in the ocean are frequently used as paleoclimate proxies and as present-day geochemical ocean tracers. In order to allow a more direct comparison of climate model results with this large and currently underutilized data set, we added a carbon isotope module to the ocean model of the Community Earth System Model (CESM), containing the cycling of the stable isotope 13C and the radioactive isotope 14C. We implemented the 14C tracer in two ways: in the "abiotic" case, the 14C tracer is only subject to air–sea gas exchange, physical transport, and radioactive decay, while in the "biotic" version, the 14C additionally follows the 13C tracer through all biogeochemical and ecological processes. Thus, the abiotic 14C tracer can be run without the ecosystem module, requiring significantly fewer computational resources. The carbon isotope module calculates the carbon isotopic fractionation during gas exchange, photosynthesis, and calcium carbonate formation, while any subsequent biological process such as remineralization as well as any external inputs are assumed to occur without fractionation. Given the uncertainty associated with the biological fractionation during photosynthesis, we implemented and tested three parameterizations of different complexity. Compared to present-day observations, the model is able to simulate the oceanic 14C bomb uptake and the 13C Suess effect reasonably well compared to observations and other model studies. At the same time, the carbon isotopes reveal biases in the physical model, for example, too sluggish ventilation of the deep Pacific Ocean.
Highlights
A large fraction of paleoclimatic reconstructions are based on isotopic measurements (e.g., Petit et al, 1999; McDermott, 2004; Curry and Oppo, 2005; Polka et al, 2013), yet there are many uncertainties associated with the interpretation of these records in terms of physical climate variables such as temperature, precipitation, and ocean circulation rates
As a reference for future studies using these new capabilities in the Community Earth System Model (CESM), we describe the model used (Sect. 2), describe the details of the implementation of the abiotic and biotic carbon isotopes (Sect. 3), and compare the simulated carbon-isotope fields to observational data to show the general performance of the model (Sect. 4)
The surface oceanic Suess effect, which is the decrease in the surface ocean δ13C due to the penetration of carbon originating from the burning of fossil fuels, has been calculated from observational data as well as from other models that include 13C, and it is often used to derive the oceanic anthropogenic carbon uptake (e.g., McNeil et al, 2001; Tagliabue and Bopp, 2008)
Summary
A large fraction of paleoclimatic reconstructions are based on isotopic measurements (e.g., Petit et al, 1999; McDermott, 2004; Curry and Oppo, 2005; Polka et al, 2013), yet there are many uncertainties associated with the interpretation of these records in terms of physical climate variables such as temperature, precipitation, and ocean circulation rates. Many isotope tracers are currently being measured in the ocean, and including them in ocean models can help us better understand the ocean circulation and diagnose model biases (e.g., Matsumoto et al, 2004) For all of these reasons, we have added a carbon isotope module to the ocean model of the Community Earth System Model (CESM) (Hurrell et al, 2013). For 14C, we added the option of a simplified representation, where the isotope is only subject to the main chemical and physical processes during gas exchange and decay, but does not cycle through the ecosystem This abiotic formulation of 14C was implemented based on the Ocean Carbon Model Intercomparison Project Phase 2 (OCMIP-2) protocol (Orr et al, 2000). Future science applications of the carbon isotopes should use the scientifically supported 1◦ horizontal resolution model version of the CESM
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