TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 61 (No. 5), pp. 803-815, 2005

Roles of Biogeochemical Processes in the Oceanic Carbon Cycle Described with a Simple Coupled Physical-Biogeochemical Model

Masahiko Fujii1*, Motoyoshi Ikeda1,2 and Yasuhiro Yamanaka1,2

1Graduate School of Environmental Earth Science, Hokkaido University, Kita-ku, Sapporo 060-0810, Japan
2Frontier Research System for Global Change, Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan

(Received 30 April 2002; in revised form 26 October 2004; accepted 19 November 2004)

Abstract: To evaluate the contribution of biogeochemical processes to the oceanic carbon cycle and to calculate the ratio of calcium carbonate to organic carbon downward export, we have incorporated biological and alkalinity pumps in the yoked high-latitude exchange/interior diffusion-advection (YOLDA) model. The biogeochemical processes are represented by four parameters. The values of the parameters are tuned so that the model can reproduce the observed phosphate and alkalinity distributions in each oceanic region. The sensitivity of the model to the biogeochemical parameters shows that biological production rates in the euphotic zone and decomposition depths of particulate matters significantly influence horizontal and vertical distributions of biogeochemical substances. The modeled vertical fluxes of particulate organic phosphorus and calcium carbonate are converted to vertical carbon fluxes by the biological pump and the alkalinity pump, respectively. The downward carbon flux from the surface layer to the deep layer in the entire region is estimated to be 3.36 PgC/yr, which consists of 2.93 PgC/yr from the biological pump and 0.43 PgC/yr from the alkalinity pump, which is consistent with previous studies. The modeled rain ratio is higher with depth and higher in the Pacific and Indian Oceans than in the Atlantic Ocean. The global rain ratio at the surface layer is calculated to be 0.14 to 0.15. This value lies between the lower and higher ends of the previous estimates, which range widely from 0.05 to 0.25. This study indicates that the rain ratio is unlikely to be higher than 0.15, at least in the surface waters.

*Corresponding author E-mail: mfujii@maine.edu

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