Program
 
General Session 2: Marine & estuarine biogeochemistry
 

 
 
1605
Direct determination of organic carbon export from N starving surface oceans
Tuesday 10th @ 1605-1625
Conference Hall
Young Ho Ko* , Pohang University of Science and Technology
Kitack Lee, Pohang University of Science and Technology
Presenter Email: sldla@postech.ac.kr
Accurate determination of the amount of organic carbon removed from the upper ocean, i.e., termed as new production, is critical in determining the carbon dioxide partial pressure (pCO2) in the atmosphere and thereby understanding the global carbon cycle. Methods for measuring new production mostly rely on the availability of fixed nitrogen (N, nitrate and to a less extent, ammonium and dissolved organic nitrogen), which has long been believed to be a limiting factor for new production. However, fixed N is rarely available to phytoplankton in the vast areas of oceans during much of the year. Therefore, new production for the N-starving oceans was mostly inferred from measurements on N2 fixation activity, directly reducing dinitrogen (N2) to ammonium and thereby circumventing N limitation. However, these estimations are uncertain owing to large temporal and spatial variability of N2 fixation rate and large uncertainty in the C:N conversion factor for organic matter (both particulate and dissolved forms) produced by N2 fixing microorganisms. Here we directly estimate new production by summing annual reduction in concentration of dissolved inorganic carbon (NCT) in the surface mixed layer, corrected for changes due to salinity (S) variations and, to less extent, corrected for changes resulting from net air-sea CO2 exchange and anthropogenic nitrogen deposition. For each pixel of 4¡ã latitude by 5¡ã longitude, the reduction of the mixed layer NCT inventory was calculated from an annual NCT cycle, deduced from unparalleled records of surface ocean pCO2 (> 6.5 million data) and total alkalinity using an established thermodynamic model. This novel method yielded 0.7 ¡À 0.2 petagrams (Pg C yr−1, Pg = 1015 grams) of global new production per year, which contribute approximately 10% of global new production. We concluded that, of several processes influencing new production in the absence of N, N2 fixing microorganisms is a major driver of this global new production.