Depth Dependent Elemental Compositions of Particulate Organic Matter (POM) in the Ocean

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The production and downward transport of particulate organic matter (POM) creates vertical nutrient and carbon gradients controlling the CO2 exchange between ocean and atmosphere. C:N:P element ratios of POM determine relative magnitudes of downward phosphorus, nitrogen and carbon fluxes. Despite observational evidence for variable element ratios, it is common practice to use the constant Redfield ratios for biogeochemical modeling, which might lead to an underestimation of downward carbon fluxes. To determine elemental ratios of POM and their impact on the marine carbon cycle, we assembled C/N data for particulate material from different sources into a single data collection for joint evaluation. The dataset contains 10200 C/N values, encompassing all major oceans and trophic levels. This dataset shows that C/N ratios are highly variable with values below the traditional Redfield ratio (C/N=6.6) to values greatly exceeding it. On a global mean, C/N ratios of marine sinking particles from the surface water amount to 7.1, and there is a systematic increase of C/N ratios with depth of 0.2 units per 1000 m. The discrepancy with results from dissolved nutrient fields (constant ratios close to Redfield's value) can be explained by implicit depth averaging caused by depth variations of the surfaces under consideration. Due to preferential remineralization of N, the C/N ratio of the dissolving component (seen on dissolved nutrient fields) is smaller than the C/N ratio of the remaining particles. For flux estimations, variable C/N ratios should be implemented in biogeochemical models to correctly represent relative strengths of carbon and nitrogen fluxes.

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Schneider, B. , Schlitzer, R. , Fischer, G. and Nöthig, E. M. (2003): Depth Dependent Elemental Compositions of Particulate Organic Matter (POM) in the Ocean , Global biogeochemical cycles, 17(2).1029/2002GB001871, 10 .

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