Carbon isotope ratios of organic matter in Bering Sea settling particles: Extremely high remineralization of organic carbon derived from diatoms

The carbon isotope ratios of organic carbon in settling particles collected in the highly-diatom-productive Bering Sea were determined. Wet decomposition was employed to oxidize relatively fresh organic matter. The amount of unoxidised organic carbon in the residue following wet decomposition was negligible. The δ13C of organic carbon in the settling particles showed a clear relationship against SiO2/CaCO3 ratio of settling particles: approximately –26‰ and –19‰ at lower and higher SiO2/CaCO3 ratios, respectively. The δ13C values were largely interpreted in terms of mixing of two major plankton sources. Both δ13C and compositional data can be explained consistently only by assuming that more than 98% of diatomaceous organic matter decays and that organic matter derived from carbonate-shelled plankton may remain much less remineralized. A greater amount of diatom-derived organic matter is discovered to be trapped with the increase of SiO2/CaCO3 ratio of the settling particles. The ratio of organic carbon to inorganic carbon, known as the rain ratio, therefore, tends to increase proportionally with the SiO2/CaCO3 ratio under an extremely diatom-productive condition.