ABSTRACT

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.