5:15 PM - 7:15 PM
[AOS20-P02] Contrasting seasonal patterns in carbon sequestration efficiency between nitrogen-excess subarctic and nitrogen-deficient subtropical gyres of the western North Pacific
Keywords:biological carbon pump, stable nitrogen isotope, western North Pacific
Moored sediment trap experiments were conducted between 2010 and 2014 at depth of 500 m at station K2 in the high-nutrient, low-chlorophyll region of the western North Pacific subarctic gyre and at S1 in the oligotrophic North Pacific subtropical gyre. The overall mean nitrogen stable isotope ratio, δ15Nsink, of the collected particles was similar at both sites, however, the monthly values showed contrasting seasonal changes: higher in winter and lower in summer at K2 and vice versa at S1. These δ15Nsink were significantly negatively correlated not only with the particulate organic carbon flux, FPOC, but also the net primary productivity, NPP, measured during cruise observations. Using those empirical regressions and the monthly averages of δ15Nsink for the entire deployments, we calculated the monthly POC sequestration efficiency to 500 m, Seq(500): FPOC/NPP, at both sites. The Seq(500) at K2 showed less seasonal change (from 7.4% in February to 8.1% in June, expressed as a percentage), while it at S1 rose to 6.5% in productive March, compared to 3.4% in June. Such regional difference in seasonality may be attributed to the divergence in cohesion, and settling velocity, SV, of the trapped POC aggregates, both of which would positively affect Seq(500). At S1, the SV and cohesion increase with increased CaCO3 mass fraction in aggregates during winter bloom, while the cohesion at K2 is enhanced with increased biogenic opal fraction during summer bloom, offsetting the decrease in SV due to reduced CaCO3 fraction.