Alkenones and particulate fluxes in sediment traps from the central equatorial Pacific

We investigated a year-long (September 1992 to August 1993) time series of total mass, calcium carbonate, organic carbon, opal, and alkenone fluxes in sinking particles collected with sediment traps moored at 1770 and 4220 m in the central equatorial Pacific. The total mass fluxes varied from 14.7 to 68.7 mg/m2/day at 1770 m, with greater fluxes in October–November and February–April, and from 14.6 to 50.4 mg/m2/day with peak fluxes during October–November at 4220 m. High flux in the spring season shown at 1770 m was not indicated at 4220 m; instead, a slight increase was shown during a broad period from March to June. The calcium carbonate fluxes varied from 10.8 to 49.1 mg/m2/day with higher fluxes in October–November and March–April at 1770 m, and from 8.9 to 37.0 mg/m2/day with a higher flux in October–November at 4220 m. The organic carbon fluxes varied from 0.36 to 5.91 mg/m2/day, with higher fluxes in October–November and March–April at 1770 m, and from 0.72 to 2.58 mg/m2/day at 4220 m. The annual mean organic carbon flux was 1.84 and 1.28 mg/m2/day at 1770 and at 4220 m, respectively. These values were less than half of those reported for the EqPac sediment trap experiment. The opal fluxes varied from 0.55 to 4.4 mg/m2/day at 1770 m and from 1.23 to 2.95 mg/m2/day at 4220 m. Alkenone fluxes varied significantly from 0.05 to 0.84 μg/m2/day, with high values in November, February–March, and June at 1770 m. For the 4220 m trap, these values ranged from 0.05 to 0.25 μg/m2/day, with slightly higher fluxes in April–May and June–July, which followed periods of high alkenone fluxes observed in February–April and June–July, respectively, at 1770 m depth. These values were remarkably low compared with those reported by the previous studies at other sites. U37K′ values were constantly high >0.95 throughout the collection period. However, relatively low U37K′ values (0.92 and 0.93) were occasionally observed during February to March. Estimated alkenone temperatures from those U37K′ values were about 27–29°C and consistent with the observed temperature of the upper layer at ca.100 m depth. The seasonal change of the U37K′ values could be affected by not only water temperature but also the relative amount of ‘warm’ and ‘cold’ types of alkenone producer in the central equatorial Pacific.