Sediment trap record of alkenones from the East Sea (Japan Sea)

Long chain (C37) alkenones in sinking particles from the East Sea were examined using time-series sediment trap records from the eastern Japan Basin (trap depth 1057 m and 3043 m) and the Yamato Basin (1057 m and 2100 m) for the period of October 2000 to August 2001. The 11month records of alkenone flux show that the flux to the upper trap appears to be higher during summer (June to August) and fall (October to November) than during the rest of the sample collection period at both stations. However, the seasonal trends in alkenone flux to the lower trap do not show such a strong seasonality. In general the alkenone sinking flux decreased with increasing depth. Despite the decomposition of the alkenones, there was little difference in U37K′ values and hence alkenone temperatures between traps. A comparison of alkenone-derived temperatures with satellite sea surface temperatures (SSTs), however, reveals that the alkenone temperatures were higher (7-11 °C) than SST during winter time and lower (2-5 °C) during summer time. This indicates that there is a time lag between the SST and trap records (about 3 months in winter and 1 month in summer). The reasons for the time lag could be (i) slow settling velocity and long residence time of alkenones in the upper water column especially during winter time when alkenone flux was low, (ii) biotic and abiotic degradation of alkenones and (iii) subsurface production of alkenones in summer. The flux-weighted alkenone temperatures are close to annual average SST at the sites, indicating that the sedimentary alkenone signal from the seafloor may represent the annual averaged temperature.