A 4-year sediment trap record of alkenones from the filamentous upwelling region off Cape Blanc, NW Africa and a comparison with distributions in underlying sediments

Abstract

We analysed long-chain alkenones in sinking particles and surface sediments from the filamentous upwelling region off Cape Blanc, NW Africa, to evaluate the transfer of surface water signals into the geological record. Our study is based on time-series sediment trap records from 730 m (1990–1991) to 2195–3562 m depth (1988–1991). Alkenone fluxes showed considerable interannual variations and no consistent seasonality. The average flux of C 37 and C 38 alkenones to the deep traps was 1.9 μg m −2 d −1 from March 1988 to October 1990 and sevenfold higher in the subsequent year. Alkenone fluxes to the shallower traps were on average twice as high and showed similar temporal variations. The alkenone unsaturation indices U K′ 37, U K 38Me and U K 38Et closely mirrored the seasonal variations in sea-surface temperature (weekly Reynolds SST). Time lags of 10–48 days between the SST and unsaturation maxima suggest particle sinking rates of about 80 and 280 m d −1 for the periods of low and high alkenone fluxes, respectively. The average flux-weighted U K′ 37 temperature for the 4-year time series of the deeper traps was 22.1°C, in perfect agreement with the mean weekly SST for the same period. This and the comparison with seasonal temperature variations in the upper 100 m of the water column suggests that U K′ 37 records principally the yearly average of the mixed-layer temperature in this region. A comparison between the average annual alkenone fluxes to the lower traps (2400 μg m −2 yr −1) and into the underlying sediments (4 μg m −2 yr −1) suggests that only about 0.2% of the alkenones reaching the deep ocean became preserved in the sediments. The flux-weighted alkenone concentrations also decreased considerably, from 2466 μg gC −1 in the water column to 62 μg gC −1 in the surface sediments. Such a low degree of alkenone preservation is typical for slowly accumulating oxygenated sediments. Despite these dramatic diagenetic alkenone losses, the U K′ 37 ratio was not affected. The average U K′ 37 value of the sediments (0.796±0.010 or 22.3±0.3°C) was identical within error limits to the 4-year average of the lower traps. The unsaturation indices for C 38 alkenones and the ratio between C 37 and C 38 alkenones also revealed a high degree of stability. Our results do not support the hypothesis that U K′ 37 is biased towards higher values during oxic diagenesis.