Temporal variability in sediment fluxes in the San Pedro Basin, southern California bight

Abstract

A 6 month sediment trap study (January–July 1988) conducted in San Pedro Basin off southern California in which bi-weekly particulate samples were obtained, indicates that significant, high frequency temporal variability exists in the flux of particulate matter to the sea floor at this site. During this period, three intervals of high fluxes were observed for both lithogenic and biogenic sediment components, which can be correlated to local climate and oceanographic events. The highest total fluxes occurred in winter (late January—early February), with up to 70% of the flux being lithogenic. The high lithogenic flux during this time of year coincides with the time of highest rainfall, and increased run-off probably contributed to the high detrital fluxes. Resuspension and basinward advection of shelf sediments may have also contributed to the winter peak in lithogenic flux. High C/N ratios and low δ 13C org values suggest the input of continentally-derived organic carbon to San Pedro Basin during the winter flux peak. Two other periods of high flux occur during April-early May and June. The April-early May flux peak is a function of both a spring bloom in early April and the onset of upwelling in late April. Both hydrographic regimes result in high biogenic fluxes (calcium carbonate, biogenic silica and organic carbon). The June increase in flux is associated with a second phase of upwelling. The average organic carbon flux at 500 m water depth during the study period was 0.032 g C m −2 day −1, which is less than 25% of the expected new production. Utilizing the empirical relationship developed by Suess [(1980) Nature, 228, 260–263], for estimating organic carbon flux as a function of primary productivity and water depth, we predict an average organic carbon flux of 0.032 g C m −2 day −1 at our trap depth (500 m) for this region. The excellent match between observed and predicted organic carbon fluxes suggests that our sediment trap sampling provides a reliable measure of changes in overlying surface water productivity. Similarly, total sediment flux estimates are in good agreement with sea-floor sediment accumulation rates, particularly when remineralization of biogenic material is taken into account.