Ocean Drilling Program, United States

Siliciclastic sedimentation at Ocean Drilling Program Site 1017 on the southern slope of the Santa Lucia Bank, Central California margin, responded closely to oceanographic and climatic change over the past approximately 130 ka. Variation in mean grain-size and sediment sorting within the approximately 25-m-thick succession from Hole 1017E show Milankovitch-band to submillenial-scale variation. Mean grain size of the "sortable silt" fraction (10-63 mu m) ranges from 17.6 to 33.9 mu m (average 24.8 mu m) and is inversely correlated with the degree of sorting. Much of the sediment has a bimodal or trimodal grain-size distribution that is composed of distinct fine silt, coarse silt to fine sand, and clay-size components. The position of the mode and the sorting of each component changes through the succession, but the primary variation is in the presence or abundance of the coarse silt fraction that controls the overall mean grain size and sorting of the sample. The occurrence of the best-sorted, finest grained sediment at high stands of sea level (Holocene, marine isotope Substages 5c and 5e) reflect the linkage between global climate and the sedimentary record at Site 1017 and suggest that the efficiency of off-shelf transport is a key control of sedimentation on the Santa Lucia Slope. It is not clear what proportion of the variation in grain size and sorting may also be caused by variations in bottom current strength and in situ hydrodynamic sorting.