A 160,000-Year High-Resolution Record of Quantity and Composition of Organic Carbon in the Santa Barbara Basin (Site 893)

Abstract

To identify the factors controlling the organic-carbon accumulation in the Santa Barbara Basin and their relationship to the (global) climatic history through the last 160,000 years, total organic carbon (TOC), Rock-Eval parameters, and C/N ratios were determined on 990 samples from Hole 893A. For a selected set of samples, n-alkanes and pristane/phytane ratios were also determined. In the major lithologies, TOC values vary between 1% and 4%, with the higher values more typical for the interglacials and the lower values more typical for the glacials. The source of the organic matter is a mixed marine/terrigenous type, with a higher marine proportion during interglacials. Gray flood/turbidity deposits and sandy turbidites are characterized by significantly lower TOC values of 0.9%—1.8% and 0.1%—0.3%, respectively, with a clear terrigenous origin. During the Holocene, increased surface-water productivity and the inflow of oxygen-depleted waters from the East Pacific probably resulted in anoxic bottom water and, thus, in the preservation of varved-like laminations and large amounts of marine organic carbon. During the warm interstadials 5e and 5a, surface-water productivity and, thus, marine organic carbon flux also increased; anoxic bottom water conditions, however, were not reached throughout, as indicated by the dominance of massive non-laminated sediments. Furthermore, distinct higher-frequency variations of a few thousands of years are present in Stages 1, 5a, and 5c. During glacial Stages 6 and 4 to 2 and cold interstadials 5d and 5b, surface-water productivity was reduced and the bottom-water conditions in the Santa Barbara Basin were more oxygenated. At those times of lowered sea level, also the supply of terrigenous organic matter was probably increased.