A 160,000-YEAR HIGH-RESOLUTION RECORD OF QUANTITY AND COMPOSITION OF ORGANIC CARBON IN THE SANTA BARBARA BASIN (SITE 893)

Ruediger Stein and Frank R. Rack

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.

Date of initial receipt: 30 June 1994
Date of acceptance: 13 February 1995


Return to Contents of Leg 146B
Return to Contents of Scientific Results