Department of Earth System Science

Surface sediments along a transect from an abyssal site in the northeastern Pacific (Station M, 34°50′N, 123°00′W) to a small mountainous river on the California coast (Santa Clara River) were studied to investigate the sources and cycling of organic matter on the continental margin. Sediment samples were separated into organic compound fractions (extractable lipids, amino acids (THAA), carbohydrates (TCHO), and the acid-insoluble fraction), and their carbon isotope ratios were measured. The Δ¹⁴C values of all the THAA and TCHO fractions were greater than −100‰, indicating relatively modern organic carbon (OC) source(s), and rapid cycling of these fractions. In contrast, the Δ¹⁴C values of extractable lipids and the acid-insoluble fraction were distinctly lower than those of the THAA and TCHO fractions. The Δ¹⁴C values of source OC to the sediments were estimated using a simple mixed layer model. These values were lower than the Δ¹⁴C signatures of pre-industrial plankton suggesting input of both old OC and contemporary plankton to the margin sediments. The source of old OC at the 2000-m site was likely from laterally transported coastal sediment. The estimated low Δ¹⁴C value of the transported OC suggests that old lipids and acid-insoluble material were selectively transported to the 2000-m site. The contribution of riverine POC to the margin sediments were estimated from Δ¹⁴C and δ¹³C values and indicate that relict OC exported by rivers was an important source of old lipids and acid-insoluble material to sedimentary OC on the shelf.