A systematic study of highly branched isoprenoids (HBI) was carried out in suspended particulate material (SPM) and Washington coastal sediments to determine their origin and fate. SPM collected at 10 m depth was filtered through Nitex membranes. C25 HBI were found only in the 1.2-40 mm range over the shelf. The particle size fractionation of SPM shows different enrichment for n-heneicosahexaene (HEH), a common hydrocarbon in phytoplankton, and the sum of C25 HBI in the finer fractions suggesting these hydrocarbons do not share a common source. The distribution of C25 and C25 HBI correlates with the chlorophyll maxima suggesting an upper-water microbial source associated with phytoplankton biomass.

It has been hypothesized that sulfur addition into specific biomarkers occurs during the early stages of diagenesis. Incorporation of the HBI into a refractory geomacromolecule via a sulfur linkage or formation of HBI-thiophenes are not evident in the sedimentary lipids. HBI show a rapid decrease in concentration with depth in both midshelf and slope sediments suggesting that biodegradation is the major pathway for their disappearance in Washington coastal sediments.

Sediment cores from a midshelf and slope locations show the existence of suboxic/anaerobic conditions within the first 5 cm in the sediments. Elemental sulfur distribution in the midshelf appears to be controlled by bioturbation. On the slope, its profile indicates a quasi steady state regime.

Phytane and phytenes (_Phy) are the major products of Raney nickel desulfurization in both midshelf and slope sediments. A strain of the coccolithophorid Emiliana huxleyi obtained from the Sargasso Sea and treated with Raney nickel showed a strikingly similar pattern to the desulfurization products of sedimentary lipids. The amount of _Phy in the slope decreases abruptly by a factor of 6 in the top 2 cm and gradually increases with depth. These results are interpreted as phytyl coming from two sources: (1) chlorophyll-a and (2) S-bound to geomacromolecules. Partial release of phytyl moieties from chlorophyll-a warrants a reevaluation of Raney nickel as a selective desulfurizing agent before its application for paleoenvironmental reconstruction.

Reducing micro-environments appear to exist within the bioturbated zone in shelf sediments. However, there is no clear evidence for phytyl moieties S-linked to macromolecules within the mixed layer. Results obtained during this study indicate that sulfur incorporation to biomarkers, although present, does not represent a significant mechanism for the preservation of organic carbon in normal marine sediments.