Diploptene: An indicator of terrigenous organic carbon in Washington coastal sediments

The pentacyclic tritcrpene 17/3(H),2 1 P(H)-hop-22(29)-ene (diploptenc) occurs in sediments throughout the Columbia River drainage basin and off the southern coast of Washington state in concentrations comparable to long-chain plantwax n-alkanes. The same relationship is evident for diploptene and long-chain n-alkanes in soils from the Willamette Valley. Microorganisms indigenous to soils and soil erosion are indicated as the biological source and physical process, respectively, for diploptene in coastal sediments. Similarity between the stable carbon isotopic composition (S13C,,,) of diploptene isolated from soil in the Willamette Valley (3 1.2 +-0.3?&1) and from sediments deposited throughout the Washington coastal environment (3 1.2 +0.5%0) supports this argument. Values of 6 for diploptene in river sediments are variable and 8-l 7% lighter, indicating that an additional biological source such as methane-oxidizing bacteria makes a significant contribution to the diploptene record in river sediments. Selective biodegradation resulting from a difference in the physicochemical association within eroded particles can explain the absence of the more-13Cdepleted form of diploptene in Washington coastal sediments, but this mechanism remains unproven. The transport of organic C to the ocean by rivers is tied closely to runoff (Meybeck 1982). The ratio of dissolved to particulate organic C (DOC : POC) varies widely beAcknowledgments We thank Louis Prahl and Howard Turin for assistance in sample collection, Bruce Caldwell for the Costa Rican soil sample, Margaret Sparrow for help with lipid extractions, Michel Rohmer and Bernd Simoneit for authentic triterpene standards, and Bernd Simoneit for use of his GUMS facility. We also thank Bruce Caldwell, Roy Carpenter, John Hedges, and four anonymous reviewers for criticism of an earlier draft of this manuscript. This research was supported in part by NSF grants OCE87-16244 and 89-15720 and by NASA grant NAGW1940. Acknowledgment is also made to the Donors of The Pctrolcum Research Fund, administered by the American Chemical Society, for partial support through PRF grant 16735-AC2. tween rivers, ranging from 0.1 to 0.9. An important factor governing this proportion is the topography of the drainage basin. Organic C in lowland rivers is typically dominated by DOC, whereas POC gains importance in highland rivers. Of the 0.59 Tg organic C yr-l exported by the Columbia River to the northeastern Pacific Ocean, 89% is DOC and 11% is POC (Dahm et al. 198 1). Riverine DOC appears to pass conservatively through estuaries (Mantoura and Woodward 1983), and its fate in the ocean is uncertain. In contrast, the fate of riverine POC is better constrained. No change of phase is required to facilitate its sedimentation from ocean waters, and the dispersion of such detrital materials is governed by the competence of water currents to transport particles away from the river mouth. Most of the POC discharged by rivers is deposited on continental shelves in close proximity to the point of input (Berner 1982). The coastal region off Washington state receives an estimated 14.3 Tg yrl of sedimentary particulate material from the Columbia River (Karlin 1980). In this region, suspended particulate material and its associated organic constituents are hydraulically sorted in the process of dispersion. As a result, distinct bands of sediment accumulate parallel to shore and extend northnorthwest from the river mouth (Fig. 1). Prahl (1985) examined the composition of lignin phenol, polycyclic aromatic hydrocarbon (PAH), and aliphatic hydrocarbon mixtures contained in a grid of samples collected from these sedimentary facies. Three independent lines of evidence indicated that the composition of terrigenous organic matter deposited in Washington continental shelf and slope sediments varies spatially as a consequence of differential dispersion of