ALGAL AND BACTERIAL HYDROCARBONS IN PARTICULATE MATTER AND INTERFACIAL SEDIMENT OF THE CARIACO TRENCH

particulate matter samples (<53 μm and > 53 μm size fractions) and sediment from the sediment-water interface were collected in the Cariaco Trench, an anoxic marine basin on the continental shelf of Venezuela, and analyzed for hydrocarbons. Concentrations of hydrocarbons were highest (10-20 ng/1) in oxic surface waters (0-150 m) and at the top of the anoxic zone (350-600 m), and were low (3-4 ng/l) in the low oxygen zone above the oxic-anoxic interface (150-250 m) and anoxic bottom waters (600-1150 m). Branched C25-trienes and -tetraenes dominated the hydrocarbon distributions in <53 μm particles in the oxic zone, while the isoprenoid C25- and C40-alkanes, 2,6,10,15,19-pentamethyleicosane and lycopane, dominated in the <53 μm particles in the anoxic waters; petrogenic C17-C40 n-alkanes dominated the > 53 μm particles. Particle size and depth distributions of the hydrocarbons demonstrate the importance of algal hydrocarbons in surface waters, coupled with increased abundances of bacterial hydrocarbons, especially in the <53 μm particles, in the anoxic zone. These observations illustrate the importance of oxic-anoxic interfaces, even in the oceanic water column, as zones of intense microbial alteration of organic matter and as sources of some of the organic compounds found in sediments. © 1990.