Algal and bacterial hydrocarbons in particulate matter and interfacial sediment of the Cariaco Trench

Abstract

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 C 25-trienes and -tetraenes dominated the hydrocarbon distributions in <53 μm particles in the oxic zone, while the isoprenoid C 25- and C 40-alkanes, 2,6,10,15,19-pentamethyleicosane and lycopane, dominated in the <53 μm particles in the anoxic waters; petrogenic C 17–C 40 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.