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Suess, E., von Huene, R., et al., 1990 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 112 33. CHARACTERIZATION OF SORBED VOLATILE HYDROCARBONS FROM THE PERU MARGIN, LEG 112, SITES 679, 680/681, 682, 684, AND 686/6871 Michael J. Whiticar2 and Erwin Suess34 ABSTRACT Bacterial and thermogenic hydrocarbons are present in the sorbed-gas fraction of Peru margin sediments. At Ocean Drilling Program (ODP) Sites 681, 682, 684, and 686, bacterial gases are restricted to the early diagenetic zones, where dissolved sulfate has been exhausted and methanogenesis occurs. Methane migrating into the sulfate zone at Sites 681, 684, 686, and possibly 682, has been consumed anaerobically by methanotrophs, maintaining the low concentrations and causing an isotope shift in S13CCH4 to more positive values. Significant amounts of C2+ hydrocarbons occur at the shelf Sites 680/681, 684, and 686/687, where these hydrocarbons may be associated with hypersaline fluids. There is evidence at Site 679 that sorbed C2+ hydrocarbons may also have been transported by hypersaline fluids. This characteristic C2+ hydrocarbon signature in the sorbed-gas fractions of sediments at Site 679 is not reflected in data obtained using the conventional "free-," "canned-," or "headspace-gas1' procedures. The molecular and isotope compositions of the sorbed-gas fraction indicate that this gas may have a thermogenic source and may have spilled over with the hypersaline fluids from the Salaverry Basin into the Lima Basin. These traces of thermogenic hydrocarbon gases are over-mature (about 1.5% Ro) and are discordant with the less-mature sediments in which they are found. This observation supports the migration of these hydrocarbons, possibly from continental sources. Sorbed-gas analyses may provide important geochemical information, in addition to that of the free-gases. Sorbed-gases are less sensitive to activities in the interstitial fluids, such as methanogenesis and methanotrophy, and may faithfully record the migration of hydrocarbons associated with hypersaline fluids. INTRODUCTION Continental margin sediments off central Peru contain an enormous amount and variety of organic matter derived from coastal upwelling productivity (Farrimond et al., Suess et al., Whelan et al., this volume). The use of biological markers, an approach that links complex organic compounds to specific biological sources, yields important environmental informa-tion from this sizable carbon reservoir (Farrimond et al., Kvenvolden et al., Repeta, ten Haven, et al., this volume). The high organic matter content also fuels vigorous syndepo-sitional and post-depositional alteration, which is dominated initially by microbial sulfate reduction (sulfate-reducing bac-teria = SRB), followed by methanogenesis through carbonate reduction (Thornburg and Suess, this volume). These pro-cesses contribute to a variety of organic compounds. Bacterial methane, in particular, reaches considerable concentrations, even sufficient to form gas hydrates (Kvenvolden and Kast-ner, this volume). Sorbed organic matter is a small pool of the total organic carbon reservoir in these sediments and is somehow associated with and stabilized by the mineral con-tent (Suess and Muller, 1980). Sorbed hydrocarbons in the volatile range (Q through C5) usually occur in low concentrations; their exact source, trans-port mobility, and stabilization against microbial and thermal alteration is poorly understood. Sediments of the Peru margin drilled during Leg 112 offer scientists the opportunity to examine the range of sorbed volatile hydrocarbons associated 1 Suess, E., von Huene, R., et al., 1990. Proc. ODP, Sci. Results, 112: College Station, TX (Ocean Drilling Program). Bundesanstalt fur Geowissenschaften und Rohstofte, Stilleweg 2, 3000 Hannover 51, Federal Republic of Germany. College of Oceanography, Oregon State University, Corvallis, OR 97331. Now at GEOMAR, Research Center for Marine Geosciences, Wischhof-strasse 1-3, 2300 Kiel 14, Federal Republic of Germany. with the mineral matrix and to determine the role of the geochemical environment on their distribution, specifically, whether (1) sorbed volatile hydrocarbons exhibit an affinity in composition and stable isotope characteristics to the free hydrocarbon gases, (2) their distribution may be controlled by clay minerals or sediment provenance, and (3) their ultimate source and mobility can reasonably be inferred. Here, we report the results of shore-based hydrocarbon gas analyses from seven drill sites occupied during ODP Leg 112. Sites 680/681, 684, and 686/687 are located in the shelf basins of Salaverry, Trujillo, and West Pisco at water depths be-tween 151 and 447 m (von Huene, Suess, et al., 1988). These sites receive direct influx from three coastal upwelling areas centered around 9°, 11°, and 13°S latitude, respectively (Fig. 1). The three basins differ in their rate of sedimentation. The fastest deposition took place at Sites 686/687, intermediate deposition occurred at Site 680, and the slowest deposition was at Site 684. The two other sites, 679 and 682, lie along a westward extension of Site 680, beyond the shelf edge on the upper slope (Site 679; water depth of 440 m) and middle slope (Site 682; water depth of 3789 m). The subsurface of the shelf basins has been permeated by a strongly saline brine that apparently moves from the central shelf northward toward Site 684 and southward toward Sites 686/687 (Kastner et al., this volume; Suess, von Huene, et al., 1988). Site 679 is located at the westernmost extent of this subsurface brine. Here, the Quaternary/ Pliocene units are influenced by the brine, while the Miocene units are unaffected. Site 682 is located outside the range of the shelf brine and, unlike the shelf sites, receives sediments that are reworked by bottom currents. The mean rate of sedimentation is high for a deep-water site and, hence, comparable to most of the shelf sites. In all respects, Site 682 may serve as a remote reference environment with which the sorbed hydrocarbons from the shelf setting may be compared. 527M. J. WHITICAR, E. SUESS 81 °W 80° 79° 78° 77° Figure 1. Location map of Leg 112 sites and basins with postulated subsurface brines. PROCEDURES Hydrocarbon gases are characterized by their molecular and stable isotope (13C/12C and D/H) compositions, in con-junction with their environmental setting; i.e., primary up-welling facies, reworked facies with and


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