ALBERT

Description: Heavy hydrocarbons (~C15 + ) are ubiquitous but minor components in sediment from three sites (692, 693, and 694) drilled during Ocean Drilling Program (ODP) Leg 113. This preliminary report is the first to describe the distribution of some of these hydrocarbons in sediment of the Weddell Sea. Samples from Sites 692 and 693, located on a midslope bench along the margin of Dronning Maud Land, span a time interval from Early Cretaceous to Pleistocene. In our samples from the Tertiary portion of the record, having an average organic-carbon content of about 0.2%, n-alkanes are common and are characterized by populations that indicate both marine and terrigenous sources. The alpha beta-hopanes and aaa-steranes are minor components with isomer ratios indicating partial to full maturity. In contrast, our samples from the Cretaceous portion of the record, having an average organic carbon content of about 4%, contain mixtures of hydrocarbons in which n-alkanes are secondary in abundance to the isoprenoid hydrocarbons, pristane and phytane. Diasterenes, sterenes, and hopenes are present in anomalously high concentrations and indicate immaturity. The Cretaceous hydrocarbons appear to be mainly primary, whereas the Tertiary hydrocarbons contain compounds that indicate that the sediment, along with its organic content, has been recycled. Our samples from Site 694, located in the Weddell Sea on the abyssal plain, range in age from late Miocene to early Pliocene, have an average organic-carbon content of about 0.2%, and contain n-alkanes with a strong terrigenous signature. Isomer ratios of alpha beta-hopanes and aaasteranes indicate intermediate maturity; diasterenes, sterenes, and hopenes are absent. The hydrocarbons at this site are mainly recycled and indicate erosion of previously buried sediment from the continent of Antarctica.

Description: Several meters of unconsolidated hydrothermal sediment were recovered from the Snake Pit hydrothermal field during ODP Leg 106. Polymetallic sulfides comprise most of the sediment with minor fragments of massive sulfide, organic debris, clay minerals, and fresh glass shards. Trace element and Sr-isotope contents of hydrothermal clays and sulfides from Holes 649B and 649G indicate that these minerals precipitated from a mixed hydrothermal fluid-seawater solution. Evaluation of the REE mineral data and the Snake Pit hydrothermal fluids shows that the REE distribution coefficients between the hydrothermal fluids and clay-sulfide mixes range from 100-500. This indicates that hydrothermal fluids originating in the root-zone of the Snake Pit hydrothermal system may be modified by the precipitation of hydrothermal minerals, either in the shallow subsurface or within chimney structures. Contrasting REE profiles of clay-sulfide aggregates and massive sulfides from Holes 649B and 649G may be accounted for by spatial and/or temporal variations in redox conditions in the plumbing system.