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Sediments from three sites in the Alboran Sea, western Mediterranean were investigated for the presence of bacteria in deep sediments. A series of 1-cm 3 sediment samples were removed from core sections of Hole 976B between 26.9 and 646.6 mbsf (16 samples), Hole 977A between 0 and 525.8 mbsf (29 samples), and Hole 978A between 346 and 644.6 mbsf (9 samples) for direct microscopic determination of bacteria. Sites 977 and 978 were only 24 km apart, contained similar sedimentary sequences and interstitial water profiles, and covered complimentary depth ranges to provide samples from the surface to 644 mbsf. Therefore, these two sites were treated as one site. Significant bacterial populations were present in the deepest samples of both cores at Sites 977/978 (644 mbsf) and 976 (647 mbsf), (10 6 cells/cm 3 and 10 6 , respectively). These bacterial depth distributions correspond well with previous data from Atlantic and Pacific Ocean sites, and they also extend considerably the previous deepest marine sediment samples that have been analyzed for bacteria (514 mbsf), thus extending the depth of the bacterial biosphere in this environment. Because the rate of decrease in bacterial populations does not change in these deep samples, it is likely that bacteria are present even deeper than 650 mbsf in marine sediments. At Site 977/978 a deep brine incursion supplies sulfate below 400 mbsf, and absence of methane in this zone suggests active bacterial anaerobic methane oxidation. Previously a brine incursion in the Peru Margin has been shown to stimulate bacterial activity and to increase bacterial viability at depths of 80 mbsf and ~1 Ma. A similar effect may be occurring at Site 977/978. This effect would explain why the bacterial profile does not decrease with depth in the deeper layers and demonstrates the viability of bacteria in ancient deposits (6 Ma).

No abstract

The oxidation and reduction that occur during early diagenesis of sediments has been studied in the interstitial waters of a rapidly accumulating sedimentary sequence from the Mediterranean margin of Spain. A series of reactions that are mediated by progressively lower free energy derived from oxidation of organic matter is evident in the sedimentary sequence. Iron and manganese are rapidly reduced. Phosphate and alkalinity maxima at a subbottom depth of 15 m indicate maximal organic matter degradation. Methane first appears at &20 m subbottom after sulfate is depleted, and its concentrations quickly climb.