Extensive research on deep sediments obtained by the Ocean Drilling Program (ODP) has demonstrated the presence of a deep bacterial biosphere in marine sediments to depths of up to 750 m below seafloor (mbsf) (Whelan et al., 1986; Tarafa et al., 1987; Parkes et al., 1990, 1993, 1994; Cragg, 1994; Cragg et al., 1990, 1992, 1996, 1998; Cragg and Parkes, 1994; Cragg and Kemp, 1995; Cragg et al., 1995a; Cragg et al., 1995b; Wellsbury et al., 1997). The presence of bacteria has also been demonstrated in basaltic rocks (Furnes et al., 1996; Giovannoni et al., 1996; Fisk et al., 1998) and Cretaceous shales and sandstones (Krumholz et al., 1997).
Previous studies on the Middle Valley on the northern Juan de Fuca Ridge (Leg 139, Site 858) demonstrated that substantial bacterial populations, including dividing cells, were present in deep sediments within submarine hydrothermal systems, although their depth distributions were limited by the high thermal gradient (1.7°-11°C m-1; Cragg and Parkes, 1994). Sites 1026 and 1027 provided an opportunity to analyze sediments in a similar geological setting, but with a markedly lower thermal gradient (~0.17°C m-1) to clarify the impact of elevated temperature gradients on bacterial depth distributions. These sites also provide "cool" control sites to contrast with bacterial distributions to be subsequently analyzed on Leg 169, which is also in the Juan de Fuca area but subject to high thermal gradients.