Site 997 is located on the crest of the Blake Ridge, which extends southeastward more than 500 km from the North American Continental Rise (Fig. 1). During Leg 164, three sites were drilled on the Blake Ridge (Sites 994, 995, and 997) that exhibit uniform, rather homogeneous silty-clayey lithologies comprising biogenic and terrigenous sediments through the upper Miocene to Holocene (Paull, Matsumoto, Wallace, et al., 1996). The Blake Ridge has been drilled during several previous DSDP cruises (DSDP Legs 11 and 76); since then much interest has developed, particularly on the gas hydrate occurrences and carbonate diagenesis (Hollister, Ewing, et al., 1972; Sheridan, Gradstein, et al., 1983). The sediment deposition on the Blake Ridge has been attributed to the Western Boundary Undercurrent along the northwestern Atlantic (Heezen et al., 1966). Contourite deposition was first noted as reddish sediment originating from upper Carboniferous sediments in southeastern Canada (Needham et al., 1969), which also has been confirmed during Leg 164 as having occurred from the late Miocene through the Pleistocene (Paull, Matsumoto, Wallace, et al., 1996).
Sedimentation rates are generally equivalent among Sites 994, 995, and 997 on the Blake Ridge, gradually decreasing from late Miocene to Holocene (i.e., 40 to 68 m/m.y. for the entire Quaternary, 89-133 m/m.y. for the late Pliocene, 140-160 m/m.y. for the early Pliocene, and 256 to 340 m/m.y. for the late Miocene; Paull, Matsumoto, Wallace, et al., 1996). Through these time periods, lithologic facies appear to be constant throughout the section. The longest composite section (750 m) was recovered from two holes at Site 997 (Holes 997A and 997B; Fig. 2) and extends into the late Miocene, based on shipboard biostratigraphy. Thus, Site 997 is the most suitable site for establishing the stratigraphic variation of the sediments of the Blake Ridge.
This paper reports the results of shore-based analyses of trace element geochemistry for sediments from Site 997. The major element geochemistry is reported separately (Lu et al., Chap. 14, this volume). The objective of this report is to present a chemical stratigraphy of the Blake Ridge section from the late middle Miocene to Holocene period, based on trace sediment geochemistry.