162 Preliminary Report

Site 984

Site 984 (BJORN-1) is located on the Bjorn Drift at a water depth of approximately 1660 m on the eastern flank of the Reykjanes Ridge. This is within the core of Glacial North Atlantic Intermediate Water (GNAIW) during the last glaciation. Obtaining a long-term history of this water mass was one of the primary scientific objectives of this site and, in conjunction with Sites 980, 981, and 982 to the east and Site 983 to the south, this site will be used to assess east-west gradients in surface-water conditions, as well as to monitor Norwegian-Greenland Sea overflows across the Greenland-Scotland Ridge. The high sedimentation rates expected (and found) here will provide us with an unprecedented record of both glacial-interglacial and millennial-scale variations in thermohaline circulation, surface-water temperatures, and ice-rafting history during the Pliocene and Pleistocene.

A continuous sequence of sediments ranging in age from lower upper Pliocene to Holocene (3.0 to 0 Ma) was recovered at Site 984. Calcareous nannofossils are the dominant fossil group at this site. However, all fossil groups exhibit variable abundance and preservation, possibly correlated to glacial-interglacial events. Sedimentation rates were determined using magnetostratigraphy combined with the biostratigraphic datums, and indicate accumulation rates of ~10-13 cm/k.y. MST data allowed construction of a continuous composite section down to about 270 mbsf. Preliminary studies done aboard ship indicate strong variance in a number of parameters on both Milankovitch and sub-Milankovitch time scales.

Sediments at Site 984, predominantly composed of rapidly accumulated fine-grained terrigenous particles, are very similar to those at Site 983. Discrete ash layers occur throughout the upper sediment column, and pale to dark brown glass commonly occurs as a constituent of the silt- and sand-sized fractions. Authigenic iron sulfides, primarily in the form of disseminated pyrite, are also commonly present in minor amounts. The dominant lithologies include silty clay, clay, clayey nannofossil mixed sediment, and clay with variable amounts of nannofossils and silt. Nannofossil oozes with variable amounts of clay and sponge spicules also occur. As at Site 983, lithologic variation on a decimeter- to meter-scale characterizes the sediment at this site, and is due to changes in the abundance of silt and biogenic materials relative to clay content.

Only one lithostratigraphic unit is defined at Site 984, with a subdivision into four subunits, IA to ID. Changes in the spectral reflectance, the character of the magnetic susceptibility signal, and the occurrence or abundance of minor lithologies interbedded within the dominant clays and silty clays, define subunit boundaries at 120, 165, and 279 mbsf. While the sediments recovered at Sites 983 and 984 share many similarities, there are notable differences. One difference is the more common occurrence of dropstones at Site 984 possibly due to iceberg trajectories and/or increased melting in this region. Another difference between the two sites is the pronounced abundance of discrete ash layers in Subunits IA and IB at Site 984. This may be due to the proximity of Site 984 to Iceland, an obvious source of ash falls from discrete eruptions.

Calcium carbonate contents in Hole 984B range from 0.4% and 32.2% with an average value of 8.0%. CaCO3 gradually decreases downhole and fluctuates, having a lower amplitude with increasing depth. As at Site 983, the carbonate cycles of Hole 984B most likely reflect glacial-interglacial fluctuations.

Calcareous nannofossils are generally abundant and well preserved in the upper 200 m at this site, with both the abundance and preservation deteriorating lower in the section. All the standard Quaternary nannofossil zones are recognized. Similar to the nannofossils, planktonic and benthic foraminifers are generally well-preserved and common throughout the upper Pliocene to Holocene at Site 984, but become progressively scarcer below 200 m. Diatoms vary in preservation and abundance while siliceous flagellates display scattered occurrences downsection in Holes 984A and 984B. The abundance of these microfossils ranges from trace to common with good to poor preservation.

TO Site 907

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