162 Preliminary Report

Site 983

Site 983 (GARDAR-1) is located on the Gardar Drift at a water depth of approximately 1995 m on the eastern flank of the Reykjanes Ridge. This is the approximate mid-depth of Glacial North Atlantic Intermediate Water (GNAIW) during the last glaciation. Obtaining a long-term history of this water mass is one of the primary scientific objectives of this site. In conjunction with Sites 980, 981, and 982 to the east, this site will also 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. In particular, Site 983 lies on the northwest margin of the Iceland Basin directly downstream of overflows from the Iceland-Faroe Ridge. The high sedimentation rates expected (and found) here will provide an unprecedented record of both glacial-interglacial and millennial-scale variations in thermohaline circulation, surface-water temperatures, and ice-rafting history during the late Pliocene and Pleistocene.

Site 983 recovered a continuous sequence of sediments ranging in age from upper Pliocene to Holocene (2.0 to 0 Ma). Sedimentation rates, determined using magnetic polarity reversals and biostratigraphic datums, range from 10 cm/k.y. in the upper Pleistocene, up to 17 cm/k.y. in the upper Pliocene section. MST data allowed the construction of a continuous composite section for this site and preliminary studies aboard ship indicate strong variance throughout the section in a number of parameters on both Milankovitch and sub-Milankovitch time scales.

The sediments at Site 983 are predominantly composed of rapidly accumulated fine-grained terrigenous particles with minor amounts of biocarbonate and biosilica. While discrete ash layers are rare, pale to dark brown glass (tachylite) commonly occurs as a constituent of the silt- and sand-sized fractions. Authigenic iron sulfides, primarily in the form of disseminated pyrite, are also typically present. 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. Lithologic variation on decimeter- to meter-scales characterizes the sediment at this site, and is due to changes in the abundance of silt and biogenic materials relative to clay content.

No major lithologic boundaries occur within the 260 m of sediment recovered at this site; therefore, only one lithostratigraphic unit is recognized. Subtle but distinct boundaries that delimit three subunits occur at depths of 120 mbsf and 180 mbsf. The shallower boundary is recognized primarily in the spectral reflectance signal. It is characterized by a downcore decrease in the amplitude of the higher frequency (decimeter- to meter-scale) reflectance signal and an absence of the lower frequency reflectance signal (>10 m-scale). The deeper boundary is recognized in visual examination of split cores and smear slides, and is characterized by a downcore absence of layers in which biocarbonate is predominant. All dropstones, which are never common, occur above this deeper horizon. There is no evidence of significant sediment disturbance, winnowing, or erosion at Site 983, although bioturbation is ubiquitous throughout the cores.

Calcium carbonate contents fluctuate between 0.7% and 43.3% (with an average value of 16.8%), and gradually decrease with increasing sediment depth. As at Sites 980, 981, and 982, the carbonate cycles of Hole 983A probably reflect glacial-interglacial fluctuations. Calcareous nannofossils are the dominant fossil group at this site and are generally abundant and well-preserved. All the standard Quaternary nannofossil zones were recognized. Planktonic foraminifers are generally common to abundant and well-preserved throughout the uppermost Pliocene to Holocene sequence, although rare barren intervals are observed. Benthic foraminifers are present at most of the levels examined and preservation is good throughout. Diatoms at Site 983 were common to abundant and exhibit moderate to good preservation. Due to the possible influence of the East Greenland Current, warmer-water species were rare, whereas many cooler-water indicators were more common. Siliceous flagellates (including silicoflagellates, ebridians, and actiniscidians) range from trace to common in abundance with good to moderate preservation.

Pore-water profiles from Site 983 are typical of sediments in which sulfate reduction and methanogenesis are occurring. Sulfate concentrations decrease from seawater values at the top of the core to zero at about 100 mbsf. Below 120 mbsf, methane begins to increase from 0 parts per million volume (ppmv), reaching a maximum of 9000 ppmv near the base of the hole. The boundary between sulfate reduction and methanogenesis is very sharp at 120 mbsf, presumably because utilization of methane by sulfate-reducing bacteria prevents significant diffusive penetration of methane into the sulfate reduction zone above. The sharp sulfate/methane boundary at 120 mbsf also corresponds with lithostratigraphic subunit boundary IA/IB and with seismic Reflector R2. Ethane and propane values occur in detectable amounts below 165 mbsf; however, the high C1/C2 ratios suggest that the source for methane is most likely in situ bacterial methanogenesis resulting from decomposition of organic matter in the sediments.

TO Site 984

162 Table of Contents

Publications Home

ODP Home