162 Preliminary Report

Sites 980/981

Sites 980 and 981 are located on the Feni Drift, east of the Rockall Bank, in the northeast Atlantic. The drift was deposited along the northwestern flank of Rockall Trough under the influence of geostrophic currents formed by Norwegian Sea overflow waters flowing across the Iceland-Scotland Ridge and deeper waters originating from the south (including Antarctic Bottom Water). The excess deposition of fine-grained sediment on the Feni Drift produced expanded sediment sections (>10 cm/k.y.) that are ideally suited for high-resolution paleoceanographic studies.

These sections consist of rapidly accumulated nannofossil ooze with variable amounts of clay and clayey nannofossil mixed sediments. The main component of lithologic variability occurs on decimeter to meter scales throughout the sections, in the form of cyclic changes in color that are mainly related to relative changes in the proportions of biogenic carbonate, detrital clay, and silt. The upper unit (Unit I), recovered at Site 980 (0 to 114 mbsf) and Site 981 (0 to 160 mbsf), includes the upper Pliocene to Holocene, and is characterized by alternating dark and light gray clays and oozes. The lower unit (Unit II), 160 to 320 mbsf at Site 981, encompasses the lower to upper Pliocene and consists of a more homogenous nannofossil ooze with subtle color changes. Carbonate content is higher and less variable than in Unit I (80%-90% vs. 20%-80%). Proceeding downhole, the boundary between the two units is marked by a sharp decrease in magnetic susceptibility and natural gamma radiation, and a sharp increase in carbonate content and spectral reflectance.

All cores recovered were measured with the multisensor track (MST). Correlation of natural gamma radiation, gamma-ray attenuation, and magnetic susceptibility records confirms that we have recovered a complete stratigraphic sequence at Site 980 and 981 to a depth of 250 meters composite depth (mcd), which is about 230 mbsf, or approximately 3.4 Ma. The MST records are easily correlated between the holes as well as between the two sites. However, at Site 981 small coring gaps may exist below 230 mbsf. Low-amplitude MST signals prevented unambiguous correlation between holes over this interval.

Age control is derived from paleomagnetic datums down to the Matuyama/Gauss boundary (where the magnetic signal deteriorates), and from nannofossil and foraminifer biostratigraphy. Site 980 extends to about 1.2 Ma whereas Site 981 extends beyond 5 Ma. At Site 980, estimated sedimentation rates (in the composite section) are ~135 m/m.y. in the Brunhes section, decreasing to about 70 m/m.y. in the Matuyama section. Based on color variations, it would appear that interglacial sedimentation rates are significantly higher than glacial rates. At Site 981, sedimentation rates average ~55 m/m.y. in the Pleistocene and ~70 m/m.y. in the Pliocene, although there may be an interval with a rate as high as 125 m/m.y. between ~2.4 and 3.0 Ma.

The highest resolution shipboard analyses are from the MST and spectral reflectance data, and both appear to exhibit millennial-scale oscillations. In all intervals, these proxies correlate extremely well with each other, as well as with shipboard measurements of carbonate percentage. The onset of major Northern Hemisphere glaciation (and ice-rafted debris [IRD] input) and the transition from 41-k.y. climate variability to 100-k.y. variability are obvious in these records. As the amplitude of the MST signals (including susceptibility) decreased significantly below 2.5 Ma, we looked to the spectral reflectance record for evidence of sub-Milankovitch-scale lithologic variability in the preglacial Pliocene. Given the 8-cm (or approximately 1000 yr.) resolution of the measurements, there appears to be a precessional signal (1-2 m cycle) and a 5-10-k.y. cycle (~50 cm cycle) within this interval.

The extremely high sedimentation rates and strong magnetic signal at Site 980 will permit high-resolution studies of paleomagnetic transitions, as well as secular variation in the intensity of the magnetic field. Likewise, these two sites, ~4 km apart, provide a natural laboratory for investigating the effects of sedimentation rate on pore-water chemistry and organic matter preservation. In particular, sulfate reduction appears to be more prevalent in the upper sections of Site 980 vs. 981, with Site 980 displaying approximately 25% higher accumulation rates. Major ion and stable isotopic studies on pore-water samples collected on the ship will be completed on shore.

TO Site 982

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