162 Preliminary Report

Site 982

Site 982 (NAMD-1), the site with the shallowest water depth (1145 m), will allow documentation of the evolution of intermediate waters of the North Atlantic during the Neogene. This record will help reconstruct water-mass behavior in the North Atlantic on glacial-interglacial time scales of the Pliocene/Pleistocene, as well as during the middle to late Miocene interval when the Iceland-Scotland Ridge subsided to depths that allowed deep water exchange between the Nordic seas and the North Atlantic. This site should also enable reconstruction of the intermediate water-mass structure of the North Atlantic during the latest Miocene Messinian events. In addition, the recovery of a lower middle Miocene section should document North Atlantic water-mass circulation at times when no North Atlantic Deep Water (NADW) was thought to exist.

Site 982 recovered a continuous, carbonate-rich sequence of sediments extending to the lower Miocene, and triple APC coring down to 235 mbsf has provided a complete composite section of approximately the last 7 Ma. Extended Core Barrel coring from 235 to 615 mbsf in Hole 982B provided good documentation of the 7-19 Ma interval. Recovery decreased below 480 mbsf where silicified layers started to appear.

The sediments have an average calcium carbonate content of about 86%, and are predominantly composed of nannofossil ooze with variable amounts of clay, clayey nannofossil mixed sediments, and clays with variable amounts of nannofossils and silt. A distinct boundary, occurring at approximately 57.4 mbsf, divides the sequence into two units. This boundary marks the initiation of major Northern Hemisphere glaciation in the late Pliocene, and is characterized by a sharp downcore decrease in siliciclastic sediments and magnetic susceptibility, as well as by pronounced downcore increases in calcium carbonate content and spectral reflectance. The sediments in Unit I are dominated by variable amounts of calcareous nannofossils, clay, and, to a lesser extent, silt and foraminifers. These variations are responsible for the considerable color contrast observed between the oozes and clays within the unit, particularly when compared with Unit II. These compositional changes form distinct high-amplitude cyclic variations which occur on a 0.5- to 3-m scale, and are mainly related to relative changes in the proportions of biogenic carbonate, detrital clay minerals, and, to a lesser extent, detrital silt. All dropstones observed in Site 982 sediments occur in Unit I.

Unit II sediments are dominated by biogenic carbonate, primarily nannofossil ooze, with only minor amounts of clay, and, to an even lesser extent, biogenic silica. The mean carbonate content of Unit II, 90.8%, is considerably higher than that of Unit I. The unit is characterized by very light gray, light gray, and very light greenish-gray nannofossil ooze. The 195.5 to 480.0 mbsf interval is Miocene in age and very homogenous with the sediments approaching a chalk-like hardness downcore. Several distinct ash layers were identified in the unit, and the lower part of the unit contains chert-like silicified layers.

Age control for the sequence is primarily based on magnetostratigraphy, and on calcareous nannofossil and foraminiferal biozonation. The magnetic signal is too weak to provide reliable polarity sequences below the Matuyama/Gauss boundary (2.6 Ma) and calcareous fossils provide the primary age control below this boundary. The bottom of the drilled sequence is about 19 Ma, and no major breaks in sedimentation are indicated by shipboard analyses. Sedimentation rates average about 25 m/m.y. for the mid-Pliocene through Pleistocene. Below this level sedimentation rates increase to about 32 m/m.y.

A sharp horizon was identified within Unit II at 268 mbsf consisting of a poorly recovered silicified foraminiferal sand cobble. This horizon, in sediments 7-8 Ma, marks the upper regional seismic reflector of the Rockall Basin (Reflector R1). Downhole logs indicate that the silicified material is a 4-m-thick sequence which we tentatively interpret as a turbidite. This silicified layer apparently formed a barrier that dampened or disabled pore-water diffusion, as indicated by distinct differences in pore-water profiles above and below the layer. This apparent lack of diffusion may open possibilities for studying differences in ocean chemistry (especially salinity) before and after the Messinian salinity crisis.

TO Site 983

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