162 Preliminary Report



Site 985

Site 985 (ICEP-3) is located on a gentle slope of the Iceland Plateau, at a water depth of 2799 m, and is part of the paleoenvironmental transect with Sites 907 and 987. The site was a second-priority site for Leg 162, and was cored due to operational constraints which required our staying in the vicinity of Iceland. With the recovered sequences we intend to (1) monitor the history of oceanic and climatic fronts moving east and west across the Norwegian Sea, (2) derive an open-ocean record of IRD and carbonate accumulation, and (3) document the history of formation of northern-source deep waters.

The sediments recovered at Site 985 are predominantly fine-grained siliciclastics. The dominant lithologies include silty clays, clays with silt, and clays. Biocarbonates are restricted to the upper parts of the sedimentary sequence. Clays and silty clays containing biosilica are encountered only between 240 and 290 mbsf. Disseminated volcanic ash, ash pods, and ash layers occur throughout the sedimentary sequence, whereas dropstones are confined to the upper sedimentary sequence (0-70 mbsf). The sequence was dated down to the latest Miocene by means of magnetic polarity records. Below the upper Miocene it became difficult to correlate to the geomagnetic polarity time scale; therefore, the underlying sequence has poor age constraints. Siliceous microfossils and arenaceous benthic foraminifers indicate that the cored sequence ends in the upper Oligocene.

Multisensor track (MST) investigations document that a complete section has been recovered over the upper 131 mbsf (Holocene to latest Miocene) with good overlap across core breaks. Within the last 7 m.y., sedimentation rates are highest in the last 3 m.y. (20-30 m/m.y.), and drop to 10-15 m/m.y. in the middle and early Pliocene. Somewhat higher sedimentation rates (about 20 m/m.y.) are documented for the latest Miocene.

Five lithostratigraphic units were recovered. Unit I (0-17.2 mbsf; Holocene to upper Pleistocene) is defined largely on the basis of relatively abundant biocarbonates (up to 30%) and higher color spectral reflectance than underlying units. The sediments are composed of interbedded layers of gray clayey nannofossil ooze with foraminifers; dark gray nannofossil clay with silt; dark gray silty clay with nannofossils; brown and dark grayish brown clay with silt; and very dark gray to dark grayish brown silty clays. The cyclic, interbedded nature of the sediments testifies to their glacial-interglacial origin.

Unit II (17.2 to 99.2 mbsf; upper Pleistocene to lower Pliocene) is defined, in part, by the diminished presence of biocarbonate which occurs as a variable sedimentary component, averaging 6.9 10.1%. The unit is composed largely of silty clay, clay with silt, and clay. X-ray diffraction (XRD) and smear slide analyses both demonstrate an increase in quartz, plagioclase, and pyroxene within Unit II relative to the underlying sediments. Both reflectance values and natural gamma counts decrease noticeably at the boundary with the underlying unit. Both Unit I and II contain dropstones, in contrast to the underlying units.

Unit III (99.2 to 155.2 mbsf; lower Pliocene to upper Miocene) is characterized by the occurrence of clay with silt, and the absence of biogenic sediments. Isolated spikes of inorganic carbonate are superimposed on a carbonate-free background. XRD analysis of the bulk sediments reveals that plagioclase, quartz, and pyroxene are present to a lesser extent than in Units I and II.

Unit IV (155.2 to 465 mbsf; upper Miocene to upper Oligocene [?]) comprises the bulk of the sedimentary sequence at Site 985. These sediments are distinguished from those of Unit III and V by the transition from silty clay and clay with silt to lithologies in which indurated clays are dominant. Other characteristics of this lithofacies include very low magnetic susceptibility values and the absence of biocarbonates. Yellowish brown carbonate concretions are observed in Unit IV, some of which are composed of fluorapatite, and may be similar in composition to yellowish orange layers found at shallower depths. Unit IV can be subdivided into three distinct subunits. Subunit IVA (155.2 - 241.5 mbsf) is distinguished from the underlying sediments by higher magnetic susceptibility and natural gamma radiation values, and by the absence of biosilica. Two carbonate-rich layers in Subunit IVA may act as barriers to the diffusion of interstitial waters. XRD analysis of the upper layer indicates that it is composed of poorly crystallized carbonate. These layers yield high-velocity measurements and bracket sediments with interstitial waters that have anomalously low chloride, sodium, and salinity content; and unusually high proportions of long-chain hydrocarbon gases relative to methane. Reduced diversity of arenaceous benthic foraminifers also characterize this interval. The cause for these anomalies are not known. Subunit IVB (241.5 - 289.6 mbsf) is defined by the presence of biosilica. Subunit IVC (289.6 - 465 mbsf) is distinguished from Subunit IVB by the absence of biosilica and by a gradual increase in natural gamma radiation counts.

Unit V (465 to 578.9 mbsf; lower Miocene [?] to upper Oligocene [?]) is comprised of indurated dark greenish gray to very dark greenish gray clay; olive gray to dark greenish gray silty clay; and very dark greenish gray clay with glauconite and glauconitic clay. These sediments are distinguished from the overlying sediments by a sharp increase in magnetic susceptibility.


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