162 Preliminary Report

Site 907

The primary objective of drilling operations at Site 907 was to recover an undisturbed pelagic sedimentary sequence with carbonate and IRD records. Shore-based studies of the one hole cored by Leg 151 at Site 907 in 1993 provided a reliable stable isotope record of the last 1 Ma, and a record of IRD back to more than 7 Ma. Given that a detailed paleoclimatic record can be extracted from these sediments, we wanted to return to this site and finish the planned triple coring to provide a complete and undisturbed high-latitude section for much of the Neogene. Thus, the site was reoccupied by Leg 162 and two additional holes were cored (Holes 907B and 907C).

High resolution shipboard multisensor track (MST) data allowed us to combine the MST records from Legs 151 and 162 to generate a spliced composite section. We were thus able to fill in recovery gaps over core breaks in Hole 907A and will now be able to complete Leg 151 for the high-resolution paleoclimate studies begun on Leg 151.

The scarcity of biogenic material in certain intervals reduces the possibility of biostratigraphic age control. However, a relatively clean magnetic polarity sequence enables correlation with confidence to the geomagnetic polarity time scale back to the upper Miocene, and with somewhat less confidence further back to approximately 16 Ma. Two short hiatuses, or condensed intervals, are indicated in the middle to upper Miocene section. Sedimentation rates averaged 15-25 m/m.y. over the last 3 Ma, 5-15 m/m.y. in the 3-14 Ma interval, and 25-30 m/m.y. in the 14-16 Ma interval.

The sediments at Site 907 are dominantly composed of silty clay, clay with silt, and clayey mixed sediment with varying amounts of biogenic material. The biogenic component, which includes calcareous nannofossils, foraminifers, diatoms, and/or spicules, is highly variable with increasing depth. The bulk calcium carbonate content at this site displays high amplitude variations from near 0% to greater than 50% within the upper 100 mbsf depth. Below this level the sediments are carbonate-free for the most part. Dropstones greater than 1 cm in size are present above 62.9 mbsf. Ash layers and ash pods are abundant throughout Site 907 sediments. Four distinct lithostratigraphic units are defined with unit boundaries at 16, 63, and 196 mbsf.

Lithostratigraphic Unit I (0 to 15.6 mbsf; Holocene to middle Pleistocene) is primarily defined by the presence of relatively abundant calcareous microfossils, and high amplitude fluctuations in spectral reflectance. The sediment consists predominantly of alternating layers of clayey nannofossil mixed sediment with silty clay and clay with silt. Quartz, feldspar, and inorganic calcite are the most common terrigenous silt-sized particles (although the calcite could possibly be authigenic). The pervasive colors of this unit are olive brown and olive gray, broken only by thin darker volcanic ash layers.

Unit II (15.6 to 63.1 mbsf; middle Pleistocene to Pliocene) is characterized by the absence of biogenic sediment. The dominant lithologies include silty clay, clay with silt and ash, and clay. The sediments are predominantly composed of clay, quartz, feldspar, mica, and accessory minerals. Unit II, as well as Unit I, contains higher amounts of quartz, feldspar, and mica than deeper intervals. Dark greenish gray, dark gray, and greenish gray-colored sediments are pervasive, although minor gray to black volcanic ash layers occur intermittently. Terrigenous components such as quartz and feldspar are relatively invariant across the boundary of Units I and II, in contrast to the downsection disappearance of biogenic material.

Unit III (63.1 to 196.1 mbsf; Pliocene to middle Miocene) is defined by the reoccurrence of biogenic sediment, in this case, biogenic silica throughout the unit with minor calcareous materials occurring within the uppermost section. The primary lithologies of Unit III are dark greenish gray to very dark greenish gray silty clay, clay with silt, clay, and clay with diatoms. With the exception of a small interval containing more than 55% nannofossils, Unit III is characterized by minor repeated occurrences of biogenic material, which increase downcore from less than 5% at the upper boundary, to 5%-20% (primarily siliceous materials) in the lower portion of the unit.

The major lithologies in Unit IV (196.1 to 214.9 mbsf; middle Miocene) are dark greenish gray to greenish gray silty clay, clay with silt, and clay. No biogenic sediment is found in the unit except for trace amounts of siliceous microfossils within the upper portion. The coarse fraction of Unit IV is similar to Unit III, except that it contains slightly smaller amounts of quartz and larger amounts of sulfides.

Biogenic material occurs sporadically and intermittently throughout the sequence. In Unit I it includes both calcareous and siliceous material, but in Unit III it consists solely of siliceous material. The alternating biogenic-bearing and nonbiogenic sediments may reflect climatically driven changes over a long time scale. Shorter-term changes of oceanographic conditions are superimposed on this long-term variation, as indicated by the variable abundance of biogenic components occurring within Units I and III. The boundary between Units II and III probably reflects the onset of increased glaciation during the Pliocene. At this level, quartz and feldspar increase markedly upsection, the first dropstones occur, and changes in the suite of clay minerals occur as indicated by changes in natural gamma-ray emissions.

The geochemistry of the sedimentary sequence is characterized by processes typical of sulfate reduction, and processes reflecting alteration of volcanic ash within the sediments and basement basalts below. Organic carbon values are generally low, but are highest in the intervals with the most siliceous microfossils. Most geochemical parameters correlate well with Leg 151 results except those involved in reduction of organic matter. Leg 151 sulfate values from Hole 907A are substantially higher than those of Holes 907B and 907C in certain intervals. Whether this reflects lateral discontinuity or analytical differences remains to be determined.

TO Site 985

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