Site 1095 is the more distal of two sites on a hemipelagic sediment drift on the continental rise off the northwestern Pacific margin of the Antarctic Peninsula. Site 1095 lies in 3840 m of water on the northwestern, lower flank of the drift (Fig. 2) and was drilled to obtain the deeper part of the stratigraphic section, where the overlying sediments are thinner than at the drift crest (Site 1096). The location was chosen on the upper part of a gentle slope bordering to the southwest a deep-sea channel system that separates adjacent drifts (Fig. 7). Drilling at Site 1095 was intended to examine the earlier stages of drift development and glacial evolution and (together with Site 1096) to answer specific questions related to the state of glaciation of the continent:
These are ambitious objectives, and not all can be addressed immediately.
About 7 days were spent recovering (effectively) a double advanced hydraulic piston corer (APC) record to about 85 meters below seafloor (mbsf), with one hole deepened by APC and extended core barrel (XCB) to 561 mbsf and logged. Drilling in the deep hole was ended after clogged jets had dramatically reduced recovery when ship heave became excessive. One APC hole was lost to iceberg approach, and part of the logging was impeded by ship heave. Recovery was complete in the double APC section and was 89% down to 484 mbsf in the deep hole. The multiple records from the upper section have been spliced, using primarily magnetic susceptibility, and a common depth provided.
The sedimentary section extends from the Holocene to the early late Miocene at 480 m (Fig. 8). It comprises alternations of predominantly fine-grained terrigenous and hemipelagic deposits, containing only trace amounts of inorganic and organic carbon (<0.1 wt%). The uppermost 50 m consists of laminated and massive, often extensively bioturbated, diatom-bearing silty clays. Diatoms, radiolarians, and benthic and planktonic foraminifers all indicate a Quaternary age, and sedimentation rates are low (2.5 cm/k.y.) (Fig. 9). These deposits show a marked cyclic pattern of alternating gray, terrigenous, and brown biogenic-rich silty clays, the stratigraphic expression of successive glacial and interglacial cycles. Weak bottom (contour) currents influenced depositional patterns, with the bulk of fine-grained sediment probably introduced by dilute muddy turbidity currents. The base of the unit is marked by an increased frequency of parallel-laminated silty clay turbidites and beds of silty clay containing abundant ice-rafted debris.
The thickest lithostratigraphic unit at Site 1095 extends from 50 down to 435 mbsf and consists mainly of green laminated silts and muds of Pliocene and late Miocene age. Sedimentation rates are higher (5 to 7 cm/k.y.). Some intervals appear barren, based on limited shipboard sampling, but biostratigraphic control is usually good. Reworking of late Miocene diatoms is common. Cycles in which structureless, intensely bioturbated sections up to 1 m thick (with marked color changes and enhanced concentrations of ice-rafted debris), alternating with sections of abundant thin, graded silt laminae (distal turbidites), are interpreted as glacial cycles that controlled sediment supply. Longer period variations in construction of the continental shelf may be reflected in coarsening- and fining-upward trends. At the top of this unit lies a possible debris flow (seen in only one of two core sections) and, from paleomagnetic and seismic reflection evidence, a possible brief hiatus. Below 285 mbsf, in the late Miocene, is seen the neritic diatom Paralia sulcata, suggesting the existence of a shallow continental shelf.
Sediments below 435 mbsf at Site 1095 consist of nonbioturbated parallel-laminated siltstone-claystones with sporadic occurrences of late Miocene diatoms. This facies (thin-bedded turbidites) does not show the second-order cyclic pattern observed in overlying sediments, which may be significant for understanding glacial history. Core recovery is very poor below 480 mbsf and the lowermost sediments appear barren, so the age of the deepest sediment cored is uncertain. Core-based magnetostratigraphic control is excellent to this point, and geologic high-sensitivity magnetic tool (GHMT) log data show signs of being able to extend control to the base of the hole. Sedimentation rates are higher still, reaching 12 cm/k.y. (Fig. 9).
The characteristics of cores retrieved at Site 1095 are consistent with a distal deep-water setting accessible (until the Quaternary) to small-scale turbidity currents. The pronounced cyclicity of the depositional record is evident in visual core description and is recorded to date in color scanner, magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE), and downhole magnetic (GHMT) logging records, as well as (probably) in clay content. Preliminary shipboard analysis has shown this record to contain Milankovic orbital frequencies, which will allow comparison with the low-latitude record of orbitally induced climate change. The downhole variation of occurrence of biogenic components is in general inversely related to sedimentation rate, owing either to variation in surface primary production or to dilution by terrigenous components. The most abundant biogenic component is diatoms, which normally account for 10%-30% but occasionally reach 60% of the sediment.
Ice-rafted debris (IRD) is ubiquitous in Site 1095 cores, a significant part of the total flux of terrigenous sediment to the site. IRD is readily identified because of the fine-grained nature of the host sediment and occurs as scattered sand grains and granules, isolated pebbles (lonestones), and as lenses of granules and sand. In the absence of core X-radiographs, estimates of IRD abundance are qualitative. A more detailed study of IRD flux through time is potentially valuable but cannot ignore changes in rates of background sedimentation. This is clearly demonstrated by low IRD content in rapidly deposited turbidite sequences and enhanced content in bioturbated intervals that most probably record slower sedimentation.
Site 1095 sediments generally contain only trace amounts of inorganic (<0.5 wt%) and organic (<0.1 wt%) carbon. Interstitial water chemistry shows evidence of organic matter decay and other diagenetic reactions, however, including carbonate and silica dissolution, cation exchange, and perhaps apatite and dolomite precipitation.
The deep hole was logged with the triple combination (TC) and GHMT tool strings, as well as a vertical seismic profile (VSP) with 12 stations. Hole and core correlation with seismic reflection profiles crossing Site 1095 is well constrained using VSP seismic traces and velocities, multisensor track (MST) and discrete sample velocities, and log and core densities. VSP data also correlate with strong reflectors on seismic profiles between 500 ms and oceanic basement at 1200 ms.
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