Site 1096 is the more proximal and the more elevated of two sites on a hemipelagic sediment drift on the continental rise off the northwestern Pacific margin of the Antarctic Peninsula (Fig. 2). Site 1096 lies in 3152 m of water close to the crest of the drift and was drilled to obtain the shallower part of the stratigraphic section, where it is most expanded (Fig. 10). With the more distal site (1095), Site 1096 was intended to answer specific questions about the state of glaciation of the continent. In particular:
Not all of these questions can be answered immediately, but shipboard work provides the basic information needed.
In almost 9 days on site, we recovered a complete double APC section to 120 mbsf, extended the cored section to 607.7 mbsf by XCB drilling, and then logged the hole. Operations were not straightforward: clogged jets required a round trip, logging was achieved in two halves (upper and lower) because of a bridge, and the Formation MicroScanner (FMS)-sonic and well seismic tools were not run because the hole had become too wide.
The sedimentary section extends from the Holocene to the early Pliocene (~ 4.7 Ma) at 607 m (Fig. 8). Sediments are predominantly fine-grained and terrigenous and are divided into three depositional units. Unit I (0 to 33 mbsf) consists of laminated and massive, often intensely bioturbated, diatom-bearing silty clays of Pleistocene age. These have a well-defined alternation of biogenic-rich and biogenic-poor horizons, which are the stratigraphic expression of glacial/interglacial cycles. Sedimentation rates averaged 7 cm/k.y. (Fig. 11). The top of Unit II is placed at the first appearance downcore of common parallel-laminated silt and mud turbidites. Unit II is a late-Pliocene to Pleistocene partly turbiditic succession some 140 m thick (33 to 173 mbsf) with a calcareous biogenic component, generally low. Sedimentation rates averaged 9 cm/k.y. Although the turbidite silts are thin and subordinate to muds, two coarsening-upward sequences may be discerned in the upper part of Unit II. A massive, well-sorted sand turbidite occurs at 112 mbsf. Unit III, Pliocene in age, extends from 173 mbsf to the bottom of the hole at 607.7 mbsf. It is composed of alternations, up to tens of meters thick, of (1) very thinly laminated and generally nonbioturbated clays derived from muddy turbidity currents and (2) intensely bioturbated homogenous silty clays with a higher biogenic component. Overall, Unit III has a sedimentation rate of 18 cm/k.y., although fluctuations in deposition rate may be recorded by intervals of intense bioturbation with more abundant ice-rafted debris. The biogenic component of Unit III is siliceous.
In Units II and III, the alternation of laminated turbidite facies and bioturbated hemipelagic facies records cyclic fluctuations in sediment supply and transport processes. Some of these fluctuations may be related to glacial/interglacial cycles along the Antarctic Peninsula margin similar to those identified within Unit I, but longer period cycles also exist. Likely proxies for glacial/interglacial variation include magnetic susceptibility, color reflectance, and clay-mineral composition.
Above 170 mbsf (Units I and II), the sediments contained calcareous nannofossils and foraminifers. The latter are rare to abundant only in this upper interval of Site 1096. Neogloboquadrina pachyderma sinistral dominates the planktonic foraminiferal assemblage. Benthic foraminifers are rare and consist mostly of shallow-water species presumed to be reworked from the continental shelf. Calcareous nannofossils provide the datums in this upper interval. Reworking is evident in the upper 170 m, with some samples containing an entire reworked assemblage. Radiolarians are present in several intervals, but the marker species for the two zones below Psi (Chi and Phi) were not found. In the upper 110 m, the diatom zonal boundary markers are not seen, and samples studied from 110 to 165 m are barren of diatoms. From 170 to 608 mbsf (Unit III) siliceous microfossils dominate, but the planktonic foraminifer N. pachyderma sinistral is seen to the base of the hole. The record of radiolarians is relatively complete for Unit III, and most of the marker species are present through the entire Upsilon Zone. The diatom record for the lower part of this site is also virtually complete, with zonal datums being available for nearly all the zones.
Magnetostratigraphic data from Site 1096 show promise. The Brunhes/Matuyama boundary is very well defined at 55 m. The Jaramillo and Olduvai Subchrons are not ideal for magnetostratigraphy because part of the interval shows a low remanent intensity resulting in a noisy inclination record. Below this, reversals are well defined, the oldest reversal observed being the onset of Chron C3n.2n (4.62 Ma) at 585 mbsf.
Physical properties and chemical data are interesting: below 170 mbsf grain density is reduced, and porosity increases steadily downhole within Unit III. Heat-flow measurements suggest that the base of the methane hydrate stability zone lies at about 290 m, but a gas hydrate bottom-simulating reflector (BSR) is not seen. The diffuse BSR at about 700 ms in the seismic section, not reached by coring, is assumed to be from silica diagenesis. A total of 172 bulk sediment and 35 interstitial water samples from Site 1096 have been analyzed. The sediment generally contains only minor amounts of inorganic (<0.1 wt%) and organic (<0.6 wt%) carbon, yet the interstitial water chemistry shows clear evidence of organic matter decay, with the sulfate reduction zone extending to 50 mbsf. Other diagenetic reactions occurring include carbonate precipitation, silica dissolution, cation exchange, and perhaps disseminated apatite precipitation. Methane concentrations were high and caused expansion in most cores, but methane/ethane ratios stayed above 200 to the base of the hole.
The deep hole was logged under unstable hole conditions, which both limited and complicated the operation. In total, the porosity-density-natural-gamma tool string was deployed through almost the entire hole (from 556 mbsf) and the GHMT (magnetic) tool between 510 and 356 mbsf. Natural-gamma activity was logged to the seafloor, and clay content can be estimated after correction for the gamma attenuation of the pipe.
Seismic unit boundaries defined at Site 1095 were traced across multichannel seismic (MCS) profiles to Site 1096. Only the upper three were penetrated at Site 1096 because of more rapid sedimentation.