Site 1101 is located in 3509 m water depth on the continental rise of the Pacific margin of the Antarctic Peninsula, centrally within a sediment drift approximately 500 km northeast of Sites 1095 and 1096 (Figs. 2, 12). Because the sediment drift (Drift 4) is much smaller than that drilled at Sites 1095 and 1096 (Drift 7), the site is no farther from the continental margin than Site 1096.
Site 1101 was chosen to answer questions raised by drilling at Sites 1095 and 1096:
The opportunity to drill Site 1101 came from the persistently unfavorable sea conditions encountered on the continental shelf (exceeding a 2-m limit on vessel heave during shallow-water drilling). In less than 2 days, a single hole was APC cored to 142.7 mbsf and extended by XCB drilling to 217.7 mbsf with 99.1% recovery. Having recovered the shallow section at Site 1101, and with improving sea conditions, the ship returned to shelf Hole 1100D.
Recovered sediments consist of 217.7 m of predominantly hemipelagic clayey silt that contains a nearly continuous distal glacial record of the past 3.1 to 3.4 m.y. (late Pliocene to present) (Fig. 21). Unit I (0-53.3 mbsf) and Unit II (53.3-142.7 mbsf) are composed of alternating biogenic-bearing massive clayey silts and laminated clayey silts that we interpret as sedimentary expression of interglacial and glacial periods, respectively. Sedimentation rates are fairly steady at about 7 cm/k.y. in the top 120 m, but a period of slow sedimentation may occur toward the base of Unit II (Fig. 13). Within Unit I, warm oxygen isotope stages are identified by diatom-bearing layers. Unit II has at least 19 discrete foraminifer-bearing layers in which carbonate concentration can be as high as 28 wt%, which alternate cyclically with barren laminated or massive intervals. Extremely well-developed cyclicity in magnetic susceptibility matches variation in color reflectance and the alternation of biogenic and terrigenous intervals in Units I and II. Unit III (142.7-217.7 mbsf) lacks the regular variation in carbonate content of the overlying units. Its upper part (above 198 mbsf) contains massive, barren clayey silt that could have originated in turbid plumes or sediment gravity flows from glaciers near the continental shelf edge as well as thin diamicts (deposited by ice rafting). The lower part of Unit III may represent a warmer interval with deposition of diatom-bearing massive and laminated facies. Sedimentation rates in Unit III are less well defined but may reach 10 cm/k.y.
Calcareous microfossils were found in the uppermost core as well as in carbonate-rich intervals between 50 and 134 mbsf (Unit II). Siliceous microfossils were found throughout the core and become more abundant in the lowest three cores. Several planktonic foraminiferal oozes were identified, mostly in Unit II. The only ooze in Unit I occurs in marine isotopic Stage 5. More than 90% of these assemblages are N. pachyderma sinistral with rare N. pachyderma dextral and Globigerina bulloides, plus benthic foraminifers. The carbonate-rich interval appears coeval at Sites 1101 and 1096. Calcareous nannofossils were recovered in the upper 120 m. The first and last occurrence of the large form of Gephyrocapsa spp. was observed, making the nannofossil record at Site 1101 more complete than at Site 1096. Diatoms were present throughout the hole, with alternating good preservation and barren intervals. The lowest six cores contain more diverse, abundant assemblages of diatoms than found above. Radiolarians were generally abundant and moderately preserved with only two barren intervals. The Psi through Upsilon Zones (Pleistocene-Pliocene) were recovered. Reworked late Pliocene assemblages (thick-shelled) occurred in the Pleistocene part of the section, which contained thin-shelled in situ specimens.
The Brunhes/Matuyama boundary occurs at55 mbsf, the same depth as at Site 1096. Site 1101 has the most complete, well-defined Matuyama epoch reversals of all of the drift sites. Coring disturbance in Core 178-1101A-16H appears to affect the record of the Reunion event. The termination of Chron C2A (2.58 Ma) was observed at approximately 170 mbsf, and the onset of C2An.1n (3.04 Ma) was found at 210 mbsf.
The interstitial water chemistry profiles at Site 1101 closely resemble those of the upper 250 mbsf at Sites 1095 and 1096, reflecting the strong similarity in moderate sedimentation rates (5-10 cm/k.y.) and low organic carbon contents (<0.4 wt%) among the three rise sites. Sulfate decreases to zero and manganese reaches a minimum concentration at 130 mbsf, where measurable concentrations of methane and ethane first occur. Other indicators of organic-matter decay, such as alkalinity and ammonium, increase with depth and reach maximum values at the bottom of the hole. They probably continue to increase at greater depths, based on comparison with Sites 1095 and 1096. Dissolved silica concentrations remain high throughout the hole and reach the solubility limit of opal-A at 150 mbsf near the top of Unit III, suggesting that biogenic opal dissolves principally between 0 and 150 mbsf and that the interstitial water becomes saturated with respect to silica in Unit III. Dissolved calcium increases significantly downhole in the upper 50 mbsf, then decreases to a minimum at 130 mbsf near the base of the sulfate reduction zone. It increases again at the bottom of the hole. The low content of calcareous microfossils in Units I and III could therefore be traceable to dissolution of carbonates.
Porosity initially decreases downhole, as might be expected under normal consolidation conditions. However, porosity rises again below 140 mbsf. At Site 1101, we found a weak positive correlation between siliceous biogenic content and porosity, which was seen previously at Sites 1096, 1098, and 1099. This suggests that the presence of significant amounts of rigid, siliceous biogenic material in the sediment, associated with a relatively fast sedimentation rate, renders the lower part of the drift sedimentary column underconsolidated.
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