At Site 1169, one hole with 26 cores was drilled. Hole 1169A reached 246.3 mbsf with a total sediment recovery of 91.4% using the APC and XCB (Fig. F2). The sediments at Site 1169 mainly consist of pelagic nannofossil ooze with minor constituents such as foraminifers and siliceous microfossils. Siliciclastic components were rarely observed. Because the entire sediment sequence at the site exhibits no major lithologic changes, only one unit is recognized. Unit I, with two subunits differentiated, shows relatively minor differences in core features, smear slides, reflectance spectrophotometry, and coulometric carbonate analyses. Two hiatuses were recognized, at ~220 and ~200 mbsf, based on multitaxa biostratigraphy.
The primary site objective was to obtain late Neogene high-resolution (i.e., Milankovitch scale) paleoceanographic data in the subantarctic. Although the overall recovery was relatively high, the piston-cored sediments (Cores 189-1169A-1H through 21H) were moderately to strongly disturbed during the drilling operation, whereas no significant drilling disturbance occurred for XCB drilled cores below Core 189-1169A-22X (Fig. F2). The coring disturbances for APC sediments resulted from heavy swell, strong winds, and related technical problems such as crushed and split liners. No further attempt was made to drill the scheduled B and C holes at this site because of the severe weather conditions. The relative intensity of sediment disturbance is shown in Figure F3A. Severely disturbed soupy (e.g., Cores 189-1169A-1H, 2H, and 11H) and vertically deformed (e.g., Cores 189-1169A-4H, 5H, 10H, and 15H) sediments hampered our measurements of several chemical and physical properties. Although high-resolution data could not be collected from the disturbed sediments, measurements of several sedimentary parameters exhibit trends that have aided interpretations of general changes in depositional environments.
Unit I consists of 246.3 m of middle Miocene to Pleistocene pelagic nannofossil ooze with rare to common foraminifers and siliceous fossils including radiolarians, diatoms, silicoflagellates, and sponge spicules. The sediments of Unit I are generally massive and exhibit no notable sedimentary structures, except for sporadic occurrences of isolated light bluish gray thin to medium laminations and small (~10 mm) in situ black pyrite concentrates. Occasionally, dark pyritic thin beds were observed (e.g., Core 189-1169A-17H). The color of sediments varies from light greenish gray to white. Calcium carbonate content is relatively high throughout the unit, ranging between 78.9 and 95.5 wt%, except for an anomalous low value of 63.8 wt% measured at 137.32 mbsf (Sample 189-1169A-15H-4, 72 cm) (see "Organic Geochemistry"). The APC-cored sediments (i.e., Cores 189-1169A-1H through 21H) show a variety of coring disturbance features in many intervals (Fig. F3A) that prevented us from observing detailed sedimentary structures. No coring disturbance was observed for the XCB-drilled cores below Core 189-1169A-22X.
In spite of the above-mentioned drilling disturbances, two subunits have been recognized within Unit I at Site 1169. Subunit IA (0-170.1 mbsf) is characterized by light greenish gray to light gray nannofossil ooze with common to abundant siliceous microfossils, mostly diatoms, sponge spicules, and bioclasts, as modifiers. The average calcium carbonate content for the Subunit IA sediments is 84.4 wt%. The difference between Subunits IA and IB is the minor biogenic components—Subunit IB (170.1-246.3 mbsf) is purely composed of nannofossil ooze. The representative lithologic color of Subunit IB is white. Its calcium carbonate content averages 93.5 wt%, higher than that of Subunit IA.
The boundary between Subunits IA and IB is placed between Cores 189-1169A-18H and 19H, based on changes in sediment color and percent content of siliceous fossils. The siliceous fossil-bearing greenish gray nannofossil ooze of Subunit IA changes to white pure nannofossil ooze of Subunit IB at the bottom of Core 189-1169A-18H. Although disturbed during coring, the sediments in Cores 189-1169A-17H and 18H are transitional in color and siliceous fossil content from Subunits IA to IB facies (Fig. F3). Biostratigraphic studies at this site suggest the presence of a major hiatus between Samples 189-1169A-23X-CC and 24X-CC (~218 mbsf), possibly spanning much of the middle Miocene to late Miocene, and a lesser one between Samples 189-1169A-21H-CC and 22X-CC (~200 mbsf), removing part the late Miocene to early Pliocene (see "Biostratigraphy").
Subunit IA consists predominantly of light greenish gray to light gray (10GY 8/1 to N 7) siliceous fossil-bearing nannofossil ooze. Throughout the subunit, siliceous fossils such as diatoms, sponge spicules, silicoflagellates, and radiolarians are present. The percent estimates of the biogenic components examined under smear slides, as plotted in Figure F3D, exhibit a sharp decrease in siliceous contents between 160 and 170 mbsf, where the subunit boundary is placed. The calcium carbonate content of Subunit IA sediments averages 84.4 wt%, ranging between 78.9 and 89.9 wt% (Fig. F3C). Minor amounts of foraminifers, calcareous bioclasts (fragments of biogenic materials such as foraminifers and ostracodes), and clay are found in the nannofossil ooze throughout the subunit. Nannofossil oozes in the upper part (Cores 189-1169A-1H through 3H) of the subunit contain noticeable amounts of foraminifers and bioclasts. These foraminifer- and bioclast-bearing nannofossil oozes are very pale brown (10YR 8/3 to 7/3), reflecting more abundant calcareous fossils (Fig. F3D).
Subunit IB consists mainly of uniform white (N 8), pure nannofossil ooze. Occasionally, minor changes to light greenish gray (10GY 8/1) were observed. The calcium carbonate content of the subunit is generally higher compared with Subunit IA, ranging between 93.3 and 95.5 wt%, with an average value of 93.5 wt%. Contents of siliceous fossils are extremely low (Fig. F3D). Spectrophotometry lightness (L*) measurements exhibit a slight increase across the Subunit IA/IB boundary (Fig. F2).
Although not apparent in calcium carbonate content or spectrophotometry lightness (L*) measurements, an interval of light yellowish white (2.5Y 8/2) nannofossil ooze is observed between 208 and 211 mbsf in Core 189-1169A-23X. This visual color feature is also recognized by the spectrophotometry (b*) signals (Fig. F3B). In Sample 189-1169A-22X-CC, of early Pliocene age, spherical microtektites were found.
Despite drilling disturbances in the Site 1169 APC cored sediments, various proxies captured long-term trends in the Neogene climatic evolution at the site. The major hiatus (or severely condensed section) recognized between Sections 1169A-23X-CC and 24X-CC, ranging from the middle Miocene (tentatively, ~12.5 Ma) to late Miocene (~6.9 Ma), implies strong deep-water circulation and resulting submarine erosion and/or dissolution of calcium carbonate at the site. It corresponds to an unconformity in seismic profiles (see "Background and Objectives"). This hiatus may be associated with the development of vigorous bottom-water circulation in the Southern Ocean that is inferred to have developed with the middle Miocene expansion of the East Antarctic Ice Sheet that started at ~14 Ma (Woodruff and Savin, 1989; Flower and Kennett, 1995). However, no microfossil datum was found below the hiatus, leaving the timing of its onset at Site 1169 unclear.
The light yellowish white nannofossil ooze observed between 208 and 211 mbsf in Core 189-1169A-23X, which encompasses or overlies the hiatus (condensed section), may reflect an incursion of oxygenated, relatively "young" water to the seafloor, supporting the hypothesis that the production of antarctic deep water intensified during the middle Miocene. Because the middle to late Miocene hiatus was not found at the shallower water depth at Site 1168, the circulation of the cold bottom water apparently affected deeper water depths only, at least between 12.5 and 6.9 Ma.
The early Pliocene pelagic sediments recovered at Site 1169 are unusually thick. Between 60 and 200 mbsf (~4.6-4.0 Ma), the linear sedimentation rate is 23.4 cm/k.y. This sedimentation rate is the highest recorded from the Southern Ocean. The transition from pure nannofossil ooze (Subunit IB) to diatom-bearing nannofossil ooze (Subunit IA) occurred at ~4.5 Ma, nearly coinciding with the onset of a high-sedimentation-rate period. Despite this increase in silica content, the majority of sediment in Subunit IA is composed of nannofossils (>84%) (Fig. F3D).
The factors that controlled this early Pliocene high-sedimentation rate are of particular interest. Because the material accumulated during this interval is mostly pelagic biogenic in origin (i.e., nannofossils and diatoms), the possibility of dilution by a nonbiogenic component is excluded. The other possible factors are increase in productivity, redeposition of winnowed materials, or a combination of both. Incursion of nutrient-rich waters to the site, probably associated with southward migration of the Subtropical Front and/or regional upwelling, may have stimulated nannoplankton productivity and induced an enormous flux of carbonate in the region. Such high-primary productivity may be responsible for the increase in siliceous fossil content at the site. Redeposition of reworked or winnowed materials may also have contributed to the elevated sedimentation rate at Site 1169.
The episode of early Pliocene amplified sedimentation rate is also documented from other areas in the Pacific Ocean. Specifically, the magnitude of the early Pliocene sedimentation rate increase in the Tasman Sea (Deep Sea Drilling Project [DSDP] Sites 590 through 594) is comparable to that of Site 1169. Nelson (1986) attributed the elevated Pliocene sedimentation rate observed in the Tasman Sea to increased calcareous biogenic productivity. Although it appears that increase in biogenic productivity is responsible for the early Pliocene high sedimentation rate, the causes of concurrent occurrence of the event in various locations remain unclear.