Advanced piston coring at Site 1211 recovered 158.9 m of sediment in Hole 1211A, 169.9 m in Hole 1211B, and 138.6 m in Hole 1211C. Excellent recovery of Holocene through lower Maastrichtian sediments provides a useful corollary to Site 305 that was RCB cored during DSDP Leg 32 at the same location.
The sediment from Holes 1211A, 1211B, and 1211C is divided into three lithologic units (Fig. F6). Unit I, extending from 0.0 to 51.95 mbsf in Hole 1211A, 53.38 mbsf in Hole 1211B, and 53.67 mbsf in Hole 1211C, ranges from the Holocene to lower Miocene. This unit consists of cyclically bedded lighter-colored nannofossil ooze and darker-colored nannofossil ooze with clay or clayey nannofossil ooze. The average carbonate content is ~77 wt% (Fig. F6). Typical biogenic silica contents are ~1% but range up to 15% in a few darker-colored intervals. Rare layers of volcanic ash are present. An unconformity at the base of Unit I separates lower Miocene clay from upper Oligocene nannofossil ooze (Fig. F7). Unit II extends from the upper Oligocene (51.95 mbsf in Hole 1211A, 53.38 mbsf in Hole 1211B, and 53.67 mbsf in Hole 1211C) to the basal, clay-rich layer at the K/T boundary (133.87 mbsf in Hole 1211A, 133.80 mbsf in Hole 1211B, and 133.02 mbsf in Hole 1211C). The general lithology of Unit II is soft, nearly homogeneous nannofossil ooze punctuated by intervals of more clay-rich sediment. Unit III extends from the upper to the lower Maastrichtian at 158.9 mbsf in Hole 1211A, 169.9 mbsf in Hole 1211B, and 138.64 mbsf in Hole 1211C (Unit III only spans upper Maastrichtian sediment in Hole 1211C) and consists predominantly of pale orange homogeneous nannofossil ooze.
Unit I at Site 1211 consists of alternating intervals of nannofossil clay and nannofossil ooze. Somewhat subtle lithologic changes support the division of Unit I into two subunits. Subunit IA is predominantly greenish gray in color and characterized by a higher clay content, the presence of siliceous microfossils, rare diagenetic color banding, pyrite, pumice, and ash layers. The division between Subunits IA and IB is placed at a color change from greenish gray sediment to yellowish brown and orange sediment. The Subunit IA/IB contact is placed at the top of the first pale yellowish brown (10YR 6/2) nannofossil ooze in Sections 198-1211A-5H-6, 70 cm, 198-1211B-5H-5, 28 cm, and 198-1211C-5H-2, 125 cm. The Subunit IA/IB color change also marks the downhole disappearance of ash beds and biogenic silica, and a decrease in pyrite.
Subunit IA extends from 0.0 to 39.51 mbsf in Hole 1211A (interval 198-1211A-1H to 5H-6, 70 cm), from 0.0 to 40.88 mbsf in Hole 1211B (interval 198-1211B-1H to 5H-5, 28 cm), and from 0.0 to 38.55 mbsf in Hole 1211C (interval 198-1211C-1H to 5H-2, 125 cm), spanning the Holocene to lower Pliocene. Below the uppermost 20-cm layer of moderate yellowish brown (10YR 5/4) clay with nannofossils lies an interval of alternating darker (light olive gray; 5Y 6/1) and lighter (yellowish gray; 5Y 8/1) nannofossil clay to clayey nannofossil ooze. The average carbonate content for Subunit IA sediment is ~75 wt%. Diatoms, and to a lesser extent radiolarians, are present in Subunit IA sediments, although they seldom exceed a few percent of the sedimentary constituents. The nature of the contacts between the Subunit IA lithologies varies from relatively distinct and horizontal to gradational and bioturbated. The cyclic alternation of light and dark lithologies tends to occur in decimeter-scale intervals of varying thicknesses with no apparent downcore trend. The sediment in Subunit IA is rarely to moderately bioturbated, although the apparent intensity of bioturbation is usually directly related to the degree of color contrast—burrows are more visible across lithologic contacts with color changes. Postdepositional Subunit IA sedimentary constituents that were observed commonly at other Leg 198 sites, such as green authigenic clay laminae (saponite) and pyrite (discrete blebs, burrow fill, and streaks), are sparse at Site 1211.
Subunit IB extends from 39.51 to 51.95 mbsf in Hole 1211A (interval 198-1211A-5H-6, 70 cm, to 7H-2, 15 cm), from 40.88 to 53.38 mbsf in Hole 1211B (interval 198-1211B-5H-5, 28 cm, to 6H-7, 28 cm), and from 38.55 to 53.67 mbsf in Hole 1211C (5H-2, 125 cm, to 6H-6, 87 cm), spanning the lower Pliocene to lower Miocene. Similar to Subunit IA, Subunit IB consists of nannofossil ooze with varying amounts of clay. The major distinction between the subunits lies in the color change from variations of greenish gray in Subunit IA to hues of yellowish brown and grayish orange in Subunit IB. Subunit IB red/blue reflectance ratios are nearly twice as high as in Subunit IA, corresponding to the overall higher carbonate contents (average Subunit IB values = ~80 wt%). Color transitions are gradational, and apparent dark-light cyclicity occurs variably as centimeter- to decimeter-scale alternations throughout this subunit. Total color reflectance percentages (L*), which generally correlate to carbonate content, exhibit a slight downcore decrease followed by an increase, locally peaking near the boundary between Units I and II (Figs. F6, F7). Pyrite is very rare in Subunit IB, and volcanic ash and green authigenic clay laminae are absent. Pumice fragments are found in Cores 198-1211B-5H and 6H. Bioturbation is rare to moderate with identifiable Zoophycos trace fossils found in Core 198-1211A-6H.
Unit II consists of nearly homogeneous very pale orange (10YR 8/2) to grayish orange (10YR 7/4) nannofossil ooze with intervals of moderate yellowish brown (10YR 5/4) and dark yellowish brown (10YR 4/4) nannofossil ooze with clay, clayey nannofossil ooze, and clay with nannofossils. Similar to the other Shatsky Rise sites, the Unit II ooze is much softer than that of Unit I, often rendered soupy during the drilling and splitting process. Carbonate content of the Unit II nannofossil ooze averages ~94 wt% (Fig. F6). Visible bioturbation is typically rare, primarily as a result of the minimal color contrast. Within the grayish orange (10YR 7/4) and very pale orange (10YR 8/2) ooze, apparent bioturbation is characterized by whitish centimeter-scale burrow fills. Color and lithologic changes in Unit II are primarily gradational and mottled; however, some light/dark contacts are relatively sharp.
The base of Unit II lies at the K/T boundary. The boundary is overlain by a basal, grayish orange (10YR 7/4) clay-enriched layer, which, in turn, is overlain by a white (N9) foraminifer-nannofossil ooze with millimeter-scale pyritized burrows.
Unit III consists of generally soft, very pale orange (10YR 8/2) nannofossil ooze, with several centimeter-scale intervals of moderate yellowish brown (10YR 4/2) clayey nannofossil ooze in the uppermost Maastrichtian (Core 198-1211B-16H). This unit is distinguished from Unit II by its relatively high abundance of foraminifers (up to 15%) and the presence of Inoceramus fragments in Cores 198-1211B-17H and 18H. Overall carbonate contents are ~95 wt%, similar to the Unit II oozes. Bioturbation is rarely apparent, characterized by discrete whitish burrow fill and slight mottling. The final core in Hole 1211B (198-1211B-19H) contains chert fragments, which may have contributed to the soupy texture and flow-in (drilling disturbance) encountered toward the base of the core.
Cyclic sedimentation during the Pleistocene and Pliocene at Site 1211 is similar to that at Site 1210, characterized by relatively low-amplitude cyclicity compared to the cyclic sedimentation recorded in the sediment farther north (e.g., Site 1209; F8). This may be attributable to overall lower surface water productivity on the southern flank of the Southern High, as indicated by the diminished abundance of siliceous microfossils compared to sites further north on the Southern, Central, and Northern Highs. In addition, the average sedimentation rate for the Pleistocene and Pliocene (8.6 m/m.y.) is significantly lower than those recorded at the Central and Northern Highs (up to 42 m/m.y.), also suggesting lower surface water productivity (Fig. F16). However, despite the lower rates of primary productivity, the cyclicity was most likely dictated by small-scale changes in the intensity of surface water productivity. Relative biogenic silica enrichment occurs within the darker, more olive-green lithologies (up to 15% diatoms), with the lighter gray intervals remaining depleted in opal (~1%). It is unlikely that the light-dark sediment cycles resulted from glacial-interglacial fluctuations in deepwater corrosiveness given the relatively shallow water depth of ~2900 m, which lies above the subtropical northern Pacific CCD (Rea et al., 1995).
The relatively low Pleistocene and Pliocene sedimentation rates may have contributed to the more yellowish sediment hue than the corresponding Subunit IA sediment cored on the Central and Northern Highs and northern part of the Southern High. The more yellowish sediment color suggests greater oxidation of the Site 1211 Subunit IA sediment than the darker, more greenish gray Subunit IA sediments found at the sites with higher sedimentation rates. As a result, the transition from the light olive-gray and yellowish gray Subunit IA sediment to the grayish orange (10YR 7/4) and very pale orange (10YR 8/2) Subunit IB sediment is more subtle than it is at the Central and Northern Highs (Sites 1207 and 1208) and at the northern portion of the Southern High (Site 1209). At every site cored on Shatsky Rise, Subunit IB is characterized by distinctly lower sedimentation rates, perhaps enabling more oxidation of Fe-bearing constituents and imparting the more reddish brown color that defines the subunit. The difference between sedimentation rates in Subunits IA and IB is less pronounced at Site 1211 than at the other sites. However, changes in the sedimentation rate (as determined by shipboard biostratigraphy; F20) do not coincide with the Subunit IA/IB boundary, suggesting that other factors contributed to the fundamental change in the redox state of Unit I sediments (e.g., bottom-water composition and age, and/or organic matter accumulation).
The upper boundary of Unit II is placed at the base of the clayey interval marking an unconformity spanning the lower Miocene to the upper Oligocene. The Unit I/II boundary is clearly seen in the magnetic susceptibility and color reflectance data (Fig. F8). The nearly homogeneous, pale orange nannofossil ooze of Unit II is punctuated by numerous darker brown clayey intervals. In contrast to the productivity-controlled sedimentation within Unit I, Unit II sedimentary variations seem to be dictated by fluctuations in the depth of the lysocline during the Paleogene. Smear slide estimates and coulometric analyses indicate that carbonate levels dropped to as low as ~50 wt%, and these clay-rich intervals are interpreted as having resulted from periodic lysoclinal and CCD shoaling. At ~2900 m water depth, Site 1211 lay only a few hundred meters above the CCD during portions of the Paleogene, as constrained by lithologic examination of similar-aged carbonate-depauperate sediment from Site 1208 (~3300 m). Thus, subtle fluctuations in either the depth of the CCD or the vertical extent of carbonate corrosivity (resulting from lysoclinal shoaling) may have caused the periodic carbonate dissolution evident at Site 1211 (e.g., Oxburgh and Broecker, 1993).
Numerous (~26) clayey intervals are present throughout Unit II. Below the Eocene-Oligocene transition, low-amplitude fluctuations in the magnetic susceptibility and color reflectance records (Fig. F9) indicate cyclic changes in the sedimentary carbonate content. Such cyclicity suggests the possibility of orbitally controlled lysocline depth changes, potentially related to variation in productivity or deepwater mass composition. Shore-based investigations hopefully will shed light on the nature and causes of Paleogene cyclicity.
Among the lysocline "events" recorded at Site 1211, Unit II sediments represent the PETM and a newly recognized, prominent level in the mid-Paleocene. Although it is widely accepted that the PETM carbonate dissolution resulted from the oxidation of methane released from sedimentary methane hydrate reservoirs (Dickens et al., 1995, 1997), the cause(s) of the mid-Paleocene event is unknown as yet.
Below the K/T boundary sequence lies Maastrichtian pale orange nannofossil ooze, very similar to Unit II ooze. Unit III oozes are distinguished by the higher abundance of foraminifers and the presence of Inoceramus fragments (prior to their mid-Maastrichtian extinction). Cyclicity, if present, is not easily observed in these somewhat soupy, very light colored, high-CaCO3 oozes.
Coring at Site 1211 represents the return to DSDP Site 305; thus, it is instructive to compare the new APC-recovered sediment to that previously recovered with RCB coring. The primary distinction lies in the quality of cores recovered within Unit I and the overall depth of coring, which determined the placement of unit boundaries. Unit I at Site 305 (Holocene to upper Miocene) consisted of the pale orange and yellowish brown sediment that is similar to Subunit IB at Site 1211. Rotary coring at that site precluded recovery of a separate and distinct interval of soft, uppermost light olive-gray sediment of Subunit IA found at Site 1211. Below Unit I, the pale orange oozes of the Paleogene and Maastrichtian were grouped together as Unit II at Site 305, whereas we place the Unit I/II boundary at the base of the unconformity spanning the upper Miocene through the upper Oligocene (Sections 198-1211A-7H-2, 15 cm [51.95 mbsf]; 198-1211B-6H-7, 28 cm [53.38 mbsf]; and 198-1211C-6H-6, 87 cm [53.67 mbsf]). Upper to lower Maastrichtian sediments at Site 1211 are classified as lithologic Unit III, based on the break in sedimentation associated with the K/T boundary coupled with the increased abundance of foraminifers and the presence of Inoceramus remains. Coring at Site 1211 terminated in the lower Maastrichtian because of the presence of chert. Rotary coring at Site 305 continued through the Maastrichtian chert intervals and terminated in the Aptian-Barremian. Basal lithologies at Site 305 consisted of porcellanite, chert, pelagic shale, and limestone (Larson, Moberly, et al., 1975).
All of the sediment recovered from the two holes at Site 1211 is ooze or clay. The lack of indurated sediment in any of the holes, with the exception of the basal chert layer, implies that the burial depths were never significantly greater than they are at present (158.9, 169.9, and 138.6 mbsf in Holes 1211A, 1211B, and 1211C, respectively).
The lack of other diagenetic features that had been relatively common in other Leg 198 Unit I sediment, such as green authigenic clay banding and pyrite, bears discussion. Several factors may have contributed to this difference. Lower overall sedimentation rates and the generally more yellow-brown hues of Subunit IA at Site 1211 suggest greater oxidation. The lower pyrite content implies that the majority of redox reactions occurred under oxic conditions, with less sulfate reduction (see "Inorganic Geochemistry") and hence less hydrogen sulfide available for pyrite formation. In addition, a lower supply of biogenic silica and the presence of fewer volcanic ash layers resulted in diminished pore water silica available for authigenic clay formation (see "Inorganic Geochemistry").