A total of 377.0 m of sediment was cored at Site 1210, representing a time span from the Holocene to the Campanian (Fig. F4). The sediment recovered consists largely of nannofossil ooze with variable amounts of clay and chert. Rare occurrences of clay-rich intervals represent condensed intervals. In general, carbonate content and bulk density increase downcore, yet no transition to chalk was observed (Fig. F4). The intercalation of several layers of chert with soft nannofossil ooze in the Cretaceous sediments led to lower recovery in this portion of the sequence. Minor and trace components observed in the nannofossil oozes include foraminifers, diatoms, radiolarians, silicoflagellates, sponge spicules, inorganic calcite, Fe oxides, opaque minerals, pyrite, quartz, volcanic glass, feldspar, and mica (see "Site 1210 Smear Slides" and "Site 1210 Thin Sections"). Other minor lithologies encountered include two discrete volcanic ash layers in the Neogene and ~0.5 m of chalk just above the K/T boundary.
Three lithologic units have been defined for the composite section of Holes 1210A and 1210B. Unit I (0-115.8 mbsf in Hole 1210A) contains clayey nannofossil ooze, nannofossil ooze with clay, and lesser amounts of nannofossil ooze. A color transition from gray and olive gray to orangish and yellowish brown marks the division between Subunits IA and IB. Subunit IC encompasses several clay-rich intervals that likely represent condensed sections. The boundary between Units I and II is defined at the base of a condensed interval of early Oligocene-middle Miocene age. Unit II (115.8-219.9 mbsf in Hole 1210A) consists primarily of very pale orange (10YR 8/2) to moderate yellowish brown (10YR 5/4) nannofossil ooze and nannofossil ooze with clay. This unit is characterized by higher carbonate content and softer consistency than Unit I. The base of Unit II is placed at the K/T boundary. Unit III (219.9-377.0 mbsf in Hole 1210B) extends from the K/T boundary to the bottom of Hole 1210B and contains interbedded white (N9) nannofossil ooze (~96-100 wt% CaCO3) and chert.
Unit I extends from the core top to the base of a clay-rich condensed interval that may represent an unconformity separating the middle Miocene and lower Oligocene. Clayey nannofossil ooze, nannofossil ooze with clay, and lesser amounts of nannofossil ooze compose the dominant lithologies present in this unit. Darker and lighter intervals in Unit I are generally defined by variable proportions of clay and biogenic carbonate. Unit I is subdivided into three lithologic subunits. The division between Subunit IA (0-83.4 mbsf in Hole 1210A) and Subunit IB (83.4-112.0 mbsf in Hole 1210A) is located at a transition from olive gray and gray to orange and yellowish brown hues. Subunit IC (112.0-115.8 mbsf) is distinct from the remainder of Unit I, because it contains several clay-rich condensed intervals.
Subunit IA (intervals 198-1210A-1H-1, 0 cm, through 10H-1, 138 cm, and 198-1210B-1H-1, 0 cm, through 9H-6, 0 cm) contains Holocene- to late Miocene-age sediment. Nannofossil ooze in this subunit has a higher clay content, and thus a lower carbonate content, than nannofossil ooze recovered downcore. The lithology predominantly grades from light olive-gray (5Y 8/1) and olive-gray (5Y 6/1) clayey nannofossil ooze to very light gray (N8), light gray (N7), and medium light-gray (N6) nannofossil ooze. These lithologies alternate throughout Subunit IA on a decimeter scale. Color transitions are generally gradational, and bioturbation ranges from rare to moderate. Where color contrasts are greater, mottling from bioturbation is more clearly visible. Rare, centimeter-scale open burrows are usually filled with pyritic nannofossil ooze, with the exception of two open burrows in Cores 198-1210A-2H and 4H that contain pyritic foraminiferal ooze. One discrete volcanic ash layer was observed in this subunit, and several pumice fragments were identified in the upper 30 m.
The upper cores in Holes 1210A and 1210B exhibit sharper contacts between colors and show some evidence of soft-sediment deformation, which may be a syndepositional feature or a product of drilling disturbance. Aside from this observation, no drilling deformation was observed in the remainder of Unit I.
Millimeter-scale pyrite blebs and nodules as well as millimeter- to centimeter-scale clay-rich, pale green laminae are both common features in Subunit IA. These diagenetic laminae sometimes appear as diffuse bands and in some cases are associated with diffuse bands that, in some cases, are purple in color. Some pale green laminae appear to be more fully developed and occur as relatively thick (up to ~1 cm) bands that are more lithified than the surrounding stiff nannofossil ooze. Pale green laminae are generally oriented horizontally but appear to be randomly distributed in space and in relation to the light-dark color cycles of the ooze. These laminae were observed to both crosscut burrows (Fig. F5) and to be crosscut by burrows (Fig. F6), implying that they are a product of early diagenesis. X-ray diffraction (XRD) analyses were performed on calcite-free samples (see "X-Ray Diffraction" in "Lithostratigraphy" in the "Explanatory Notes" chapter) from several pale green laminae, as well as from nannofossil ooze above and below the laminae to assess the clay mineralogy (Fig. F7). Shipboard XRD results indicate that pale green laminae are concentrated layers of saponite (saponite-15A; JCPDS 29-1491), a Mg-Ca-bearing smectite group clay. X-ray diffractograms display a shift to 18.7 Å due to expansion from glycolation and to 10.2 Å upon heating, consistent with the behavior of saponite (Fig. F8). This mineralogy is consistent with XRD results from pale green laminae at Site 1209 (Table T2). The clay of the surrounding ooze contains saponite, illite, and smectite-kaolinite (relative abundances not determined). Some illite also appears to be present in the green laminae; however, we suspect that this may be the result of a contamination of the sample with some of the surrounding sediment.
Thin sections were made of two well-defined green laminae (intervals 198-1210B-6H-7, 56-64 cm, and 6H-3, 57-65 cm). In these thin sections, clay is observed to have precipitated in foraminiferal tests, particularly in the pores (Fig. F41 in the "Leg 198 Summary" chapter). Clay also is present in irregular thin seams and wisps and as a product of partial alteration of disseminated shards of volcanic glass. Some burrows are entirely filled with grains of volcanic glass that are only slightly altered around the edges. Authigenic clay is growing in the interstices of these glass grains, and there is much less clay in the center of the burrow.
Subunit IB (intervals 198-1210A-10H-1, 138 cm, through 13H-1, 112 cm, and 198-1210B-9H-6, 0 cm, through 12H-6, 30 cm) includes sediment of middle to late Miocene age. Subunit IB has the same lithologic composition as Subunit IA but is distinguished by pale yellowish brown (10YR 6/2) to moderate yellowish brown (10YR 5/4) clayey nannofossil ooze and very pale orange (10YR 8/2) to grayish orange (10YR 7/4) nannofossil ooze with clay as opposed to the olive gray and light gray hues characteristic of Subunit IA. Red/blue reflectance ratios are ~1.5 times greater in Subunit IB than in Subunit IA. Color transitions are gradational, and dark and light intervals alternate in decimeter-scale cycles in the upper part of this subunit and increase to meter-scale cycles in the lower portion of the subunit. A notable shift in the reflectance data to lower values occurs within Subunit IB (Fig. F4). Pyrite is a very rare component in Subunit IB, and pale green diagenetic laminae are absent. A pumice fragment and a discrete layer of volcanic ash are both observed in Core 198-1210A-11H. Bioturbation is rare to moderate, as in Subunit IA. Zoophycos trace fossils were identified in Core 198-1210A-10H.
Subunit IC (intervals 198-1210A-13H-1, 112 cm, through 13H-4, 90 cm, and 198-1210B-12H-6, 30 cm, through 13H-1, 104 cm) includes several clay-rich condensed intervals that occur in the middle Miocene, near the base of Unit I (Fig. F9). Intervals of dark yellowish brown (10YR 4/2) clay with nannofossils in Core 198-1210A-13H contain phillipsite, barite, and Fe oxides as minor components, as well as trace amounts of quartz, volcanic glass, and opaques. Clay-rich intervals (~24-35 wt% carbonate) in Subunit IC are interbedded with very pale orange (10YR 8/2) nannofossil ooze. The basal clay interval in this subunit contains nannofossils early Oligocene to middle Miocene in age.
Unit II extends from the early Oligocene to the K/T boundary and consists predominantly of homogeneous very pale orange (10YR 8/2) to grayish orange (10YR 7/4) nannofossil ooze with lesser amounts of pale yellowish brown (10YR 6/2) to dark yellowish brown (10YR 4/2) nannofossil ooze with clay, clayey nannofossil ooze, and clay with nannofossils. Color gradations are subtle throughout this unit. The ooze in Unit II is much softer than in Unit I, and it was probably made soupier by the drilling and splitting process. Carbonate content of the nannofossil ooze is generally higher in Unit II (~73-100 wt%) than it is in Unit I (~50-91 wt%) (Fig. F4). Bulk density also shifts to higher values in Unit II (Fig. F34). Bioturbation is rare to moderate and is characterized by white (N9) to very pale orange (10YR 8/2) centimeter-scale burrow fills. Some tektites were recovered from a core catcher in Unit II (Section 198-1210A-14H-CC), which is late Eocene in age. A sharp contact between a clay-rich interval overlying a relatively carbonate-rich interval in Sections 198-1210A-20H-6, 52 cm, and 198-1210B-20H-3, 110 cm, has been identified as correlative to the PETM based on shipboard biostratigraphy (Fig. F10). The lowermost Paleocene portion of the section is characterized by lighter colors (very pale orange [10YR 8/2] to white [N9]) and contains pyritized vertical burrows. A more lithified foraminifer-nannofossil chalk directly overlies the K/T boundary. Most of the foraminifers have chambers filled with pyrite.
Unit III consists of interbedded chert and white (N9) nannofossil ooze and nannofossil ooze with foraminifers. Chert layers were drilled with the XCB center bit, and then the APC was used to recover the softer nannofossil ooze interbeds. As a result, the chert was intensely brecciated by the drilling process. This is particularly evident in the top of several cores where the residue from center-bit drilling collected. Carbonate content is ~96-100 wt% in the ooze intervals. Small patches of granular green material consist of foraminifers with green (possibly glauconite?) infill. Rare patches of pyrite blebs were observed, as well as larger (centimeter-scale) green patches of unknown mineralogy. One Maastrichtian-age core (Core 198-1210B-28H) contained shell fragments of Inoceramus.
Notable features of the section recovered at Site 1210 include an unconformity between the middle Miocene and lower Oligocene and several boundary sections of particular interest to the shipboard party, including the Eocene/Oligocene, Paleocene/Eocene, and K/T boundaries. Comparison of the sedimentary record through these critical intervals to those recovered at other sites drilled on Leg 198 will allow for interesting comparisons along paleolatitudinal and paleodepth transects.
Sedimentation accumulation rates throughout Unit I average between 14.4 and 16.0 m/m.y. and are similar to those of the time-equivalent section at Site 1209 (Fig. F18). The prevalence of bioturbation in Unit I indicates that sediment at Site 1210 was deposited under sufficiently oxic conditions. Dark-light color cycles that prevail throughout this sequence result from varying proportions of clay and biogenic carbonate content. This pattern is a manifestation of variation in carbonate production and/or dissolution. Modern water depth at Site 1210 is slightly deeper than at Site 1209 (~2.6 vs. ~2.4 km), yet both sit well above the lysocline and the CCD that are regionally located at ~3.5 and 4.1 km, respectively. Because the CCD deepens in the Pacific basin during glacial periods (Farrell and Prell, 1989), it is unlikely that the cyclic variation in carbonate content observed is produced by shoaling cycles of the lysocline/CCD that occur in concert with glacial-interglacial transitions. In contrast to Sites 1207 and 1208, there is very little biogenic silica at Site 1210; rare occurrences of biogenic silica in smear slide analysis amount to only ~1%-2%. Although Site 1209 is also largely devoid of biogenic silica, there is a peak in biosiliceous deposition (up to 25% in smear slide estimates) in the mid-Pliocene that does not appear at the slightly more southerly Site 1210. This may reflect the effects of dissolution at Site 1210 or greater rates of deposition of biogenic silica at Site 1209 in the Pliocene.
Substantially fewer ash layers are present in the Neogene section at Site 1210 than at previous Leg 198 sites. Regional studies (e.g., Natland, 1993) of ash distribution across Shatsky Rise suggest that beds of ash are wind-borne from volcanic eruptions along the Japan and/or Kurile magmatic arc systems. Differences in the frequency of ash beds between these drilling sites could be linked to distance from the arc system or differences in prevailing wind patterns at these localities. The combination of differences in volcanic ash frequency and in biogenic silica composition suggest the possibility that the prevailing wind and current dynamics at Site 1210 were distinct from the more northerly Sites 1207-1209 on Shatsky Rise.
Below 83.4 mbsf in Subunit IB, the sediment is more oxidized, characterized by shades of pale orange and grayish orange rather than olive gray and gray hues. This color transition is evident in the color reflectance data by an increase in red/blue ratios. Weight percent carbonate increases in Subunit IB, giving rise to higher bulk density (Fig. F4). Subunit IC contains several clay-rich condensed intervals, which possibly are the result of shoaling of the CCD in the early to middle Miocene (Rea et al., 1995). These dark horizons contain approximately a few percent phillipsite, barite, and Fe oxides based on smear slide estimates. It is unclear whether the dark, clay-rich intervals in Subunit IC represent periods of nondeposition, erosion, or relatively continuous but slow sedimentation rates.
This unit is composed of sediment that is remarkably homogenous in composition and physical properties such as bulk density and color reflectance (Fig. F4). Sedimentation rates in the Paleogene are much lower than those in Unit I, ranging from ~2.1 to 7.6 m/m.y., most likely a result of lower carbonate production, higher rates of carbonate dissolution, and consequent development of condensed sequences. The homogenous texture of the ooze, disrupted only by rare burrow fills, is evidence that oxic conditions prevailed during sedimentation. High carbonate content throughout Unit II indicates that deposition generally occurred well above the CCD during the Paleogene. Tektites recovered from the upper Eocene in Hole 1210A (Section 198-1210A-14H-CC) are closely correlative with the timing of the Popigan impact in Russia and the Chesapeake Bay bolide.
The PETM interval is hypothesized to represent enhanced carbonate dissolution, triggered by the release of large quantities of methane clathrate (Dickens et al., 1995). This interval is slightly bioturbated at Site 1210 but otherwise fully intact and will provide a valuable resource for shore-based studies comparing the geochemical record preserved on Shatsky Rise during the PETM. Although it appears that the paleodepth of Shatsky Rise at Site 1210 was well above the CCD during much of the Paleogene, the CCD must have shoaled rapidly during the PETM. There is a clear contrast between the clay-rich layer at the base of the event and the underlying carbonate-rich layer. The clay-rich interval just above this event grades back into more carbonate-rich sediment upsection, likely due to the deepening of the CCD.
The nannofossil ooze in Unit III is nearly pure carbonate and remains white in color throughout the Maastrichtian and Campanian section recovered at Site 1210. The ooze alternates with chert, and in two places the chert appears to exist as nodules. Because the drilling process brecciated the chert, it is difficult to interpret whether the chert is primarily nodular or occurs in layers. The presence of chert in Unit III is in sharp contrast to Units I and II, which are almost entirely composed of carbonate and clay.
Inoceramus fragments are locally present in the mid-Maastrichtian (Sections 198-1210B-28H-5 and 28H-6). The occurrence of Inoceramus is restricted to sediment of mid-Maastrichtian and older age (MacLeod et al., 1996), but visible shell fragments are restricted to the mid-Maastrichtian on Shatsky Rise at Sites 1210 and 1209.
Bulk density increases downcore with carbonate content (Fig. F4), yet carbonate sediment throughout the entire sequence at Site 1210 is unlithified with the exception of a short interval of chalk just above the K/T boundary. Relatively low redox conditions persist through the upper 70 m of sediment, where it is olive gray in color and there are frequent occurrences of pyrite. Decreasing sulfate concentrations in the pore waters through Unit I (see "Inorganic Geochemistry") are consistent with sulfate reduction and, therefore, with pyrite formation. Other interesting diagenetic features that may be associated with reducing pore waters through this section are frequent pale green laminae, which are sometimes associated with diffuse purple banding. Smear slide analyses suggest that the well-developed green laminae are nearly 100% clay. XRD analysis of the clay fraction of the bulk sediment indicates the presence of illite, saponite, and smectite-kaolinite (relative abundances undetermined). However, the mineralogy of the green diagenetic laminae is primarily saponite (Ca0.2Mg0.8(Si,Al)4O10(OH)2), a clay belonging to the smectite group (Table T2). Similar features have been observed in previous Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores (e.g., Winterer, Riedel, et al., 1971; Kennett, von der Borch, et al., 1986) and have been rationalized as altered volcanic ash (Gardner et al., 1986; Lind et al., 1993). Yet several lines of evidence lead us to suggest that these green laminae are authigenically produced rather than altered volcanic glass grains:
We note, however, that the temporal distribution of the green diagenetic laminae is similar to that of volcanic ashes at Site 1210 as well as at the previous sites (1207-1209). It is possible that volcanic glass present in the sediment column provides a source of silica and cations for saponite authigenesis. Alternatively, dissolution of biogenic silica could also generate a source of silica. Pore water profiles at Site 1210 demonstrate a depletion of Mg in the upper portion of the section, which is consistent with the formation of saponite through this interval (see "Inorganic Geochemistry").
Laths of inorganic calcite are present as a minor component in some of the smear slides from Unit II. It is probable that these crystals represent incipient cementation of carbonate from the pore waters.
Carbonate sediment of Cretaceous age in Unit III is remarkably unlithified, despite ~220 m of sediment overburden. Some of the ooze in this unit is stiff, while other portions are very soft and likely became soupy during drilling and splitting. In places, the ooze seems somewhat more indurated, but this may have occurred as sections were compacted and stretched during drilling.
Although there is no evidence for biosiliceous material in the nannofossil oozes, this component was likely concentrated through diagenetic dissolution and remobilization of the silica, which was eventually converted to chert. We suspect that the chert recovered from this sequence exists primarily as layers, in part due to evidence for layered chert at Site 1207 (see "Downhole Measurements" in the "Site 1207" chapter). However, in two places where the stratigraphy within the ooze appears to be preserved, chert nodules within the core were observed. Chert recovered from Site 1210 differs from that at Site 1207 in that no porcellenite coatings or inclusions were observed at Site 1210. The chert was also relatively a monochrome medium dark gray (N4) to black (N2) and did not have the variety in color that was observed at Site 1207.