6. Site 1210¹

Shipboard Scientific Party²

PRINCIPAL RESULTS

Site 1210 is located on the southern flank of the Southern High of Shatsky Rise on seismic line TN037-17A. Because of the proximity of this site to the seismic lines that are calibrated with drill holes and characteristic sets of reflectors on these profiles, the seismic units of Sliter and Brown (1993) can be identified with confidence. Seismic Unit 1 (Neogene) is relatively condensed, and Units 2 (Paleogene) and 3 (Upper Cretaceous) are moderately expanded (Fig. F1). Although the site was selected at a location where the sedimentary section looks relatively expanded, the section was expected to contain a number of minor disconformities as indicated by prominent horizontal reflectors.

At 2574 m, Site 1210 is the second shallowest site in the Shatsky Rise Paleogene-Late Cretaceous depth transect that ranges from 2387 to 3346 m. As part of this depth transect, cores from Site 1210 will be used to address a number of leg-related objectives focused on abrupt and long-term climate change during the Cretaceous and Paleogene.

Holes 1210A and 1210B were cored largely with the advanced piston corer (APC). Extended core barrel (XCB) center bit drilling was used to punch through chert layers in the Upper Cretaceous section. Hole 1210A terminated at 249.3 meters below seafloor (mbsf) in the core below a major chert horizon in the upper Maastrichtian. In Hole 1210B, a greater effort was made to penetrate the chert layers and to core the surprisingly soft sediment in between them with the APC. In this hole, 11 chert layers were penetrated with XCB center bit drilling, and a total depth of 376.5 mbsf was achieved.

Coring at Site 1210 recovered three lithologic units that have been separated based on composition (Fig. F2). Lithologic Unit I ranges from Holocene to lower Oligocene (0 to ~32 Ma; 0-115.8 mbsf) and consists of clayey nannofossil ooze, nannofossil ooze with clay, and nannofossil ooze. This unit is split into three subunits. Subunit IA (Holocene to upper Miocene; 0 to 5.5 Ma; 0-83.4 mbsf) is olive gray to gray and was deposited at higher rates than Subunit IB (upper Miocene to upper middle Miocene; 5.5 to 11.5 Ma; 83.4-112.0 mbsf), whose color ranges from shades of yellowish brown to pale orange to grayish orange. Subunit IC (upper middle Miocene to lower Oligocene; 11.5 to ~32 Ma; 112.0 to 115.8 mbsf) encompasses several dark yellowish brown clay-rich intervals that represent condensed sections or unconformities, interbedded with pale orange nannofossil ooze. A significant unconformity from lower Miocene to lower Oligocene occurs within this interval (see "Biostratigraphy" in "Specialty Syntheses" in the "Leg 198 Summary" chapter). Lithologic Unit II ranges from lower Oligocene to lowermost Paleocene (~32 to 65 Ma; 115.8-219.9 mbsf) and consists of shades of orange and yellowish brown nannofossil ooze, nannofossil ooze with clay, and minor amounts of clay with nannofossil ooze. The unit has a generally higher carbonate content than Unit I. Lithologic Unit III ranges from uppermost Maastrichtian to lower Campanian (65 to ~77 Ma; 219.9-377.0 mbsf) and consists of white nannofossil ooze, nannofossil ooze with foraminifers, and chert. Eleven chert layers were penetrated in lithologic Unit III.

The recovered section at Site 1210 is remarkably similar to that cored at Site 1209 located 29 km to the northeast on the summit of the Southern High. On a large scale, the ages of unconformities at the two sites are similar. On a small scale, critical boundaries—for example, the Paleocene/Eocene and Cretaceous/Tertiary (K/T) boundaries—show a similar, detailed sequence of lithologies. Thus, the two sites have highly comparable sedimentation histories. There are a number of subtle differences between the Site 1209 and 1210 sedimentary sections that yield important interpretations in the depth transect framework of Leg 198. Site 1210 is 200 m deeper than Site 1209. Preliminary biostratigraphy suggests that the middle Miocene to upper Miocene interval at Site 1209 is highly condensed with a number of diastems, whereas the same interval at Site 1210 more likely corresponds to an unconformity (see "Biostratigraphy" and "Sedimentation and Accumulation Rates"). Increased dissolution at Site 1210 in the middle to late Miocene is the most likely mechanism to explain this difference.

The highlights of coring at Site 1210 are also similar to those at Site 1209, namely the recovery of all of the critical intervals, most in both holes. These include the Eocene/Oligocene boundary, the Paleocene-Eocene Thermal Maximum (PETM), a biological event in the mid Paleocene, the K/T boundary, and the mid-Maastrichtian deepwater event (MME).

Recovery of Critical Events

As at Site 1209, a number of critical events have been recovered at Site 1210 in multiple holes. The lithologic record of each of these intervals at Site 1210 appears to be remarkably similar to that at Site 1209. The correlation is especially compelling based on magnetic susceptibility data of the composite section (Fig. F3). These data show similar shaped peaks for the events at the two sites, but perhaps more remarkably, a broadly similar number of peaks in between them. Multisensor track (MST) data will provide precise correlations between the two sites as well as an internal chronology. Site 1210 in general shows a slightly more expanded as well as more complete section in certain intervals. However, in the absence of detailed data and analysis, discussion of the significance of these events would be broadly similar for both Sites 1209 and 1210. More detail on the critical events is presented in "Site 1209" in "Principal Results" in the "Leg 198 Summary" chapter.

The Eocene-Oligocene transition has been recovered in an interval of continuous recovery from Holes 1210A and 1210B. The record for both holes show a gradual increase in carbonate content that is indication for a deepening in the calcite compensation depth (CCD), similar to Site 1209 and sites from the Atlantic and Indian Oceans (e.g., Zachos et al., 1996). The Site 1210 record shows alternating dark and light lithologic cycles throughout this interval that indicate an orbital control on dissolution. In intervals of the uppermost Eocene at Site 1210, planktonic foraminifers are extremely dissolved, suggesting that the site was toward the base of the lysocline. Comparison of carbonate content and microfossil records between the Leg 198 sites will provide important information on changes in the level of the lysocline and CCD through this major cooling event.

The interval recording the PETM was recovered in Holes 1210A and 1210B. The lithologic record of this event is very similar in the two holes; this similarity is borne out by the magnetic susceptibility records (Fig. F3). The sharp base of the event coincides with an abrupt change from a very pale orange nannofossil ooze to a thin (1-2 mm), dark yellowish brown clayey nannofossil ooze. This is overlain by ~18 cm of moderate yellowish brown nannofossil ooze with clay that grades slowly into a pale orange nannofossil ooze. There is a noticeable color change from below the PETM clay-rich horizon to directly above it that persists upsection for at least 10 m. The significance of this color change has not been determined. The clay-rich units show signs of dissolution, although this does not appear to be more pervasive than at Site 1209. Nannofossils appear highly dissolved in the lowest 1 cm of the event, but preservation improves significantly in the middle and upper part of the nannofossil ooze with clay. Blade-shaped, ~10-20 µm calcite grains are observed throughout the clay-rich units. Transient "excursion" planktonic foraminifers that correlate with the interval represented by the negative carbon isotope shift (e.g., Kelly et al., 1996) are observed within and just above the clay-rich units. The lack of significant difference between the PETM records at Sites 1209 and 1210 suggests that these sections were located in a depth range that was relatively insensitive to carbonate solubility changes across the PETM. The Paleocene-Eocene transition at Site 1208, on the other hand, shows a significant amount of dissolution and intervals lacking carbonate, suggesting that it was at a depth (~800 m deeper than Site 1210) far more sensitive to solubility changes.

The record of the K/T boundary at Site 1210 is similar to that at Site 1209. The boundary succession includes uppermost Maastrichtian (nannofossil Zone CC26) pale orange nannofossil ooze overlain by lowermost Paleocene (planktonic foraminiferal Zone P) grayish orange foraminiferal ooze that grades into a white foraminiferal nannofossil chalk then back into a grayish orange nannofossil ooze. The boundary between the uppermost Maastrichtian and the lowermost Paleocene is clearly bioturbated, and careful sampling of burrows yields planktonic foraminifers dominated by Guembelitria with rare Hedbergella holmdelensis that suggest a possible Zone P0 age. Light brown to amber, spherical particles ~50 µm in diameter found in a sample from these burrows may be altered tektites. As at Site 1209, perhaps the most exciting aspect of this boundary succession is the excellently preserved, and apparently expanded, Danian section that will allow us to investigate the detailed record of the recovery and adaptive radiation of floras and faunas after this major extinction event.

The MME also appears to have been recovered at Site 1210. Large Inoceramus prisms can be seen over a 2-m interval of the mid-Maastrichtian in Core 198-1210B-28H but disappear above this level. The first occurrence (FO) of the planktonic foraminifer Abathomphalus mayaroensis lies in Section 198-1210B-26H-CC, which is consistent with the age of the event at other sites (i.e., MacLeod and Huber, 1996).

Anomalous Lithification of Shatsky Rise Sediments

One of the most interesting results to emerge from Site 1210 and other Southern Rise sites is that the sediment has undergone little lithification, even at comparable burial depths to sediment at other sites that are indurated. For example, at the base of Hole 1210B at 377 mbsf, the predominant lithology is a nannofossil foraminiferal ooze. This sediment is soft, plastic in behavior, and almost uncemented. Nannofossils and foraminifers at this level have suffered a greater amount of dissolution than overgrowth. Chalk is found at a comparable depth in a typical carbonate sequence on the Ontong Java Plateau; in fact, the chalk/ooze boundary is located between 181 and 339 mbsf (Berger et al., 1991). The soft nature of the upper part of the Shatsky Rise section has been discussed by Matter et al. (1975).

At Site 1210 between 200 and 300 mbsf, there are not the expected changes in physical properties that go hand in hand with compaction (e.g., Schlanger and Douglas, 1974). In this interval, gamma ray attenuation (GRA) density decreases, P-wave velocity is constant, and porosity increases (see "Physical Properties"). What are the major anomalies at Shatsky Rise that might be responsible for the relative lack of induration of the Cretaceous and Paleogene section? One possible factor that may have played a role in keeping the sediment soft is that sedimentation rates for most of the burial history of the deep section have been relatively slow, except for the last 5 m.y. (see "Sedimentation and Accumulation Rates"). Thus, the time integrated overburden for the deep section has been far less than for most comparable sections.

A second factor is sediment composition. Cretaceous and Paleocene sediments at Site 1210 and other Leg 198 sites on the Southern High of Shatsky Rise are unusually enriched in foraminifers. Qualitative estimates of the foraminiferal abundance range up to 30% by volume, whereas typical deep-sea sediments never exceed 15%-20% foraminifers. The bulk of the remaining volume is composed of nannofossils. The Site 1210 foraminiferal nannofossil oozes do not have anomalous porosities, densities, or P-wave velocities. However, the predominantly subspherical shape of foraminifers (vs. the predominantly flat shape of nannofossils) provides a smaller surface of exposed carbonate grains and a lower amount of grain-to-grain contact than in typical nannofossil ooze. Thus, typical nannofossil ooze will experience more pressure solution, and this will lead to a greater amount of available carbonate for overgrowth on particles and for cement. This general relationship is borne out by results from the Ontong Java Plateau. Shipboard microfossil abundance estimates show a substantial difference in the relative abundance of nannofossils and planktonic foraminifers, especially around the ooze-chalk transition. Although these data are semiquantitative, an apparent relationship exists between the depth of the transition and the relative abundance of foraminifers. Sections with generally higher abundances of foraminifers (Sites 805, 806, and 807) have deeper ooze-chalk transitions, between 264 and 339 mbsf, than those with lower percentages of foraminifers (Sites 803 and 804) where this transition is at 217 and 181 mbsf, respectively.

¹Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
²Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 198IR-106