Site 1207 is the northernmost site in the Shatsky transect, lying some 5° north of the sites on the Southern High. The site is located in lower bathyal (3103 m) water depth close to the most elevated, central part of the Northern High of Shatsky Rise. The paleodepth history of this site is not well known, but subsidence was likely rapid in the interval immediately after its formation in the Valanginian (135 Ma) then slowed considerably. Paleoreconstructions suggest that the site was formed and remained in equatorial latitudes for the first 50 m.y. of its history (Fig. F4).
The Northern High has not been drilled before; thus, the stratigraphy was unknown prior to drilling Site 1207. The sedimentary section at Site 1207 is ~1200 m thick. Seismic profiles show a generally horizontal stratigraphy, with several prominent reflectors. Drilling objectives were to recover a Paleogene and Cretaceous section to investigate climate change during an interval of long-term global warmth. Correlation with reflectors on the Southern High defined by Sliter and Brown (1993) was tentative. The most prominent reflectors on the Southern High, Reflectors R1 of Cenomanian-Coniacian age and R2 of Barremian-Aptian age, are apparent on seismic profiles. The drilling strategy was to core through R2 in a first hole using the advanced piston corer/extended core barrel (APC/XCB), to core a second APC hole to refusal, and then to core a hole using the rotary core barrel (RCB) through R2. Finally, a full suite of logs were to be collected through the whole sequence.
A 622.5-m upper Barremian to Holocene section was penetrated at Site 1207 (Table T1). Recovery was excellent in the upper Campanian to Holocene section, but poor in the upper Barremian to lower Campanian where chert horizons were extremely common. Results show a surprisingly different sedimentary sequence than the one previously documented at DSDP and ODP sites on the Southern High of Shatsky Rise. The Paleogene and uppermost Cretaceous sequence likely is missing as a result of major early Neogene slumping and erosion (Fig. F11; see also lines 5A and 5B in oversized Figures F7 and F10 in Klaus and Sager, this volume). Thus, we were unable to achieve our Paleogene objectives at this site. The middle Miocene to Pliocene section, on the other hand, is expanded (161 m thick), apparently complete, and composed of nannofossil ooze with diatoms that also contain planktonic foraminifers and radiolarians (Fig. F12). The section contains prominent carbonate cycles that appear to record orbital climate fluctuations. The section should provide a valuable biochronology as well as important information on the nature of ocean circulation changes.
At the opposite end of the stratigraphic column, a horizon of highly carbonaceous (up to 34.7 wt% Corg) lower Aptian claystone that correlates to the global OAE1a, including the Selli level of the southern Alps and Apennines of Italy, was recovered. Organic matter in these horizons is exceptionally well preserved. Organic geochemical, stable isotopic, and paleontological analyses of this Corg-rich interval will help constrain the environmental changes during OAE1a in the Pacific Ocean. Drilling at Site 1207 was impeded by the presence of chert throughout the lower Campanian to Barremian section. Drilling and logging at Site 1207 revealed important information about the nature and stratigraphy of this chert that will help develop strategies to improve recovery in such chalk/chert sequences.
Upper Miocene to Holocene sediments (lithologic Subunit IA) recovered at Site 1207 contain common to abundant diatoms (between 10% and 40%), a far higher proportion than in contemporaneous units from sites on the Southern High of Shatsky Rise. At Southern High Site 305, the relative proportion of diatoms is 5%-15% in the Miocene to Holocene section, but it decreases rapidly at the unconformity between the upper Miocene and the upper Oligocene (Larson, Moberly, et al., 1975). Diatoms were described as rare in samples from Sites 47 and 577 (Koizumi, 1975; Koizumi and Tanimura, 1985). Approximately 5° of latitude separates Site 1207 from the previously drilled sites on the Southern High, and it is feasible that this distance is associated with a significant change in oceanographic regime.
The abundance of diatoms in lithologic Subunit IA (0-131.3 mbsf) sediments suggests that surface waters over Site 1207 were moderately to highly productive from the late Miocene to the Holocene. Alternatively, high biosiliceous production at Site 1207 may be an effect of local topographic upwelling. This option is not appealing given the absence of significant diatom concentration in sediments on the Southern High. More likely, the abundance of diatoms at Site 1207 reflects a component of productive subarctic surface waters over the site in the late Neogene. The relative proportion of diatoms is considerably lower in Subunit IB (131.5-162.5 mbsf) sediments, likely as a result of lower surface water productivity in the early middle to early late Miocene. The abundance of Discoaster, a nannoplankton genus thought to thrive in oligotrophic waters (e.g., Chepstow-Lusty et al., 1989) appears to be antithetical to the proportion of diatoms in the Neogene section.
One of the most prominent features of the Neogene section at Site 1207 is the decimeter-scale lithologic cycles in Cores 198-1207A-1H to 18H between dominantly darker green-gray horizons and lighter tan-gray-white horizons. These cycles show significant changes in the relative percentage of diatoms (see "Lithostratigraphy" in "Specialty Syntheses") and marked differences in the nature of carbonate preservation. Reconstructions suggest that the site was above the CCD after at least the early Pliocene but close to this level in the Miocene (Rea et al., 1995); thus, the cycles could represent variations in dissolution as a result of changes in the depth of the lysocline. Variation in biosiliceous production may also be responsible for the cycles.
Today, Site 1207 lies in a subtropical water mass toward the north of the range of the warm water Kuroshio Extension current. To the north of the site lies a significant front, a transition region between subtropical and subarctic water masses. Transition zone waters are derived from off the coast of northern Japan, where the cold, nutrient-rich Oyashio Current mixes with the warm, nutrient-poor Kuroshio Current. These waters move eastward across the Pacific in the West Wind Drift at a latitude of 40°-42°N. The location of Site 1207 is highly sensitive to past climatic variations because of its proximity to the transition zone.
Significant oceanographic changes occurred during the Neogene that had profound effects on circulation and distribution of water masses in the Pacific Ocean; these changes appear to be reflected in the sedimentary record at Site 1207. An event at 14.5 Ma has been associated with the formation of the East Antarctica Ice Sheet (Kennett et al., 1985); another event at 11 Ma is related to the closure of the Indo-Pacific seaway (Romine and Lombari, 1985). These events led to a steepening of temperature gradients and intensification of North Pacific gyral circulation including the ancestral Kuroshio Current. The modified circulation resulted in the development of a distinct North Pacific transitional water mass, separated from the northern subpolar region, and the northward displacement of temperate organisms. The development of distinct water masses in the North Pacific combined with long-term cooling may also be responsible for the marked facies change between lithologic Subunits IB and IA that corresponds to a sharp increase in biosiliceous material in sediments at ~9 Ma.
Koizumi (1985) correlated fluctuations in Pliocene-Pleistocene diatom communities at DSDP Sites 579 and 580 in the abyssal plain northwest of Shatsky Rise to climatic fluctuations that caused variation in the amount of subarctic waters at these sites. In colder intervals of the late Miocene to Pleistocene, Site 1207 may have been within the transition zone between the subtropical water mass and the subarctic water mass (Fig. F13), even though the site was well to the south of its current location (32°N at 10 Ma) (R. Larson, pers. comm., 2001). Thus, fluctuations in the proportion of diatoms, which are more abundant in subarctic than subtropical waters, may reflect latitudinal shifts in the position of the transition zone.
Shipboard biostratigraphic data suggest that nannofossils have suffered a greater amount of dissolution in greenish layers that are enriched in diatoms; preservation is considerably better in the white-gray ooze layers that have fewer diatoms. If the greenish layers correspond to cool intervals as argued above, then the pattern of preservation is opposite to that of most sites in the Pacific (e.g., Farrell and Prell, 1991; Zahn et al., 1991). A body of evidence suggests that during glacial stages intermediate deep waters were produced in the Pacific and that these young, nutrient-poor waters caused little dissolution close to their source. An opposite pattern was noted at Site 882 on Emperor Seamount by Haug et al. (1995), who argued that the upwelling of nutrient- and CO2-rich waters during glacial stages increased carbonate dissolution. Microfossils suggest a similar mechanism for dissolution patterns in the upper Miocene to Pleistocene at Site 1207.
A significant peak in opal accumulation rates occurs between 3.2 and 2.75 Ma at other sites from the northern Pacific (Haug et al., 1995; Maslin et al., 1995). The origin of this peak and subsequent decline is uncertain, but the base of this peak corresponds to the beginning of long-term global cooling that culminated in Northern Hemisphere glaciation, and the top of the peak corresponds to the rapid advance of Northern Hemisphere glaciers. Although qualitative in nature, a sharp rise in the abundance of diatoms occurs in the middle of Core 198-1207A-6H and continues up to the lower part of Core 198-1207A-3H. The age model indicates that this interval corresponds approximately to the 3.2- to 2.75-Ma period.
One of the surprises of drilling at Site 1207 was a major unconformity between the middle Miocene and upper Campanian. This hiatus represents ~60 m.y. of stratigraphic record. The unconformity lies at the base of a 3.3-m interval of zeolitic nannofossil clay (lithologic Subunit IC; 162.5-163.8 mbsf), at the center of which are a 5-cm Mn nodule and a few chert nodules. The interval has common micronodules of phosphate, volcanic glass, and grains of quartz, feldspar, and heavy minerals, as well as authigenic phillipsite crystals. Microfossils of Campanian and Miocene ages are mixed throughout the clay interval, possibly because of drilling disturbance. Very rare Paleogene nannofossils and foraminifers were observed in a few samples from this interval.
The age of the event(s) that caused the unconformity is difficult to interpret from the fossil record. The most likely age is an interval prior to the middle Miocene (16 Ma), the age of immediately overlying sediments. The presence of an Mn crust and micronodules, chert nodules, and zeolite minerals suggests an extended interval of seafloor exposure. A mixture of nannofossils of Campanian and Miocene ages suggests winnowing of underlying units after the hiatus. Paleogene microfossils hint at the presence of some intermediate section before removal during the hiatus interval.
Major unconformities are also found at Sites 49 and 50 near the base of the southwestern flank of the Southern High (4282 and 4487 m, respectively) (Table T2). At these sites, Pleistocene ooze directly overlies uppermost Jurassic or lowermost Cretaceous chalk. At Site 306 on the southern flank of the Southern High, Pleistocene sediments directly overlie those of Albian age. All of these sites are located on significant slopes; thus, it is likely that unconformities were produced by the removal of sediments by mass wasting. The unconformities at Sites 49 and 50 are characterized by zeolitic clay, common phillipsite, and Mn-coated rock fragments, chert, and ash pebbles. The core with mixed Albian and Quaternary at Site 306 contains Mn nodules and chert fragments.
Several possibilities exist for the origin of the major unconformity at Site 1207. A number of prominent hiatuses in the deep sea are caused by changes in the CCD resulting from shifts of carbonate deposition to shallow-water environments coincident with sea-level rise (e.g., Loutit et al., 1988) or changes in deepwater circulation that increases the age of a water mass at a particular location. Although the subsidence history of Site 1207 is undetermined at the current time, and the history of the CCD in the Pacific is still somewhat uncertain, available information (Thierstein, 1979; Rea et al., 1995) suggests that the CCD was substantially deeper than the current depth of Site 1207 in early Miocene to late Eocene times (Rea et al., 1995). The curve of Rea et al. (1995) for the northwest Pacific indicates that Site 1207 was likely near or slightly below the CCD at some point in the Maastrichtian-middle Eocene. However, the sediment record on the Southern High suggests that the CCD history for Shatsky Rise is different from the history for the northwest Pacific developed by Rea et al. (1995). Indeed, a separate study (Thierstein, 1979) shows that the current depth of Site 1207 was substantially above the Pacific CCD in the Campanian-Maastrichtian. Given that the site must have been shallower than its current depth throughout the Campanian to Miocene interval, it is unlikely that dissolution is responsible for the unconformity.
A second possible mechanism for the unconformity is that sediments were removed from the top of the Northern High during one or a series of erosion, slumping, or mass wasting events. Seismic line TN037-5A shows an interval of disturbed, diagonal reflectors with thicknesses ranging to 100 m up to 10 km to the north and south of Site 1207. These disturbed horizons may represent beds truncated by erosion or slumping (Fig. F14).
The depth and age distribution of Neogene deep-sea hiatuses have been studied in detail by Keller and Barron (1987). These authors found a prominent hiatus between 15 and 16 Ma in CN4, which corresponds to the minimum age of the major unconformity at Site 1207. Keller and Barron (1987) also observed that hiatuses in the depth range of Site 1207 are concentrated on plateaus, rises, and seamounts and suggested that they were most likely due to slumping events.
Site 1207 lies between basement highs to the east and west. The unconformity lies substantially below the level of the highs. Thus it is possible that this paleotrough served as a north-south conduit for bottom-water flow from the Cretaceous to the early Neogene. This flow may have steepened and undercut the sediments leading to slumping on the north and south of the present summit. In fact, a prominent canyon, which may have been produced by mass wasting, lies just to the north of Site 1207. Erosion would have been efficient while sedimentation rates were lower in the middle Miocene, but once productivity and, hence, sedimentation rates increased in the late Miocene, a sediment drape was established over the paleotrough.
In summary, results from Site 1207 show a drastically different sequence than anywhere on the Southern High of Shatsky Rise. Available seismic profiles suggest that sediments of Paleogene and latest Cretaceous age are not present on the Northern High as opposed to the Southern High, where these units are widespread. Conversely, the Miocene-Holocene section on Northern High is relatively expanded, whereas this sequence is thin or absent on the Southern High.
A major highlight of Site 1207 is the recovery of a 45-cm-thick, dark brown, finely laminated Corg-rich claystone of early Aptian age in lithologic Unit III (335.3-622.8 mbsf). The presence of lamination indicates dysoxic or anoxic conditions at the seafloor for the duration of the event. Organic carbon contents are extremely high (up to 34.7 wt%); characterization of this organic matter indicates it is algal and bacterial in origin. Gamma ray logging data suggest that an additional 50 cm of claystone was not recovered. Biostratigraphy indicates that this distinct horizon was deposited during OAE1a (Schlanger and Jenkyns, 1976; Bralower et al., 1993).
Corg-rich horizons of OAE1a age are also found at Site 305 on Southern High, Shatsky Rise, Site 463 on the Mid-Pacific Mountains, and Site 866 on Resolution Guyot (Sliter, 1989; Jenkyns, 1995). Only Sites 463 and 866 have decent recovery. The Site 1207 record extends the latitudinal and depth transect of this event in the Pacific Ocean. At Site 463 (2525-m water depth), carbonaceous limestones with up to 7.6 wt% Corg are associated with volcanic ash. At Site 866 (1373-m water depth), thin organic carbon-rich (14.2 wt%) claystones are found in a sequence of shallow-water carbonates. Organic matter characterization, including stable carbon isotope analysis indicate that the Corg in both sites is of algal marine origin (Dean et al., 1981; Baudin et al., 1995), similar to that at Site 1207.
Detailed microfossil biostratigraphic studies (Sliter, 1989; Bralower et al., 1994; Erba, 1994) have demonstrated that Site 463 Corg-rich horizons correlate exactly with carbonaceous units of early Aptian age known as the Niveau Goguel in southeastern France (Bréhéret, 1988), the Livello Selli type-level in central Italy (Coccioni et al., 1987), and the Livello Selli equivalents from the Italian and Swiss Alps, Sicily, DSDP Site 641 in the eastern North Atlantic, and northern Mexico (i.e., Weissert and Lini, 1991; Bralower et al., 1994; Erba, 1994; Menegatti et al., 1998; Erba et al., 1999; Bralower et al., 1999).
OAE1a is associated with abrupt changes in calcareous nannofossil assemblages, especially among a rock-forming group known as the nannoconids, which decline sharply in abundance just prior of the onset of the event (Erba, 1994). Other robust nannofossil taxa radiate and increase in size (Tremolada and Erba, 2002). Diversity and abundance of planktonic foraminifers decline rapidly prior to OAE1a. Planktonic foraminifers are absent at the base of the event as a result of dissolution then fluctuate from rare to few and from low to moderate diversity through the rest of the event. When present, taxa are mainly adapted to the poor oxygen contents in the upper water column (Magniez-Jannin, 1998; Premoli Silva et al., 1999). Radiolarians are abundant throughout but exhibit a marked compositional change at the beginning of the event (Premoli Silva et al., 1999).
The early Aptian event is marked by a distinctive carbon isotopic profile consisting of a ~2
-3 negative shift at the base of the event followed by a ~4-5 increase. The pronounced negative excursion has been recognized at Site 866 on Resolution Guyot (Jenkyns, 1995), the southern Alps of Italy (Menegatti et al., 1998; Erba et al., 1999), the Isle of Wight (Gröcke et al., 1999), and northern Mexico (Bralower et al., 1999).
Early Aptian Corg-rich units that correlate to OAE1a have been found in a limited number of locations compared to the Cenomanian/Turonian boundary OAE (OAE2). This has led to some uncertainty as to whether the event was global in scale. Recovery of the early Aptian Corg-rich horizon at Site 1207 provides additional evidence that OAE1a was a global event. The record of OAE1a at Site 1207 is more complete than any other deep-sea record except possibly Site 463. The Corg contents are exceedingly high, and the burial depth is relatively shallow (565.5 mbsf). Hence, the organic geochemical records should be nearly pristine. These and biotic records should provide key information on the environmental changes that occurred during the anoxic event.
Chert is a fundamental part of the Cretaceous stratigraphic section on Shatsky Rise. Unfortunately, little was known about the distribution and nature of chert. For example, it was unclear whether these units are nodular or layered, or whether they are distributed randomly through the section or in individual zones.
The occurrence of chert in the Shatsky Rise section is thought to reflect its path across the equatorial divergence, where high productivity led to accumulation of opaline siliceous microfossils in the sediment. As the sediment was subsequently buried, this opal was progressively transformed to quartz. The stratigraphic distribution of chert in the sediment may yield important information about the width and strength of the equatorial divergence and whether this productivity varied in a predictable fashion (i.e., on orbital timescales); chert morphology may provide indications of the nature of the diagenetic process.
The uppermost chert horizon at Site 1207 lies in upper Campanian sediments at 76 Ma. Common chert is found from the lower Campanian (79-80 Ma) to the base of the hole (lithologic Units II and III; 163.8-335.3 and 335.3-622.8 mbsf, respectively). Assuming the reconstruction of R. Larson (pers. comm., 2001), these data suggest that the equatorial divergence was significantly wider than of today, ranging to ~15° north of the equator (Fig. F4). The presence of chert throughout the section indicates that the site remained within the equatorial divergence from the Aptian up to the early Campanian. In fact, the reconstructed path predicts that the site remained within the divergence for at least the first 60 m.y. of its history.
Anecdotal drilling information and logging data show that chert levels are extremely close together for the entire lower Campanian to upper Barremian section. Formation MicroScanner (FMS) data indicate that the cherts are generally finely layered as opposed to nodular and that most layers are <10 cm thick. However, interbedded soft sediment layers are <1 m thick and most often under 30 cm. These interbeds are mainly composed of ooze or soft chalk down to the Aptian, suggesting that the burial depths were never much greater than those at present. The softness of these interbeds exacerbated recovery efforts in the mid- and Upper Cretaceous interval.
Site 1208 is located at lower bathyal (3346 m) water depth, close to center of the Central High of Shatsky Rise. The site lies on seismic line TN037-8 (Fig. F15). Sediments of the Central High have not been cored before; thus, the stratigraphy was unknown prior to drilling at Site 1208. Moreover, correlation of reflectors and the major seismic units with the Southern and Northern Highs, where units and reflectors are calibrated with drill holes, is speculative. Site 1208 coring was designed to provide knowledge of the stratigraphy of the Central High, as well as correlation of units and reflectors with the Southern and Northern Highs. The site was located at the point where the stratigraphic sequence appears to be most complete.
The objective at Site 1208 was to recover a Paleogene through Upper Cretaceous section for paleoceanographic investigation. Highly tentative predrilling correlation with the Southern High seismic units of Sliter and Brown (1993) suggested relatively thick seismic Units 1 (Neogene) and 2 (Paleogene) characterized by predominantly weak, largely horizontal reflectors and a relatively thin Unit 3 (Upper Cretaceous). The Upper Cretaceous to Holocene sequence was expected to contain a number of minor disconformities as indicated by prominent, but horizontal reflectors. At depth, a major angular unconformity suggests erosion of a significant part of the mid-Cretaceous sequence (Fig. F15). The total thickness of the sedimentary section at Site 1208 was estimated at ~785 m. Basement underlying the site was formed during Chron CM15 in the Berriasian (Nakanishi et al., 1989). The drilling strategy was to double core down to the uppermost chert horizon in the Upper Cretaceous using the APC/XCB.
Coring at Site 1208 revealed a dramatically different sequence than predicted. The upper ~260 m is an expanded upper Miocene to Holocene section below which lies almost 60 m of less expanded lower and middle Miocene section. Reflectors that were thought to represent transitions between different units are probably individual horizons with substantially different bulk density values, probably diatom-rich levels. The Paleogene is almost entirely missing at this site and only a short segment of the Upper Cretaceous was encountered beneath a prominent, deep reflector at 328.2 mbsf (Fig. F15; see line 8 in oversized Figure F7 in Klaus and Sager, this volume). We drilled a total of 392.3 m through a major angular unconformity where Campanian ooze and chalk rest unconformably on middle Albian chalk and chert (Table T1). This chert prevented further drilling with the XCB. Because only a small portion of the section will contribute to the major goals of the leg, we decided not to core a second hole.
A thick, apparently complete upper Miocene to Holocene section was recovered between 0 and 251.6 mbsf (Fig. F16; lithologic Subunit IA). This section is composed of nannofossil ooze and nannofossil clay. Most intervals have significant (5%-20%) amounts of diatoms and minor amounts of foraminifers, radiolarians, and silicoflagellates, and numerous discrete ash horizons. The entire Neogene section is composed of prominent lithologic cycles on a decimeter to meter scale. These cycles were also detected in multisensor track (MST) and color reflectance data throughout the section; preliminary biochronology suggests that they have dominant frequencies of ~40 and 100 k.y. and thus represent obliquity and eccentricity rhythms. The section shows highly promising magnetostratigraphy as well as properties suitable for paleointensity records. The average sedimentation rate of the upper Miocene to Holocene section is ~42 m/m.y; thus, it will be an excellent candidate for high-resolution biochronologic and paleoceanographic investigations.
The recovered section contains an interval (lithologic Subunit IC; 311.65 to 328.15 mbsf) with at least four unconformities separated by sediments (claystone to nannofossil ooze and chalk) deposited at extremely slow rates (0.08 to 4.2 m/m.y.). These highly condensed claystones contain some zeolite minerals (phillipsite), manganese micronodules, and rare foraminifers. Nannofossil biostratigraphy allows us to piece together the history of this highly disjointed section. The section shows some striking similarities to the major unconformity at Site 1207, as well as some minor differences. Combining results from both sites, will enable us to reconstruct the regional depositional history for the Central and Northern Highs of Shatsky Rise.
The zeolitic claystone at the base of lithologic Subunit IC at Site 1208 suggests that the site was close to or below the CCD for much of the late Paleocene, Eocene, and Oligocene. Directly above the unconformity between the Campanian and Paleogene, distinct nannofossils allow us to identify a narrow time slice across the P/E boundary in Section 198-1208A-36X-CC. This section may contain a highly condensed interval of the PETM and thus may provide important information on the response of the deep ocean to this abrupt global warming event. An overlying interval of nannofossil ooze indicated an abrupt drop in the CCD in the earliest Oligocene coincident with the earliest Oligocene (Oi-1) cooling step (Zachos et al., 1993, 1996).
Sedimentation rates at Site 1208 average 42.4 m/m.y. from the Holocene to upper lower Pliocene (0 to 3.82 Ma), 22.3 m/m.y. from the upper lower Pliocene to the lower upper Miocene (3.82 to 8.28 Ma), then decrease to 5.9 m/m.y. in the lower upper Miocene to upper lower Miocene (8.28 to 18.2 Ma). The rates in the upper Miocene to Holocene are far higher than typical pelagic sedimentation. The detrital clay and silt component of the sediment may have been derived by eolian transport. However, we suspect that a large component of the sediment must have been delivered by bottom currents. A number of recent ODP legs have targeted sediment drifts for high-resolution paleoceanographic investigation. These include Leg 162 in the Iceland Basin and Norwegian-Greenland Sea and Leg 172 on the Blake-Bahama Outer Ridge and Bermuda Rise. The Site 1208 sedimentation rates, although significant, are not as high as the rates in the majority of these sites.
Sedimentation rates in the lower Pliocene to Holocene interval are more or less constant. Sedimentation rates in contemporaneous intervals at sites drilled during Leg 145 in the North Pacific peaked in the Pliocene and decreased above this interval (Barron et al., 1995). The Pliocene peak, which is considerably higher than rates at Site 1208, is associated with a massive flux of diatoms, the so-called "diatom dump."
The Neogene section at Site 1208 has a number of additional advantages for high-resolution biochronology. These include a combination of siliceous and calcareous microfossils, a high-resolution magnetostratigraphy, a marked orbital cyclicity, numerous ash layers with potential for radiometric dating and intrasite correlation, and a potential magnetic paleointensity record.
Foraminiferal preservation is generally poor, but sufficient for stable isotope stratigraphy in selected intervals. Nannofossil and planktonic foraminiferal assemblages show considerable variation that appears to record climatic change. Diatoms, radiolarians, and silicoflagellates also show sharp changes in abundance that are likely related to changing water-mass properties. Thus, the section has significant potential for high-resolution paleoceanographic investigations.
Marked cyclic variations are observed in MST data throughout the upper Miocene to Holocene section at Site 1208. These variations are expressed as strong lithologic cycles that have frequencies at the decimeter to meter scale. Preliminary shipboard biochronology suggests that the dominant periodicities correspond to eccentricity (~100 k.y.) cycles for the last 0.6 m.y. and to a combination of eccentricity and obliquity (~40 k.y.) cycles in the interval from 0.6 to 2.7 Ma. These cycles are marked by relatively subtle to sharp changes in color that are associated with variations in the amount of clay, pyrite, and different biogenic particles. For most of the upper Miocene to Holocene section, the cycles are predominantly between nannofossil clay with diatoms and nannofossil ooze with clay and diatoms. The darker gray to green interbeds tend to have more abundant diatoms and clay, more dissolved nannofossil assemblages, and more abundant reduced iron minerals (i.e., pyrite). The lighter, gray, tan, and white interbeds contain fewer diatoms, less clay, and a better-preserved nannofossil assemblage.
Upper Miocene to Holocene sediments (lithologic Subunit IA) recovered at Site 1208 contain few to common diatoms (up to 20%)—lower percentages than in sediments recovered at Site 1207, but higher percentages than contemporaneous units from sites on the Southern High of Shatsky Rise, where diatoms are usually <5%. Site 1208 is ~1° south of Site 1207 and ~4° north of the Southern High sites. As at Site 1207 (see "Site 1207"), diatom-rich layers are thought to represent intervals during which colder, more productive, transitional, and subarctic water masses shifted southward over the site (Fig. F13). Lighter-colored layers that are poorer in diatoms represent warmer intervals during which Site 1208 was located in a subtropical water mass, similar to its location today and similar to sites on the Southern High through most of the Neogene. As the site is considerably higher than the surrounding deep-ocean floor, there may also be a topographic effect to the productivity and productivity variation.
Diatoms increase markedly in abundance across the transition from lithologic Subunit IB to IA at 251.6 mbsf (~8 Ma). This interval lies just after a change in Pacific Ocean circulation associated with the closure of the Indonesian seaway that caused intensification of North Pacific gyral circulation (Fig. F17) (Kennett et al., 1985); a strengthened west wind drift likely increased upwelling along this boundary and created a more well-established North Pacific transitional water mass separated from the northern subpolar region.
Changes in the geographic distribution of water masses through time also affected other fossil groups at Site 1208. In particular, Neogene planktonic foraminifers show distinct stratigraphic changes between assemblages dominated by subtropical and tropical taxa and those dominated by taxa with cool, temperate affinities. Moreover, faunas at Site 1208 are considerably richer in warmer, tropical taxa than at Site 1207, which is located only ~1° to the north. This suggests that for much of the Neogene, Sites 1207 and 1208 were located in a region with sharp temperature gradients.
The interval between the lower Miocene and the upper Campanian (lithologic Subunit IC; Cores 198-1208A-35X and 36X; 311.65-328.15 mbsf) is marked by at least three distinct unconformities (see "Biostratigraphy" in the "Site 1208" chapter). These unconformities separate the upper Oligocene and lower Oligocene, the upper Eocene and lower Eocene, and the P/E boundary interval and the upper Campanian. Additionally, most of these intervals are characterized by extremely slow sedimentation that locally resulted in the postdepositional precipitation of zeolites and manganese micronodules. Carbonate layers occur sporadically and are generally composed of dissolved nannofossils and sparse benthic and planktonic foraminifers. The only coring gap that affects interpretation lies between Sections 198-1208A-36X-2 and 36X-CC in the lower Eocene between nannofossil Subzone CP9b and combined Zones CP10 and CP11. The estimated sedimentation rates for the key intervals containing unconformities are 0.09 m/m.y. for the late Paleocene to early Eocene transition and 4.2 m/m.y. for the earliest Oligocene.
The lower Miocene to Campanian interval at Site 1208 shows stratigraphic and lithologic similarities to the contemporaneous interval at Site 1207 on the Northern High. This suggests that events that caused the unconformity were regional in scale. However, the unconformity record at Site 1208 is more complex than that at Site 1207. The near continuous record at Site 1208 allows us to distinguish highly condensed intervals from major breaks in sedimentation. The presence of carbonate provides a record of CCD variations through time.
Several of the unconformities at Site 1208 appear to result from shoaling of the CCD. The history of the CCD in the North Pacific shows a slow rise from 70 to 50 Ma (early Maastrichtian to early Eocene), a rapid rise from 50 to 30 Ma (early Eocene to early Oligocene), and a long-term deepening from 30 to 14 Ma (early Oligocene to middle Miocene) (Rea et al., 1995). The CCD remained above the current depth of Site 1208 from 39 to 15 Ma (middle Eocene to middle Miocene). One of the clearest lithologic changes lies close to the E/O boundary in interval 198-1208A-36X-2, 0-20 cm, where a distinct color change reflects a sharp upsection increase in carbonate content that likely corresponds to an abrupt drop in the CCD. This interval, within nannofossil Subzone CP16a and the lower part of Subzone CP16b, corresponds to a global deepening of the CCD (Zachos et al., 1996) that is thought to correspond to the Oi-1 cooling event.
The uppermost Paleocene to lowermost Eocene interval appears highly condensed. Preliminary biostratigraphy suggests that this interval is complete to the limits of resolution. The presence of highly dissolved nannofossil assemblages and depauperate foraminifers in some samples and other samples that are devoid of carbonate suggests that the site rested close to the CCD through this interval.
The absence of zonal markers in the lower Miocene to lower Oligocene transition complicates age interpretation (see "Biostratigraphy" in the "Site 1208" chapter). This transition lies in an interval of nannofossil ooze, chalk, and claystone that is dark orange to brown in color, indicative of slow sedimentation. However, given poor biostratigraphic resolution, it is also possible that one or more unconformities lie within this transition. The similarity in the age of the sediment overlying this interval (early Miocene foraminiferal Zone N9; 14.7-15.1 Ma) at Sites 1207 and 1208 also suggests that the unconformity represents a regional event. Seismic reflection profiles at Site 1207 indicate that a major Oligocene-early Miocene interval of erosion and slumping removed much of the section underlying this unconformity. The stratigraphy at Site 1208 suggests that this erosive event may have been regional, perhaps driven by the intensification of deepwater circulation during long-term early Neogene cooling (e.g., Kennett et al., 1985).
The record of hiatuses at Site 1208 and other sites in the North Pacific also shows a number of similarities. The section at Site 883 on Emperor Seamount has unconformities in the upper to middle Miocene, the lower Miocene to lower Oligocene and the lower Eocene (Barron et al., 1995). Keller and Barron (1987) show four widespread hiatuses in the uppermost Oligocene to lowest middle Miocene interval. Thus, the early Miocene to early Oligocene erosive event is not limited to Shatsky Rise but is a regional phenomenon.
A major angular unconformity occurs between the Campanian and the middle Albian (Fig. F15). This unconformity contact was not recovered at Site 1208, but its nature can be inferred from regional seismic interpretations. Seismic stratigraphy indicates that the age of the units overlying the Campanian becomes younger to the east of Site 1208. In this direction, the truncation of the inferred mid-Cretaceous section beneath the unconformity suggests a lengthy phase of nondeposition and erosion at some stage during the Late Cretaceous or Paleogene (Fig. F18). Campanian horizons appear to depositionally onlap the unconformity surface at Site 1208. The mid-Cretaceous horizons appear to have draped the rough basement topography across the Central High and the Campanian horizons appear to have smoothed out this topography. The horizons on either side of the unconformity between the upper Campanian and the upper Paleocene are parallel. This suggests most likely that the unconformity resulted from an interval when the site was below the CCD, an interval that was not recovered in Core 198-1208A-36X. We cannot rule out the possibility that a minor amount of erosion complimented this dissolutional episode. The similarity in age of the Campanian horizons underlying the unconformity at Sites 1207 and 1208 (nannofossil Zone CC22; 75-76 Ma) suggests that the Late Cretaceous and early Paleogene interval of dissolution was regional in scope.
The interval of time surrounding the P/E boundary was characterized by rapid changes in climate and ocean circulation that led to profound changes in marine and terrestrial biotas (i.e., papers in Aubry et al., 1998). Superimposed on this interval of long-term transition was an abrupt global warming event at the P/E boundary known as the PETM. The deep-sea and high-latitude oceans warmed by 4° and 8°C, respectively, during the PETM. The warming, in turn, led to profound changes in precipitation and continental weathering patterns (Gibson et al., 1993; Robert and Kennett, 1994). The climatic changes also affected biota on a global scale triggering rapid turnover of benthic and planktonic organisms in the ocean (i.e., Thomas, 1990; Kelly et al., 1996).
The carbon isotopic composition of the ocean decreased by 3-4 coeval with the warming event, suggesting a massive perturbation to the global carbon cycle (Kennett and Stott, 1991; Bains et al., 1999). The large magnitude and rate (~-3-4 per 5 k.y.) of the carbon isotope excursion is consistent with a sudden injection of a large volume of isotopically depleted carbon into the ocean/atmosphere system. Dickens et al. (1995, 1997) suggested that the largest source of depleted carbon was the vast reservoir of methane clathrates stored in continental slope sediments. Much of this methane would have quickly converted to CO2, stripping O2 from deep waters and lowering alkalinity. The expected response of this massive input of CO2 into the ocean-atmosphere system is a sharp rise in the level of the CCD. The response of the CCD in the Pacific can be determined by comparison of carbonate preservation in Shatsky Rise sediments at various water depths.
At Site 1208, the upper Paleocene-lower Eocene transition rests unconformably above the Campanian in Section 198-1208A-36X-CC. Preliminary nannofossil biostratigraphy suggests that this section lies close to the P/E boundary and the PETM. Three events that lie close to the boundary—the last occurrence (LO) of the genus Fasciculithus and the first occurrences of Tribrachiatus bramlettei and Discoaster diastypus—occur within 6 cm of each other (see "Biostratigraphy" in the "Site 1208" chapter). These events lie just within or just above the PETM and are separated by ~330 k.y. (Aubry et al., 1996; Bralower et al., 1995).
Poorly preserved nannofossil assemblages and intervals barren of carbonate suggest that the CCD was close to the depth of the site during the P/E boundary transition interval. Combined with results from other Leg 198 drill sites, the recovered section may hold important clues about the Paleocene-Eocene transition in the North Pacific. High-resolution bulk carbon isotope analyses and carbonate variations will be used to identify the event. In addition, the clay-rich lithology will provide an excellent opportunity to derive detailed clay mineralogical records that could be used to monitor changes in atmospheric circulation.
Site 1209 is located in middle bathyal (2387 m) water depth close to the most elevated, central part of the Southern High of Shatsky Rise. The site is located on seismic line TN037-14A (Fig. F19). This profile is hard to correlate with other profiles on the southern and western flanks of the Southern High that are tied to drill holes. A tentative predrilling correlation with the Southern High seismic units of Sliter and Brown (1993) suggests a moderately thick Unit 1 (Neogene), an expanded Unit 2 (Paleogene), and a moderately thick Unit 3 (Upper Cretaceous). The site is close to the point where the stratigraphic sequence appears to be most complete; however, the section was expected to contain a number of minor disconformities as indicated by prominent horizontal reflectors. The total thickness of the sedimentary section at Site 1209 is estimated at ~1147 m. Basement underlying the site was formed during Magnetochron CM20 in the Tithonian (Nakanishi et al., 1989).
The major goals of Site 1209 drilling were to core a shallow, relatively expanded Paleogene and uppermost Cretaceous section. Holes 1209A and 1209B were cored largely with the APC (Table T1). Only two cores were taken with the XCB. Hole 1209C was drilled down to the lower Miocene, and then the Oligocene to Maastrichtian section was cored. The three holes terminated at different levels in the Maastrichtian because of difficulty penetrating chert horizons with the XCB center bit. The deepest recovered sediment was early Maastrichtian in age from 297.6 mbsf in Hole 1209B.
Coring recovered a relatively thin (111.2 m) lower Miocene to Holocene (0-16.4 Ma) section of nannofossil ooze, clayey nannofossil ooze, and nannofossil ooze with clay (lithologic Unit I) (Fig. F20). This unit has an unconformity (11.19 to 15.1 Ma) separating the uppermost middle Miocene and the uppermost lower Miocene. Lithologic Unit I rests unconformably on lithologic Unit II (111.2 and 235 mbsf), which consists of nannofossil ooze and nannofossil ooze with clay of early Oligocene to early Paleocene age (28.6-65 Ma). Preliminary nannofossil and planktonic foraminiferal biostratigraphy suggests that the succession is largely complete. However, minor unconformities may be present, especially in the upper to middle Eocene interval (between 38 and 42 Ma) that is characterized by low sedimentation rates. Underlying lithologic Unit III of Maastrichtian age (65 to 70 Ma) is composed of nannofossil ooze and chert. Three prominent chert layers were encountered in the ~50-m Maastrichtian section in Hole 1209C. However, no horizons were found in the same interval in Hole 1209B, suggesting that chert layers are laterally discontinuous. The ooze surrounding the chert at the bottom of each hole was often still fluid and almost completely unlithified.
The upper middle Miocene to Holocene section at Site 1209 is apparently continuous and similar to contemporaneous sequences at Sites 1207 and 1208. Unconformities from the lower to middle Miocene and the lower Miocene to the lower Oligocene are partially equivalent to those observed at Sites 1207 and 1208 (Fig. F21), suggesting that regional oceanographic processes controlling erosion and dissolution had a major effect on sedimentation. The Neogene section at Site 1209 was deposited at much lower sedimentation rates than at the other sites, at least partially as a result of the lower production of biosiliceous and carbonate materials. Moreover, sedimentation at Site 1209 was exclusively pelagic, whereas the other two sites appear to have received a large supply of fine sediment from bottom water currents and eolian transport. A progression of decimeter- to meter-scale orbital cycles is observed in the sedimentary record at Site 1209. Preliminary ages suggest that dominant frequencies are eccentricity (100 k.y.) subsequent to 0.6 Ma, obliquity (40 k.y.) from 0.6 to 2.5 Ma, and then a gradual transition from long obliquity (1.25 m.y.) to long eccentricity (400 k.y.) to eccentricity (100 k.y.) through the lower Neogene and Paleogene section.
The lower Maastrichtian to lower Oligocene section at Site 1209 is also apparently complete. The highlights of Site 1209 coring were clearly associated with the recovery of the sedimentary record of several critical events in this interval, most of them in multiple holes. These include the Eocene-Oligocene transition, the PETM, a prominent biological event in the early late Paleocene, the K/T boundary, and the MME. Sediments are almost completely unlithified, and the site appears to have remained above the CCD for most of this time interval; hence, foraminiferal preservation is good to excellent. The stable isotope and paleontological records from Site 1209 will provide important information on the nature of environmental changes during the critical events and their effect on marine biotas. Site 1209 will provide a firm shallow end-member in the Shatsky Rise depth transect.
The Eocene to Oligocene transition represents the true end of the "greenhouse" world of the Mesozoic and early Cenozoic. Although this transition occurred over a period of several million years, stable isotopic records reveal that much of the cooling occurred over a relatively brief (350 k.y.) interval in the earliest Oligocene known as Oi-1 (33.15-33.5 Ma) (e.g., Miller et al., 1991; Zachos et al., 1996). The deep oceans cooled by ~3°C during Oi-1, and large permanent ice sheets became established in Antarctica (Zachos et al., 1992a, 1992b). Current reconstructions of ocean temperature and chemistry for the Eocene-Oligocene transition, however, are based primarily on pelagic sediments collected in the Atlantic and Indian Oceans (Miller et al., 1987a; Zachos et al., 1996). Very few sections suitable for such work have been recovered from the Pacific (e.g., Miller and Thomas, 1985). As a consequence, we still lack a robust understanding of how global ocean chemistry or circulation evolved in response to high-latitude cooling and glaciation.
The E/O boundary was recovered in Cores 198-1209A-14H, 198-1209B-14H and 15H, and 198-1209C-4H. Preliminary nannofossil and planktonic foraminiferal biostratigraphy suggests that the boundary interval is complete. The most marked feature in the transition record at Site 1209 is a gradual change, over ~7.5 m in the lowermost Oligocene and uppermost Eocene, from light brown to tan nannofossil ooze with clay to a light gray to white nannofossil ooze. A similar lithologic transition was observed in an identical stratigraphic position at Site 1208. However, the Site 1208 record is far more condensed than the Site 1209 record.
The distinctive color change in the Site 1209 record reflects a pronounced deepening in the CCD. This oceanwide event is thought to be related to the intensification of ocean circulation and/or to increased continental weathering (Zachos et al., 1996). Cycles from within the transition show that the transition was nonlinear and likely affected by orbital climatic variations. This observation will help constrain the cause of deepening of the CCD. Planktonic and benthic foraminiferal preservation in this interval is moderate. Thus, stable isotope stratigraphies from Site 1209 have the potential to provide a firm understanding of the evolution of Pacific surface and deep waters through this important climatic transition.
At the other end of the climatic spectrum from the Eocene-Oligocene transition, the PETM was an abrupt and short-term (~210 k.y.) warming event at ~55.5 Ma that led to major transformation of marine plankton and benthos. This climatic event involved an increase of some 8°C of high-latitude surface water temperature and some 5°C of deep ocean water temperature. Warming is thought to have been driven by the input of a massive quantity of greenhouse gas, most likely methane, into the ocean-atmosphere system (e.g., Dickens et al., 1995). The response of the tropics and the Pacific Ocean to this climatic event is currently poorly known (Bralower et al., 1995).
The PETM at Site 1209 corresponds to a 12.5-cm-thick medium brown layer of clayey nannofossil ooze with a sharp basal contact and a gradational upper contact. This horizon was recovered in Sections 198-1209A-21H-7, 198-1209B-22H-1, and 198-1209C-11H-3. The detailed lithostratigraphy of the three PETM records recovered varies significantly on a millimeter scale as a result of deformation of the soft horizons during coring and splitting. Core 198-1209C-11H shows the most complete upper Paleocene to lower Eocene transition as indicated by MST data. The event corresponds to a sharp change from a white nannofossil ooze to a brown nannofossil ooze with clay (Fig. F22). In Hole 1209B, these two lithologies are separated by an extremely thin (1 mm) dark brown clay seam. In the other two holes, this seam has been disturbed during coring and splitting.
The micropaleontologcal record of the PETM interval is summarized in Figure F22. Preliminary biostratigraphy shows that the event lies toward the top of nannofossil Zone CP8 and planktonic foraminiferal Zone P5. In addition, a single specimen of Gavelinella beccariiformis, a benthic foraminiferal species that goes extinct at the onset of the PETM (e.g., Thomas, 1990) was found in Section 198-1209A-21H-CC several decimeters below the event (see "Biostratigraphy" in the "Site 1209" chapter). The abrupt decrease in the nannofossil Fasciculithus that occurs just above the PETM in other sections (Bralower et al., 1995, 1997b; Aubry et al., 1996; Monechi et al., 2000) lies at the top of the clayey nannofossil ooze layer at Site 1209 (Fig. F22). The PETM layer corresponds to a prominent magnetic susceptibility peak followed by a 6- to 7-m interval of relatively high susceptibility that shows cyclic variation. The top of the event is currently undefined. However, preliminary biostratigraphy—in particular the first occurrence (FO) of the nannofossil Discoaster diastypus that lies 1.73 m above the base of the PETM (between Samples 198-1209A-21H-5, 130 cm, and 21H-6, 30 cm), and the LO of the plantkonic foraminifer Morozovella velascoensis that lies 2.38 m above the base of the event (between 198-1209A-21H-5, 129-130 cm, and 49-50 cm)—indicates that the section is condensed compared to continental margin records from the Atlantic and Tethys (i.e., Kennett and Stott, 1991; Norris and Röhl, 1999; Röhl et al., 2000).
The PETM interval is associated with dramatic turnover in nannofossil assemblages. One of the dominant nannolith genera, Fasciculithus, is replaced by Zygrhablithus bijugatus, a nannolith that is often a highly abundant or dominant component of Eocene assemblages (Fig. F22). The genus Discoaster is often highly abundant within the event itself, likely as a result of warming or increased oligotrophy (Bralower, in press). Common to abundant calcispheres are found in sediments from the PETM interval at Site 1209. These fossils are likely produced by calcareous dinoflagellates during intervals of adverse surface water conditions.
Low-latitude planktonic foraminiferal assemblages in the PETM also experienced a significant transformation with the sudden appearance of an ephemeral group of ecophenotpyes or new species of the genera Acarinina and Morozovella (Kelly et al., 1996). The range of these transient taxa is limited to the interval of the carbon isotope excursion; hence, they are known as the "excursion" taxa. Although preliminary observations did not yield any of the end-member excursion taxa, several forms that are intermediate between the true excursion taxa and their ancestors were observed. No detailed observations were made on benthic foraminifers at Site 1209 due to sampling limitations.
The response of the CCD is a sensitive indicator of change in carbon cycling during the PETM, likely as a result of the input of large quantities of methane into the ocean-atmosphere system (e.g., Dickens et al., 1997). This response can be monitored by changes in carbonate content and preservation. Nannofossil preservation below the event at Site 1209 is moderate, indicating that the site was located in the broad range of the lysocline. Preservation declines markedly in the 1-mm clay seam at the base of the event as shown by pervasive etching of coccolith shields and nannolith rims. Preservation remains poor for several centimeters above this level, but nannofossils never completely disappear from the record. This interval of poor preservation is accompanied by a high abundance of bladey rhombs that may be reprecipitated calcite. The deterioration in nannofossil preservation is evidence for an abrupt rise in the level of the CCD and lysocline during the PETM. The effects of this rise are not nearly as dramatic at Site 1209 as at Site 1208, which was situated almost 1 km deeper (see "Site 1208").
A prominent, 23-cm-thick, dark brown nannofossil ooze with clay was found in Sections 198-1209A-23H-3, 198-1209B-23H-5, and 198-1209C-12H-CC. This layer shows a sharp magnetic susceptibility increase and a slight density decrease. Preliminary micropaleontological investigations suggest that this interval may represent a previously unrecognized event of considerable evolutionary significance. This interval lies within planktonic foraminiferal Zone P4 and coincides exactly with the evolutionary FO of the nannolith Heliolithus kleinpellii, an important component of late Paleocene assemblages and a marker for the base of Zone CP5. The interval immediately below the brown ooze layer contains significant numbers of the species Bomolithus elegans, which may be the ancestor of H. kleinpellii. The abundance of H. kleinpellii increases sharply in a transitional interval at the base of the brown ooze layer. Planktonic foraminifers in the ooze layer are characterized by a low-diversity, largely dissolved assemblage dominated by representatives of the genus Igorina (mainly I. pusilla and I. tadjikistanensis). This low-diversity assemblage suggests some kind of oceanic perturbation.
A contemporaneous and prominent magnetic susceptibility peak was found on the Blake Nose at Site 1051 (U. Röhl et al., unpubl. data); this peak is associated with a dark brown chalk and correlates almost exactly with the FO of H. kleinpellii. Isotopic and further paleontological investigations are required to understand the oceanographic conditions that gave rise to this mid-Paleocene event as well as the detailed evolutionary relationships of the key taxa.
At Site 1209 the base of lithologic Unit II coincides with the K/T boundary, marked by a significant lithologic change from uppermost Maastrichtian, white to very pale orange nannofossil ooze to basal Paleocene, darker grayish orange foraminiferal nannofossil ooze. This boundary is located in Sections 198-1209A-25H-6 and 198-1209C-15H-3 but occurs within an unrecovered interval in Hole 1209B. The most complete boundary sequence is found in Hole 1209C. Here the top of the Maastrichtian is slightly indurated, possibly indicating an incipient hardground, and overlain by lowermost Paleocene mottled, light orange, slightly indurated foraminiferal nannofossil ooze that grades into softer and paler tan-gray nannofossil ooze. The lowermost Paleocene layer is strongly bioturbated as shown by the pale orange roots within the irregular surface of the top of the white Maastrichtian ooze (Fig. F23). The basal Paleocene unit is ~14 cm thick and overlain by a 23-cm-thick pure white ooze. In Hole 1209A, the uppermost Maastrichtian white nannofossil ooze is separated from the lowermost Paleocene light orange foraminiferal nannofossil ooze by a watery, disturbed, orange-brown clay horizon. This horizon was formed by disturbance during coring and splitting and does not correspond to the "clay horizon" that defines the K/T boundary in classic sections such as El Kef, Tunisia; Caravaca, southern Spain; and Gubbio (Bottaccione), Italy.
The micropaleontological record of the K/T interval is summarized in Figure F23. Preliminary biostratigraphy shows the well-known abrupt change in nannofossil and planktonic foraminiferal assemblages. The white nannofossil ooze (Hole 1209A: sample F2) yields a diverse, but dissolved, highly fragmented fauna of the latest Maastrichtian Abathomphalus mayaroensis planktonic foraminiferal Zone; the washed residue also contains rare, minute, well-preserved heterohelicids, predominantly Guembelitria, that suggest a possible preservation of Zone P0 fauna in the deepest burrows. Nannofossil assemblages below this interval (Hole 1209A: sample N1) are diverse, well preserved, and include relatively common Micula prinsii, indicating correlation to the uppermost Maastrichtian nannofossil Zone CC26. The light orange foraminiferal nannofossil ooze burrows (Hole 1209C: sample F1) yield highly abundant, minute (<100 µm), and well-preserved planktonic foraminiferal assemblages that are dominated by heterohelicids and belong to the basal Paleocene Parvularugoglobigerina eugubina (P
) Zone. Nannofossils in the gray-orange ooze (Hole 1209C: sample N2) are limited to "disaster" taxa (calcispheres) and reworked Cretaceous taxa. In the overlying pale nannofossil ooze horizon, the average size of the foraminiferal assemblage increases associated with increasing abundance of trochospiral forms with respect to heterohelicids (Hole 1209A: sample F3; Hole 1209C: sample F2). The lower part of the white ooze unit (Hole 1209C: sample N3, Hole 1209A: Sample N4) is dominated by ultrafine micrite, calcispheres, and the survivor coccolith taxon Cyclagelosphaera reinhardtii. Finally, the upper part of the white ooze unit (Hole 1209C: sample N4) contains fine micrite, C. reinhardtii, another survivor, Markalius inversus, and small species of the Danian coccolith genus Neobiscutum. This whole interval thus belongs to nannofossil Subzone CP1a. In Hole 1209A, the orange-brown soupy clay, containing a late Danian planktonic fauna, coats indurated ooze above the burrows fragmented by drilling.
The lowermost 2-3 cm of basal Paleocene darker grayish orange foraminiferal nannofossil ooze contains common spherules, probably altered tektites. The thin (1-2 cm) boundary clay unit that corresponds to planktonic foraminiferal Zone P0 in a few deep-sea and shelf sites (e.g., El Kef, Tunisia; and Caravaca, southern Spain) is not found at this site. However, Zone P0 fauna seems to be present in the deepest burrows.
We postulate that this interval has been affected by bioturbation soon after the boundary (Fig. F23). Nevertheless, the substantial thickness of the uppermost Maastrichtian M. prinsii (CC26) Zone and the lowermost Danian P Zone at Site 1209 indicates that the K/T boundary is paleontologically complete. In most deep sea-sites, the P Zone is either unrecovered or poorly preserved. Thus, the section represents one of the best-preserved and least-disrupted deep-sea records of this major extinction event, as well as of the subsequent radiation.
The long-term cooling of the Late Cretaceous was interrupted by a dramatic event in the mid-Maastrichtian (~69-70 Ma; nannofossil Zone CC24), when oceanic deep waters appear to have switched abruptly from low- to high-latitude sources (e.g., MacLeod and Huber, 1996; Frank and Arthur, 1999). This event appears to have coincided with the extinction of the inoceramid bivalves. Growing evidence, however, suggests that this biotic event is distinctly diachronous in the Atlantic, Tethys, and Pacific Oceans (MacLeod et al., 1996). Moreover, the magnitude and direction of stable isotope changes are quite variable from site to site (Frank and Arthur, 1999), possibly as a result of uncertainties in stratigraphic correlation or of true differences in deepwater properties.
The MME appears to have been recovered in Section 198-1209C-21H-1, which lies in nannofossil Zone CC24. In this section (Fig. F24) and several below it, large inoceramid prisms can be seen with the naked eye. Although the LO of inoceramids has not yet been determined in Hole 1209C, the abrupt disappearance is a likely sign of the event. Shore-based stable isotopic and benthic assemblage studies will help us refine our understanding of the origin and implications of this deepwater change.
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. F25). 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 APC. XCB center bit drilling was used to punch through chert layers in the Upper Cretaceous section (Table T1). Hole 1210A terminated at 249.3 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. F26). 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"). 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 P/E and 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" in the "Site 1210" chapter). 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 E/O boundary, the PETM, the K/T boundary, and the MME.
As at Site 1209, a number of critical events were 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. F27). 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. 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".
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 (Fig. F28) that is indication for a deepening in the 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. F27). 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 saturation changes.
A prominent, 18-cm-thick magnetic susceptibility increase was found in the mid-Paleocene corresponding to a a dark brown nannofossil ooze with clay layer. Preliminary micropaleontological investigations suggest that this interval as already identified in cores from Site 1209 may represent a previously unrecognized event of considerable evolutionary significance. Planktonic foraminifera are characterized by a low diversity and largely dissolved assemblage dominated by representatives of the genus Igorina. The composition of the assemblage suggests some kind of oceanic perturbation.
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 (Fig. F24). The 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).
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" in the "Site 1210" chapter). 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" in the "Site 1210" chapter). Thus, the time integrated overburden for the deep section has been far less than 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.
PRINCIPAL RESULTS continued next file.
