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Site 1208

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 Sager line 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 Rise, 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 magnetochron 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 a 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). We drilled a total of 392.3 m through a major angular unconformity where Campanian ooze and chalk rests 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.

Summary of Results

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. This unit contains 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) 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 approximately 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 Paleocene/Eocene boundary in Section 198-1208A-36X-CC. This section may contain a highly condensed interval of the LPTM 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 Oligicene time (Oi-1) cooling step (Zachos et al., 1993, 1996).

Expanded Neogene Section for Paleoceanography and Chronology

Sedimentation rates at Site 1208 averaged 42.4 m/m.y. from the Holocene to late early Pliocene (0 to 3.82 Ma), 22.3 m/m.y. from the late early Pliocene to the early late Miocene (3.82 Ma to 8.28 Ma), then decreased to 5.9 m/m.y. in the early late Miocene to late early Miocene (8.28 to 18.2 Ma). The rates in the late 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, a large component of the sediment must have been delivered by sediment drift. 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 peak in the Pliocene and decrease 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 also has significant potential for high-resolution paleoceanographic investigations.

Orbital Rhythms: A Strong Climate Signal

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 0.8 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 sediments recovered at Site 1207, but higher percentages than contemporaneous units from sites on the Southern High of Shatsky Rise, where diatoms are usually less than 5%. Site 1208 is ~1° south of Site 1207 and ~4° north of the Southern High sites. As at Site 1207 (see "Site 1207" in "Principal Results"), 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.

Origin of Regional Unconformities

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. These unconformities separate the upper Oligocene and lower Oligocene, the upper Eocene and lower Eocene, and the Paleocene/Eocene 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 of Shatsky Rise. 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 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 50 to 70 Ma (early Maastrichtian to early Eocene), a rapid rise from 30 to 50 Ma (early Eocene to early Oligocene), and a long-term deepening from 14 to 30 Ma (early Oligocene to middle Miocene) (Rea et al., 1995). The CCD remained above the current depth of Site 1208 from 15 to 39 Ma (middle Miocene to middle Eocene). One of the clearest lithologic changes lies close to the Eocene/Oligocene 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. Thus, it is not possible to determine whether this section contains one or more unconformities or if it is merely highly condensed. 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 deep-water 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 (9.8 to 14 Ma), the lower Miocene to lower Oligocene and the lower Eocene (Barron et al., 1995). Keller and Barron (1987) show four widespread hiatuses in the latest Oligocene to earliest 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 Rise 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.

Deep-Water Record of the Paleocene–Eocene Transition

The interval of time surrounding the Paleocene/Eocene 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 in the late Paleocene, known as the LPTM. The deep sea and high latitude oceans warmed by 4° and 8°C, respectively, during the LPTM. The warming, in turn, led to profound changes in precipitation and continental weathering patterns (Robert and Kennett, 1994; Gibson et al., 1993). 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‰ /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 Paleocene/Eocene boundary and the LPTM. Assemblages appear to contain a small component derived from downhole contamination during drilling or core handling. The contaminated component can be separated from the in situ component by observing the continuity of ranges; contaminated taxa tend to be highly abundant upsection and occur discontinuously. 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. These events lie just within or just above the LPTM 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 Paleocene/Eocene 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.

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