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.
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 approximately 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 CenomanianConiacian age and R2 of BarremianAptian 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). 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.Unique Neogene Section
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 late Miocene and the late 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 (0131.3 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 (131.5162.5 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 until at least the early Pliocene but close to this level until the early 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 water 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 PliocenePleistocene 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 with 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 3.22.75 Ma period.Origin of the 57-m.y. Hiatus
One of the surprises of drilling at Site 1207 was a major unconformity between the middle Miocene and upper Campanian. This hiatus represents approximately 57 m.y. of stratigraphic record. The unconformity lies at the base of a 3.3-m interval of zeolitic nannofossil clay (lithologic Subunit IC [162.5163.8 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 (15 Ma), the age of immediately overlying sediments. The presence of an Mn crust and micronodules, chert nodules, and zeolite crystals suggests an extended interval of seafloor exposure. A mixture of nannofossils of Campanian and Miocene ages suggests winnowing of underlying and overlying 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 unconformities 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 deep-water circulation that increases the age of a water mass. 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 Maastrichtianmiddle Eocene. However, the sediment record on the Southern Rise 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 CampanianMaastrichtian. 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 Rise 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 MioceneHolocene section on Northern High is relatively expanded, whereas this sequence is thin or absent on the Southern High.Early Aptian Oceanic Anoxic Event
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 (335.3622.8 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%); 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 Sites 305 on Southern High, Shatsky Rise, 463 (MPM), and 866 (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% Corg are associated with volcanic ash. At Site 866 (1373-m water depth), thin organic carbon-rich (14.2%) 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, in press). 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 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 ~23 negative shift at the base of the event followed by a ~45 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 CenomanianTuronian 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 horizon is found at a relatively shallow burial depth (565.5 mbsf). Hence, the organic geochemical and stable isotopic records should be nearly pristine and highly informative. These and biotic records should provide key information on the environmental changes that occurred during the anoxic event.Chert
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 the path of the site 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 timescale); 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 (7980 Ma) to the base of the hole (lithologic Units II and III [163.8335.3 and 335.3622.8 mbsf, respectively]). Assuming the reconstruction of R. Larson (pers. comm., 2001), these data suggest that the equatorial divergence was significantly wider at this time than 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 show that the cherts are generally finely layered as opposed to nodular and that most layers are less than 10 cm thick. However, interbedded soft sediment layers are less than 1 m thick and most often under 30 cm. These interbeds are mainly composed of ooze or very 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.
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