13C shift (Weissert and Lini, 1991).
The record of 140 m.y. of sedimentation on Shatsky Rise holds clues about the nature of oceanic environments during long intervals of global warmth and the transition to colder climates of the more recent geological past. The focus of Leg 198 scientific objectives was to understand the long-term transitions into and out of the warm-climate "greenhouse," as well as transient but critical events that involved major changes in ocean circulation, geochemical cycling, and marine biota. The record of this shifting climate was cored during Leg 198 in a depth transect that provides a multidimensional picture of the ocean during periods of relative stability and intervening episodes of environmental perturbation. The signature of both abrupt and gradual change is graphically displayed in the lithology, biota, and geochemistry of the sediments and sedimentary rocks.
The deep-sea sedimentary record of Shatsky Rise contains evidence of brief intervals of open-ocean dysoxia during the early and middle parts of the Cretaceous period. These are followed by more stable times in the Late Cretaceous and Paleogene, when the rain of calcareous skeletal material from calcareous microplankton formed almost pure biogenic ooze on the seafloor. Three highly selective extinction events interrupted these stable times, one that affected a long-ranging group of bottom-dwelling clams at ~69 Ma in the mid-Maastrichtian; another, the well-known K/T boundary event at 65 Ma that almost eradicated surface-dwelling plankton; and a third at ~55.5 Ma (PETM) that profoundly affected benthic organisms and caused the abrupt reorganization of calcareous plankton communities. Throughout the Paleogene and the Neogene, evidence for fluctuations in climate, oceanic circulation and chemistry, and/or marine productivity forced by orbital variations are expressed in prominent lithologic cycles.
The warm mid- to Late Cretaceous and early Paleogene climate came to an end, beginning ~50 Ma, but wholesale cooling did not begin until an abrupt event at ~33.5 Ma when ice sheets developed on Antarctica and the deep oceans suddenly filled with cold water. The youngest Shatsky Rise sediments contain clear evidence of the onset and peak of Northern Hemisphere glaciation in the last 2.7 m.y. Here we present a preliminary summary of the highlights of Leg 198 beginning with the Cretaceous and moving forward in time.
A major highlight of Leg 198 is the recovery of Corg-rich sedimentary rocks of early Aptian age at Sites 1207 and 1213 (Fig. F38). These sedimentary rocks were deposited during OAE1a and provide the best truly pelagic record of this event outside of the Tethys. At Site 1207 the event is found within 45 cm of finely laminated, dark brown radiolarian claystone with up to 34.7 wt% Corg. The Site 1213 Corg-rich units include olive-black to greenish black, clayey porcellanites, and radiolarian porcellanites with up to 25.2 wt% Corg. At both sites, gamma ray and uranium logs show pronounced highs and indicate the true thickness of the critical levels and the incomplete recovery. These data suggest that at Site 1207, ~50% of the organic-rich unit was recovered; in the more siliceous section at Site 1213, this percentage was much lower, certainly <30% (Fig. F45). At Site 1214, laminated black claystone in the lower Aptian has <1.4 wt% Corg. The distinctive radiolarian assemblage (e.g., Erbacher and Thurow, 1997; Premoli Silva et al., 1999) in these sediments suggests that they lie within the OAE1a interval.
Shipboard biostratigraphy indicates that the Corg-rich units lie in lower Aptian nannofossil Zone NC6 and planktonic foraminiferal Globigerinelloides blowi (KS6) Zone, thus correlating to Corg-rich units in other localities that lie within OAE1a (e.g., Bralower et al., 1994; Erba et al., 1999; Premoli Silva et al., 1999). Rock-Eval analyses and gas chromatography-mass spectrometry (GC-MS) of extractable hydrocarbons and ketones have been used to characterize organic matter from the Corg-rich unit at Sites 1207 and 1213 (Fig. F51). These data indicate that the organic matter is almost exclusively algal and bacterial in origin. GC-MS data, in particular, have been used to identify biomarkers that are associated with cyanobacteria in material from both locations. The prevalence and character of bacterial biomarkers suggest the existence of microbial mats at the time of deposition. Compounds produced by hapthopyte algae include the oldest known alkenones. Organic matter of algal origin was also reported in the lower Aptian Corg-rich units of Sites 463 and 866 (Dean et al., 1981; Baudin et al., 1995), suggesting that production by these organisms was widespread during OAE1a.
At Sites 1207 and 1213, the interval within and directly above and below the Corg-rich units lacks carbonate. Both organic-rich intervals are associated with minor amounts of tuff. The records of the two sites are different in a number of ways, however. At Site 1207 the presence of lamination in the Corg-rich units is clear evidence for dysoxic or anoxic deepwater conditions. Lamination is not present in the recovered, highly bioturbated Corg-rich units at Site 1213. This could indicate a slightly higher level of oxygenation of deep waters during deposition of the carbonaceous sediments, although conditions still must have been poorly oxygenated due to the high flux of organic matter. In fact, the exceptional preservation of organic compounds in the Site 1207 and 1213 lower Aptian samples indicates that conditions were highly dysoxic at the time of deposition. The units at Site 1213 are more siliceous and radiolarian rich; however, this might be a result of the lack of recovery of the softer, less siliceous interbeds.
Using basement ages from Nakanishi et al. (1989), a paleodepositional depth track (based on normal crustal subsidence) from Thierstein (1979) for Site 306, and correcting for differences in the thickness of sediment between basement and the lower Aptian yields paleodepths of 1.3 km for Site 1207 on the Northern High and 2.8 km for Site 1213 on the Southern High. The true depths might have been slightly deeper given faster than normal subsidence rates, but the relative difference should be similar. These depths indicate remarkable shoaling of the CCD during OAE1a. Calcareous sediments are found directly underneath the Corg-rich sediments at Site 1213, indicating that the CCD shoaled by at least 1.5 km during the event. The magnitude of the change of the CCD during OAE1a is a result of a combination of oxidation of a steady flux of organic matter over a fairly long time period (~1 m.y.) (Larson and Erba, 1999) and CO2 outgassing (e.g., Arthur et al., 1985).
Corg-rich horizons of OAE1a age have been found in a number of other locations in the Pacific Ocean. These include Sites 305 on Shatsky Rise (location of Site 1211), 463 (Mid-Pacific Mountains), and 866 (Resolution Guyot) (Sliter, 1989; Jenkyns, 1995). Of these, only Sites 463 and 866 have decent recovery, and both of these sites have a shallow-water influence: one is located in shallow-water carbonates (Site 866), and the other has a considerable fraction of material derived from shallow-water environments (Site 463). The only known record of OAE1a in the Atlantic Ocean, Site 641, is on the continental margin of Spain. Thus, the new Shatsky Rise organic-rich units represent the most pelagic records outside of the Tethys (e.g., Coccioni et al., 1992).
The extremely high Corg contents of the Site 1207 and 1213 units reflect their pelagic depositional environment where dilution by clastic material was minimal. The only interval with a comparably high carbonaceous level derives from an algal mat horizon slightly above OAE1a at Site 866 (Baudin et al., 1995). Maximum Corg values at levels that are correlative with OAE1a are limited to a few Pacific sites: 14.2 wt% at Site 866 (Jenkyns, 1995), 7.6 wt% at Site 463, and 9.3 wt% at Site 305 (Sliter, 1989) (Fig. F51). Onshore sections are much less enriched. The Selli level in the Cismon core from Italy contains 5 wt% Corg (Erba et al., 1999); and the Goguel level in southern France contains <3 wt% Corg (Bréhéret, 1988). Corg-rich lower Aptian horizons in the Santa Rosa Canyon of Mexico also contain <3 wt% Corg (Bralower et al., 1999).
Early Aptian Corg-rich units that correlate to OAE1a have been found in a limited number of locations outside the Tethys and are not as widely distributed as levels that correspond to OAE2 (Schlanger et al., 1987). This has led to some uncertainty as to whether the OAE1a event was global in scale. Recovery of the early Aptian Corg-rich horizons at Sites 1207 and 1213 provides additional evidence that OAE1a was indeed a global event.
Several Corg-rich levels were recovered below the lower Aptian at Site 1213. One of these levels in the upper Valanginian correlates to an anoxic event within the Tethys (Lini et al., 1992). Organic carbon contents are considerably lower (2.5 wt%) than those of the lower Aptian horizons and the facies are clearly bioturbated. Thus, environments on Shatsky Rise during the deposition of this unit were oxygenated. However, this is the first record of the Valanginian event outside the Tethyan region, supporting the contention that this was a widespread, even global, event that resulted in a positive 13C shift (Weissert and Lini, 1991).
There is some indirect evidence for the presence of other Corg-rich mid-Cretaceous levels in the unrecovered sections on Shatsky Rise. At Site 1207, a minor gamma ray peak around the Cenomanian/Turonian boundary and a subsidiary U peak signify either a thin organic-rich unit or an unconformity with an associated Fe-Mn hardground. This evidence is equivocal; however, gamma ray logs from both Sites 1207 and 1213 show background levels with a few insignificant increases throughout the Albian, suggesting that Corg-rich units are absent. Although the lack of recovery prohibits firm conclusions concerning the continuity of the section, the expanded nature of the Albian, especially at Site 1207, where it corresponds to 140 m of section, suggests that the sequence for most of this stage should be relatively complete. As such, the gamma ray data indicate that Corg-rich levels representing the Albian OAEs (OAE1b-1d) are not found on Shatsky Rise. On a regional scale, the only significant Corg-rich sediments (~9 wt% Corg) of Albian age are found in the upper Albian section (Rotalipora appenninica and R. ticinensis Zones) at Site 465 on Hess Rise (Dean et al., 1981). Thus, the event that produced OAE1a appears to have had a more profound effect on the pelagic realm in the Pacific than did events of the Albian.
Although chert was a significant hindrance to coring during Leg 198, it still provided significant information that fulfilled several leg objectives. The poorly recovered Berriasian to Santonian sedimentary section on Shatsky Rise is dominated by chert and associated porcellanite with minor amounts of ooze, chalk, claystone, and limestone. In the overlying Maastrichtian-Campanian interval, fewer chert horizons are found in soft nannofossil ooze. The dominant source of silica for the chert and porcellanite is thought to be radiolarians; no evidence of diatoms was found in the Cretaceous section on Shatsky Rise.
Despite poor recovery, the occurrence and character of chert and porcellanite at all Leg 198 sites show marked trends through the Cretaceous that provide important information on the nature of depositional environments on Shatsky Rise. Stratigraphic information is obtained from FMS-sonic log data from Site 1207 and, more crudely, from drilling penetration rates at other sites. The FMS-sonic log from Hole 1207B clearly shows that chert layers have variable thickness and spacing through the stratigraphic column (Fig. F46). Changes in the spacing of chert layers also clearly affected penetration rates. These rates suggest that chert is concentrated in the upper Albian (Rotalipora appenninica to R. ticinensis Zones), near the Aptian/Albian boundary and around the lower Aptian OAE1a at Sites 1207, 1213, and 1214. Chert is also abundant in the lower Berriasian at Site 1213, and a minor peak is present in the lower Santonian at Sites 1207 and 1213. The Turonian-Coniacian interval at Site 1207 has a lower abundance of chert layers.
A more detailed record of chert occurrence was obtained from the Campanian-Maastrichtian section on the Southern High of Shatsky Rise. At Sites 1211 and 1212, the uppermost, scattered layer or nodules of chert are present in the middle part of the upper Maastrichtian (middle part of the Abathomphalus mayaroensis foraminiferal Zone [KS31]). At Sites 1209 and 1210, the uppermost chert horizon is slightly older, occurring in the mid-Maastrichtian (Racemiguembelina fructicosa/Contusotruncana contusa foraminiferal Zone [KS30]). At Site 1207 on the Northern High, the youngest chert was found in the mid-Campanian (nannofossil Zone CC20 corresponding to the middle part of the Globotruncana ventricosa foraminiferal Zone). The age of the shallowest chert level can be used to predict when Shatsky Rise left the equatorial divergence zone on its northward migration toward its current location. Thus, it appears that the Northern High was out the divergence zone ~9 m.y. earlier than the Southern High.
The color of the cherts and porcellanites is variable, ranging from very light yellow hues to black hues. A compilation of chert color for the Leg 198 sites, including data from Sites 305 and 306 (Larson, Moberly, et al., 1975), shows regional trends through time (Fig. F37). We suspect that the various colors may indicate specific redox conditions; for instance, orange, red, and brown hues are indicative of deposition and diagenesis in oxidizing environments, whereas olive-green to black hues are indicative of more reducing conditions during deposition and burial. The color stratigraphy suggests that oxygenated conditions prevailed in the earliest Berriasian, in the late Aptian through early middle Albian, and from the late Cenomanian through the Maastrichtian. Reducing conditions within the sediment prevailed in the Berriasian through the early Aptian and in the late Albian to middle Cenomanian. Overall, similar trends were observed in all of the sites for coeval portions of the sequence, suggesting that the entire rise experienced generally similar redox conditions at bathyal depths. One possibility is that redox was a function of sedimentation rate, with higher sedimentation rates causing higher accumulation rates of organic matter and more reducing conditions.
Comparison of the color redox patterns to the inferred frequency of chert in the Cretaceous section on Shatsky Rise suggests that the higher frequency is associated with predominantly reducing conditions, at least in the Berriasian, around OAE1a, and in the late Albian. This relationship indicates that higher siliceous production may have contributed to reducing conditions during deposition and diagenesis by increasing sedimentation rates.
An unusual record of the MME was observed in the sedimentary record at two sites on the Southern High of Shatsky Rise. At Sites 1209 and 1210, clusters of large Inoceramus prisms are seen in the cores for several meters (Fig. F24) but disappear abruptly. This disappearance is in the same stratigraphic position in both holes, in the Racemiguembelina fructicosa-Contusotruncana contusa Zone at ~69 Ma. Furthermore, Inoceramus prisms were observed at Sites 1208, 1211, and 1212, and shells are recorded in Hole 47.2 (Fischer, Heezen, et al., 1971) in the same foraminiferal zone as at Sites 1209 and 1210. The significance of the short range of visible specimens in this open-ocean setting is not currently understood. However, the position of the event is similar to that of the Inoceramus extinction and the isotopic shifts that mark the MME at Site 305 and other deep-sea locations (MacLeod, 1994; Frank and Arthur, 1999).
Growing evidence, however, suggests that this biotic event is diachronous in the Atlantic, Tethys, and Pacific Oceans. For example, the Inoceramid extinction lies slightly higher in the Bottaccione Section (Gubbio, central Italy) in the overlying Abathomphalus mayaroensis Zone (Chauris et al., 1998). Moreover, the magnitude and direction of stable isotope changes are quite variable from place to place, possibly as a result of uncertainties in stratigraphic correlation or of true differences in deepwater properties. Benthic and planktonic data from Shatsky Rise will help to accurately characterize the changes in deep- and surface water properties as well as the timing of this transition in the Pacific.
Close to the MME, the more specialized planktonic foraminifers start to decrease in diversity and more generalized groups increase, indicating a shift to less oligotrophic conditions, a trend that strongly accelerated near the end of the Maastrichtian (Premoli Silva and Sliter, 1999). This suggests change in the structure of oceanic surface waters. Shore-based stable isotopic and foraminiferal assemblage studies will help us refine our understanding of the origin and implications of this climatic transition.
Although the K/T boundary was not a prime focus of investigation during Leg 198, a remarkable set of cores was taken across this critical interval. The K/T boundary was cored at four sites on the Southern High: Sites 1209, 1210, 1211, and 1212. Double and triple coring at these sites recovered a total of nine separate K/T records: three from Site 1211, and two each from Sites 1209, 1210, and 1212 (Fig. F52). The lithologic sequence in the K/T boundary interval is similar at all of these sites. The boundary succession includes uppermost Maastrichtian (nannofossil Zone CC26) white to very pale orange, slightly indurated nannofossil ooze overlain by lowermost Paleocene (foraminiferal Zone P
) grayish orange foraminiferal ooze (Fig. F52). The basal 8- to 12-cm-thick Paleocene layer grades into a 19- to 23-cm-thick white foraminiferal nannofossil chalk, then into a grayish orange nannofossil ooze. The boundary between the uppermost Maastrichtian and the lowermost Paleocene is clearly bioturbated as shown by the irregular nature of the contact and the pale orange burrows that extend up to 10 cm into the white Maastrichtian ooze. The K/T boundary interval exhibits a strong magnetic susceptibility peak that allows detailed correlation among holes and sites.
The lithostratigraphy of the boundary succession is remarkably similar at all sites on Southern High, including Site 577 (near Site 1212) and to some extent Hole 47.2, although the latter section was badly disturbed by coring. The main difference between the sections is the degree of bioturbation, thickness of the bioturbated layer, and the thickness and color of the lowermost Paleocene foraminiferal ooze layer.
Preliminary biostratigraphy at all of the K/T boundary sites drilled during Leg 198 shows the well-established, abrupt change in nannofossil and planktonic foraminiferal assemblages across the boundary (e.g., Luterbacher and Premoli Silva, 1964; Percival and Fischer, 1977; Thierstein, 1982; Monechi, 1985; Gerstel et al., 1986; Pospichal, 1991). The white nannofossil ooze yields diverse assemblages of the uppermost Maastrichtian Abathomphalus mayaroensis planktonic foraminiferal Zone and Micula prinsii nannofossil Zone (CC26). Sampling of the deepest sections of the burrows of Paleocene ooze within the uppermost Maastrichtian yields highly abundant, minute planktonic foraminiferal assemblages that are dominated by Guembelitria with rare Hedbergella holmdelensis, suggesting a possible Zone P0 age. The upper parts of the burrows contain foraminiferal assemblages dominated by Guembelitria with rare, small Parvularugoglobigerina eugubina that identifies the basal Paleocene (P) Zone. Well-preserved planktonic foraminifers of Zone P dominate the lowermost 10 cm of the Paleocene (Fig. F23). In the overlying white foraminiferal nannofossil ooze horizon, the average size of the foraminiferal assemblage increases as the assemblages become increasingly dominated by P. eugubina.
Nannofossils in the basal Danian grayish orange ooze are limited to "disaster" taxa (calcispheres), survivor taxa, and reworked Cretaceous taxa. The lower part of the overlying white ooze unit is dominated by ultrafine micrite, calcispheres and the "survivor" coccolith taxa, including Cyclagelosphaera reinhardtii and Markalius inversus. Finally, the upper part of the white ooze unit contains fine micrite, small early species of the coccolith Neobiscutum, C. reinhardtii, and M. inversus (Fig. F23). This whole interval thus belongs to nannofossil Subzone CP1a. Comparable evolutionary trends in both nannoflora and planktonic foraminifers from Site 577 were described by Monechi (1985) and Gerstel et al. (1986).
A significant feature of the K/T boundaries recovered at Leg 198 sites is the widespread occurrence of light brown to amber spherules up to 100-150 µm in diameter that are concentrated in the first few (2-3) cm of the basal Paleocene and in the shallower burrows into the uppermost Maastrichtian white nannofossil ooze. Spherules are rarely found in the overlying 30 cm of the basal Danian. These spherules show textures similar to the spherules composed of glauconite and magnetite that have been described by Smit and Romein (1985) from the K/T boundary in other locations and referred to as "microtektite-like" spherules.
The uppermost Maastrichtian planktonic foraminifers at all sites are characterized by a fair amount of etching and fragmentation. The amount of dissolution appears to be unrelated to paleodepth. The minute, thin-walled earliest Paleocene faunas, however, are well preserved. This suggests that the lysocline and CCD over Shatsky Rise shoaled in the latest Maastrichtian, just prior to the K/T boundary, and deepened in the earliest Paleocene.
The K/T boundary sequence at Shatsky sites bears similarities to records at other deep-sea sites. The boundary at Site 1049 in the western North Atlantic corresponds to the base of a graded spherule bed (ejecta fallout), capped by an orange-brown limonitic layer. This layer is overlain by (1) a dark, burrow-mottled clay that contains planktonic foraminifers diagnostic of Zone P and (2) a 5- to 15-cm-thick white foraminiferal nannofossil ooze that also correlates to P. The same white unit is found in varying degrees of lithification directly above the boundary at less complete sections including DSDP Site 536 (Gulf of Mexico), and ODP Sites 999 and 1001 in the Caribbean. The ultra-fine micrite in this oceanwide white layer may be related to the collapse of the marine biosphere that would have caused a substantial drop in the CCD (e.g., Caldeira and Rampino, 1990).
The K/T boundaries on Shatsky Rise have been mixed by bioturbation in the interval after the boundary. Nevertheless, the substantial thickness of the uppermost Maastrichtian M. prinsii (CC26) Zone and the lowermost Danian P. eugubina (P) Zone indicates that the K/T boundary is paleontologically complete. The P Zone is either unrecovered or poorly preserved in most other deep sea sites. Thus the sections represent some of the best-preserved and least-disrupted deep-sea records of this major extinction event as well as the subsequent biotic radiation.
Sediments on Shatsky Rise show that deep-ocean circulation fluctuated in a highly regular fashion in the Paleogene. The record from Sites 1209, 1210, 1211, and 1212 is strongly cyclic as a result of regular changes in the properties of the deep waters bathing Shatsky Rise. Decimeter-scale alternations in color that correspond to minor changes in clay and carbonate content are relatively faint in cores but striking in color reflectance records. Magnetic susceptibility records also show these fluctuations clearly (Fig. F47). The cycles occur in bundles that can be easily correlated between sites. The cycles are likely produced by minor changes in the dissolution of carbonate in the deep ocean. Variation in the flux of eolian material may have also played a minor role.
Preliminary biostratigraphy suggests that the frequency of cycles corresponds to orbital periodicities. The predominant frequencies appear to be short- (100 k.y.) and long- (400 k.y.) period eccentricity. However, the nature of cycles varies among sites and through time in subtle ways. The amplitude of the cycles does not appear to correlate well with paleodepth. In fact, the two shallowest sites, Sites 1209 and 1210, show the highest amplitude cycles for significant periods of time. Magnetic susceptibility and color reflectance data indicate that the two shallowest sites with the most expanded Paleogene records also have the most cycles. This suggests that the two deeper sites (1211 and 1212) have a number of condensed intervals and/or diastems.
The lack of a clear correlation of cycle amplitude with depth suggests that the cycles are not the result of a simple dissolutional scenario in which deep waters were more corrosive. If this relationship held, the deepest site should have the largest amplitude fluctuations between the dissolved and less dissolved end-members. Instead, the cycles may represent repeated shoaling and deepening of the boundary between two water masses, with the boundary lying close to the depths of the shallower two sites (1209 and 1210) at some times and in deeper waters close to Site 1211 at others. This situation would produce higher-amplitude chemical and physical fluctuations (i.e., corrosiveness and oxygenation) at depths close to the water mass boundaries than in shallower and deeper water where conditions were less variable. Alternatively, the cycles might represent variation in surface water productivity that produced subtle changes in chemical composition. For instance, the shallower sites might have had more variable productivity for a number of reasons. Additional compositional and isotopic data are required to shed further light on the origin of Paleogene cycles.
At this preliminary stage, the cyclic Paleogene record has helped considerably in the construction of composite sections. These composites show that through multiple coring at the Southern High sites we have completely sampled the Paleogene sedimentary section (see "Downhole Measurements, Physical Properties, and Core Logging" in "Specialty Syntheses"). Furthermore, with the exception of a gap close to the Oligocene/Miocene boundary, the combination of Sites 1209, 1210, 1211, and 1212 of Leg 198 has recovered a composite section from the Pleistocene down to below the K/T boundary.
One of the new discoveries of Leg 198 is the recognition of an event of evolutionary and perhaps paleoclimatological significance in the early late Paleocene at ~58.4 Ma. We use mid-Paleocene to refer to this event to distinguish it from the subsequent PETM. A prominent clay-rich ooze found at Sites 1209, 1210, 1211, and 1212 corresponds to lower planktonic foraminiferal Zone P4 and coincides with the evolutionary first occurrence of the nannolith Heliolithus kleinpellii, an important component of late Paleocene assemblages and a marker for the base of Zone CP5. Planktonic foraminifers in the clay-rich layer are characterized by a low-diversity, largely dissolved assemblage, dominated by representatives of the genus Igorina (mainly I. tadjikistanensis and I. pusilla). Nannofossils are often strongly etched but do not show a change in diversity.
The clay-rich layer contains common crystals of phillipsite and fish teeth and corresponds to a prominent peak in magnetic susceptibility that probably reflects Fe-Mn coating of grains. The abundance of phillipsite and fish teeth indicates very slow sedimentation or intervals of seafloor exposure, possibly resulting from dissolution of carbonate. Moreover, the biostratigraphic record at Site 1211 is more complex than at the shallower sites with the first appearance of discoaster taxa including Discoasteroides bramlettei and Discoaster mohleri within and even below the event. These occurrences suggest a significant hiatus (~1 m.y.) in the interval surrounding the event.
Even though microfossil assemblages are clearly altered by dissolution, the biotic event appears to have been triggered by environmental perturbation in surface waters. Even though the igorinids appear to be a solution-resistant taxa, their ecological affinities are not well known; thus, it is not currently possible to determine the nature of the environmental changes in surface waters. The mid-Paleocene event does not appear to correspond to a major change in benthic assemblages based on range data for the late Paleocene interval (e.g., Tjalsma and Lohmann, 1983); however, more detailed investigations around the event are clearly required to determine the effect on the benthos.
Sediments cored on Shatsky Rise show evidence of a strong deep-ocean response to warming around the P/E boundary (PETM). The PETM interval was cored at four sites on the Southern High (Sites 1209, 1210, 1211, and 1212) (Table T3). Double and triple coring at these sites recovered a total of 10 separate PETM records: three each from Sites 1209 and 1211, and two each from Sites 1210 and 1212 (Fig. F53). The P/E boundary interval was also recovered at Site 1208 on the Central High. The range of present depths, from 2387 m at Site 1209 to 2907 m at Site 1211, provides a 520-m depth transect to observe the sedimentary response to this abrupt warming event as a function of depth. Although the Site 1208 sequence is highly condensed and it is not currently possible to determine whether the PETM is present, some inferences can be made from this section that extend the transect some 440 m to 960 m.
At the Southern High sites, the PETM corresponds to an 8- to 23-cm-thick layer of clayey nannofossil ooze with a sharp base and a gradational upper contact. The clay-rich layer is generally yellowish brown in color and is often bioturbated into the underlying sediment. At several sites an extremely thin (1 mm) dark brown clay seam lies at the base of the PETM. Carbonate contents have been measured in detail across the PETM at Site 1210. These data show a decrease from 96 to 89 wt% CaCO3 at the base of the event, a decrease that would involve a substantial increase in dissolution. Color reflectance and magnetic susceptibility data (Figs. F39, F47) allow detailed correlation between holes and sites in this time interval.
Preliminary biostratigraphic investigations show that the event lies toward the top of nannofossil Zone CP8 and planktonic foraminiferal Zone P5. At several sites, rare specimens of Gavelinella beccariiformis, a benthic foraminiferal species that goes extinct at the onset of the PETM (i.e., Thomas, 1990) were found below the event. The abrupt decrease in the nannofossil Fasciculithus that occurs just above the PETM in other sections (Bralower et al., 1995; Aubry et al., 1996; Monechi et al., 2000) lies near the top of the clay-rich layer. The stratigraphic level of the top of the PETM is currently undefined, but the FO of the nannofossil Discoaster diastypus and the LO of the plantkonic foraminifer Morozovella velascoensis indicate that the record in all sections is apparently complete, with the exception of Hole 1211B. This biostratigraphy shows that the PETM interval at Sites 1209, 1210, and 1212 are condensed compared to continental margin records from the Atlantic and Tethys (e.g., Kennett and Stott, 1991; Norris and Röhl, 1999; Röhl et al., 2000) but somewhat expanded compared to other deep-sea sites such as Site 865 on Allison Guyot (Bralower et al., 1995) and Site 527 on Walvis Ridge (Thomas et al., 1999). At Site 1211, the PETM interval was recovered across the break between two sections in Holes 1211A and 1211C. The clay-rich bed is more prominent and condensed than in the other records (Fig. F53). In Hole 1211B, the basal clay seam appears to be present, but the occurrence of D. diastypus 1-2 cm above this level and the concurrent decline in the abundance of Fasciculithus strongly suggests an unconformity. The PETM in both holes at Site 1212 appears complete; however, the presence of Globanomalina pseudomenardii suggests a slight unconformity immediately below the event in Hole 1212B.
The PETM interval at all of the sites contains a clear record of nannofossil and planktonic foraminiferal assemblage transformation at this time of major environmental upheaval (Fig. F22). One of the dominant nannolith genera, Fasciculithus, is replaced by Zygrhablithus bijugatus, a holococcolith that is often a highly abundant or dominant component of Eocene assemblages. The genus Discoaster is highly abundant, likely as a result of warming or increased oligotrophy (Bralower, in press). Calcispheres, possible resting cysts produced by calcareous dinoflagellates at times of environmental changes, are also found. Planktonic foraminiferal assemblages within the clay-rich interval contain an ephemeral group of ecophenotypes or short-lived species of the genera Acarinina and Morozovella (Kelly et al., 1996). These "excursion" taxa include both end-member as well as transitional morphologies.
The depth transect strategy of Leg 198 was specifically designed to address the response of the ocean to the greenhouse forcing mechanism proposed for the PETM. This warming is generally thought to have resulted from input of a massive burst of methane into the ocean-atmosphere system (e.g., Dickens et al., 1997). Methane is the only agent that can explain both the warming and the rate of carbon isotopic change at the onset of the excursion. The oceanic response to this methane input is predictable but currently untested (e.g., Dickens, 2000). Regardless of how the transfer to the ocean took place, oxidation of methane would generate CO2, which would lower the dissolved CO32- content of seawater and cause a dramatic shoaling in the depth of the lysocline and CCD. This response should be recorded in changes in carbonate content and preservation in all marine sections. Over a depth range, shallow sections should show less change in dissolution and carbonate content than deep sections.
Nannofossil preservation below the event in all of the Southern High sites is moderate indicating that the sites were located in the broad range of the lysocline. All sites show a general deterioration in preservation at the onset of the event and abundant 10- to 20-µm-sized, needle-shaped calcite crystals that are thought to have been derived from precipitation of dissolved carbonate are found in smear slides. The detailed response of fossil preservation is complex and different from site to site, as is the absolute change in carbonate content as indicated by reflectance data.
The general changes in lithology suggest a transition from paleodepths in the shallower sites that were less sensitive to changes in carbonate solubility in the deep ocean (Sites 1209, 1210, and 1212) to those that were at depth ranges highly sensitive to changes across the PETM (Sites 1208 and 1211) (Fig. F53). The decrease in carbonate content and deterioration in nannofossil preservation are evidence for an abrupt rise in the level of the CCD and lysocline during the PETM. Determination of the magnitude of this change awaits postcruise analysis; however, the preliminary lithostratigraphic and biostratigraphic results from the PETM interval in the Shatsky Rise depth transect are highly consistent and thus support the expected ocean response to massive methane input.
Sediment recording the response of the tropical Pacific ocean to cooling in the Eocene-Oligocene transition was recovered across a large depth range on Shatsky Rise (Sites 1208, 1209, 1210, and 1211 in a total of nine holes). Preliminary nannofossil and planktonic foraminiferal biostratigraphy suggests that the boundary interval is complete. At the Southern High sites (1209, 1210, and 1211), the transition records a gradual, subtle but distinctive change over a 4- to 7.5-m interval in the uppermost Eocene and lowermost Oligocene, from light brown to tan nannofossil ooze with clay to a light gray to white nannofossil ooze (Fig. F32). This transition is associated with marked changes in color reflectance data (Fig. F28) but fairly minor changes in percent carbonate from ~90 to 96 wt% (in Hole 1210A). Superimposed on the gradual change in color and increase in carbonate content are marked cycles that appear to represent orbital rhythms (see "Downhole Measurements, Physical Properties, and Core Logging" in "Specialty Syntheses").
The lithologic record of the Eocene-Oligocene transition at Site 1208 on the Central Rise is markedly different from the Southern High sites. A lithologic transition from a dark brown zeolitic claystone with extremely low carbonate content to a gray-orange nannofossil ooze was observed in an identical stratigraphic position to the gradual changes observed on the Southern High. However, the Site 1208 transition is far more condensed than the other records, occurring over an interval of 1-2 cm.
The E/O boundary is associated with a marked increase in sedimentation rates at all of the sites. In general, the lithologic change at all of the sites is accompanied by a general improvement in microfossil preservation; late Eocene nannofossils in most of the sections show a high degree of etching, whereas early Oligocene assemblages show less dissolution and slightly more overgrowth. Planktonic foraminifers are rare and badly fragmented in the upper Eocene, but preservation improves and abundance increases in the Oligocene. At Site 1208, the deepest site in the transect that contains the transition interval, planktonic foraminifers are largely absent from the sequence. The benthic foraminifer Nuttallides truempyi, the LO of which shortly precedes the E/O boundary, is found below the color transition. Sparse nannofossils below the boundary are extremely etched.
The age model for the Eocene-Oligocene transition at all of the Leg 198 sites is preliminary, and thus the exact correlation of the change in carbonate content with the series boundary and the sharp Oi-1 cooling event (33.15-33.5 Ma) (e.g., Miller et al., 1991; Zachos et al., 1996) has yet to be determined. However, the preliminary data show that the prominent change in lithology occurs just before or within the time of cooling.
The distinctive color change in all of the Leg 198 records reflects a pronounced deepening in the CCD in the Eocene-Oligocene transition. In the latest Eocene, the CCD on Shatsky Rise was between the paleodepths of Sites 1208 and 1211, probably closer to the former site based on the sporadic occurrence of nannofossils. After the event, the depth was substantially greater than Site 1208. This significant change is observed in other ocean basins (e.g., Zachos et al., 1996) and possibly reflects an increase in mechanical and chemical weathering rates on continents associated with cooling. Alternatively, deepening of the CCD may be associated with an intensification of deep-ocean circulation and a consequent decrease in the age of deep waters. The magnitude of the change in the transition interval as shown by color reflectance data varies as a function of depth (see Fig. F28). This is consistent with the deepening of both the lysocline and CCD.
Our current understanding of the change in climate and circulation in the Eocene-Oligocene transition is based almost entirely on records from the mid- and high-latitude Atlantic and Indian Oceans (Miller et al., 1991; Zachos et al., 1996). Changes observed across this transition in the tropical-subtropical Pacific from the Shatsky Rise depth transect have the potential to add another dimension to this understanding.
A surprising side note to the major Cretaceous and Paleogene objectives on Leg 198 is the recovery of two expanded late Neogene sections on the Northern High of Shatsky Rise at Site 1207 and the Central High at Site 1208. These sections could be important from both stratigraphic and paleoceanographic perspectives. The two sections have apparently complete upper middle Miocene (~12 Ma) to Holocene sections that are composed of nannofossil ooze and nannofossil clay. Both sections have a mixture of nannofossils and significant amounts of diatoms (10%-40% at Site 1207; 5%-20% at Site 1208), and minor amounts of foraminifers, radiolarians, and silicoflagellates. Numerous discrete ash horizons are found at both sites, predominantly in the Pliocene-Pleistocene interval.
Sedimentation rates at Site 1207 average 18.4 m/m.y. from the Holocene to latest Miocene. Rates at Site 1208 range between 22 and 42 m/m.y. from the Holocene to the early late Miocene (Fig. F42). These rates are far higher than typical pelagic sedimentation. The detrital clay and silt component of the sediment may have been delivered by eolian transport. However, a large component of the sediment could be a drift deposit.
Marked cyclic variations are observed in MST and color reflectance data throughout the upper Miocene to Holocene section at both sites. These variations are expressed as strong lithologic cycles that have frequencies at the decimeter to meter scale. These cycles are marked by relatively subtle to sharp color changes that are associated with variations in the amount of clay, pyrite, and different biogenic particles. The darker interbeds tend to have more abundant diatoms and clay, more dissolved nannofossil assemblages, and more abundant pyrite. The lighter interbeds contain fewer diatoms, less clay, and a better-preserved nannofossil assemblage. The cycles might represent variations in carbonate dissolution as a result of changes in the depth of the lysocline, fluctuations in biosiliceous production, or a combination of the two. Preliminary biostratigraphy indicates that these variations represent eccentricity and obliquity cycles. The section recovered at Site 1208 contains an extraordinary expanded magnetic stratigraphy extending back to Chron C5An in the late middle Miocene (Fig. F44). This section also shows great promise for establishing a paleointensity record for the North Pacific.
The Site 1207 and particularly the Site 1208 Neogene section has significant potential for establishing a high-resolution biochronology and astrochronology back to at least the late Miocene. This potential derives from the combination of siliceous and calcareous microfossil biostratigraphy, high-resolution magnetostratigraphy, a marked orbital cyclicity, numerous ash layers with potential for radiometric dating, and a potential magnetic paleointensity record. With high sedimentation rates and foraminifers found throughout the section, the sites also have significant potential for reconstructing climate and paleoceanography of the northwestern Pacific over the last 12 m.y.
During Leg 198, we cored the first igneous rocks from Shatsky Rise, probing the top of the igneous pile. Coring in Hole 1213B terminated in mafic igneous rocks on the flanks of southern Shatsky Rise, with 46.4 m of igneous section drilled, and 33.4 m of recovered core. Six cores, 198-1213B-28R through 33R contain mostly massive diabase and minor basalt from three subunits (IVA to IVC), with each subunit thought to be a separate sill. The igneous rocks are predominantly hypocrystalline, fine-grained diabase (97.6%) with a small amount of sparsely phyric, aphanitic basalt (2.0%) at contacts. The diabase groundmass consists mainly of euhedral to subhedral plagioclase and intervening subhedral pyroxene and olivine, with minor glass. Alteration in the igneous section ranges from minor to moderate. Plagioclase and pyroxene crystals are locally altered to clay, and in thin section, glassy groundmass has been ubiquitously devitrified and/or altered to clay minerals. Basalt occurs at subunit "chilled" contacts, symmetrically disposed around fragments of metasediment that mark the subunit boundaries. From the chilled contacts, the basalt grades toward more coarse-grained diabase in the unit centers, where the groundmass approaches medium- grained.
The sills must be early Berriasian age or younger, since this is the age of the host sediment. Paleomagnetic data show two subunits that have fairly steep positive magnetic inclinations, whereas the third, basal subunit has a lower, negative inclination, implying both normal and reversed magnetic polarities are recorded in the igneous section. This mixture indicates that the sills must have formed either before or after the Cretaceous Long Normal Superchron (i.e., the Cretaceous Quiet Period; 121-83 Ma). On the seismic profile along which Hole 1213B was drilled, the seismic "basement" has an odd character that may be related to the presence of intrusive, rather than extrusive, igneous rock at the sediment/igneous rock contact. The "basement" reflector, that being the deepest continuous seismic horizon, is weaker than elsewhere on Shatsky Rise, and other, stronger reflectors occur beneath it. These deeper reflectors were not considered "basement" because they are not continuous all along the line, as is the weaker, shallower horizon. The cored section suggests that the weak "basement" horizon denotes the top of the sills, whereas the deeper reflectors may be the top of the extrusive lava pile.
The stratigraphy of sites drilled on the Southern, Central, and Northern Highs during Leg 198 can be integrated to interpret the geologic history of Shatsky Rise from its formation as a LIP in the Late Jurassic and Early Cretaceous, through multiple depositional episodes and different sedimentological regimes separated by short and sometimes quite lengthy hiatuses (Fig. F42). These gaps were produced by erosion and dissolution, but distinguishing between these two processes can be difficult. Comparison of the stratigraphy of the cored sections of sites on the Northern and Central Highs shows similarities and differences with the sequence cored on the Southern High. This suggests that a combination of local and regional-scale processes controlled sedimentation. The following history is compiled from the stratigraphic data gathered during Leg 198. Although we have cored a complete record from the Southern High, drilling was terminated in the mid-Cretaceous on the other highs.
Although Leg 198 did not core "true" extrusive basement, igneous rocks intruded soon after the formation of extrusive basement were recovered at Site 1213. Similar sills are widespread above basement in the Pacific (e.g., Larson, Schlanger, et al., 1981; Plank, Ludden, Escutia et al., 2000) and argue for a multistage origin for the igneous foundations of Shatsky Rise. The Cretaceous history of the rise was dominated by fairly continuous deposition on the Southern and Northern Highs (Fig. F42). Unconformities found in more than one location suggest an interval of erosion on the Southern High during much of the latest Hauterivian and Barremian. The upper Cenomanian to Santonian interval was also characterized by sporadic sedimentation, as noted by Sliter (1992) (Fig. F54), and widespread erosion on the Central and Southern Highs, at least in most locations (Sites 1208, 1212, and 1214). The deepest site in the depth transect (Site 1213) may have also rested below the CCD (e.g., Thierstein, 1979). The sequence on the Northern High appears to be complete in this interval. Continuous deposition resumed at the shallower sites in the Campanian and Maastrichtian whereas dissolution likely continued at deeper locations; an unconformity above the upper Campanian at Sites 1207 and 1208 was likely caused by a regional erosional episode at that time or in the early Cenozoic.
Deposition in the lower Paleogene was nearly continuous at the shallow sites of the Southern High (Sites 1209-1212), but even at these sites, a lengthy hiatus in the upper part of the Paleogene continued into the lower Miocene in most locations (Table T3). The onset of this hiatus lies in the middle Eocene at Site 1212 and in the Oligocene at the other shallower sites. At the shallower sites, this hiatus was likely a result of removal of sediment by erosion during the late Oligocene and early Miocene. Very slow sedimentation characterized the late early and early middle Miocene (except at Sites 1213 and 1214, where no sediment from this interval was recovered), and the rate of sedimentation increased through the rest of the Neogene at most locations. Sites on the Northern and Central Highs lay in a different oceanographic regime in the late Neogene with a significant biosiliceous sediment component as well as clastic material concentrated into sediment drifts. Continuous sedimentation on the deep rise (Sites 1213 and 1214) began again in the early Pliocene.
Like other open-ocean plateaus and rises, Shatsky Rise shows many aspects of stratigraphy and sedimentation that are different from other open-ocean, abyssal settings. Open-ocean plateaus and rises are prone to erosion, and sedimentation rates are not as high as areas along the continental margins. Thus, their sedimentary sections are not as thick, and older deposits often rest at shallow burial depths. This has important implications for fossil preservation and the fidelity of paleoceanographic proxies. Because open-ocean deposits tend to be mostly biogenic and are less complex sedimentologically, their geologic record is often easier to interpret than in areas along continental margins. Vertical changes in physical and chemical oceanography can be determined using depth transects down the flanks of the rises. The cycles recovered on Shatsky Rise are a good example. Leg 198 has proven that oceanic plateaus hold some of the best records for investigating climate change through extended intervals of geologic time, particularly based on a depth transect approach.
