The record of 145 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 occurred periodically, events that involved major changes in ocean circulation, geochemical cycling, and marine biotas. 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.
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 Paleocene, 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 Ma that profoundly affected benthic organisms and caused the abrupt reorganization of calcareous plankton communities. Each of these events appears to have been associated with a unique environmental perturbation. 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 about 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 sediments were deposited during OAE1a and provide the best truly pelagic record of this event outside of 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 and associated minor tuff (Site 1213). At both sites, gamma ray and uranium logs show pronounced highs, and at the same time indicate the true thickness of the critical levels and the incomplete recovery. These data suggest that at Site 1207, approximately 50% of the organic-rich unit was recovered; in the more siliceous section at Site 1213, this percentage was much lower, certainly less than 30% (Fig. F44). At Site 1214, the highest measured Corg content in the lower Aptian is 1.4 wt% in a black claystone, but the presence of lamination in sediments, discrete tuff layers, and a distinctive radiolarian assemblage (e.g., Erbacher and Thurow, 1997; Premoli Silva et al., 1999) suggest that the recovered sediments lie within the OAE1a interval.
Shipboard biostratigraphy indicates that the organic-rich units lie in lowermost 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 (Figs. F50, 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 sections (Fig. F51). The prevalence and character of bacterial biomarkers suggest the existence of microbial mats at the time of deposition. Compounds produced by haptopyte algae include the oldest known alkenones (Fig. F51). Organic matter of algal origin was also reported in the lower Aptian organic-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 deep-water 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 dysaerobic 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 organic-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 (MPM), 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 Tethys. For example, the original Selli Level in Italy is in a truly pelagic section (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 the OAE1a interval at Site 866 (Baudin et al., 1995). Maximum Corg values at levels that are correlative with the OAE 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. F50). 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 less than 3 wt% Corg (Bréhéret, 1988). Corg-rich lower Aptian horizons in the Santa Rosa Canyon of Mexico also contain less than 3 wt% (Bralower et al., 1999).
Early Aptian Corg-rich units that correlate to OAE1a have been found in a limited number of locations outside Tethys and are not as widely distributed as 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 organic-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). Environments on Shatsky Rise during the deposition of this unit were not as dysoxic as during the early Aptian; organic carbon contents are considerably lower (2.5 wt%) than those of the lower Aptian horizons and the facies are clearly bioturbated. 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 at 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 (OAE1b1d) are not found on Shatsky Rise. On a regional scale, the only significant Corg-rich sediments (<9 wt% Corg) of Albian age are found within the upper Albian section (Rotalipora appenninica and R. ticinenesis 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.
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