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Learning from Chert on Shatsky Rise

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 for 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 can be 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. F45). Changes in the spacing of chert layers also clearly affected penetration rates. These rates (Fig. F52) suggest that chert is concentrated in the upper Albian (R. appenninica to R. ticinensis Zones), near the Aptian/Albian boundary and around the lower 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 A. mayaorensis foraminiferal Zone [KS31]). At Sites 1209 and 1210, the uppermost chert horizon is slightly older, occurring in the mid-Maastrichtian (C. contusa/R.fructicosa 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 G. ventricosa foraminiferal Zone). The age of the shallowest chert level can be used to predict when Shatsky Rise cleared the equatorial divergence zone on its northward migration toward its current location. Thus, it appears that the Northern High was out the divergence zone about 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). 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 those with 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.

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 the 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.

Mid-Maastrichtian Event: Death on the Seafloor

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, at Site 1211, Inoceramus prisms have been collected from core catcher samples and shells are recorded in Hole 47.2 (Fischer, Heezen, et al., 1971) within 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 distinctly diachronous in the Atlantic, Tethys, and Pacific Oceans. For example, the inoceramid extinction lies slightly higher in the Bottaccione Section (Gubbio, central Italy) within 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 deep-water 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 changes 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.

Multicore Record of the Cretaceous/Tertiary Boundary on Shatsky Rise

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 1211, and two each from Sites 1209, 1210, and 1212 (Fig. F53). The lithologic sequence in the K/T boundary interval is similar at all of these sites. At Sites 1209–1212, 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. F53). 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 on 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 latest 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, which is characterized by its unique populations of Cretaceous "survivors." 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 of up to 100–150 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 the record 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) Zones 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.

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