The mid-Cretaceous (~125-85 Ma) and early Paleogene (~65-34 Ma) were characterized by some of the most equable climates of the Phanerozoic and are among the best known ancient "greenhouse" climate intervals. In addition, these intervals contain some of the most abrupt and transient climatic changes in the geologic record, including the Paleocene-Eocene Thermal Maximum (PETM), the mid-Maastrichtian deepwater event (MME), and the early Aptian Oceanic Anoxic Event (OAE1a). These critical transitions involved dramatically modified oceanic circulation patterns, profound changes in geochemical cycling, and abrupt turnover in marine biotas. Recent ocean drilling efforts have led to profound advances in our understanding of the ocean and climate dynamics of a warm Earth; however, we have yet to gain a firm knowledge of how atmospheric or deep-ocean circulation operates in the apparent absence of substantial thermal gradients, how rapid removal of important elements such as nutrients in some of these events is maintained for a long period of time, and exactly how environmental changes cause extinction and speciation of marine biotas.
Ocean Drilling Program (ODP) Leg 198 on Shatsky Rise was designed to address the causes and consequences of Cretaceous and Paleogene global warmth. The objectives were to address the origin of the long-term climatic transition into and out of "greenhouse" climate as well as abrupt climatic events. Shatsky Rise, a medium-sized large igneous province in the west-central Pacific, contains sediments of Cretaceous and Paleogene age at relatively shallow burial depths on three prominent highs. As a result, sediments of both ages can be reached readily through drilling, and fossil materials are sufficiently well preserved for stable isotope and trace element analyses and for faunal and floral assemblage studies.
Eight sites were cored during Leg 198 on a broad depth transect (Table T1) designed to characterize changes in the nature of surface and deep waters through time, including vertical gradients of temperature, oxygenation, and corrosiveness. Six sites were cored on the Southern High, and one each on the Central and Northern Highs. The Southern High had been cored during previous Deep Sea Drilling Project (DSDP) and ODP legs; thus, the general stratigraphy of this area was known prior to drilling. The Central and Northern Highs, however, had not been cored previously.
A virtually complete section from the Holocene to the lowermost Berriasian (lowermost Cretaceous) was cored on Shatsky Rise. Leg 198 also cored the first igneous rocks from Shatsky Rise, predominantly diabase sills that may represent a regional intrusive event. Multiple coring produced a composite section for all but a small portion of the Cenozoic. Prominent orbital cycles throughout the Cenozoic hold significant potential for both the precise correlation between holes and sites and for improving the resolution of the Cenozoic timescale.
A number of critical transitions were cored during Leg 198, most in multiple holes from several sites. Dark-colored claystones rich in organic carbon (Corg) and porcellanites of early Aptian OAE1a were recovered at three different sites. Samples from two of these sites have extremely high organic carbon contents, up to 34.7 wt%. This organic matter is marine in origin and of exceptional preservation, containing evidence for cyanobacteria and the oldest known alkenones, compounds produced by haptophyte algae. The preservation of organic matter and common lamination suggests deposition and diagenesis in highly dysoxic or anoxic environments. Nannofossil and radiolarian biostratigraphy indicate that the three sections correlate with sediments representing OAE1a at other sites in the Pacific and southern Europe, including the well-known Selli level of Italy. The recovered organic-rich sequences are possibly the best pelagic records of OAE1a.
A significant representation of the MME was recovered at two sites. Clusters of large Inoceramus prisms are seen in the cores for several meters but disappear abruptly. This disappearance is in the same stratigraphic position in two sites at ~69 Ma and correlates to the Inoceramus extinction and the isotopic shifts that mark the MME at other deep-sea locations. Prisms have been noticed in three additional sites as well. The significance of the short range of visible specimens in this open-ocean setting is not currently understood. Benthic foraminiferal data from the Shatsky Rise depth transect will help to accurately characterize the changes in deepwater properties across this event.
A remarkable set of cores was taken across the Cretaceous/Tertiary (K/T) boundary at four sites on the Southern High and in nine separate holes. The lithostratigraphy of the boundary succession is similar in all of the holes, allowing precise correlation between records. The uppermost Maastrichtian whitish nannofossil foraminiferal ooze contrasts strongly with the pale orange, clay-rich lowermost Danian foraminiferal ooze. The contact is affected by bioturbation, but otherwise the boundary interval is undisturbed.
Preliminary biostratigraphy shows the well-established, abrupt change in nannofossil and planktonic foraminiferal assemblages across the boundary at all sites. Careful sampling of burrows of Danian ooze within the top of the Maastrichtian yields spherules that are probably altered tektites. The same reaches of the burrows contain highly abundant, minute planktonic foraminiferal assemblages that possibly represent basal Paleocene Zone P0. This is the first time the basal Paleocene zone has been identified in the deep sea. Even though the K/T boundaries on Shatsky Rise have been affected by bioturbation, the substantial thickness of the uppermost Maastrichtian Micula prinsii (CC26) Zone and the lowermost Danian Parvularugoglobigerina eugubina (P) Zones indicates that the boundary is paleontologically complete. The Shatsky Rise sections represent one of the best preserved and least disrupted deep-sea records of this major extinction event, as well as the subsequent radiation.
The PETM, one of the primary targets of Shatsky Rise coring, was recovered in 10 separate holes at four sites on the Southern High. These sites provide a depth range of ~500 m, designed to test the response of the ocean to the hypothesized massive input of methane hydrate. In all but one of the holes, the PETM corresponds to an 8- to 23-cm-thick layer of yellowish brown, clayey nannofossil ooze with a sharp base and a gradational top. At several sites, an extremely thin (1 mm) dark brown clay seam lies at the base of the PETM, corresponding to a sharp decrease in carbonate content as indicated by color reflectance data. One hole at the deepest site, Site 1211, has an unconformity right above the clay seam. In all of the other holes, preliminary biostratigraphy suggests that the PETM interval is complete. The Paleocene/Eocene boundary interval was also recovered at Site 1208 on the Central High, although it is currently uncertain whether this record contains the PETM. This deep site will extend the depth transect another 450 m.
The PETM interval in all of the Southern High sites contains a clear record of nannofossil and planktonic foraminiferal assemblage transformation at this time of major environmental upheaval. Planktonic foraminiferal assemblages within the clay-rich interval contain an ephemeral group of ecophenotypes or short-lived species of the genera Acarinina and Morozovella observed at other tropical and subtropical locations. All sites show a general deterioration in nannofossil preservation at the onset of the event and contain abundant 10- to 20-Ám-sized, bladey calcite crystals that are thought to have been derived from precipitation of dissolved carbonate.
The general changes in lithology suggest a transition from paleodepths at 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 interval (Sites 1208 and 1211). The decrease in carbonate content and deterioration in nannofossil preservation are evidence for an abrupt rise in the level of the calcite compensation depth (CCD) and lysocline during the PETM, the predicted response for a massive release of methane into the ocean-atmosphere system.
A major deepwater cooling event that coincides with intensification of glaciation on Antarctica occurred during the Eocene-Oligocene transition. Leg 198 recovered records in a total of nine holes at four sites across a large depth range to test the response of the tropical Pacific Ocean to this cooling event. Cooling is thought to have indirectly led to a gradual deepening in the CCD as a result of increased physical weathering on continents or intensification of deep-sea circulation. In all of the holes, the transition is associated with a lightening in the color of the sediment associated with an increase in carbonate content. This transition takes place over 4 to 7.5 m at the Southern High sites, but over 1 to 2 cm at Site 1208 on the Central High. The distinctive color change in the all of the records reflects a pronounced deepening in the CCD during the Eocene-Oligocene transition. The sharp boundary at Site 1208 indicates a considerable deepening in the CCD in this region. Preliminary data show that the prominent change in lithology occurs just before or within the time of cooling. However, further analysis is required to determine the exact timing of this event and the nature of the deepwater changes on Shatsky Rise.
Coring on Shatsky Rise also provided a number of surprises. At several sites on the Southern High, a prominent clay-rich horizon in the mid-Paleocene (~58 Ma) corresponds to increased carbonate dissolution. The layer occurs very close to the evolutionary appearance of Discoaster, a genus that dominated assemblages for a considerable duration in the Cenozoic. Planktonic foraminifers are characterized by a nearly monogeneric assemblage of Igorina. The level represents an abrupt environmental change on Shatsky Rise, the nature of which will be investigated during postcruise research. On the Central and Northern Highs, expanded, mixed siliceous-calcareous Neogene sequences were recovered that are potentially significant for advances in biochronology and magnetochronology of this time interval. These sections are strongly cyclic and have potentially significant high-quality paleoceanographic records. Site 1208 on the Central High is an excellent candidate for the development of a paleomagnetic intensity record for the Neogene of the Pacific Ocean. At the opposite end of the stratigraphic column, an organic-rich horizon in the Valanginian at Site 1213 may represent the first deep ocean record of an anoxic event that was previously thought to be confined to the Tethys.
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