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SITE SUMMARIES (continued)

Site 1218

Site 1218 (8°53.378'N, 135°22.00'W) is the only Leg 199 drill site to be located over ~40-Ma crust. It was chosen for drilling in order to investigate paleoceanographic processes in the equatorial Paleogene Pacific Ocean during the inferred transition of Earth's climate from the early Paleogene greenhouse into the late Paleogene icehouse state. Site 1218 is situated on a basement swell ~3°N of the Clipperton Fracture Zone in the central tropical Pacific. Pacific plate motion carried it across the equator at ~40 Ma based upon a fixed hotspot model (Gripp and Gordon, 1990, for 0- to 5-Ma Pacific hotspot rotation pole and Engebretson et al., 1985, for older poles). Based upon the same model the site remained within 2° of the equator from the time crust was formed until 27 Ma. The sediments recovered from Site 1218 should therefore record equatorial oceanographic conditions from the middle Eocene until the early Oligocene. The precise age of basement at the site was poorly constrained prior to Leg 199 because little magnetic anomaly data are available between the Clipperton and Clarion Fracture Zones (Cande et al., 1989).

At Site 1218 we recovered a complete sediment section to within 15 m of basalt basement. Cores recovered from three holes were spliced into a continuous section from the seafloor to 263 mbsf (~41–42 Ma, nannofossil biostratigraphic Subzone CP14a). The sediments immediately overlying basalt are from near the CP14a/CP13 boundary (42 Ma), which indicates that basement Site 1218 is slightly older than expected. A full suite of downhole logs were obtained to the base of the Hole 1218A, and the data are of excellent quality.

The sediment column at Site 1218 is made up of four sedimentary units. At the top of the sedimentary column is 52 m of yellowish brown radiolarian clay with occasional barren intervals and intervals with nannofossils. The age of this unit is Pleistocene–middle Miocene. Below this unit are nannofossil ooze and chalk of early Miocene–Oligocene age from 52 to 217 mbsf. The boundary below with the Eocene section is abrupt. The upper and upper middle Eocene sediments (217–250 mbsf) are composed of radiolarites and nannofossil chalk with occasional chert beds. The cherty sections were completely recovered by XCB coring. The base of the sediment column is middle Eocene chalk (250–274 mbsf). The lower 7 m of the chalk is dolomitized with up to 20% dolomite in the coarse fraction.

Planktonic foraminifers occur sporadically through the lower Miocene, Oligocene, and middle Eocene sediments at Site 1218 with generally less consistent occurrence than other calcitic groups such as benthic foraminifers and calcareous nannofossils. Most samples show at least some dissolution of planktonic foraminifers with preferential preservation of thick-walled, large specimens in many cases. Middle Oligocene sediments (Zones P21 and P20) were generally the best preserved and most species rich. Benthic foraminifers are present in core catchers throughout the cored interval except in the upper radiolarian clays and the upper Eocene interval. Calcareous nannofossils, in contrast, are present at varying concentrations and states of preservation from uppermost middle Miocene Zone NN8 (CN6; 37mbsf) to the base of the sediment section. Radiolarians were found in all recovered cores.

Pore water chemical profiles are subtle. Sulfate concentrations are near seawater values throughout the section, indicating the lack of significant organic matter diagenesis, whereas trends in Ca and Mg reflect modest basement alteration. Bulk sediment concentrations (measured every 1.5 m) outline the major lithologic units described above. Only Ca and Sr concentrations are high in the lower Miocene–Oligocene nannofossil oozes, and all other elements tend to be higher in the clays and radiolarite sediments. Biogenic enrichment of Ba can be detected through Ba/Ti ratios. High Mg concentrations can be found in the basal dolomitized chalk.

Natural remanent magnetization (NRM) intensity of the sediments at Site 1218 is relatively strong, and magnetic overprint from drilling can be mostly removed by AF demagnetization. An excellent record of magnetic reversals was made for the entire APC-cored sediment section (0–189 mbsf) to Chron C12r of the early Oligocene. In addition, magnetic inclinations were determined on discrete samples after more thorough demagnetization. Middle and early Miocene samples have an average inclination of 5.2° (+2.2°–8.4°), whereas the Oligocene samples have an average inclination of 3.8°
(95 = 6.2), which is indistinguishable from the paleoequator.

Composite Section

Recovered sediments at Site 1218 have high signal-to-noise ratio MST data sets such that it was possible to direct drilling in real time in Holes 1218B and 1218C to construct a complete composite sediment section to a depth of ~263 mbsf (~287 mcd). The sediments recovered at Site 1218 retain high-quality paleomagnetic reversal stratigraphy for the entire APC-cored sedimentary section (to C12r; ~31 Ma; 188.7 mbsf). In the presence of well-preserved siliceous and carbonate microfossils, this section will help to define and calibrate Cenozoic radiolarian stratigraphic zonation, to develop an astronomically tuned Cenozoic timescale, and to generate high-resolution paleoceanographic and paleoclimatic records of the Paleogene–Neogene transition.

Oligocene–Miocene Transition

The short-range of nannofossil species Sphenolithus delphix provides a reliable marker close to the O/M boundary between Chrons C6Cn.3n and C6Cn.2n2 (Raffi, 1999), and on this basis the O/M boundary in Site 1218 occurs at ~98 mbsf. The first occurrence of the planktonic foraminifer Paragloborotalia kugleri (the marker for the O/M boundary) and first occurrence of the radiolarian Cyrtocapsella tetrapera also occur at this depth as well.

Calcareous benthic foraminifer assemblages indicate lowermost bathyal and upper abyssal paleodepths at Site 1218. Examination of test walls under transmitted light indicates that most of the benthic foraminifers at this site have suffered little or no postburial diagenetic alternation. Average sedimentation rates across the Oligocene–Miocene transition (~1–2 cm/k.y.) are relatively high for a deep-ocean Pacific setting.

Eocene–Oligocene Transition

The E/O boundary interval at Site 1218 is characterized by a major lithologic change involving a two-step downcore shift from pale nannofossil chalk to dark radiolarite. This transition from carbonate-rich to carbonate-poor sediments is also evident as a two-step decrease in GRA bulk density and a decrease in magnetic susceptibility values in MST data. Based upon biostratigraphic information, this transition occurs across or just above the Eocene–Oligocene transition.

The extinction of the calcareous nannoplankton Discoaster saipanensis is estimated to have occurred ~0.3 m.y prior to the E/O boundary event sensu stricto (extinction of the planktonic foraminifer genus Hantkenina) and the last representative of the Paleogene rosette-shaped discoasters, Discoaster barbadiensis, disappeared ~0.2 m.y. before D. saipanensis. Discoaster barbadiensis and D. saipanensis are constrained to have disappeared over narrow (~20–30 cm) intervals in Site 1218 sediments, shortly below the major change in lithology. These findings reveal that the entire two-step change in lithology occurred within Zone NP21 (CP16c), above the extinction of the last Eocene discoasters. By assuming a linear sedimentation rate within Zone NP21 in this composite section, an age estimate of 33.3 Ma was obtained for the initial change (midpoint of transition) in lithology, and an estimate of 32.9 Ma for the midpoint of the second, final step. The boundary condition change of the ocean-climate system that caused the first step of this drastic deepening of the CCD and accompanying change in sedimentation in the tropical Pacific Ocean thus occurred in middle Oi-1 (33.5–33.1 Ma) (Zachos et al., 2001a) on the common timescale used (Cande and Kent, 1995).

The abrupt change in the lithologies at the Eocene–Oligocene interval is a reflection of the rapid deepening of the CCD (van Andel et al., 1975) in the Oligocene. Site 1218 demonstrates that the change in CCD occurred the earliest Oligocene in two steps, as a rapid increase in CaCO3 over 10–20 k.y. followed by a pause of ~100 k.y. and then another rapid increase in CaCO3 over 10–20 k.y. The CCD was shallower than ~3600 mbsl in the latest Eocene radiolarite interval based on the paleodepth of Site 1218.

Middle and Late Eocene Radiolarites

Coring results from Site 1218 show that radiolarian-rich sediments are characteristic of the late and late–middle Eocene Pacific Ocean even near the equator. Nannofossils occasionally dominate the sediments during parts of this interval, showing that the CCD deepened beyond the paleodepth of Site 1218 at times within the late and middle Eocene. The absence of planktonic foraminifers in the upper Eocene section is most probably related to dissolution, but it is unknown whether planktonic foraminifers were common in the plankton originally. The presence of radiolarians and occasional intervals of nannofossils and the absence of diatoms (except in specific intervals near the E/O boundary) is more typical of Neogene plankton assemblages found at the fringes of the equatorial high productivity zone than in the near-equatorial position that Site 1218 probably occupied during the middle and late Eocene.

Middle Eocene Basal Sediments

The basal nannofossil chalks of Site 1218 abruptly become radiolarites upcore at ~40 Ma (base of nannofossil Subzone CP14b; Core 199-1218C-21X). The transition from chalks to radiolarites occurs over only 1.5 m when the site was at a paleodepth of 3000–3100 m. The middle Eocene rise in CCD at 40 Ma is almost as abrupt as the drop in CCD at the E/O boundary.

Finally, the basal 7 m of nannofossil chalk at Site 1218 that overlie basement are, like their Site 1215 and 1217 counterparts, dolomitized—an intriguing discovery given their proximity to what is generally considered to be a kinetically more favorable geochemical sink for Mg (alteration minerals in the upper oceanic crust).

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