Next Section | Table of Contents

SITE SUMMARIES (continued)

Site 1217

Site 1217 is situated approximately 1° latitude north of the Clarion Fracture Zone on abyssal hill topography typical of the Central Pacific. The site was chosen for drilling because it is thought to have been located just outside of the equatorial region at 56 Ma (~5°N, 106°W, 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). At 40 Ma the site was located at ~8°N, 111°W. Thus, Site 1217 should help define the paleoceanography of the northern tropical Pacific and, in particular, help to locate the ancient North Equatorial Countercurrent (NECC) region. Based on site survey seismic data and piston coring together with results from the nearest drill site (DSDP Site 162, situated on 48-Ma crust ~300 km south and west), we expected the sedimentary sequence at Site 1217 to comprise a relatively thick (25–35 m thick) section of red clays, overlying a radiolarian ooze. We also expected a basal carbonate section with possible chert near basement (estimated total depth of ~125–150 mbsf) deposited when the site was near the ridge crest in the late Paleocene and early Eocene.

The sedimentary section overlying basalt at Site 1217 is ~138 m thick and records a lower Eocene nannofossil chalk overlain by a poorly recovered lower–middle Eocene chert-clay sequence. Middle Eocene–Holocene deposition is represented by radiolarian ooze followed by red clays at the surface. The recovered sediments from all three holes at Site 1217 were affected by coring disturbance, downhole debris, and flow-in associated with chert fragments blocking the core liner. One stratigraphic interval in the APC-cored section was highly disturbed or not recovered in all three holes (~37–49 mbsf), preventing the recovery of a complete continuous sedimentary section. Nevertheless, it was possible to generate a spliced, but discontinuous, record in the upper 90 m of the section.

The uppermost red clays at Site 1217 (Unit I) are ~39 m thick. The clays show a downhole mineralogical transition from illite to smectite at~7 mbsf, indicating a change in wind-blown dust provenance from American to Asian sources associated with the movement northward of the Pacific plate. These clays also contain a thin (~2.5 m thick, ~30 mbsf) subunit of nannofossil ooze interbedded with nannofossil clay. Below the red clays are a radiolarian ooze with clay grading to clayey radiolarian ooze (Unit II; ~39–90 mbsf). The radiolarian ooze is of middle Eocene age (~38–43 Ma) and contains a rich and well-preserved middle Eocene radiolarian fauna. Below 90 mbsf we encountered a unit of chert and interbedded clay (Unit III; l90 and 128 mbsf) before recovering a thin (~1.13 m thick) section of early Eocene chalk (Unit IV; 128–129 mbsf). The chalk is partially to extensively dolomitized (washed core catcher samples yield abundant near-perfectly euhedral rhombs ~100 mm in size) but contains nannofossils and planktonic and benthic foraminifers. Basement basalt was encountered at 138 mbsf.

Wet bulk density values at Site 1217 are high between the seafloor and 22 mbsf (mean values = 1.37 g/cm3) and decrease sharply to a minimum of 1.18 g/cm3 at 25.58 mbsf—a value that is very similar to the density of the radiolarian oozes at the site. In contrast, the wet bulk densities of the Oligocene nannofossil ooze subunit and lower Paleocene nannofossil chalk are distinctly higher (~1.28 and 1.79 g/cm3, respectively).

The magnetic intensity of the sediments is relatively strong, and drilling-induced magnetization was mostly removed with mild alternating-field (AF) demagnetization. Site 1217 sediments provided a good record of geomagnetic reversals that could be easily interpreted as magnetozones. Characteristic remanent magnetization (ChRM) inclinations are usually shallow, as expected in these latitudes. A reversal record from Chron 20 to Chron 12 was established for the middle Eocene–early Oligocene sediments, but the reversal stratigraphy in the upper 15 m (late Oligocene to recent) could not be established because of the low sedimentation rate of the upper sediments and the lack of either core orientation or an independent age model.

High levels of sulfate and concomitant low levels of ammonium in interstitial pore waters at Site 1217 indicate a relatively oxic system, consistent with very low levels of labile organic matter, occurrences of metalliferous oxides, and the strong and stable magnetic signals within the host sediments. Relatively high pore water silica concentrations are consistent with dissolution of biogenic silica within the sediments. The solid phase chemical content reflects the microfossil content and the low level of reductive diagenesis. The whole section at Site 1217 shows relatively high levels of Mn and Fe in the solid phase. Within the upper red clays, solid phase Si levels generally increase southward in the Site 1215, 1216, and 1217 transect, presumably indicating higher export of biogenic debris to the ocean floor closer to the paleoequator.

The average linear sedimentation rates in the red clays (shallower than 23 mbsf) are very low (~0.8 m/m.y.). Sedimentation rates in the underlying fossiliferous and cherty lower sections are substantially higher but are, nevertheless, relatively modest (~5 m/m.y.).

Lower Oligocene Nannofossil Ooze

Assuming that a distinct polarity reversal seen in paleomagnetic data at 23.27 mbsf in Hole 1217A represents the base of Chron 12n, the succeeding downhole normal polarity interval is interpreted to represent Chron 13n, which lies at the base of the Oligocene. This interpretation is consistent with the underlying spacing and length of several normal and reversed intervals and places the thin subunit of nannofossil-rich carbonate ooze (all from nannofossil Zone NP22) in the lowermost Oligocene. We hypothesize that this brief interval of carbonate sediment preservation represents the dramatic deepening of the CCD recorded elsewhere in the deep-sea tropical Pacific Ocean (e.g., DSDP Sites 42, 70, 161, and 162) associated with the Eocene–Oligocene transition.

Middle Eocene Radiolarian Ooze

At Site 1217 we collected the first near-continuously drilled sequence of Eocene radiolarian oozes by deep-sea drilling. In the presence of a well-defined magnetostratigraphy, this section will help to define and calibrate radiolarian stratigraphic zonation. Radiolarian oozes have no modern analog but are prominent sedimentary features of the middle Eocene low-latitude Pacific Ocean having been recovered from piston cores and drill sites up to 4° to the north (e.g., DSDP Sites 40 and 41) and 10° to the south (e.g., DSDP Sites 70 and 162). In contrast, middle Eocene radiolarian oozes were absent at Site 1216. Thus, Sites 1216 and 1217, along with DSDP Sites 40 and 41, appear to define the northern extent of this type of biogenic sedimentation. We anticipate that further recovery of middle and upper Eocene sediments will better define the oceanographic conditions that lead to radiolarian ooze formation.

Lower to Middle Eocene Chert and Dolomitized Nannofossil Chalk

By coring to basement at Site 1217 we recovered sediments needed to address a number of Leg 199 objectives, including an improvement in our understanding of the following: (1) the location of the Eocene/Paleocene paleoequator, (2) the biotic composition and rate of accumulation of sediments within the tropical Eocene Pacific Ocean, (3) the behavior of the silica budget and CCD during the early Eocene, and (4) the sedimentary expression of the P/E boundary in the central tropical Pacific Ocean. Broadly speaking, the sequence drilled at Site 1217 conforms to the classical sedimentary succession predicted for a deep-sea drill site situated on relatively old oceanic crust in the Central Pacific (red clays overlying siliceous and, in turn, carbonate biogenic sediments). However, the lowermost sediments drilled (lower to middle Eocene chert and chalk) surprised us in two ways. First, these sediments show average linear sedimentation rates are relatively slow (<5 m/m.y.), consistent with results from Site 1216. Second, the basal lower Eocene chalks that overlie basement are, like their Site 1215 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).

Next Section | Table of Contents