Next Section | Table of Contents

SITE SUMMARIES (continued)

Site 1219

Site 1219 is the southernmost site to be drilled during Leg 199 on the 56-Ma transect and is situated ~3° north of the Clipperton Fracture Zone at a depth of 5063 mbsl on abyssal hill topography. The age of basement at Site 1219 was poorly constrained prior to Leg 199 because little reliable magnetic anomaly data are available between the Clipperton and Clarion Fracture Zones (Cande et al., 1989). At the outset of Leg 199, based on one interpretation of the location of magnetic anomaly C24r (~57 Ma) (Petronotis et al., 1994), previous drilling, and assumed spreading rates, we estimated the age of basement at Site 1219 to be ~55 Ma. Site 1219 is the only site to be drilled during Leg 199 that features all of the seismic horizons identified for a Paleogene equatorial seismic stratigraphy and our estimate of sediment thickness prior to drilling based upon the seismic reflection profile was 270–290 mbsf, depending on which reflector was chosen as basement.

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) Site 1219 should have been within 2° of the equator between 40 and 21 Ma and should have crossed the equator at 29 Ma. Thus the sediments should record equatorial conditions from the late middle Eocene through the early Miocene. In addition, Site 1219 should provide an analog for Site 1218, except that it is on older, deeper crust.

Two holes were drilled at Site 1219. Hole 1219A was a remarkable operations success. We advanced to ~225 mbsf using ODP's APC technology and thereby achieved one of our high-priority objectives by recovering sediments suitable for whole-core paleomagnetic analysis below the lower Oligocene (including the E/O boundary). Basement was reached at ~250 mbsf, shortly after switching to XCB coring. A full suite of downhole logs were obtained to the base of Hole 1219A, and the data are of high quality. In addition to these downhole logs, MST data from the latest Eocene to the earliest Miocene interval in this hole bore a striking resemblance to those recovered from Site 1218 such that it was possible to correlate between Sites 1218 and 1219 (which are separated by 7° of longitude and 1° latitude, or ~800 km) to a submeter scale.

In contrast to Hole 1219A, Hole 1219B was terminated when an APC core jammed in the bottom-hole assembly (BHA) at the depth of the E/O boundary (~155 mbsf). In light of these difficulties in Hole 1219B and the successes of both Hole 1219A and Site 1218, the shipboard party took a collective decision to abandon Site 1219 earlier than planned in order to target additional and more complete programs at forthcoming sites having early and middle Eocene objectives.

Holes 1219A and 1219B can be spliced to form a continuous section to 130 mcd (~30 Ma) with two apparent gaps at ~90 and ~100 mcd. Excellent correlations between Site 1218 and 1219 allow us to estimate the properties of the unrecovered intervals, whereas correlations between logging data and the cores can be used to estimate the size of core gaps deeper in the sedimentary section. Sedimentation rates over the Oligocene interval were ~16% slower than at Site 1218, based upon the site-to-site correlation.

The sediment column at Site 1219 has a strong resemblance to that of Site 1218. 30 m of clay (lithologic Unit I) overlie lower Miocene–Oligocene nannofossil ooze (Unit II; 30–151 mbsf). Strong cyclic variations in nannofossil content are apparent in both the upper and lower parts of Unit II. An abrupt change in lithology from nannofossil ooze to radiolarian clay and clayey radiolarian ooze occurs at 151 mbsf within the E/O boundary interval. The lithologic change marks the upper boundary of Lithologic Unit III, an upper and middle Eocene unit composed of radiolarian ooze and radiolarian clay, becoming radiolarite, chert, and zeolitic clay at the base. A short section in Unit III within magnetic Chron C18r (40.1–41.3 Ma) contains alternating diatom and nannofossil ooze. Below the cherty, clay-rich section at the base of Unit III are the chalks of lithologic Unit IV (234–243 mbsf; ~53–54.8 Ma). The oldest sediments above basalt are slightly younger than the P/E boundary.

NRM intensity of the sediments at Site 1219 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 (0–223 mbsf), from Pleistocene Chron C1n to early middle Eocene Chron C20r (43.8–46.3 Ma). This remarkably clean magnetic reversal stratigraphy allows us to calibrate biostratigraphic events and to develop detailed sedimentation rate curves downhole through to lower middle Eocene sediments. Overall, declination and inclination patterns in discrete samples show that at least the lower part of the sedimentary section was located in the Southern Hemisphere during deposition. The mean inclination depicts a time-averaged paleolatitude of 1.6°S for the site, but this result is preliminary and will require further testing. The paleolatitude inferred from the inclination is consistent with the expected latitudes as calculated from both paleomagnetic pole positions and those based upon a fixed hotspot model.

Nannofossils are present to abundant in the middle Miocene through Oligocene sediments (~10–151 mbsf) but absent from upper Eocene sediments. Nannofossils reappear briefly in magnetic Chron C17r (~38.3 Ma) in the middle Eocene, disappear, and then reappear and are present between the middle of magnetic Chron C18r to the base of C20n (~40.5–43.8 Ma) and are found in the chalk in the lower Eocene interval. Planktonic foraminifers are present in the lower Miocene to Oligocene sediments and in the lower Eocene chalks. Preservation quality and abundance are highest in the lower Miocene (Zones M4–M2) and the middle part of the Oligocene (Zones P20 and P21). Benthic foraminifers are consistently present and well-preserved through the Miocene and Oligocene at Site 1219 but are scarce and very poorly preserved through much of the Eocene sections. Radiolarians were found in all cores except the deepest one (Core 199-1219A-27X), which recovered chalk over basalt basement.

The magnitude of the downhole calcium concentration increase and magnesium and potassium concentration decrease in pore waters at Site 1219 is the greatest seen at all sites (Sites 1215–1219) drilled so far drilled during Leg 199. This pattern is consistent with the extensive alteration (e.g., chlorite formation) of basement rocks observed at Site 1219. Depth gradients of pore water alkalinity, pH, sulfate, and ammonium at Site 1219 reflect the small amount of organic matter degradation occurring in these sediments, whereas the chlorinity profile may reflect the diffusion of the more saline Pacific bottom water of the last glacial maximum. Bulk sediment geochemical analyses from Site 1219 reflect the shifts in lithology back and forth between sediments dominated by silica and carbonate. Clay-rich units are high in Ti and Al, but are also high in Fe and Mn, presumably reflecting the deposition of authigenic ferromanganese oxyhydroxides. Physical properties of the sediments also primarily reflect lithology. The carbonate sediments are higher in density, lower in porosity, and lower in magnetic susceptibility than the clay or radiolarian ooze lithologies. This is true even on a fine scale . The radiolarian ooze has higher compressional wave velocity than the carbonates despite having higher average porosity.

Paleomagnetic Reversal Stratigraphy

The clean record of magnetic reversal history from Site 1219 (for the entire APC-cored sediment section; 0-223 mbsf) from the Pleistocene down to Chron C20r (43.8–46.3 Ma) is remarkable for a tropical site and, together with the superb cyclostratigraphic correlations to Site 1218 (see below), will provide invaluable time control for shore-based high-resolution paleoceanographic studies.

MST Correlation to Site 1218

The MST data from Site 1219 bear a striking resemblance to those recovered from Site 1218 from the middle Eocene to the lowermost Miocene interval. The excellent match in these data sets between the two sites made it possible to align both records on a common (Site 1218 mcd) depth scale. The two records show such high quality correlation (down to the submeter scale) that successful a priori prediction of biostratigraphic zones and paleomagnetic reversals was possible at Site 1219. The mapping from Site 1219 mcd to Site 1218 mcd results in relative sedimentation rates at Site 1218 that are ~16% higher than at Site 1219 over the Oligocene interval. The remarkable fidelity of the correlation between these two sites, separated by more than 1° of latitude and 7° of longitude, suggests that drilling results from these two sites are representative of large-scale paleoceanographic forcing functions in the late Paleogene eastern equatorial Pacific Ocean. We anticipate that the continuously cored sediments from Site 1218 with supplementary control from correlative sediments in Site 1219 will provide a paleoceanographic reference section for the late Paleogene tropical Pacific Ocean.

Oligocene–Miocene Transition

Calcareous nannofossils suggest a placement of the O/M boundary ~55 mcd at Site 1219, where the range of Sphenolithus delphix was observed and correlates to Chron C6Cr. On the Cande and Kent (1995) timescale, an age estimate of 24.28 S. delphix at Site 1219. Conversion to the orbitally tuned timescale of Shackleton et al. (2000) by subtracting 0.9 m.y. from the Cande and Kent (1995) timescale estimate, an age of 23.38 Ma is obtained for the base of S. delphix. This value is 0.14 m.y. older than the orbitally tuned estimate for this datum derived from the eastern South Atlantic (DSDP Site 522) and the western equatorial Atlantic (Sites 926, 928, and 929) (Shackleton et al., 2000). Yet the calibration of the S. delphix event to the geomagnetic polarity record is remarkably consistent in upper Chron C6Cr, from the South Atlantic Ocean to the Mediterranean region (Raffi, 1999) and to the tropical Pacific Ocean (Site 1219).

Eocene–Oligocene Transition

The Eocene–Oligocene transition at Site 1219 is associated with an abrupt lithological change from lithology at Site 1219 from radiolarian clays below to nannofossil ooze above. This transition is similar to but sharper than that observed at Site 1218, presumably reflecting higher rates of carbonate dissolution at Site 1219 (the contemporaneous paleowater depth is ~400 m deeper at Site 1219 than Site 1218). Together with drilling results from Site 1217, this finding offers exciting prospects for shore-based investigation of the first Pacific Ocean depth and latitudinal transect across this important paleoceanographic boundary.

Eocene Sediments

The Eocene sediments at Site 1219 are dominated by radiolarian oozes from the period between ~45 Ma through the E/O boundary. Nevertheless, there are intervals of carbonate within the section, most notably at ~40.5 Ma but also at ~38.3 and ~43.5 Ma. These carbonate units can be detected by density, reflectivity, velocity, and LAS mineralogy as well as by chemical analyses and microscopic examination of the sediments. The 40.5-Ma event and 38-Ma event are also found at Site 1218. The oldest of these events is older than basement at Site 1218.

The lower middle Eocene interval is represented by cherty sediments that were not recovered by drilling. This is the third site on the 56-Ma transect in which we encountered cherts in roughly the same interval. Immediately below the cherts at Site 1219 are zeolitic clays barren of microfossils, suggesting that the chertified interval was deposited slowly. The uppermost lower Eocene chalk in contact with the zeolitic clay has an age of ~53 Ma (nannofossil Subzone CP9b) whereas the lowermost radiolarian ooze above the cherty interval is dated to ~46 Ma. The average sedimentation rate over the 10-m-thick cherty interval could thus be as low as 1.4 m/m.y. The interval has been condensed by diagenesis, but an estimate of using sediment density measured by downhole logging (~2 g/cm3) is about half the bulk mass accumulation rate of the sediments immediately above it.

The lower Eocene chalks were poorly recovered but appear to be typical basal carbonate-rich sediments. There is little evidence of hydrothermal sediments within the chalks.

Next Section | Table of Contents