SUMMARY

Site 1220 (10°10.601´N, 142°45.491´W) forms a southerly component of the 56-Ma transect to be drilled during Leg 199. It is situated halfway between the Clipperton and Clarion Fracture Zones at a water depth of 5218 mbsl in typical abyssal hill topography. On the basis of regional magnetic anomalies, we anticipated basement age at Site 1220 to be equivalent to Anomaly An25n (~56 Ma) (Cande et al., 1989), slightly older than at Site 1219.

Based upon a fixed hotspot model (Gripp and Gordon, 1990, for the 0- to 5-Ma Pacific hotspot rotation pole; Engebretson et al., 1985, for older poles) Site 1220 should have been located ~3°S of the equator at 56 Ma and in an equatorial position at 40 Ma. Thus, Site 1220 should have been situated beneath the South Equatorial Current in the early Eocene. Site 1220 data will be used as a deeper analog to Site 1218. Both sites are thought to have been located in an equatorial position at ~40 Ma, but Site 1220 was ~400 m deeper at this time.

Three holes were drilled at Site 1220. Hole 1220A was terminated when an advanced piston corer (APC) core jammed in the bottom-hole assembly (BHA) at ~100 mbsf, but basement was reached in Holes 1220B and 1220C at ~200 mbsf. Cores from Site 1220 overlap and form a continuous sedimentary sequence down to ~144 meters composite depth (mcd) (base of Core 199-1220B-10H). The sedimentary sequence recovered at the site is divided into five major sedimentary units. The uppermost unit (~0-19 mbsf) consists of very dark grayish brown clay with zeolites and is underlain by a lower Miocene-Oligocene Unit II (~19-40 mbsf) of radiolarian and nannofossil oozes with varying clay content. The underlying Oligocene radiolarian and nannofossil oozes of Unit III (~40-70 mbsf) are notable by the presence of a significant (~15%-45%) diatom component toward the base. Unit IV (~70-185 mbsf) consists of upper Eocene radiolarian oozes with clay and middle-lower Eocene chert with clayey radiolarian ooze. These sediments are underlain by a lower Eocene-upper Paleocene unit (~185-200 mbsf) of partially dolomitized nannofossil ooze, radiolarian nannofossil ooze, radiolarian ooze, calcareous chalk, and black clay atop an aphanitic to fine-grained phaneritic basalt.

Paleomagnetic data from Site 1220 gave excellent results and a reliable record of geomagnetic reversals from the early-middle Eocene to the early Miocene. The composite depth record from Holes 1220A, 1220B, and 1220C shows a remarkable match of the cores between the different holes. In fact, the virtual geomagnetic pole (VGP) latitude changes were used as a basis to help fit the cores to the mcd depth scale. Correlation of the magnetic stratigraphy at Site 1220 to the geomagnetic polarity timescale (GPTS) shows a record that spans from the top of Chron C21n to Subchron C6An.1n (~20.5-46.3 Ma).

Biostratigraphic results indicate that we recovered a nearly complete sequence of lower Miocene-lower Eocene radiolarian zones at Site 1220, interrupted only by a poorly recovered chert sequence from the uppermost lower Eocene and the lowermost middle Eocene. Calcareous fossils are generally poorly preserved or absent through much of the sequence. Calcareous nannofossils are sufficiently well preserved in the lowermost Miocene and Oligocene to provide a basic zonation. Planktonic foraminifers are almost entirely absent above the lower Eocene, but dissolution-resistant species allow the lower/upper Oligocene boundary to be approximated. Both planktonic foraminifers and calcareous nannofossils provide a detailed zonation of a condensed sequence of lower Eocene nannofossil oozes and chert in the basal 10 m of Site 1220. The extinction of Paleocene benthic foraminifers, the appearance of the nannofossil genus Rhomboaster, the extinction of the nannofossil genus Fasciculithus, and the presence of "excursion fauna" of planktonic foraminifers provide a detailed biostratigraphy of the Paleocene/Eocene (P/E) boundary in Unit V. The nannofossil events occur 0.8-1.4 m above the extinction of Paleocene benthic foraminifers. The excursion fauna of planktonic foraminifers is present in sediments below the level of the benthic foraminifer extinction in sediments immediately overlying basalt. Thus, none of these events are precisely synchronous with the benthic foraminifer extinction, the marker we used for the P/E boundary, at this site.

Interstitial pore water profiles from Site 1220 are very similar to the profiles of all other Leg 199 sites except Site 1219 and primarily reflect minor organic matter degradation, the dissolution of biogenic silica, and minor alteration of underlying basalt. The bulk geochemistry of the sediments from Site 1220 reflects the shifts in lithology between sediments dominated by silica and carbonate.

Physical properties of the sediments also primarily reflect lithology. The carbonate sediments are higher in density, lower in porosity, and lower in magnetic susceptibility (MS) than the clay or radiolarian ooze lithologies. The radiolarian-rich sediments of Units II and IV are marked by high porosities, which average 88% and 85%, respectively. The radiolarian oozes maintain their porosity despite burial. The Eocene radiolarian oozes have the highest P-wave velocities of the unconsolidated sediment lithologies.

Highlights

Magnetic Reversal Stratigraphy

The clean record of magnetic reversal history from Site 1220 (for the entire APC-cored sediment section; 0-150 mbsf) spans the lower Miocene to the lower-middle Eocene (Subchron C6An.1n to the top of Chron C21; 20.05-46.3 Ma). This record is remarkable for a tropical site and will provide invaluable time control for calibration of radiolarian biostratigraphy. Because there are cyclic variations in radiolarian content of the Site 1220 Eocene section, presumably driven by orbital forcing of insolation, this record could be highly important for orbital tuning of the GPTS from the middle Eocene through the early Miocene.

Eocene/Oligocene Boundary

A combination of magnetostratigraphy and nannofossil biostratigraphy indicates that we recovered a further Eocene/Oligocene (E/O) boundary at Site 1220 (~70 mbsf). Shore-based work on this section, together with those recovered at Sites 1217, 1218, and 1219, will allow us to improve existing constraints on the links between global cooling, Antarctic ice sheet growth, and a deepening CCD across this important paleoceanographic boundary.

P/E Boundary

In Hole 1220B just above basement basalt, we recovered a lithologically striking interval of calcareous chalk and clay (199-200 mbsf). Based on biostratigraphic data, this sequence represents the P/E boundary. Layers of calcareous chalk and clay display a large range in composition and color downcore. Alternating intervals of white and very pale brown calcareous chalk are present between 198.9 and 199.4 mbsf and contain a minor amount (5%-10%) of poorly preserved nannofossils. White layers are present between 199.15 and 199.23 mbsf and contain 15%-30% planktonic foraminifers. The sediments below this interval consist of faintly banded dark yellowish brown calcareous chalk but are barren of microfossils. Dolomite (up to 10%) and clay (10%-35%) are also present in this lithology. Below 199.50 mbsf is a 4-cm-thick layer of very dark brown (7.5YR 2.5/3) calcareous chalk underlain by a black clay. Major components of both sediments are dolomite, iron manganese oxides, and clay. Volcanic glass is present as a minor component. Dolomite content decreases over the interval from 199.52 to 199.54 mbsf, and no calcareous or siliceous fossils were observed in corresponding smear slides. Directly beneath the black clay is a 4.5-cm-thick interval of yellowish red calcareous chalk. In this interval is a 1-cm-thick brownish yellow layer of calcareous chalk. Sediments in the yellowish red layers are weakly laminated and contain clay, dolomite, calcite, and a minor percentage of nannofossils. Iron manganese oxides and opaque minerals are minor components. The brownish yellow chalk layer contains clay, dolomite, iron manganese oxides, volcanic glass, and abundant small mottles. Nannofossils are rare in these layers.

The base of the sedimentary section contains a 9-cm-thick interval of brownish yellow clayey calcareous chalk (199.59-199.68 mbsf) with thin black laminations. Dolomite comprises up to 10% of these sediments. Zeolites and nannofossils are also present as minor components. Iron manganese oxides are rare. Laminations contain clay, iron manganese oxides, dolomite, zeolites, and small amounts of volcanic ash and are barren of calcareous fossils.

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