Next Section | Table of Contents

SITE SUMMARIES (continued)

Site 1220

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 and is situated about midway 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 C25n (~56 Ma; Cande et al., 1989), slightly older than at Site 1219. At the outset of drilling at Site 1220, our estimate for total sediment thickness was ~225 m.

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 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. A nearby piston core (EW9709-13PC) taken during the site survey cruise recovered more than 16 m of red clay with the base of the core dated as middle lower Miocene on the basis of radiolarian biostratigraphy (Lyle, 2000).

Site 1220 was drilled in order to study equatorial ocean circulation from the late Paleocene to the late Eocene during the early Cenozoic thermal maximum. Sediment records from this site will help to define the CCD and lysocline during the Paleocene–Eocene and Eocene–Oligocene transitions. In this and other respects, Site 1220 will act as an interesting analog to Site 1218. Both sites are thought to have been located in an equatorial position at ~40 Ma, but the older crustal age anticipated at Site 1220 dictates a greater paleowater depth than for contemporaneous sediments accumulating at Site 1218.

Three holes were drilled at Site 1220. Hole 1220A was terminated when an APC core jammed in the 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 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 (~19–40 mbsf) of radiolarian and nannofossil oozes with varying clay content. The underlying Oligocene radiolarian and nannofossil oozes of Unit III (~40–67 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 to lower Eocene chert with clayey radiolarian ooze. These sediments are underlain by a lower Eocene to 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 Miocene to the early–middle Eocene. 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 geomagagnetic pole (VGP) latitude changes were used in real time 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 Chron C6An.1n to the top of Chron C21n (~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 upper/lower 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 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 of 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 reflect 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 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.

Paleomagnetic 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 lower–middle Eocene (Chron C6An.1n to the top of Chron C21; 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 will be highly important for orbital tuning of the magnetic reversal timescale from the middle Eocene through the Miocene.

Eocene/Oligocene Boundary

A combination of paleomagnetic and nannofossil biostratigraphy indicates that we recovered a further 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.

Paleocene/Eocene 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 occur between 198.9 and 199.4 mbsf and contain a minor amount (5%–10%) of poorly preserved nannofossils. White layers occur 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%) also occur 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, clay, and possibly sulfides. Volcanic glass occurs 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 smear slides. Directly beneath the black clay is a 4.5-cm-thick interval of yellowish red calcareous chalk. Within this interval is a 1-cm-thick brownish yellow layer of calcareous chalk. Sediments within 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 the sediment. Zeolites and nannofossils also occur 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.

Next Section | Table of Contents