During Leg 199, a transect was drilled across the Eocene equatorial region following 56-Ma crust, with one additional site located at the 40-Ma paleoequatorial position on 42-Ma crust (Lyle, Wilson, Janecek, et al., 2002) (Fig. F1). The primary objective of the drilling was to capture a latitudinal transect at the Paleocene/Eocene boundary. Nevertheless, the combination of sites on younger and older crust for the middle and late Eocene provided an opportunity to study the CCD after in detail 42 Ma.
Site 1218 (Fig. F1; Table T1) was sited at the 40-Ma equator to study middle Eocene equatorial processes and to provide a late Eocene to early Miocene record of paleoceanographic evolution of the tropical Pacific (Lyle, Wilson, Janecek, et al., 2002). One of the primary objectives of drilling at this site was to obtain a good quality record of the Eocene–Oligocene transition from "greenhouse" to "ice house" Earth conditions. Because of the shallow CCD, Eocene sediments from Site 1218 had relatively low carbonate compared to Oligocene sediments. The carbonate content of the 42- to 34-Ma sediments averages 40 wt% but ranges between 0 and 90 wt%. The lower Oligocene and Eocene sedimentary section was drilled with the extended core barrel (XCB) because burial had lithified sediments of this age and advanced piston coring (APC) was not possible. The sediments drilled with the XCB are thus more disturbed than the overlying lower Oligocene–Holocene APC-cored section. However, a complete sediment column was recovered to 41.2-Ma sediments (287 meters composite depth [mcd]), and only small gaps were uncored between 287 mcd and the 42-Ma basement (11 m deeper).
Site 1219 (Fig. F1; Table T2) was located ~4.6° south of the equator at the time of crust formation (54.5 Ma). At 42 Ma, it was located at ~2.2°S, and the site was carried northward across the equator at ~30 Ma by the Pacific plate. Because the Oligocene and Neogene sediments at Site 1219 were thinner than those at Site 1218, it was possible to APC core to a depth of 224.5 meters below seafloor (mbsf), or an age of ~48.9 Ma (H. Pälike, unpubl. data). The sediment column was completely drilled to a depth of ~245 mbsf (a basal age of ~54.5 Ma). Only one hole was drilled to basement and through the Eocene section because an APC core jammed in the drill bit as the Eocene/Oligocene boundary section was being drilled in Hole 1219B. The site was abandoned because we believed that we could drill a better section that also contained the Paleocene/Eocene boundary section farther north, nearer the paleoequator. Because Site 1219 was APC cored, it was possible to measure paleomagnetic vectors and determine a paleomagnetic chronostratigraphy to constrain age models within the Eocene. In addition, the similarity of sedimentation between Sites 1218 and 1219 made it possible to develop a precise stratigraphic correlation between the two sites and allowed the development of a common timescale (H. Pälike, unpubl. data). The Eocene sediment section is primarily radiolarian ooze, but intervals of carbonate deposition are present in the upper and middle Eocene sediments (see "Results").
Site 1220 (Fig. F1) was located at the equator at 40 Ma but on deeper and older crust than at Site 1218. The basal sediment age is just slightly older than the Paleocene/Eocene boundary, or just older than 55 Ma. The sediment column is 200 m thick and was APC cored to a depth of 152 mbsf (46.3 Ma). The seafloor at Site 1220 is 200 m deeper than that at Site 1219, but the offset was mostly due to a thicker Oligocene and lower Miocene sediment section at Site 1219. The depth offset between the two sites at 40 Ma was 80 m. We expected to find carbonates within the same middle Eocene intervals noted at Site 1219 because Site 1220 was closer to the Eocene equator than Site 1219 and should have experienced higher biogenic sedimentation. We were surprised to discover that the entire middle and upper Eocene section of Site 1220 was barren of CaCO3 (Lyle, Wilson, Janecek, et al., 2002).
We calculated paleopositions reported in Tables T1 and T2 using the hotspot-Pacific plate motion model used in the Leg 199 Initial Reports volume (Lyle, Wilson, Janecek, et al., 2002; see the "Supplementary Materials" contents list). We checked this model against the best new piece of information from Leg 199—the crossing of the equator observed at Site 1219. Paleomagnetic inclination, magnetization intensity, and sedimentation rates all indicate that the equatorial crossing occurred when sediments at 130 mcd ± 10 m in the sediment column were deposited. This translates to an age of 29.5 Ma for the equatorial crossing, with a range between 29.0 and 30.4 Ma. The fixed-hotspot estimation predicts an equator crossing at the same time and thus appears to be adequate to estimate paleopositions in the central and eastern Pacific through the Eocene.
Sedimentation rate is highly dependent upon latitude and has a strong peak at the equator from the Oligocence to the Holocene because of biogenic deposition from the upwelling system (Murray et al., 1993; Pisias et al., 1995; Lyle, 2003; Rea and Lyle, 2005). Because of peak productivity and Corg particulate rain at the equator (Honjo et al., 1995), there should also be a minimum in magnetic intensity. Provided that iron-bearing minerals are uniformly supplied to sediments at a relatively low rate and that the Corg particulate rain is strongly dependent on proximity to the equator, magnetization should show a minimum at the equator (Karlin et al., 1987; Musgrave et al., 1993).
These indicators show that the position difference for the equatorial region from the Site 1219 lower Oligocene sediments compared to the fixed hotspot approximation estimate is <1°. Tarduno et al. (2003) showed that the Hawaiian hotspot moved significantly before 47 Ma but argue that the hotspot became fixed at about that time. Estimated positions by the fixed hotspot model before 47 Ma thus may be offset to the south of their actual position because of the overcompensated movement of the Pacific plate. The moving hotspot problem should have minimal effect on our estimated positions because we report sediments between the ages of 49 and 33 Ma. Moore et al. (2004) reached a similar conclusion about equatorial position based on sedimentation rate patterns from the entire DSDP and ODP set of drill sites, recalibrated using Leg 199 chronostratigraphy.