Biological productivity in the equatorial Pacific Ocean is a significant part of the Holocene carbon cycle and likely was a major component of this cycle throughout the geologic past. Because the volume of the Pacific Ocean has shrunk from the early Cenozoic to modern conditions, the influence of the Pacific equatorial region on the carbon cycle was probably much greater in the Eocene than the present. Processes in the modern equatorial Pacific upwelling region are an important part of the global carbon cycle (e.g., Hammond et al., 1996) and this was probably true throughout the Cenozoic. This region is nearly equal in size to the Pacific continental margins (~1.1 x 1013 m2) for the area bounded by 5°N to 5°S latitude and 90°–180°W longitude (Chavez and Barber, 1987), and carbon respiration rates in the Pacific upwelling region are equal to ~25% of the Pacific margin area (shelf + slope). Two primary factors (sedimentation rates and oxygen exposure time) account for the fact that the continental margins are the location of 95% of the total oceanic Corg mass accumulation rate (MAR) (Gershanovich et al., 1974; Hedges and Kiel, 1995). The higher Corg MAR is a consequence of much higher sedimentation rates and lower oxygen exposure times (Hartnett et al., 1998) for margin sediments relative to pelagic regimes.
The Pacific Ocean has been the largest ocean basin throughout the Cenozoic, and the Pacific equatorial upwelling region was increasingly larger further back in time. Therefore, it is reasonable to expect that throughout the Eocene, the Pacific equatorial upwelling region could have exerted a much greater influence on the global carbon cycle, nutrient cycling, and climate change than it does today.
This paper examines the organic carbon and biogenic barium (Bio-Ba) sedimentation history for the Eocene at two sites in the equatorial Pacific drilled during Ocean Drilling Program (ODP) Leg 199. During Leg 199, a transect was drilled across the Eocene equatorial region following 56-Ma crust, with one 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 cruise was to capture a latitudinal transect at the Paleocene/Eocene boundary, and the combination of sites on younger and older crust for the middle and late Eocene provided an opportunity to study the sedimentation history in detail after 42 Ma. Detailed site descriptions are in Shipboard Scientific Party (2002) and Lyle et al. (this volume). Paleoposition calculations, based on a fixed hotspot model, are reported in detail in Lyle et al. (this volume).
Organic carbon analyses from our laboratory focused on the Eocene and constitute a time series between 33.8 and 54.5 Ma. The behavior of barium and other elements (Shipboard Scientific Party, 2002) was examined to provide a perspective of the sedimentation history that spanned most of the Cenozoic. Together, Sites 1218 and 1219 span a paleolatitude and paleolongitude history for a large region of the equatorial Pacific—from 9°N to 5°S and 104°–135°W. This area, 1500 km2, represents one-third of the total modern upwelling region in the Pacific. Organic carbon data are reported at a resolution as small as 10 k.y. for the paleogeographic area bounded by 1°N to 5°S latitude and 104°–114°W longitude, an area representing ~7% of the modern Pacific upwelling region.