The eastern equatorial Pacific (EEP) is one of the most dynamic regions of the open ocean. To fully appreciate the history of this area in the time domain, correlative and complete sedimentary records are required from multiple drill sites. One essential step for each site is the construction of an accurate composite depth scale, whereby selected intervals of successive cores from proximal holes are spliced together to render a full stratigraphic section. Here, we generate revised composite depth scales for Integrated Ocean Drilling Program (IODP) Sites U1336, U1337, and U1338, recovered during IODP Expeditions 320 and 321. Composite sections were generated using physical properties data overlain on high-resolution scanned images of adjacent core sections from all holes cored at a site. Coring disturbance, particularly deeper in the holes, prevented composite construction to total depth at each site. At Site U1336, utilizing two holes, the composite record reaches almost 135 m core composite depth below seafloor (CCSF). At Site U1337, with four holes, a depth of close to 450 m CCSF was reached with only three gaps. Using the three holes of Site U1338, a composite section of almost 400 m CCSF was developed with only two breaks. Composite depth records are crucial for working on these sites because sediment composition varies considerably over short (<30 cm) depth intervals. The composite gamma ray attenuation density records will be particularly important to a range of studies in the region because they can be coupled to those collected at earlier drill sites in the EEP.
To reconstruct the climate history of the equatorial Pacific, one major objective of the Pacific Equatorial Age Transect (PEAT) program is to compile a Cenozoic megasplice that integrates all available bio-, chemo-, and magnetostratigraphic data including key records from Ocean Drilling Program (ODP) Leg 199. In order to do so, extended postcruise refinements of the shipboard composite depth scales and composite records are required. Here, we present a revised depth scale of Integrated Ocean Drilling Program (IODP) Expedition 320 Sites U1331, U1332, U1333, and U1334 as well as Leg 199 Sites 1218, 1219, and 1220. The revised composite records were used to perform site-to-site correlation and integration of Leg 199 and Expedition 320 sites. Based on this decimeter-scale correlation, a high-resolution integrated paleomagnetic, calcareous nannofossil, and radiolarian stratigraphy for the equatorial Pacific is established that covers the time from 20 to 40 Ma. This sedimentary compendium from the equatorial Pacific will be the backbone for paleoceanographic reconstructions for the late Paleogene.
Integrated Ocean Drilling Program Expedition 320/321, “Pacific Equatorial Age Transect” (Sites U1331–U1338), was designed to recover a continuous Cenozoic record of the equatorial Pacific by coring above the paleoposition of the Equator at successive crustal ages on the Pacific plate. These sediments record the evolution of the equatorial climate system throughout the Cenozoic. As we gained more information about the past movement of plates and when in Earth’s history “critical” climate events took place, it became possible to drill an age transect (“flow-line”) along the position of the paleoequator in the Pacific, targeting important time slices where the sedimentary archive allows us to reconstruct past climatic and tectonic conditions. The Pacific Equatorial Age Transect (PEAT) program cored eight sites from the sediment surface to basement, with basalt aged between 53 and 18 Ma, covering the time period following maximum Cenozoic warmth, through initial major glaciations, to today. The PEAT program allows the reconstruction of extreme changes of the calcium carbonate compensation depth (CCD) across major geological boundaries during the last 53 m.y. A very shallow CCD during most of the Paleogene makes it difficult to obtain well-preserved carbonate sediments during these stratigraphic intervals, but Expedition 320 recovered a unique sedimentary biogenic sediment archive for time periods just after the Paleocene/Eocene boundary event, the Eocene cooling, the Eocene–Oligocene transition, the “one cold pole” Oligocene, the Oligocene–Miocene transition, and the middle Miocene cooling. Expedition 321, the second part of the PEAT program, recovered sediments from the time period roughly from 25 Ma forward, including sediments crossing the Oligocene/Miocene boundary and two major Neogene equatorial Pacific sediment sections. Together with older Deep Sea Drilling Project and Ocean Drilling Program drilling in the equatorial Pacific, we can delineate the position of the paleoequator and variations in sediment thickness from ~150°W to 110°W longitude.