There is a distinct maximum in hiatuses in the deep-sea sedimentary record within the upper Eocene-lower Oligocene interval [Moore et al., 1978]. This is one of several breaks in the stratigraphic record of deep-sea sediments. The presence of these hiatuses has generally been attributed to the dissolution rate of the carbonate and siliceous microfossils in deep-sea pelagic sediments being greater than their supply rate to the seafloor; thus, temporal variation in the presence of hiatuses was caused by variations in these two rates. In some sites, erosion/nondeposition near boundary currents and flow around large bathymetric features may have also played a role. As recovery of sediments has improved and the stratigraphy of Paleogene sections have become more finely resolved during the Ocean Drilling Program (ODP) and the Integrated Ocean Drilling Program (IODP), this peak in a missing section has been narrowed down to the Eocene-Oligocene (E/O) boundary region itself, the boundary that marks the rather abrupt shift from the “hothouse” world to the “icehouse” world. Of the 127 sites drilled in the Pacific basin proper, sites that penetrated below the E/O boundary and contained fossiliferous sediments, only 25 actually captured the upper Eocene-lower Oligocene boundary in a core (Table S1 in the Auxiliary Material). The boundary appeared to be between cores in 40 of these sites [Moore, 1972]. The 62 remaining sites contained a hiatus at the E/O boundary, sometimes representing a lacuna of millions of years or more and sometimes only a few hundred thousand years. Only where detailed sampling and high-resolution lithostratigraphy or isotope stratigraphy were performed (e.g., ODP Site 1219) could these rather small hiatuses be detected. Thus, there are likely to be small breaks in the stratigraphic record in at least some of the 22 cores that captured the E/O boundary but for which such detailed stratigraphy is not available.
 There have been only three tropical Pacific sites in which a complete section across the E/O boundary appears to have been recovered (Table S1 in the Auxiliary Material). The first site, ODP Site 1218, shows the characteristic “two-step” shift in all major lithostratigraphic and isotopic changes across the boundary [Coxall et al., 2005; Coxall and Pearson, 2007; Dunkley Jones et al., 2008; Lear et al., 2008], which was linked to a two-phase development of the Antarctic ice sheets [DeConto and Pollard, 2003]. More recently, two more sites that recovered very similar sections in the near-equatorial region of the tropical Pacific have been cored: IODP Sites U1333 and U1334 [Pälike et al., 2010] (Figure 1).
 Work on these three sites has revealed the multifaceted problems in resolving the E/O boundary, even when using paleomagnetic stratigraphy, as well as the more highly resolved biostratigraphies now available. First, the paleomagnetic stratigraphy indicates that the E/O boundary lies just below the base of Chron C13n, which coincides with the base of the second (youngest) step in lithologic and isotopic changes [Dunkley Jones et al., 2008; Coxall and Wilson, 2011]. The boundary itself is contained within the uppermost part of Chron C13r [Coxall et al., 2005]. There are marked changes in the microfossil assemblages across the E/O boundary [Funakawa et al., 2006; Dunkley Jones et al., 2008; Wade and Pearson, 2008], so it should be easy to precisely locate this boundary. But here we run into some difficulty. Preservation of carbonate is generally poor in the Eocene deep-sea sediments of the Pacific, so Eocene carbonate microfossils are generally rare, poorly preserved, or absent altogether. In the deep Pacific basin, you are just as safe assuming that if the sediments are carbonate rich, they are Oligocene; if they are siliceous clays, they are Eocene. But exactly where in the Eocene?
 This problem could be resolved with the radiolarian microfossils as their stratigraphy in the E/O boundary region has been greatly improved by the work of Nigrini et al.  using material from ODP Leg 199. The material studied by Nigrini et al.  included Site 1218 as well as Sites 1219 and 1220. These latter sites unfortunately do not have complete E/O boundary sections. This causes some uncertainty in the placement of datum levels, which in turn causes some uncertainty in defining the E/O boundary with radiolarian stratigraphy. This uncertainty is exacerbated by the apparent reworking of older radiolarian species, many of which became extinct somewhere near the E/O boundary [Funakawa et al., 2006; Moore and Kamikuri, 2012]. The plethora of reworked radiolarians found near the E/O boundary is an almost unique event in the Cenozoic stratigraphy of the tropical Pacific. The only other interval in which such extensive reworking is commonly found is the uppermost part of the section on the northern flank of the equatorial Pacific biogenic sediment mound [Moore et al., 2012].
 The pervasive reworking of the upper Eocene radiolarians was investigated by Moore and Kamikuri  in a detailed sampling of ODP Site 1218 as well as IODP Sites U1333 and U1334 (Figure 1 and Table 1), which also recovered stratigraphically complete E/O boundary sections. In their study, they identified Site U1334 as generally having the deepest (oldest) last appearance datums (LAD) for most of the stratigraphically important Eocene species. Furthermore, the older reworked radiolarians in the upper Eocene section were usually lower in abundance in U1334 than in either 1218 or U1333. Using Site U1334 as a guide and looking for discontinuous appearances of the older Eocene radiolarians that became extinct near the E/O boundary, Moore and Kamikuri were able to quantify the number of older reworked radiolarians in samples from all three sites.
|Site||Latitude||Longitude||Water Depth (m)||Section Thickness Pre-E/O (m)||Basement Relief (m)||Basement Age (Ma)||Oldest Reworked Radiolarian LAD (Ma)|
|1218||8°53.378′N||135°22′W||4828||58||370||42.3||P. trachoides (41.23)|
|U1333||10°30.996′N||138°25.159′W||4853||66||360||45.8||E. lagena (43.05)|
|U1334||7°59.998′N||131°58.408′W||4799||36||395||~38||P. chalara (38.74)|
 In this paper, we look at the record of reworked older radiolarians in Sites 1218, U1333, and U1334 and compare their changes in abundance through the interval from ~30 to ~40 Ma (i.e., the E/O boundary interval as used herein). We then try to relate these changes to possible paleoceanographic changes that might have led to this reworking as well as to the widespread presence of hiatuses within this boundary interval. As used herein, the E/O boundary zone spans the two-step change seen in isotopic and lithologic data.