One of the important objectives of Leg 199 was to investigate the structure of the Paleocene/Eocene boundary in the pelagic Pacific, to compare with other boundary sections worldwide. Such comparisons have now led to a hypothesis that deep circulation reversed at the P/E boundary during the PETM event (Nuņes and Norris, 2006).

P/E boundary sections were sampled at three of the Leg 199 drill sites (Sites 1215, 1220, and 1221; Lyle, Wilson, Janecek, et al., 2002), all of which are notable for high Mn contents, partly because all the sections were collected from basal sediment sections near ocean basement, where it is common to find deposition from mid-ocean-ridge hydrothermal plumes (Ruhlin and Owen, 1986; Lyle et al., 1987). Site 1215, at ~12°N when basement was formed, has >5% Mn in the basal section (Knoop, this volume) and Mn/Fe near the hydrothermal ratio for modern East Pacific Rise sediments (0.3) (Dymond, 1981). Site 1216 also shows evidence of enrichment with hydrothermal sediments in the lower sediment column (Ito et al., this volume).

Sites 1220 and 1221, the two sites near the equator at the P/E boundary, show evidence for significant remobilization of Mn after deposition but no evidence that the Mn escapes the sediments. In the middle of a multicolored P/E boundary section at both Sites 1220 and 1221 there lies a black Mn oxide–rich sediment containing as much as 14.6% Mn and having Mn/Fe as high as 3.7 (Fig. F10) (Lyle, Wilson, Janecek, et al., 2002; Knoop, this volume). Such high Mn contents are similar to sediments from Guatemala Basin, which have high Mn inputs but are also subject to suboxic diagenesis (Finney et al., 1988). Under anoxic conditions, Mn is almost quantitatively removed from the sediments. The retention of high amounts of Mn oxide is strong evidence that the bottom waters and the uppermost sediments always contained oxygen. If bottom waters had become depleted in oxygen, the Mn oxides would not have been trapped.

There are other unusual but consistent geochemical signatures of the two P/E boundaries from Sites 1220 and 1221 (Lyle, Wilson, Janecek, et al., 2002; Faul and Paytan, this volume). Because the two sites are >200 km apart, the geochemical patterns must result from regional, not local, trends. Both P/E boundary intervals are marked by two high-Ba intervals, which Faul and Paytan (this volume) showed at Site 1221 to be primarily composed of barite.

Bains et al. (2000) previously reported high Ba in the P/E interval of two sites, one from the Blake Nose in the North Atlantic (Site 1051) and one from Maude Rise (Site 690) in the Atlantic sector of the Southern Ocean. They attributed the Ba peaks to paleoproductivity. Faul and Paytan (this volume) investigated Site 1221 and found that the Ba peaks consisted of barite and that the barite crystals resembled those derived from biogenic processes, not hydrothermal barite (observed near some marine hydrothermal vents). However, Faul and Paytan (this volume) and Murphy et al. (this volume) found that other elements associated with biogenic rain to the sediment were not as enriched as Ba. There was a P enrichment associated with the barite peaks (Faul and Paytan, this volume) but evidence of significant P loss from the sediments and more loss of P in the warmer conditions later in the boundary event. Murphy et al. (this volume) found no change in Corg or biogenic Si content across the Site 1221 P/E boundary on splits of samples from Faul and Paytan (this volume). Murphy et al. (this volume) attribute the difference between the Corg and Ba signal to metabolic burndown of the Corg.

Nuņes and Norris (this volume, 2006) investigated the change in stable isotope composition across the PETM at Sites 1220 and 1221 as well as 12 other P/E boundary sections around the world. From the distribution of carbon isotopes they hypothesize a switch from Southern Hemisphere deepwater sources to Northern Hemisphere sources during the excursion. Such a switch, if confirmed by further analyses in boundary sections, suggests an important way to quickly warm up the deep oceans. High-latitude northern sources of deep water were probably warmer than Antarctic sources, so provided a way to pump heat downward and provide a huge thermal reservoir to maintain global warmth.

The rapid change in both deepwater temperatures and global surface temperatures created terminal stresses for a variety of organisms—the largest benthic extinction of foraminifers in the Cenozoic is not associated with the Cretaceous/Tertiary boundary, for example, but with the PETM (Thomas, 2003). Large extinctions of marine plankton also mark the boundary. These ecological studies of the boundary sections from Leg 199 are only beginning, but important results are already surfacing. For example, Nomura and Takata (this volume) note that the extinction level of Paleocene benthic foraminifers is 30 cm below the carbon isotope excursion characteristic of the PETM, as found by Nuņes and Norris (this volume), suggesting that significant environmental changes led the postulated methane hydrate release that caused the carbon isotope event. The good P/E boundary section at Site 1215, combined with data from other P/E boundary intervals, allowed Raffi et al. (2005) to begin to explore the evolution of the calcareous nannoplankton from a biogeographic perspective. They find that some evolutionary events at the boundary are associated with the equatorial Atlantic, Tethys, and eastern Pacific (Rhomboaster spp.–Discoaster araneus association), some events appear global (loss of diversification of the genus Fasciculithus), while some are restricted to temperate latitudes (occurrence of Zygrhablithus bijugatus).

Leg 199 has clearly added important new P/E boundary sections to the growing global collection. The combined isotope, evolutionary, biogeographic, and paleoceanographic records are reaching a new level for which to understand this important global transient, and furthermore to get a better understanding of global Earth systems during extreme warmth.