Our purpose here is not to review the details of postcruise stratigraphic studies, but rather to point readers to the literature that has enhanced the wealth of shipboard stratigraphic information contained in the Leg 177 Initial Reports volume (Gersonde, Hodell, Blum, et al., 1999).
Studies of diatom biostratigraphy by Censarek and Gersonde (2002) and Zielinski and Gersonde (2002) have enhanced the Miocene and Pliocene-Pleistocene zonation of Leg 177 sites. Both studies revealed latitude-dependent differences in the stratigraphic ranges of diatoms, including key biostratigraphic marker species that are related to development and migration of surface-water masses within the ACC during the Neogene and Pleistocene. New biostratigraphic marker species were defined that assist in the identification of marine isotope Stages (MISs) 6 and 8 in the highest-latitude Leg 177 Sites 1093 and 1094 (Zielinski et al., 2002). Shipboard nannofossil biostratigraphy has been revised for the Pleistocene of all seven Leg 177 sites (Flores and Marino, 2002), the Miocene and Pliocene of Sites 1088 and 1090 (Marino and Flores, Chap. 7, this volume), and the middle Eocene to lower Oligocene of Site 1090 (Marino and Flores, Chap. 8, this volume). Galeotti et al. (2002) refined the planktonic foraminiferal stratigraphy of the middle Eocene to lower Pliocene section of Site 1090.
While aboard ship, we used variations in sediment physical properties (mainly weight percent carbonate) in conjunction with biomagnetostratigraphy to predict the position of marine isotope stages (see fig. F11 in Shipboard Scientific Party, 1999, p. 45). Hodell et al. (Chap. 9, this volume) compiled the oxygen isotope stratigraphies for Sites 1088, 1090, 1090, 1093, and 1094 and compared the results to the shipboard color reflectance data (Figs. F3, F4). In all but Site 1089, the shipboard prediction of the position of isotope stages proved to be fairly accurate. Kleiven and Jansen (Chap. 12, this volume) developed oxygen isotope stratigraphies for MISs 18 through 26 (early-mid Pleistocene) at Sites 1091 and 1094. Andersson et al. (2002) identified the major oxygen isotope events of the lower and lower upper Pliocene section of Site 1092 (Fig. F4). Billups et al. (2002) used the benthic oxygen and carbon isotope record at Site 1090 between ~25 and 16 Ma to correlate to the astronomically tuned isotopic signal at Site 929 on the Ceara Rise (Fig. F5) (Shackleton et al., 1999, 2000).
Channell and Stoner (2002) refined the polarity reversal stratigraphy of the high-resolution Pliocene-Pleistocene sections of Sites 1089, 1091, 1093, and 1094 by analyzing discrete samples that underwent complete demagnetization. Two studies have augmented the paleomagnetic results obtained onboard ship on the middle Eocene-lower Miocene section at Sites 1090 and the upper Miocene section of Site 1092 by measuring discreet samples and U-channel samples from the composite sections (Channell et al., in press; Evans and Channell, in press). In addition to geomagnetic polarity reversals, variations in the paleointensity of the Earth's dipole magnetic field are proving useful for long-distance correlation. Studies of Leg 177 site survey piston cores demonstrated that a paleointensity signal could be extracted from South Atlantic sediments (Fig. F6) (Channell et al., 2000; Stoner et al., 2002). One of the objectives of Leg 177 was to extend these records further back in time. To date, a long paleointensity record has been produced from Site 1089 for the past 450 k.y. (Stoner et al., in press). Although some diagenetic alteration of magnetization intensity has occurred downcore, the relative paleointensity signal from Site 1089 can be correlated to similar high-resolution paleointensity records from the North Atlantic and lower-resolution globally stacked composites (Fig. F7). This study demonstrated that together with traditional oxygen isotope stratigraphy, paleointensity is a powerful new tool for optimizing global stratigraphies.
The stratigraphy of the outstanding middle Eocene-lower Miocene section at Site 1090 deserves special mention because it ranks among one of the best deep-sea sections recovered for this time interval. The polarity reversal stratigraphy at Site 1090 is superb and holds much promise for calibrating biostratigraphic and geochemical records to the geomagnetic polarity timescale (GPTS) (Channell et al., in press; Billups et al., 2002). On the basis of calcareous nannofossils, Marino and Flores (2002a) provided evidence for a hiatus or strongly condensed section in the lower Oligocene at Site 1090. Marino and Flores (2002a) placed the Eocene/Oligocene (E/O) boundary at 259 meters composite depth (mcd) and correlated the polarity reversal stratigraphy to the GPTS with the insertion of a cryptochron in C13r. In contrast, planktonic foraminifers point to a continuous record and placement of the E/O boundary at ~269 mcd, although foraminiferal assemblages are poorly preserved across the boundary (Galeotti et al., 2002). On the basis of oxygen isotopic measurements of bulk carbonate and the identification of isotope Event Oi-1 (correlated to Chron 13n) (Zachos et al., 1996), Diekmann et al. (submitted a [N1]) placed the E/O boundary at ~247 mcd and suggested a disconformity in the lower Oligocene. Additional evidence for a lower Oligocene hiatus is provided by multichannel seismic records, which indicate that ~80 m of sediment deposited nearby is missing at Site 1090 (Wildeboer Shut et al., 2002). Channell et al. (in press) provide two alternative interpretations of the polarity reversal stratigraphy at Site 1090 across the Eocene/Oligocene boundary. Option 1 is consistent with the nannofossil biostratigraphy, whereas Option 2 is favored by planktonic foraminiferal stratigraphy. Oxygen and strontium isotope stratigraphy strongly support Option 1, resulting in the identification of a cryptochron within C13r and an apparent hiatus in the early Oligocene affecting the C11n-C11r interval (Channell et al., in press).
Another stratigraphic discrepancy is present below the hiatus at ~70 mcd that removed the lower Pliocene to middle Pliocene section at Site 1090. The section immediately below the hiatus is correlated to Chron C5n near the base of the middle Miocene (Channell et al., in press). This interpretation is supported by strontium, oxygen, and carbon isotope stratigraphy, but is inconsistent with the presence of the planktonic foraminifer Globorotalia sphericomiozea, which ranges from the latest Miocene to earliest Pliocene (Galeotti et al., 2002). It is possible that the section immediately below the hiatus at Site 1090 represents a mixing/reworking zone associated with the unconformity (Channell et al., in press).