Fundamental to all paleoceanographic studies are good age models to determine rates of change and age order of events at widely separate sites. The primary need of most paleoceanographic drilling legs is to tie newly recovered sediment columns into an accurate chronostratigraphy. The problem is compounded in the early Cenozoic because Paleogene reference chronostratigraphies are still in a reconnaissance stage. The early Cenozoic, until recently, has lacked recovery of good high-resolution continuous sediment sections on which orbital variations, paleomagnetic variation, and biostratigraphy can be measured. Postcruise studies of Leg 199 sediments have fundamentally advanced both our understanding of chronostratigraphy in the Paleogene tropical regions and our ability to intercalibrate different stratigraphic events by studying the continuous sections of lower Miocene through middle Eocene sediments recovered during the leg.
Shipboard scientists were able to develop a continuous magnetostratigraphy from the Pleistocene to the middle Eocene on Leg 199 sediments (Lyle, Wilson, Janecek, et al., 2002; Lanci et al., 2004, 2005; Parés and Lanci, 2004). With this new paleomagnetic record it has been possible to intercalibrate paleomagnetic stratigraphy with equatorial Pacific biostratigraphy, cyclostratigraphy, and Paleogene isotope stratigraphy. Development of the continuous paleomagnetic record was possible because relatively undisturbed sediments were recovered during Leg 199 that had not been diagenetically altered to lose their initial magnetic signal. The equatorial sediment bulge has moved north over time, so relatively old sediment sections were not deeply buried, cemented, or strongly overprinted with diagenetic chemical signals. It was therefore possible to core Paleogene sections with the advanced piston corer and recover relatively undisturbed sediments amenable to paleomagnetic analysis. In addition, the sediments have remained oxidized throughout their history and preserve a good paleomagnetic signal.
Shipboard paleomagnetic measurements were good, making it easy to correlate between cores and assemble a shipboard paleomagnetic stratigraphy. However, postcruise U-channel studies significantly sharpened the record and allowed the search for short magnetic polarity reversals (Fig. F4) in the Miocene and Oligocene using records from Sites 1218 and 1219 (Lanci et al., 2004, 2005). Parés and Lanci (2004) continued the record on into the Eocene using Site 1220.
The high resolution of the U-channel paleomagnetic record allowed the search for short magnetic polarity changes, termed cryptochrons by Cande and Kent (1992, 1995). Interestingly, the high-resolution records from Sites 1218 and 1219 identified five short polarity reversal events but none that were of the age suggested by Cande and Kent (1995). Lanci et al. (2004, 2005) thus conclude that if the Cande-Kent cryptochrons exist, they must have lasted <5 k.y., the resolution of the Leg 199 records.
Both the shipboard and improved shore-based paleomagnetic stratigraphy provided the basis for recalibration of other stratigraphic datum levels and provided an initial age model for higher resolution cyclostratigraphic studies.
Leg 199 provided the first intercalibration of paleomagnetic and radiolarian biostratigraphy for the entire Paleogene (Nigrini et al., this volume), and provided the first Oligocene low-latitude diatom biostratigraphy (also paleomagnetically intercalibrated; Barron et al., this volume). Recovery and analysis of upper Paleocene–lower Eocene carbonates also proved important to understand nannofossil biostratigraphy at the Paleocene/Eocene boundary (Raffi et al., 2005).
Cenozoic tropical radiolarian stratigraphy was for the first time intercalibrated with paleomagnetic chronostratigraphy to radically revise zone boundary ages (Fig. F5) (Nigrini et al., this volume). Three different sites (1218, 1219, and 1220) were intercalibrated in age using paleomagnetic reversals and distinctive cyclical variations in physical properties. Biostratigraphic datum levels were established by their occurrence at the three different sites. Ages for 305 morphological first and last occurrences were established using paleomagnetics and cyclostratigraphy of the common data set. Ages for the established radiolarian zonation were then adjusted based on the new data. These new data allowed Moore et al. (2004) to develop modern age models for earlier Deep Sea Drilling Project (DSDP) drilling in the equatorial Pacific and explore changes in equatorial sedimentation through the Cenozoic.
Funakawa et al. (this volume, 2006) also studied radiolarian biostratigraphic datum levels in more detail around the Eocene/Oligocene boundary at Sites 1218, 1219, and 1220. Kamikuri et al. (this volume) performed detailed radiolarian counts around the Oligocene/Miocene boundary at Site 1219. Funakawa et al. (2006) note that the Eocene/Oligocene boundary is marked by large faunal turnovers and a large increase in cosmopolitan (probably cool water) forms. However, other radiolarian turnovers occurred in the late Eocene and early Oligocene. The E/O boundary appears to be the largest of several important events in this interval. Kamikuri et al. (this volume) note high variability in abundances of individual radiolarian species in the late Oligocene and early Miocene. These results suggest that important environmental information can be extracted with further study.
Similarly, the high-quality paleomagnetic stratigraphy of Site 1220 was used to intercalibrate low-latitude diatom stratigraphy for the interval between 33.5 and 21.5 Ma (Oligocene to early Miocene) (Barron et al., this volume). Prior to Leg 199 the diatom datums had been calibrated to nannofossils, which were in turn calibrated to the paleomagnetic age model of Cande and Kent (1995). More than 35 new age estimates of diatom datum levels were assigned in this study.
Site 1219 provided an opportunity to establish an Eocene–Oligocene low-latitude silicoflagellate biostratigraphy (Engel and McCartney, this volume; McCartney et al., this volume). Leg 199 was the first leg to study equatorial Eocene silicoflagellate biostratigraphy and was one of five that have investigated the Oligocene equatorial region. Takata and Nomura (this volume) also gathered new data on the abundance of benthic foraminifers in the Oligocene equatorial Pacific.
Detailed biostratigraphic studies across the Paleocene/Eocene boundary for benthic foraminifers (Nomura and Takata, this volume) and calcareous nannofossils (Raffi et al., 2005) add important new information to our understanding of equatorial biostratigraphy and bioevents around this important stratigraphic boundary. Nomura and Takata (this volume) counted time series of benthic foraminiferal assemblages across the Paleocene/Eocene boundary at Sites 1215, 1220, and 1221. In this preliminary study, they found that the last occurrence of Paleocene benthic foraminifers at Site 1221 occurs ~30 cm below the location of the carbon isotope event in the same core, rather than coincidental with it. They also found high abundances of benthic foraminifers in the two sites (1220 and 1221) near the Paleocene/Eocene equatorial position.
Raffi et al. (2005) conducted a detailed study of the Paleocene/early Eocene boundary primarily from Site 1215 to determine biostratigraphy, biochronology, and evolutionary history of nannofossil assemblages across this important event in the central equatorial Pacific. Using carbonate cycles at Site 1215 they established a floating cyclostratigraphy (i.e., relative to the first appearance of Tribrachiatus bramletti) assuming that the carbonate cycles correlate to orbital variations in solar insolation. They then compared P/E intervals between the low-latitude Pacific and Atlantic Oceans and found almost identical sequences of evolutionary change in the two interconnected oceans.
Cyclostratigraphic studies are extremely important to develop Cenozoic age models. These types of correlative stratigraphy allow development of high-resolution age models and better study of the timing of important paleoceanographic events. Pälike et al. (this volume) describe the first step in this process for Leg 199, development of an integrated stratigraphic correlation using Sites 1218 and 1219 and improving the spliced records at each site by revising the composite depth scale and stacking physical property records.
Better timescales than the shipboard age models based on magneto- and biostratigraphy are now being constructed by identifying cyclic variations of carbonate and correlating the cycles to age models of orbital variations. For example, Wade and Pälike (2004) correlated the refined Leg 199 physical property data with recently developed Paleogene orbital insolation models (Laskar et al., 2004) to develop an age model for the Site 1218 record between 26.4 and 30 Ma. They combined the new age model with stable oxygen and carbon isotope data reported in Wade and Pälike (this volume) to better date Oligocene glaciations and to investigate their relationship to longer period insolation variability. Pälike et al. (submitted [N1]) extended this correlation to the entire Oligocene using Site 1218 as the Oligocene reference section.
Cyclostratigraphy also helps to discriminate between alternate solutions for the evolution of planetary movements in the solar system (Pälike et al., 2004). Data from Sites 926, 929, and 1218 were used to search for a chaotic change from a 2:1 resonance to a 1:1 resonance for the 2.4- and 1.2–m.y. eccentricity modulations. One model (Laskar et al., 1993) predicted such a change some time after 30 Ma, whereas a newer model (Laskar et al., 2004) did not. The sedimentary records showed no evidence of such a change in signal since the Eocene/Oligocene boundary. Although this finding does not prove the Laskar et al. (2004) model, it is strong evidence that the Laskar et al. (1993) model is incomplete.
It is well known that the Neogene equatorial Pacific has a distinctive pattern of seismic horizons that extend for >1000 km (Mayer et al., 1985; Bloomer et al., 1995). These horizons mark large-scale changes in carbonate deposition (Mayer et al., 1985, 1986), and they extend over such large distances because of the size of the equatorial Pacific biome and the related scale over which coherent paleoceanographic changes occur. An example of the fine-scale coherence over large distances is the submeter correlation in physical properties that was possible between Sites 1218 and 1219, separated by ~740 km (Pälike et al., this volume).
Lyle et al. (2002) showed that a distinctive seismic stratigraphy also exists for the Paleogene equatorial Pacific. Several Leg 199 researchers are working to tie seismic horizons to sedimentary horizons so that they may be used to study regional sedimentology (e.g., Mitchell and Lyle, 2005). The first-order problem is to compare a seismic response, measured in two-way traveltime, to a change in physical properties measured in depth below seafloor. Rea and Gaillot (this volume) explored the problem of expansion of the sedimentary section caused by drilling and carrying the sediments from high subseafloor confining pressures to the ocean surface. They developed a new methodology to estimate rebound based on comparing thicknesses of distinctive density (or other physical properties) features between the logged interval and the multisensor track (MST) splice. This allows shipboard physical property measurements to be corrected for changes caused by depressurization of the sediment column.
Busch et al. (this volume) assessed the ability of shipboard measurements to predict acoustic impedance, needed for determining the seismic reflection profile. Their work was hampered by the lack of adequate velocity data from downhole logging. Corrections to produce in situ velocity from shipboard measurements were thus based on published relationships. Nevertheless, density variations proved to be very good predictors of acoustic impedance for seismic stratigraphy applications. For the equatorial Pacific, where carbonate content is a good predictor of density (e.g., Mayer, 1991), seismic reflection profiles can be used to estimate subsurface carbonate content.
Vanden Berg and Jarrard (2004, this volume) also showed how sediment composition could be estimated from physical property data from cores and used light absorption spectroscopy (Vanden Berg and Jarrard, 2002) to develop detailed downcore records of mineralogy. These records use more physical property information than just density but demonstrate two concepts—how mineralogical records of moderate accuracy can be quickly generated for a basic lithostratigraphy and how much of the mineralogical information is encoded in the physical properties of the bulk sediments. Vanden Berg and Jarrard (this volume) also found that density is always the strongest predictor for carbonate in their stepwise regressions.