STRATIGRAPHY

The purpose of this chapter is to point the readers to the publications that significantly complement shipboard biostratigraphic information contained in the Leg 202 Initial Reports volume (Mix, Tiedemann, Blum, et al., 2003) rather than discussing the details of postcruise stratigraphic work.

Studies on calcareous nannofossil biostratigraphy (Flores et al., this volume) and radiolarian stratigraphy (Weber and Pisias, this volume) enhanced the Miocene–Pleistocene zonation of Leg 202 sites. Stable isotope records in combination with orbitally tuned cyclostratigraphy refined the Pleistocene stratigraphy at Sites 1233 and 1234 (Lamy et al., 2004; Heusser et al., 2006a); the Miocene–Pliocene stratigraphy at Sites 1236, 1237, 1239, and 1241 (Tiedemann et al., this volume; Holbourn et al., 2005); and the Oligocene stratigraphy around the late Oligocene climate optimum at Site 1237 (Flower and Chishom, this volume).

The stratigraphies at Sites 1233 and 1237 deserve closer attention, as they rank among the best hemipelagic and pelagic reference sections from the South Pacific. Site 1237 provided a complete pelagic Oligocene (~31 Ma) to Holocene sediment sequence that was relatively unmodified by burial diagenesis and was fully recovered using the advanced piston corer (APC). Good preservation of calcareous microfossils to the base of the Oligocene, the late Miocene–Holocene presence of siliceous microfossils, and a nearly complete magnetostratigraphy provided an excellent basis for postcruise stratigraphic refinements. Although the completion of a high-resolution upper Paleogene and Neogene stratigraphy at Site 1237 will take several more years, substantial progress has been made within particular time intervals. The work of Flores et al. (this volume) significantly improves the Pleistocene calcareous nannofossil biostratigraphy by confining 11 events of key biostratigraphic marker species using a sampling resolution of 10–30 cm for the last 0.5 m.y. and 75–150 cm for the time interval from 0.5 to 2.0 Ma. Weber and Pisias (this volume) present a first radiolarian biostratigraphic framework for the time interval of the last 11.5 m.y. that is consistent with the biostratigraphic zonation of Sanfilippo and Nigrini (1998) and Moore (1995). The ages for key radiolarian datums are derived from shipboard magnetostratigraphy using the age assignments of Cande and Kent (1995).

The goal to establish a complete stable isotope chronostratigraphy for Site 1237 represents a joint effort of various groups and has not yet been completed. So far, high-resolution benthic stable isotope records in combination with orbitally tuned cyclostratigraphy provide detailed age control for the time intervals from 2.0 to 6.0 Ma (Tiedemann et al., this volume) and 12.7 to 14.7 Ma (Holbourn et al., 2005), although sedimentation rates were relatively low (1–3 cm/k.y.). Flower and Chisholm, this volume) established a benthic isotope record for the time interval from 25.2 to 27.3 Ma.

Figure F4A provides an example of integrating bio-, magneto-, and isotope stratigraphy on the basis of an orbitally tuned timescale for the time interval 2–6 Ma. Tiedemann et al. (this volume) generated an orbitally tuned timescale by correlating the high-frequency variations (precession- and obliquity-related cycles) in GRA density and benthic ¹³C and ¹⁸O to the orbital solution of Laskar et al. (1993). The orbitally derived ages of Pliocene magnetic reversal boundaries between the base of the Réunion and the top of the Thvera coincide with those of the ATNTS2004 timescale (Lourens et al., 2004), except for the top of Kaena and the base of Sidufjall. At Site 1237, the astronomical age for the top of Kaena is about one obliquity cycle older. The base of Sidufjall appears to be about one precession cycle younger. The astronomically tuned isotope stratigraphy is in agreement with that from Atlantic Site 925/926 (Tiedemann and Franz, 1997; Shackleton and Hall, 1997) but deviates from the orbitally tuned LR04 benthic ¹⁸O stack (0–5.3 Ma) (Lisiecki and Raymo, 2005) prior to MIS Si6 (4.9 Ma). The isotope nomenclature from MIS 96 to Si6, the stage identification and age assignments of recognized oxygen isotope stages are almost identical between the LR04 benthic ¹⁸O stack and the Leg 202 ¹⁸O records from 2.4 to 4.9 Ma. However, the correlation of identical isotope stages is ambiguous in the time interval from 4.9 to 5.3 Ma, which comprises the Thvera Chron, although the age models of Tiedemann et al. (this volume) and Lisiecki and Raymo (2005) provide nearly identical ages for the top and base of Thvera. Within this interval, the LR04 ¹⁸O stack represents an average of five aligned (globally distributed) benthic ¹⁸O records, whereas three of them were aligned to an initially produced transitional high-quality stack created from Sites 846 and 999. The LR04 ¹⁸O stack identifies MIS T7 (~5.05 Ma) as the most pronounced ¹⁸O minimum of the last 5.3 m.y (Fig. F4A). This minimum may correspond to MIS T5 or T3 at Sites 1237 and 1241, which are one or two obliquity cycle(s) younger, respectively. At Sites 1237 and 1241, the ¹⁸O signal-to-noise ratio is relatively low between 4.8 and 5.3 Ma (except for MIS T5 and T3), and thus the benthic ¹⁸O records are far from being an excellent tuning medium. Spectral analyses in the depth domain (Tiedemann et al., this volume) suggest that the Leg 202 records of benthic ¹³C, GRA density, and sand content are better suited for orbital tuning. The benthic ¹³C amplitudes provide a surprising clarity of the 41-k.y. signal across this interval, in contrast to the ¹⁸O records. In addition, the GRA density record from Site 1237 and the sand content record from Site 1241 provide significant variability at precession-related frequencies. Thus, Tiedemann et al. used a multi-proxy tuning approach (taking priority over ¹⁸O) to create the Leg 202 age model for this interval. This may explain the possible deviation between the LR04 ¹⁸O stack and the Leg 202 records, as the orbital tuning of the LR04 ¹⁸O stack is solely based on oxygen isotope variability. The deviation between the two age models becomes evident when comparing the ¹³C records from Leg 202 (Sites 1237 and 1240) with the ¹³C record from Site 846 (Fig. F4B), which was, in addition to that from Site 999, one of the two key records used to create the LR04 ¹⁸O stack. (As the ¹³C record from Site 999 has no composite depth, it is not considered in Fig. F4B). Within the critical time interval from 5.3 to 4.6 Ma, all ¹³C records are well correlated between ~4.9 Ma (MIS ST1) and 4.6 Ma (MIS N8) and vary nearly in phase with orbital obliquity. Prior to ~4.9 Ma, however, the ¹³C record from Site 846 lacks correlation with both orbital obliquity and the Leg 202 ¹³C records. This argues for a possible mismatch between the LR04 ¹⁸O stack and the orbital record prior to 4.9 Ma.

Holbourn et al. (2005) developed an orbitally tuned benthic isotope stratigraphy for the middle Miocene time interval (12.7–14.7 Ma) at Site 1237, which marked the Earth's final transition into an icehouse climate ~13.9 m.y. ago. The new chronology was generated by initially matching the 400- and 100-k.y. amplitude variations in the ¹⁸O series to the latest astronomical solution (Laskar et al., 2004) and then adjusting individual obliquity-scale cycles. The most surprising feature of the ¹⁸O time series is the transition from high-amplitude obliquity-paced variations dominant between 14.7 and 13.9 Ma to eccentricity-paced fluctuations between 13.8 and 13.1 Ma. The timing of this change in ¹⁸O frequency parallels the mid-Miocene expansion of the Antarctic ice sheet and is consistent with a change in the amplitudes of orbital obliquity and eccentricity. Flower and Chishom (this volume) generated a benthic stable isotope record for the late Oligocene time interval from 25.2 to 27.3 Ma and tied the onset of the late Oligocene climate optimum to GTS 2004 (Gradstein et al., 2004) by using isotope data and shipboard magnetostratigraphy. Their results suggest an age of 26.35 Ma for the final decrease in ¹⁸O, marking the initiation of the late Oligocene climate optimum. This climatic shift is closely associated with the LO of the planktonic foraminifer P. opima. These studies demonstrate the potential for completing a high-resolution, orbitally tuned stable isotope chronostratigraphy for the upper Cenozoic and Neogene that in addition includes a tight framework of bio- and magnetostratigraphy at a single site.

Although the 135.7-mcd composite sequence from Site 1233 only spans the time interval of the last 70 k.y., it comprises a time resolution that allows centennial-scale reconstructions of climate variability. This in combination with an excellent and unprecedented high-resolution magnetostratigraphy makes it an outstanding stratigraphic reference section for the South Pacific (Lund et al., this volume a). Comparison of detailed rock magnetic and paleomagnetic records from three different holes at Site 1233 clearly demonstrate reproducible, pervasive, and distinctive centennial- (~150–300 yr) and millennial-scale environmental, climatic, and geomagnetic field variability (Lund et al., this volume b). In addition, more than 20 AMS ¹⁴C age datings led to a detailed age model (Fig. F5) (Lamy et al., 2004; Kaiser et al., 2005; Kaiser, 2005) that suggests extremely high sedimentation rates, ranging between ~1.4 m/k.y. in the Holocene to an average of ~2.2 m/k.y. during MIS 2–4.