STRATIGRAPHY AND CHRONOLOGY

The recovered sequence at Site 1014 provides a continuous Pliocene to Quaternary record of hemipelagic sedimentation underlain by a poorly dated upper Miocene sequence. The upper Quaternary sequence at Site 1014 consists of hemipelagic clay interbedded (every 30 to 120 cm) with nannofossil ooze, with foraminifers throughout (Lyle, Koizumi, Richter, et al., 1997). Bedding contacts are gradational, and the sediments are slightly bioturbated. An initial well-constrained biostratigraphy and chronology are provided by a combination of calcareous nannofossil, planktonic foraminifer, and radiolarian datums for the upper Pliocene and Quaternary (Lyle, Koizumi, Richter, et al., 1997). Further chronology is also provided by the paleomagnetic record through the identification of the Brunhes/Matuyama boundary and the Jaramillo Subchronozone (Lyle, Koizumi, Richter, et al., 1997).

A more detailed chronology is given using the well-established global changes in the relative abundance of stable oxygen isotopes (SPECMAP) (Martinson et al., 1987). The benthic oxygen isotope record mainly represents the global removal and storage of the light oxygen isotope (¹⁶O) from seawater during cool intervals through major ice-sheet accumulation (and thus sea-level change). A ¹⁸O record obtained from the benthic foraminiferal species Uvigerina for the upper 20 mcd of Site 1014 exhibits the familiar sawtooth pattern of the late Quaternary ¹⁸O deep-sea records from marine isotope Stage (MIS) 6 to the present (Fig. 2). Clearly expressed are the glacial maxima (MISs 6 and 2) represented by the highest ¹⁸O values (~4.3), interglacial MISs 5 and 1 by low ¹⁸O values (2.6 to 3.6), and the climate amelioration of MIS 3 and the glacial MIS 4. The last full interglacial marine isotope Substage 5e (Eemian) is clearly recorded as the warmest interval during the last interglacial. The warm Substages 5a and 5c are well pronounced and are separated by the intervening cooler intervals of marine isotope Substages 5b and 5d (Fig. 3). A detailed chronology with at least 20 datums for the last 160 k.y. has been established (Fig. 2) by comparison with a globally averaged record whose fluctuations have been well described (Imbrie et al., 1984; Pisias et al., 1984; Prell et al., 1986) and dated with an average error of ±5 k.y. (Martinson et al., 1987).

Paleoclimatic events at Site 1014 correlated with standard deep-sea oxygen isotope chronology are described in Table 2 (Imbrie et al., 1984). Ages were calculated using linear interpolation between each datum. All stable isotopic values are plotted against kilo years (k.y.) using this standard age model (in standard units of Martinson et al., 1987). Depths of isotopic events identified in this study (Table 2) are graphically correlated (Fig. 3) with the standard deep-sea oxygen isotopic reference section dated by Martinson et al. (1987). In this age vs. depth plot, datums deviate only slightly from a straight line, thus supporting the oxygen isotope stratigraphy for Site 1014. Changes in slope between line segments are interpreted to represent changes in sedimentation rate in the core relative to the reference section. Because these changes in slope are slight, it can be assumed that sedimentation rates were relatively constant. This plot also suggests that there are no significant hiatuses or drilling gaps in the upper 20 mcd at Site 1014. These are predictable results since splicing between the four holes drilled was expected to produce the most complete record at the site. Thus, it is demonstrated that sedimentation rates experienced little change in Tanner Basin during the late Quaternary.