BACKGROUND AND PREVIOUS WORK

Major paleoceanographic and climate changes that occurred during the middle Miocene represent a key step in the evolution of the Cenozoic. The middle Miocene ¹⁸O increase has been interpreted to primarily record the intensification of continental glaciation in Antarctica (Shackleton and Kennett, 1975; Savin et al., 1975; Savin and Woodruff, 1990; Woodruff et al., 1981). In this interpretation, cooling of high southern latitude surface waters increased the production of deep and intermediate waters and enhanced vertical stratification throughout the world ocean. Others have proposed a different interpretation in which large ice sheets may have existed prior to the middle Miocene and that the middle Miocene ¹⁸O increase was entirely caused by deep-water cooling, unaccompanied by Antarctic ice growth (Matthews and Poore, 1980; Prentice and Matthews, 1988). Drilling in the Ross Sea region of Antarctica (Barrett et al., 1987) confirmed the existence of intermittent continental ice sheets on Antarctica between the early Oligocene and the early Miocene. In addition, it remains controversial whether the middle Miocene step represented the development of a permanent ice sheet in East Antarctica (Matthews and Poore, 1980; Kennett and Barker, 1990). The relative proportions of ¹⁸O increase attributable to Antarctic ice storage or to bottom-water cooling remain uncertain.

Miocene climate changes may be related to changes in deep-water circulation. Several hypotheses have linked late Pliocene and Pleistocene climate changes (glacial/interglacial intervals) with changing fluxes of North Atlantic Deep Water (NADW) (e.g., Shackleton et al., 1993). The middle Miocene ¹⁸0 increase has been linked to changes in Northern Component Water (NCW) (Schnitker, 1980) or Tethyan water (Woodruff and Savin, 1989). However, there is still much discussion regarding circulation patterns of deep water during the Miocene (see summary in Wright et al., 1992). Planktonic-benthic foraminiferal ¹⁸O covariance at low latitudes associated with major oxygen-isotope events (Miller et al., 1991; Wright and Miller, 1993) suggests that Antarctic ice sheets waxed and waned throughout the early and middle Miocene. More recently, Zachos et al. (1997) have shown a strong 40 ka periodicity in an equatorial oxygen-isotope record, consistent with a high-latitude orbital control on ice volume and temperature.

Miocene paleoceanographic changes were accompanied by major variations in mean ocean ¹³C, involving redistribution between carbon reservoirs (Vincent and Berger, 1985; Miller and Fairbanks, 1985; Kennett, 1986). The mean ¹³C/¹²C ratio is generally controlled by the proportions of carbon deposited as organic carbon vs. calcium carbonate in the deep sea. Assuming that the rate of delivery to the ocean of terrestrial organic carbon, depleted in ¹³C, did not vary greatly during the Cenozoic, a higher mean ¹³C/¹²C ratio reflects an increase in organic carbon storage (e.g., Vincent and Berger, 1985). Two major maxima in mean ocean ¹³C occurred during the late Oligocene-Miocene: the first near the Oligocene/Miocene boundary, ~24 Ma (Zachos et al., 1997), and the second during the late early to middle Miocene from 17 to 13.5 Ma, termed the Monterey Carbon Isotope Excursion (Vincent and Berger, 1985). The Monterey ¹³C maximum has been attributed to the storage of large volumes of organic carbon in the Monterey Formation of California, circum-North Pacific, and the southeastern shelf of the United States, and it is postulated as a major contributor to global cooling through drawdown of atmospheric CO₂ and a series of positive-feedback mechanisms (Vincent and Berger, 1985). Although a time lag between the inception of the Monterey Formation deposition at 17.85 ± 0.1 Ma (DePaolo and Finger, 1991) and major global cooling at 14.8-14.0 Ma represents a difficulty with this hypothesis, it has been proposed that episodic increases in organic carbon burial within the Monterey Formation may have contributed to accelerated atmospheric drawdown of CO₂ and global cooling (Flower and Kennett, 1993a, 1993b). Strong covariance between deep-sea ¹⁸O and ¹³C records from 16 to 13.5 Ma (Woodruff and Savin, 1991) and the ¹⁸O correlation between the Monterey Formation at Naples Beach and the deep-sea record from 14.5 to 14.1 Ma (Flower and Kennett, 1993a) suggest a linkage between organic carbon burial, deep-water cooling, and ice-volume changes.