Site 1256 (6°44.2´N, 91°56.1´W) (Fig. F1), drilled during ODP Leg 206, lies in 3635 m water depth in the Guatemala Basin on Cocos plate crust that formed at ~15 Ma on the eastern flank of the East Pacific Rise (EPR) when the site experienced a superfast spreading rate (Wilson, 1996). This site formed at an equatorial latitude within the equatorial high-productivity zone and initially experienced very high sedimentation rates (39.1 m/m.y.) (e.g., Jiang and Wise, this volume). The high core recovery (89% with two-thirds of the cores taken by the advanced piston corer) and high sedimentation rates through critical time intervals provide an excellent opportunity for high-resolution paleoceanographic study.
The modern oceanographic setting in the equatorial world oceans is illustrated in Figure F1. This circulation pattern is a product of tropical atmospheric circulation and the Coriolis effect across the equator. Because the southeastern trade winds are stronger than the northeastern antithesis, they converge north of the equator to form the Intertropical Convergence Zone (ITCZ), a belt with weak winds and heavy rainfall forming a barrier for eolian dust between southern and northern sources (e.g., Rea, 1994; Pettke et al., 2002). As southeastern trade winds blow across the equator, the change in direction of the Coriolis effect causes divergence along the equator that results in a depression in surface topography and creates the pressure gradient that together produce a geostrophic flow (i.e., the South Equatorial Current [SEC]). In the Pacific, the waters flowing in the SEC originate from the Peru-Chile Current (PCC) and Equatorial Undercurrent (EUC) (Kessler, 2002, 2006). The strength of the SEC mimics the strength of the southeastern trade winds, whereas the North Equatorial Current and North Equatorial Countercurrent change intensity in response to the position of northeastern trade winds (Wyrtki, 1974).
The sediments of the eastern equatorial Pacific Ocean are known to be sensitive recorders of oceanographic changes and record a complex interplay of ocean chemistry, productivity, climate, and plate tectonics (e.g., van Andel et al., 1975). The high sedimentation rates are engendered by upwelling-driven, high biological productivity (e.g., van Andel et al., 1975; Murray et al., 1994; Lyle, 2003), which is estimated to contribute 18%–56% to the global new production (Chavez and Barber, 1987) (more recently revised to 26% [Chavez and Toggweiler, 1995]), although only corresponding to 3% of the global ocean area. This upwelling, resulting from the equatorial divergence under the influence of the Coriolis effect, provides major nutrients as well as elemental iron for phytoplankton (Landry et al., 1997), the primary producers in ocean surface waters and major contributors to deep-sea sediments. The bulk nutrients, however, are brought to the equator by the PCC with a shallow-water source (~50 m) (Wyrtki, 1981) of a subantarctic origin. Thus, the primary productivity in this region depends on the intensity of upwelling and the supply of nutrients imported at thermocline depths (e.g., Rea et al., 1991; Weber and Pisias, 1999). The latter, in turn, is controlled by the wind stress along the equator that affects the depth of the thermocline. Nutrients in this region, however, are not depleted by phytoplankton, a condition known as "high nutrient, low chlorophyll" (HNLC) (Minas et al., 1986), which is believed to be a result of iron limitation on large phytoplankton (Martin, 1990; Martin et al., 1991; Barber and Chavez, 1991) and/or intense grazing control on small phytoplankton (Landry et al., 1997). Dissolved iron plays an essential role in controlling phytoplankton growth in the oceans (e.g., Weinberg, 1989; Coale et al., 1996; Hutchins and Bruland, 1998). This need for phytoplankton in open oceans can usually be met by the fallout of Fe-rich eolian dust (including volcanic ash) (e.g., Martin and Gordon, 1988; Frogner et al., 2001), and/or by upwelling of deep water (e.g., Landry et al., 1997), although iron concentrations are very low due to its insolubility in oxygenated seawater, hence its fragile and transient bioavailability in the marine ecosystem.
Samples were taken aboard JOIDES Resolution at a spacing of two per section (~75 cm) for all cores from Hole 1256B. Stable oxygen and carbon isotopic analyses were carried out on each sample above 150.50 meters below seafloor. Below this depth, one sample per 4–10 m was selected for isotopic analysis to illustrate isotopic fluctuations prior to the carbonate crash.
Samples for oxygen and carbon isotope study were prepared on bulk sediments by grinding dried sediment to a homogeneous powder and baking at 425°C for 2 hr in vacuum. The stable carbon and oxygen isotopic ratios of the baked samples were analyzed using a Gas Bench II Auto-Carbonate device interfaced to a Finnigan MAT Delta plus XP mass spectrometer. All isotope results were calibrated against international and internal laboratory standards and expressed in standard delta notation relative to the Peedee belemnite standard. Analytical precision for isotope analyses was better than centration in each sample was derived by calibrating signal amplitude of the first CO2 peak against those of carbonate standards, which are assumed to be pure carbonate. CaCO3 MARs (in grams per square centimeter per thousand years) were calculated in using the following equation:
The age model by Jiang and Wise (this volume) was followed in this study, which integrated paleomagnetic data from the upper 100 m and biostratigraphic datums from the entire sequence by best linear fit. Assuming a constant sedimentation rate, the age of an individual sample was obtained by extrapolating the sedimentation rate in the specific interval to the depth of this sample.