Sites drilled during Leg 202 will provide an opportunity to examine the details of regional climate responses to the onset and amplification of Pleistocene ice age cycles at orbital scales over the last 5 m.y. Evidence from the Southern Ocean (Imbrie et al., 1993) and from the equatorial Pacific (Pisias and Mix, 1997; Lea et al., 2000) suggests that near-surface changes in these areas precede those at high northern latitudes. Thus, climate changes here do not passively respond to Northern Hemisphere glaciation but could be part of the chain of responses that led to Northern Hemisphere glaciation.
Especially important for Leg 202 will be to assess the linkages between changes observed at higher southern latitudes (e.g., Leg 177) with those along the equator (Sites 1238-1240). Site 1237 will provide a useful monitor of the advective link between the high and low latitudes, by monitoring the strength of the cool Humboldt Current at orbital scales of 104-106 yr. Comparison of benthic water mass tracers from sites at intermediate depths (Sites 1239 and 1242) with those at greater depths (Sites 1238 and 1240) will help to document the role of changing Pacific intermediate waters of northern and southern sources in large-scale climate change (Mix et al., 1991).
Within Pleistocene time, the origin of the large 100-k.y. climate cycle ~1 m.y. ago remains puzzling. A number of mechanisms have been proposed, including a threshold response of high-latitude glaciers to gradual long-term cooling associated with uplift of mountain ranges (Ruddiman and Raymo, 1988) or reduction of greenhouse gases (Maasch and Saltzman, 1990); a transition from land-based to marine-based ice sheets (Pisias and Moore, 1981; Berger and Jansen, 1994); erosion of soft sediment below Northern Hemisphere glaciers to expose bedrock, allowing larger glaciers to grow by increasing basal friction (Clark and Pollard, 1998); atmospheric loading of cosmic dust to trigger a response to rhythmic changes in the plane of Earth's orbit (Muller and MacDonald, 1997); and long-term cooling of the deep sea at polar outcrops, which influenced sea ice distributions (Gildor and Tziperman, 2001).
A 100-k.y. cycle of climate may also originate independently of polar climate changes via the responses of tropical climate systems to orbital changes in seasonal insolation (Crowley et al., 1992). Evidence exists for rhythmic 100-k.y. cycles of sedimentation in the eastern tropical Pacific that, if climatically significant, could have provided a "template" for a climate cycle that was later picked up by the global ice sheets (Mix et al., 1995). A range of evidence suggests that tropical climate changes at orbital scales preceded those of the Northern Hemisphere ice sheets and must vary independently from the high northern latitudes (Imbrie et al., 1989; McIntyre et al., 1989; Pisias and Mix, 1997; Harris and Mix, 1999; Lea et al., 2000).
One prediction of climate models that create climate cycles of a 100-k.y. period from a tropical response to orbital precession is that strong cycles of ~400-k.y. duration must also be found. Although shipboard age models are not sufficiently refined to allow interpretation of all the orbital cycles, preliminary data from Leg 202 demonstrate the likely existence of orbital-scale variability in lithologies. Evolutive spectra of the optical lightness parameter L* at Site 1239 (which here tends to mimic calcium carbonate concentrations in the sediment) reveal substantial changes in orbital-scale cycles through time (Fig. F37). Prior to ~1.2 Ma, large variations in sediment L* occurred most strongly within a long period (~400-k.y.). Near ~1.0 Ma, this ~400-k.y. cycle broadens to include variability near the ~100-k.y. period. Rhythmic variations with ~400-k.y. periodicity are also present in reflectance-predicted TOC content at Site 1237 (Fig. F11), and in an interval with a well-constrained paleomagnetic age model, variations in GRA density at the orbital periods of tilt (~41 k.y.) and precession (~23 and 19 k.y.) are well defined (Fig. F19).