Under this award the PIs will test the hypotheses regarding the origin of the 100-kyr climate cycle - the dominant feature of Earth's climate system over the past ~1 Ma. This knowledge is fundamental to understanding the inner workings of the global Earth system. The PIs will employ detailed oxygen isotope and Mg/Ca paleotemperature estimates to deconvolve the history of the ice-volume and temperature components on d18O over the past two million years, a time of transition commonly referred to as the "Mid-Pleistocene Transition" (MPT). Prior to that time, benthic foraminiferal oxygen isotope data reveal a dominant 41-kyr climate, followed by a relative abrupt onset of a 100-kyr cycle that persists to the present. The relative influence of temperature and ice volume across this transition, and in the relatively stationary regimes before and after the MPT, remains unknown. Tests of hypotheses on the origin of the 100 kyr climate cycle, whether a result of ice dynamics, regolith erosion, gradual CO2 change, or tropical triggering, have relied on interpretation of the marine isotopes as primarily an ice volume record. This interpretation is now in questions and thus, true tests of these hypotheses await the development of more reliable records of global ice volume. As a temperature proxy the PIs will analyze Mg/Ca using a new and innovative method called flow through time resolved analysis, or FT-TRA which virtually eliminates effects of contaminant phases and provided precision and accuracy of estimates of calcification temperature in the same species of foraminifera analyzed for d18O. They will focus our analyses on two new drill sites, recovered by ODP Leg 202. These sites (1238 and 1242) are placed in the eastern tropical Pacific "cool tongue" and "warm pool", and are at water depths situated to monitor Pacific Central Water.
