Pleistocene evolution: Northern hemisphere ice sheets and North Atlantic Ocean

Abstract

We analyze five high‐resolution time series spanning the last 1.65 m.y.: benthic foraminiferal δ 18 O and δ 13 O, percent CaCO 3, and estimated sea surface temperature (SST) at North Atlantic Deep Sea Drilling Project site 607 and percent CaCO 3 at site 609. Each record is a multicore composite verified for continuity by splicing among multiple holes. These climatic indices portray changes in northern hemisphere ice sheet size and in North Atlantic surface and deep circulation. By tuning obliquity and precession components in the δ 18 O record to orbital variations, we have devised a time scale (TP607) for the entire Pleistocene that agrees in age with all K/Ar‐dated magnetic reversals to within 1.5%. The Brunhes time scale is taken from Imbrie et al. [1984], except for differences near the stage 17/16 transition (0.70 to 0.64 Ma). All indicators show a similar evolution from the Matuyama to the Brunhes chrons: orbital eccentricity and precession responses increased in amplitude; those at orbital obliquity decreased. The change in dominance from obliquity to eccentricity occurred over several hundred thousand years, with fastest changes around 0.7 to 0.6 Ma. The coherent, in‐phase responses of δ 18 O, δ 13 O, CaCO 3 and SST at these rhythms indicate that northern hemisphere ice volume changes have controlled most of the North Atlantic surface‐ocean and deep‐ocean responses for the last 1.6 m.y. The δ 13 O, percent CaCO 3, and SST records at site 607 also show prominent changes at low frequencies, including a prominent long‐wavelength oscillation toward glacial conditions that is centered between 0.9 and 0.6 Ma. These changes appear to be associated neither with orbital forcing nor with changes in ice volume.