Showing papers by "Douglas G. Martinson published in 1989"

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 δ18O and δ13O, percent CaCO3, and estimated sea surface temperature (SST) at North Atlantic Deep Sea Drilling Project site 607 and percent CaCO3 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 δ18O 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 δ18O, δ13O, CaCO3 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 δ13O, percent CaCO3, 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.

Late Pliocene variation in northern hemisphere ice sheets and North Atlantic deep water circulation

Abstract: High-resolution records of δ18O, δ13O, and percent CaCO3 from the late Pliocene North Atlantic (Deep Sea Drilling Project sites 607 and 609) are presented and oxygen isotope stages are formalized back to stage 116 at 2.73 Ma. From 2.8 to 1.6 Ma, the interval studied, variations in these records were dominated by the 41-kyr component of orbital obliquity. Significant variation at the orbital frequencies of eccentricity (96-kyr) and precession (23-kyr) are observed in the δ18O record between 1.6 and 2.1 Ma, but not before. Prior to 2.4 Ma (stage 100), δ18O variations suggest ice sheet growth 1/4 to 1/2 as large as late Pleistocene ice volumes; however, these events are below the threshold needed to result in extensive ice-rafting to the open North Atlantic Ocean. After 2.4 Ma, ice sheets appear to be, on average, 1/2 as large as those of the late Pleistocene. The δ18O record indicates that some glacial suppression of North Atlantic Deep Water occurred both before and after 2.4 Ma and that glacial-interglacial transfers of 12C between the continents and oceans appear to have been larger in the late Pliocene relative to the late Pleistocene. In addition, the strong 23-kyr power observed in δ18O between 2.75 and 2.10 Ma suggests that deep-sea circulation (or changes in biomass) is controlled, in part, by climatic variations unrelated to ice sheets.