| Abstract: | The evolution of interocean carbon isotopic gradients over the last 2.5 m.y. is examined using high-resolution δ13C records from deep sea cores in the Atlantic and Pacific Oceans. Over much of the Northern Hemisphere ice ages, relative reductions in North Atlantic Deep Water production occur during ice maxima. From 2.5 to 1.5 Ma, glacial reductions in NADW are less than those observed in the late Pleistocene. Glacial suppression of NADW intensified after 1.5 Ma, earlier than the transition to larger ice sheets around 0.7 Ma. At a number of times during the Pleistocene, δ13C values at DSDP Site 607 in the North Atlantic were indistinguishable from eastern equatorial Pacific δ13C values from approximately the same depth (ODP Site 677), indicating significant incursions of low δ13C water into the deep North Atlantic. Atlantic/Pacific δ13C values converge during glaciations between 1.13-1.05 m.y., 0.83-0.70 m.y., and 0.46-0.43 m.y. This represents a pseudo-periodicity of approximately 300 kyr which cannot easily be ascribed to global ice volume or orbital forcing. This partial decoupling, at low frequencies, of the δ18O and δ13C signals at Site 607 indicates that variations in North Atlantic deep water circulation cannot be viewed simply as a linear response to ice sheet forcing. |
