Obscuring of long eccentricity cyclicity in Pleistocene oceanic carbon isotope records

Long eccentricity (400-kyr) cycles in carbon isotope records from the Pacific and Atlantic oceans and the Mediterranean sea of the past 5.0 Ma are compared. All records show maximum delta delta C-13 values (delta(13)Cmax) at eccentricity minima during the Pliocene, but this relationship obscured in the Pleistocene after similar to 1.6 Ma in particular for the open ocean deep-water delta C-13 records. Since a clear anti-phase relationship was set up between oceanic 6180 and delta C-13 in the 100-kyr hand from this time, we attribute the obscured 400-kyr signal to a major change in the oceanic carbon reservoir probably associated with restructure of the Southern Ocean. A similar change occurred in the Miocene at 13.9 Ma when the 400-kyr cyclicity in delta C-13 records flattened out together with a drastic cooling and Antarctic ice-sheet expansion. A remarkable exception is the Mediterranean surface water delta C-13 record, which remained paced by the long-term eccentricity cycle throughout the Pliocene and Pleistocene, suggesting a low-latitude climatic origin of the 400-kyr signal that is independent of glacial-interglacial forcing. Since the Earth is currently passing through an eccentricity minimum, it is crucial to understand the nature of the delta(13)max events. (C) 2009 Elsevier B.V. All rights reserved.