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 δδ13C values (δ13Cmax) at eccentricity minima during the Pliocene, but this relationship obscured in the Pleistocene after ∼ 1.6 Ma in particular for the open ocean deep-water δ13C records. Since a clear anti-phase relationship was set up between oceanic δ18O and δ13C in the 100-kyr band 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 δ13C records flattened out together with a drastic cooling and Antarctic ice-sheet expansion. A remarkable exception is the Mediterranean surface water δ13C 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 δ13Cmax events.