Pacific CaCO3 Preservation and δ18O Since 4 Ma: Paleoceanic and Paleoclimatic Implications

Abstract

The Pliocene‐Pleistocene history of CaCO3 preservation in the central equatorial Pacific is reconstructed from a suite of deep‐sea cores and is compared to fluctuations in global ice volume inferred from δ18O records. The results are highlighted by: (1) a strong covariation between CaCO3 preservation and ice volume over 104 to 106 year time scales; (2) a long‐term increase in ice volume and CaCO3 preservation since 3.9 Ma demonstrated by a deepening of the lysocline and the carbonate critical depth; (3) a dramatic shift to greater CaCO3 preservation at 2.9 Ma; (4) distinctive ice‐volume growth and CaCO3 preservation events at 2.4 Ma, which are associated with the significant intensification of northern hemisphere glaciation; (5) a mid‐Pleistocene transition to 100‐kyr cyclicity in both CaCO3 preservation and ice volume; and (6) a 600‐kyr Brunhes dissolution cycle superimposed on the late Pleistocene glacial/interglacial 100‐kyr cycles. CaCO3 preservation primarily reflects the carbonate chemistry of abyssal waters and is controlled by long‐term (106 year) and short‐term (104 to 105 year) biogeochemical cycling and by distinct paleoclimatic events. We attribute the long‐term increase in CaCO3 preservation primarily to a fractionation of CaCO3 deposition from continental shelf to ocean basin, and secondarily to a gradual rise in the riverine and glaciofluvial flux of Ca++. On shorter time scales, the fluctuations in CaCO3 preservation slightly lag ice volume fluctuations and are attributed to climatically induced changes in the circulation and chemistry of Pacific deep water.