Abstract:
The evolution of early Paleogene climate began with a long-term warming trend culminating in peak Cenozoic warmth, followed by intense cooling that ultimately led to the onset of the icehouse. While various mechanisms have been proposed, this long-term climatic evolution remains largely unexplained. Changes in the magnitude of poleward heat transport via thermohaline circulation may have played a role in the long-term evolution of Cenozoic climate. For example, one mechanism proposed to explain warm polar temperatures during the greenhouse interval invokes increased equator-pole heat transport through more vigorous deep-sea convection. However, intensified oceanic circulation is difficult to reconcile with diminished meridional thermal gradients. Neodymium isotope records from Ocean Drilling Program Leg 198 (Shatsky Rise, northwest Pacific Ocean) shed light on the relationship between thermohaline circulation and global climate evolution. The (super 143) Nd/ (super 144) Nd ratio is a tracer of deep-water mass circulation because of Nd's short residence time relative to oceanic mixing, and fossil fish debris record the deep-water Nd isotopic composition at a given location. Three fish-debris Nd isotopic records generated from a depth-transect of sites at Shatsky Rise (ODP Sites 1208, 1209, and 1211) indicate that a single deep-water mass encompassed the depth range of at least approximately 2400 to 3300m. The Nd isotopic composition (expressed in epsilon units) of this water mass varied considerably over the time interval investigated. The primary trend begins with an increase in epsilon (sub Nd) values from approximately -4.5 to approximately -2.5 from approximately 70 to approximately 58 Ma. From approximately 58 to approximately 45 Ma, epsilon (sub Nd) values fluctuate somewhat but remain relatively high ( approximately -3). Subsequently, epsilon (sub Nd) values decrease to approximately -4.5 by approximately 35 Ma. The major transitions in water mass composition at Shatsky Rise correspond to the major shifts in global climate as deduced from the delta (super 18) O compilation of Zachos et al. (2001). One explanation for the relationship between deep-water mass composition and climate is that the intensity of thermohaline circulation was diminished during periods of extreme global warmth and then intensified during the late Eocene as cooling progressed. This suggests that thermohaline circulation responded to, rather than forced, long-term global climate change.