Evidence For Deep-water Production In The North Pacific During The Early Cenozoic

The evolution of Cenozoic climate began with a long-term warming trend culminating in peak warmth during the Early Eocene. This was followed by intense cooling that ultimately led to the onset of the "icehouse" climate of the late Cenozoic. Changes in poleward heat transport via thermohaline circulation may have played a role in the general evolution of Cenozoic climate. To investigate the relationship between the evolution of global climate and thermohaline circulation over the interval ∼70 to 30 million years ago (Ma), I generated paleo-seawater neodymium isotopic records from Ocean Drilling Program (ODP) sites in the present-day northern Pacific Ocean. Fish debris Nd isotope data from ODP Sites 1209 and 1211 (paleodepths ∼2300m and ∼2900m) indicate that ɛ _Nd values in the deep waters of the central subtropical Pacific were characterized by a composition of ∼-4.5 to -5 during the latest Cretaceous and the Early Paleocene ( ∼70 to 64 Ma). Beginning ∼64 Ma, the composition of Pacific deep waters became more radiogenic ( ∼-3 to -3.5), and remained so for the next ∼20 million years. From ∼46 to 33 Ma, deep-water ɛ _Nd values subsequently became more nonradiogenic ( ∼-4.5 to 5). The similarity in the overall ɛ _Nd trends at both sites indicates that they were bathed by a common deep-water mass. Nd isotopic data from ODP Leg 199 Sites (1215, 1219, and 1221; north-central Pacific), which lie ∼6000 km to the west and south of Shatsky Rise, demonstrate a trend similar to the upper portion of the records from Shatsky Rise. Although the basal age of sediments from the Leg 199 Sites is younger than 56 Ma, ɛ _Nd values from Sites 1215, 1219 and 1221 demonstrate the shift from relatively radiogenic (North Pacific) values of -3.5 at 53.7 Ma to more non-radiogenic (Southern Ocean) values of -4.9 by 44.8 Ma. The ∼1.5 epsilon unit shifts toward more radiogenic values recorded at both sites reflect a fundamental change in the source of deep waters bathing the northern Pacific sites from ∼65 to 45 Ma, involving a switch in deep-water production from the Southern Ocean to the North Pacific. Production of North Pacific deep waters lasted ∼20 million years, and coincided with the warmest climatic interval of the Cenozoic Era. The timing of the deep-ocean circulation changes lagged changes in global temperatures implying that climate exerts long-term control over thermohaline circulation patterns. Thus Cenozoic thermohaline circulation patterns evolved in response to long-term changes global climate, and were not the cause of extreme Cenozoic warmth.