Abstract:
The production and flow of deep-water masses is a key component of heat transport in the modern climate system. The present mode of meridional overturning circulation (MOC) is characterized by deep-water formation in the North Atlantic and the Southern Ocean, while the North Pacific is characterized by slow deep-water renewal. However, the present bipolar mode of MOC dominated by the Atlantic is a geologically recent development, the culmination of millions of years of tectonic and climatic evolution. But what role did deep-water circulation play in heat transport during warm climates such as the early Cenozoic? In order to address this question, we must understand the role of the Pacific Ocean.The first step toward assessing the role of the Pacific MOC in global heat transport is a comprehensive reconstruction of deep-water mass composition and circulation patterns. A growing body of Nd isotope data from Fe-Mn crusts and fossil fish debris is beginning to provide a reconstruction of the ancient Pacific water column. Recent Nd isotope evidence suggests that a bipolar mode of MOC may have operated in the Pacific during a portion of the early Cenozoic. Deep-sea Nd isotope data from Shatsky Rise in the present-day northwestern Pacific Ocean suggest a fundamental switch in deep-water production from the Southern Ocean to of North Pacific deep waters lasted approximately 20 million years, and coincided with the warmest climatic interval of the Cenozoic Era. New Nd isotope data from ODP Sites 1215, 1217, 1219, and 1221 further constrain the evolution of water mass composition and structure of the deep Pacific during the Paleogene. These sites were ideally situated to monitor any contributions of deep-water from the Atlantic via the Caribbean gateway, and the data indicate no influence of Atlantic waters on the deep-water composition of the tropical Pacific. This finding reinforces the hypothesis that the Pacific was a bimodal MOC system that operated independently of the Atlantic throughout the Paleogene.Comparison of the records of deep-water mass composition and global climate allow us to speculate on the possible role of MOC modes in ancient greenhouse climate. Deep-water production in the North Pacific was limited to the warmest portion of the Cenozoic. As global deep-waters cooled during the Eocene, the source of deep waters shifted back to the Southern Ocean. The relative timing of the switch from southern sector deep-water production to the North Pacific during the period of general global warming, and the subsequent reversion back to a Southern Ocean source of deep waters during the protracted Eocene cooling trend implies that the long-term evolution of thermohaline circulation patterns was a consequence and not a cause of global climate change. Thus major changes in the mode of MOC likely don't force major transitions in climate.