Paleogene deepwater mass composition of the tropical Pacific and implications for thermohaline circulation in a greenhouse world

The Pacific Ocean likely exerted the greatest marine influence on climate during the greenhouse interval of the Paleogene. The first step toward assessing the role of Pacific deepwater circulation in global heat transport is a comprehensive reconstruction of deepwater mass composition and circulation patterns. We present new deepwater mass Nd isotope data from ODP Leg 199 Sites 1215, 1217, 1219, and 1221 to augment the existing coverage provided by ferromanganese crust and fossil fish debris data. The new data corroborate the notion that the Pacific Ocean operated in a bimodal pattern with deepwater sources in both the Southern Ocean and North Pacific for much of the early to middle Paleogene. In addition, the data indicate that there was little to no influence of Atlantic waters on the deepwater composition of the tropical Pacific. The available deepwater Nd isotope data provide refined constraints on the geographic and depth range of influence of both a Southern Ocean and North Pacific deepwater mass. From ∼65 to ∼45 Ma, the deep tropical Pacific was the mixing location of deep waters sourced from the Southern Ocean and the North Pacific. The Southern Ocean deepwater mass extended from ∼1800 m to abyssal depths. Until 53.6 Ma it circulated northward in the western Pacific at least as far as the paleolocation of Fe‐Mn Crust CLD01 (∼5°N paleolatitude) and in the eastern portion of the Pacific as far as Site 1215 (∼10°N paleolatitude). A North Pacific deepwater mass extended from at least ∼2400 m to 2900 m water depth from ∼65 to 54 Ma, south to Shatsky Rise (paleolatitude ∼10–15°N), and possibly as far as Site 1215, 700 km to the south of Shatsky Rise Sites 1209 and 1211.