Abstract

Two Nd and Pb isotope time series of hydrogenous ferromanganese crusts, one from the Tehuantepec Ridge in the deep eastern equatorial Pacific and the other from Blake Plateau in the shallow northwestern Atlantic, which cover the past 7–8 m.y., show no variations coincident with the final closure of the Panama gateway, estimated as ca. 3.5 Ma. The record of the Atlantic crust located in the present-day Gulf Stream shows a shift in isotope composition from ca. 8 to 5 Ma that is explained by a diminishing supply of Pacific water. It is argued that the major restriction of water-mass exchange through the Panama gateway occurred before 5 Ma and thus cannot serve as a direct cause of the onset of Northern Hemisphere glaciation. The absence of a significant signature in the isotope records from the Pacific crust suggests that the volume of water exchanged with the Atlantic through shallow archipelagic straits of the gateway during the 3–4 m.y. prior to closure was too small to influence the radiogenic isotope composition of Pacific deep water.

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