Paleogeography, Paleobiogeography and the History of Circulation in the Atlantic Ocean
The history of surface and bottom current circulation is delineated in the Atlantic and Tethys-Mediterranean Ocean over the past 200 million years within the framework of continental drift and sea-floor spreading.
An intimate relationship between changing paleogeography, paleobiogeography and paleo-circulation is demonstrated for the Mesozoic and Cenozoic. Major Mesozoic events include: a) initial opening of the North Atlantic Ocean about 180—190 my ago and the subsequent separation of Eurasia and Africa (about 160-170 my ago) allowing a Tethyan-North Atlantic faunal exchange; 2) separation of Africa and South America (about 95 my ago) allowing North-South Atlantic surface water mass and faunal exchange.
Distinct latitudinal control of the distribution of marine microfaunas in the North Atlantic Ocean began in the early part of the Tertiary (Paleogene) and is related to the opening of the seas in the North Atlantic about 50 my ago when the deep-water circulation pattern was probably first initiated in the Atlantic basins. At that time a boreal zoogeographic province was established in the Atlantic Ocean for the first time.
Seismic Horizon A is both a depositional and erosional feature of late early and middle Eocene age. In the central portion of the North Atlantic basin it correlates with beds of chert that were deposited when the ocean was unusually enriched with silica and its geographic distribution is a reflection of the subtropical North Atlantic gyre (Gulf Stream) and westward flowing North Equatorial Current which flowed into the Pacific Ocean along the paleoequator. Along the margin of the basins drilling has shown that the seismic horizon correlates with an unconformity or hiatus between mid-Cenozoic and early to middle Cretaceous sediments. Here Horizon A is related to the initiation of strong bottom circulation that followed the opening of the North Atlantic seas and the formation, for the first time, of cold North Atlantic bottom water.
Caribbean and Mediterranean henthonic foraminiferal faunas exhibit a marked degree of similarity during the early Tertiary. The gradual displacement of west-east current migration routes into higher latitudes and the compression of Spain against North Africa brought an end to this amphiatlantic distribution pattern about 15 my ago. The late Miocene evaporation of the western remnant of the once extensive Tethys Sea exacerbated this situation but in the early Pliocene, 5 my ago, normal marine connections were resumed with the Atlantic.
In the Pliocene two events occurred which significantly affected circulation in the Atlantic Ocean: 1) the elevation of the Isthmus of Panama about 3.5 my ago brought an end to a long history of Atlantic and Pacific marine faunal interchange and allowed the free passage of continental mammals between North and South America; 2) the initiation of glaciation in the Northern Hemisphere 3 my ago generated the Labrador Current which displaced the Gulf Stream southwards to its present position at about latitude 45°N. At this time the Polar Faunal Realm developed, thus effectively completing the process of biogeographic provincialization which began in the early Cenozoic, about 60 my ago.
The Western Boundary Undercurrent has played a major role in controlling fine-grained sediment deposition only since the early Tertiary. Data from piston coring and deep drilling in the Pacific and Indian oceans as well as the Atlantic Ocean suggests that during early to middle Tertiary times extensive unconformities and redistribution of sediment occurred, and it is proposed here that during this time vigorous deep and bottom circulation on a global scale was first initiated in response to the nearly synchronous opening of the circum-Antarctic and North Atlantic basins. Thus the deep cold circulation of the Circumpolar Current and Antarctic Bottom Current in the Pacific, Indian, and Atlantic oceans and the Western Boundary Undercurrent in the North Atlantic probably is no older than about 50 -60 million years.