The history of sedimentation and abyssal circulation on the Greater Antilles outer ridge

Abstract

The Greater Antilles Outer Ridge is an 1800 km long, submarine sedimentary ridge which lies below 5000 m in the southwestern North Atlantic Ocean. Seismic reflection profiles and core data indicate that the ridge is composed of more than 6 x 104 km3 of acoustically transparent sediment which has accumulated above a sequence of acoustically stratified sediments deposited before late Eocene time. The sediments consist of low-carbonate, homogeneous, terrigenous lutites which have accumulated at rates of up to 30 cm/1000 yr since the middle Eocene. Clay-mineral analyses indicate that the chlorite-enriched sediment is derived from the northeastern continental margin of North America. Abyssal contour-following currents which flow around the Greater Antilles Outer Ridge are interpreted as an extension of the Western Boundary Undercurrent (WBUC) found along the continental rise of eastern North America. This current system is proposed to be the agent which has transported sediment southward for more than 2500 km and deposited it on the Greater Antilles Outer Ridge. Sediment is presently carried in concentrations up to 65 ug/liter in the currents flowing around the outer ridge, and mineral analyses show that the suspended sediment has a northern provenance; it is similar in composition to the bottom sediment and is interpreted as the source of sediment deposited on the Greater Antilles Outer Ridge. The Puerto Rico Trench began to form in middle Eocene time, and it cut off direct downslope sedimentation to the Greater Antilles Outer Ridge. At the same time, the newly formed WBUC interacted with existing sea-floor topography and the Antarctic Bottom Water (AABW) flowing in from the South Atlantic, and it began to deposit acoustically transparent sediment on the eastern outer ridge. This depositional pattern persisted until the middle or late Miocene, when increased AABW flow diverted the WBUC to the northwest and initiated deposition of the western sector of the Greater Antilles Outer Ridge. Shortly thereafter, decreased AABW flow and lower current speeds allowed rapid deposition of sediment on the Greater Antilles Outer Ridge and on the Caicos Outer Ridge to the west. The bottom topography has controlled the abyssal current pattern, and current-controlled deposition has continued to construct the Greater Antilles Outer Ridge since early Pliocene time.