The East Scotia Sea: Mantle to Microbe

Abstract

The East Scotia Sea is an important geological, oceanographic, and biological gateway between the Pacific and Atlantic realms, containing a particularly good example of back-arc spreading, the East Scotia Ridge. Recent surveys show that the ridge consists of nine segments, separated by nontransform offsets. The five middle segments (E3-E7) have the morphology of slow-intermediate spreading centers and erupt mainly normal mid-ocean ridge basalt lavas. End segments, E2 and E9, are much shallower, have the inflated appearance of faster-spreading ridges, and erupt slightly enriched, plume-influenced rocks with small to moderate subduction components. Spreading rates vary from 62 mm/yr in the north to 70 mm/yr in the south and cannot explain the along-axis variations. Rapid slab retreat is believed to cause flow of Atlantic asthenosphere around the ends of the subducting slab at the South Sandwich Trench, advecting fertile mantle into the back-arc region. Eastward rollback of the slab brings this mantle within 150 km of the volcanic front, where excess melting occurs as a result of fluxing by subduction-related hydrous fluids. This leads to the initiation of magmatically inflated end segments, characterized by thicker crust and elevated topography. Reconnaissance work has detected hydrothermal plumes above the most inflated parts of segments E2 and E9, suggesting a close relationship between mantle inflow, magmatism, and hydrothermal venting. Vent sites at the East Scotia Ridge may be affected by currents from the Weddell Sea and the South Pacific. Hence, the at present unknown vent fauna could be of either Pacific or Atlantic type, or conceivably a combination of both.