Charlie-Gibbs fracture zone: bottom-water transport and its geological effects

Abstract

Hydrographic sections of the northern valley of the Charlie-Gibbs fracture zone show that below 2000 m this passage through the Mid-Atlantic Ridge is filled mainly with Iceland-Scotland Overflow Water (ISOW). Two current meters moored within this water mass for 9 months, at sites just downstream from the 3675-m sill of the passage, recorded mean westerly currents of 3.8 and 4.4 cm s −1 These averages include mean westerly velocities of 7 to 8 cm s −1 during fall and spring and almost zero during the winter. Another long-termmeter and an array of meters deployed within 100 m of the sea floor showed steering of the Overflow Water current by the complex topography near the intersection of the transform fault and the spreading center. West of 35°W a different, denser water mass fills the deepest parts of the valley below about 3400 m; it also flows westward, as shown by meters moored 10 and 20 m above the valley floor for 1.5 to 4 days. However, its properties (potential temperature <2.3°C; salinity < 34.93% oxygen 6.4 ml 1 −1; silica <16 μ mol 1 −1) indicate an origin in the Newfoundland Basin west of the Mid-Atlantic Ridge. It probably enters the northern valley from the south, via a leak in the bounding transform ridge near 34°30′W, and is entrained westward, back into the western Atlantic, by the overlying ISOW current. The total westward transport across the sill, below 2000 m, is estimated at 2.4 × 10 6 m 3 s −1. The ISOW carries a significant load of suspended sediment (25 μg 1 −1), and there is an intense 100-m-thick bottom nepheloid layer. The seabed, which was examined with a deeply towed instrument package, has been locally scoured downstream from the passage sill, and there are patches of ripples and moated rock fragments. However, most of the floor of the transform valley is a depositional environment.