Geomorphological and geochemical evidence (230Th anomalies) for cross-equatorial currents in the central Pacific

Shallow broad elongated sediment depressions and ridges are revealed in multibeam echo-sounder data collected over the carbonate ooze in the central equatorial Pacific. These features, otherwise called “furrows”, have orientations that appear locally distorted by seabed topography as expected of contour-trending currents but at regional scale typically cross contours at high angles. In places, complex patterns suggest that formative currents have a strong time-varying component. From direction indicators, the movement of bottom waters is north to south on average, though with some movement locally south to north. There is a modest 18° average change in orientation crossing from north to south of the equator, with features to the south oriented clockwise of those to the north. This is as expected for a partly developed bottom Ekman layer, with currents in the layer deflected by the Coriolis effect with opposing senses either side of the equator. The features are less prominent on and immediately south of the equator. We evaluated these observations along with reported 230Th accumulation rates in sediment cores, which are curiously enhanced along the equator, an observation that has been previously interpreted as suggesting transport of 230Th bound to particles to the equator. Limited current meter and other data and physical oceanographic models help to explain these observations. Data from current meters 1° north of the equator show a highly asymmetric mesoscale eddy motion here, aligned with the furrows. Phase relationships between near-bed and upper ocean currents suggest an indirect coupling of upper-ocean eddies with the lower ocean. The bottom Ekman layer is predicted theoretically to thicken towards the equator. The resulting reduced bed shear stress may explain the 230Th deposition and more weakly developed furrows at the equator. Given evidence that equatorial accumulation rates of 230Th and extraterrestrial 3He both fluctuated over the Late Pleistocene, we explore how the ideas presented here could help to explain how the geochemical anomalies relate to physical oceanographic processes