Vigorous deep-sea currents cause global anomaly in sediment accumulation in the Southern Ocean

The vigorous current systems in the Southern Ocean play a key role in regulating the Earth's oceans and climate, with the record of long-term environmental change mostly contained in deep-sea sediments. However, the well-established occurrence of widespread regional disconformities in the abyssal plains of the Southern Ocean attests to extensive erosion of deep-sea sediments during the Quaternary. We show that a wide belt of rapid sedimentation rates (>5.5 cm/k.y.) along the Southeast Indian Ridge (SEIR) is a global anomaly and occurs in a region of low surface productivity bounded by two major disconformity fields associated with the Kerguelen Plateau to the east and the Macquarie Ridge to the west. Our high-resolution numerical ocean circulation model shows that the disconformity fields occur in regions of intense bottom-current activity where current speeds reach 0.2 m/s and are favorable for generating intense nepheloid layers. These layers are transported toward and along the SEIR to regions where bottom-current velocities drop to <0.03 m/s and fine particles settle out of suspension, consistent with focusing factors significantly greater than 1. We suggest that the anomalous accumulation of sediment along an 8000-km-long segment of the SEIR represents a giant succession of contourite drifts that is a major extension of the much smaller contourite east of Kerguelen Plateau and has occurred since 3-5 Ma based on the age of the oldest crust underlying the deposit. These inferred contourite drifts provide exceptionally valuable drilling targets for high-resolution climatic investigations of the Southern Ocean.