Calculating erosion by deep-sea turbidity currents during initiation and flow

Abstract

TURBIDITY currents transport massive amounts of sediment from continental margins to the deep sea. Individual flows can catastrophically remove and redeposit (as turbidites) many hundreds of cubic kilometres of material1,2, with the larger events reaching the bottom of the continental slope to form the abyssal plains3. Here we show that the age range of sediments in individual turbidites can be used directly to estimate both the thickness of failed sediment in the source region (even when its exact location is unknown) and the extent to which the turbidity current caused erosion of the sea bed. Our method involves the comparison of the abundance ratios of microfossil (coccolith) species in turbidites with those in the ocean margin sediments of the source region. Analysis of a recently emplaced turbidite on the Madeira Abyssal Plain shows that it contains a mixture of sediments with an age range of about 200,000 years, equivalent to the failure of a block of sediment about 15 m deep. Radiocarbon dating and coccolith ratios show that the turbidite contains only about 12% of recent, near-surface sediment, indicating that this turbidity current caused surprisingly little erosion en route.