Separation of primary ice-rafted debris from lag deposits, utilizing manganese micronodule accumulation rates in abyssal sediments of the Southern Ocean

Abstract

Changes in the abundance of ice-rafted debris in abyssal sediments of the Southern Ocean have been interpreted as evidence of variations in Antarctic glacial activity. When accompanied by a simultaneous increase in Mn micronodules, however, it is proposed that ice-rafted debris maxima may represent residual lag deposits, created during a period of reduced sedimentation rate caused by selective removal of the finer or lighter fraction by high-velocity bottom currents. An empirical relationship between ice-rafted debris and Mn micronodules in the southeast Pacific Ocean has been used to suppress the lag component in the total ice-rafted debris signal in order to isolate the ice-rafted debris caused by increased iceberg supply associated with increases in glacial activity in Antarctica. The spatial distribution of the mean accumulation rate of ice-rafted debris during the past 690,000 yr is complicated by local bottom-scouring activity, which causes erratic latitudinal variations of total ice-rafted debris accumulation rates. When the residual portion of ice-rafted debris is removed, a coherent latitudinal decrease of the mean accumulation rate of debris is revealed. The average northern limit of debris during the past 690,000 yr is not different from present conditions. A sharp maximum in debris accumulation occurs in the vicinity of the present Antarctic convergence, suggesting that the factors governing iceberg supply and melting rate have not changed appreciably during the period involved. Six periods of increases in Mn micronodule production are defined at intervals of 100,000 to 110,000 ± 20,000 yr. The linear correlation coefficient between ice-rafted debris and Mn micronodules during the past 690,000 yr defines two areas of intense winnowing, separated by the East Pacific Rise. The southernmost zone corresponds to an area independently described as one of high bottom-current activity. Our results indicate that the intensity of bottom-current winnowing on the north flank of the East Pacific Rise may be as great as in the southern zone. This could mean that the Eltanin Fracture Zone serves as a conduit for high-velocity Antarctic Bottom Water flowing onto the northern flank of the East Pacific Rise.