Sedimentation patterns and accumulation rates in the Tasman Basin

Sediment accumulation rates determined by nondestructive gamma spectrometry on 22 piston cores taken in the Tasman Basin vary from 0·2 to 2·7 g cm^-2 (1000 yr)^-1. Near the South Tasman Rise, winnowing keeps the sedimentation rate of foraminiferal ooze low on the rise crests [0·15 to 0·90 g cm^-2 (1000 yr)^-1] while rates on the flanks and base of the rise are faster [1·4 to 2·0 g cm^-2 (1000 yr)^-1]. The rateson the Tasman abyssal plainand central basin are controlled by proximity to a sediment source area and local scour. The effect of submarine canyons off the Bass Strait is evident in the high rate of clly accumulation near the terminus of the canyons. Rates in the central basin are low [0·2 to 1·0 g cm^-2 (1000 yr)^-1] because of bottom currents. The ²³⁰Th balance is studied by relating the accumulation rate of ²³⁰Th (= F_a) to its rate of production from ²³⁰U in the water column (= F_p). Sediment settling through the water column scavenges ²³⁰Th to produce an ideal ratio of F_a to F_p of 1·0. Winnowing and bottom transport can alter the F_a/F_p ratio. Cores from rise crest and the central basin have F_a/F_p ratios < 1·0 while areas of high clay accumulation rate have F_a/F_p > 1·0. The region as a whole is in balance with respect to ²³⁰Th accumulation.