Sedimentation and continental slope processes in the vicinity of an ocean waste-disposal site, southeastern Tasmania

Digital echo sounding, SeaBeam swath bathymetry data and sediment cores wer e collected on the continental slope (1500-3700 m water depth) off southeas tern Tasmania in order to study sedimentary processes in the vicinity of an ocean disposal site. The new bathymetry data show that the shallower limit s of the disposal site are positioned on the seaward edge of a gently dippi ng (3 degrees) mid-slope shoulder, between 1200 and 2100 m water depth. The slope below the disposal site is relatively steep (6.5 degrees) and is cut by submarine canyons which lead into the adjacent East Tasman Saddle. The SeaBeam bathymetry data show a small submarine canyon traversing the slope in 2400 m water depth directly downslope from the disposal site, with local slopes of up to 22 degrees. The canyon feeds into a perched basin at 2450 m, which could be acting as a local sediment trap. Short (<90 cm) gravity c ores indicate that indurated erosional surfaces characterise the slope envi ronment. The cores contain Upper Cretaceous (upper Campanian) sandstones an d siltstones, which in places crop out on the sea floor where they are loca lly draped by a thin (0-30 cm), modern layer of hemipelagic calcareous ooze . Five cores collected from the vicinity of the disposal site had lead and zinc concentrations in the surface 1 cm of 10.3 +/- 5.0 and 39.5 +/- 19.6 m g/kg, respectively, significantly greater than the background values (2.9 /- 1.4 for lead and 21.2 +/- 5.4 for zinc) which characterise the underlyin g unit that is composed of the same hemipelagic calcareous ooze. Lead and z inc are constituents of the dumped material, jarosite, which, after mixing with slope sediments, can be used as sediment tracers. One core contains a fining-upwards bed which is also elevated in lead and zinc. This is interpr eted as evidence for dispersal of the jarosite from the disposal site downs lope to depths >3000 m via turbidity flows sometime during the past 24 year s. Current meter data collected from 30 m above the sea floor over one year at the disposal site show that bottom currents attain speeds of up to 0.46 m/s. The current events are attributed to eddies shed by the East Australi a Current. The measured bottom currents are capable of transporting fine-gr ained hemipelagic muds and could provide a trigger mechanism for turbidity flows.