Influence of sediment transport dynamics and ocean floor morphology on benthic foraminifera, offshore Fraser Island, Australia

Abstract

The extensive longshore sediment transport system along the SE margin of Australia transports yearly 500,000 m 3 of sand from the Gold Coast of southern Queensland north towards Fraser Island. Fraser Island, which consists of 203 km 3 of quartz sand, is presently not increasing in size and north of the island quartz sand is replaced by carbonate sand. Recent multibeam sonar seafloor imagery and sediment analysis has established that the transported sand is being diverted by strong ebb tidal flow over the continental shelf edge onto the Tasman Abyssal Plain through a series of active gullies that incised the continental slope. Foraminiferal distribution patterns on the shelf and slope are closely linked to the variable ocean floor morphology and associated physical processes. In sample locations outside the continental slope gullies, areas unaffected by downslope sediment transport, foraminiferal assemblages gradually change in accordance with bathymetric zones and their prevailing ecological parameters. Species diversity, evenness, the proportion of infaunal species and the abundance of agglutinated taxa all increase with depth. Assemblages within gullies of the clastic sand transport route are significantly different. Estuarine and shelf species are transported over the shelf break and mix with typical slope species along the transport path, resulting in continued high species diversities. As the sediment and faunal load enters the abyssal plain, a faunal portion continues to travel within the Capricorn Sea Valley to over 4000 m depth. The erosional nature of gullies results in reduction of agglutinated species. Subtle topographic features such as ridges or levees within the canyon and deep-sea valley act as protection from the main erosional sand transport pathway and support the presence of fragile and erect suspension feeders. Foraminiferal distribution patterns would have received an entirely different biofacies interpretation without linking them to ocean floor processes as revealed through multibeam sonar imagery.