Origin of high density pockmark fields in a marine reserve and their use in inferring near-seabed currents
Authors / CoAuthors
Pockmarks have been observed around the world's ocean and lake beds for decades. They indicate shallow and/or deep sub-surface fluid seepage, and may occur in isolation or in groups. Dense fields of pockmarks were identified in three areas (510 km2) mapped using multibeam sonar in the Oceanic Shoals Commonwealth Marine Reserve, located in the tropical Timor Sea on the Australian continental shelf. The pockmarks occur in flat, barren, silty plains (~105 m water depth) which surround extensive carbonate banks and terraces (~40-75 m water depth). The banks hosted rich communities of benthic organisms including sponge gardens and corals. A distinctive feature of many of the pockmarks in this area is a linear scour mark that extends up to 200 m from pockmark depressions. Previous numerical and flume tank simulations have shown that scouring of pockmarks occurs in the direction of the dominant near-seabed flow. These geomorphic features may therefore serve as a proxy for local-scale bottom currents, which may in turn provide information on sediment processes influencing biodiversity patterns in the region.
In this presentation, we: 1) provide information on the methods used to characterise and count the scoured and non-scoured depressions (i.e. an automated method involving ArcGIS spatial analyst tools); (2) draw on other datasets to provide information on why the pockmarks developed (e.g. multibeam backscatter and geochemical variables); and 3) investigate their potential as an environmental proxy (oceanographic) for benthic habitat studies.