Unveiling the Perth Canyon and its deep-water faunas

Abstract. The Perth Canyon is a prominent submarine valley system in the southeast Indian Ocean that incises the southwest Australian continental shelf. It is characterised by two main steep-sided valleys forming a V-shaped configuration that extend from a depth of ~ 600 m to the abyssal plain at ~ 4000 m. Despite its prominence and location of only ~ 27 nautical miles (50 km) offshore, this study represents the first ROV-based exploration of the canyon and its inhabitants. ROV surveys revealed quiescent environments, the structure essentially representing a fossil canyon system with localised occurrences of significant mega- and macrobenthos in the depth range of ~ 680 to ~ 1800 m. The patchy distribution of canyon life comprised corals, sponges, molluscs, echinoderms, crustaceans, brachiopods, and worms, as well as plankton and nekton (various fish species) especially near benthic communities. High definition video surveys and biomass sampling were complemented by ship-based multi-beam bathymetry, and seawater CTD profiling and chemical analyses.

ROV transects were conducted at six geomorphologically distinct locations, from the head to the mouth of the canyon and on the northern shelf plateau. The dives traversed the generally featureless muddy canyon floor, along near vertical walls, and onto the canyon rim. ROV imaging revealed typically massive and well-bedded sedimentary units that are variably lithified and mostly friable. Biostratigraphic and palaeoecological analysis of foraminifers from rock and sediment samples (~ 700 to 1600 m) indicate that they were deposited from the Late Paleocene to Early Oligocene within upper-middle bathyal (~ 200 to ~ 700 m) water depths, thus implying that significant subsidence has occurred. Strontium isotope (⁸⁷Sr/⁸⁶Sr) dating also suggests the presence of Early Miocene sediments at the shallower sites.

Settlement of large benthic sessile organisms is largely limited to indurated substrates mostly along the canyon walls. Corals were specifically targeted, with solitary (Desmophyllum dianthus, Caryophyllia sp., Vaughanella sp., and Polymyces sp.) and colonial (Solenosmilia variabilis) scleractinians found sporadically distributed along the walls and beneath overhangs in the deeper canyon valleys as well as along the canyon rims. Gorgonian, bamboo, and proteinaceous corals were also present with noticeable patches of live Corallium hosting a diverse community of organisms. Extensive coral graveyards were discovered between ~ 690–720 m and 1560–1790 m comprising colonial (S. variabilis) and solitary (D. dianthus) scleractinians, which had flourished during the last ice age between ~ 18 ka to 33 ka (BP).

Faunal sampling (674 m to 1815 m) spans the intermediate and deep waters, which were identified as Antarctic Intermediate Water and Upper Circumpolar Deep Water, with temperatures of ~ 2.5 to ~ 6 °C. The carbonate chemistry of those water depths show supersaturation (Ω_calc ~ 1.3 to 2.2) with respect to calcite, but mild saturation to undersaturation (Ω_arag ~ 0.8 to 1.4) with respect to aragonite. Notably, some scleractinians inhabit depths below the aragonite saturation horizon (~ 1000 m). Depth profile measurements of δ¹³C and nuclear bomb produced Δ¹⁴C show decreases within the upper canyon waters of up to ~ 0.8 ‰ (< 800 m) and 95 ‰ (< 500 m) respectively, relative to measurements taken nearby in 1978, thereby reflecting the ingress of anthropogenic carbon into upper intermediate waters. Thus, the canyon waters and its inhabitants are already being subject to the influences of CO₂ induced climate change and ocean acidification.