Robin J. Beaman

New Evidence for the Holocene Sea-Level High from the Inner Shelf, Central Great Barrier Reef, Australia

Radiocarbon dates from fossil oyster beds of intertidal origin on Magnetic Island, north Queensland indicate that the local Holocene maximum of relative sea-level was attained no later than 5660 +/- 50 B.P. (conventional uncorrected age) and remained at 1.6-1.7 m above modern levels until 4040 +/- 50 B.P. Given the tectonic stability of the area, this implies that eustatic sea-level remained at its Holocene peak for at least ca. 1600 yr. The new high-precision sea-level data indicate sea levels 1-5 m higher than those of the same age inferred from buried mangrove deposits on the inner shelf in north Queensland. Uncertainties in deriving relative sea-level from such mangrove deposits may be a significant source of error in worldwide attempts to distinguish the eustatic and crustal warping components off relative sea-level change, especially in the tropics.

A new high-resolution bathymetry model for the Terre Adélie and George V continental margin, East Antarctica

Abstract The Collaborative East Antarctic Marine Census (CEAMARC) surveys to the Terre Adélie and George V continental margin highlight the requirement for a revised high-resolution bathymetry model that can be used as a spatial tool for improving information on the physical environment of the region. We have combined shiptrack singlebeam and multibeam bathymetry, coastline data, and land and ice sheet topographic data to develop a new regional-scale bathymetry grid, called GVdem (short for George V digital elevation model). The GVdem grid spans an area between 138–148°E and 63–69°S, with a cell pixel size of 0.001-arcdegree (c. 100 m). The revised digital elevation model is a large improvement over previously available regional-scale grids from the area, and highlights seabed physiographic detail not formerly observed in this part of East Antarctica. In particular, the extent and complexity of the rugged inner-shelf valleys are revealed, and their spatial relationship with large shelf basins and adjacent flat-topped banks. The new grid also reveals further insight into the spatial distribution of the submarine canyons found on the continental slope.

Continental shelf drift deposit indicates non-steady state Antarctic bottom water production in the Holocene

A late Quaternary, current-lain sediment drift deposit over 30 m in thickness has been discovered on the continental shelf of East Antarctica in an 850 m deep glacial trough off George Vth Land. Radiocarbon dating indicates that a period of rapid deposition on the drift (averaging 290 cm/kyr) occurred in the mid-Holocene, between about 3000 and 5000 yr before present. Slower rates of around 10 cm/kyr, during the past 0-3000 yr and from 5000 to about 13000 yr BP, coincides with deposition of bioturbated, ice-rafted debris (IRD) rich, sandy mud under an energetic bottom current regime. In contrast, the mid-Holocene (3000-5000 yr BP) sediments are fine-grained, laminated to cross-laminated with minimal IRD content, and are contemporaneous with a period of warmer marine conditions with less sea ice production. This pattern suggests that bottom currents were weaker than present day in the mid-Holocene, and that the rate of dense bottom water production was reduced at that time. This scenario is consistent with the hypothesis of non-steady state rates of Antarctic bottom water production through the Holocene as recently proposed by Broecker and his colleagues.

Geology–benthos relationships on a temperate rocky bank, eastern Bass Strait, Australia

To better understand the possible relationships between the geology of the seabed and the associated biological communities, a multibeam sonar survey over New Zealand Star Bank in the eastern Bass Strait was conducted. A hierarchical method of benthic habitat mapping was applied to the secondary biotope and biological facies levels at the site (<10 km) scale. Four secondary biotopes and four biological facies have been defined on the basis of geomorphology revealed by the bathymetry model and the results of statistical analysis of the sediment and underwater video transect data over the bank. The major differences that control the distribution of biological communities in the New Zealand Star Bank area appear to be related to variations in substrate. (1) Hard-ground features related to high-relief granite outcrops are associated with diverse and abundant sessile and motile fauna. These faunal communities may be biologically modified to patchy barrens habitat by grazing urchins. (2) Unconsolidated sediment on a flat seabed is associated with sparse small sponges on the inner shelf. On the middle shelf and seaward of bank, the flat and muddy seabed supports a community dominated by infauna. (3) Unconsolidated sediment on a low-relief seabed is associated with an increase in the density and sizes of sponges concentrated on any low-relief feature raised above the surrounding flat seabed.

Physical controls on deep water coral communities on the George V Land slope, East Antarctica

Abstract Dense coral-sponge communities on the upper continental slope at 570–950 m off George V Land, East Antarctica have been identified as Vulnerable Marine Ecosystems. The challenge is now to understand their probable distribution on other parts of the Antarctic margin. We propose three main factors governing their distribution on the George V margin: 1) their depth in relation to iceberg scouring, 2) the flow of organic-rich bottom waters, and 3) their location at the head of shelf cutting canyons. Icebergs scour to 500 m in this region and the lack of such disturbance is a probable factor allowing the growth of rich benthic ecosystems. In addition, the richest communities are found in the heads of canyons which receive descending plumes of Antarctic Bottom Water formed on the George V shelf, which could entrain abundant food for the benthos. The canyons harbouring rich benthos are also those that cut the shelf break. Such canyons are known sites of high productivity in other areas due to strong current flow and increased mixing with shelf waters, and the abrupt, complex topography. These proposed mechanisms provide a framework for the identification of areas where there is a higher likelihood of encountering these Vulnerable Marine Ecosystems.

Seafloor morphology and acoustic facies of the George V Land shelf

To study the seafloor morphology on the George V Land shelf, East Antarctica, over 2000km of high-frequency 3.5-27 kHz, echo-sounder data were collected between February and March 2000. The shelf can be divided into four acoustic facies: (a) Type IA-2 seabed is ice-keel turbate found on mid- to outer-shelf banks on seafloor less than 500m deep; (b) Type IB seabed is siliceous mud and diatom ooze drift, drape and fill deposits within the George V Basin between 750 and 850m depth; (c) Type IIB seabed is smooth diamicton below 500m depth, and occasionally has lowrelief megaflutes or ridge and swale features; (d) Type IIIC seabed is high relief ridges and canyons from the coast to the deepest part of the George V Basin. The acoustic facies are explained in terms of glacial and oceanographic influences on the shelf since the Last Glacial Maximum.

Mesophotic coral ecosystems on the walls of Coral Sea atolls

A research cruise was undertaken in October 2010 to explore potential mesophotic coral communities (30–150 m) in the recently established Coral Sea Conservation Zone (CSCZ). The CSCZ covers an area of almost one million square kilometres east of the Great Barrier Reef (Australia), with its reefs and atolls located hundreds of kilometres from the nearest landmass and surrounded by deep oceanic water. Three of the atolls in the CSCZ (West Holmes Reef [16.243°S, 147.874°E], East Holmes Reef [16.459°S, 148.024°E] and Flora Reef [16.755°S, 147.738°E]) were assessed using SCUBA and a Seabotix ROV. Shallow reef areas (<30 m) consisted largely of bare substrate with predominantly juvenile corals and very low coral cover due to past cyclone damage and thermal bleaching events. In contrast, the steep walls in 40–100 m depth were covered by extensive Halimeda curtains (Fig. 1a), which harboured diverse scleractinian coral communities, including Acropora, Astreopora, Fungia, Galaxea, Goniastrea, Porites, Mycedium (Fig. 1c), Seriatopora and Turbinaria spp., with Pachyseris (Fig. 1d), Leptoseris and Montipora spp. recorded to 102 m depth. At least one of the collected specimens represents a new species record for Australia: Echino- morpha nishihirai (Fig. 1b). Diverse communities of azooxanthellate octocorals were also observed to 150 m, the maximum depth of the ROV. These observations confirm the presence of mesophotic coral ecosystems (MCEs) along the walls of Coral Sea atolls and indicate that MCEs may form extensive features in the CSCZ. The deep-water coral communities may play an important role in the recovery of shallow reef areas on these isolated atolls by functioning as refugia from the repeated disturbances that have affected these reefs.

From Corals to Canyons: The Great Barrier Reef Margin

The significance of submerged fossil coral reefs as important archives of abrupt global sea level rise and climate change has been confirmed by investigations in the Caribbean [Fairbanks, 1989] and the Indo-Pacific (see Montaggioni [2005] for a summary) and by recent Integrated Ocean Drilling Program (IODP) activities in Tahiti [Camoin et al., 2007]. Similar submerged (40-130 meters) reef structures are preserved along the margin of the Great Barrier Reef (GBR), but they have not yet been systematically studied. The submerged reefs have the potential to provide critical new information about the nature of past global sea level and climate variability and about the response of the GBR to these past and perhaps future environmental changes [Beaman et al., 2008]. Equally important for GBR Marine Park managers is information about the role of the reefs as habitats and substrates for modern biological communities. Here we summarize the highlights and broader implications of a September- October 2007 expedition on the R/V Southern Surveyor (Australian Marine National Facility, voyage SS07/2007) to investigate the shelf edge, upper slope, and submarine canyons along the GBR margin.

Predicting the Location and Spatial Extent of Submerged Coral Reef Habitat in the Great Barrier Reef World Heritage Area, Australia

Aim Coral reef communities occurring in deeper waters have received little research effort compared to their shallow-water counterparts, and even such basic information as their location and extent are currently unknown throughout most of the world. Using the Great Barrier Reef as a case study, habitat suitability modelling is used to predict the distribution of deep-water coral reef communities on the Great Barrier Reef, Australia. We test the effectiveness of a range of geophysical and environmental variables for predicting the location of deep-water coral reef communities on the Great Barrier Reef. Location Great Barrier Reef, Australia. Methods Maximum entropy modelling is used to identify the spatial extent of two broad communities of habitat-forming megabenthos phototrophs and heterotrophs. Models were generated using combinations of geophysical substrate properties derived from multibeam bathymetry and environmental data derived from Bio-ORACLE, combined with georeferenced occurrence records of mesophotic coral communities from autonomous underwater vehicle, remotely operated vehicle and SCUBA surveys. Model results are used to estimate the total amount of mesophotic coral reef habitat on the GBR. Results Our models predict extensive but previously undocumented coral communities occurring both along the continental shelf-edge of the Great Barrier Reef and also on submerged reefs inside the lagoon. Habitat suitability for phototrophs is highest on submerged reefs along the outer-shelf and the deeper flanks of emergent reefs inside the GBR lagoon, while suitability for heterotrophs is highest in the deep waters along the shelf-edge. Models using only geophysical variables consistently outperformed models incorporating environmental data for both phototrophs and heterotrophs. Main Conclusion Extensive submerged coral reef communities that are currently undocumented are likely to occur throughout the Great Barrier Reef. High-quality bathymetry data can be used to identify these reefs, which may play an important role in resilience of the GBR ecosystem to climate change.

Processes controlling the formation of the Mertz Drift, George Vth continental shelf, East Antarctica: evidence from 3.5 kHz sub-bottom profiling and sediment cores

Recently discovered sediment drift deposits on the Antarctic continental shelf provide access to information on the Holocene palaeoceanography of the bottomcurrent regime within deep shelf basins that were previously inaccessible. The George Vth Basin on the East Antarctic margin has been identified by oceanographers as an important source of Antarctic BottomWater, hence the Holocene history of bottomcurrent activity here may be relevant to variations in bottom water export. The analysis of seismic and sediment core data indicates that the Holocene history of sedimentation on the Mertz Drift occurred in response to a progressively changing bottom current regime. The stratigraphic horizons that mark the onset of different phases of deposition are diachronous within the Mertz Drift. Rapidly accumulating, laminated siliceous mud and diatom ooze (SMO) that comprises the bulk was deposited first on the eastern side of the drift (5000-3500 years BP) and later on the lobate, southwestern side (3000 and 2000 years BP). This spatial variation in timing of rapid SMO deposition is attributed to a gradual increase in bottom current speed over the mid- to late Holocene. The deposition of true drift-style sedimentary features is restricted to a small area in the southwestern corner of the Mertz Drift, with the remaining parts characterised by drape and fill deposits. An estimation of the mass flux of sediment reaching the drift suggests that the modern, stronger bottom currents need carry only a very low suspended sediment concentration to explain the measured 10-fold reduction in sediment accumulation rates after 3000 years BP. Cores fromthe Mertz Drift and froma perched basin located 60 kmto the east contain the same lithologic units and have the same approximate age. Thus the palaeoenvironmental interpretations based on the Mertz Drift can be extrapolated over the entire George Vth Basin region.