Less than one year after the spectacular calving of the Mertz Glacier tongue, scientists were collecting the first ever images of the seafloor where the glacier tongue once sat.

This report is the second of three reports that provide the scientific analyses and interpretations resulting from a four-year collaborative habitat mapping program undertaken within the Darwin and Bynoe Harbour region by Geoscience Australia (GA), the Australian Institute of Marine Science (AIMS) and the Northern Territory Government Department of Environment and Natural Resources (DENR). This program was made possible through offset funds provided by the INPEX-operated Ichthys LNG Project to DENR, and co-investments from GA and AIMS.

Geoscience Australia marine reconnaissance survey GA2476 to the west Australian continental margin was undertaken as part of the Australian Government's Offshore Energy Program between 25 October 2008 and 19 January 2009 using the German research vessel RV Sonne. The survey acquired geological, geophysical, oceanographic and biological data over poorly known areas of Australia's western continental margin in order to improve knowledge of frontier sedimentary basins and marginal plateaus, and allow assessment of their petroleum prospectivity and environmental significance. Four key areas were targeted: the Zeewyck and Houtman sub-basins (Perth Basin), the Cuvier margin (northwest of the Southern Carnarvon Basin), and the Cuvier Plateau (a sub-feature of the Wallaby Plateau). These areas were mapped using multi-beam sonar, shallow seismic, magnetics and gravity. Over the duration of the survey a total of 229,000 km2 (26,500 line-km) of seabed was mapped with the multibeam sonar, 25,000 line-km of digital shallow seismic reflection data and 25,000 line-km of gravity and magnetic data. Sampling sites covering a range of seabed features were identified from the preliminary analysis of the multi-beam bathymetry grids and pre-existing geophysical data (seismic and gravity). A variety of sampling equipment was deployed over the duration of the survey, including ocean floor observation systems (OFOS), deep-sea TV controlled grab (BODO), boxcores, rock dredges, conductivity-temperature depth profilers (CTD), and epibenthic sleds. Different combinations of equipment were used at each station depending on the morphology of the seabed and objectives of each site. A total of 62 stations were examined throughout the survey, including 16 over the Houtman Sub-basin, 16 over the Zeewyck Subbasin, 13 in the Cuvier margin, 12 over the Cuvier Plateau and four in the Indian Ocean. This dataset comprises total chlorin concentrations and chlorin indices measured on the upper 2 cm of seabed sediments. For more information: Daniell, J., Jorgensen, D.C., Anderson, T., Borissova, I., Burq, S., Heap, A.D., Hughes, M., Mantle, D., Nelson, G., Nichol, S., Nicholson, C., Payne, D., Przeslawski, R., Radke, L., Siwabessy, J., Smith, C., and Shipboard Party, (2010). Frontier Basins of the West Australian Continental Margin: Post-survey Report of Marine Reconnaissance and Geological Sampling Survey GA2476. Geoscience Australia, Record 2009/38, 229pp

Lord Howe Rise is a deep sea marginal plateau located in the Coral Sea and Tasman Sea, ~125,000 km2 in area and 750 to 1200 m in water depth. An area of the western flank of northern Lord Howe Rise covering ~25,500 km2 was mapped and sampled by Geoscience Australia in 2007 to characterise the deep sea environments and benthic habitats. Geomorphic features in the survey area include ridges, valleys, plateaus and basins. Smaller superimposed features include peaks, moats, holes, polygonal furrows, scarps and aprons. The physical structure and biological composition of the seabed was characterised using towed video and sampling of epifaunal and infaunal organisms. These deep sea environments are dominated by thick depositional soft-sediments (sandy mud), with local outcrops of volcanic rock and mixed gravel-boulders. Ridge, valley and plateau environments were moderately bioturbated but few organisms were directly observed or collected. Volcanic peaks were bathymetrically complex hard-rock structures that supported sparse distributions of suspensions feeders (e.g. cold water corals and glass sponges) and associated epifauna (e.g. crinoids and brittlestars). Isolated outcrops along the sloping edge of one ridge also supported similar assemblages, some with high localised densities of coral-dominated assemblages.

Benthic sediment sampling of Inner Darwin Harbour (GA0358) and shallow water areas in and around Bynoe Harbour (GA0359) was undertaken between May 29 and June 19, 2017. Partners involved in the surveys included Geoscience Australia (GA), the Australian Institute of Marine Science (AIMS) and the Department of Environment and Natural Resources within the Northern Territory Government (NT DENR) (formerly the Department of Land and Resource Management (DLRM)). These surveys form part of a four year (2014-2018) science program aimed at improving knowledge about the marine environments in the regions around Darwin and Bynoe Harbour’s through the collection and collation of baseline data that will enable the creation of thematic habitat maps to underpin marine resource management decisions. This project is being led by the Northern Territory Government and is supported by the INPEX-led Ichthys LNG Project, in collaboration with - and co-investment from GA and AIMS. This dataset comprises sediment oxygen demand measurements made on seabed sediments.

The Petrel Sub-basin Marine Environmental Survey GA-0335, (SOL5463) was undertaken using the RV Solander during May 2012 as part of the Commonwealth Government's National Low Emission Coal Initiative (NLECI). The survey was undertaken as a collaboration between the Australian Institute of Marine Science (AIMS) and GA. The purpose was to acquire geophysical and biophysical data on shallow (less then 100m water depth) seabed environments within two targeted areas in the Petrel Sub-basin to support investigation for CO2 storage potential in these areas. This 10 sample data-set comprises specific surface area and bulk (%) carbonate data from surface seabed sediments (~0-2 cm) in the Timor Sea.

The Petrel Sub-basin Marine Environmental Survey GA-0335, (SOL5463) was undertaken using the RV Solander during May 2012 as part of the Commonwealth Government's National Low Emission Coal Initiative (NLECI). The survey was undertaken as a collaboration between the Australian Institute of Marine Science (AIMS) and GA. The purpose was to acquire geophysical and biophysical data on shallow (less then 100m water depth) seabed environments within two targeted areas in the Petrel Sub-basin to support investigation for CO2 storage potential in these areas. This 10 sample dataset comprises chlorophll abc measurments from surface sediments (0-2 cm) in the Timor Sea.

This dataset comprises an amalgamation of State, Territory and Commonwealth priorities in terms of Seabed mapping as of February 2017. Data that were identified as important for: safe navigation, environmental baseline assessment, resource competition and baseline data, and urgently required to support policy and government decisions were given priority. Agencies then ranked their priority areas 1 – very high/high (red), 2 – moderate (blue), and 3 – low (green), based on the need for the data and the impact of the data. These priority sets were compiled by Geoscience Australia and submitted to the Australian Hydrographic Office (AHO) for consideration as supplementary material for their "Hydroscheme" acquisition plan.

In September and October of 2011 Geoscience Australia surveyed part of the offshore northern Perth Basin in order to map potential sites of natural hydrocarbon seepage. The primary objectives of the survey were to map the spatial distribution of seepage sites and characterise the nature of the seepage at these sites (gas vs oil, macroseepage vs microseepage; palaeo vs modern day seepage) on the basis of: acoustic signatures in the water column, shallow subsurface and on the seabed; geochemical signatures in rock and sediment samples and the water column; and biological signatures on the seabed. Areas of potential natural hydrocarbon seepage that were surveyed included proven (drilled) oil and gas accumulations, a breached structure, undrilled hydrocarbon prospects, and areas with potential signatures of fluid seepage identified in seismic, satellite remote sensing and multibeam bathymetry data. Within each of these areas the survey acquired: water column measurements with the CTD; acoustic data with single- and multi-beam echosounders, sidescan sonar and sub-bottom profiler (sidescan not acquired in Area F as it was too deep in places); and sediment and biological samples with the Smith-McIntyre Grab. In addition, data were collected with a remotely operated vehicle (ROV), integrated hydrocarbon sensor array, and CO2 sensor in selected areas. Sampling with the gravity corer had limited success in many of the more shallow areas (A-E) due to the coarse sandy nature of the seabed sediments. This dataset comprises total sediment metabolism (CO2 production) and porewater pH and salinity measurments of seabed sediments.

Increases in atmospheric CO¬2 cause the oceanic surface water to continuously acidify, which has multiple and profound impacts on coastal and continental shelf environments. Here we present the carbonate mineral composition in surface sediments from a range of continental shelf seabed environments and their current and predicted stability under ocean acidifying conditions. Samples come from the following four tropical Australian regions. 1. Capricorn Reef (southern end of the Great Barrier Reef). 2. The Great Barrier Reef Lagoon. 3. Torres Strait. 4. The eastern Joseph Bonaparte Gulf. Outside of the near-shore zone, these regions typically have a carbonate content in surface sediments of 80 wt% or more. The abundance of high magnesium-calcites (HMC) dominates over aragonite (Arag) and low magnesium-calcite (LMC) and makes up between 36 and 50% of all carbonate. HMC is significantly more soluble than Arag and LMC and the solubility of HMC positively correlates with its magnesium concentration. Using the solubility data by Plummer and Mackenzie (1974) (1), 96% of HMC in the four regions is presently in thermodynamic equilibrium or slightly supersaturated relative to global mean tropical sea surface water. When the modelled saturation state for aragonite in equatorial areas for this century (2) is converted into HMC saturation state curves, HMC is predicted to become undersaturated in the four regions between 2040 to 2080 AD with typical HMC decline rates between 2 and 5% per year. The range of respective estimated carbonate dissolution rates is expected to exceed current continental shelf carbonate accumulation rates leading to net dissolution of carbonate during the period of HMC decline. In a geological context, the decline in HMC is a global event in tropical continental shelf environments triggered by reaching below-equilibrium conditions. The characteristic change in carbonate mineral composition in continental shelf sediments will serve as a geological marker for the proposed Anthropocene Epoch.