Dolomite is a magnesium rich carbonate mineral abundant in ancient coral reef formations [1-3] yet very little is found forming in modern sedimentary environments. For over 150 years this conundrum has led to various theories being put forward about dolomite formation, however none have solved the so called `Dolomite Problem'[1]. It has generally been considered a post-depositional diagenetic process [2, 3], despite little experimental success at replicating dolomite formation in normal sea water conditions [4]. Here we show dolomite is in fact forming with living crustose coralline algae Hydrolithon onkodes, a species growing prolifically in coral reefs globally. Chemical micro-analysis of the coralline skeleton reveals that not only are the cell walls calcitised, but that the cell spaces are typically filled with magnesite, rimmed by dolomite, or both. Thus there are at least three mineral phases present (magnesium calcite, dolomite and magnesite) rather than one or two (magnesium calcite and brucite) as previously thought[5-7]. Both the magnesium calcite and dolomite phases comprise a continuum of magnesium to calcium compositions, whereas magnesite is near ideal composition. Using a mass balance approach we quantify potential dolomitisation of the coralline algae and can account for the total amount of dolomite found in a raised Pleistocene reef [2]. Our results are consistent with observed dolomites in coralline-algal rich environments in fossil reefs. This is the first time the presence of dolomite in living coralline algae has been confirmed.

This study was undertaken as part of a program to collect baseline data from the seabed environments over the Van Diemen Rise, which comprises a series of carbonate platforms and banks in the eastern Joseph Bonaparte Gulf, northwest of Darwin. Samples were collected during a survey on board the RV Solander in August and September 2009. The state of saturation for the different carbonate minerals (aragonite, calcite, high-magnesium calcite) was calculated for each sampling site from the ratio of the ion activity product and the solubility product. The carbonate ion concentration used for the ion activity product was calculated from total alkalinity and pH. The solubility products of the carbonate minerals were derived from literature data, e.g. the solubility for high-magnesium calcite as a function of the mol% MgCO3 was based on experimental results by Plummer and Mackenzie (1974, American Journal of Science vol. 274, p. 61-83). The calculated average state of saturation was 1.4 (range: 0.8-1.9) for high-magnesium calcite, 4.2 (range: 3.4-4.6) for aragonite, and 6.4 (range: 5.1-6.8) for calcite. Values close to 1 suggest the mineral is in thermodynamic equilibrium with ambient water, which is the case for high-magnesium calcite. In contrast, aragonite and calcite are distinctively supersaturated. Given the near-equilibrium state of high-magnesium calcite, this mineral phase will likely be lost over a time scale of decades as ocean acidification progresses. This ongoing process will alter the sediment composition significantly given the high abundance of high-magnesium calcite. This study supports the concept of using high-magnesium calcite as an indicator for the progression of ocean acidification where surface sediments have been sampled and preserved over time.

Dense hydrocoral-sponge communities have been identified on the upper continental slope of George V Land, East Antarctica and declared Vulnerable Marine Ecosystems. Analysis of physical and biological datasets collected during the 2007/08 CEAMARC survey identified that the richest communities are found in the heads of canyons which receive Antarctic Bottom Water formed on the George V shelf, and the canyons harbouring rich benthos are also those that cut the shelf break. This led to several hypotheses regarding their distribution and three main factors were identified. These hypotheses were tested during a recent marine science voyage in January 2011 to the same region. Initial analysis of the new data supports the hypotheses regarding the physical controls on hydrocoral-sponge community distribution.

This movie describes the main findings of two Geoscience Australia surveys to the Gulf of Carpentaria in 2003 (survey 238) and 2005 (survey 276). The story relates to the discovery of submerged coral reefs across the southern Gulf and how they were identified using new multibeam sonar technology. The age of the reefs was determined using drill-core samples collected from the reefs, and measured by the Uranium/Thorium method at the Australian National University. The submerged reefs in the Gulf were not known to exist prior to this study and their discovery adds a new coral reef province to be managed and protected as part of Australia's marine zone.

High-CO2 gas fields serve as important analogues for understanding various processes related to CO2 injection and storage. The chemical signatures, both within the fluids and the solid phases, are especially useful for elucidating preferred gas migration pathways and also for assessing the relative importance of mineral dissolution and/or solution trapping efficiency. In this paper, we present a high resolution study focussed on the Gorgon gas field and associated Rankin trend gases on Australia's Northwest Shelf of Australia. The Gorgon field is characterized by a series of stacked reservoirs (Figure 1), and is therefore well placed to characterize CO2 migration, dissolution and reaction by looking at geochemical signatures in the different reservoirs. Hydrological data at the Gorgon field also suggests that many of the major faults possess very low transmissivities, which should prevent or limit mixing of reservoir fluids with different chemical imprints. The gas data we present here reveal correlatable trends for mole %-CO2 and --C CO2 both areally and vertically as observed by Edwards et al. (2007). We suggest that the observed relationships are imparted due to mineral carbonation reactions that occurred along the CO2 migration pathway. These results have important implications for carbon storage operations and suggest that under certain conditions mineral sequestration might occur over longer migration distances and on shorter timescales than previously thought.

Multibeam sonar mapping, drill cores and underwater video data have confirmed the existence of a previously unknown reef province in the Gulf of Carpentaria, Australia. Seven reefs, comprised of coral limestone that support living corals have been mapped so far and as many as 50 other reefs may exist in the region. U/Th ages show that reef growth commenced shortly after limestone pedestals were submerged by rising sea level around 10.5 kyr BP, making them the oldest reefs known in Australia. Reef growth persisted for ~2.0 kyr but it had ceased at most locations by ~8.0 kyr BP. Measurements of reef growth rates (0.95 to 4 m kyr-1), indicate that the reefs were unable to keep pace with contemporaneous rapid sea level rise (>10 m kyr-1), which is consistent with a 'give up' reef growth history. Core samples from reef platforms demonstrate that Pleistocene limestone is exposed in depths of 27 and 30 m below present mean sea level. These depths represent regionally significant phases of reef growth during a prolonged sea level still stand. We conclude that the reefs are therefore mostly relict features, whose major phase of growth and development relates to an earlier, pre-Holocene sea level stillstand.

Concern about the impact of ocean acidification on organisms secreting high magnesium calcite skeletons has led to renewed interest in the mineralogy of these organisms. The identification of minerals making up the skeletons of tropical coralline algae, and in particular the determination of the Mg-content of calcite, is most commonly performed with X-ray diffraction. This method, based on XRD peak position, attracted criticism in the past because it produced Mg-contents that were in some cases lower compared to those based on chemical analyses of the bulk sample (in solution). The recent discovery of dolomite and magnesite in living coralline algae skeletons in addition to Mg-calcite explained this issue, and it is our goal in the present study to reinstate XRD as a reliable, quick and affordable method for the study of the mineral make-up of coralline algae species. In this paper we review the history of mineralogical analyses on tropical coralline algae and identify physical preparation methods that can affect results. We build on existing XRD methods to develop simple sampling and analytical methods to identify the presence of dolomite and magnesite, and numerically assess peak asymmetry that is caused by the overlapping reflections of calcite, dolomite and magnesite. These methods do not require specialist crystallographic knowledge or expensive or time consuming processes. The additional information our methods produce can be used to study intra-cellular calcification, and helps to rapidly assess and compare the mineral make-up of large numbers of samples. We conclude that XRD should be an integral part of any mineralogical analysis of coralline algae skeletons, which may be composed of not only Mg-calcite (Ca1.0-0.6Mg0-0.4CO3), but also dolomite (Ca0.5Mg0.5CO3), magnesite (MgCO3) and aragonite (CaCO3).

The Coompana Project is a collaborative project between Geoscience Australia and the Geological Survey of South Australia, co-funded by Geoscience Australia’s Exploring for the Future Programme and the South Australian Government’s PACE Copper Initiative. The Coompana Project aims to provide new precompetitive geological, geophysical and geochemical data in the under-explored Coompana Province in South Australia. The pre-drilling geophysics program was undertaken to assist the drilling process by reducing the uncertainty associated with intersecting the targeted stratigraphy. Seismic data were acquired at eight proposed drilling sites for the Coompana Drilling Program in February 2017. Seismic data were collected using vertical and horizontal geophones. An accelerated weight-drop source was tested with metal and plastic strike plates. The plastic strike plate was preferred and used for all sites. P-wave reflection images were used to images subsurface layers and estimate bedrock depth at each drill site. Horizontal geophone data indicated S-wave data had higher resolution than the vertical geophone P-wave data, but initial testing did not provide good bedrock imaging. Fan shots can indicate zones of low velocity weathering at the near surface. Multichannel Analysis of Surface Waves provided near surface weathering profiles. Near surface weathering can be an indication of caves in this limestone environment. The S-wave shot records may indicate the presence of caves with S-wave signal degradation at cave locations, as S-waves do not travel through voids, not noticed at these sites. The seismic reflection method can be used to show subsurface variations and provide bedrock depth estimates, though the bedrock can be difficult to identify. The data is very noisy and data quality changes at different locations. Seismic data quality can be improved with a higher energy source.

This record contains the preliminary results of Geoscience Australia marine survey 238 (SS04/2003) to southeast Gulf of Carpentaria. The survey was completed between 9 May and 10 June 2003 using Australia's national facility research vessel Southern Surveyor. The survey included Geoscience Australia and CSIRO and Marine and Atmospheric Research scientists.

Lord Howe Island in the southwest Pacific Ocean is surrounded by a shallow (20 - 120 m) sub-tropical carbonate shelf 24 km wide and 36 km long. On the mid shelf a relict coral reef (165 km2) extends around the island in water depths of 30 - 40 m. The relict reef comprises sand sheet, macroalgae and hardground habitats. Inshore of the relict reef a sandy basin (mean depth 45 m) has thick sand deposits. Offshore of the relict reef is a relatively flat outer shelf (mean depth 60 m) with bedrock exposures and sandy habitat. Infauna species abundance and richness were similar for sediment samples collected on the outer shelf and relict reef, while samples from the basin had significantly lower infauna abundance and richness. The irregular shelf morphology appears to determine the distribution and character of sandy substrates and local oceanographic conditions, which in turn influence the distribution of different types of infauna communities.