Holocene reef growth in Torres Strait

The platform and fringing reefs of Torres Strait are morphologically similar to reefs of the northern Great Barrier Reef to the south, except that several are elongated in the direction of the strong tidal currents between the Coral Sea and the Gulf of Carpentaria. Surface and subsurface investigations and radiocarbon dating on Yam, Warraber and Hammond Islands reveal that the initiation and mode of Holocene reef growth reflect antecedent topography and sea-level history. On the granitic Yam Island, fringing reefs have established in some places over a Pleistocene limestone at about 6 m depth around 7000 years BP. Emergent Holocene microatolls of Porites sp. indicate that the reefs have prograded seawards while sea level has fallen gradually from at least 0.8 m above present about 5800 years BP. On the Warraber Island reef platform drilling near the centre indicated a Pleistocene limestone foundation at a depth of about 6 m over which reefs established around 6700 years BP. Reef growth lagged behind that on Yam Island. Microatolls on the mature reef flat indicate that the reef reached sea level around 5300 years BP when the sea was around 0.8–1.0 m above present. On the reef flat on the western side of Hammond Island bedrock was encountered at 7–8 m depth, overlain by terrigenous mud. A progradational reef sequence of only 1–2 m thickness has built seaward over these muds, as sea level has fallen over the past 5800 years. Reef-flat progradation on these reefs is interpreted to have occurred by a series of stepwise buildouts marked by lines of microatolls parallel to the reef crest, marking individual coalescing coral heads. Detrital infill has occurred between these. This pattern of reef progradation is consistent with the radiocarbon dating results from these reefs, and with seismic investigations on the Torres Reefs.     

Holocene stratigraphic and morphological evolution of the Wandandian Creek delta,St Georges Basin,New South Wales

Subaerial exposure beside the Wandandian Creek channel during the last glacial maximum led to the development of red and orange mottling and, in some areas, produced a palaeosol over the Pleistocene land surface. Incision of the palaeo-Wandandian Creek, during the Late Pleistocene, formed a relatively deep steep-sided channel partially infilled with medium-grained fluvial sand. This palaeovalley became drowned as the post-glacial marine transgression impounded the western portion of St Georges Basin and the basal prodelta/lagoonal mud facies was deposited from ca 7 ka. The Wandandian Creek delta prograded down the palaeovalley and reached the study area ～3500 – 4000 years ago with the deposition of delta-front sandy silt and the overlying prograded sand facies. The subaerial portion of the delta is characterised by well-developed floodplains, levees, mouth bars and backswamps. Dredging in Wandandian Creek and land clearing for rural and urban development have had little long-term effect on the growth and morphology of Wandandian Creek delta.     

Speleothem preservation and diagenesis in South African hominin sites implications for paleoenvironments and geochronology

Plio‐Pleistocene speleothems from australopithecine‐bearing caves of South Africa have the potential to yield paleoenvironmental and geochronological information using isotope geochemistry. Prior to such studies it is important to assess the preservation of geochemical signals within the calcitic and aragonitic speleothems, given the tendency of aragonitic speleothems to recrystallize to calcite. This study documents the geochemical suitability of speleothems from the principal hominin‐bearing deposits of South Africa. We use petrography, together with stable isotope and trace element analysis, to identify the occurrence of primary aragonite, primary calcite, and secondary calcite. This study highlights the presence of diagenetic alteration at many of the sites, often observed as interbedded primary and secondary fabrics. Trace element and stable isotopic values distinguish primary calcite from secondary calcite and offer insights into geochemical aspects of the past cave environment. δ¹³C values of the primary and secondary calcites range from +6 to −9‰ and δ¹⁸O values range from −4 to −6‰. The data are thus typical of meteoric calcites with highly variable δ¹³C and relatively invariant δ¹⁸O. High carbon isotope values in these deposits are associated with the effects of recrystallization and rapid outgassing of CO₂ during precipitation. Mg/Ca and Sr/Ca ratios differ between primary and secondary calcite speleothems, aiding their identification. Carbon and oxygen isotope values in primary calcite reflect the proportion of C₃ and C₄ vegetation in the local environment and the oxygen isotope composition of rainfall. Primary calcite speleothems preserve the pristine geochemical signals vital for ongoing paleoenvironmental and geochronological research. © 2009 Wiley Periodicals, Inc.     

Assessing the recent (1834 – 2002) morphological evolution of a rapidly prograding delta within a GIS framework: Macquarie Rivulet delta,Lake Illawarra,New South Wales

The recent morphological evolution of Macquarie Rivulet delta, Lake Illawarra, New South Wales, has been assessed from historical parish maps from 1834, 1892, 1916 and 1927 along with aerial photographs from 1938 to 2002. These images were digitised, georeferenced and analysed in a GIS framework. The 1834 sketch map could not be georeferenced and it was excluded from the calculation of delta progradation areas. Between 1892 and 1927 morphological changes were minimal, probably because the areas were not resurveyed between these map editions. A significant change in morphology between 1927 and 1938 reflects a change in source material from maps to aerial photographs. The research highlights that great care must be taken when utilising historical data. Major morphological changes observed between 1938 and 2002 included the development of a large crevasse splay in the 1960s to 1970s and the abandonment and infilling of the delta's previously active distributary channels. The research shows that the morphological changes observed on the delta can be related to both natural processes and anthropogenic modifications to the catchment and thus could be used in the development of catchment management plans.     

Anthropogenic influences on Australia's Great Barrier Reef

Although legislation has been introduced to allow stricter control of activities within the Great Barrier Reef region, paradoxically there are increasing pressures due to greater use of the Reef, and population expansion and economic development on the adjacent mainland. In addition, greater understanding of the functioning of the reef system has allowed for recognition of more subtle anthropogenic effects on the Reef. On the outer Reef some localised degradation may result from intense use, accidents or from pre‐legislation activities. Runoff from the mainland has the potential to introduce increased turbidity levels, reduced salinities and some chemical pollutants particularly on nearshore reefs and in the Cairns region. Through global scale atmospheric and oceanic circulation the Great Barrier Reef is open to more distant sources of perturbation.     

An investigation of morphological zonation of beach rock erosional features

Application of one-dimensional Fourier analysis to the erosional morphology of intertidal beach at a number of contrasting sites in North Queensland, Australia, confirms that there exists a zonation of hollows. Although different combinations of processes may be responsible for the morphology of each zone it is suggested that increasing case hardening and joint development with associated intermittent exposure of the beach rock may be the greatest influence on the development of erosion hollow zonation.     

The significance of coral reefs as global carbon sinks— response to Greenhouse

Coral reefs are net sinks for C, principally as CaCO₃ accretion. It is possible to predict quite accurately the rate of production, given adequate information about any particular reef environment. The best data set for an extensive region is that for the Great Barrier Reef (GBR). Careful analysis of this region and the incorporation of previously documented present day system calcification rates suggest net production (G) from G = 1 (kg CaCO₃ m⁻² yr⁻¹) for fringing reefs, to G = 1.9 for planar (infiled platform) reefs, G = 3 for ribbon reefs and lagoonal reefs. The 20,055 km² of reefs in the GBR are thus estimated to average G = 2.4, resulting in a total production of 50 million tonnes yr⁻¹. In a 50–100 year Greenhouse scenario of rising sealevel, we predict that recolonisation of present day reef flats will be extensive and prolific. Production will increase substantially, and this could be by as much as 40% (ranging from 0% for deep shoals to 180% for fringing reefs) to give 70 million tonnes yr⁻¹ if the rate of sealevel rise reaches or exceeds 6–8 mm yr⁻¹We estimate 115,000 km² of oceanic atolls worldwide. Drawing on points equivalence from the detailed analysis of the GBR, we estimate the atolls presently produce 160 million tonnes yr⁻¹. We predict that a similar 40% increase could be possible in the next 100 years or so resulting in a production of 220 million tonnes.Accepting an existing estimate of 617,000 km² for reefs worldwide, drawing from our projections from the GBR and the atolls, and making some assumptions about the remaining reef types (we suggest fringing reefs to dominate) we estimate global reef production at the present time to be 900 million tonnes yr⁻¹. Within the next 100 years or so, we suggest this rate could almost double to 1800 million tonnes. In the long term (several centuries) we predict that the continuing trend of recolonisation, particularly of fringing and planar reefs could result in the production of > 3000 million tonnes yr⁻¹ if rates of sealevel rise approaching or exceeding 6–8 mm yr⁻¹ are achieved. Eventually (> 500 yr), reefs could actually “drown” due to inability to match the rate of sealevel increase if that rate significantly exceeds 6–8 mm yr⁻¹.Thus, coral reefs at present act as a sink for 111 million tonnes C yr⁻¹, the equivalent of 2% of present output of anthropogenic CO₂. In the short term Greenhouse scenario (100 yr) we predict this could increase to the equivalent of 4% of the present CO₂ output. In the much longer term (several centuries), if all trends continue, this could increase to the equivalent of as much as 9% of the present CO₂ output.Unfortunately, we also predict that this considerable sink for C will be most likely of negative value in alleviating Greenhouse because of the immediate effect of CaCO₃ precipitation is to raise the P_CO2 of the surface oceans — ie, ot encourage CO₂ efflux to the atmosphere. We do not attempt to quantify this effect.Other Greenhouse changes such as seawater temperature increase, changes in cloud cover, increased rainfall and runoff, increased storm activity, and changes in dissolved CO₂ concentration and surface ocean circulation may complicate the reef response. However, we suggest that sealevel rise will be the dominant influence, at least during the next 100 years or so.