Evolution of the inter-reef Halimeda carbonate factory in response to Holocene sea-level and environmental change in the Great Barrier Reef

The inter-reef Halimeda bioherms of the northern Great Barrier Reef (GBR) have accumulated up to 25 m of positive relief and up to four times greater volume of calcium carbonate sediment than the nearby coral reefs during the Holocene. Covering >6000 km2, the Halimeda bioherms represent a significant contribution to the development of the northeast Australian continental shelf geomorphology, neritic carbonate factory, and sedimentary archive of post-glacial environmental changes. However, the geochronological record of initiation and development of the Halimeda bioherm carbonate factory was poorly constrained and based on very few datapoints. A comprehensive age dataset is presented, comprising sixty-three new AMS radiocarbon measurements of Halimeda and foraminifera grains, mollusc shells and bulk soil from twelve inter-reef sediment cores, and ten previously published Halimeda ages. Facies transitions and environmental changes are recorded from lithological and palynological analyses spanning Last Interglacial Halimeda deposits, Last Glacial Maximum terrestrial palaeosols, transgressive mangrove vegetated shorelines, and the turn-on of the Halimeda bioherm carbonate factory. Mangrove pollen and coral records from the study area provide additional spatial, temporal and environmental context.Halimeda had established by 11.1 cal kyr BP, just 450 years after transgressive shelf inundation and approximately 1000 years earlier than previous inferred estimates. The outer-shelf carbonate factory was initially dominated by benthic foraminifera, then Halimeda was productive for at least 2100 years prior to the turn-on of Holocene coral reefs in the study area at 8.9 cal kyr BP from Boulder Reef (15.4°S, 145.4°E). Inter-reef Halimeda bioherms play a major role in carbon and nutrient cycling and potentially preserve a near-continuous geochemical record of northeast Australian Holocene oceanographic and climatic changes, filling spatial and temporal gaps not covered by coral and other marine sediment proxies.</p

Cellular adaptations leading to coral fragment attachment on artificial substrates in Acropora millepora (Am-CAM)

Reproductive propagation by asexual fragmentation in the reef-building coral Acropora millepora depends on (1) successful attachment to the reef substrate through modification of soft tissues and (2) a permanent bond with skeletal encrustation. Despite decades of research examining asexual propagation in corals, the initial response, cellular reorganisation, and development leading to fragment substrate attachment via a newly formed skeleton has not been documented in its entirety. Here, we establish the first "coral attachment model" for this species ("Am-CAM") by developing novel methods that allow correlation of fluorescence and electron microscopy image data with in vivo microscopic time-lapse imagery. This multi-scale imaging approach identified three distinct phases involved in asexual propagation: (1) the contact response of the coral fragment when contact with the substrate, followed by (2) fragment stabilisation through anchoring by the soft tissue, and (3) formation of a "lappet-like appendage" structure leading to substrate bonding of the tissue for encrustation through the onset of skeletal calcification. In developing Am-CAM, we provide new biological insights that can enable reef researchers, managers and coral restoration practitioners to begin evaluating attachment effectiveness, which is needed to optimise species-substrate compatibility and achieve effective outplanting.</p

Reef core insights into mid-Holocene water temperatures of the southern Great Barrier Reef

The tropical and subtropical oceans of the Southern Hemisphere are poorly represented in present-day climate models, necessitating an increased number of paleoclimate records from this key region to both understand the Earth's climate system and help constrain model simulations. Here we present a site-specific calibration of live collected massive Porites Sr/Ca records against concomitant in situ instrumental water temperature data from the fore-reef slope of Heron Reef, southern Great Barrier Reef (GBR). The resultant calibration, and a previously published Acropora calibration from the same site, was applied to subfossil coral material to investigate Holocene water temperatures at Heron Reef. U-Th-dated samples of massive Porites suggest cooler water temperatures with reduced seasonal amplitude at ~5.2 ka (2.76–1.31�C cooler than present) and ~7 ka (1.26�C cooler than present) at Heron Reef. These results contrast the previous suggestion of a mid-Holocene Thermal Maximum in the central GBR around 5.35 ka and 4.48 ka, yet may be explained by differences in temperature of the shallow ponded reef flat (central GBR) and the deeper reef slope waters (this study) and potential large reservoir correction errors associated with early radiocarbon dates. Combining coral-based water temperature anomaly reconstructions from the tropical and subtropical western Pacific indicates a coherent temperature response across the meridional gradient from Indonesia and Papua New Guinea down to the southern GBR. This similarity in reconstructed temperature anomalies suggests a high probability of an earlier expression of a mid-Holocene Thermal Maximum on the GBR between ~6.8 and 6.0 ka

Significance of shallow core transects for reef models and sea-level curves, Heron Reef, Great Barrier Reef

A sequence of shallow reef cores from Heron Reef, Great Barrier Reef, provides new insights into Holocene reef growth models. Isochron analysis of a leeward core transect suggests that the north-western end of Heron Reef reached current sea-level by ca 6·5 kyr bp and then prograded leeward at a rate of ca 19·6 m/kyr between 5·1 kyr and 4·1 kyr bp (pre-1950) to the present reef margin. A single short core on the opposing margin of the reef is consistent with greater and more recent progradation there. Further to the east, one windward core reached modern sea-level by ca 6·3 kyr bp, suggesting near ‘keep-up’ behaviour at that location, but the opposing leeward margin behind the lagoon reached sea-level much more recently. Hence, Heron Reef exhibited significantly different reef growth behaviour on different parts of the same margin. Mean reef accretion rates calculated from within 20 m of one another in the leeward core transect varied between ca 2·9 m and 4·7 m/kyr depending on relative position in the prograding wedge. These cores serve as a warning regarding the use of isolated cores to inform reef growth rates because apparent aggradation at any given location on a reef varies depending on its location relative to a prograding margin. Only transects of closely spaced cores can document reef behaviour adequately so as to inform reef growth models and sea-level curves. The cores also emphasize potential problems in U-series dates for corals within a shallow (ca 1·5 m) zone beneath the reef flat. Apparent age inversions restricted to that active diagenetic zone may reflect remobilization and concentration of Th in irregularly distributed microbialites or biofilms that were missed during sample vetting. Importantly, the Th-containing contaminant causes ages to appear too old, rather than too young, as would be expected from younger cement

Morphotype differentiation in the Great Barrier Reef Halimeda bioherm carbonate factory:Internal architecture and surface geomorphometrics

The calcareous Halimeda bioherms of the northern Great Barrier Reef, Australia are the largest actively accumulating Halimeda deposits worldwide. They contribute a substantial component of the Great Barrier Reef neritic carbonate factory, as well as the geomorphological development of Australia's northeast continental shelf. Halimeda bioherm geomorphology is complex, expressing three distinct variations in morphotype patterns: annulate, reticulate and undulate. Similar regular and irregular geomorphological patterning often results from scale-dependent biophysical feedback mechanisms. Therefore, a better understanding of morphotype differentiation can inform the biotic and abiotic drivers of spatial heterogeneity in the bioherm ecosystem. Here, 3D LiDAR bathymetry is integrated with 2D sub-bottom profile datasets to investigate surface topography and internal sedimentary architecture of Halimeda bioherms through space and time. Using the ESRI ArcGIS 3D Analyst and Benthic Terrain Modeller extensions, the bioherm surface and subsurface geomorphometric characteristics were quantified for the annulate, reticulate and undulate morphotypes. Significant variation was found between the three bioherm morphotypes in their surface topography, internal structure, volume, slope gradients and terrain complexity. Therefore, their geomorphology is probably influenced by differing processes and biophysical feedback mechanisms. The complex surface topography does not appear to be inherited from the antecedent substrate, and preferred aspect orientations resulting from hydrodynamic forcing appear to be limited. It is suggested here that autogenic dynamics or biotic self-organization similar to patterns and processes in other marine organo-sedimentary systems modulates Halimeda bioherm geomorphology, and some hypotheses are offered towards future studies. Morphotype differentiation has implications for the development of the Halimeda bioherm carbonate factory, rates of sediment aggradation and progradation, and variable capacity to fill accommodation space. Self-organization dynamics and morphology differentiation in Modern bioherm systems could potentially inform palaeo-environmental interpretations of fossil bioherms and phylloid algal mounds on geological timescales.</p

Inter-reef Halimeda algal habitats within the Great Barrier Reef support a distinct biotic community and high biodiversity

Tropical marine biodiversity studies have been biased towards more accessible coastal habitats and shallow coral reefs, while deeper inter-reef habitats are less studied due to different survey challenges. One such inter-reef habitat is the ‘bioherms’ dominated by the calcareous Halimeda macroalgae. In the northern section of Australia’s Great Barrier Reef, Halimeda algal bioherms occupy >6,000 km2 of the inter-reef seabed, more than twice the area of adjacent shallow coral reefs. Here, we describe the biodiversity of the plant, vertebrate and invertebrate communities inhabiting Halimeda bioherms. By combining previous spatial mapping with legacy benthic biodiversity datasets, we find that Halimeda bioherms are a critically important complex habitat that hosts higher average species richness and diversity for both plants and invertebrates than the surrounding inter-reef (non-coral reef) seascape. Furthermore, at the community level, the structure of the bioherm-associated biotic assemblage is distinct from the non-bioherm community, with 40% of Halimeda bioherm-associated species not recorded at any non-bioherm sites. These findings improve estimates of the biodiversity of the Great Barrier Reef and elevate Halimeda bioherms as a critically important inter-reef habitat. Regular long-term monitoring is required to detect potential impacts to inter-reef biodiversity and ecosystem structure and function under future climate change scenarios.</p

Has Nitrogen Supply to Coral Reefs in the South Pacific Ocean Changed Over the Past 50 Thousand Years?

Tropical islands can facilitate surface ocean biological productivity by enhancing the supply of nitrogen to the euphotic zone. Yet in the world's most oligotrophic oceanic region, the South Pacific Subtropical Gyre (SPSG), this “island mass effect” appears diminished. If this is the case, where did island coral reefs in the SPSG get their nitrogen from, and has this changed over time? Here we use coral skeleton isotopes (δ15N and δ18O) and element/Ca ratios to identify the sources of nitrogen to a late Pleistocene coral reef in the SPSG (Cook Islands); we then compare these results to modern corals in the same region. The supply of nitrogen to the late Pleistocene reef appears dominated by upwelling of subsurface nitrogen during cool dry events, supplemented with nitrogen from island-induced N2 fixation (27 ± 3%) during warm wet periods. For the modern corals, N2 fixation supplies nitrogen to the island reefs during cool dry periods with groundwater providing nitrogen during wet periods. We propose that the subsurface supply of nitrogen to the modern reefs has declined as a result of reduced upwelling but this supply has been replaced with increasing nitrogen discharge from groundwater and an increase in island-induced N2 fixation.</p

Holocene microbialite geochemistry records > 6000 years of secular influence of terrigenous flux on water quality for the southern Great Barrier Reef

Anthropocene climate change and water quality degradation represent unprecedented challenges to modern coral reefs. Although declining reef health after European colonization is well documented around the world and increased terrigenous sediment flux is known to have terminated deglacial reefs in the Great Barrier Reef (GBR), longer-term patterns of water quality are poorly understood. Here we present the first direct proxy-based Holocene water quality reconstruction for any reef. The unique geochronological framework provided by cores from Heron and One Tree reefs (offshore, southern GBR) allowed reconstruction of offshore water quality from 8200 to 1800 years before present (BP) using centennially resolved microbialite-based geochemical proxies. Terrigenous sediment-sourced trace elements were measured in microbialites from a well dated succession of reef rock and in paleosol (ancient soil) formed at the Pleistocene-Holocene unconformity. Microbialite-hosted rare earth element and yttrium distributions (e.g., Nd/YbSN = 0.36; Y/Ho = 57) are consistent with precipitation from shallow oxygenated seawater but show a non-linear trend through the Holocene with distinct intervals of higher and lower terrigenous influence relative to average values. Immediately following reef initiation (>8300 years ago) our data suggest increasing terrigenous influence by 8000 ka. Surrounding reef seawater became less affected by terrigenous runoff from ~7000 years ago, but showed marked mid-Holocene variability related to regional climatic factors. Major fluctuations between intervals of high and low relative terrigenous influence correlate well with particular regional and more global climate records. These include local relative sea level fluctuations, fluctuations in Indian-Australian Summer Monsoon (IASM) strength, and dampened El Niño Southern Oscillation (ENSO) frequency corresponding to greater terrigenous influence in the southern GBR at ~7.0, 5.4, and 2.7 ka BP. Water quality then improved significantly after 3200 years BP. More broadly, it is well established that water quality has a major effect on reefs and reef communities, but for past reef history, inferences about water quality are commonly highly speculative. Reefal microbialite geochemistry provides an independent, high-quality proxy for ambient water quality that can be used to directly compare contemporaneous reef growth dynamics and ecological shifts to changing water quality. The high concentrations of trace elements in reefal microbialites, relative to other marine carbonates, provide a very robust, if time averaged, proxy for investigating ancient seawater chemistry, even in offshore reefs, such as Heron Reef. At the same time, the proxy provides a new independent data set for that may aid interpretation and model of climate change relevant to reef growth at centennial to millennial scales. As reefal microbialites are common in many global reef systems, where associated with high quality dating, they may provide useful proxies for investigating secular changes in water quality and associated climatic drivers at various temporal scales in other regions of the world.</p