Abstract Inshore coral reefs are threatened globally by decreased water quality resulting from an... more Abstract Inshore coral reefs are threatened globally by decreased water quality resulting from anthropogenic land clearing and other drivers. Understanding the timing and rate of past native vegetation clearing and forest firing is critical for improved catchment management to protect reefs and other coastal ecosystems. However, commonly used coral-based sediment proxies (e.g., Ba/Ca and Y/Ca) are controlled partly by variable precipitation and discharge rates. No current marine proxy provides a temporal record of rates and areas of land clearing itself. Here we present a high-resolution temporal record (1957–2010) of Vanadium/Calcium (V/Ca) ratios in a Porites coral colony from the southern Great Barrier Reef (GBR) that correlates uniformly with historical clearance rates of woody vegetation in the local catchment, despite discordant stream discharge. We found that the V/Ca record could not be explained by terrestrial flux, nutrient flux, hydrocarbon pollution or upwelling and propose that the reaction between hot alkaline ash and the upper soil layer under oxidizing conditions during clearing-associated burning may favour formation and desorption of soluble anionic VV species (e.g., HVO 4 2 − ) from soil-based iron oxyhydroxides containing less mobile VIII. The newly soluble V components can then mobilise quickly into the earliest surface runoff and reach coastal seawater, where they provide a distinct temporal record in corals. That record is temporally decoupled from the sediment-bound elements, such as Y and Ba, which require greater degrees of erosion to mobilise, and thus better reflect particulate sediment flux. The Porites V/Ca record documented here captured the temporal effects of specific legislations aimed at regulating and conserving vegetation in the local catchment and may provide a means for evaluating catchment management practices elsewhere. The new proxy also may provide records of catchment clearing and firing in prehistoric times, with implications for both natural and anthropogenic fire regimes in coastal areas.
Abstract Degradation of inshore water quality associated with catchment modification is threateni... more Abstract Degradation of inshore water quality associated with catchment modification is threatening global coral reef ecosystems. Coral trace element proxies are widely applied to document the magnitude and timing of historical changes in river runoff and other disturbances. However, conflicting interpretations of commonly used coral proxies (Ba/Ca, Y/Ca and Mn/Ca) complicate their application for examining historical changes in coastal water quality. The exploration of other coral trace element proxies (such as rare earth elements, REE) is limited in space and time, and to few coral genera. This study examined the dynamics of dissolved REE and yttrium (REEY) in the Fitzroy and Burdekin river tributaries and estuaries, Queensland, Australia. In addition, monthly-resolution long-term temporal records of Ba/Ca, Mn/Ca, Y/Ca and REEY proxies were investigated in two Porites and one Cyphastrea coral colonies from the central and southern Great Barrier Reef in order to test the reliability of these proxies to record the variability of river runoff and local anthropogenic disturbances. The results showed large scale removal of REEY (55 – 86 %) in the low salinity mixing zones of both estuaries with significant fractionation of Nd-Yb and Y-Ho, confirming the ability of coral REEY to act as a terrestrial runoff proxy. Coralline Ba/Ca, Y/Ca and Mn/Ca records generally lack coherence with proximal discharge. However, temporal fluctuations of REEY proxies, irrespective of geographic location and coral genus, showed consistent behaviour relative to regional discharge with progressive increase of total REE/Ca and shale normalized Nd/Yb and decrease of Y/Ho during high-flow summer periods. The shifting baselines of REEY proxies in Cyphastrea grown in the turbid water setting of Rat Island effectively captured the timing of Gladstone Port dredging activities. Our findings suggest that shale normalized coral REEY distributions outperformed other commonly used trace element proxies and are robust indicators of changing inshore water quality. Longer-term records of coral REEY, even covering the period before European settlement from the 1850s, may provide a means to identify shifting baselines and evaluate and quantify the impacts of catchment alteration on coastal water quality and specific coral reef communities.
Analysis of high resolution diversity data for Mississippian corals in the western interior Unite... more Analysis of high resolution diversity data for Mississippian corals in the western interior United States yielded mild latitudinal diversity gradients despite the small geographic area covered by samples and a large influence on diversity patterns by geographic sampling intensity (sample bias). Three competing plate tectonic reconstructions were tested using the diversity patterns. Although none could be forcefully rejected, one reconstruction proved less consistent with diversity patterns than the other two and additional coral diversity data from farther north in Canada would better discriminate the two equivalent reconstructions.Despite the relatively high sampling intensity represented by the analyzed database, diversity patterns were greatly affected by sample abundance and distribution. Hence, some effort at recognizing and accounting for sample bias should be undertaken in any study of latitudinal diversity gradients. Small-scale geographic lumping of sample localities had only small effects on geographic diversity patterns. However, large-scale (e.g., regional) geographic lumping of diversity data may not yield latitudinally sensitive diversity patterns. Temporal changes in coral diversity in this region reflect changes in eustacy, local tectonism, and terrigenous sediment flux, far more than they do shifting latitude. Highest regional diversity occurred during the interval when the studied region occupied the highest latitude. Therefore, diversity data from different regions may not be comparable, in terms of latitudinal inference. Small-scale stratigraphic lumping of the data caused a nearly complete loss of the latitudinal diversity patterns apparent prior to lumping. Hence, the narrowest possible stratigraphic resolution should be maintained in analyzing latitudinal diversity gradients.
The coral fauna of the Upper Mississippian (Chesterian) Pitkin Formation was described by Easton ... more The coral fauna of the Upper Mississippian (Chesterian) Pitkin Formation was described by Easton (1943) and, more recently, by Webb (1987), who defined two coral faunules. The lower faunule occurs throughout the Pitkin outcrop belt from northeastern Oklahoma to north-central Arkansas. The upper faunule has been recognized only in north-central Arkansas where a higher part of the Pitkin is preserved. Equivalent strata have apparently been removed to the west by the pre-Pennsylvanian regional unconformity that differentially truncates Chesterian strata across the Ozark uplift. Thus the upper faunule is less well known than the lower faunule. The purpose of this note is to describe a new tabulate coral from the upper faunule in Searcy County, Arkansas.
With rubble predicted to increase on coral reefs worldwide, we review the physical, biological, a... more With rubble predicted to increase on coral reefs worldwide, we review the physical, biological, and ecological dynamics of rubble beds, with a focus on how rubble generation, mobilization, binding, and coral recruitment is expected to change on future reefs. Major disturbances, including storms and coral bleaching, are predicted to increase in intensity and frequency, and—like localized impacts including blast fishing and ship groundings—generate large quantities of coral rubble. Reefs will have increasingly smaller recovery windows between successive disturbances, leading to persistence of unstable rubble beds on reefs. With more severe storms and increased bioerosion on future reefs, rubble mobilization thresholds will be met more often as smaller, less complex rubble pieces are generated. If rubble remains stable for adequate time, it can be bound by organisms including sponges and coralline algae, and eventually be cemented. However, increasing rubble mobilization frequencies wi...
Coral Reefs and Sea-Level Change: Quaternary Records and Modelling
Understanding of global sea-level changes and coral reef development is poorly constrained during... more Understanding of global sea-level changes and coral reef development is poorly constrained during Marine Isotope Stage 3 (MIS 3; ~ 60 to 30 ka). Australia’s North West Shelf (NWS), at depths of ~ 50 to 120 m below present sea-level (mbsl), represents an ideal natural laboratory to address these knowledge gaps. In this study, the authors investigate a unique suite of sea-bed rock drill (PROD) cores recovered as part of a geotechnical survey from the NWS ~ 150 km south-east of Ashmore Reef. Twenty cores, penetrating to 28 m below sea floor, were collected from the top of the now drowned platform complex in similar water depths (74.8 to 81.6 mbsl), forming two transects ~ 17 km apart. High-resolution 3D seismic and multibeam bathymetry data reveal three distinct, multigenerational platforms that are rimmed by smaller reef terraces and bisected by deeper channels, placing the core transects into a robust, regional geomorphic context that includes a succession of linear palaeo-shorelines...
We employ high-throughput thermal-neutron tomographic imaging to visualise internal diagnostic fe... more We employ high-throughput thermal-neutron tomographic imaging to visualise internal diagnostic features of dense fossiliferous breccia from three Pleistocene cave localities in Sumatra, Indonesia. We demonstrate that these seemingly homogeneous breccias are an excellent source of data to aid in determining taphonomic and depositional histories of complex depositional sites such as tropical caves. X-ray Computed Tomographic (CT) imaging is gaining importance amongst palaeontologists as a non-destructive approach to studying fossil remains. Traditional methods of fossil preparation risk damage to the specimen and may destroy contextual evidence in the surrounding matrix. CT imaging can reveal the internal composition and structure of fossils contained within consolidated sediment/rock matrices prior to any destructive mechanical or chemical preparation. Neutron computed tomography (NCT) provides an alternative contrast to X-rays, and in some circumstances, is capable of discerning den...
Proceedings of the National Academy of Sciences, 2013
Around 88 large vertebrate taxa disappeared from Sahul sometime during the Pleistocene, with the ... more Around 88 large vertebrate taxa disappeared from Sahul sometime during the Pleistocene, with the majority of losses (54 taxa) clearly taking place within the last 400,000 years. The largest was the 2.8-ton browsingDiprotodon optatum, whereas the ∼100- to 130-kg marsupial lion,Thylacoleo carnifex, the world’s most specialized mammalian carnivore, andVaranus priscus, the largest lizard known, were formidable predators. Explanations for these extinctions have centered on climatic change or human activities. Here, we review the evidence and arguments for both. Human involvement in the disappearance of some species remains possible but unproven. Mounting evidence points to the loss of most species before the peopling of Sahul (circa 50–45 ka) and a significant role for climate change in the disappearance of the continent’s megafauna.
Abstract Inshore coral reefs are threatened globally by decreased water quality resulting from an... more Abstract Inshore coral reefs are threatened globally by decreased water quality resulting from anthropogenic land clearing and other drivers. Understanding the timing and rate of past native vegetation clearing and forest firing is critical for improved catchment management to protect reefs and other coastal ecosystems. However, commonly used coral-based sediment proxies (e.g., Ba/Ca and Y/Ca) are controlled partly by variable precipitation and discharge rates. No current marine proxy provides a temporal record of rates and areas of land clearing itself. Here we present a high-resolution temporal record (1957–2010) of Vanadium/Calcium (V/Ca) ratios in a Porites coral colony from the southern Great Barrier Reef (GBR) that correlates uniformly with historical clearance rates of woody vegetation in the local catchment, despite discordant stream discharge. We found that the V/Ca record could not be explained by terrestrial flux, nutrient flux, hydrocarbon pollution or upwelling and propose that the reaction between hot alkaline ash and the upper soil layer under oxidizing conditions during clearing-associated burning may favour formation and desorption of soluble anionic VV species (e.g., HVO 4 2 − ) from soil-based iron oxyhydroxides containing less mobile VIII. The newly soluble V components can then mobilise quickly into the earliest surface runoff and reach coastal seawater, where they provide a distinct temporal record in corals. That record is temporally decoupled from the sediment-bound elements, such as Y and Ba, which require greater degrees of erosion to mobilise, and thus better reflect particulate sediment flux. The Porites V/Ca record documented here captured the temporal effects of specific legislations aimed at regulating and conserving vegetation in the local catchment and may provide a means for evaluating catchment management practices elsewhere. The new proxy also may provide records of catchment clearing and firing in prehistoric times, with implications for both natural and anthropogenic fire regimes in coastal areas.
Abstract Degradation of inshore water quality associated with catchment modification is threateni... more Abstract Degradation of inshore water quality associated with catchment modification is threatening global coral reef ecosystems. Coral trace element proxies are widely applied to document the magnitude and timing of historical changes in river runoff and other disturbances. However, conflicting interpretations of commonly used coral proxies (Ba/Ca, Y/Ca and Mn/Ca) complicate their application for examining historical changes in coastal water quality. The exploration of other coral trace element proxies (such as rare earth elements, REE) is limited in space and time, and to few coral genera. This study examined the dynamics of dissolved REE and yttrium (REEY) in the Fitzroy and Burdekin river tributaries and estuaries, Queensland, Australia. In addition, monthly-resolution long-term temporal records of Ba/Ca, Mn/Ca, Y/Ca and REEY proxies were investigated in two Porites and one Cyphastrea coral colonies from the central and southern Great Barrier Reef in order to test the reliability of these proxies to record the variability of river runoff and local anthropogenic disturbances. The results showed large scale removal of REEY (55 – 86 %) in the low salinity mixing zones of both estuaries with significant fractionation of Nd-Yb and Y-Ho, confirming the ability of coral REEY to act as a terrestrial runoff proxy. Coralline Ba/Ca, Y/Ca and Mn/Ca records generally lack coherence with proximal discharge. However, temporal fluctuations of REEY proxies, irrespective of geographic location and coral genus, showed consistent behaviour relative to regional discharge with progressive increase of total REE/Ca and shale normalized Nd/Yb and decrease of Y/Ho during high-flow summer periods. The shifting baselines of REEY proxies in Cyphastrea grown in the turbid water setting of Rat Island effectively captured the timing of Gladstone Port dredging activities. Our findings suggest that shale normalized coral REEY distributions outperformed other commonly used trace element proxies and are robust indicators of changing inshore water quality. Longer-term records of coral REEY, even covering the period before European settlement from the 1850s, may provide a means to identify shifting baselines and evaluate and quantify the impacts of catchment alteration on coastal water quality and specific coral reef communities.
Analysis of high resolution diversity data for Mississippian corals in the western interior Unite... more Analysis of high resolution diversity data for Mississippian corals in the western interior United States yielded mild latitudinal diversity gradients despite the small geographic area covered by samples and a large influence on diversity patterns by geographic sampling intensity (sample bias). Three competing plate tectonic reconstructions were tested using the diversity patterns. Although none could be forcefully rejected, one reconstruction proved less consistent with diversity patterns than the other two and additional coral diversity data from farther north in Canada would better discriminate the two equivalent reconstructions.Despite the relatively high sampling intensity represented by the analyzed database, diversity patterns were greatly affected by sample abundance and distribution. Hence, some effort at recognizing and accounting for sample bias should be undertaken in any study of latitudinal diversity gradients. Small-scale geographic lumping of sample localities had only small effects on geographic diversity patterns. However, large-scale (e.g., regional) geographic lumping of diversity data may not yield latitudinally sensitive diversity patterns. Temporal changes in coral diversity in this region reflect changes in eustacy, local tectonism, and terrigenous sediment flux, far more than they do shifting latitude. Highest regional diversity occurred during the interval when the studied region occupied the highest latitude. Therefore, diversity data from different regions may not be comparable, in terms of latitudinal inference. Small-scale stratigraphic lumping of the data caused a nearly complete loss of the latitudinal diversity patterns apparent prior to lumping. Hence, the narrowest possible stratigraphic resolution should be maintained in analyzing latitudinal diversity gradients.
The coral fauna of the Upper Mississippian (Chesterian) Pitkin Formation was described by Easton ... more The coral fauna of the Upper Mississippian (Chesterian) Pitkin Formation was described by Easton (1943) and, more recently, by Webb (1987), who defined two coral faunules. The lower faunule occurs throughout the Pitkin outcrop belt from northeastern Oklahoma to north-central Arkansas. The upper faunule has been recognized only in north-central Arkansas where a higher part of the Pitkin is preserved. Equivalent strata have apparently been removed to the west by the pre-Pennsylvanian regional unconformity that differentially truncates Chesterian strata across the Ozark uplift. Thus the upper faunule is less well known than the lower faunule. The purpose of this note is to describe a new tabulate coral from the upper faunule in Searcy County, Arkansas.
With rubble predicted to increase on coral reefs worldwide, we review the physical, biological, a... more With rubble predicted to increase on coral reefs worldwide, we review the physical, biological, and ecological dynamics of rubble beds, with a focus on how rubble generation, mobilization, binding, and coral recruitment is expected to change on future reefs. Major disturbances, including storms and coral bleaching, are predicted to increase in intensity and frequency, and—like localized impacts including blast fishing and ship groundings—generate large quantities of coral rubble. Reefs will have increasingly smaller recovery windows between successive disturbances, leading to persistence of unstable rubble beds on reefs. With more severe storms and increased bioerosion on future reefs, rubble mobilization thresholds will be met more often as smaller, less complex rubble pieces are generated. If rubble remains stable for adequate time, it can be bound by organisms including sponges and coralline algae, and eventually be cemented. However, increasing rubble mobilization frequencies wi...
Coral Reefs and Sea-Level Change: Quaternary Records and Modelling
Understanding of global sea-level changes and coral reef development is poorly constrained during... more Understanding of global sea-level changes and coral reef development is poorly constrained during Marine Isotope Stage 3 (MIS 3; ~ 60 to 30 ka). Australia’s North West Shelf (NWS), at depths of ~ 50 to 120 m below present sea-level (mbsl), represents an ideal natural laboratory to address these knowledge gaps. In this study, the authors investigate a unique suite of sea-bed rock drill (PROD) cores recovered as part of a geotechnical survey from the NWS ~ 150 km south-east of Ashmore Reef. Twenty cores, penetrating to 28 m below sea floor, were collected from the top of the now drowned platform complex in similar water depths (74.8 to 81.6 mbsl), forming two transects ~ 17 km apart. High-resolution 3D seismic and multibeam bathymetry data reveal three distinct, multigenerational platforms that are rimmed by smaller reef terraces and bisected by deeper channels, placing the core transects into a robust, regional geomorphic context that includes a succession of linear palaeo-shorelines...
We employ high-throughput thermal-neutron tomographic imaging to visualise internal diagnostic fe... more We employ high-throughput thermal-neutron tomographic imaging to visualise internal diagnostic features of dense fossiliferous breccia from three Pleistocene cave localities in Sumatra, Indonesia. We demonstrate that these seemingly homogeneous breccias are an excellent source of data to aid in determining taphonomic and depositional histories of complex depositional sites such as tropical caves. X-ray Computed Tomographic (CT) imaging is gaining importance amongst palaeontologists as a non-destructive approach to studying fossil remains. Traditional methods of fossil preparation risk damage to the specimen and may destroy contextual evidence in the surrounding matrix. CT imaging can reveal the internal composition and structure of fossils contained within consolidated sediment/rock matrices prior to any destructive mechanical or chemical preparation. Neutron computed tomography (NCT) provides an alternative contrast to X-rays, and in some circumstances, is capable of discerning den...
Proceedings of the National Academy of Sciences, 2013
Around 88 large vertebrate taxa disappeared from Sahul sometime during the Pleistocene, with the ... more Around 88 large vertebrate taxa disappeared from Sahul sometime during the Pleistocene, with the majority of losses (54 taxa) clearly taking place within the last 400,000 years. The largest was the 2.8-ton browsingDiprotodon optatum, whereas the ∼100- to 130-kg marsupial lion,Thylacoleo carnifex, the world’s most specialized mammalian carnivore, andVaranus priscus, the largest lizard known, were formidable predators. Explanations for these extinctions have centered on climatic change or human activities. Here, we review the evidence and arguments for both. Human involvement in the disappearance of some species remains possible but unproven. Mounting evidence points to the loss of most species before the peopling of Sahul (circa 50–45 ka) and a significant role for climate change in the disappearance of the continent’s megafauna.
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Papers by Gregory Webb