Browsing by Author "Smithers, SG"

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    Benthic foraminifera: their importance to future reef island resilience
    (International Coral Reef Society, 2012-07-19) Dawson, JL; Hua, Q; Smithers, SG
    The provenance, age and redistribution of sediments across Raine Reef (11°35’28”S 144°02’17”E), northern Great Barrier Reef (GBR) are described. Sediments of both the reef flat and sand cay beaches are composed predominantly of benthic foraminifera (35.2% and 41.5% respectively), which is a common occurrence throughout the Pacific region. The major contemporary sediment supply to the island was identified as Baculogypsina sphaerulata, a relatively large (1-2 mm exclusive of spines) benthic foraminifera living on the turf algae close to the reef periphery, and responsible for beach sand nourishment. Radiometric ages of foraminiferal tests of ranging taphonomic preservation (pristine to severely abraded) included in surficial sediments collected across the reef flat were remarkably young (typically <60 years). Results indicate rapid transport and/or breakdown of sand with a minimal storage time on the reef (likely <102 years), inferring a tight temporal link between the reef island and sediment production on the surrounding reef. This study demonstrates the critical need for further research on the precise residence times of the major reef sediment components and transport pathways, which are fundamental to predicting future island resilience. © Copyright belongs to the authors.
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    The importance of large benthic foraminifera to reef island sediment budget and dynamics at Raine Island, northern Great Barrier Reef
    (Elsevier, 2014-10-01) Dawson, JL; Smithers, SG; Hua, Q
    Low-lying reef islands are among the most vulnerable environments on earth to anthropogenic-induced climate change and sea-level rise over the next century because they are low, composed of unconsolidated sediment that is able to be mobilised by waves and currents, and depend on sediments supplied by reef organisms that are particularly sensitive to environmental changes (e.g. ocean temperatures and chemistry). Therefore, the spatial and temporal links between active carbonate production and island formation and dynamics are fundamental to predicting future island resilience, yet remain poorly quantified. In this paper we present results of a detailed geomorphological and sedimentological study of a reef and sand cay on the northern Great Barrier Reef. We provide an empirical investigation of the temporal linkages between sediment production and reef island development using a large collection of single grain AMS 14C dates. Large benthic foraminifera (LBF) are the single most important contributor to contemporary island sand mass (47%; ranging from 36% to 63%) at Raine Island, reflecting rapid rates of sediment production and delivery. Standing stock data reveal extremely high production rates on the reef (1.8 kg m− 2 yr− 1), while AMS 14C dates of single LBF tests indicate rapid rates of sediment transferral across the reef. We also demonstrate that age is statistically related to preservation and taphonomic grade (severely abraded tests > moderately abraded tests > pristine tests). We construct a contemporary reef and island sediment budget model for Raine Island that shows that LBF (Baculogypsina, Marginopora and Amphistegina) contribute 55% of the sediment produced on the reef annually, of which a large proportion (54%) contribute to the net annual accretion of the island. The tight temporal coupling between LBF growth and island sediment supply combined with the sensitivity of LBF to bleaching and ocean acidification suggests that islands dominated by LBF are likely to be very sensitive to short and long term climate change projections. Potential outcomes of this work relate to improving the understanding of the future change dynamics of reef islands in response to climate change. © 2014, Elsevier B.V.
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    Marine reservoir correction for the Cocos (Keeling) Islands, Indian Ocean
    (Cambridge University Press, 2004) Hua, Q; Woodroffe, CD; Barbetti, M; Smithers, SG; Zoppi, U; Fink, D
    Known-age corals from the Cocos (Keeling) Islands, Indian Ocean, have been analyzed by accelerator mass spectrometry (AMS) for radiocarbon to determine marine reservoir age corrections. The ΔR value for the Cocos (Keeling) Islands is 66 ± 12 yr based on the analyses undertaken for this study. When our AMS and previously published dates for Cocos are averaged, they yield a ΔR of 64 ± 15 yr. This is a significant revision of an earlier estimate of the ΔR value for the Cocos (Keeling) Islands of 186 ± 66 yr (Toggweiler et al. 1991). The (revised) lower ΔR for the Cocos (Keeling) Islands is consistent with GEOSECS 14C data for the Indian Ocean, and previously published bomb 14C data for the Red Sea, Gulf of Aden, and Cocos Islands. The revised ΔR is also close to values for the eastern Indian Ocean and adjacent seas. These suggest surface waters that reach the Cocos Islands might be partly derived from the far western Pacific, via the Indonesian throughflow, and might not be influenced by the southeast flow from the Arabian Sea. Copyright © The Arizona Board of Regents on behalf of the University of Arizona
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    Microatolls and the record of Holocene sea level that can be derived from them
    (Australian Geosciences Council, 2012-08-05) Woodroffe, CD; Smithers, SG; McGregor, HV; Fink, D; Lambeck, K
    The Great Barrier Reef expedition in 1973 described living and fossil microatolls on the Great Barrier Reef and recognised their significance as sea-level indicators. John Chappell subsequently surveyed and dated Porites microatolls along the mainland and interpreted them in the context of hydro-isostatic adjustment, inferring gradual falling sea level during the past 6000 years. We re-examine the significance of microatolls, providing a detailed account of more than 100 fossil microatolls from Christmas Island in the central Pacific. We relate their elevations to living modern equivalents and compare the sea-level record derived from them with the modelled Holocene isostatically-corrected sea level for this equatorial location. These long-lived massive corals, up to 9 metres in diameter, preserve a biologically-mediated multi-decadal record of sea-level over the past 5000 years. The upper surface of fossil microatolls on the reef flat lie in a narrow elevation range similar to that observed for their modern, living counterparts, implying negligible change of sea level. Further evidence for the lack of major fluctuations in sea level over the period 5000–1000 years BP is provided by corals from the interior of the island. This second population grew prolifically in a large lagoonal setting and is offset from those in open water by about 50 cm. The continuity of microatoll growth precludes significant oscillations of sea level during this time, and accords with geophysical modelling for this site, implying that the ‘eustatic’ contribution from post-6ka ice melt and the isostatic adjustment of the ocean floor to loading cancel each other at this site.
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    Mid-Pacific microatolls record sea-level stability over the past 5000yr
    (Geological Society of America, 2012-10-01) Woodroffe, CD; McGregor, HV; Lambeck, K; Smithers, SG; Fink, D
    There has been geographical variation in sea level since rapid postglacial melting of polar ice ceased similar to 6 k.y. ago, reflecting isostatic adjustments of Earth and ocean surfaces to past (and ongoing) redistribution of ice and water loads. A new data set of over 100 fossil microatolls from Christmas (Kiritimati) Island provides a Holocene sea-level record of unparalleled continuity. Living reef-flat corals grow up to a low-tide level. Adjacent fossil microatolls, long-lived Porites corals up to several meters in diameter, occur at similar elevations (+/-0.1 m), and extensive fossil microatolls in the island interior are at consistent elevations within each population. Collectively, they comprise an almost continuous sequence spanning the past 5 k.y., indicating that, locally, sea level has been within 0.25 m of its present position, and precluding global sea-level oscillations of one or more meters inferred from less stable locations, or using other sea-level indicators. This mid-Pacific atoll is tectonically stable and far from former ice sheets. The precisely surveyed and radiometrically dated microatolls indicate that sea level has not experienced significant oscillations, in accordance with geophysical modeling, which implies that the eustatic contribution from past ice melt and the isostatic adjustment of the ocean floor to loading largely cancel each other at this site. © 2012, Geological Society of America
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    The potential application of taphonomy-AMS 14C analysis in modelling reef island geomorphic response to climate change
    (18th INQUA Congress, 2011-07-21) Dawson, JL; Hu, Q; Smithers, SG
    The Great Barrier Reef (GBR) is the largest known reef system in the world with more than 2900 individual reefs and over 900 reef islands. As climate change progresses reef islands become increasingly vulnerable to sea-level rise, increased SST’s and ocean acidity, and more frequent tropical storms, through alterations to sediment production, transport and deposition. We focus on large benthic foraminifera, an important sediment component of reefs and reef islands globally. Using taphonomy coupled with more than 35 AMS 14C ages of individual foraminifera tests, we develop a novel approach to understanding the spatial and temporal patterns and rates of sediment production and delivery across a reef flat and how this relates to island formation and future change. Our study site was a platform reef and vegetated sand cay in the northern GBR (Raine Island, 11°35’28”S 144°02’17”E). Only four reef flat samples predate 1950 AD while all beach sediments are younger than ~1970 AD, indicating a remarkably short residence time for sediment on the reef flat. Calibrated 14C ages increase from the outer reef flat to the island consistent with the main transport pathways. At all sample sites, percent modern carbon (pMC) decreases as shell degradation increases (poorly abraded tests are ca. 10-20 yrs older than pristine tests). While the maximum residence time for foraminiferal sediment on the reef flat is in the order of 100-200 yrs, the bulk of sediment reaches the island within 10-15 yrs. These findings are of significant importance to the future stability and sustainable management of reef islands because such rapid exchange of sediment from the reef flat to the island would suggest a high degree of sensitivity to environmental change and an immediate island geomorphic response to any alteration to the reefs ability to produce sediment. Copyright (c) 2011 INQUA 18
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    Radiocarbon in corals from the Cocos (Keeling) Islands and implications for Indian Ocean circulation
    (American Geophysical Union (AGU), 2005-11-02) Hua, Q; Woodroffe, CD; Smithers, SG; Barbetti, M; Fink, D
    Annual bands of a Porites coral from the Cocos (Keeling) Islands, eastern Indian Ocean, were analysed by radiocarbon for 1955–1985 AD. A rapid oceanic response of the site to bomb 14C is found, with a maximum Δ14C value of 132‰ in 1975. This value is considerably higher than those for the northwestern Indian Ocean, suggesting that surface waters reaching Cocos are not derived from the Arabian Sea. Instead, Δ14C values for Cocos and those for Watamu (Kenya) agree well over most of the study interval, suggesting that the South Equatorial Current carries 14C-elevated water rather than 14C-depleted water westward across the Indian Ocean. This implies that oceanic upwelling in the northwestern Indian Ocean is spatially confined with little contribution to the upper limb of the global thermohaline circulation. © 2005 by the American Geophysical Union.