GBR - Dating and mapping historical changes in Great Barrier Reef coral communities, 2007 - 2010 (MTSRF 1.1.4, UQ)

Trajectories of decline have been observed in coral reefs throughout the Caribbean and Indo-Pacific region attributable to the synergistic effects of human-induced disturbances. Whilst direct and indirect evidence suggests that inshore reefs from the Great Barrier Reef (GBR) are showing signs of regional decline following European settlement in the mid 18th century, it has proven difficult to ascertain the link between anthropogenic disturbance and coral degradation on a regional scale. This is primarily due to the absence of detailed ecological baselines on the past condition of many of these reefs prior to the advent of long-term monitoring in the mid 1980’s.An understanding of the timing, frequency, duration and cause of mortality events in hard coral communities is necessary to help direct management efforts towards ameliorating potential impacts. Furthermore, assessing the spatial and temporal variability of changes in community structure before and after European settlement, will provide an invaluable management tool to overcome the ‘shifting baseline’ syndrome.By dating and mapping historical changes in coral communities of inshore reefs of the GBR, the purpose of this study is to provide a detailed baseline assessment on coral community structure and an accurate chronology on the history and nature of mortality events.Dead coral skeletons have been dated using the highly-precise (2? ± up to 1 year) thermal ionisation mass spectrometry (TIMS) uranium-series (U-series) dating method in order to determine 1) the timing of historical mortality in coral communities and massive Porites corals in the Palm Islands region (central Inshore GBR) and reefs adjacent to catchments in the southern GBR (Mackay region) and far northern GBR, 2) historical patterns of storm occurrence on the offshore GBR (One Tree and Heron Reefs), and 3) Holocene sea level changes from the inshore GBR (Magnetic Island).So far, more than 320 samples have been dated at the Radiogenic Isotope Facility, University of Queensland. Our results have revealed shifts in coral community structure and a loss of previously dominant Acropora corals in the early 20th century in the Palm Islands region. It is theorized that this loss may be attributable to the synergistic impacts of declining water quality and climatic related factors. Analysis of uplifted reef blocks from the offshore GBR reveals a period of high storm activity throughout the 19th & 20th centuries. Furthermore, U-series dating of fossil massive corals from Magnetic Island suggests that sea level was higher than present around 2200 to 7000 years ago during the Holocene.Overall, our results show that U-series dating and palaeoecological approaches can provide unique insight into the GBR’s past.Data Units: U-series age (AD) ± 2-sigma errorReferences:- Yu K. F. and Zhao J. X. (2010) U-series dates of Great Barrier Reef corals suggest at least +0.7 m sea level ~7000 years ago. The Holocene 20, 1-8.- Zhao J. X., Neil D. T., Feng Y. X., Yu K. F., and Pandolfi J. M. (2009) High-precision U-series dating of very young cyclone-transported coral reef blocks from Heron and Wistari reefs, southern Great Barrier Reef, Australia. Quaternary International 195, 122-127.- Zhao J. X., Yu K. F., and Feng Y. X. (2009) High-precision 238U-234U-230Th disequilibrium dating of the recent past - a review. Quaternary Geochronology 4, 423–433.

在加勒比海及印度-太平洋区域的珊瑚礁中，已观测到由人为干扰协同效应导致的衰退轨迹。尽管直接和间接证据表明，自18世纪中期欧洲殖民定居以来，大堡礁（Great Barrier Reef, GBR）的近岸礁已显现区域衰退迹象，但在区域尺度上确定人为干扰与珊瑚退化之间的关联仍被证明是困难的。这主要是因为，在20世纪80年代中期长期监测开始之前，许多此类珊瑚礁过去状态的详细生态基线缺失。

了解硬珊瑚群落中死亡事件的发生时间、频率、持续时间及原因，对于指导管理措施以缓解潜在影响至关重要。此外，评估欧洲殖民定居前后群落结构变化的时空变异性，将为克服“基线漂移”综合征（shifting baseline syndrome）提供宝贵的管理工具。

通过对大堡礁近岸礁珊瑚群落的历史变化进行定年与制图，本研究旨在提供珊瑚群落结构的详细基线评估，以及死亡事件历史与性质的准确年表。已采用高精度（2σ ± 可达1年）的热电离质谱法（thermal ionisation mass spectrometry, TIMS）铀系（uranium-series, U-series）定年法对死亡珊瑚骨骼进行定年，以确定：1）棕榈岛区域（大堡礁中部近岸）及大堡礁南部（麦凯区域）集水区邻近礁、大堡礁远北部珊瑚群落和块状滨珊瑚的历史死亡时间；2）大堡礁离岸礁（One Tree Reef和Heron Reef）的历史风暴发生模式；3）大堡礁近岸（Magnetic Island）的全新世海平面变化。

截至目前，已有超过320个样本在昆士兰大学放射性同位素实验室完成定年。我们的结果显示，20世纪初棕榈岛区域的珊瑚群落结构发生转变，先前占主导地位的鹿角珊瑚（Acropora）消失。推测这一消失可能归因于水质下降与气候相关因素的协同影响。对大堡礁离岸礁抬升礁块的分析显示，19世纪至20世纪期间存在一个高风暴活动期。此外，对Magnetic Island化石块状珊瑚的铀系定年表明，全新世期间（约2200至7000年前）海平面高于当前水平。

总体而言，我们的结果表明，铀系定年与古生态学方法可为大堡礁的过去提供独特见解。

数据单位：铀系年龄（AD）± 2σ误差

参考文献：
- Yu K. F. 和 Zhao J. X. (2010) 大堡礁珊瑚的铀系定年表明，约7000年前海平面至少比现在高+0.7米。《The Holocene》20卷，1-8页。
- Zhao J. X., Neil D. T., Feng Y. X., Yu K. F., 和 Pandolfi J. M. (2009) 澳大利亚大堡礁南部Heron Reef和Wistari Reef极年轻的飓风搬运珊瑚礁块的高精度铀系定年。《Quaternary International》195卷，122-127页。
- Zhao J. X., Yu K. F., 和 Feng Y. X. (2009) 近期高精度238U-234U-230Th不平衡定年——综述。《Quaternary Geochronology》4卷，423–433页。