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.\n\nAn 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.\n\nBy 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.\n\nDead 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).\n\nSo 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.\n\nOverall, our results show that U-series dating and palaeoecological approaches can provide unique insight into the GBR¿s past.\n\n\nData Units: U-series age (AD) ± 2-sigma error\n\n\nReferences:\n\n- 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.\n- 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.\n- 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.

整个加勒比海及印度-太平洋海域的珊瑚礁均呈现衰退轨迹，其诱因为人为扰动的协同效应。尽管有直接与间接证据表明，自18世纪中叶欧洲人定居以来，大堡礁（Great Barrier Reef, GBR）的近岸珊瑚礁已显现区域性衰退迹象，但要在区域尺度上明确人为扰动与珊瑚退化之间的关联仍颇具挑战。这主要是因为在1980年代中期长期监测体系建立之前，多数珊瑚礁的历史生态基线数据存在缺失。

明确造礁珊瑚群落死亡事件的发生时间、频率、持续时长与诱因，有助于指导管理工作以缓解潜在影响。此外，评估欧洲人定居前后珊瑚群落结构变化的时空变异特征，可为克服“基线漂移综合征（shifting baseline syndrome）”提供极具价值的管理工具。

本研究通过对大堡礁近岸珊瑚礁的珊瑚群落历史变化进行定年与制图，旨在对珊瑚群落结构开展详细的基线评估，并构建珊瑚群落死亡事件的历史与本质的精确年表。

本研究采用高精度（2σ误差≤1年）热电离质谱法（thermal ionisation mass spectrometry, TIMS）铀系（uranium-series, U-series）测年法对死亡珊瑚骨骼进行定年，以达成三个研究目标：1）明确大堡礁中部近岸海域棕榈群岛（Palm Islands region）、南部大堡礁麦凯（Mackay）流域周边礁体以及大堡礁远北部海域的珊瑚群落与块状滨珊瑚（Porites）的历史死亡时间；2）解析大堡礁外海海域单树礁（One Tree Reef）与苍鹭礁（Heron Reef）的历史风暴活动规律；3）重建大堡礁近岸磁岛（Magnetic Island）的全新世海平面变化历史。

截至目前，研究团队已在昆士兰大学放射性同位素实验室（Radiogenic Isotope Facility）完成了320余份样品的测年工作。研究结果显示，棕榈群岛海域在20世纪早期发生了珊瑚群落结构转变，此前占优势的鹿角珊瑚（Acropora）出现大规模消亡。研究推测，该消亡事件可能与水质下降与气候相关因素的协同影响有关。对大堡礁外海抬升礁体的分析表明，19至20世纪期间存在一段高强度风暴活动期。此外，对磁岛化石块状珊瑚的铀系测年结果显示，全新世时期的2200至7000年前，海平面较现今更高。

总体而言，本研究结果证实，铀系测年与古生态学方法可深入揭示大堡礁的历史演变过程。

数据单位：铀系测年年龄（公元纪年）±2倍标准偏差误差

参考文献：
- Yu K. F. 与 Zhao J. X.（2010）《大堡礁珊瑚的铀系测年结果显示约7000年前海平面至少高出0.7米》，《The Holocene》第20卷，第1-8页。
- Zhao J. X.、Neil D. T.、Feng Y. X.、Yu K. F. 与 Pandolfi J. M.（2009）《澳大利亚南部大堡礁苍鹭礁与威斯塔里礁风暴搬运年轻珊瑚礁块的高精度铀系测年》，《Quaternary International》第195卷，第122-127页。
- Zhao J. X.、Yu K. F. 与 Feng Y. X.（2009）《近代高精度238U-234U-230Th不平衡测年研究综述》，《Quaternary Geochronology》第4卷，第423-433页。