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 error

References:

- 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.

加勒比海及印太海域的珊瑚礁均出现衰退轨迹，这一现象可归因于人为扰动的协同效应。尽管有直接与间接证据表明，自18世纪中期欧洲殖民以来，大堡礁（Great Barrier Reef, GBR）的近岸珊瑚礁已显现区域性衰退迹象，但仍难以确定区域尺度上人为扰动与珊瑚退化之间的关联。这主要是因为在20世纪80年代中期长期监测体系建立之前，多数这类珊瑚礁的历史生态基线数据缺失。

明确造礁石珊瑚群落死亡事件的发生时间、频次、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，评估欧洲殖民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」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精细化基线评估，并构建珊瑚群落死亡事件的历史进程与本质特征的精确年表。

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（U-series）测年技术对死亡珊瑚骨骼进行定年，以实现三大研究目标：1）明确棕榈群岛区域（大堡礁近岸中部）以及南堡礁（马凯流域周边礁体）、远北堡礁近岸珊瑚群落与块状滨珊瑚（Porites）的历史死亡时间；2）解析外堡礁（一塔礁与赫伦礁）的历史风暴活动规律；3）重建大堡礁近岸磁岛区域的全新世海平面变化历史。

截至目前，昆士兰大学放射同位素实验室已完成320余份样品的测年工作。研究结果显示，棕榈群岛区域在20世纪早期出现了珊瑚群落结构转变，且此前占据优势地位的鹿角珊瑚（Acropora）出现消亡。研究推测该消亡现象可能与水质下降与气候相关因素的协同影响有关。对外堡礁抬升礁体的分析表明，19至20世纪存在一段高强度风暴活动期。此外，对磁岛区域化石块状珊瑚的铀系测年结果显示，全新世时期约2200至7000年前的海平面高于当前海平面。

综上，本研究结果证实，铀系测年与古生态学方法可为揭示大堡礁的历史演变提供独特视角。

数据单位：铀系测年年龄（公元纪年）±2σ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表明约7000年前海平面至少高出当前0.7米》，《全新世》，20卷，1-8页。
- Zhao J.X.、Neil D.T.、Feng Y.X.、Yu K.F. 与 Pandolfi J.M.（2009）《澳大利亚南堡礁赫伦礁与维斯塔里礁的风暴搬运年轻珊瑚礁块体的高精度铀系测年》，《第四纪国际》，195卷，122-127页。
- Zhao J.X.、Yu K.F. 与 Feng Y.X.（2009）《近期高精度238U-234U-230Th不平衡测年研究综述》，《第四纪地质年代学》，4卷，423-433页。