Holocene evolutionary history and accumulation rates for the granite based MacGillivray Reef System, Northern Great Barrier Reef

MacGillivray Reef (formerly known as Shadwell Reef) is located in the Northern Great Barrier Reef province, approximately 270km north of Cairns. Reef drilling was undertaken to investigate the Holocene evolution of the reef using radiocarbon dating of the cored coral material.\n\nTwo windward and two leeward cores were recovered from MacGillivray Reef. Two of the cores were initiated through thrown up coral blocks on the leeward margin which allowed a longer drilling window during the tidal range. Cores Mac 3 and 5 were abandoned due to the rising tide and equipment failure respectively; Mac 4 was abandoned after encountering a thick sand layer. The maximum depth reached was 11 m and 15 m an the windward and leeward margins respectively.\n\nThe cores were logged and photographed, and thin sections were made and petrographically examined. Corals that were determined to be in situ (by the orientation of their corallites) and free from internal cement and detritus by microscope examination were selected for radiocarbon dating.\n\nRadiocarbon dating suggests that MacGillivray platform reef developed on a topographic high, which is assumed to be granite, at a depth of 15 m below the reef flat approximately 8.2 cal kyr BP. The reef only approached present day sea-level within the last 500 years (260 - 80 cal yr BP). Rates of vertical accretion ranged from 1.4 to 5.8 m/kyr.

麦吉利夫雷礁（MacGillivray Reef）原名为沙德韦尔礁（Shadwell Reef），坐落于大堡礁北部海域，距凯恩斯以北约270公里。为借助取芯珊瑚材料的放射性碳定年法（radiocarbon dating）研究该礁的全新世（Holocene）演化历程，研究人员开展了礁体钻探工作。

研究人员从麦吉利夫雷礁采集了2个迎风侧（windward）与2个背风侧（leeward）的钻探岩芯（core）。其中2个岩芯以背风缘处的隆起珊瑚块为起始钻进点，可在潮汐周期（tidal range）内获得更长的钻进窗口期。麦吉利夫雷3号（Mac 3）与5号（Mac 5）岩芯分别因涨潮和设备故障中途停钻；麦吉利夫雷4号（Mac 4）则在钻遇厚砂层后终止钻进。迎风侧与背风缘的最大钻进深度分别为11米与15米。

研究人员对所有岩芯进行了编录与拍照，并制备薄片开展岩相学（petrographically）分析。经显微镜镜检，选取那些经鉴定为原位生长（依据珊瑚萼（corallites）的取向判断）且无内部胶结物与碎屑的珊瑚样品，用于放射性碳定年。

放射性碳定年结果显示，麦吉利夫雷台礁发育于礁坪下方约15米处的地形高地，该高地被推测为花岗岩岩体，其形成年代约为距今8.2个校准千年（cal kyr BP）。该礁体直至近500年内（距今260~80校准年（cal yr BP））才接近现今海平面。其垂向加积速率介于1.4~5.8米/千年。