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.

Two 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.

The 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.

Radiocarbon 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.

麦吉利夫里礁（MacGillivray Reef，前称谢德韦尔礁Shadwell Reef）坐落于大堡礁（Great Barrier Reef）北部海区，距凯恩斯以北约270公里。为探究该礁的全新世（Holocene）演化历程，研究团队通过对取芯获取的珊瑚材料开展放射性碳定年（radiocarbon dating），实施了礁体钻探工作。

本次钻探从麦吉利夫里礁共获取4根岩芯，其中2根取自迎风岸（windward），2根取自背风岸（leeward）。其中2根岩芯以背风缘出露的珊瑚碎块作为开孔点位，该条件在潮汐周期（tidal range）内可提供更长的钻探作业窗口。Mac 3与Mac 5号岩芯分别因涨潮与设备故障中途废弃；Mac 4号岩芯则在钻遇厚砂层后终止钻探。迎风岸与背风缘的最大钻探深度分别为11米与15米。

所有岩芯均完成编录与拍照，并制备薄片（thin sections）开展岩石学镜下鉴定。经镜下观测，那些被判定为原生原位（in situ，依据珊瑚体的珊瑚石（corallites）排列方向）且无内部胶结物与碎屑（detritus）的珊瑚样品，被选定用于放射性碳定年测试。

放射性碳定年结果表明，麦吉利夫里台礁发育于一处被推断为花岗岩（granite）的地形高地（topographic high）之上，该高地位于礁坪（reef flat）下方约15米处，年代约为距今8.2校准千年BP（cal kyr BP）。该礁体直至近500年内（260~80校准年BP，cal yr BP）才接近现今海平面高度。其垂直加积速率介于1.4至5.8米/千年之间。