Pleistocene sedimentary deposits on subantarctic Macquarie Island: Contribution to IGBP-Pages Pole Equator Pole Transect II

Macquarie Island is an emergent part of the Macquarie Ridge Complex composed of ocean-floor rocks of Miocene age now 4000 m above the ocean floor. A number of landforms, including palaeobeaches now above sea level (a.s.l.) on Macquarie Island, were formed by marine erosion during uplift of the island. During the last Pleistocene period of low sea level (c. 20 ka) the island was three times larger than now. Thermoluminescence (TL) dating of two palaeobeaches indicates Pleistonce ages: 172 plus or minus 40 ka for one at 100 m a.s.l. and 340 plus or minus 80 ka for another at 263 m a.s.l. Matching the altitude sequence of palaeobeaches on Macquarie Island with the pattern of peaks in world sea level determined from deep sea cores allows an independent estimate of beach ages. Comparison of the altitude and sea level sequences most plausibly places the 100 m palaeobeach in Oxygen Isotope Stage 5e (130-125 ka) and the 263 m palaeobeach in Stage 9 (340-330 ka), matching reasonably with the TL dates. Other palaeobeaches at about 50 m and 170-190 m a.s.l. then correlate with high sea levels. We calculate an average rate of uplift for the island of 0.8 mma-1. At this rate, 4000 m of Macquarie Ridge uplift would have taken about five million years and the top of the island may first have emerged some 700 to 600 ka. During the six Pleistocene glacial-interglacial cycles since then, there has been periglacial rather than glacial activity on cold uplands, but conditions suitable for vegetation of the present type persisted close to sea level.

麦夸里岛（Macquarie Island）是麦夸里海脊复合体（Macquarie Ridge Complex）的出露部分，该复合体由形成于中新世的洋底岩石构成，这些岩石现今出露于洋底以上4000米处。岛上发育有多种地貌，包括现今高于海平面（a.s.l.）的古海滩（palaeobeach），这些地貌均形成于岛屿抬升过程中的海洋侵蚀作用。在末次更新世低海平面时期（约20 ka），该岛的面积是现今的三倍。对两处古海滩的热释光（Thermoluminescence, TL）测年结果显示其形成于更新世：海拔100米a.s.l.处的古海滩测年为172±40 ka，海拔263米a.s.l.处的古海滩测年为340±80 ka。将麦夸里岛古海滩的海拔序列与深海岩芯重建的全球海平面峰值模式进行匹配，可独立估算古海滩的形成年代。通过对比海拔序列与海平面序列，最合理的拟合结果显示：海拔100米处的古海滩对应氧同位素阶段（Oxygen Isotope Stage）5e期（130~125 ka），海拔263米处的古海滩对应氧同位素阶段9期（340~330 ka），该结果与热释光测年结果较为吻合。其余海拔约50米、170~190米a.s.l.的古海滩则可对应高海平面时期。我们测算得出该岛的平均抬升速率为0.8毫米/年。按照该抬升速率，麦夸里海脊抬升4000米耗时约500万年，而该岛的出露初始时间可追溯至约70万~60万年前。在此之后的6次更新世冰期-间冰期旋回中，寒冷高地以冰缘作用而非冰川作用为主，但海平面附近仍维持着适宜现今植被生长的环境条件。