NOAA/WDS Paleoclimatology - Heron Reef Mid-Holocene Porites and Acropora Sr/Ca Data

The tropical and subtropical oceans of the Southern Hemisphere are poorly represented in present-day climate models, necessitating an increased number of paleoclimate records from this key region to both understand the Earth's climate system and help constrain model simulations. Here we present a site-specific calibration of live collected massive Porites Sr/Ca records against concomitant in situ instrumental water temperature data from the fore-reef slope of Heron Reef, southern Great Barrier Reef (GBR). The resultant calibration, and a previously published Acropora calibration from the same site, was applied to subfossil coral material to investigate Holocene water temperatures at Heron Reef. U-Th-dated samples of massive Porites suggest cooler water temperatures with reduced seasonal amplitude at ~5.2 ka (2.76–1.31°C cooler than present) and ~7 ka (1.26°C cooler than present) at Heron Reef. These results contrast the previous suggestion of a mid-Holocene Thermal Maximum in the central GBR around 5.35 ka and 4.48 ka, yet may be explained by differences in temperature of the shallow ponded reef flat (central GBR) and the deeper reef slope waters (this study) and potential large reservoir correction errors associated with early radiocarbon dates. Combining coral-based water temperature anomaly reconstructions from the tropical and subtropical western Pacific indicates a coherent temperature response across the meridional gradient from Indonesia and Papua New Guinea down to the southern GBR. This similarity in reconstructed temperature anomalies suggests a high probability of an earlier expression of a mid-Holocene Thermal Maximum on the GBR between ~6.8 and 6.0 ka.

当前气候模式对南半球热带及亚热带海洋的表征不足，因此亟需从这一关键区域获取更多古气候记录，以增进对地球气候系统的认知，并助力约束气候模式的模拟结果。本研究针对大堡礁（Great Barrier Reef, GBR）南部赫伦礁（Heron Reef）的前礁坡，将现场采集的块状孔珊瑚（massive Porites）的Sr/Ca比值记录，与同步原位实测的海水温度数据开展单点校准。将本次得到的校准结果，与该站点此前已发表的轴孔珊瑚（Acropora）校准结果一同应用于亚化石珊瑚样品，以重建赫伦礁的全新世（Holocene）海水温度。经铀钍（U-Th）测年的块状孔珊瑚样品显示，赫伦礁在约5.2 ka（较现代低2.76~1.31℃）和约7 ka（较现代低1.26℃）时期的海水温度更低，且季节振幅有所减弱。上述结果与此前提出的大堡礁中部在约5.35 ka和4.48 ka出现全新世中期热盛期（mid-Holocene Thermal Maximum）的观点相悖，但这一差异可通过以下因素解释：浅水区礁坪（大堡礁中部）与深水区礁坡（本研究区域）的水温差异，以及早期放射性碳测年（radiocarbon dating）潜在的较大储层校正误差。整合西太平洋热带及亚热带海域基于珊瑚重建的海水温度异常（temperature anomaly）记录后发现，从印度尼西亚、巴布亚新几内亚向南至大堡礁南部的经向梯度（meridional gradient）上，水温响应具有一致性。重建得到的温度异常的这一相似性表明，大堡礁的全新世中期热盛期（mid-Holocene Thermal Maximum）可能更早出现，时间区间约为6.8~6.0 ka。