Submerged Coral Reefs and Benthic Habitats of the Southern Gulf of Carpentaria: Post-Survey Report - GA Survey 276, RV Southern Surveyor

In 2003, Geoscience Australia discovered three large patch reefs in the southern Gulf of Carpentaria (GA Survey 238; SS-03/2004; Harris et al., 2004). The submerged platform reefs (R1, R2 and R3) are located east of Mornington Island and appear to have been formed when sea level was ~30 m below its present position, however as the ship did not come prepared with a drill-core sampler, the sub-surface composition of the reefs was not determined. The submerged platforms support live hard corals in many locations and their discovery raised the question of the possibility of widespread reef occurrence in that region. Survey 276 was designed to deliver some answers to these questions.

The current survey used rotary drilling of reefs R1, R2 and R3 which recovered coral material from 8 sites and confirmed the coral reef composition of these features. Multibeam sonar bathymetry and rotary drill cores were collected over two sections (R4 and R5) of a large (>100 km long) submerged platform that extends westwards from Mornington Island. The platform exhibits a Karst erosion surface, exhibiting drainage and depressions with raised rims, overprinting relict reef-growth geomorphic features. Reef growth features include raised rims, spur and groove reef front and elevated back-reef mounds. Other platform reefs were mapped in the south-western Gulf (R6 and R7) and in the Arafura Sea (R8). Rotary drilling has confirmed the coral reef composition of these features.

Preliminary assessments of the recovered drill cores indicate that reef growth has persisted in the region for several glacial cycles, extending over at least the past 120,000 years. Dating of Holocene corals by the U/Th method demonstrates that a phase of rapid (1-2 m per kyr) reef growth occurred at most sites between 9 and 7 kyr before present, with zero or much reduced growth rates occurring after 7 kyr ago. Although coral growth occurs in many areas, the production of carbonate has not been sufficient to build the reef-tops upwards to the present sea level. The observations of live corals, but low carbonate production rates, are consistent with a 'catch-up' reef growth pattern, in which the upper surfaces of the reefs are submerged 20 to 30 m below present sea level, with isolated local reef-tops having reached to within 18 m of the sea surface. An analysis of the hypsometry of the reef surfaces indicates that platform surfaces at all sites (R1 to R8) are confined to two narrow depth intervals, centred at 26.8 ± 1 m and 30.7 ± 0.3 m. The good correspondence of hypsometric peaks indicates regionally significant phases of carbonate deposition during a prolonged, Pleistocene sea level still stand.

This voyage has proved that the southern Gulf of Carpentaria contains a previously unknown major coral reef province in Australia. The reefs support locally diverse and luxuriant coral growth. From a management perspective, the slow rates of coral growth point to the need for protection of these reef systems because of their limited capacity to recover from natural or human-induced disturbances.

2003年，澳大利亚地质调查局（Geoscience Australia）在卡奔塔利亚湾（Gulf of Carpentaria）南部发现了三处大型斑礁（GA Survey 238；SS-03/2004；Harris等人，2004）。这些淹没台地礁（R1、R2和R3）位于莫宁顿岛东侧，推测形成于海平面较当前低约30米的时期，但由于科考船未配备钻孔取样器，未能探明这些礁体的地下组成。该区域多处存活有硬质珊瑚，此次发现引发了“该区域可能广泛分布礁体”的疑问，为此开展了第276次科考调查以解答相关问题。

本次第276次科考对R1、R2、R3三处礁体开展了回转钻探（rotary drilling），从8个点位获取了珊瑚样品，证实了这些构造的珊瑚礁属性。科考团队还针对一处从莫宁顿岛向西延伸、长度超100千米的大型淹没台地的两个区段（R4和R5）开展了多波束测深声呐（multibeam sonar bathymetry）测量与回转钻探。该台地存在喀斯特侵蚀面（Karst erosion surface），可见水系与带有凸起边缘的洼地，叠加了残存的礁体发育地貌特征，包括凸起边缘、礁前脊沟（spur and groove reef front）地貌以及抬升的礁后丘体。此外，在卡奔塔利亚湾西南部（R6、R7）与阿拉弗拉海（Arafura Sea）还测绘到其他台地礁（R8），回转钻探证实了这些构造同样属于珊瑚礁。

对获取岩心的初步评估显示，该区域的礁体发育已历经多个冰期循环，时间跨度至少达12万年。通过铀钍（U/Th）测年法对全新世（Holocene）珊瑚进行定年，结果表明绝大多数点位在距今9千至7千年期间经历了快速礁体发育（速率为每千年1~2米），7千年后发育速率降至零或大幅降低。尽管多处区域仍有珊瑚生长，但碳酸盐产出量不足以将礁体顶部抬升至当前海平面高度。现场观测到活珊瑚但碳酸盐产出速率较低，这与“追赶型”礁体发育模式相符：礁体上表面较当前海平面低20~30米，仅少数局部礁顶距离海面不足18米。对礁体表面的高程分布统计（hypsometry）分析显示，所有点位（R1至R8）的台地表面均集中在两个狭窄深度区间，分别为26.8±1米与30.7±0.3米。两个高程统计峰值的良好对应性，表明在长期的更新世（Pleistocene）海平面稳定期，该区域曾发生区域性的大规模碳酸盐沉积。

本次科考证实，卡奔塔利亚湾南部存在一处此前未被发现的澳大利亚大型珊瑚礁省。该区域礁体局部生长着多样且繁茂的珊瑚。从管理角度而言，缓慢的珊瑚生长速率意味着这些礁体系统从自然或人为干扰中恢复的能力有限，因此亟需对其加以保护。