Significance of Halimeda bioherms to the global carbonate budget based on a geological sediment budget for the Northern Great Barrier Reef, Australia

Since the correlation between carbon dioxide levels and global temperatures was established in the ice core records, quantifying the components of the global carbon cycle has become a priority with a view to constraining models of the climate system. The link between the marine carbonate budget and atmospheric CO2 levels draws attention to the fact that this budget is still not adequately quantified. \n\nIt is widely accepted that the calcareous green alga Halimeda is a prominent contributor of carbonate to reef sediment facies in the tropics, however, Halimeda bioherms remain one of the main reservoirs of unaccounted for carbonate. In fact, it has been suggested that Halimeda bioherms on the shelf of the Northern Great Barrier Reef may contain an equal or greater volume of carbonate than the reefs themselves.\n\nHere we attempt to determine the quantitative relationship between Halimeda bioherm and coral reef derived carbonate accumulated during the Holocene on the outer shelf of the Northern Great Barrier Reef Province using data from seismic profiles, reef drill cores and Halimeda bioherm piston and vibrocore cores from the published literature. \n\nThe methodology used is detailed in the document availble for download through this record.\n\nCalculations reveal that the ribbon reefs of the Lizard Island and Cooktown regions cover areas of approximately 18.5 and 20.5 km2 and comprise approximately 0.52 and 0.92 Gt CaCO3 respectively. The Halimeda bioherms of the Lizard Island and Cooktown regions cover areas of approximately 184 and 118 km2 and comprise approximately 2.06 and 0.86 Gt CaCO3 respectively. \n\nThus, that ratio of ribbon reef to Halimeda bioherm CaCO3 is approximately 1:4 and 1:1 for the Lizard Island and Cooktown regions respectively. If we assume that the carbonate content of the Lizard Island region bioherms ranges from 20 to 90 %, we estimate that the mass balance of Halimeda bioherm CaCO3 is between 0.55 and 2.5 Gt. The ratio of ribbon reef to Halimeda bioherms CaCO3 may vary between 1:1 to 1:5 depending on the carbonate content of the Lizard Island bioherms.

自冰芯记录（ice core records）确立了二氧化碳浓度与全球气温之间的关联以来，量化全球碳循环的各组分便成为约束气候系统模型的核心研究要务。海洋碳酸盐收支与大气二氧化碳浓度之间的联系也凸显出一个事实：该碳收支仍未得到充分量化。

学界普遍认为，钙化绿藻仙掌藻（Halimeda）是热带珊瑚礁沉积相中碳酸盐的重要贡献者，但仙掌藻生物礁仍是未被充分核算的碳酸盐储库之一。事实上，已有研究指出，北大堡礁陆架上的仙掌藻生物礁所含碳酸盐体积可能与珊瑚礁相当，甚至更高。

本研究借助已发表文献中的地震剖面（seismic profiles）、珊瑚礁钻探岩芯以及仙掌藻生物礁活塞岩芯与振动岩芯数据，尝试明确北大堡礁省外陆架区全新世（Holocene）沉积的仙掌藻生物礁与珊瑚礁源碳酸盐之间的定量关系。

本研究采用的方法细节可通过本数据集记录获取下载。

计算结果显示，蜥蜴岛与库克敦区域的带状珊瑚礁面积分别约为18.5与20.5平方千米，其碳酸钙（CaCO₃）储量分别约为0.52与0.92 Gt。该区域的仙掌藻生物礁面积分别约为184与118平方千米，其碳酸钙储量分别约为2.06与0.86 Gt。

因此，蜥蜴岛与库克敦区域的带状珊瑚礁与仙掌藻生物礁的碳酸钙储量比分别约为1:4与1:1。若假设蜥蜴岛区域生物礁的碳酸盐占比为20%至90%，则本研究估算仙掌藻生物礁的碳酸钙质量平衡介于0.55至2.5 Gt之间。根据蜥蜴岛生物礁的碳酸盐占比差异，带状珊瑚礁与仙掌藻生物礁的碳酸钙储量比可在1:1至1:5之间波动。