Great Barrier Reef Coral and Halimeda Cd/Ca Ratios: Holocene Paleochemical Records

在全新世过去的1万年里，大堡礁（澳大利亚昆士兰陆架）的珊瑚礁发育模式和时间可能受到地表水营养物质浓度变化和季节性上升流模式的控制。钙质绿藻在陆架后无礁带沉积文石板而形成的生物礁也可能受营养富集事件的控制。没有关于古营养水平的直接记录可以用来检验这些假设。然而，海水中溶解的镉含量与磷酸盐（一种必需的限制性营养物质）的含量密切相关。镉在珊瑚文石中的含量与海水中钙离子的比例成正比，钙离子是文石中的主要阳离子，目前已经开发出了精确测量珊瑚文石中微量含量的Cd/Ca比率的技术，并应用于研究季节性和年度地表水变化，主要是在加拉帕戈斯群岛的珊瑚中。珊瑚和哈利米达的Cd/Ca测量对确定大堡礁营养历史的效用尚不清楚，然而，这项可行性研究解决了几个目标，以确定大堡礁珊瑚和哈利米达的镉钙比如何在全新世期间作为该地区海洋条件的记录者。这些目标包括：(1)确定陆地影响可能影响大堡礁珊瑚中测量到的Cd/Ca信号的程度；（二）调整和验证哈利梅达文石的样品处理和分析方案；(3)记录了哈利梅达文石现代和下芯微量元素信号和保存状态与环境的关系。

The pattern and timing of coral reef development in the Great Barrier Reef (Queensland shelf, Australia) during the past 10,000 y of the Holocene may have been controlled by changing surface water nutrient concentrations and seasonal upwelling patterns as driven by rising seal level. The bioherms developed by accumulation of the aragonite plates deposited by the calcareous green algae Halimeda in the reefless tract behind the outer shelf may also be controlled by nutrient enrichment events. There are no direct records of paleo- nutrient levels available with which to test these hypotheses. However, dissolved cadmium levels in seawater correlate well with those of phosphate, an essential limiting nutrient. Cadmium is incorporated in coral aragonite in proportion to its seawater ratio to calcium, the major cation in aragonite, and techniques to accurately measure Cd/Ca ratios in coral aragonite at the trace levels found have been developed and applied to study seasonal and annual surface water variability, primarily in corals from the Galapagos. The utility of Cd/Ca measurements in coral and Halimeda for determining the nutrient history of the Great Barrier Reef is not yet known, however, this feasibility study addresses several objectives to define how cadmium-calcium ratios in Great Barrier Reef coral and Halimeda act as recorders of oceanographic conditions in that region during the Holocene. These objectives involve: (1) defining the degree to which terrestrial influences may affect measured Cd/Ca signals in Great Barrier Reef corals; (2) adapting and verifying sample handling and analytical protocols for use with Halimeda aragonite; and (3) documenting the modern and down-core trace element signals and preservational states in Halimeda aragonite in relationship to their environment.