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

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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.