Trace and minor element ratios inHalimeda aragonite from the Great Barrier Reef

Abstract

The calcareous green algaHalimeda can be a substantial contributor to aragonite sediment in reef ecosystems. In contrast to coral aragonite, little is known about the trace and minor element composition ofHalimeda aragonite, so it is difficult to test oceanographic hypotheses about factors controlling its past growth. We investigated adapting trace element cleaning protocols for modern and HoloceneHalimeda aragonite, modern and HoloceneHalimeda trace and minor element compositions, and the potential utility ofHalimeda aragonite for paleoceanographic investigations. We successfully adapted and applied sample treatment protocols developed for measuring trace elements in coral aragonite (generally less than 500 y old) toHalimeda aragonite (modern to approximately 5000 y old in this study). ModernHalimeda aragonite from John Brewer Reef in the Central GBR had mean Cd/Ca ratios of 5.19 ± 1.68 nmol/mol\(( \pm 2\sigma /\sqrt n )\) forHalimeda micronesica and 2.35 ± 0.38 nmol/mol for three closely related species important in bioherm accumulationHalimeda copiosa, Halimeda hederacea, andHalimeda opuntia. Mn/Ca ratios, with means from 89–239 nmol/mol for these four species, showed both intra-and inter-specific variability. Sr/Ca ratios (10.9 ± O.1 mmol/mol) and Mg/Ca ratios (1.35 ± 0.26 mmol/mol) were similar for all samples. HoloceneHalimeda aragonite samples from cores of two bioherms in the northern GBR seemed well preserved on the basis of mineralogy and Sr/Ca and Mg/Ca ratios similar to those in modernHalimeda aragonite. Cd/Ca ratios (overall mean 0.96 ± 0.15 nmol/mol) were lower than those measured in the modernHalimeda from the central GBR location. However, Mn/Ca ratios in both cores were substantially higher than in modernHalimeda aragonite. While it may be possible to extract paleoceanographic information fromHalimeda aragonite, substantial care is needed to evaluate and avoid the effects of post-depositional alteration.