Use of skeletal Sr/Ca ratios to determine growth patterns in a branching coral Isopora palifera

Abstract

Studies on the growth response of corals to changing climate have largely focused on long-lived corals with relatively distinct density bands such as massive Porites corals. Little is known about the climatic response of other more abundant growth forms, such as branching Acropora corals, largely because of the absence of a clear annual density banding pattern. Using a combination of X-radiography, gamma densitometry, Sr/Ca analysis, and Uranium–Thorium (U-Th) dating, we quantified patterns of annual growth in the robust branching coral Isopora palifera from the central Great Barrier Reef (GBR), Australia (18°16′S 147°22′E) collected in May 2013. While visual analysis of the positive X-radiographs revealed alternating patterns of high- and low-density bands along the central growth axis, gamma-densitometry analysis suggests that these bands do not exhibit a clear annual cycle. In contrast, skeletal Sr/Ca ratios consistently revealed clear patterns of seasonality matching local sea surface temperatures (SST), and provided a growth chronology to calculate linear extension rate, skeletal density and calcification rate. Comparisons between SST-Sr/Ca calibrations derived from our I. palifera samples with (i) massive Porites from the same location and (ii) Isopora spp. from the GBR and Papua New Guinea revealed significant differences of up to 3.5 °C, implying palaeoclimate reconstructions should use site-specific and species-specific Sr/Ca-SST calibrations. Our approach provides a robust method for assessing changes in growth for a common Indo-Pacific branching coral, and provides a valuable framework for quantifying past and future changes in skeletal growth in response to climate change.