Organisation Internationale pour le Bambou et le Rotin

Organisation Internationale pour le Bambou et le Rotin

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Scale dependence of environmental and physiological correlates of delta O-18 and delta C-13 in the magnesium calcite skeletons of bamboo corals (Gorgonacea; Isididae)













Growth rate; Kinetic effects; Lines method; Mg/Ca; Salinity; Temperature

We examine in detail delta O-18 and delta C-13 in the calcite internodes of bamboo corals as potential proxies of physiological and environmental variability, through (a) a « core top » calibration that includes specimens from a wide range of habitats and environmental conditions and (b) a comparison of high resolution serial point analyses along radial growth axes of a sub-set of specimens with each other, with instrumental temperature and salinity records, with growth rates and with a nominal skeletal proxy for temperature (Mg/Ca) in the same specimens. At the whole-of-specimen level, delta O-18 and the intercept of the strong within-specimen regression of delta O-18 against delta C-13 correlates highly with ambient temperatures at slopes that are identical to those reported for other marine biogenic carbonates (-0.22 per degrees C). delta C-13 varies predominantly with apparent specimen-mean growth rate. It also correlates with the slope of the within-specimen covariance between delta O-18 and delta C-13, which in turn is distributed bi-modally among specimens and linked to differences in apparent growth rates. Within-specimens, variability in delta C-13, and to a lesser extent delta O-18, correlates between specimens collected in the same region and differs between regions, implying an environmental effect, but the factors involved for either isotope ratio are unclear. Correlations between delta O-18 and temperature (and Mg/Ca) range from positive to negative among specimens and appear to vary over time even within specimens. The mismatch between the consistent temperature-dependence of delta O-18 at the whole-of-specimen level and the mixed relationship within-specimens can be reconciled by assuming an unknown temperature-dependent factor affecting delta O-18 during the growth of Isidid calcite. The contrast between the results of the « core top » temperature calibration for delta O-18, which are consistent with studies of other carbonates, and the apparently more complex suite of factors affecting both delta C-13 and delta O-18 within specimens strongly suggests that calibrations based on a « core top » approach be applied cautiously to environmental reconstructions based on variations of a nominal proxy within specimens. (C) 2016 Elsevier Ltd. All rights reserved.