Barium variation in Acropora palmata and Montastrea annularis

The barium contents of Acropora palmata and Montastrea annularis are distinct both in their averages (6 and 9 ppm, respectively) and, more impressively, in their variability (5 to 7 ppm and 6 to 15 ppm). A. palmata contained about 50% more Ba²⁺ than M. annularis and exhibited more than three times as much variation, as measured by their respective coefficients of variation. In contrast, the means and coefficients of variation of Sr²⁺ in these groups differed by only 6% and 15%. No obvious environmental or post-depositional causes for the inter-and intra-specific variation of Ba²⁺ could be found. Previous experiments indicate that the partitioning of Ba²⁺ into aragonite depends directly on the rate of precipitation. This suggests that Ba²⁺ does not substitute for Ca²⁺ but instead is incorporated by occlusion. Since growth rates in A. palmata often exceed those of M. annularis, this appears to be the mechanism which generates higher and more variable concentrations of Ba²⁺ in A. palmata.