Radiocarbon variability in modern deep-sea bamboo coral skeletons from the North Atlantic

Geochemical records from modern and recent deep-sea corals can provide new opportunities for understanding how changes in intermediate to deep ocean chemistry and circulation relate to climatic changes during the Common Era. Of critical importance for such comparisons are well-constrained coral growth rates (and hence chronology). Bamboo coral specimens of the genus Keratoisis are widely distributed in the modern oceans, but their calcitic skeletons exhibit open system U-Th behavior and many specimens show obscure growth banding, limiting available techniques for growth rate determinations. We measured radial transects of radiocarbon across the skeletons of six Keratoisis specimens that were live-collected in the northwestern North Atlantic Ocean between 1879 and 2005. Initial results show that Δ¹⁴C from the outer surface of three modern corals is within error of seawater Δ¹⁴C estimates at the site of coral collection from GLODAP. Most specimens show increasing ¹⁴C age toward the coral center, implying near-linear growth rates of 40 to 60 μm/¹⁴C year that are consistent with growth rates for North Atlantic Keratoisis specimens established by skeletal ²¹⁰Pb and organic (gorgonian) ¹⁴C measurements. However, other specimens show radiocarbon variability that precludes accurate growth rate determinations for parts of the skeleton. In particular, multiple specimens exhibit systematic radiocarbon age reversals within 1-3 mm of the central growth axis, which is assumed to be the ontogenetically oldest part of the skeleton. This observation is consistent with previously published records of elevated unsupported ²¹⁰Pb around the central growth axis of some North Atlantic Keratoisis specimens. We discuss potential physical and biological causes for the observed radiocarbon variability and implications for accurate growth rate determinations for bamboo coral skeletons.