LASER-ABLATION AMS-14C MEASUREMENTS OF BOMB RADIOCARBON IN ARCTIC AND SUB-ARCTIC BAMBOO CORALS FROM THE NORTHWEST ATLANTIC

Bamboo corals are a cosmopolitan polyphyletic group of octocorals characterized by alternating proteinaceous nodes and calcitic internodes. In high latitudes and deep waters, most bamboo corals are members of the slow-growing monophyletic Keratoisididae. Carbon in the gorgonin nodes is derived metabolically, hence reflecting surface primary productivity, while carbon in the calcitic internodes is derived inorganically, hence reflecting ambient water DIC. In the Arctic bamboo coral Keratoisis flexibilis, which is typically less than 5 mm thick and very slow-growing, growth rings in the organic nodes are typically too small to be sub-sampled for ¹⁴C age determination in separate growth rings using standard milling or band separation techniques. Ring-count derived ages indicated a maximum of 189 years for the Arctic bamboo coral K. flexibilis from SE Baffin Bay, validated using a limited number of AMS-¹⁴C ages of calcite at different points along the stem.

In order to further validate ages and growth rates determined by ring counts through the proteinaceous nodes of K. flexibilis, we used an experimental laser-ablation AMS-¹⁴C system at ETH-Zurich to subsample cross-sections through the gorgonin nodes and calcitic internodes from K. flexibilis samples,including 1 sample of K. grayi that had been previously validated using ²¹⁰Pb and standard AMS-¹⁴C. Dual laser ablation transects were measured across cross-sections of skeletons. F¹⁴C measurements typically had error ratios < 5%. Repeatability between transects across the same coral cross-section was generally good, but showed occasional large exceptions.

Laser-ablation F¹⁴C measurements in a previously measured sampled of K. grayi were generally consistent with those from previously subsampled AMS-¹⁴C determination, and generally separated pre-bomb from post-bomb samples in both calcite and gorgonin. Laser-ablation F¹⁴C measurements across organic nodes of F. flexibilis had greater difficulty separating pre-bomb F¹⁴C values below 1 from post-bomb values greater than 1. High variability along radial transects in K. flexibilis may relate to interspersed calcite bands within the gorgonin nodes, particularly in the hollow centres of stems. Laser ablation F¹⁴C data quality may be improved by decalcification of organic cross-sections, or by analysis of the calcitic internodes farther from gorgonin nodes.