Varying growth rates in bamboo corals: sclerochronology and radiocarbon dating of a mid-Holocene deep-water gorgonian skeleton (Keratoisis sp.: Octocorallia) from Chatham Rise (New Zealand)

Abstract

A branched mid-Holocene bamboo coral skeleton of the isidid gorgonian genus Keratoisis (Octocorallia) recovered at southwestern Chatham Rise (New Zealand) from an average water depth of 680 m is described with respect to sclerochronology and age determination. Growth rates of the Mg-calcitic internodal increments were investigated by the counting of colour bands and radiocarbon dating. Growth banding is produced by varying orientations of crystal fan bundles towards the image plane. The skeleton shows three growth interruptions, which are documented in all branches. AMS 14C ages decrease from base to top of the trunk and from the central axes to the margins of the branches, documenting a simultaneous vertical and lateral growth. The data provide a maximum age of 3,975 ± 35 years BP, and a record spanning 240 ± 35 years. While calculated longitudinal growth rates amount to an average of 5 mm year−1 during a 55-year record, average lateral linear extension rates of 0.4 mm year−1 are an order of magnitude lower, still allowing for a seasonal to annual resolution of colour bands on a macroscopic scale and for a daily to monthly resolution on microscales of individual crystal generations to fascicle bundles. Hence, the isidid skeleton provides a high-resolution archive of paleoceanographic dynamics in deeper water masses. Concentric incremental accretion around the central axis in the early growth stages changed into a unilaterally asymmetric growth during late-stage evolution, probably triggered by the establishment of a stable system of unidirectional currents and nutrient flux. While colour band counts, related to the AMS 14C ages, support a seasonal to annual accretion of macroscopic growth bands in the inner concentric and complete outer parts of the skeleton, incremental growth rates at the condensed side are highly variable, as documented by hiatuses and unconformities. Thus the specimen proves that growth rates of bamboo corals may vary within individual skeletons and strongly deviate from the annual mode, hence showing implications on paleoceanographic proxy analyses.