210Pb-226Ra chronology reveals rapid growth rate of Madrepora oculata and Lophelia pertusa on world's largest cold-water coral reef

Here we show the use of the Pb-210-Ra-226 excess method to determine the growth rate of two corals from the world's largest known cold-water coral reef, Rost Reef, north of the Arctic circle off Norway. Colonies of each of the two species that build the reef, Lophelia pertusa and Madrepora oculata, were collected alive at 350 m depth using a submersible. Pb and Ra isotopes were measured along the major growth axis of both specimens using low level alpha and gamma spectrometry and trace element compositions were studied. Pb-210 and Ra-226 differ in the way they are incorporated into coral skeletons. Hence, to assess growth rates, we considered the exponential decrease of initially incorporated Pb-210, as well as the increase in Pb-210 from the decay of Ra-226 and contamination with Pb-210 associated with Mn-Fe coatings that we were unable to remove completely from the oldest parts of the skeletons. Ra-226 activity was similar in both coral species, so, assuming constant uptake of Pb-210 through time, we used the Pb-210-Ra-226 chronology to calculate growth rates. The 45.5 cm long branch of M. oculata was 31 yr with an average linear growth rate of 14.4 +/- 1.1 mm yr(-1) (2.6 polyps per year). Despite cleaning, a correction for Mn-Fe oxide contamination was required for the oldest part of the colony; this correction corroborated our radiocarbon date of 40 yr and a mean growth rate of 2 polyps yr(-1). This rate is similar to the one obtained in aquarium experiments under optimal growth conditions. For the 80 cm-long L. pertusa colony, metal-oxide contamination remained in both the middle and basal part of the coral skeleton despite cleaning, inhibiting similar age and growth rate estimates. The youngest part of the colony was free of metal oxides and this 15 cm section had an estimated a growth rate of 8 mm yr(-1), with high uncertainty (similar to 1 polyp every two to three years). We are less certain of this Pb-210 growth rate estimate which is within the lowermost ranges of previous growth rate estimates. We show that Pb-210-Ra-226 dating can be successfully applied to determine the age and growth rate of framework-forming cold-water corals if Mn-Fe oxide deposits can be removed. Where metal oxides can be removed, large M. oculata and L. pertusa skeletons provide archives for studies of intermediate water masses with an up to annual time resolution and spanning over many decades.