Abstract
Abstract. Here we show the use of the 210Pb-226Ra excess method to determine the growth rate of two corals from the world's largest known cold-water coral reef, Røst 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. 210Pb and 226Ra differ in the way they are incorporated into coral skeletons. Hence, to assess growth rates, we considered the exponential decrease of initially incorporated 210Pb, as well as the increase in 210Pb from the decay of 226Ra and contamination with 210Pb associated with Mn-Fe coatings that we were unable to remove completely from the oldest parts of the skeletons. 226Ra activity was similar in both coral species, so, assuming constant uptake of 210Pb through time, we used the 210Pb-226Ra 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 (~1 polyp every two to three years). We are less certain of this 210Pb growth rate estimate which is within the lowermost ranges of previous growth rate estimates. We show that 210Pb-226Ra 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.
Highlights
Cold-water corals have been known since the 18th century, but much less is known about their ecology and growth patterns compared to their shallow water counterparts (Roberts et al, 2009)
210Pb-226Ra chronology was applied in this study for the first time to large branching specimens of L. pertusa and M. oculata, two constructional deep-sea scleractinian corals which form large deep-sea reefs that are of great ecological and conservation importance in the North Atlantic. 210Pb and 226Ra were not incorporated the same way into the deepsea corals due to their different chemical behaviors in the aquatic environment
Since 226Ra activities in both deepsea corals were fairly constant, a constant uptake of 210Pb with time was assumed and the 210Pb-226Ra chronology was applied to calculate the linear growth rate expressed in mm per year or polyp generation per year
Summary
Cold-water corals have been known since the 18th century, but much less is known about their ecology and growth patterns compared to their shallow water counterparts (Roberts et al, 2009). Advances in acoustic survey techniques and more widespread use of ROVs and submersibles have allowed detailed in situ studies of cold-water coral habitats showing their ecological importance for a diverse range of invertebrates and fish Knowledge of cold-water coral reefs growth is central to inform policy makers who decide about fishing impacts and reef management in the face of ocean acidification and ocean warming
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