Bamboo coral (Keratoisis sp.) at Davidson Seamount (1455 meters). Note the rather impressive diversity of animals associated with the coral. Click image for larger view and image credit.
Deep-sea Corals and How to Measure Their Age and Growth
Allen H. Andrews
Research Associate
Moss Landing Marine Laboratories
Andrew DeVogelaere
Research Coordinator
Monterey Bay National Marine Sanctuary
Corals in deep, cold water
Most people imagine tropical waters and snorkeling when they think of corals. They are surprised to learn that corals are also found in ice-cold, dark waters of the deep ocean, throughout the world. Cold-water, or deep-sea corals are part of the taxonomic group called Cnidaria, related to animals like sea anemones and jellyfish. They can live as individuals, or as colonies that form extensive reefs. For food, they wait for small particles to flow past, and use stinging cells to capture them. They also provide habitat for other species. These beautiful animals are among the oldest living organisms; some reefs have been found to be several thousand years old, and some individual corals live to several hundred years old.
Radiometric dating
Information about the life history of habitat forming corals, such as age, growth, and longevity, is necessary before their sensitivity and importance can be fully understood. Age determination studies to date have found that deep-sea corals can attain ages that are on the order of a hundred to perhaps thousands of years. Age and growth of deep-sea corals is typically determined from outgrowth studies in the field, growth zone counts in the skeletal structure, a radiometric technique (e.g. lead-210 dating), or a combination of these techniques.
Cross section of bamboo coral from Davidson Seamount that was sampled for lead-210 dating. The series of holes followed circular patterns that could be seen in the growth structure of the cross section. Age was determined based on the decay of lead-210 from the edge to near the center. Click image for larger view and image credit.
Lead-210 dating is a technique that uses the radioactive decay of lead-210 as a natural clock that can reveal estimates of age. How this process works begins with the natural incorporation of lead-210 from seawater into the coral skeleton. As the coral grows like a tree, laying down growth rings, the radioactivity of lead-210 decreases from the youngest to the oldest part of the skeleton. The reason lead-210 activity decreases is because it slowly decays away (radioactive decay) at a known rate (half-life of 22.26 years). To measure this change in radioactivity and relate it to age, a series of samples are taken from the edge to the center of a skeletal cross section. By taking this series of lead-210 measurements, the decrease of lead-210 activity can be used to determine the age and growth of the coral. This approach is useful to about 100 years of age at which time the activity of lead-210 decreases to background or supported levels.
Coral age estimates
Age was estimated in a recent study of bamboo coral from Davidson Seamount (Andrews et al. 2005a). The results from various growth zone counting interpretations led to a wide range of estimated age. For a cross section 8 to 11 mm in radius, estimates of 80 to 220 years were determined for the bamboo coral colony. From a series of lead-210 dating samples taken in that study, and a recent follow up study (Andrews et al. 2005b), support for the older age estimates prevailed. This line of work has been further applied to another species of bamboo coral from New Zealand and related to possible indicators of ocean climate change (Neil et al. 2005, Tracey et al. 2005).
Diagrammatic representation of the decay of lead-210 over time. Note that the activity is reduced to half at about 22 years. This trend continues to approach an activity of zero (or supported levels) in an exponential manner. It is this decrease in lead-210 activity that is a measure of age as activity approaches the asymptote. Click image for larger view and image credit.
In the current study, additional specimens will be collected from Davidson Seamount to better refine estimates of age using lead-210 dating. The aim of this portion of the study is to collect two relatively large colonies, along with a series of upper limbs or tips from colonies within and between various locations. The two large colonies will be sampled more extensively for lead-210 dating to provide age determinations with minimal uncertainty. The series of colony tips will be used to relate growth rates within and between colonies and locations on the seamount.
References
Andrews, A.H., G.M. Cailliet, L.A. Kerr, K.H. Coale, C. Lundstrom, and A. DeVogleare. 2005a. Investigations of age and growth for three species of deep-sea coral from the Davidson Seamount off central California . In : Cold-water Corals and Ecosystems. A. Freiwald and J.M. Roberts eds . Proceedings of the Second International Symposium on Deep Sea Corals. Erlangen, Germany. September 8 - 13, 2003. pp. 965-982
Andrews, A.H., D.M. Tracey, H. Neil, G.M. Cailliet, and C.M. Brooks . 2005b Abstract : Lead-210 dating bamboo coral (family Isididae) of New Zealand and California . Third International Symposium on Deep-Sea Corals: Science and Management. University of Miami, Florida. November 28 – December 2, 2005.
Tracey, D.M., J.A. Sanchez, H. Neil, P. Marriott, A.H. Andrews, and G.M. Cailliet. 2005. Abstract : Age and growth, and age validation of deep-sea coral family Isididae. Third International Symposium on Deep-Sea Corals: Science and Management. University of Miami, Florida. November 28 – December 2, 2005.
Helen Neil, D.M. Tracey, P. Marriott, R. Thresher, A.H. Andrews, J. Sanchez. 2005. Abstract : Preliminary evidence of oceanic climate change around New Zealand over the last century: the pole-equator seesaw. Third International Symposium on Deep-Sea Corals: Science and Management. University of Miami, Florida. November 28 – December 2, 2005.
