May 30, 2017, 1:21 am

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Dating Methods

Lophelia pertusa has a skeleton made of aragonite (carbonate), which allows the use of radiocarbon dating and 230Th/U dating.

Radiocarbon dating is a technique based on the decay of the radioactive isotope of carbon, 14C. The radioactive isotope of carbon, 14C, is produced by cosmic ray bombardment of 14N in the upper atmosphere. After their formation, 14C isotopes are oxidized to CO2 and exchange between the different carbon reservoirs. In the North Atlantic, where North Atlantic Deep Water (NADW) is formed, the 14C isotopes from the surface water are carried into the deep ocean, closing off carbon exchange with the atmosphere and allowing the radiocarbon decay clock to start ticking. The radiocarbon age on a marine (bio)carbonate includes both the age of the mineral and the age of the water mass from which the mineral formed. 

As a dating method, once the coral skeleton is formed, the 14C isotopes present are in equilibrium with the ocean and start decaying to become stable. By measuring the remaining quantity of 14C compared to the carbon total, the period of carbonate formation can be estimated. 

The other method, U/Th dating is based on the radioactive decay series of the isotopes of Uranium (238U), which is also present in coral’s skeleton. Eventually these isotopes will become stable as 230Th isotopes. The age of the aragonite mineralization can be calculated by measuring the present-day activity ratios of the isotopes from the radioactive decay series. 

The difference between the ages obtained by these two methods (radiocarbon and 230Th/U) allows the estimation of the water mass age (reservoir age), which is a function of how quickly the ocean is overturning and mixing with atmospheric CO2. Since the aragonite of cold-water corals is ideally suited for analysis by both U-series and radiocarbon methods, several studies have used this technique to reconstruct past ventilation.