Bomb radiocarbon dating


Bomb radiocarbon dating is a technique that has evolved as a unique application in the age validation of marine and freshwater fishes and invertebrates. The approach relies on a conserved record of the rapid increase in radiocarbon (14C) that occurred in the aquatic  environments (rivers, lakes, and oceans) of the world as a result of atmospheric testing of thermonuclear devices in the 1950’s and 1960’s. The figure shown here from Hua and Barbetti (2004) provides a comprehensive view of the rise in 14C with time with the nuclear bomb events that led to the atmospheric pulse. 

The change in 14C due to bomb-produced 14C is reported as Δ14C in reference to an established prenuclear 14C standard.  The series of nuclear test events that led to the signals 14C we see in the atmosphere, largely circulated globally in the stratosphere, worked their way back into the troposphere and aquatic environments through precipitation (rain) and diffusion of radio-CO2 at the water surface.  These environmental signals are reflected in the various freshwater environments of North America using fish otolith records and in the marine environments of the Northern Hemisphere using both otolith and coral records. 

The bomb-produced 14C signal of the marine environment was virtually synchronous in terms of the initial rise in Δ14C in the mixed layer of mid-latitude oceans. This oceanic signal was first recorded from marine carbonates in hermatypic (reef-building) corals and has since been shown to be regionally specific in terms of the magnitude and timing of the post-bomb rise, as shown in the plot from Druffel (2002) of Δ14C records across the tropical Pacific Ocean. 

Application of the method to fishes began with an innovative comparison of Δ14C values recorded in otolith carbonate relative to regional Δ14C records from hermatypic corals. The temporal specificity of the measured levels provided an independent determination of age for corroboration of age estimates from growth zone counting in otoliths, as can be seen in this plot from Kalish (1993).  Note that the rise in Δ14C measured from aged fish otoliths coincides with the recorded pulse from regional hermatypic corals (see Bomb radiocarbon dating of three important reef-fishes of the Indo-Pacific for a recent application).

The use of bomb produced 14C to make valid estimates of age, growth, and longevity for various marine organisms has now been in practice for for more than 25 years (Kalish 1993). Early studies were crude and involved instrumentation that is currently outdated.  Not only have the instrumentation and analytical approach used to measure 14C increased in precision and accuracy, but the amount of material required for an otolith measurement is greatly reduced and continues to improve. In concert with these improvements, extraction techniques are more precise — most studies now employ a micromilling machine (Elemental Scientific Lasers, Bozeman, MT, USA; — and there is a more thorough understanding of bomb produced 14C in freshwater and marine environments.  As a result, questions of age and longevity have been answered for fishes throughout the world (e.g., rockfishes of the Northeastern Pacific, snapper of the Indo-Pacific, red steenbras of South Africa, bigmouth buffalo and alligator gar of North America), and in some cases the limits were pushed for species with very small otoliths and geographical origins that were not well constrained (tuna of the Atlantic and blue marlin of the Pacific).  The utility of the method has expanded to the vertebrae of sharks and rays (e.g., tiger shark and sand tiger shark), and is also being used for other marine organisms (e.g., calcareous marine alga, deep-sea coral, whale teeth, and mollusks). In many cases the applications are complicated (e.g., gray snapper of the Gulf of Mexico) or incomplete information can lead to necessary to assumptions for conclusions to be drawn; however, the method is evolving and some of the work presented addresses some of the potential pitfalls. Stay tuned for more developments. 

In December 2013 I gave an interview with Carlie Wiener on the work that I do in Hawaii and abroad with NOAA Fisheries. The program is called 'All Things Marine' and it aired on the local news talk radio KGU760 Honolulu, Hawaii. 

Link: COSEE Earth Island Interview

See how this method revealed a 50 year lifespan

for bluespine unicornfish (Naso unicornis) by

clicking the picture above.

References cited:

Hua and Barbetti 2004 C-14 Atm.pdf

Druffel 2002 RadiocarbonCoral.pdf

Kalish 1993 C-14 Fish Otoliths.pdf