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Bomb radiocarbon otolith ageing

Learn about difficulties we face with precisely ageing otoliths and how using bomb radiocarbon overcomes some of those issues.

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Radiocarbon chronology in marine waters showing a similar year of increase (around 1956) whether from corals, bivalves or otoliths.

Difficulties with precise ageing

The otolith annuli is globally accepted as the best means for estimating the age of most fish species. However, correct interpretation of the annuli is far from trivial and can result in serious and systematic ageing error.

Confirming the accuracy of a method of annulus interpretation for marine fish species is often problematic, unless you’re using tagged, hatchery-reared fish released into the wild. There’s a well-defined gap in our ability to confirm age interpretations of the majority of marine fish species, particularly those that are long-lived.

Other methods

We generally consider mark-recapture of chemically-tagged fish to be the most accurate means of confirming the frequency of formation of presumed annuli. You must compare the time at liberty with the number of annuli deposited distal to the chemical check for proper ageing.

This approach is sound, but low recapture rates for fish at liberty more than 2 to 3 years makes it difficult to acquire sufficient samples for adequate testing. Moreover, the technique validates the time elapsed since tagging, not the absolute age of the fish.

You can differentiate between very different age interpretations with radiochemical dating-based ratios, including 210Pb : 226Ra or 228Th : 228Ra. However, these assays are too imprecise for detailed or individual age confirmations.

The seasonal progression of marginal increments is the most widely used approach. However, it’s only well-suited to fast-growing fish and suffers from the lack of an objective means of evaluation.

Atmospheric radiocarbon

The widespread atmospheric testing of atomic bombs in the 1950s and 1960s produced a 100% increase in atmospheric radiocarbon, which was quickly incorporated into the world's oceans.

Analysis of annular growth rings in coral demonstrated that bomb radiocarbon incorporated into the accreting coralline structure in concentrations proportional to those present in the water column. Thus, the time series of bomb radiocarbon recorded in the coral was shown to reflect that present in the marine environment. This increased by about 20% between 1950 and 1970.

Using accelerator mass spectrometry as a sensitive and accurate assay tool, we learned that a New Zealand fish species also incorporated carbon-14 in their otoliths. The time series of radiocarbon reconstructed from the presumed otolith annuli was similar to that present in nearby corals.

Thus, we can infer that the otolith annuli had been interpreted and aged correctly. This is because systematic under- or over-ageing would have resulted in a phase shift between the otolith carbon-14 and the coral carbon-14 time series.

We’ve confirmed the value of the bomb radiocarbon technique for solving problems of age validation in a variety of fish species. Furthermore, recent work has confirmed that the uptake of carbon-14 in young fish otoliths is synchronous with that of both corals and bivalves in the North Atlantic.

Such large-scale synchronicity implies that the carbon-14 time series reconstructed from the otolith cores of old fish can be compared to that of the other North Atlantic time series. Errors in annulus-based age determinations would manifest themselves as non-coherent time series.

Interpretation

In light of the sharp rate of increase of the carbon-14 signal associated with the onset of nuclear testing, interpretation of the carbon-14 chronology in otolith cores is relatively simple. The otolith chronology should match other published chronologies for the region as long as the annular age assignments (equaling year minus class) are correct.

Any under-ageing would phase shift the otolith carbon-14 chronology towards more recent years. Over-ageing would phase shift it towards earlier years.

Marine waters with D14C values greater than 00/00 didn’t generally exist prior to the late 1950s. Therefore, coastal fish otolith cores with sub-zero values must have formed before the late 1950s.

Even contamination with material of more recent origin could only increase the carbon-14 value, not decrease it. Thus, the carbon-14 value sets a minimum age to the sample. The years 1958 to 1965 become the most sensitive years for carbon-14-based ageing.

Advantages

The only ageing method with the potential to confirm both annulus formation and absolute age in individual fish is radiocarbon from nuclear testing.

All studies suggest that bomb radiocarbon can be used to confirm the accuracy of an ageing method to:

Constraints

Constraints to this procedure include:

The availability of suitable otolith samples may limit the applicability of this approach to specific stocks and species. However, this method is still one of the best for providing accurate and logistically feasible age validation of long-lived species.

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