Bomb-produced radiocarbon validation of growth-increment crossdating allows marine paleoclimate reconstruction

The bivalve Pacific geoduck (Panopea generosa) has been used in the eastern North Pacific Ocean to create proxies for environmental factors such as temperature and oceanographic conditions. This type of research depends upon accurate age determination of Pacific geoducks, which historically was based on shell growth-increment counts. A recent study comparing age estimates generated by the dendrochronology (tree-ring science) procedure of crossdating to those estimated from growth-increment counts found a significant difference between the methods for geoduck older than 30years. Compared to the traditional age determination method of counting growth increments, the crossdating method estimates a greater longevity in this species, with some individuals living in excess of 150years. In the present study, the accuracy of each method was independently assessed using bomb-produced radiocarbon (¹⁴C) techniques. Specimens whose birth years were estimated to be within the era of the bomb-produced marine ¹⁴C increase and where the differences between ages estimated by the two methods were greatest were selected for ¹⁴C analysis. The difference between age estimates from traditional growth-increment counts and those from crossdating was evaluated using their respective ¹⁴C chronologies in comparisons to a reference chronology as a standard. The comparisons relied on Bayesian nonlinear models using Markov Chain Monte Carlo simulation. This method of analysis showed that with a 50% probability geoducks were aged correctly when using the crossdating method, compared to the growth increment counts which had a 50% probability of underestimating the age by 4years. Therefore, the crossdated age estimates were found to be more accurate than increment counts. Furthermore, these results provide new confidence in using Pacific geoduck biochronologies for paleoclimate and paleoenvironmental reconstruction.