Ba/Ca ratios in shells of Arctica islandica-Potential environmental proxy and crossdating tool

Ba/Cashell time-series of marine bivalves typically show flat background levels which are interrupted by erratic sharp peaks. Evidence from the literature indicates that background Ba/Cashell ratios broadly reflect salinity conditions. However, the causes for the Ba/Cashell peaks are still controversial and widely debated although many researchers link these changes to primary productivity, freshwater input or spawning events. The most striking feature is that the Ba/Cashell peaks are highly synchronous in contemporaneous specimens from the same population. For the first time, we studied Ba/Cashell in mature and ontogenetically old (up to 251 year-old) specimens of the long-lived Arctica islandica. Also, we analyzed specimens from surface water and deeper water. The typical pattern of low background and erratic peaks persisted throughout ontogeny. However, due to decreasing sampling resolution and greater time-averaging in older, slower growing shell portions, the background Ba/Cashell values appeared to gradually increase with ontogenetic age, whereas the peaks became attenuated and broader. Despite that, Ba/Cashell maxima were still highly synchronous among contemporaneous specimens from the same locality and habitat confirming previous reports from short-lived species. Computing of annual Ba/Cashell averages largely eliminated any bias introduced by time-averaging and sampling resolution. Strongly elevated annual Ba/Cashell peaks in specimens from surface waters (Iceland, Faroe Islands, Isle of Man) during the 1980s appear to coincide with an extreme primary productivity pulse recorded by remote sensing. However, due to the lack of in vivo experiments, we cannot ultimately test a causal link between annual Ba/Cashell excursions and primary productivity. We propose that Ba/Cashell time-series, specifically the highly synchronous Ba/Cashell peaks and annual Ba/Cashell values in contemporaneous specimens from the same locality can serve as a tool to verify crossdating and facilitate the construction of statistically robust growth increment width master chronologies. Long-term environmental reconstructions based on bivalve shell growth chronologies can likely greatly benefit from this new technique.