The exaggerated radiocarbon age of deposit‐feeding molluscs in calcareous environments

Abstract

Throughout northernCanada, live‐collected, pre‐bomb, deposit‐feeding marine molluscs from calcareous sediments yield greater apparent radiocarbon ages than do suspension feeders. We explore the size of this effect in a set of 57 paired datings of deposit feeders, mainlyPortlandia arctica, and suspension feeders, mainlyHiatella arcticaandMya truncata, collected from both calcareous and non‐calcareousHolocene sediments. Deposit feeders from calcareous sediments are older than their suspension‐feeding counterparts by as much as 2240±13014Cyears. This is attributed to the uptake of ‘old’ bicarbonate derived from calcareous bedrock. The age discrepancy between suspension and deposit feeders in calcareous terrain is non‐systematic in space and time, thereby invalidating the application of a correction. In contrast, the age comparisons are concordant at sites located on thePrecambrian Shield. In terrestrial environments underlain by carbonate, previous acceptance of dates on deposit feeders led to erroneous interpretations of deglaciation and relative sea‐level history, in both theNorthAmerican and theEurasianArctic. This has prompted several researchers to exclude deposit feeders from their lateQuaternary reconstructions. The same chronological problem of deposit‐feeding molluscs now needs to be more widely acknowledged by the marine community.