| Summary: | Marine sediment records from the north Icelandic shelf, which rely on tephrochronological age models, reveal an average R (regional deviation from the modeled global surface ocean reservoir age) of approximately 150 yr for the last millennium. These tephra-based age models have not hitherto been independently verified. Here, we provide data that corroborate R values derived from these sediment archives. We sampled the youngest portion (ontogenetic age) of a bivalve shell, Arctica islandica (L.), for radiocarbon analysis, which was collected alive in 2006 from the north Icelandic shelf in ~80 m water depth. Annual band counting from the sectioned shell revealed that this clam lived for more than 405 yr, making it the longest-lived mollusk and possibly the oldest non-colonial animal yet documented. The 14C age derived from the umbo region of the shell is 951 27 yr BP. Assuming that the bivalve settled onto the seabed at AD 1600, the corresponding local value of R is found to be 237 35 yr by comparison of the 14C age with the Marine04 calibration curve (Hughen et al. 2004) at this time. Furthermore, we cross-matched a 287-yr-old, dead-collected, A. islandica shell from AD 1601 to 1656 from the same site with the live-caught individual. 14C analysis from the ventral margin of this shell revealed a R of 186 50 yr at AD 1650. These values compare favorably with each other and with the tephra-based R values during this period, illustrating that 14C from A. islandica can effectively record 14C reservoir changes in the shelf seas.
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