Intrashell Radiocarbon Variability in Marine Mollusks

We demonstrate variable radiocarbon content within 2 historic (AD 1936) and 2 prehistoric (about 8200 BP and 3500 BP) Mytilus californianus shells from the Santa Barbara Channel region, California, USA. Historic specimens from the mainland coast exhibit a greater range of intrashell variability (i.e. 180–240 14 C yr) than archaeological specimens from Daisy Cave on San Miguel Island (i.e. 120 14 C yr in both shells). δ 13 C and δ 18 O profiles are in general agreement with the up welling of deep ocean water depleted in 14 C as a determinant of local marine reservoir correction (ΔR) in the San Miguel Island samples. Upwelling cycles are difficult to identify in the mainland specimens, where intrashell variations in 14 C content may be a complex product of oceanic mixing and periodic seasonal inputs of 14 C-depeleted terrestrial runoff. Though the mechanisms controlling ΔR at subannual to annual scales are not entirely clear, the fluctuations represent significant sources of random dating error in marine environments, particularly if a small section of shell is selected for accelerator mass spectrometry (AMS) dating. For maximum precision and accuracy in AMS dating of marine shells, we recommend that archaeologists, paleontologists, and 14 C lab personnel average out these variations by sampling across multiple increments of growth.