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 ¹⁴C yr) than archaeological specimens from Daisy Cave on San Miguel Island (i.e. 120 ¹⁴C yr in both shells). Δ¹³C and Δ¹⁸O profiles are in general agreement with the upwelling of deep ocean water depleted in ¹⁴C 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 ¹⁴C content may be a complex product of oceanic mixing and periodic seasonal inputs of ¹⁴C-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 ¹⁴C lab personnel average out these variations by sampling across multiple increments of growth.