INTRASHELL RADIOCARBON VARIABILITY IN MARINE MOLLUSKS

Abstract

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. 180240 14C yr) than archaeological specimens from

Daisy Cave on San Miguel Island (i.e. 120 14C yr in both shells). d13C and d18O 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.