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. 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 up welling 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.

New Marine ΔR Values for the South Pacific Subtropical Gyre Region

“…From each of these shells, we removed a 5-mm cross-section perpendicular to the edge across multiple increments of growth to avoid intrashell variations in 14 C (cf. Culleton et al 2006) and provide an average value over a maximum period of 5 yr (i.e. 1 increment in the Marine04 data set).…”

The freshwater reservoir effect in radiocarbon dating

“…Small fluctuations in the Kilen reservoir age might be result of these variations. However, this is more likely to happen in regions with variations in the upwelling of deep-sea water [95,99,100].…”

Life among the tides : recent archaeology on the Georgia Bight : proceedings of the Sixth Caldwell Conference, St. Catherines Island, Georgia, May 20-22, 2011.

“…For one thing, the sequential, high-precision dating of this single Mercenaria valve makes it crystal clear that the marine reservoir surrounding St. Catherines Island is extraordinarily stable on a season-to-season basis. This portion of the Georgia Bight is totally lacking in the radical fluctuations within the annual cycle caused by varying supplies of old carbon resulting from upwelling, as noted elsewhere in the world (e.g., Kennett et al, 1997;Culleton et al, 2006). We have previously assumed this to be true, but this is the first empirical evidence that effectively tests this important assumption.…”