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Intrashell Radiocarbon Variability in Marine Mollusks

Published online by Cambridge University Press:  18 July 2016

Brendan J Culleton*
Affiliation:
Department of Anthropology, University of Oregon, Eugene, Oregon 97403-1218, USA.
Douglas J Kennett
Affiliation:
Department of Anthropology, University of Oregon, Eugene, Oregon 97403-1218, USA.
B Lynn Ingram
Affiliation:
Departments of Earth and Planetary Science and Geography, University of California, Berkeley, California 94720, USA.
Jon M Erlandson
Affiliation:
Department of Anthropology, University of Oregon, Eugene, Oregon 97403-1218, USA.Museum of Natural and Cultural History, University of Oregon, Eugene, Oregon 97403-1224, USA.
John R Southon
Affiliation:
WM Keck Carbon Cycle AMS Facility, Earth System Science Department, University of California, Irvine, California 92697-3100, USA.
*
Corresponding author. Email: bculleto@uoregon.edu.
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Abstract

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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.

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