Centuries of marine radiocarbon reservoir age variation within archaeological Mesodesma Donacium shells from Southern Peru

Mollusk shells provide brief (<5 yr per shell) records of past marine conditions, including marine radiocarbon reservoir age (R) and upwelling. We report 21 ¹⁴C ages and R calculations on small (∼2 mg) samples from 2 Mesodesma donacium (surf clam) shells. These shells were excavated from a semi-subterranean house floor stratum ¹⁴C dated to 7625±35 BP at site QJ-280, Quebrada Jaguay, southern Peru. The ranges in marine ¹⁴C ages (and thus R) from the 2 shells are 530 and 170 ¹⁴C yr; R from individual aragonite samples spans 130±60 to 730±170 1⁴C yr. This intrashell ¹⁴C variability suggests that ¹⁴C dating of small (time-slice much less than 1 yr) marine samples from a variable-R (i.e. variable-upwelling) environment may introduce centuries of chronometric uncertainty.