RADIOCARBON RESERVOIR EFFECTS AND 14C DATING OF AQUATIC MOLLUSK SHELLS IN THE MIDWESTERN US

Freshwater mollusks (i.e. aquatic gastropods and bivalves) are common in stream deposits in the midwestern U.S. Because these shells are composed of calcium carbonate, they can be radiocarbon dated to determine the age of geologic strata or archaeologic deposits. Over much of the Midwest, however, the presence of limestone bedrock may result in ¹⁴C reservoir effects in shells and cause ¹⁴C ages to be too old. The magnitude of any such ¹⁴C reservoir effect must be determined before shells can be reliably dated. To determine potential ¹⁴C reservoir effects in a small stream in southwestern Ohio, Four Mile Creek, we analyzed 1) the ¹⁴C concentrations of aquatic gastropods and bivalves that were collected live, and 2) the ¹⁴C age offset between matched pairs of organics and shells within late Pleistocene and Holocene stream deposits.

Seven modern aquatic gastropod and bivalves collected live in 2020 and 2021 yielded apparent ¹⁴C ages that ranged from 443 to 595 ¹⁴C years for the five shells collected in 2020 and from 641 to 689 ¹⁴C years for the two shells collected in 2021. Five matched pairs of organics and aquatic shells from Late Pleistocene and Holocene deposits yielded ¹⁴C reservoir effects that ranged from 536 to 1,333 ¹⁴C years with a weighted mean of 948±332 ¹⁴C years. Each method for determining ¹⁴C reservoir effects has advantages and limitations. Determining the ¹⁴C activity of specimens collected live determines the modern ¹⁴C reservoir effect, but does not include the temporal variability and may be hindered by anthropogenic modifications to the hydrology of the watershed. Using matched pairs of organics and shells in the geologic record incorporates the temporal variability of climate on ¹⁴C reservoir effects and avoids potential anthropogenic modifications to watershed hydrology, but also includes errors associated with the re-working of samples and open system behavior of samples subsequent to burial. In general, these results indicate that the ¹⁴C reservoir effect for aquatic shells in a humid midwestern stream is in the range of 500 to 1,200 ¹⁴C years.