Joint 56th Annual North-Central/ 71st Annual Southeastern Section Meeting - 2022

Paper No. 2-7
Presentation Time: 9:35 AM


BALDASARE, Alexander, RECH, Jason, TENISON, Christina N. and CURRIE, Brian, Department of Geology and Environmental Earth Science, Miami University, 118 Shideler Hall, Oxford, OH 45056

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.