CONSTRAINING RADIOCARBON RESERVOIR EFFECTS OF AQUATIC MOLLUSK SHELLS IN STREAM DEPOSITS IN THE MIDWESTERN US

Stream deposits, which are critical for understanding changes in fluvial processes, as well as for inferring climate, tectonic, and environmental change, are notoriously difficult to date. Freshwater mollusks (i.e. aquatic gastropods and bivalves) are common in fluvial deposits, and as their shells are composed of calcium carbonate, they can be radiocarbon dated to determine the age of fluvial deposits. The presence of limestone bedrock, however, may result in ¹⁴C reservoir effects in the 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, and assess the viability of using aquatic mollusks to date fluvial deposits in the Midwest, 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 435 to 595 ¹⁴C years for the five shells collected in 2020 and from 640 to 690 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 1,030±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 is hindered by anthropogenic modifications to the hydrology of the watershed and the unknown influence of the bomb spike. 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 and the carbon cycle, 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.