Testing for the 14C Reservoir Effect in Submerged Aquatic Macrophytes: The Good, the Bad and the Ugly of 14C Dating Plant Macrofossils

Obtaining accurate radiocarbon (¹⁴C) chronologies for lake-sediment cores and outcrops are crucial for improving reconstructions of the timing of past climate and landscape changes. Common use of the AMS method of ¹⁴C dating has allowed us to obtain ages from small amounts of carbon, but still the accuracy of these ages ultimately depends on the types of material dated. Many scientists generally assume that all “seeds” recovered from lake-sediment cores or exposed outcrops are “good” for ¹⁴C dating. However, others have suspected that the fossils of submerged aquatic macrophytes provide erroneously older ages, because of a “reservoir effect,” the incorporation of ancient carbon (derived from calcareous bedrock and tills) into their tissues while alive. But this assumption was not tested, until our study.

Our test involved a series of 9 matched sets, each pair comprised of a ¹⁴C date obtained from fossil seeds of a submerged aquatic plant vs. an age from terrestrial plant macrofossils for the same stratigraphic level of a lake-sediment core, and comparison of the paired results using a T-test as well as a graphical approach (plotted normal probability density functions). Consistently, the aquatic macrofossils provided erroneously older ages (“bad dates”), the offset ranging from 445 to 2225 ¹⁴C years, with a greater disparity with depth. The exception was during the “radiocarbon plateau,” at about 10,000 ¹⁴C yr BP, when the difference was insignificant. Based on these results, we recommend that 1) all fossil seeds considered for ¹⁴C dating be identified to the genus or species level and 2) that macrofossils of submerged aquatic plants not be submitted for dating.