IMPROVING THE ACCURACY OF RADIOCARBON-DATING PLANT MACROFOSSILS: A PICTORIAL AND TEXTUAL GUIDE TO DISTINGUISH SUBFOSSIL SEEDS OF TERRESTRIAL, AQUATIC EMERGENT, AND SUBMERGED AQUATIC PLANTS

Radiocarbon (¹⁴C) dating of organic carbon materials is the most common method used to obtain chronologies for lacustrine sediment cores and terrestrial outcrops of Late Quaternary age. Previous researchers have demonstrated that the ¹⁴C dating of aquatic mosses, submerged aquatic plant (macrophyte) seeds, mollusk shells, and organic sediment in carbonate terrain produces erroneously older ages, compared to dates from terrestrial plant remains from the same sample depths. They attributed this discrepancy to the “reservoir effect,” that these aquatic organisms when alive fixed carbon under water, and this carbon had two sources: (1) total dissolved inorganic carbon in ground water and runoff; and (2) atmospheric carbon entering the lake via exchange at the surface. Given that ¹⁴C dating assesses only an atmospheric carbon source, terrestrial plant remains obviously provide accurate ¹⁴C ages for their stratigraphic contexts (in the absence of taphonomy). Less commonly known is that aquatic emergent plants, such as sedges and bulrushes located along the shorelines of lakes, ponds, and streams, also intake only atmospheric carbon, and hence their subfossil seeds do provide accurate ¹⁴C ages.

Most earth scientists, however, lack botanical training to distinguish the subfossil seeds of submerged aquatic macrophytes that provide erroneously older ages from terrestrial and aquatic emergent plants that should yield accurate ages for their stratigraphic contexts. This is a serious concern because, to non-botanists, the moderate-to-large seed size of submerged aquatic plants make them attractive candidates for AMS ¹⁴C dating – and the dating of these seeds can produce ages too old by several hundred to a few thousand years compared to the dating of seeds from terrestrial and aquatic emergent plants extracted from the same samples. Presented here for the first time is a guide with photographs and textual descriptions to aid non-botanists in distinguishing between the subfossil seeds of submerged aquatic, aquatic emergent, and terrestrial plants. The broader objective of this guide is to assist these researchers in selecting the appropriate materials to yield accurate ages and thus improve their chronological interpretations of past climate and environmental events.