Implications for the dating of Wisconsinan (Weichselian) late-glacial events of systematic radiocarbon age differences between terrestrial plant macrofossils from a site in SW Ireland

AMS radiocarbon dates were obtained from Salix herbacea leaves, Carer seeds , and bulk organic detritus from a lake sediment profile of Wisconsinan (We ichselian) Lateglacial age in SW Ireland. There is a systematic age differe nce between the dated series from the two types of macrofossils, with ages obtained from Salix herbacea leaves being 900 to 1500 C-14 years younger th an those obtained from Carer seeds. The latter tend to be more in accord wi th dates from the total organic detritus in the lake sediment, although the bulk organic fraction invariably registered the older ages. Intact surviva l of the fragile Salix leaves indicates that they are unlikely to have been physically transferred within the sediment matrix and/or otherwise reworke d from the surrounding catchment, Hence, these macrofossils are the more li kely to be contemporaneous with the time of deposition, However, there is n o significant correlation between measured C-14 age and depth in the Salix values, which scatter over a range of 700 C-14 years. In contrast, the age/ depth relationship for Carer shows a significant reversal, possibly reflect ing the redeposition of these macrofossils, and therefore giving radiocarbo n ages that are anomalously old, The data have important implications for t he dating of lake sediment sequences by AMS radiocarbon measurement of terr estrial plant macrofossils. (C) 2000 University of Washington.