AMS 14C dating of Holocene estuarine deposits: consequences of high energy and reworked foraminifera

Benthic foraminiferal species were used to AMS date Holocene sediments adjacent to transgressive overlap boundaries (TOB) in two high energy estuaries in North Spain. It had previously been recognised that the foraminifera could be divided into indigenous and exotic components. Whereas it was known that the exotic forms had been transported into the estuaries from the adjacent inner shelf, it was assumed that the indigenous forms were in situ. The AMS dates based on indigenous forms obtained from pairs of samples adjacent to the TOB showed an inverse relationship in three out of four boreholes (i.e., the age of the higher sample was older than that of the lower). The most probable explanation of this unexpected result is that there had been significant reworking of indigenous foraminifera especially associated with the transgressive episode. It is concluded that, in high energy estuaries, AMS dating on foraminiferal shells is influenced by transport and reworking even of the indigenous component. Therefore, if only foraminifera are available for dating, we recommend that closely spaced pairs of samples should be dated in order to assess the reliability of the results.