Optimizing sampling strategy for radiocarbon dating of Holocene fluvial systems in a vertically aggrading setting

In this paper we address the question of how to determine the period of activity (sedimentation) of fossil (Holocene) fluvial systems in vertically aggrading environments. Our data base consists of almost 100 14 C ages (partly AMS) from the Rhine–Meuse delta in the central Netherlands. Radiocarbon samples from the tops of lithostratigraphically correlative organic beds underneath overbank deposits (sample type 1) yield consistent ages, indicating a synchronous onset of overbank deposition over distances of at least up to 20 km along channel belts. Similarly, 14 C ages from the base of organic residual channel fills (sample type 3) generally indicate a clear termination of within‐channel sedimentation. In contrast, 14 C ages from the base of organic beds overlying overbank deposits (sample type 2), commonly assumed to represent the end of fluvial sedimentation, show a large scatter reaching up to 1000 14 C years. This setting usually produces 14 C ages significantly younger than residual channels of the same fluvial system, indicating the presence of non‐depositional unconformities. These usually occur on top of slightly elevated sediment bodies (especially natural levee deposits), which often contain palaeosols. Such hiatuses appear to be much more abundant than hitherto supposed. We conclude that a combination of sample types 1 and 3 generally yields a satisfactory delimitation of the period of activity of a fossil fluvial system. The problems associated with 14 C samples of type 2 may also be present in coastal areas with an alternation of tidal deposits and organic beds.