Abstract

In Holocene marine deltaic sequences there is no simple relationship between radiocarbon age and depth of sampling. The Nile and Ganges-Brahmaputra deltas show remarkably similar radiocarbon age-depth relationships: (1) most radiocarbon dates, by conventional and accelerator mass spectrometric (AMS) analyses, are older (some by 4000 yr) than reasonably expected; (2) dates are commonly inverted (i.e., older upcore); and (3) sample dates at delta surfaces commonly exceed 2000 yr B.P. These anomalous age-depth phenomena are in part a response to reservoir effects, introduction of old carbon, and, emphasized herein, influence of sediment storage and remobilization that prevail in fluvial and delta plains. In fluvial systems, sediment generally is not transported in a single episode from its original source area to the sea. Rather, material transported from upland reaches and river valley to coast is subject to a repeated series of depositional, burial, and erosional events induced by episodic flooding, overbank sedimentation, and channel migration. Because sediment is likely to be temporarily stored in the delta plain, it is difficult to determine the time of a sediment's final deposition and burial at a delta site. A successful strategy requires use of several independent dating methods, including AMS analyses of in situ basal peats and fossils and, where possible, archaeological evaluation of associated artifacts that are in place. Accurate dates are needed for reliable measurement of rates of delta plain aggradation and land subsidence, information to help implement effective coastal protection measures.

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