Differences in radiocarbon ages among molluscan shells, plant materials, and total organic carbon: An example from the paleo-Changjiang incised-valley fill, China

Abstract

Radiocarbon dating of samples from the CM97 cores of Late Pleistocene to Holocene sediments in the paleo-Changjiang incised valley, China, was performed, and age differences among molluscan shells, wood and other plant materials, and bulk total organic carbon (TOC) were evaluated. The incised-valley fill consists of fluvial, estuarine, and deltaic deposits in ascending order. In the dating results, samples of wood and other plant materials yielded similar ages to molluscan shell samples. In contrast, in samples from estuarine and deltaic sediments, radiocarbon ages of TOC were systematically older, by up to 6500 calibrated years, than those of molluscan shells. This result suggests that the paleo-Changjiang River transported aged terrigenous organic matter to its mouth. The magnitude of the age offset increased upward within the transgressive estuarine sediments, and the largest offset, ∼4500 calibrated years on average, was observed in the prodelta sediments deposited after the maximum transgression. Grain size, the C/N ratio, δ13C values, and the sediment accumulation rate of the incised-valley-fill sediments and the paleo-water depth indicated a weak terrestrial influence and a strong marine influence in the prodelta sediments. Therefore, the large age offsets in the prodelta sediments likely reflect reworking and resuspension in a strongly tide-dominated setting of sediments deposited earlier on the inner shelf, resulting in the supply of both fine-grained resuspended sediments and old organic carbon to the prodelta in addition to the supply of terrigenous organic carbon from the paleo-Changjiang River. The upward (deepening) increase of the age offset in the estuarine sediments supports this interpretation. The magnitude of the age offset in the past prodelta sediments was comparable to the old radiocarbon ages of the surface sediments in the present prodelta to inner shelf area. This result suggests that the supply, dispersal, recycling, and burial of terrestrial organic carbon on the inner shelf has been mostly stable during the past 8000 years.