Radiocarbon Dating of Organic and Inorganic Carbon in Coastal Lake sediments

What is it about?

Coastal lake sediments are a valuable archive of past climates provided that they can be accurately dated. Here, we report radiocarbon ages of various carbon-containing substrates, including bulk sediment organic matter, plants, shells and particulate organic matter, in coastal lakes along the Gulf Coast of Florida. The data reveal significant radiocarbon deficiencies in sedimentary organic matter in these lakes, resulting in the radiocarbon ages of bulk sediments being too old and erroneous. The data also show that the radiocarbon deficiency of sedimentary organic matter varies significantly over time and with location, making it impossible to determine a proper correction factor for radiocarbon ages of bulk sediments from these lakes. Thus, we consider the radiocarbon ages obtained from the bulk sediment samples to be unreliable. Furthermore, our data show that sedimentation rate can vary considerably with location in a lake, resulting in very different age-depth relationships. This implies that the age model based on dating of one sediment core cannot be directly applied to other nearby cores from the same coastal lake despite their close proximity. The age reversals in the sediment cores are most likely caused by rapid influx of eroded materials from upland soils into the lake resulting from large storm events. Our data from snail shells from one of the lakes suggest that fresh/brackish water shells may serve as a useful substrate for obtaining reliable radiocarbon dates. But, radiocarbon ages of shell fragments associated with sand deposits in these coastal lakes are likely a few hundred years too old due to their possible marine origin and the influence of marine radiocarbon reservoir effect.

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Why is it important?

Coastal sediment cores have been used to reconstruct long-term records of paleohurricanes and environmental changes over the last millennia. Chronologies of such records are often based on radiocarbon dating of bulk sediment organic matter. In this study, we show that radiocarbon ages of bulk sediment organic matter in coastal lakes are unreliable due to significant and variable radiocarbon deficiency of sedimentary organic matter in these lakes. We also show that fresh/brackish water shells in coastal lakes in NW Florida may serve as a useful substrate for obtaining reliable radiocarbon dates in the absence of other more suitable dating substrates (such as plant fragments). The results have important implications for developing reliable radiocarbon-based age models for coastal lake sediment cores in paleoclimate research.