Sedimentological and microfossil records of modern typhoons in a coastal sandy lagoon off southern China coast

Abstract

To determine the characteristics and potential indicators of modern typhoon deposition in a sandy lagoon off the coast of Guangdong Province (southern China), we analysed the 210Pb, sedimentology, and microfossils of samples from ten cores obtained before and after the passage of Typhoon Rammasun in 2014. Typhoon deposition showed a thinning trend from internal areas of the lagoon to its mouth, with the maximum thickness inside the lagoon of ∼35 cm. These typhoon deposits are dominated by overwash and differ from sediments deposited under normal weather conditions. Under normal weather conditions, lagoon sediment has a 210Pb curve that follows a model of exponential decay, has a unimodal granularity frequency curve, and lacks organic matter and microfossils (diatoms and foraminifera). However, 210Pb is low in the typhoon deposits, the grain size is coarse, and the granularity frequency curve is obviously bimodal. There are also abundant foraminifera in the typhoon deposits. We found a clear double-layered structure in the typhoon deposits, which was caused by strong hydrodynamic disturbance that mixed sediments originally from the offshore area with those of the lagoon. The lower layer has coarse-grained particles with medium sorting, low organic matter content, and low diatom content. The upper layer has fine-grained particles with poor sorting, high organic matter content, and abundant diatoms. The rate of fragmentation of diatoms in the upper layer was very high (40%–60%). The diatom assemblage contained offshore and freshwater species carried by storm runoff. Therefore, we believe that the sediments of this typical sand bar–lagoon environment retain evidence of typhoon events along the southern China coast that is displayed in the marked sedimentological and microfossil characteristics.