Nepartak Typhoon Influenced Bottom Sediments From the Yangtze River Estuary and Adjacent East China Sea-Foraminiferal Evidence

Foraminiferal taphocoenose characteristics, notably allochthonous tests in surface sediment samples from the Yangtze River Estuary and adjacent areas in the East China Sea, were examined during (July, 2016) and after typhoon Nepartak (February, 2017) crossed this region. A comparison of transport indices from five sample stations reveals that this typhoon significantly strengthened sediment transport, although the transport in predominate direction (TWCnorth branch) occurs all year. Moreover, findings from a comprehensive impact of currents, topography, test size and shape, as well as other abiotic factors, show that transport occurs in the order (from stronger to weaker): south-north path ≫ southeast-northwest path > nearshore-northeast path. Along the south-north path, the TWCnorth branch transports larger tests of taxa Ammonia compressiuscula and Cavarotalia annectens as bed load along the gentle seabed in a rolling or saltating process. Results from the southeast-northwest path show smaller and bulging tests of the dominant species Epistominella naraensis and Bolivina robusta may be transported as suspension load along the steep submarine valley by the TWCnorthwest branch. On the nearshore-northeast path, only part of allochthonous tests Ammonia beccarii vars. was transported, probably due to distance travelled and weakening of the typhoon. Furthermore, we also infer the northeastern area as a sink zone that accumulates grains of allochthonous tests predominantly from the southern source zone, and a selection of tests from littoral species originating from the nearshore source zone. It is important to identify and independently count allochthonous test samples if a large number of these tests are recorded. Findings from this study provide a reference to identify and investigate typhoon modern effects in areas with complex currents and frequent strong physical dynamic events, as well as their geological records.