Sedimentary evidence for a period of rapid environmental change in the Yangtze River Delta, China around 150 years ago

Abstract

The mechanism of channel bifurcation in tide-dominated deltas is poorly understood. Here, we evaluate the function of extreme events on the channel bifurcation by examining the lithology, grain size, and organic carbon stable isotope (δ13C) composition, as well as the AMS 14C and OSL ages in two sediment cores (KZ01-A and KZ02) collected close to a site of a shipwreck of a known age at the offshore section of the North Channel in the Yangtze River mouth, China. The sedimentary sequence in both cores is mostly typical of deltaic succession, changing vertically from a prodelta to a subtidal mouth shoal, and finally to a subaqueous distributary channel. In KZ01-A, a shipwreck layer underlies the succession of the subaqueous distributary channel, and the porcelain collected from the sunken ship constrains the formation of the North Channel to post-1862, which is consistent with historical literature and marine charts though not with the OSL ages of the shipwreck layer, which are overestimated, likely owing to the rapid deposition during the event. After the formation of the North Channel, the consistently more negative δ13C values in the facies of subaqueous distributary channel in KZ01-A evidence the prevalence of fluvial input. Based on the shipwreck's location in the flood-dominated tidal channel of the outer delta front platform, the ship likely sank due to a large typhoon rather than a river flood. In core KZ02, significant fluctuations in both grain size and δ13C values, such as shifting from less negative to more negative δ13C in the mouth shoal facies, indicate periods of frequent river floods and typhoon events before formation of the channel. We therefore suggest that the frequent extreme events including both typhoons and river floods in the late 19th century played an important role in the geomorphological evolution of the North Channel in the tide-dominated Yangtze River Delta.