Long-chain alkyl diols as indicators of local riverine input, temperature, and upwelling in a shelf south of the Yangtze River Estuary in the East China Sea

Abstract

Sedimentary processes on continental shelves are affected by complex interactions between a variety of riverine and oceanographic processes. In the shelf of the East China Sea (ECS) south of the Yangtze River estuary, we investigated source-to-sink processes relating to local riverine input as well as the effects of the Kuroshio intrusion by examining the distribution of long-chain alkyl diols in surface and core sediments. The results show that at present the main source of 1,14-diols is most likely diatoms belonging to the genus Proboscia. Most C32 1,15-diol is produced in situ in local rivers (rather than originating from the Yangtze River) and its distribution is mainly restricted to the front of local river mouths along the coast (water depth < 50 m) due to the hydrodynamism of the nearshore circulation system. Although the sources of the 1,13- and 1,15-diols remain uncertain, principal component analysis and their distribution patterns suggest that 1,14-, 1,13- and 1,15-diols have different biological sources, and that their producers may bloom in different seasons. The long chain diol index (LDI), a proxy for sea surface temperature (SST), was used in this study to reconstruct records of SST (with a bias towards autumn temperatures) in different parts of the ECS shelf, except for shallow water areas (water depth < 50 m), which are more affected by riverine input. The diol index (DI), based on 1,14-diols, worked well as a upwelling tracer in our study area, and our results show that upwelling prevails in the ECS shelf and that zones of strengthened upwelling are present to some extent at ~28.8°N–29.6°N and ~ 27.0°N–27.8°N during the summer due to the interaction of geomorphology, monsoons, and different water masses (i.e., the Kuroshio branch and the Yangtze River diluted water). The nutrient diol index (NDI) shows potential as an indicator for nutrient concentration in the surface waters in this study area during the warm season, but further work is needed.