Sediment Resuspension as a Major Contributor to Sinking Particles in the Northwestern South China Sea: Evidence From Observations and Modeling

Lihua Ran,Martin G Wiesner,Yuntao Wang,Rong Xiang,Jingjing Zhang,Lanlan Zhang,Jian Ren,Wentao Ma,Erick Fredj,Hongliang Li,Zhi Yang,Jianfang Chen

doi:10.3389/fmars.2022.819340

Abstract

The lateral advection of sinking particles is a well-known phenomenon in the South China Sea (SCS) and has a significant impact on the estimation of the efficiency of the biological carbon pump. However, little is known about the sources and pathways of sinking particles. Here, we present benthic and freshwater diatom fluxes and relative abundances collected by a sediment trap deployed at a water depth of 1,000 m and more than 500 m above the seafloor in the northwestern SCS, indicating that laterally transported resuspended sediment accounts for a significant part of the particle flux to the deep sea. A Lagrangian particle tracking model (LPTM) revealed that the resuspended particles likely originated from the neighboring continental slope, approximately 12–145 km to the west of the study site. Sediment trap observations and the LPTM together indicated that the impact of resuspended sediment occurred mainly in the deep water, and especially strong sediment resuspension was related to summer monsoon-induced coastal upwelling. The results suggest that particle resuspension has an important impact on the biological pump as well as on paleoenvironmental reconstructions of the SCS.

Highlights

Sediment traps are used as an effective tool for monitoring marine organic carbon export, i.e., the biological carbon pump, by collecting time-series sinking particles in the deep ocean, which are the major carrier of organic carbon from the surface ocean to the deep sea
The frequent appearance of benthic diatoms indicated that some particles originated from the seafloor of coastal areas and the shallow continental shelf, and the frequent appearance of freshwater diatoms indicated that some sinking particles originated from the nearshore estuarine area
There are several small rivers draining Hainan Island that may serve as potential sources of freshwater diatoms collected in the sediment trap

Summary

Introduction

Sediment traps are used as an effective tool for monitoring marine organic carbon export, i.e., the biological carbon pump, by collecting time-series sinking particles in the deep ocean, which are the major carrier of organic carbon from the surface ocean to the deep sea. Basin-wide cyclonic circulation occurs in response to the northeast monsoon in the winter, while anticyclonic circulation occurs mainly in the southern SCS in response to the southwest monsoon in the summer (Hu et al, 2000). This significant seasonal environmental variation results in seasonal changes in the upper ocean nutrient content, primary productivity, and organic carbon export (Liu et al, 2002, 2013; Ran et al, 2015a,b; Li et al, 2017). To understand the complex biogeochemical processes and biological carbon pump in the SCS, bottom-tethered sediment traps have been widely used since the 1980s, in the northern and central parts of the basin (Wiesner et al, 1996; Lahajnar et al, 2007; Gaye et al, 2009; Ran et al, 2015a,b; Li et al, 2017; Zhang et al, 2019; Tan et al, 2020)

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