Topographic and Climatic Control on Chemical Weathering of Mountainous Riverine Sediments of Hainan Island, South China Sea

Fangjian Xu,Xiting Liu,Jianping Huang,Jingtao Zhao,Bangqi Hu,Xue Ding,Guifeng Wang,Ruyong Cui

doi:10.3389/feart.2021.770236

Abstract

Hainan Island, the largest island in the northwestern South China Sea (SCS), is characterized by many tropical mountainous rivers that flow into the SCS. The geochemistry of these riverine sediments and the factors controlling the weathering intensity are still not well understood. In this study, sedimentary geochemistry has been investigated by using 45 riverine sediments collected from 18 major rivers on Hainan Island. The Hainan riverine sediments are only first-cycle rather than polycyclic sediments, and they faithfully reflect the present weathering intensity. The high and steady values of the Chemical Index of Alteration (CIA) indicate that the riverine sediments at different parts of the Hainan Island have overall undergone intensive chemical weathering. The low values of Weathering Index of Parker (WIP) and high αCa, αK, αSr, and αBa values of north Hainan indicate the highest weathering degree, which is mainly influenced by the flat topography. The values of αNa, αK, αSr, and αBa of southwest Hainan which are higher than those of east Hainan suggest that the leaching of elements such as Na, K, Sr, and Ba is more extreme (i.e., more intensive weathering). These are mainly caused by the different physical denudation due to different summer precipitation. Overall, the Hainan Island is featured by intensive chemical weathering and is classified as a typical transport-limited weathering regime. Therefore, the geochemistry of the riverine sediments of the Hainan Island is different from that of sediments in tectonically active regions (e.g., Taiwan Island).

Highlights

Rivers are the main passages of land-derived sediments into the sea
We suggest that the highest chemical weathering intensities of sediments in north Hainan are mainly influenced by the flat topography under a favorable monsoon climate
The αNa, αK, αSr, and αBa values of southwest Hainan are much higher than those of the east Hainan. These suggest that the overall weathering degree of the southwest Hainan is comparable with the east Hainan, the leaching of elements such as Na, K, Sr, and Ba is more extreme

Summary

Introduction

Numerous studies have focused mainly on world-class rivers, such as the Amazon (Gaillardet et al, 1997; Bouchez et al, 2011), Mekong (Egashira et al, 1997), Congo (Dupré et al, 1996; Dinis et al, 2020), Yangtze and Yellow (Yang et al, 2004), and Pearl (Liu et al, 2007) rivers, because they play leading roles in delivering the great mass of water and terrestrial materials to the sea. The characteristics of sediment geochemistry in small rivers, for example, central Japan (Ohta et al, 2005), western Korea (Xu et al, 2009), southeast China (Zhou et al, 2010; Su et al, 2017; Jian et al, 2020), Taiwan Island (Li et al, 2013; Deng et al, 2019), Malaysia, Sumatra, Borneo (Liu et al, 2012), Luzon, and Philippines (Liu et al, 2009), have aroused wide concern

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