Clay composition heterogeneity in sediments from mountainous catchments with contrasting bedrock lithology in SE China coast

Abstract

Low-latitude mountainous rivers in South and East Asia play a crucial role in material cycles on the Earth’s surface and are ideal targets to study how mountainous river sediments are generated, supplied and transported to continental margins. The Zhe-Min mountainous rivers in SE China coast are characterized by similar monsoon- and typhoon-induced high rainfall conditions to those islands in western Pacific (e.g., Taiwan) but have different geological backgrounds. With rather diverse bedrock types, intra- and inter-catchment differences of discharged sediment compositions of the Zhe-Min mountainous rivers remain poorly known. Here, we present clay mineral data of sediments from eight small–mesoscale mountainous rivers (drainage area < 80000 km² and maximum altitude > 1000 m) and two estuarine bays in this region to investigate spatial variations in sediment compositions and characterize supply, transport and deposition processes of the fine-grained sediments in these catchments. Significant spatial heterogeneity of clay minerals in the analyzed river sediments is observed, not only among these rivers, but also among non-tidal influenced reach, tidal reach and estuarine bay from the single river, implying complex accumulation process of the riverbed fine-grained sediments. Our quantitative analysis results reveal major controls of previously-underrated climate differences (e.g., temperature) and contrasting bedrock lithology on the clay mineral heterogeneity. The relatively low heterogeneity in clay mineral compositions between tidal reaches and bays, along with the widespread appearance of smectite in these regions, are distinctively different from those in the non-tidal influenced reaches. This reveals well-mixing of the offshore-sourced and mountain-supplied fine-grained sediments and highlights the major role of the tidal effects. This study demonstrates that determining terrigenous sediment output signals for small–mesoscale mountainous rivers with contrasting bedrock lithology and prevailing tidal water intrusion is a challenging job. Our findings have broad implications for chemical weathering intensity evaluation and sediment source-to-sink system studies.