Provenance versus weathering control on sediment composition in tropical monsoonal climate (South China) - 1. Geochemistry and clay mineralogy

Abstract

This study presents an integrated mineralogical and geochemical database on mud and sand transported by the Pearl River (Zhujiang) and coastal rivers of SE China. Clay mineralogy and bulk-geochemistry of fluvial sediments are used to trace changes in weathering intensity across South China, a vast tropical to subtropical region characterized by a strong longitudinal rainfall gradient. The control of climate-induced weathering is manifest in the mineralogy and geochemistry of Pearl River mud, although mud and, to a greater extent, sand composition are also largely affected by the lithology of parent rocks. The depletion in mobile alkali and alkaline-earth metals increases steadily from the drier western part of the Pearl River catchment to the wetter eastern part and is most marked in easternmost tributaries of the Pearl River and in southern coastal rivers. Na and Ca are the most strongly depleted elements in Pearl River sediments, whereas the low concentration of Ca and Mg in river sand of coastal SE China also reflects the felsic character of parent rocks in the Cathaysia block. Eastward-increasing weathering conditions are most clearly highlighted by the progressive eastward increase in kaolinite across the Pearl River catchment, whereas the weathering effect in sand is blurred by quartz addition owing to widespread recycling of quartz-rich siliciclastic source rocks.This actualistic case study from a region characterized by monsoonal climate underscores the care that should be taken while inferring climatic conditions from the geochemical composition of sediments and sedimentary rocks, which is controlled by multiple geological factors including the felsic versus mafic character of parent rocks and the presence of extensive carbonate or siliciclastic covers in source areas. Most reliable indications are provided by the kaolinite/(illite+chlorite) ratio and by αAl indices for Na, Ca, Mg and Sr, which consistently increase eastward across South China thus paralleling the marked eastward increase in monsoonal precipitation towards the coast.