UNEXPECTEDLY LOW CHEMICAL WEATHERING RECORDED IN FLUVIAL SEDIMENTS OF A HOT-HUMID SYSTEM

Humid tropical and subtropical regions are commonly covered with thick laterite profiles reaching up to several tens of meters that provide a major sediment source for adjacent watersheds. Application of well-known indices for chemical weathering, such as the Chemical Index of Alteration (CIA), in interpreting paleoclimate using sediment is based on the known relationship between climate and weathering intensity; e.g., deep weathering and correspondingly high CIA values are thought to characterize warm and humid climates. This study investigates chemical weathering of modern fluvial sediment of the Rio Guayanés, SE Puerto Rico. It provides an excellent opportunity to study weathering and sediment production in a hot (~26 ºC) and humid (~2107 mm MAP) tropical climate. The sediment samples were collected from slackwater areas of bars along the main channel (~20 km), in a region that drains exclusively granodioritic and quartz dioritic bedrock of the San Lorenzo Batholith. The simple lithology of plutonic bedrock excludes any input of recycled material from sedimentary rock and enables characterization of sediment produced by bedrock weathering in the tropical climate. Thick saprolite and soil profiles occur throughout the study area, yet the degree of chemical weathering inferred from CIA (58 – 72) of the fluvial muds is significantly lower than expected. Compilation of CIA values of soil from varying climates reveals that the studied fluvial sediment is similar to granite soil developed in high latitudes (e.g. Norway). These data indicate that deep weathering profiles do not provide the main source of riverine sediment. Rather, comparison with results of previous studies on chemical weathering in small watersheds of eastern Puerto Rico suggest that the riverine sediment records significant input from weathering rinds of plutonic bedrock (i.e. rindlets) mobilized by landslides, rather than surface transport of soil. Landslides triggered by rainfall are the dominant process of hillslope erosion in montane humid-tropical settings and intense and/or prolonged rainfall has been the principal cause of landslides in Puerto Rico. This study underlines the caveats in interpreting paleoclimate from sediment and the importance of understanding complexities of sediment production and transportation in a watershed.