Caura River basin: Weathering rates, CO₂ consumption, and chemistry of major and trace elements in an Orinoco River tributary coming from the Precambrian Guayana Shield, Venezuela

Abstract

This study describes the geochemistry of dissolved elements in the Caura River and gives information about weathering rates and associated CO 2 consumption in an Orinoco River subbasin. Physicochemical variables (pH, conductivity, HCO 3 − , dissolved O 2 , and temperature), dissolved elements (Na, K, Ca, Mg, Si, Al, Fe, Cu, Mn, Zn, and Cr), total suspended sediments (TSS), and dissolved organic carbon (DOC) concentrations were measured monthly from June 2007 to August 2008 in the Caura River. Major element concentrations (Na, Ca, Mg, and Si) showed an inverse correlation with discharge, whereas K showed high concentration values during the rising waters, probably due to biogenic sources. All these elements are provided mainly from weathering of plagioclases and K feldspars. The weathering rate (riverine flux of dissolved major cations and SiO 2 derived from weathering per unit area) and the CO 2 consumption rate in the Caura basin (15.4 tons km −2 yr −1 and 1.1 × 10 5 mol km −2 yr −1 , respectively) were higher than those reported for the Orinoco basin and other black water river basins. This fact can be due to several factors such as lithology (volcanic rocks), high runoff, and the presence of organic acids, which can enhance the chemical weathering. The variability of the trace elements showed a different behavior than major elements. Fe and Al concentrations were correlated with DOC. Dissolved Mn content was correlated with pH, whereas the low concentrations of Cu and Cr are possibly associated with the low content of small size organic colloids. The high values of Zn observed during the decreasing stage suggest biogenic input of Zn to river waters.