A comparative overview of weathering intensity and HCO 3− flux in the world's major rivers with emphasis on the Changjiang, Huanghe, Zhujiang (Pearl) and Mississippi Rivers

Abstract

In this paper, general relationships of riverine bicarbonate concentrations and fluxes as a function of drainage basin mineral content and runoff are examined using a database of the 25 largest rivers in the world. Specific HCO 3 − flux normalized to unit basin area, which peaks in the mid latitudes, was found to be strongly correlated with the carbonate mineral content of river basins, while river HCO 3 − concentration was related to the balance of precipitation and evaporation. Within this global context, the weathering patterns of CO 2 in a few large rivers (Changjiang, Huanghe, Pearl, and Mississippi rivers) were examined in further detail. The Zhujiang (Pearl River), especially its largest branch (Xijiang), was characterized by the highest specific weathering rate among all the world's large rivers due to an exceptionally high carbonate mineral content (over 80%) in its drainage basin and its warm and wet environment. It has a moderate level of HCO 3 − concentration, however, due to dilution by relatively high precipitation in the watershed. In stark contrast, the Huanghe (Yellow River) has one of the lowest specific weathering rates because of low carbonate mineral content and a dry climate. However, it has a high HCO 3 − concentration due largely to the concentrating effects of high evaporative water loss, as a result of arid weather and the agricultural use of water through irrigation systems, as well as carbonate-containing surficial deposits (i.e., loess). The strong correlation between specific HCO 3 − fluxes and discharge in all four rivers with different discharge seasonality suggests that higher precipitation in drainage basins promotes higher weathering rates. For the 25 large rivers studied here, rivers in low (<30°), mid (30–60°) and high (>60°) latitudes have an average HCO 3 − concentration of 0.584, 1.649, and 1.154 mM, respectively, and they account for 42.6%, 47.3% and 10.1%, respectively, of the total global dissolved inorganic carbon flux to the ocean. Thus the mid-latitude rivers carry a disproportionally high dissolved inorganic carbon flux with a relatively small (26%) amount of freshwater discharge. The discharge-averaged global river HCO 3 − concentration was estimated to be 1.1 mM.