Geochemistry of the Amazon: 2. The influence of geology and weathering environment on the dissolved load

Abstract

In the Amazon Basin, substrate lithology and erosional regime (seen in terms of transport-limited and weathering-limited denudation) exert the most fundamental control on the chemistry of surface waters within a catchment. Secondary effects, such as the precipitation of salts within soils and in stream beds, biological uptake and release, and cyclic salt inputs, are more difficult to discern. Samples can be separated into four principal groupings based on relationships between total cation charge (TZ+) and geology. (1) Rivers with O<TZ+<200 •eq/1 drain the most intensely weathered materials (Upper Tertiary sediments, soils of the Negro Basin, and similarly weathered regions). These rivers show high levels of Fe, A1, H +, and coloration and are enriched in Si relative to other major species; they exhibit cation ratios similar to those of substrate rocks. (2) Rivers with 200<TZ+<450 •eq/1 drain siliceous terrains. These rivers are also rich in silica relative to other species. Rivers draining weathering-limited siliceous terrains exhibit the highest TZ+, and their cation load is typically preferentially enriched in Na over X and Ca over Mg when compared to the rocks in their catchments. (3) Rivers with 450<TZ+<3000 •eq/1 drain marine sediments or red beds with high cation concentrations (resulting from the presence of carbonates and minor evaporites in the Peruvian Andes and reduced shales and minor carbonates in the Bolivian Andes). These rivers exhibit relatively high levels of Ca, Mg, alkalinity, and SO 4 (in rivers draining reduced shales and minor evaporites). (4) Rivers with TZ+>3OOO •eq/1 drain massive evaporites. These rivers are rich in Na and C1. In the third and fourth categories, rivers tend to have 1:1 (equivalent) ratios of Na:C1 and (Ca+Mg):(alkalinity+S04) , caused primarily by the weathering of carbonates and evaporites. [1972] use factor analysis to deduce that for the major dissolved species, clear relationships exist between the geology of the sample catchment and the chemistry of the samples. Relatively high concentrations of certain species have been related to catchment geologies in the following associations: Mg, Ca, Sr, HC03, and SO 4 with carbonate rocks and gypsum; X, B, and Na with illitic bedrock; Si with bentonites and hot spring areas; Na and C1 with evaporites; and Fe and B with glauconitic substrates. Both U and PO 4 show river-specific chemical behavior which, however, could not be related unambiguously to geology, while F, NO3, Ni, Cu, and Zn show no clear geological relationships or exceptional behavior. The present work focuses on the relationship between the chemistry of major dissolved species in rivers of the Amazon system, and the geology, topography, and soils of their catchments. Data were collected in conjunction the 1976-1977 R/V Alpha Helix Amazon expedition. Like the Mackenzie project, the bulk of the Amazon samples was collected during one part of.the year. Precipitation chemistry, lacking in the Mackenzie work, was also studied [Stallard and Edmond, 1981], so that at...

Damming the rivers of the Amazon basin

“…The Andean Cordillera (approximately 12% of the Amazon basin area), provides more than 90% of the detrital sediment to the entire system 12,18 , out of which wetlands are constructed, and supplies most of the dissolved solids and nutrients transported by the mainstem Amazon River to its floodplains, estuary and coastal region 19 (Fig. 1).…”

Yields of suspended sediment and dissolved solids from the Andean basins of Ecuador

“…Due to the great number of calcareous rock and salt rock outcrops in this part of the basin, the Peruvian Andes are the largest source of dissolved material in the Amazon basin (Gibbs 1972, Stallard andEdmond 1983).…”

Ionic characterization of the Xingu River water's in the region of the future UHE Belo Monte (PA)

“…The Xingu River is one of the main right bank tributaries of the Amazon Basin and is characterized by having clear and transparent waters. The Xingu basin drains 509,685 km 2 of land with basic geological features related to the chemical composition of the water (Stallard and Edmond, 1983) that present clear, transparent, with a small amount of suspended solids, pH between 4.5 to 7.0. The Xingu River has its origin in pre-Cambrian land and drains crystalline rocks of the Guiana Shield and the Central Plateau of Brazil.…”