Major element geochemistry of the Amazon river system / Robert F. Stallard.

Tese (Ph.D.) - Massachussets Institute of Technology, 1980

Samples of surface waters and precipitation were collected on the Amazon main channel, its major tributaries, and headwater rivers in the Peruvian and Bolivian Andes. Collection was done between 1976 and 1978. The bulk of the samples were obtained during traverses of the Amazon Basin, on the main channel, in the May-July peak discharge period of 1976 and 1977. The intent of the sampling was to identify the dominant processes controlling the chemistry of rivers within the Amazon River system and to quantify the relative importance of these processes. Analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the Amazon Basin draining past Óbidos (80% of the basin). Amazon precipitation chemistry can be divided into two principal components, marine and terrestrial. The marine component (determined from analyses of marine rain) consists of Na, K, Mg, Ca, and Cl in approximately seasalt proportions, while S is doubly enriched relative to seasalt. The excess sulfur is probably derived from gas-phase inputs. The terrestrial component makes an important contribution of K, Ca, S, and N, and is in part related to biological emissions. The chloride content of lowland rivers, which drain regions lacking significant geological contributions of chloride, shows a systematic decrease with increasing distance from the Atlantic Ocean. This trend is used to define the cyclic salt background for Amazonian surface waters. Cyclic salts, in general, make only a minor contribution, relative to terrestrial inputs, to the chemistry of Amazon Basin rivers, even those draining intensely weathered terrains. An estimated 19.0%-Cl, 7.9%-Na, 1.4%-Mg, 3.8%-S, 0.5%-K, and 0.1%-Ca of the dissolved load at Obidos during peak discharge is cyclic. Within the Amazon Basin, the regional geology of highland and lowland areas contrasts markedly. Major exposures of marine and brackish water sedimentary rocks are concentrated in the Andes and the southwest lowlands. The Peruvian and south Ecuadorian Andes exhibit abundant exposures of carbonates and evaporites. The latter are expressed as numerous salt springs, salt extrusions, and intercalations in red beds. The Bolivian Andes contain a thick section of Paleozoic grey-green to black shales. Carbonates and evaporites (CaSO4) are found in the southwest lowlands. The remainder of the lowlands are covered by purely siliceous rock types, excepting narrow strips of Paleozoic carbonates and evaporites (CaSO4), exposed along the lower Amazon Valley. The contrast between the highlands and the lowlands is also reflected in differing denudation regimes. In flatlands, denudation rates are controlled largely by the capacity of transport processes to remove dissolved and solid materials (the latter generally accumulate as soils). In the highlands and hilly areas, denudation rates are limited by the rates at which weathering processes can mobilize materials. Samples can be separated into four principal groupings based on relationships between total cation charge (TZ+) and geology: (1) Rivers with OTZ+200 µEq/1 drain the most intensely weathered materials (Upper Tertiary sediments, soils of the Negro Basin and similarly weathered regions). (2) Rivers with 200TZ+450 µEq/1 drain siliceous terrains. (3) Rivers with 450TZ+3000 µEq/1 drain marine sediments 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). (4) Rivers with TZ+3000 µEq/1 drain evaporites. Rivers in categories (1) and (2) exhibit an approximately 2:1 (mole) relationship between Si and (Na+K) (corrected for cyclic salt inputs), which characterizes the weathering of many major primary silicate minerals to kaolinite. In categories (3) and (4), rivers tend to have 1:1 (equivalent) ratios of Na:Cl and (Ca+Mg):(alkalinity+SO4), caused primarily by the weathering of carbonates and evaporites. A mass conservation model is used to predict discharges along the main channel and on its tributaries. The predicted discharges agree with measured values, showing that the transport of major dissolved species down the main channel is conservative with respect to chemical removal. The calculated discharges are used to estimate preliminary denudation rates for various parts of the Amazon Basin. For Na, Mg, Ca, alkalinity, SO4 and Cl, great contrasts in denudation rates are observed between areas which drain cation-rich lithologies, such as carbonates, marine shales, and evaporites (e.g., the Andes and the southwest lowlands), and areas which drain only siliceous rock types. The Peruvian Andes contribute 50-60% of the terrestrially derived Na, Mg, Ca, alkalinity, and SO4 and 90% of the Cl in the Amazon dissolved load. Denudation rates for these species are 10-20 times greater in the Perovian Andes than in lowland areas having exclusively siliceous lithologies. In contrast, Si and K show only smell variations in denudation rates over the basin, reflecting the widespread distribution of their siliceous parent phases. This information is used to calculate the contributions that various lithological groupings (silicates carbonates, evaporites) make to the dissolved lond for different species. .