Isotopic constraints on sources and cycling of riverine dissolved inorganic carbon in the Amazon Basin
Mayorga, Emilio, 1968-
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Dissolved inorganic carbon (DIC) and CO2 gas are fundamental components of riverine biogeochemical functioning. Amazonian and humid tropical rivers represent a large, neglected source of CO2 to the atmosphere. Carbon isotopes are invaluable tools for examining sources and cycling of inorganic carbon. However, 14C-DIC has never been analyzed in Amazonia, while 13C-DIC studies have concentrated on the mainstem and surroundings, despite large basin landscape heterogeneity.I used geochemical and 13C-14C isotopic measurements on basin-wide samples to address the following DIC and CO 2 topics: isotopic variability and its primary causes; sources and age of respired organic carbon (OC) dominating CO2 production; terrestrial sources; and downstream evolution. To facilitate interpretation within a consistent river system view, I developed an Amazon river network dataset and algorithm. I also compiled into a common database new and previously published geochemical data. I characterized lithological sources through major ions geochemistry.Dominant sources of riverine DIC isotopic variability are: presence of carbonate lithologies (common throughout the Andes, especially Peru), relative contribution of C3 and C4 plant material to respiration, and atmospheric CO2 invasion in turbulent rivers. Lowland rivers are predominantly supersaturated in pCO2 and carry young DIC. Aged lowland DIC occurs only in carbonate lithologies, some small streams, and low-oxygen environments. A small, contemporary OC fraction dominates respiration; C4 vegetation exerts a disproportionate influence on lowland fluvial respiration. CO2 is typically more supersaturated and depleted in delta13C at high water. In the shields, sandy soils and deforested regions export contemporary inorganic carbon; certain forested clayey soils export aged inorganic carbon.Extensive Andean erosion carries unweathered carbonate sediments into depositional areas. Dissolution then results in slowed replacement of 14C-depleted DIC in the mainstem; delta13C-DIC reflects equilibrium between evasion and respiration in addition to very gradual downstream increase in contributions from C3 respiration.Respiration maintains CO2 supersaturation, flushing out terrestrial DIC via exchange and gas evasion. DIC gradually approaches atmospheric 14C-CO2 downstream. Turbulence in steep Andean rivers and rocky Shield rivers accelerates gas exchange. In the Andes, tectonic activity and lithological heterogeneity leads to localized influence by marginal processes such as black shale oxidation and lithospheric CO2 degassing.
- Oceanography