Application of uranium and strontium isotopes as salinity and paleo-environmental conditions tracers: Insight from the Rio Grande River and pedogenic carbonates in drylands soils of Southwest, USA
South New Mexico and west Texas regions of the northern Chihuahua Desert are characteristic of low annual rainfall and high potential evapotranspiration rates. Soils in these arid and semi-arid regions are abundant of pedogenic carbonates of various stages of development. Both surface and ground water resource in the area have high salt content; the Rio Grande river, which flows through the region is known for its high salinity downstream. Due to increasing population in cities of the region thus high food demand, irrigation activities along the river valley have enhanced crop productivity. High evapotranspiration rates and intense saline flood irrigation have caused excessive salt accumulation (e.g. pedogenic carbonates) in irrigated soils. Long term accumulation of pedogenic carbonates impair plant root and soil water infiltration, lower crop yield, and sequester carbon thus influencing carbon cycle and ultimately climate over time. It is important to monitor the chemistry of water in drylands and its impact on soil quality. This dissertation focuses on three main areas: (1) establishing factors that control formation rates of pedogenic carbonates in soils of the southwest USA, (2) reconstructing paleomoisture information archived in pedogenic carbonates within the Jornada Basin, New Mexico and its environs, and (3) tracing salinity into the Lower Rio Grande river between Elephant Butte Reservoir, NM and El Paso, TX. Using major and minor element geochemistry (including carbon), U-series, strontium, and carbon isotopes, this study shows that: (1) Land management practices such as irrigation load high Ca2+ and HCO3− ions resulting in high pedogenic carbonate accumulation rate in irrigated soils (9±6 gCaCO3/m2/yr) along the Rio Grande valley compared to low rates in naturally forming carbonates at the Jornada Basin (2.5±1.0 to 3.5±1.5 gCaCO3/m2/yr), (2) Paleo-moisture conditions have varied between ∼200 ka and 6 ka based on the pedogenic carbonate U-series chronology. Older carbonates at soil depth formed under moist conditions while younger shallower carbonates precipitated in drier conditions, (3) significant salts are loaded into various sections of the lower Rio Grande River depending on the irrigation or non-irrigation season. Water released from Elephant Butte Reservoir dominated the river chemistry in the irrigation season. In the non-irrigation season, deep ground water salinity was distinct in the river a few kilometers after the reservoir while mixed anthropogenic salinity sources (e.g. shallow irrigation groundwater, agricultural drains, wastewater, and city effluents) were significant in the downstream locations near the Rio Grande at El Paso, TX. This work has demonstrated the effects of anthropogenic factors on water quality and carbon budgets in dryland areas. ^
Nyachoti, Syprose K, "Application of uranium and strontium isotopes as salinity and paleo-environmental conditions tracers: Insight from the Rio Grande River and pedogenic carbonates in drylands soils of Southwest, USA" (2016). ETD Collection for University of Texas, El Paso. AAI10118191.