Stable-isotope (H, O, and Si) evidence for seasonal variations in hydrology and Si cycling from modern waters in the Nile Basin: implications for interpreting the Quaternary record
Seasonal variations in hydrology and Si cycling in the Nile Basin were investigated using stable-isotope (H, O, and Si) compositions and dissolved Si (DSi) concentrations of surface waters, as a basis for interpreting lacustrine diatom sequences. δ18O ranged from -4.7 to +8.0‰ in the wet season and +0.6 to +8.8‰ in the dry season (through 2009-2011). Higher δ18O values during the dry season reflected increased evapotranspiration and open water evaporation under conditions of lower humidity. Progressive downstream enrichment in the heavy isotope 18O also occurred in response to cumulative evaporative losses from open water bodies and swamps. δ30Si values of DSi ranged from +0.48 to +3.45‰ during the wet season and +1.54 to +4.66‰ during the dry season, increasing the previously reported global upper limit for δ30Si values in natural waters by 1‰. Si-isotope fractionation was most intense during the dry season when demand for DSi by aquatic ecosystems exceeded supply. Progressive downstream enrichment in the heavy isotope 30Si, coupled with decreasing DSi concentrations, represented cumulative Si uptake by diatoms, macrophytes and other Si-accumulating aquatic organisms. The pronounced seasonal variations in DSi concentrations and Si-isotope compositions in the River Nile suggest that its DSi flux to the ocean may have varied significantly on a glacial/interglacial time scale, with important consequences for the marine Si budget and consequently the global C cycle. Anthropogenic impacts were evident in both the water- and Si-isotope datasets, especially during the dry season and along the Main Nile, where water management is most intensive.