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

Cockerton, H. E. and Street-Perrott, F. A. and Leng, M. J. and Barker, P. A. and Horstwood, M. S. A. and Pashley, V. (2013) 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. Quaternary Science Reviews, 66. pp. 4-21. ISSN 0277-3791

Abstract

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. delta O-18 ranged from -4.7 to +8.0 parts per thousand in the wet season and +0.6 to +8.8 parts per thousand. in the dry season (through 2009-2011). Higher delta O-18 values during the dry season reflected increased evapotranspiration and open water evaporation under conditions of lower humidity. Progressive downstream enrichment in the heavy isotope O-18 also occurred in response to cumulative evaporative losses from open water bodies and swamps. delta Si-30 values of DSi ranged from +0.48 to +3.45 parts per thousand. during the wet season and +1.54 to +4.66 parts per thousand during the dry season, increasing the previously reported global upper limit for delta Si-30 values in natural waters by 1 parts per thousand. Si-isotope fractionation was most intense during the thy season when demand for DSi by aquatic ecosystems exceeded supply. Progressive downstream enrichment in the heavy isotope Si-30, 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. (C) 2013 Elsevier Ltd. All rights reserved.