Diffusive Gradients in Thin Films (DGT) in Amazonian Rivers ¿ Analysis of Strontium, Barium, and Rare Earth Elements

Strontium (Sr) and barium (Ba) isotope ratios as well as rare earth element (REE) patterns are valuable tracers of hydrogeochemical processes in freshwater systems. Especially the radiogenic 87Sr/86Sr isotope ratio provides unmatched insights into biogeochemical cycles. The Amazon river basin, a network of over 1000 tributaries accounting for 20 % of the global freshwater, is characterized by partly low Sr and Ba levels (c < 2 ¿g L¿1), high Rb/Sr ratios (up to 1), and low ionic strengths (I < 10¿4 mol L¿1), which are challenging conditions for subsequent Sr isotope ratio analysis by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Water and sediment grab samples were taken from three main rivers of the Amazon basin (Rio Negro, Solimões, and Amazon) at low and high water levels, revealing visibly different REE patterns in the Rio Negro compared to the Solimões. A pronounced 87Sr/86Sr variation (¿(87Sr/86Sr)max¿min = (42.22±0.91)¿) between the rivers was observed, reflecting the geological diversity of the Amazon basin. The 87Sr/86Sr ratio of the Amazon river results from binary mixing of Solimões and Rio Negro, with the latter exhibiting much stronger seasonal 87Sr/86Sr variation. Conventional grab sampling methods often fail to capture temporal variability in dynamic river environments, necessitating alternative approaches, e.g., passive sampling by diffusive gradients in thin films (DGT). The TK100 DGT technique for time-integrated Sr and Ba quantification and Sr isotope ratio assessment by (MC-)ICP-MS was applied under extreme hydrochemical conditions in laboratory experiments with synthetic and natural river water and in situ in the mentioned rivers. Up to 16-fold Sr preconcentration during DGT sampling enabled 87Sr/86Sr isotope ratio measurements without additional evaporation. A logarithmic relation found between the effective diffusion coefficients and the ionic strength was used to correct Sr and Ba cDGT values at high and low ionic strengths, resulting in an average ratio of cDGT/csoln = 1.05 ± 0.16 (n = 53). The challenging sampling conditions necessitated additional column-chromatographic matrix removal for the TK100 DGT eluates. Still, no isotopic fractionation was caused by the DGT sampling and elution procedure (¿(87Sr/86Sr)DGT¿grab = (¿0.07 ± 0.43)¿, n = 33), reiterating the method¿s potential for accurate Sr isotope ratio analysis in natural waters. For the first time, TK100 DGT devices were used to assess DGT induced Sr and Ba fluxes from sediments, indicating a yet unexplored potential for understanding sediment-water-biota interactions.