Seasonal floodplain dynamics control Fe-rich colloid characteristics

High concentrations of Fe-rich colloids have been detected in the anoxic zones of our Slate River floodplain field site (CO, USA) since 2018. We speculated that the composition and abundance of the colloids is controlled by the seasonal dynamics and spatial heterogeneities of the subsurface. Therefore, our goals were to 1) decipher the structure and chemical composition of Fe-rich colloids, 2) identify mechanisms of colloid transformation and 3) understand their biogeochemical function.

TEM analysis revealed nano-spheres and nano-assemblages that consist mainly of Fe, O, Si, and C, with lower contributions from Al, S and Ca. Based on this elemental distribution, we hypothesized that the colloids are composed of Fe minerals that are associated with organic matter and Si. This was further confirmed through Mössbauer spectroscopy and Fe-EXAFS that indicated the colloids consist ferrihydrite associated with organic matter and Si. NanoSIMS imaging detected co-localities of Fe, S, Si, and O, as well as C and N, which demonstrate once more that these are ferrihydrite-based colloids that are embedded in an organic matter-Si matrix.

 These findings are intriguing as they demonstrate high abundance of ferrihydrite-based colloids in anoxic depths of our field site. The stability of the colloids is likely attributed to the coating of organic matter and Si that serves as a protective layer against the reducing conditions. Nevertheless, our data also showed that colloids collected during snowmelt (Spring 2021) contained a higher proportion of Fe(II) than colloids collected during baseflow conditions (Summer 2021). XPS analysis measured higher atomic percentages of C and Si compared to Fe and O in baseflow versus snowmelt colloids, indicating a decrease in the organic-Si protective layer under baseflow conditions, allowing for Fe(II) oxidation and an increase in Fe(III)/ferrihydrite content. The fact that there is an occurrence of S species only in the more reduced snowmelt colloids illustrates the dynamic and delicate composition of these environmental colloids during seasonal changes in hydrology and porewater chemistry.

We are also in the process of interpreting our seasonal data that will shed light on the controls over the seasonal dynamics of the colloids. We have already linked Fe(II) colloid abundance to lower vertical porewater velocities and higher organic matter levels, and are working on additional analyses including data acquired from FT-ICR-MS analysis of the organic composition of the colloids.