Benthic dissolved silicon and iron cycling at glaciated Patagonian fjord heads

Glacier meltwater supplies a significant amount of silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these essential nutrients reach the ocean is regulated by the processes occurring within fjords, which act as conduits from glacial rivers and subglacial discharge and the ocean. One key – but understudied – component of biogeochemistry within fjords is benthic cycling, especially in regions of rapid deposition of reactive particulates at fjord heads. Here, we explore the benthic cycling of Si and Fe at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including stable Si and Fe isotopes (δ³⁰Si and δ⁵⁶Fe respectively), and novel reaction-transport modelling for Si. A high diffusive flux of dissolved Fe from the fjord sediments compared to open ocean sediments is supported by both reductive and non-reductive dissolution of glacially-sourced reactive Fe phases, as reflected by the range of pore water stable Fe isotopes (δ⁵⁶Fe from -2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments is relatively low. High pore water δ³⁰Si (up to +3.3‰) observed near the Fe(II)-Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe but suppresses the release of dissolved Si at glaciated fjord heads. The redox sensitive coupling of Si and Fe has significant implications for our understanding of how essential nutrients are transport along fjord systems.