Benthic dissolved silicon and iron cycling at glaciated Patagonian fjord heads

Abstract

Glacier meltwater supplies silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However,the extent to which these nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Siand Fewithin fjord systems,given the rapid deposition of reactive particulate fractions at fjord heads. Here, we examine the benthic cycling of the two nutrients at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including Siand Feisotopes (δ³⁰Si and δ⁵⁶Fe), and reaction-transport modeling for Si.A highdiffusive fluxof dissolved Fefromthe fjord sediments (up to 0.02mmol m^-2 day^-1) compared to open ocean sediments (typically <0.001 mmol m^-2day ^-1) is supported by both reductive and non-reductive dissolution of glacially-sourced reactive Fe phases, as reflected by the range ofpore water δ⁵⁶Fe (-2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments (0.02–0.05 mmol m^-2day^-1) is relatively low (typical ocean values are>0.1 mmol m^-2day^-1). High pore water δ³⁰Si (up to +3.3‰) observed near the Fe(II)-Fe(III) redox boundaryis likely associated with the removal of dissolved Si byFe(III)mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Siat the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe, yet suppresses the release of dissolved Si at glaciated fjord heads, which has significant implications for understanding the downstream transport of these nutrients along fjord systems.