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 Si and Fe within 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 Si and Fe isotopes (δ30Si and δ56Fe), and reaction‐transport modeling for Si. A high diffusive flux of dissolved Fe from the fjord sediments (up to 0.02 mmol m−2 day−1) compared to open ocean sediments (typically <0.001 mmol m−2 day−1) is supported by both reductive and non‐reductive dissolution of glacially‐sourced reactive Fe phases, as reflected by the range of pore 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−2 day−1) is relatively low (typical ocean values are >0.1 mmol m−2 day−1). 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, 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.

Silicon Isotopes Highlight the Role of Glaciated Fjords in Modifying Coastal Waters

“…Sediment efflux has been shown to be a significant source of DSi in the region (Ng et al, 2020), which could help to explain some of the elevation in particulate concentrations combined with isotopically light DSi. However, porewater δ 30 Si DSi values in glaciated fjords are generally very high (Ng et al, 2022) Therefore, the most realistic explanation remains the dissolution of isotopically light ASi, derived from glacial meltwaters. Glacially derived ASi from a range of glaciers has been shown to be enriched in 28 Si, with an average δ 30 Si ASi composition is −0.39‰ (range: −0.01‰ to −0.86‰, Hatton et al, 2019aHatton et al, , 2019b.…”

Detrital Input Sustains Diatom Production off a Glaciated Arctic Coast

“…Specifically, subglacial-routed meltwaters are known to be rich in several key macro-and micronutrients, including the dissolved silicon or silicic acid (DSi) (Meire et al, 2016), reactive amorphous solid phases of silica (ASi) (Hawkings et al, 2017), and iron (Fe) (Bhatia et al, 2013;Hawkings et al, 2014). A significant proportion of these dissolved inorganic nutrients are trapped within fjords by biological utilization (Hopwood et al, 2015(Hopwood et al, , 2020 and abiotic processes such as adsorption and precipitation (Ng et al, 2022). Despite these consumption processes, fjord mouth waters have high concentrations of dissolved micronutrients (e.g., Fe) relative to coastal and open ocean waters (Tonnard et al, 2020).…”