Silicon Cycling in the High-latitude Glacier-Fjord-Coast System

Abstract

The Greenland Ice Sheet is experiencing the most widespread surface melting in recent years, resulting in accelerated mass loss and enhanced freshwater runoff to adjacent marginal area, including fjords and coastal regions. This glacial meltwater discharge delivers substantial amounts of bioavailable silicon (Si) in forms of both dissolved silicon (DSi) and dissolvable amorphous silica (ASi) to the downstream ecosystem. These glacier-sourced Si phases play an important role in sustaining local primary production and so also impact the biogeochemical cycling of nutrients and carbon in the high latitudes. The glacially-derived ASi dominates the glacial Si export and sustains a large portion of coastal productivity by promoting diatom growth in DSi-depleted seawater. However, the degree to which fjord processes, including sediment diagenesis, modifies the Si flux and budget before reaching the coast is still poorly constrained, preventing the prediction of future changes in primary production and carbon drawdown in this climate-sensitive region, following the inevitable widespread retreat of Greenland glaciers.

This study produces a new set of Si isotope data (δ30Si) of reactive Si pools in the fjords, including the dissolved phase in pore water and core-top water, and reactive solid phases attached to bottom and suspended sediments, which enables the construction of sedimentary (benthic) Si cycling via reaction-transport models. Model simulations reveal that fjords experience low return DSi fluxes but high burial efficiencies of ASi relative to the adjacent coastal area, due to their high sedimentation rates. The build-up of the pore water DSi pool is controlled by competing Si uptake and release processes during early diagenesis. Si uptake processes are found to be more intense in fjord head sediments, contributing to the suppressed benthic DSi fluxes. The thesis supports that fjords act as a ‘trap’ for reactive Si during the sought-after glacier-fjord-coast transport. My results suggest that approximately half of the ASi sourced from the glacial export is deposited at the fjord head with the remaining half being transported downstream and sustaining diatom production in both fjord and the coastal region. In particular, over one-third of the coastal detrital ASi is contributed by these glacially-derived ASi. The results also imply a reduction in downstream transport of glacial ASi following the impending large-scale glacier retreats, which likely impacts high-latitude diatom production, carbon drawdown, and coastal ecosystems.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Sponsors	China Scholarship Council
Supervisor	Katharine Hendry (Supervisor) & Hong Chin Ng (Supervisor)