Early Diagenetic Silicon Cycling in a Changing Arctic Ocean

Abstract

The Arctic Ocean silicon (Si) cycle is under considerable pressure from natural and anthropogenic forcings. For example, the Barents Sea has experienced a 20-30% reduction in the concentration of dissolved silicic acid (DSi) in inflow waters since 1990, in addition to the most rapid rates of warming and sea-ice retreat in the Arctic, largely driven by a northward expansion of Atlantic Water. These factors have contributed to a shift in phytoplankton spring bloom community compositions towards temperate flagellate species, as conditions become less favourable for Si-based diatoms. The Arctic seafloor represents an important biogeochemical reactor, supplying a similar flux of DSi to the ocean as pan-Arctic rivers. However, current understanding of the regional Si budget is partially limited by a lack of data in this area, leading to an isotopic imbalance. It is crucial to resolve this discrepancy, in order to better anticipate the implications of further perturbations.

This thesis presents a dataset of stable Si isotopes (d30Si) measured in the sediment pore water DSi pool and reactive solid phases, coupled to steady state and transient reaction-transport models, used to gain a comprehensive understanding of the current state of early diagenetic Si cycling in the Barents Sea. This research uncovers a strong mineralogical control on the benthic Si cycle, with DSi derived from the dissolution of lithogenic silicate minerals contributing an estimated 13-98% of the pore water DSi pool alongside biogenic silica (BSi), as well as a coupling of the Si cycle with iron-redox cycling. Furthermore, pore water DSi d30Si and cation concentrations reveal a reprecipitation of dissolving DSi (3-37%) as authigenic clays (AuSi). By using a reaction-transport model to quantify these processes, this thesis presents an isotopically balanced budget for the Barents Sea, pointing to AuSi burial as a possible missing sink of isotopically light Si in the pan-Arctic Ocean budget. Transient reaction-transport modelling also reveals that seasonal variability in BSi deposition is reflected within the seafloor through tight benthic-pelagic coupling. The dissolution of fresh phytodetritus is estimated to contribute approximately one-third of the total annual DSi benthic flux, which will be subject to change as pelagic phytoplankton community compositions further adjust to changing conditions.

Date of Award	21 Mar 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Katharine Hendry (Supervisor), Sandra Arndt (Supervisor) & Christian März (Supervisor)