Configuration of the cGENIE Model with Marine Biogeochemical Silicon Cycling

Silicon is an essential nutrient to diatoms, photosynthesising marine organisms responsible for up to half of global carbon export to the deep ocean. Formal configuration of silicon cycling within an ocean circulation model is vital to understand biogeochemical silicon and carbon cycling since the proliferation of diatoms, and the subsequent role of diatoms in carbon/climate processes. Here, we show the capability of the intermediate complexity model, cGENIE, to reproduce uptake and remineralisation processes of marine silicon over timescales >100 yrs. The model is enabled with water column and sediment silicon cycling, and related isotope processes.

cGENIE is tuned primarily to reproduce modern dissolved silicon (DSi) concentrations and biogenic silica burial, given a range of organism Si:C uptake ratios, and particulate organic matter sinking rates. The model captures important features of silicon cycling globally, achieving an AMS score of ≤ 0.76 (where 1 = perfect fit) against observational DSi data. Elevated Southern Ocean and North Pacific biogenic opal rain and burial fluxes contribute 3.2 Tmol Si yr^-1 to a total burial flux of 4.4 - 4.6 Tmol Si yr^-1 in the model. Observed trends of surface silicon isotopes are also captured by cGENIE, reflecting utilisation and circulatory processes: high δ³⁰Si values of up to +4.7‰ are observed in surface waters of high utilisation, whilst Southern Ocean δ³⁰Si is < +2 ‰. Mean deep water δ³⁰Si values remain low with an average of +1.1 ‰.

We illustrate the utility of the model as a tool for investigating potential changes in dissolved silicon concentrations, isotopic compositions and siliceous production on a basin-to-global scale, using a North Atlantic hosing experiment as a case study. A glacially derived meltwater and accompanying weathered silicon flux is modelled, based on predicted Greenland ice-loss over the next century. Observed changes include a decrease in global organic matter export of 4% by 2100, largely isolated to the North Atlantic. Furthermore, enriched in light δ³⁰Si ( 0 ‰), glacially-derived silicon perturbs the Atlantic surface ocean δ³⁰Si. These results could have implications for the interpretation of seafloor opal δ³⁰Si as a proxy for glacial-interglacial silicon cycling.