Isotopic model of oceanic silicon cycling: The Kerguelen Plateau case study

A box model is presented describing the time evolution for the three stable Si isotopes (or total concentration and natural isotopic compositions), both in the dissolved and biogenic pools. Temporal variations are controlled by uptake, dissolution (both with isotopic fractionation), settling/export and mixing/advection (without isotopic fractionation). The basic building blocks of the model are combined to form a setup for the Kerguelen Plateau where distinct “plateau” and “out-plateau” areas exist and where measurements were made at the end of the growth season (early 2005, KEOPS cruise: Kerguelen Ocean and Plateau compared Study). In addition, we distinguished between surface (0–100 m) and subsurface (100–400 m) water. This resulted in a model composed of eight compartments, each containing three variables (the three Si isotopes) whose time evolution can be modelled. The model does not assume steady state, and can therefore be used to simulate transient events like blooms. We applied the model to simulate the 2004–2005 growth season. The model parameterisations were kept as simple as possible. Still, the KEOPS measurements were satisfactorily reproduced and estimates of instantaneous and seasonally integrated fluxes compared well with previous literature.