VARIATIONS IN THE DELIVERY OF SI TO THE OCEANS FROM THE CONTINENTS: IMPLICATIONS OF A DYNAMIC TERRESTRIAL SI CYCLE

Abstract:

A common assumption is that the modern day continental Si cycle is at steady-state. However, a new paradigm is emerging regarding the terrestrial biogeochemical Si cycle and its importance. We now know that the terrestrial Si cycle is strongly biologically mediated, which creates and recycles a pool of reactive Si in soils and sediments that ultimately buffers the export of dissolved silicate (DSi) to the oceans. Our recent results show that imbalances in the continental Si pool on millennial scales may have caused systematic mis-estimation of silicate weathering rates, particularly when using river chemistry at large spatial scales to calculate weathering rates. Quantifying this imbalance is essential to understand ocean-terrestrial biogeochemical interactions. On short (human) timescales large-scale changes in land cover, erosion and eutrophication have significantly altered the fluvial flux of Si. Likewise, periods of glacial-interglacial transitions produce new weathering products from the continents with subsequent impacts on oceanic DSi. Over still longer timescales significant changes in fluvial Si fluxes have probably occurred with large-scale changes in chemical weathering due to orogeny, climate change, the emplacement of large igneous provinces, or changes in volcanic activity. We will discuss the timescale dependency of processes and the assumption of steady state. In addition, we will examine how a changing terrestrial Si cycle impacts the ocean Si cycle. Finally, we will address unresolved questions regarding the delivery of Si to the oceans from the continents.