Contribution of sandy beaches to the global marine silicon cycle

Abstract

Dissolved silicon levels in the ocean, which can shape marine carbon cycling owing to silicon’s role as a nutrient, are largely controlled by influxes from land. While riverine and groundwater silicon fluxes are relatively well understood, this is not the case for inputs stemming from the intense physical mixing of beaches made up of silicon-rich minerals. Here we investigate how energy dissipation due to breaking waves influences quartz dissolution rates in an experimental setup simulating a sandy beach made of pure α-quartz. The concentrations of dissolved silicon obtained show a substantial increase in the dissolution rate due to wave action, supporting related previous findings. The observed laboratory physico-chemical mechanism is upscaled to the worldwide sandy coastlines using global reanalysis. Overall, controlling for differences in wave power and sea surface temperature, this suggests that beaches contribute 8.4 ± 3.0 Tmol of dissolved silicon to the ocean each year, which is similar to the flux coming from rivers. This suggests, on the basis of a statistical analysis, that the global abiotic silicon cycle may not be in steady state as had previously been assumed and that sandy beaches must be considered when developing silicon budgets for the global ocean. Waves breaking on sandy beaches globally contribute a similar amount of dissolved silicon to oceans as that from rivers, according to a global analysis informed by experiments performed on a simulated quartz sand beach.