The contribution of sandy beaches to the oceanic silica cycle

Wave action at the coast significantly increases the dissolution rate of quartz grains on sandy beaches. By combining laboratory experiments and satellite observations, the global flow of silica to the ocean from beaches is estimated at 300 million tonnes of dissolved silicon per year, equivalent to that from rivers. This new and substantial contribution challenges the commonly accepted hypothesis of an oceanic silica cycle in equilibrium.

Dissolved silicon is an essential nutrient for the development of phytoplankton. Its abundance controls the biological pump, the mechanism by which the ocean captures and sequesters atmospheric CO2. Until now, the main sources of dissolved silicon in the ocean have been rivers, groundwater discharges and hydrothermal springs. This new study confirms and pushes forward previous findings suggesting the potential contribution of a silicate-rich medium: sandy beaches.

Resulting from an interdisciplinary collaboration between three laboratories in Toulouse (LEGOS, IRAP & IMFT), this study measured the impact of wave action on the dissolution kinetics of the quartz grains that make up beaches in a laboratory swell channel. These experiments showed that the intense agitation caused by waves favours quartz dissolution kinetics, considerably increasing the concentration of dissolved silicon in the water compared with an inert medium. Once the increased dissolution mechanism has been demonstrated in the laboratory, a dissolution model can be used to estimate a silica flux proportional to (1) water temperature and (2) wave energy.

Using satellite data (global mapping of sandy beaches, ocean surface temperature and wave power at the coast), it is possible to extrapolate the results of the experiments on a global scale. Thus, a global flow of silica to the ocean has been estimated at around 8.3 Tmol of silicon (or 300 million tonnes) per year, a quantity comparable to that contributed by rivers, hitherto considered the main source of silica to the ocean.

This discovery calls into question the commonly accepted idea of an oceanic silicon cycle in equilibrium. Indeed, if a new source of a chemical element to the ocean is identified, it requires an additional sink to maintain this equilibrium, unless we admit that the silicon cycle is not at steady state. The only “output” of chemical elements from the ocean is marine sediments, fed by the snow of marine particles, the debris of surface biological activity. These additional silicon inputs are likely to impact marine ecosystem dynamics and sediment carbon sequestration, reinforcing the hypothesis that biological pump activity remains underestimated.

Further Resource

• Scientific article : Aparicio, M., Le Bihan, A., Jeandel, C. et al. Contribution of sandy beaches to the global marine silicon cycle. Nat. Geosci. 18, 154–159 (2025). doi: 10.1038/s41561-024-01628-6

IRAP Contact

• Sébastien Fabre, sebastien.fabre@irap.omp.eu