Incipient weathering by Stereocaulon vulcani at Réunion volcanic island

The impact of early land plants and fungi in increasing global weathering is still debated, particularly before the advent of vascular plants during the Devonian. Here we present a study of the incipient weathering of basalt by Stereocaulon vulcani, a native colonizing lichen on Réunion Island (Indian Ocean). We analyzed the chemistry and mineralogy of a 24-year-old flow located at low altitude that was mostly covered by S. vulcani with aboveground biomass of 6249kgha−1. The chemical composition of S. vulcani showed that besides C and N, Si and Fe were the dominant elements. The Si stored in the aboveground pioneer vegetation gives 27kgha−1, comparable to the Si stored in the old-growth at the Marelongue Reserve (Meunier et al., 2010). On thin sections, the inner part of S. vulcani was mostly composed of Si while Fe coatings observed at the base of the thallus may be the result of wind blow dust interception as suggested by Cochran and Berner (1992). Using BSE images on SEM, we showed evidence of dissolution of the glass matrix at the basalt–thallus contact. The quantification of porosity by digital imagery showed a variation between 7% in the unweathered zone to near to 40% at the surface. A maximum denudation rate of 6.7μmyear−1 is estimated to fall within the range of the values reported in the literature. Using our data for analog to the past, we suggest that early land plants would have been capable to mobilize Si in a similar proportion as vascular plants and should have significantly affected the weathering of land before the advent of vascular plants. •Major elements from Stereocaulon vulcani and rocks were analyzed on a 24-year-old lava flow at Réunion Island.•Si stored in Stereocaulon vulcani is comparable to Si stored in the mature forest above a 500-year-old lava flow.•In basalt at the contact with Stereocaulon vulcani, the glass matrix has been preferentially weathered.•We estimate a significant denudation rate of about nearly 7μmyear−1.•Early land plants may have contributed significantly to weathering before the advent of vascular plants.