Lava-seawater interaction effects in the carbonate system during the eruption of the La Palma volcano 2021

Abstract

The effect of the lava arriving to the sea in the carbon dioxide system was studied during the eruption of the volcano located in Cabeza de Vaca (La Palma, Canary Islands) on September 19th. The affected area was a shoreline below a cliff with heights between 80- 100m with a sandy beach in the southern part. The lava reached the coastal waters three times since the eruption began. An increase in temperature and salinity was observed due to seawater heating and water vapor formation, followed by an important decrease in pHT, related to the release of magmatic volatiles and water-rock reactions. A portion of the magmatic acidic volatiles (CO2, SO2, HCl, HF, HBr) that remains after the lava transit to the ocean is released in contact with seawater and moving out, mixed with the water vapor, but also affecting the seawater pH. Changes in the alkalinity (AT) and total dissolved inorganic carbon (CT) were also observed, producing a decrease in both variables. In September 30th, the NCT decreased by 90 μmol kg-1 while the NAT decreased in 224 μmol kg-1, reaching 2180 μmol kg-1. This result indicates the effect is not only due to strong acid addition but to the loss of carbonate species, affecting both variables, together with silicate and phosphate precipitation with metals from the lava, changing the AT. The anomalies found in the carbon dioxide and hydrography properties were localized in the frontal seawater that moved up and displaced out as a tongue of 4-6m depth with anomalous values observed as far as 1.5 km in the first meter. When the lava was not arriving to the sea or was falling on previously formed rocks, the area affected was strongly limited to the first few tens of centimeters of the column and at distances of 10-15 m.