The erosion and suspended matter/seawater interaction during and after the 1996 outburst flood from the Vatnajökull Glacier, Iceland

The Gjálp subglacial eruption 1996 within the Vatnajökull Glacier, Iceland triggered a catastrophic outburst flood, bringing at least 180 million tonnes of suspended solids to the sea in only 42 h. This amounts to 1% of the total annual global river suspended flux to the oceans. The specific BET-surface area of the suspended solids was measured to be 11.8-18.9 m2/g, translating to the average total BET-surface area of 2.8 × 10 9 km2, providing enormous potential for adsorption/desorption and precipitation/dissolution fluxes at the suspended solids-ocean water interface. Altered basalt glass was the major constituent of the suspended matter (80%), secondary minerals such as zeolites and calcite amounted to 11%, but only 5% was fresh volcanic glass. The suspended grains were generally rounded. The glass carried by the flood is different in chemical composition from the glass produced by the Gjálp eruption. The Gjálp material has higher FeOtotal / TiO2 and TiO2 / P2O5 ratios than the suspended glass in the flood waters. The majority of the flood samples match the composition of the volcanic system, down stream from the eruption site. The large amount of altered material in the flood and its chemical composition suggests erosion conforming to a 2 m deep, 1000 m wide and 50 000 m long channel in less than 42 h. The behaviour of 28 elements on the surface of the suspended solids exposed to seawater was quantified by experiments in the laboratory. The altered basaltic glass dissolved in seawater, as recorded by the Si release from the glass. The dissolved concentrations of Na, Ca, Si, Ba, Cd, Co, Cu, Hg, Mn, Ni, and total dissolved inorganic N increased considerably when the suspended solids come into contact with the seawater, but the concentrations of Mg, K, S, Sr, Fe, Pb and Zn decreased. The experimental seawater solutions were supersaturated with respect to calcite, Mg-montmorillonite and amorphous iron-hydroxide. The rate of release (mol/m2/s) of Si, Mn, Ba, Co, Ni and Cd decreased continuously during the one week exposure to sweater. After one week, the logarithm of the dissolution rate of the altered basaltic glass was − 11.9 to − 11.6 (Si mole/m2/sec). Significantly lower than the steady-state rates for fresh basaltic glass at similar conditions. Calculated one day desorbed/dissolution suspended material fluxes are greater than the integrated dissolved flood fluxes for Mn, Ba, Ni, Co and Cd, but the Si dissolved food flux was greater than the one day desorbed/dissolved suspended material flux.