Surveillance of Ruapehu Crater Lake, New Zealand, by aqueous polythionates

Abstract

Aqueous sulfur speciation of samples of crater lake water from Crater Lake collected at Mt. Ruapehu, New Zealand since 1968 has been determined using high-performance liquid chromatography. Polythionates (S x O 6 2− ) are produced in the lake through reactions between sulfur dioxide and hydrogen sulfide gases from the subaqueous fumaroles. Their concentrations vary from 0 to a few hundred mg/l, corresponding to the activity of the volcano. The concentration of the total polythionates ( Σ S x O 6 2− = S 4 O 6 2− + S 5 O 6 2− + S 6 O 6 2− ) is found to be a good indicator of changes in the subaqueous fumarolic activity at Crater Lake. The molar S x O 6 2− /Cl − ratios of the lake water can be used to divide the volcanic activity of Crater Lake into four stages: 1. Stage I: Period of weak volcanic activity with no hydrothermal explosions. This stage is characterized by low total S x O 6 2− with a distribution order of S 5 O 6 2− > S 4 O 6 2− > S 6 O 6 2− . H 2 S is predominant among dissolved gases in the lake water. 2. Stage IIa: Quiescent period with minor hydrothermal explosions. The lake water contains high ΣS x O 6 2− with a distribution order of S 5 O 6 2− > S 4 O 6 2− > S 6 O 6 2− . Dissolved H 2 S and SO 2 gases are very low. 3. Stage IIb: Increased fumarolic activity with frequent hydrothermal explosions. Low ΣS x O 6 2− with a distribution order of S 4 O 6 2− > S 5 O 6 2− > S 6 O 6 2− . Sulfur dioxide but no H 2 S is detected in the water. 4. Stage III: Period of phreatomagmatic eruptions with lahars and intensive seismic activity. No S x O 6 2− exists but dissolved SO 2 is high in the lake water. The moderate-sized explosion of December 8, 1988, which was not preceded by any changes in lake water polythionate chemistry or seismic activity, is hypothesized to be a vent-cleaning eruption caused by blockage of ascending magmatic gases that was caused by solidification of a molten sulfur pool at the bottom of Crater Lake.