Convection and stratification in Ruapehu Crater Lake, New Zealand: Implications for Lake Nyos-type gas release eruptions

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The first of a series of water-borne surveys of bathymetry, water column temperature and water column chemistry of Ruapehu Crater Lake was undertaken in February of 1991 to ascertain the behaviour of the lake during quiescent periods. Results show the present maximum depth of the lake to be 134 m, and the central vent to be overlain by a large pool of molten sulfur (>50 m in diameter and ≥6 m deep). Maximum recorded temperature in the sulfur was 177°C. The lake showed little stratification with respect to volatile constituents, indicating that the vent was in effect totally sealed with respect to the transfer of magmatic gases during the 3 month quiescent period prior to sampling. Minor thermal inversions characterised the upper 60 m of the water column in several locations, whereas water column chemical analyses showed this interval to be largely isochemical. A small but abrupt increase in TDS below 60 m depth and a thermal gradient of 0.2°C/m between 60 and 130 m suggest that the lake was thermally convecting over this interval. This convection is thought to be driven by thermal inputs from the underlying sulfur, including conductive heat transfer and discharge of recirculating lake water from the upper portion of the vent. The results suggest that a gas buildup in the water column is unlikely to form in the present state of the vent-lake system, and therefore that Lake Nyos-type gas release eruptions are unlikely to occur.