Publication Date

April 2019


Johan Varekamp


Earth & Environmental Sciences


English (United States)


The Monticchio crater lakes, Lago Grande and Lago Piccolo, in southern Italy formed ~140,000 years ago during eruptions that produced carbonatitic and melilitic ashes, which form part of the sediment bed in the lakes. Both lakes have carbonate rich waters and are thermally stratified most of the year. The lakes occasionally turn over, which created Fe flocculates in Lago Piccolo at the surface. Chemical cross sections of the water column indeed indicate an Fe-rich bottom water layer in Lago Piccolo (up to 120 ppm Fe) which is also enriched in Silica and the major cations. The bottom layer is also warmer, but because of the high dissolved contents still denser than overlying waters. Lago Piccolo has both thermal fluids and CO2-rich gas bubbles entering at the lake bottom, leading to mild Hg enrichments in the sediment (~ 100 ppb Hg). Lago Grande seems to have only a CO2-rich gas input, leading to slightly higher Hg in the sediment (up to 1000 ppb Hg). Both lakes degass CO2 at the surface, creating high d13C(DIC) values in the surface waters. The bottom waters show even higher d13C(DIC), presumably the result of methane generation in these anoxic waters. The Piccolo sediments are enriched in Fe, V, As and Ba, presumably brought in by the hydrothermal fluids. Hg accumulation rates in Piccolo are on the order 12 ng Hg/cm2 year, creating a total accumulated mass of Hg in the sediment of ~ 5 tonnes. The Lago Grande sediment host presumably at least 10 tonnes Hg. These calculations are minimum estimates because it is likely that older sediments are more enriched in Hg (closer in time to the volcanic activity) than the young sediment.



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