The chemistry of Crater Lake sediments: Definition of sources and implications for hydrothermal activity

Dymond, J; Collier, R

"Crater Lake sediments exhibit variations in composition that can largely be explained by mixtures of three distinct sources: aluminosilicate debris from the caldera walls, biogenic materials which settle from the euphoric zone, and an iron-rich precipitate. The origin of the precipitate component is uncertain. Redox mobilization of iron and other metals driven by decomposition of organic matter is unlikely, because of the spatial variability of metal enrichments are incompatible with this mechanism. Precipitation of iron from anoxic springs is a feasible explanation. Since the metal deposits have only been observed in a region of the lake which has thermal and chemical anomalies in the water column, the precipitation may be from thermal springs rather than cold springs. Precipitation from hydrothermal is the favored hypothesis because recent submersible observations have demonstrated that iron-rich precipitates are currently forming in the lake in association with warmer waters, anomalous concentrations of tracers of hydrothermal venting, and bacterial communities. Comparison between the composition of surficial sediments and materials collected with sediments and materials collected with sediment traps moored in the mid-water column indicates that most of the recycling of biogenic debris occurs at the sediment-water interface. Less than10 % of the organic carbon and nitrogen that reaches the bottom is preserved in the sediments. Approximately 20 % of biogenic opal is preserved. The sediment trap data also reveal that resuspension of sediments and downslope transport of material are important mechanisms for introducing sediments to the deep basin of the lake. Downcore analysis of the lake sediments indicate the presence of manganese reduction and mobilization. Manganese mobilization is most pronounced in sediments from the portion of the lake that exhibits surficial sediment enrichment of iron and water column anomalies. We propose that the strong manganese mobilization, iron precipitates, and observed iron-rich deposits on the lake bottom are a consequence of slow and dispersed advection of thermal fluids through the lake sediments and rock outcrops. Downcore variations in opal content may indicate variations in biological productivity in the lake in the past. These variations are positively correlated with variations in the abundance of the hydrothermally-associated element, lithium; this observation suggests that hydrothermal inputs may contribute to the biological productivity of the lake."

The chemistry of Crater Lake sediments: Definition of sources and implications for hydrothermal activity

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