DIAGENETIC MODIFICATIONS OF FILAMENTOUS MICROFOSSILS AND ASSOCIATED ORGANIC MATTER IN SILICEOUS HOT SPRING DEPOSITS FROM PUCHULDIZA, CHILE

On Earth, hot springs host diverse microbial communities. The precipitation of silica in these environments enables the capture and preservation of these communities over geologic timescales. Therefore, hot spring deposits (sinter) are important analogs for studying the presence of life on early Earth and other planets. While rapid mineralization is essential for microfossil preservation, mineralization in hot springs begins with the precipitation of metastable silica that, over time, undergoes diagenetic transformations to more stable, ordered phase(s). In sinters, diagenesis involves systematic, structural transformation of amorphous opal-A, to opal-CT, to opal-C, to quartz. Because of this, sinters provide a natural laboratory to study the effects of diagenesis on the fossilized communities. We hypothesize that there is systematic degradation of microfossils and loss of organic carbon during crystallization of opal-A through quartz. 30 samples were collected from Puchuldiza hot springs in Chile. The mineralogy was determined by XRD. Using thin-section petrography and bulk organic carbon analysis we characterized the morphology and organic content at each silica phase. Preliminary thin-section analysis showed that microfossil abundance and morphology changed with increased crystallinity. The most notable changes are 1) loss of microfossil yellow pigment at the opal-A to opal-A/CT transition, 2) decrease in the number of filaments at opal-CT and opal-C, and 3) an increase in filament diameter and distortion of the filament surface at opal-C and quartz. C_org ranged from 0.0007±0.023 to 0.8373±0.046 wt%. The highest C_org was measured in opal-A samples with yellow-pigmented fossils. But, the C_org content did not systematically decrease for the other phases as expected. This suggest that the taphonomic and diagenetic processes in these systems may influence the morphological characteristics of microfossils differently than the organic material. Further research will be conducted to understand the relationships between the organic matter and microfossils by creating elemental maps using an electron microprobe. Documenting the morphological and chemical changes that occur to microfossils after deposition is necessary to better interpret the fossil record on Earth and potentially, Mars.