Spatial Distribution and Preservation of Carbon Isotope Biosignatures in Freshwater Microbialite Carbonate
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Modern microbialites provide the opportunity to explore the influences of biology on microbialite formation and understand how biosignatures can be preserved in these structures. In this study, we compared δ13Ccarb values from nodule and surface biofilm carbonates on microbialite structures across depths and locations throughout Pavilion Lake to evaluate whether variable light exposure produced limitations to biosignature formation. At depths below 21 m, vertical profiling of δ13Ccarb across colour transitions of surface biofilm on microbialite structures was performed to identify spatial arrangement of autotrophic and heterotrophic biosignatures. Finally, preservation of the photosynthetic biosignature over time was investigated by collecting carbonates beneath the microbialite surface. These investigations were performed in order to better characterize the factors controlling biosignature formation, distribution, and preservation within Pavilion Lake.Decreasing trends of δ13Ccarb with depth across study sites indicated that attenuated sunlight in the water column is likely the primary control on biosignature formation. Below 21 m, photosynthetic enrichments representing biosignatures on microbialite surfaces were reduced and recorded δ13Ccarb values that fell within the predicted equilibrium range. Biosignature loss is suggested to result from the relative proportions of autotrophic and heterotrophic processes changing at depths and producing average δ13Ccarb values. Variability of where biosignatures are lost on the microbialite surface indicated that the spatial extent of photosynthetic communities producing enrichments is potentially influenced by variable incidences of light at these depths. Although no definitive biosignatures of heterotrophy were identified, several interfaces were identified where the balancing proportions of autotrophic and heterotrophic processes influenced by light variability potentially mediate biosignature loss. Decreasing trends of δ13Ccarb beneath the microbialite surface and estimates of past microbialite growth rates indicated that surface biosignatures are lost within 100 – 400 years. It is suggested that infilling processes overprint enrichments and deplete δ13Ccarb values due to heterotrophic abundance below the microbialite surface. This is supported by an isotopic mass balance that predicts smaller inputs of heterotrophically-depleted DIC are required to sufficiently overprint δ13Ccarb enrichments. These results concluded that the photosynthetic biosignature identified in Pavilion Lake is short-lived and mitigated by biological processes.