Understanding the Impact of a Modern Euxinic Spring on Sediment Using Bulk Sediment and Smectite Clay-Preserved Proxies

Modern euxinic environments are useful proxies for early Earth. We investigated the morphological and chemical properties preserved in sediments impacted by a modern euxinic spring, an early Earth proxy, as compared to adjacent unimpacted sediments. We used X-ray diffraction (XRD) to analyze the mineralogy of bulk sediment and isolated clay fractions. We further examined the morphology and chemistry of smectite clay nanoparticles using scanning/transmission electron microscopy (S/TEM) and energy-dispersive X-ray spectroscopy (EDX). XRD results show that pyrite and barite are present in all of the impacted sediments but absent in the unimpacted sediments. Although grain size analysis shows the impacted samples have more clay-sized particles, the mineralogy of impacted and unimpacted clay fractions is composed of smectite, chlorite, kaolinite, and illite clays. We further focused on smectite clay nanoparticles, as they are known to be sensitive proxies for redox conditions. Smectites with a platy and cornflake texture were prevalent in all samples. However, EDX results reveal differences between the smectites in the impacted and unimpacted samples: impacted smectites additionally incorporate S, Ba, and Sr. Furthermore, STEM–EDX shows evidence of nanoscale barite and pyrite precipitation on the impacted smectites. Ultimately, this study demonstrates that (1) XRD-detectable sulfur bearing minerals in a soil profile exposed to euxinic groundwater are present outside the zone with visible sediment iron reduction and (2) Al-smectite nanoparticles primarily respond to euxinic conditions by immobilizing cations via adsorption or interlayer cation exchange rather than significantly changing their octahedral sheet cation composition.