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After 4.5 billion years as an evolving and dynamic planet, the Earth continues to evolve but with human‐altered dynamics. Earth scientists have special opportunities and responsibilities to accelerate our understanding of Earth's changes that are transforming our most remarkable home. 

Oxidative weathering of pyrite plays an important role in the biogeochemical cycling of Fe and S in terrestrial environments. While the mechanism and occurrence of biologically accelerated pyrite oxidation under acidic conditions are well established, much less is known about microbially mediated pyrite oxidation at circumneutral pH. Recent work (Percak-Dennett et al., 2017, Geobiology, 15, 690) has demonstrated the ability of aerobic chemolithotrophic microorganisms to accelerate pyrite oxidation at circumneutral pH and proposed two mechanistic models by which this phenomenon might occur. Here, we assess the potential relevance of aerobic microbially catalyzed circumneutral pH pyrite oxidation in relation to subsurface shale weathering at Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania, USA. Specimen pyrite mixed with native shale was incubated in groundwater for 3 months at the inferred depth of in situ pyrite oxidation. The colonized materials were used as an inoculum for pyrite-oxidizing enrichment cultures. Microbial activity accelerated the release of sulfate across all conditions. 16S rRNA gene sequencing and metagenomic analysis revealed the dominance of a putative chemolithoautotrophic sulfur-oxidizing bacterium from the genus Thiobacillus in the enrichment cultures. Previously proposed models for aerobic microbial pyrite oxidation were assessed in terms of physical constraints, enrichment culture geochemistry, and metagenomic analysis. Although we conclude that subsurface pyrite oxidation at SSCHZO is largely abiotic, this work nonetheless yields new insight into the potential pathways by which aerobic microorganisms may accelerate pyrite oxidation at circumneutral pH. We propose a new “direct sulfur oxidation” pathway, whereby sulfhydryl-bearing outer membrane proteins mediate oxidation of pyrite surfaces through a persulfide intermediate, analogous to previously proposed mechanisms for direct microbial oxidation of elemental sulfur. The action of this and other direct microbial pyrite oxidation pathways have major implications for controls on pyrite weathering rates in circumneutral pH sedimentary environments where pore throat sizes permit widespread access of microorganisms to pyrite surfaces. 

Abstract. Endmember mixing analysis (EMMA) is often used by hydrogeochemiststo interpret the sources of stream solutes, but variations in streamconcentrations and discharges remain difficult to explain. We discoveredthat machine learning can be used to highlight patterns in stream chemistrythat reveal information about sources of solutes and subsurface groundwaterflowpaths. The investigation has implications, in turn, for the balance ofCO2 in the atmosphere. For example, CO2-driven weathering ofsilicate minerals removes carbon from the atmosphere over ∼106-year timescales. Weathering of another common mineral, pyrite, releases sulfuricacid that in turn causes dissolution of carbonates. In that process,however, CO2 is released instead of sequestered from the atmosphere. Thus, understanding long-term global CO2 sequestration by weatheringrequires quantification of CO2- versus H2SO4-drivenreactions. Most researchers estimate such weathering fluxes from streamchemistry, but interpreting the reactant minerals and acids dissolved in streams has been fraught with difficulty. We apply a machine-learningtechnique to EMMA in three watersheds to determine the extent of mineraldissolution by each acid, without pre-defining the endmembers. The resultsshow that the watersheds continuously or intermittently sequester CO2, but the extent of CO2 drawdown is diminished in areas heavily affectedby acid rain. Prior to applying the new algorithm, CO2 drawdown wasoverestimated. The new technique, which elucidates the importance ofdifferent subsurface flowpaths and long-timescale changes in the watersheds,should have utility as a new EMMA for investigating water resourcesworldwide.