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Mars Reconnaissance Orbiter HiRISE images and Opportunity rover observations of the ~22 km wide Noachian age Endeavour Crater on Mars show that the rim and surrounding terrains were densely fractured during the impact crater-forming event. Fractures have also propagated upward into the overlying Burns formation sandstones. Opportunity's observations show that the western crater rim segment, called Murray Ridge, is composed of impact breccias with basaltic compositions, as well as occasional fracture-filling calcium sulfate veins. Cook Haven, a gentle depression on Murray Ridge, and the site where Opportunity spent its sixth winter, exposes highly fractured, recessive outcrops that have relatively high concentrations of S and Cl, consistent with modest aqueous alteration. Opportunity's rover wheels serendipitously excavated and overturned several small rocks from a Cook Haven fracture zone. Extensive measurement campaigns were conducted on two of them: Pinnacle Island and Stuart Island. These rocks have the highest concentrations of Mn and S measured to date by Opportunity and occur as a relatively bright sulfate-rich coating on basaltic rock, capped by a thin deposit of one or more dark Mn oxide phases intermixed with sulfate minerals. We infer from these unique Pinnacle Island and Stuart Island rock measurements that subsurface precipitation of sulfatedominated coatings was followed by an interval of partial dissolution and reaction with one or more strong oxidants (e.g., O 2 ) to produce the Mn oxide mineral(s) intermixed with sulfate-rich salt coatings. In contrast to arid regions on Earth, where Mn oxides are widely incorporated into coatings on surface rocks, our results demonstrate that on Mars the most likely place to deposit and preserve Mn oxides was in fracture zones where migrating fluids intersected surface oxidants, forming precipitates shielded from subsequent physical erosion.

41 Naturally occurring Mn(IV/III) oxides are often formed through microbial Mn(II) 42 oxidation, resulting in reactive phyllomanganates with varying Mn(IV), Mn(III), and vacancy 43 contents. Residual aqueous Mn(II) may adsorb in the interlayer of phyllomanganates above 44 vacancies in their octahedral sheets. The potential for interlayer Mn(II)-layer Mn(IV) 45 comproportionation reactions and subsequent formation of structural Mn(III) suggests that 46 aqueous Mn(II) may cause phyllomanganate structural changes that alters mineral reactivity or 47 trace metal scavenging. Here we examine the effects of aging phyllomanganates with varying 48 initial vacancy and Mn(III) content in the presence and absence of dissolved Mn(II) at pH 4 and 49 7. Three phyllomanganates were studied: two exhibiting turbostratic layer stacking (δ-MnO 2 with 50 high vacancy content and hexagonal birnessite with both vacancies and Mn(III) substitutions) 51 and one with rotationally ordered layer stacking (triclinic birnessite containing predominantly 52 Mn(III) substitutions). Structural analyses suggest that during aging at pH 4, Mn(II) adsorbs 53 above vacancies and promotes the formation of phyllomanganates with rotationally ordered 54 sheets and mixed symmetries arranged into supercells, while structural Mn(III) undergoes 55 disproportionation. These structural changes at pH 4 correlate with reduced Mn(II) uptake onto 56 triclinic and hexagonal birnessite after 25 days relative to 48 hours of reaction, indicating that 57 phyllomanganate reactivity decreases upon aging with Mn(II), or that recrystallization processes 58 involving Mn(II) uptake occur over 25 days. At pH 7, Mn(II) adsorbs and causes limited 59 structural effects, primarily increasing sheet stacking in δ-MnO 2 . These results show that 60 aging-induced structural changes in phyllomanganates are affected by aqueous Mn(II)

Selenium (Se) redox chemistry is a determining factor for its environmental toxicity and mobility. Currently, millions of people are impacted by Se deficiency or toxicity, and in geologic history, several mass extinctions have been linked to extreme Se deficiency. Importantly, microbial activity and interactions with other biogeochemically active elements can drastically alter Se oxidation state and form, impacting its bioavailability. Here, we use wet geochemistry, spectroscopy, and electron microscopy to identify a cryptic, or hidden, Se cycle involving the reoxidation of biogenic volatile Se compounds in the presence of biogenic manganese [Mn­(III, IV)] oxides and oxyhydroxides (hereafter referred to as “Mn oxides”). Using two common environmental Ascomycete fungi, Paraconiothyrium sporulosum and Stagonospora sp., we observed that aerobic Se­(IV and VI) bioreduction to Se(0) and Se­(-II) occurs simultaneously alongside the opposite redox biomineralization process of mycogenic Mn­(II) oxidation to Mn oxides. Selenium bioreduction produced stable Se(0) nanoparticles and organoselenium compounds. However, mycogenic Mn oxides rapidly oxidized volatile Se products, recycling these compounds back to soluble forms. Given their abundance in natural systems, biogenic Mn oxides likely play an important role mediating Se biogeochemistry. Elucidating this cryptic Se cycle is essential for understanding and predicting Se behavior in diverse environmental systems.

Layered Mn oxide minerals (phyllomanganates) often control trace metal fate in natural systems. The strong uptake of metals such as Ni and Zn by phyllomanganates results from adsorption on or incorporation into vacancy sites. Mn(II) also binds to vacancies and subsequent comproportionation with structural Mn(IV) may alter sheet structures by forming larger and distorted Mn(III)O octahedra. Such Mn(II)-phyllomanganate reactions may thus alter metal uptake by blocking key reactive sites. Here we investigate the effect of Mn(II) on Ni and Zn binding to phyllomanganates of varying initial vacancy content (δ-MnO, hexagonal birnessite, and triclinic birnessite) at pH 4 and 7 under anaerobic conditions. Dissolved Mn(II) decreases macroscopic Ni and Zn uptake at pH 4 but not pH 7. Extended X-ray absorption fine structure spectroscopy demonstrates that decreased uptake at pH 4 corresponds with altered Ni and Zn adsorption mechanisms. These metals transition from binding in the interlayer to sheet edges, with Zn increasing its tetrahedrally coordinated fraction. These effects on metal uptake and binding correlate with Mn(II)-induced structural changes, which are more substantial at pH 4 than 7. Through these structural effects and the pH-dependence of Mn(II)-metal competitive adsorption, system pH largely controls metal binding to phyllomanganates in the presence of dissolved Mn(II).