Amapari Marker Band, Gale Crater, Mars: Event Horizon With Highest Bedrock Iron and Zinc Concentrations Detected by Curiosity's Alpha Particle X‐Ray Spectrometer.

Alpha Particle X‐ray spectrometer (APXS) analyses of the distinct Amapari Marker Band (AMB), Gale crater, Mars reveal the highest, in situ, FeO and Zn abundances (47.51, 2.23 wt%), and elevated MnO associated with a lower rippled unit. APXS analyses also reveal a marked shift in provenance, to a generally basaltic composition, compared to the underlying Mg‐sulfate‐bearing strata, which persists into the overlying stratigraphy. The AMB also records perturbation in the MgSO4‐forming conditions present above and below. AMB chemistry could be consistent with a volcanic ash source; high metal concentrations resulting from volatile reactions within an ash cloud. Alternatively, syn‐ and/or post‐depositional precipitation processes within a primary lake setting and/or a later diagenetic event or events may have played a role. Ongoing and future work will aim to further constrain processes responsible for deposition of the AMB, the high metal concentrations and its regional and global implications. Plain Language Summary: The Amapari Marker Band (AMB) at Gale crater forms a distinct, dark‐toned, resistant horizon identified from orbit within rock layers of the Mg‐sulfate‐bearing, central mound. Curiosity recently investigated the AMB and found a lower rippled layer, consistent with a shallow lake, contrasting with windblown sediment deposition above and below it. Analysis of the AMB by the Alpha Particle X‐ray spectrometer also revealed a marked change in the chemistry of the rocks, with the highest in situ FeO and Zn abundances measured on Mars, elevated MnO, and a composition consistent with input of different sediment compared to underlying rocks. The change in bulk chemistry persists into the overlying rocks indicating that the AMB marks a significant event in the evolution of Gale crater, and possibly beyond. The AMB may record deposition of basaltic volcanic ash into a lake; the high metal concentrations resulting from gas reactions within an ash cloud. Alternatively, the high metals may be the result of water/sediment interactions: either in the lake, and/or after deposition and possibly after becoming a solid rock. Ongoing and future work will aim to further constrain processes responsible for deposition of the AMB, the high metal concentrations and its regional and global implications. Key Points: APXS analysis reveals a marked change in chemistry and provenance associated with the Amapari Marker Band, Gale craterHighest in situ iron and zinc, and elevated manganese detected by APXS within the Amapari Marker BandThe Amapari Marker Bands marks a significant event in the evolution of Gale crater, and possibly beyond [ABSTRACT FROM AUTHOR]