PRECIPITATION OF MN-BEARING NODULES IN SHALLOW SOFT SEDIMENTS IN GALE CRATER, MARS

Nodule-like features with unusually high Mn- and P-rich chemistries were observed by the NASA Curiosity rover payload in Gale crater in the Glen Torridon (GT) region between sols 2829-2923 of the mission, notably at target named Groken. A working group led by A. Treiman and N. Lanza and composed of a small group of Curiosity team members was established to determine the nature and origin of these materials. Several hypotheses arose from the group’s discussions. Here we outline the hypothesis that these materials may have formed in shallow lake waters in organic-bearing soft (pre-lithified) sediments at a redox interface.

Regionally in the Groken area, the grain size is progressing from mudstone to sandstone, suggesting a transition from low-energy lacustrine deposits to a higher-energy lake margin environment. There is evidence for shallow water wave ripples in nearby stratigraphy. Nodules at Groken occupy thin horizons ~1.4 mm thick. Results from the ChemCam instrument from 85 nodules show that 71% of nodules contained both Mn and P, showing a strong relationship between Mn, P, and nodule morphology. Results from the Sample Analysis at Mars (SAM) instrument at the nearby Mary Anning 3 target point to the local presence of S-bearing organic molecules and high molecular weight (HMW) molecules. Results from the CheMin instrument suggest that the high Mn and P materials are currently amorphous.

Manganese-rich nodular features are common in terrestrial lacustrine environments. Diagenetic ferromanganese concretions that are sub-mm to a few mm in diameter can form within a distinct sediment layer at or below the sediment surface in fine-grained sediments containing organic carbon. In this scenario, Mn-oxides are first deposited in sediments where oxygen is absent below the top layer. The oxides then dissolve by reduction with organic carbon to form Mn²⁺, which diffuses upward to the oxidizing interface and re-precipitates as Mn-oxides along a thin horizon. Due to the slightly lower redox potential of Fe²⁺, Fe-oxides and peak concentrations of dissolved Fe²⁺ are found just below the Mn-oxides. Amorphous Mn and Fe oxides have been shown to sorb P if it is available in solution. A similar diagenetic pathway may be applicable to the mm-scale Mn nodules that outcrop on the sediment surface at Groken.