We investigated the equatorial layered deposits (ELDs) of Arabia Terra, Mars, inFirsoff crater and on the adjacent plateau. We produced a detailed geological map that included a survey of the relative stratigraphic relations and crater count dating. We reconstructed the geometry of the layered deposits and inferred some compositional constraints. ELDs drape and onlap the plateau materials of late Noachian age, while they are unconformably covered by early and middle Amazonian units. ELDs show the presence of polyhydrated sulfates. The bulge morphology of the Firsoff crater ELDs appears to be largely depositional. The ELDs on the plateau display a sheet-drape geometry. ELDs show different characteristics between the crater and the plateau occurrences. In the crater they consist of mounds made of breccia sometimes displaying an apical pit laterally grading into a light-toned layered unit disrupted in a meter-scale polygonal pattern. These units are commonly associated with fi ssure ridges suggestive of subsurface sources. We interpret the ELDs inside the craters as spring deposits, originated by fl uid upwelling through the pathways likely provided by the fractures related to the crater formations, and debouching at the surface through the fi ssure ridges and the mounds, leading to evaporite precipitation. On the plateau, ELDs consist of rare mounds, fl atlying deposits, and cross-bedded dune fi elds. We interpret these mounds as possible smaller spring deposits, the fl at-lying deposits as playa deposits, and the cross-bedded dune fi elds as aeolian deposits. Groundwater fl uctuations appear to be the major factor controlling ELD deposition. Wind-related erosionYardangs 100 m B HiRISE PSP_004355_1830_RED Res. 0.25 m/pixel Main wind direction 100 m HiRISE ESP_016776_1810_RED Res. 0.5 m/pixel C 500 m CTX P06_003221_1815_XN_01N009W Res: 5.38 m/pixel Main wind direction Elevated lineament D Rim A B C D Figure 8. Erosional versus depositional morphologies. (A) Example of aeolian erosion with yardangs on equatorial layered deposits (ELDs). (B) ELD irregularly shaped layers. ELDs show rounded edges and lengthen in a direction and with geometries not consistent with formation by aeolian erosion. Main wind direction as inferred by yardang alignment is indicated at the top left. (C) ELD irregularly shaped layers. Some of the layers are characterized by the presence of rims. (D) Northwest-southeast-trending depression. Its regularity and trend do not seem consistent with an erosional formation by wind action. Main wind direction as inferred by yardang alignment is shown at the bottom left. Inside the depression, elevated lineaments (fi ssure ridges?), emphasized by mounds, are present. The location of the images is indicated in the HRSC mosaic at the top right of the fi gure.as

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Highlights  In situ calcification (aragonite) of a 3.3 Ga microbial biofilm  Calcification probably due to the activities of a photosynthetic consortium of microorganisms within the biofilm including heterotrophic sulphur reducing bacteria  Preservation of the metastable carbonate phase by the organic matrix and secondary silicification  Oldest known occurrence of aragonite.