Siderite-Rich Laminae at the Marker Band Contact in Gale Crater
Résumé
The apparent near absence of carbonate minerals in Noachian and Hesperian strata [1,2], despite predictions on their abundance [3], has cast doubt on the mechanisms proposed for their formation and the nature of early Mars environments [4]. Carbonate formation can sequester CO2 from volcanic outgassing, a key driver in early climate evolution and habitability [5,6]. Understanding the role and fate of carbonates and its impact on early climate regimes requires both orbital mapping and in situ geological investigations. Mg-rich carbonates have been identified in a few locations in situ at Gusev and Jezero craters [7,8] as isolated replacements by latestage diagenetic fluids, but more recently the Curiosity rover showed the extensive presence of siderite (FeCO3) within strata of the hundreds-meters thick sulfate-bearing unit [9]. Whereas chemical sedimentation is hypothesized as the mode of formation and deposition, a clear sedimentological context is so far lacking for the upper sulfate-bearing unit and the deposition of siderite [10]. In addition, its abundance (510 wt.%) makes it a significant though not a dominant component of the sediments. Here we report on a previously unidentified unit of the Amapari Marker Band (AMB) [11,12] that is composed mostly of siderite. This siderite-rich bed occurs at the base of the main wave-rippled AMB interval which itself has a comparatively lower Fe-content [13]. Examining this type of carbonate deposit within a regional sedimentary context (Fig. 1) provides new insights on the mechanisms for carbonate formation on early Mars.
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