Understanding the climate history of Mars, one of our closest planetary neighbours, has important implications for understanding the environmental evolution of Earth and other rocky planets in general. The widespread recognition of an extensive sedimentary record modified, at least in part, by liquid water holds much promise to provide evidence about Mars' past. However, unravelling the pressure and compositional history of Mars' Noachian atmosphere has remained problematic. Based on expected outgassing behaviour, observed atmospheric isotope ratios, and climate models, carbon dioxide is widely considered a main constituent of the early Martian atmosphere. If this was indeed the case, it is surprising that carbonate minerals, the expected sinks for this carbon, are only abundant in a few isolated localities across the Martian surface. Three broad possibilities may account for this apparent inconsistency: (1) carbonates formed under pCO₂ higher than present but were buried, or subsequently destroyed; (2) Noachian atmospheric pCO₂ was significantly lower than current estimates; or (3) low-temperature carbonate formation was kinetically controlled under the aqueous conditions that characterised much of the early Martian surface. While orbital spectroscopic investigations have yielded an increasing inventory of isolated carbonate-bearing deposits, their relative rarity at the near-surface has prompted suggestions that the Noachian atmosphere was commonly characterised by low pCO₂. Without a more complete investigation of the carbonate-forming processes occurring within the ancient crust, no hypothesis can be tested robustly. Here, we examine the controls on Fe(II)-carbonate precipitation in low-temperature, anoxic water-rock systems as a function of pCO₂. In experiments lasting up to 85 days, no measurable Fe(II)-carbonate precipitation occurred within the limits of analytical detection, despite significant and sustained supersaturation with respect to siderite. These observations are quantitatively consistent with recent investigations highlighting a significant supersaturation threshold for Fe(II)-carbonate nucleation. Reaction path models that incorporate these constraints indicate that Fe(II)-carbonate supersaturation thresholds would have been commonly met in low water:rock ratio systems isolated from the Noachian atmosphere as opposed to high water:rock ratio, open-systems exposed at the near surface. These results suggest the rarity of carbonates exposed at or near the surface of Mars may be controlled, at least in part, by a lack of deep crustal exposures and lack of the geochemical conditions conducive to carbonate formation, rather than insufficient atmospheric pCO₂. Such geochemical constraints are in turn consistent with available orbital data indicating that carbonates may be more abundant within the deep crust, potentially representing a significant subsurface carbon sink.

理解火星的气候历史是我们最接近的行星邻居之一，对理解地球和其他岩石行星的环境演变具有重要意义。人们广泛认可至少部分地被液态水改变的大量沉积记录，这为提供有关火星过去的证据提供了广阔的前景。但是，如何解开火星诺亚纪大气的压力和组成历史仍然存在问题。根据预期的放气行为，观测到的大气同位素比和气候模式，二氧化碳被广泛认为是火星早期大气的主要成分。如果确实如此，那么令人惊讶的是，碳酸盐矿物（这种碳的预期汇）仅在整个火星表面的少数几个偏远地区丰富。p CO ₂高于目前水平，但被掩埋或随后销毁；（2）诺亚大气p CO ₂明显低于当前估计；或（3）在早期火星表面大部分特征的含水条件下，动力学控制了低温碳酸盐的形成。尽管轨道光谱研究已经增加了孤立的含碳酸盐矿床的存量，但它们在近地表的相对稀有性提示人们，诺亚大气通常具有低p CO _{2的特征。}。如果不对古地壳内发生的碳酸盐形成过程进行更全面的调查，就无法可靠地检验假设。在这里，我们研究了低温缺氧水岩系统中Fe（II）-碳酸盐沉淀的控制与p CO _{2的关系。}。在长达85天的实验中，尽管菱铁矿存在明显且持续的过饱和状态，但在分析检测范围内未发生可测量的Fe（II）-碳酸盐沉淀。这些观察结果与最近的研究在定量上是一致的，后者突出了Fe（II）-碳酸盐成核的显着过饱和阈值。包含这些约束的反应路径模型表明，Fe（II）-碳酸根过饱和阈值通常会在与Noachian大气隔离的低水：岩石比率系统中得到满足，而不是高水：岩石比率，在近距离暴露的开放系统表面。这些结果表明，至少可以部分控制火星表面或其附近暴露的碳酸盐的稀有性，p CO ₂。这样的地球化学约束反过来与可用的轨道数据一致，表明深部地壳内的碳酸盐可能更为丰富，可能代表了一个重要的地下碳汇。