Fe(II)-carbonate precipitation kinetics and the chemistry of anoxic ferruginous seawater

The chemical evolution of anoxic seawater is chronicled in part by the abundance of sedimentary Fe minerals. Our view of the chemical state of the oceans throughout much of Earth's history, therefore, hinges on understanding the behaviour of Fe2+ in seawater and sedimentary pore water. However, the formation pathways and precipitation rates of a number of sedimentary Fe(II)-minerals are poorly understood. For example, because siderite is widely distributed in Precambrian rocks, it is frequently assumed that seawater Fe2+ concentrations would have been poised at siderite equilibrium. Yet a variety of studies indicate that siderite precipitation would have been strongly controlled by kinetic processes rather than equilibrium behaviour, prompting a reconsideration of the Precambrian Fe cycle and those element cycles to which it was connected. To address this issue, we experimentally investigated spontaneous (or homogeneous) precipitation of Fe(II)-carbonate across a range of pH and pCO(2) conditions, and in the presence/absence of SiO2(aq). Our data show that the formation of siderite directly from seawater is initiated by the formation of precursor amorphous Fe carbonate (AFC). The precursor, once formed, rapidly transforms into either siderite or chukanovite (Fe2CO3(OH)2; metastable with respect to siderite), dependent on the availability of carbonate ions. Precipitation rates estimated from our experiments reveal a strong exponential dependence on the saturation state of the solution, consistent with theoretical predictions. This relationship, in turn, delineates a critical supersaturation threshold for homogeneous Fe(II)-carbonate precipitation from anoxic and ferruginous water. Incorporation of our experimental data with constraints on the ocean-atmosphere carbon system shows that significant accumulations of siderite in Precambrian sedimentary rocks require an origin from fluids that were distinct from seawater in equilibrium with the ocean-atmosphere inorganic carbon reservoir. In addition, these data show that the maximum Fe2+ concentration in Precambrian oceans was controlled by Fe(II)-silicate precipitation and not by Fe(II)-carbonate, consistent with the sedimentary record of Fe-rich Precambrian rocks. More broadly, these results contribute to a quantitative framework that underpins the utility of Fe(II)-carbonate as a sensitive palaeo-environmental proxy for ferruginous systems where CO2 may have been present. (C) 2018 Elsevier B.V. All rights reserved.