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Abstract: The phyllomanganate birnessite is the main Mn-bearing phase in oxic marine sediments where it exerts a primary control on the concentration of micronutrient trace metals in seawater. However, during sediment diagenesis and under mild hydrothermal conditions birnessite transforms into the tectomanganate todorokite. We have recently shown that the transformation of birnessite to todorokite proceeds via a four-stage nucleation and growth mechanism, beginning with todorokite nucleation, then crystal growth from solution to form todorokite primary particles, followed by their self-assembly and oriented growth via oriented attachment to form crystalline todorokite laths, culminating in traditional crystal ripening (Atkins et al., 2014, GCA 144, 109-125). Here we determine the fate and mobility of Ni sorbed by birnessite during this transformation process. Specifically, in our recent work we predict that the presence of Ni within the phyllomanganate matrix will disrupt the formation of todorokite primary particles. As such, contrary to current understanding, we suggest that Ni sorbed by birnessite will slow the transformation of birnessite to todorokite and/or be released to marine porewaters during sediment diagenesis. Here we transform a synthetic, poorly crystalline, Ni-sorbed (~1 wt% Ni) hexagonal birnessite, analogous to marine birnessite, into todorokite under a mild reflux procedure, developed to mimic marine diagenesis and mild hydrothermal conditions. We characterize our birnessite and reflux products as a time series, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy. In addition we determine Ni speciation and mineral phase associations in a suite of natural marine ferromanganese precipitates, containing intermixed phyllomanganate and todorokite. Our work shows for the first time that Ni significantly slows the transformation of birnessite to todorokite and reduces the crystallinity of the neo-formed todorokite phase, but does not alter the mechanism and pathway of todorokite formation, compared to a Ni-free system. Furthermore, in systems tending towards todorokite as the final diagenetic product, we see that up to 50 % of the Ni originally sequestered by birnessite is released to solution during the transformation. Our work indicates that the transformation of birnessite to todorokite in oxic marine sediments likely provides a significant source of Ni to marine sedimentary porewaters and potentially a hitherto unrecognized benthic flux of Ni to seawater.

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Abstract: The phyllomanganate birnessite is the dominant Mn-bearing phase in oxic marine sediments and through coupled sorption and redox reactions exerts a strong control on the oceanic concentrations of micronutrient trace metals. However, during oxic diagenesis and under mild hydrothermal conditions, birnessite undergoes transformation to the tectomanganate todorokite. The mechanistic details of the transformation are important for the speciation and mobility of metals sequestered by birnessite, and are necessary in order to quantify the role of marine sediments in global trace element cycles. This study provides new insight into the crystallization pathway and mechanism of todorokite formation from birnessite under conditions analogous to those found in marine diagenetic and hydrothermal settings. Using a combined approach employing X-ray diffraction, electron microscopy, infrared spectroscopy, X-ray absorption spectroscopy and wet chemical methods, I propose a new four-stage process for the transformation of birnessite to todorokite, beginning with todorokite nucleation, then crystal growth from solution to form todorokite primary particles, followed by their self-assembly and oriented growth via oriented attachment to form crystalline todorokite laths, culminating in traditional crystal ripening. Furthermore, the results of this study indicate that contrary to current understanding, the bioessential trace metal Ni impedes the transformation of birnessite to todorokite, and is eventually released into sediment porewaters. This mineralogical transformation may therefore provide a benthic flux of Ni and possibly other micronutrient trace metals to seawater. Finally, I find that the uptake of Ni to the phyllomanganate birnessite under varying physiochemical conditions is accompanied by Ni stable isotope fractionation. During fractionation, the light Ni isotope is preferentially sorbed to birnessite, leaving the remaining solution heavy with respect to its Ni isotopic composition. These findings raise important questions about the mechanisms and processes responsible for the heavy δ60Ni isotopic compositions recently measured in marine ferromanganese-rich sediments.