Publication Date:
2022-10-26
Description:
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 21(11), (2020): e2020GC009074, doi:10.1029/2020GC009074.
Description:
Marine ferromanganese deposits, often called the scavengers of the sea, adsorb and coprecipitate with a wide range of metals of great interest for paleo‐environmental reconstructions and economic geology. The long (up to ∼75 Ma), near‐continuous record of seawater chemistry afforded by ferromanganese deposits offers much historical information about the global ocean and surface earth including crustal processes, mantle processes, ocean circulation, and biogeochemical cycles. The extent to which the ferromanganese deposits hosting these geochemical proxies undergo diagenesis on the seafloor, however, remains an important and challenging factor in assessing the fidelity of such records. In this study, we employ multiple X‐ray techniques including micro–X‐ray fluorescence, bulk and micro–X‐ray absorption spectroscopy, and X‐ray powder diffraction to probe the structural, compositional, redox, and mineral changes within a single ferromanganese crust. These techniques illuminate a complex two‐dimensional structure characterized by crust growth controlled by the availability of manganese (Mn), a dynamic range in Mn oxidation state from +3.4 to +4.0, changes in Mn mineralogy over time, and recrystallization in the lower phosphatized portions of the crust. Iron (Fe) similarly demonstrates spatial complexity with respect to concentration and mineralogy, but lacks the dynamic range of oxidation state seen for Mn. Micrometer‐scale measurements of metal abundances reveal complex element associations between trace elements and the two major oxide phases, which are not typically resolvable via bulk analytical methods. These findings provide evidence of post‐depositional processes altering chemistry and mineralogy, and provide important geochemical context for the interpretation of element and isotopic records in ferromanganese crusts.
Description:
This research is supported by NASA Exobiology NNX15AM046 to Scott D. Wankel and Colleen M. Hansel, NASA NESSF NNX15AR62H to Kevin M. Sutherland, and WHOI Ocean Exploration Institute to Colleen M. Hansel. The Stanford Synchrotron Radiation Lightsource was utilized in this study. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE‐AC02‐76SF00515.
Description:
2021-04-26
Keywords:
Diagenesis
;
Ferromanganese crust
;
Manganese oxide minerals
;
X‐ray absorption spectroscopy
Repository Name:
Woods Hole Open Access Server
Type:
Article
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