Monday, April 29, 2013

Highlights—Microbes Pumping Iron(stone)

Oolitic ironstones occur quite frequently in condensed sections in the stratigraphic record and can serve as important iron resources. In this paper, Barale et al. use a Cretaceous condensed section to explore the conditions in which mixed Fe-oxide / Ca-phosphate ooids form, why they usually show an ellipsoidal shape in spite of a sub-equidimensional nucleus, and why Fe-oxide cortical layers intimately alternate with Ca-phosphate layers. Their results, which integrate cathodoluminescence, EDS microprobe, backscattered electron imaging, epifluorescence, XRD, and micro Raman spectroscopy, reveal a complex history of prolonged alterations of oxic and post-oxic conditions, interpreted to be related to alternating accumulation and winnowing, and related stop-and-start microbial activity. Collectively, the results provide new insights into the sedimentological and paleoenvironmental conditions in which oolitic ironstones form, detailing the sedimentation, shallow burial, reworking and sea-floor exposure of sediment that ultimately is composed almost exclusively of authigenic grains. 

The Role of Microbial Activity In the Generation of Lower Cretaceous Mixed Fe-Oxide–Phosphate Ooids from the Provençal Domain, French Maritime Alps by Luca Barale, Anna D'atri, and Luca Martire

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