Assessment of the diagenetic overprint of sediments within the deep subseafloor biosphere in the Nankai Trough, Japan – IODP Expedition 370

male.koester [ at ]


IODP Expedition 370 established Site C0023 down to 1180 mbsf in the Nankai Trough, Japan, to explore the upper temperature limit of microbial life in deep subseafloor sediments. The area is characterized by strongly changing sedimentation rates. The diagenetic iron (Fe) cycling is an important process within the deep biosphere. However, microbial and abiotic alterations of Fe phases in deep subseafloor sediments are poorly understood. Fe (oxyhydr)oxides are important carriers of sedimentary magnetic signals. Diagenetic cycling, especially the reductive dissolution of Fe (oxyhydr)oxides driven by microbial degradation of organic matter and/or by reactions with hydrogen sulfide, may lead to transformations of primary ferrimagnetic Fe (oxyhydr)oxides to secondary Fe sulfides, and thus, to modification of rock magnetic properties. In this study, we aim at assessing the alteration of the primary sedimentary record at Site C0023, including authigenic formation of secondary minerals, pyritization as well as effects on rock magnetic properties. To investigate the Fe mineralogy, sequential extractions of Fe (oxyhydr)oxides and sulfides are combined with rock magnetic analyses and SEM-EDS observations. The reactive Fe pool includes Fe carbonates and Fe (oxyhydr)oxides. Amorphous Fe oxides are the dominant reactive Fe fraction at Site C0023. Fe sulfides, mainly pyrite, are present in all investigated intervals. However, pyritization only affects 5 to 15% of the reactive Fe pool. Rock magnetic properties also show the presence of metastable Fe sulfides in the upper interval between 200 and 450 mbsf. Their preservation might be caused by limited hydrogen sulfide availability, an increase in sedimentation rates, which prevents a complete pyritization by decreasing the time the interval was located in the sulfidic zone, or by recent authigenesis. Combining our geochemical and rock magnetic data improves the understanding of iron cycling in subseafloor sediments and the role of iron minerals in maintaining life in the deep biosphere.

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Conference (Talk)
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ICYMARE 2019, 24 Sep 2019 - 27 Sep 2019, Bremen.
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Köster, M. , Kars, M. A. , Tsang, M. Y. , Heuer, V. B. , Morono, Y. , Inagaki, F. , Kasten, S. and Henkel, S. , and Expedition 370 Scientists (2019): Assessment of the diagenetic overprint of sediments within the deep subseafloor biosphere in the Nankai Trough, Japan – IODP Expedition 370 , ICYMARE 2019, Bremen, 24 September 2019 - 27 September 2019 .


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