Eintrag von gelöstem Fe in das Oberflächenwasser von Ozeanen - quantitative Ansätze von der ICDP-Bohrung Hawaii durch Untersuchung des grenzflächengesteuerten Fe-Transfers aus vulkanischen Gesteinen des tieferen Untergrundes
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
Within this project the main goals were to characterize differentially altered subsurface basaltic rocks with respect to Fe containing phases and to study the potential biotic and abiotic Fe mobilization from these rocks and basaltic glasses. Moreover, it was tried to elucidate the effects of Fe(II)/Fe(III) and thermal history on basaltic glass dissolution. The findings of this study demonstrate the significance of volcanic islands for geochemical element cycles between lithosphere and hydrosphere and provide a better understanding of microbial mediated alteration and the relationship between compositional and structural properties of basaltic glasses and their dissolution behavior. A sequential extraction was applied to differentially altered subsurface basaltic rocks to determine the type of chemical bonding of Fe that is a prerequisite for its release from the rocks. It was shown that amorphous to weakly crystalline secondary phases (based on Feo/Fed ratios) dominate. Their abundance was strongly related to the depositional environment with highest contents in seawater influenced rocks. Here, altered pillow basalts and hyaloclastites have to be emphasized from which a high release of limiting nutrients and especially Fe is expected. However, circulation of groundwater is fast within the highly porous rocks of volcanic islands and leached elements are transported away from their source and discharged into the ocean. Evidence was provided from microbial alteration experiments that the microorganisms acquired essential nutrients by direct microbe-surface interactions which is supposed to be an adapted strategy under nutrient limiting conditions. Due to the high groundwater flow within volcanic islands and mid-ocean ridges these locations may provide conditions were microbe-surface interactions and microbial alteration are supported. Moreover, microbial alteration experiments indicated that the Fe-redox state and thermal history of basaltic glasses have a direct influence on the microbial activity. Dissolution was accelerated by microbial activity and increased in the order annealed Fe(III)-rich < quenched Fe(III)-rich < quenched Fe(II)-rich basaltic glass. Furthermore, the abundance of microbial cells attached to glass surfaces was directly correlated with the thermal history and increased from annealed to quenched glasses. Within the percolation experiments dissolution of quenched basaltic glasses at pH 2 was enhanced in the presence of an organic ligand (oxalate) relative to annealed glasses of the same composition. This allows to conclude that in the microbial alteration experiments the microorganisms have most likely produced an organic ligand that enhanced the dissolution of quenched relative to annealed glasses similar to what was observed in the percolation experiments. Finally, it was demonstrated in percolation experiments that the long term durability of basaltic glasses is affected by the Fe redox state and decreases with increasing Fe(III) content. This observation was surprising since a higher chemical durability with increasing network former content and, thus, for Fe(III)-rich glasses was expected. For aluminosilicate glasses it is well documented that dissolution increases as the number of Al atoms per Si tetrahedral increases from albite to jadeite to nepheline glasses. A similar influence of Fe(III) in basaltic glasses is assumed but more systematic studies are needed. The investigations on Fe mobilization suggest that major portions of the HSDP2 drill core (about 2/3) comprise highly reactive secondary Fe-bearing phases. The mobilization of Fe from these phases is assumed to be relatively constant because of the slight changes of the mineralogical composition over time and might be a long-lasting source of Fe supply to the hydrosphere. The potential for high amounts of mobilizable Fe from basaltic rocks altered under seawater dominated conditions suggests that the submarine part of volcanic ocean islands represent an underestimated source of Fe supply to ocean surface waters.
Projektbezogene Publikationen (Auswahl)
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(2017) Interface driven Fe transfer from volcanic rocks of ICDP site Hawaii to ocean surface waters. IODP/ICDP Kolloquium Braunschweig, 2017
Stranghöner, M., Behrens, H., Dultz, S., Schippers, A.
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(2017): Iron specification in lithium iron phosphate glasses. 12th Pacific Rim Conference on Ceramic and Glass Technology, Hilo, Big Island (Hawaii)
Stranghoener, M., Behrens, H., Welsch, A.M.
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(2018) Experimental microbial alteration and Fe mobilization from basaltic rocks of the ICDP HSDP2 drill core, Hilo, Hawaii. Frontiers in Microbiology 9: 1252
Stranghöner, M., Schippers, A., Dultz, S., Behrens, H.
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(2018) Microbially mediated alteration and Fe mobilization from basaltic rocks of the ICDP HSDP2 drill core, Hilo, Hawaii. IODP/ICDP Kolloquium Bochum, 2018
Stranghöner, M., Schippers, A., Dultz, S., Behrens, H.
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(2019) Implications of subsurface basaltic rock alteration on the supply of soluble Fe to ocean surface waters – a study on ICDP site Hawaii. IODP/ICDP Kolloquium Köln, 2019
Stranghöner, M., Dultz, S., Behrens, H., Schippers, A.
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(2020) Far from equilibrium basaltic glass alteration: The influence of Fe redox state and thermal history on element mobilization. Geochimica et Cosmochimica Acta 273, 85-98
Stranghöner, M., Dultz, S., Behrens, H., Schippers, A.
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(2020) Potential mobilizable Fe from secondary phases of differentially altered subsurface basaltic rock– a sequential extraction study on ICDP site Hawaii. Applied Geochemistry 121, 104705
Stranghöner, M., Dultz, S., Behrens, H., Schippers, A.
