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Supply of soluble Fe to ocean surface waters - quantitative approaches from ICDP site Hawaii by determining interface driven Fe transfer from subsurface volcanic rocks

Leitung:Prof. Dr. Harald Behrens
Laufzeit:2016-2019
Förderung durch:DFG

Fe as a key limiting micro-nutrient for phytoplankton growth in the surface ocean has impact on the global C cycle. Volcanic rocks are thought to have a marked contribution to the Fe budget in some parts of the oceans, but for quantification of the effect, as stated in a recent review (Ayris & Delmelle, 2012), knowledge on the complex interface driven mechanisms of Fe release/solubility is too limited. In particular the understanding how microbial activity works to release Fe is poor. Inventories on soluble Fe in the surface ocean show marked increases in the vicinity of Hawaii and other volcanic islands. Samples from ICDP site HSDP2 are an archive for different almost fresh to altered subsurface volcanic rocks from hyaloclastites, lava flows and pillow basalts. Thus, these samples are excellently suited for the detection of relevant mechanisms of Fe release at the solid-liquid interface, modes of Fe solubility and quantitative approaches on this process. In the first year suitable samples were selected based on different criteria such as type of rock, age, degree of alteration, fresh- or seawater influenced and temperature. Quantification of four different secondary Fe binding forms in core samples was done using a sequential extraction procedure. A colorimetric wet-chemistry method was used to quantify Fe(II) and total Fe. Fe transfer from core samples to a soluble form induced by the bacterium Burkholderia fungorum was investigated in microbiological experiments. Colonization experiments with fresh synthetic basaltic glasses were performed which show a strong promoting effect of strain and ferrous iron on biological activity. Ionic effects on the chemistry of the solid-liquid interface of basaltic glass during alteration traced by measurements of the zeta potential and determination of the specific surface area (SSA) of samples from laboratory experiments as well as core samples are planned for the second year of the first proposal.In the third year, for which funding is requested here, we want to study porosity and pore connectivity in the rock samples by T-dependent water-release, Woods metal intrusion and penetration. Tomographic studies and Raman spectroscopy will provide additional information on pore distribution and phase abundance. Additionally, the measurement of water-release in function of temperature will yield data about contents of secondary minerals. Influence of various parameters (solution composition, strain in glasses, redox state of iron) on bacterical colonization will be studied using XPS, ESEM and Laser microscopy. Combining the different findings with literature data will improve our understanding of interdependencies of mineralogical composition, solution composition, biochemical factors and alteration rates on release of soluble Fe.

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