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Project Progress

Metals such as Fe, Mn, Ni, Cu and Mo are extremely low in abundance in natural waters, but these metals are used in bacterial enzymes, coenzymes, and cofactors. Strategies for targeted metal uptake by organisms evolved as bioavailability of trace elements changed, and these uptake processes may have left biosignatures in the rock record. We recently published a review article (Liermann et al. 2007) summarizing investigations into microbial influences on trace metal mobilization from mineralogical materials. Studies investigating Fe dissolution and isotopic fractionation by microorganisms have shown promise for detecting biosignatures, and N₂-fixing microorganisms (diazotrophs) which require Mo as well as Fe may produce “molybdophores”. Recent evidence indicates that when the N2-fixing soil bacterium Azotobacter vinelandii is grown in the laboratory ± fixed nitrogen, Fe in solution (δ⁵⁶Fe/⁵⁴Fe) becomes isotopically light, and Mo in solution (δ⁹⁷Mo/ ⁹⁵Mo) becomes isotopically heavy (Wasylenki et al. 2007). Mechanisms of fractionation as these metals are assimilated into the organisms are the subject of ongoing investigation. Additional studies examining the effects of A. vinelandii and organic ligands on the dissolution of two central PA shales have shown that while a strong siderophore, desferrioxamine B, enhances release of Fe from both shales, and a siderophore analog, 2,3-dihydroxybenzoic acid, enhances release of Mn from one of the shales, A. vinelandii has little affect on metal release from either shale and does not grow well in their presence. Fe and Mo isotopic signatures of samples from these experiments will allow comparison of fractionation of these metals under different experimental conditions.

Although the importance of iron in subsurface environments is widely recognized, it is difficult to study because of the complex biogeochemical pathways resulting in distinct patterns of isotopically light and heavy Fe pools within a soil profile. Experiments have been conducted to explore fractionation of Fe during ligand binding and subsequent release from the ligands in the presence of a reductant. Desferrioxamine B (DFMB), acetohydroxamic acid (aha), and EDTA were used as ligands due to their different binding affinities for Fe(III) (Shenker et al. 1999). To investigate the stability of these iron complexing ligands, we studied the release of iron in time series using ascorbate as the reductant. Our results showed significant differences in the kinetics of iron release and its complexation to ferrozine, a complexing agent for ferrous iron. We found that the reductive dissociation of ferric iron from all of the ligands follows a pseudo first order rate model, and the dissociation kinetics are strongly pH-dependent. The calculated half-life of iron dissociation is on the order of seconds for aha, minutes to hours for EDTA, and days for DFMB. These differences may be due to differences in coordination geometry. Furthermore, a reduction step is necessary to dissociate iron from these strongly binding environments. In our system, ferric iron, ferrous iron and protons are competing for the donor sites of aHA, EDTA, or DFMB. The addition of gallium catalyzes the dissociation reaction of iron from desferrioxamine B, and the half-life decreases by three orders of magnitude in the presence of Ga. Overall, Ga is more effective in the competition for the donor sites of DFMB than ferrous iron or H+ and accelerates the dissociation of ferrous iron from DFMB.

Our results show that the initial dissociative ferrous iron fraction is depleted in ⁵⁶Fe in comparison to the starting isotopic composition of ferrioxamine B and iron-acetohydroxamic acid. In contrast, we showed that initial release of iron from EDTA results in an enrichment of ⁵⁶Fe. These results show that dissociation reactions have a strong influence on iron cycling and therefore on iron isotope fractionation processes.

Libby Hausrath, a former graduate student, has investigated basalt and olivine weathering in a Mars analog environment, the Sverrefjell volcano in Svalbard. Weathered basalt samples were collected for analysis of biotic and abiotic weathering, and the potential identification of biomarkers. The main source of biotic weathering in the arctic is considered to be lichens, which are known to release organic acids and “lichen acids”, which have been shown to enhance weathering and complex metals from a mineral substrate. Data generated by microscopic and spectroscopic methods indicate that physical weathering plays a larger role than chemical weathering in this environment. Studies of basalts by Hausrath et al. (2005) documented low-temperature alteration in samples from ~30 cm below the surface, with olivine dissolution < glass dissolution, and silica-rich coatings (see also Dorn et al. 1998; Dixon et al., 2002). More recent olivine weathering studies in Svalbard confirmed that olivine dissolves more slowly than glass at low temperatures at pH 7 -9, whereas olivine dissolves more quickly than glass at ~50 C and similar pH (Hausrath et al. 2008, in press). These same studies showed surface alterations on buried olivine and glass samples as determined by XPS and SEM analyses, which were interpreted as indicating biological activity.

Relative mineral dissolution rates at different pH values can be predicted from laboratory dissolution experiments. We have compared relative mineral weathering rates observed in the field with laboratory predicted trends. Relative mineral weathering rates observed for basalt in Svalbard (Norway), Pennsylvania,and Costa Rica are explainable by pH. These results suggest that the pH-dependence of laboratory rates can be used to interpret relative mineral persistence on Mars to yield information about the pH of the reacting fluid. We also interpret both terrestrial and Martian weathering profiles using reactive transport modeling, which can yield insights into the duration of weathering. The interpretation of weathering profiles on Mars is a promising approach to study that planet’s aqueous history.