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Geochemical Highlights (#138)

"Geochemical Highlights" is a new feature at Geochemical News that we are hoping catches on in popularity. Here you will find short summaries of recent journal articles that the GN editors (in future issues, this hopefully will include guests too) found particularly interesting. Some scientific journals do this for a wide-range of research topics (e.g. Nature's Research Highlights) but we thought it would be nice to have this specifically for the geochemistry community. This first sampling admittedly lacks representation from all geochemical research areas, but we hope to address this in future installments. If you have a suggestion for a recent paper that you think should be highlighted (or would like to write one yourself), contact us at gnews@geochemsoc.org.

Mineral nanofractures influence transport
Kerisit and Liu (2009) Environ. Sci. Technol. DOI: 10.1021/es8016045 (in press, available online)

The influence of porosity on the movement of water through rocks has been taught in introductory geoscience classrooms worldwide for decades, yet researchers are still revealing surprising natural phenomena that change how we view this fundamental concept.

Sebastien Kerisit and Chongxuan Liu from Pacific Northwest National Laboratory in Richland, WA recently used molecular dynamics simulations of nano-fractures in orthoclase (KAlSi3O8) to show that the diffusivity of water and transport of electrolyte ions are significantly lower when confined to spaces up to several tens of nanometers wide when compared to bulk water. The researchers note that such nano-fractures may be important pathways for the sequestration of environmental contaminants across many mineral-water interfaces. The results may also have implications for understanding the surface reactivity of mineral nanoparticle aggregates where confined spaces of just a few nanometers are likely abundant. --NW

Some bugs like it hot
Takai et al. (2008) PNAS 105 (31), 10949-10954

One of the holy grails of geomicrobiology is to discover the microorganism that lives at the highest temperature. For a long time, the record holder has been Pyrolobus fumarii at 113°C. In 2003, Kashefi and Lovely reported in a Brevia article in Science the growth of an uncharacterized organism that grows at 121°C (the temperature at which steam sterilizers operate), which they dubbed Strain-121. Now, Takai et al. report the growth of a methanogen, Methanopyrus kandleri strain 116, at 122°C. Normally, Methanopyrus attains a maximum growth temperature of up to 110°C. The increase, according to Takai et al., results from culturing Methanopyrus under a pressure of 20 MPa instead of just enough pressure to keep water from boiling (or 0.4 MPa).

Perhaps more significantly, Takai et al. report that the isotope fractionation of carbon by Methanopyrus to methane at 122°C and 40 MPa (-9.4‰) is much smaller than previously reported for methanogens under conventional conditions (-34‰ to -27‰). The relatively heavy values have significant implications for interpreting Earth's carbon isotope record and the search for life outside Earth. --MS

Removing oxygen to study Isotope adsorption
Hu et al (2008) Geochem. Trans. 9(12).

The United States tested 828 nuclear weapons tests at the Nevada Test Site (NTS) between 1951 and 1992. Radiogenic isotopes in groundwater at the NTS include iodine, neptunium, technetium, and a number of plutonium and uranium isotopes. Sorption of aqueous complexes of the radiogenic nuclides has been characterized under aerobic conditions in groundwater, but few studies address sorption under anaerobic conditions.

New experiments by Hu et al. show that more Np and Tc adsorb on to devitrified tuff where redox conditions correspond to sulfate reduction (pε -5) than where aerobic conditions prevail (pε 10). Behaviors of the other tested isotopes were less clear suggesting we need more laboratory and field investigations to define how redox conditions affect the behavior of aqueous radionuclides in groundwater. --SK

Nitrogen fixers on the shelf
Rees et al (2009) MEPS 374, 7-12

Researchers from the National Oceanography Centre in Southampton and the Plymouth Marine Laboratory appear to have found the first evidence of nitrogen fixation in a shelf environment, which might lead to some revisions with regard to nitrogen and carbon budgets.

Until now, most marine research into nitrogen fixation had been dedicated to tropical and subtropical seas although it was known, for instance, that the Baltic Sea is a site where tremendous nitrogen fixation takes place every summer. Rees et al. took samples at two stations in the English Channel off Plymouth during a mixed phytoplankton bloom dominated by Emiliania huxleyi and Karenia mikimotoi. They incubated the samples with 15N- N2 and also carried out nifH identification in DNA and mRNA. The authors conclude that "there is clear evidence of nitrogen fixation in mesotrophic waters of the Western English Channel during summer conditions of thermal stratification and nutrient depletion." --AS

A new look at ionic gold
Lintern et al (2009) Geochim. Cosmochim. Acta doi: 10.1016/j.gca.2008.12.019 (in press, available online)

Gold can cause obsession in man and nations, but, from a prosaic, geochemical viewpoint, gold could be conceived as less fascinating due to its inertness. However, Au does not only occur in nature in its metallic form but can also be incorporated in other minerals in the form of solid solutions. Incorporation of Au in pyrite or other chalcogenides has been reported and is not completely unexpected. However, a high correlation of Au with earth alkaline metals, as observed in some carbonate rich Australian soil profiles, is surprising. Elemental soil profiles suggest the concurrence of Au and earth alkaline metals relocation in these soils and point to the existence of an alternative binding form of non metallic Au. Analysing elemental distribution in these soils on micrometer scale reveals that the correlation of Au with earth alkaline metals breaks down at this scale and association of Au with Br indicates that immobilization of ionic Au in these soils is related to evapotranspiration processes. Local enrichment of Au in a root tubule suggests that Au mobility is influenced by the activity or living organisms and that Au might undergo biogeochemical cycling similar to less noble trace metals. --TB

Note: The study is a good example of the complimentary use of a variety of microanalysis techniques, including LA-ICP-MS, synchrotron micro-XRF, and electron microscopy. The potential of these techniques to provide new insight into biogeochemical processes becomes evident but also the limitation of the individual techniques can be recognized. Readers, who became interested in micro analysis techniques based on the tabletop micro-XRF article in this issue, will find supplementary information on the above mentioned techniques in this article.