Sorption of ferric iron from ferrioxamine B to synthetic and biogenic layer type manganese oxides

Siderophores are biogenic chelating agents produced in terrestrial and marine environments that increase the bioavailability of ferric iron. Recent work has suggested that both aqueous and solid-phase Mn(III) may affect siderophore-mediated iron transport, but scant information appears to be available about the potential roles of layer type manganese oxides, which are relatively abundant in soils and the oligotrophic marine water column. To probe the effects of layer type manganese oxides on the stability of aqueous Fe-siderophore complexes, we studied the sorption of ferrioxamine B [Fe(III)HDFOB⁺, an Fe(III) chelate of the trihydroxamate siderophore desferrioxamine B (DFOB)] to two synthetic birnessites [layer type Mn(III,IV) oxides] and a biogenic birnessite produced by Pseudomonas putida GB-1. We found that all of these predominantly Mn(IV) oxides greatly reduced the aqueous concentration of Fe(III)HDFOB⁺ at pH 8. Analysis of Fe K-edge EXAFS spectra indicated that a dominant fraction of Fe(III) associated with the Mn(IV) oxides is not complexed by DFOB as in solution, but instead Fe(III) is specifically adsorbed to the mineral structure at multiple sites, thus indicating that the Mn(IV) oxides displaced Fe(III) from the siderophore complex. These results indicate that layer type manganese oxides, including biogenic minerals, may sequester iron from soluble ferric complexes. We conclude that the sorption of iron-siderophore complexes may play a significant role in the bioavailability and biogeochemical cycling of iron in marine and terrestrial environments.     

The exceptionally stable cobalt(III)–desferrioxamine B complex

The biogeochemistry of trivalent iron, manganese, and cobalt in the oceans is dominated by soluble complexes formed with high-affinity organic ligands that are believed to be microbial siderophores or similar biogenic chelating agents. Desferrioxamine B (DFOB), a trihydroxamate siderophore found in both terrestrial and marine environments, has served as a useful model for a large class of microbial siderophores in studies of 1:1 complexes formed with trivalent iron and manganese. However, no data exist concerning DFOB complexes with Co(III), which we hypothesize should be as strong as those with Fe(III) and Mn(III) if the current picture of the ocean biogeochemistry of the three trivalent metals is accurate. We investigated the complexation reaction between DFOB and Co(III) in aqueous solution at seawater pH using base and redox titrations, and then characterized the resulting 1:1 complex Co(III)HDFOB⁺ using X-ray absorption, resonance Raman spectroscopy, and quantum mechanical structural optimizations. We found that the complex stability constant for Co(III)HDFOB⁺ (log K _{[Co(III)HDFOB⁺]} = 37.5 ± 0.4) is in fact five and seven orders of magnitude larger than that for Fe(III)HDFOB⁺ (log K_{[Fe(III)HDFOB⁺]} = 32.02) and Mn(III)HDFOB⁺ (log K_{[Mn(III)HDFOB}⁺_] = 29.9), respectively. Spectroscopic data and the supporting theoretical structural optimizations elucidated the molecular basis for this exceptional stability. Although not definitive, our results nevertheless are consistent with the evolution of siderophores as a response by bacteria to oxygenation, not only because of sharply decreasing concentrations of Fe(III), but also of Co(III).     

Back analysis of landslide susceptibility zonation mapping for the 2005 Kashmir earthquake: an assessment of the reliability of susceptibility zoning maps

The October 2005 earthquake triggered several thousand landslides in the Lesser Himalaya of Kashmir in northern Pakistan and India. Analyses of ASTER satellite imagery from 2001 were compared with a study undertaken in 2005; the results show the extent and nature of pre- and co-/post-seismic landsliding. Within a designated study area of ~2,250 km², the number of landslides increased from 369 in 2001 to 2,252 in October 2005. Assuming a balanced baseline landsliding frequency over the 4 years, most of the new landslides were likely triggered by the 2005 earthquake and its aftershocks. These landslides mainly happened in specific geologic formations, along faults, rivers and roads, and in shrubland/grassland and agricultural land. Preliminary results from repeat photographs from 2005 and 2006 after the snowmelt season reveal that much of the ongoing landsliding occurred along rivers and roads, and the extensive earthquake-induced fissuring. Although the susceptibility zoning success rate for 2001 was low, many of the co-/post-seismic landsliding in 2005 occurred in areas that had been defined as being potentially dangerous on the 2001 map. While most of the area in 2001 was (very) highly susceptible to future landsliding, most of the area in 2005 was only moderate to low susceptible, that is, most of the landsliding in 2005 actually occurred in the potentially dangerous areas on the 2001 map. This study supports the view that although susceptibility zoning maps represent a powerful tool in natural hazard management, caution is needed when developing and using such maps.     

Inherited drainage - paleochannels and preferential groundwater flow

It is suggested that in a localized remnant of Kalahari sand at Dufuya, central Zimbabwe, groundwater flows in an integrated pattern inherited from the paleochannel network of the underlying gneiss. Contact springs occur at discrete localities along the Kalahari sand/gneiss boundary and are associated with spring sapping and land surface subsidence. Subsidence is presumed to be due to preferential solute removal by leaching and dissolution as a result of concentration of groundwater flow within the buried paleochannel network and the location of the springs is presumed to occur where the paleochannel network intersects the Kalahari sand/gneiss boundary. Over time the surficial Kalahari sand is preferentially removed along these buried drainage lines by spring sapping and headwards erosion, exposing the gneiss. Multi-electrode direct current resistivity profiling and radar have been used to map the sub-surface, revealing the topography of the basement and nature of the Kalahari cover. Coincidence of gneiss basement depressions with the spring sites, leached sands and subsidence zones suggests inheritance of the gneiss fluvial paleochannel network pattern by the present day groundwater flow. Washed sand and gravel intersected in shallow boreholes in these areas provides further evidentiary support for the concept of inherited drainage.     

Summary of the Mars recent climate change workshop NASA/Ames Research Center,May 15–17, 2012

This note summarizes the results from the Mars recent climate change workshop at NASA/Ames Research Center, May 15–17, 2012.     

Fractionated martian atmosphere in the nakhlites?

Abstract— Considerable evidence points to a martian origin of the SNC meteorites. Noble gas isotopic compositions have been measured in most SNC meteorites. The ¹²⁹Xe/¹³²Xe vs. ⁸⁴Kr/¹³²Xe ratios in Chassigny, most shergottites, and lithology C of EETA 79001 define a linear array. This array is thought to be a mixing line between martian mantle and martian atmosphere. One of the SNC meteorites, Nakhla, contains a leachable component that has an elevated ¹²⁹Xe/¹³²Xe ratio relative to its ⁸⁴Kr/¹³²Xe ratio when compared to this approximately linear array. The leachable component probably consists in part of iddingsite, an alteration product produced by interaction of olivine with aqueous fluid at temperatures lower than 150 °C. The elevated Xe isotopic ratio may represent a distinct reservoir in the martian crust or mantle. More plausibly, it is elementally fractionated martian atmosphere. Formation of sediments fractionates the noble gases in the correct direction. The range of sediment/atmosphere fractionation factors is consistent with the elevated ¹²⁹Xe/¹³²Xe component in Nakhla being contained in iddingsite, a low temperature weathering product. The crystallization age of Nakhla is 1.3 Ga. Its low-shock state suggests that it was ejected from near the surface of Mars. As liquid water is required for the formation of iddingsite, these observations provide further evidence for the near surface existence of aqueous fluids on Mars more recently than 1.3 Ga.     

The composition and origin of Titan's atmosphere

The discovery that Titan had an atmosphere was made by the identification of methane in the satellite's spectrum in 1944. But the abundance of this gas and the identification of other major constituents required the 1980 encounter by the Voyager 1 spacecraft. In the intervening years, traces of C₂H₂, C₂H₄, C₂H₆ and CH₃D had been posited to interpret emission bands in Titan's i.r. spectrum. The Voyager Infra-red Spectrometer confirmed that these gases were present and added seven more. The atmosphere is now known to be composed primarily of molecular nitrogen. But the derived mean molecular weight suggests the presence of a significant amount of some heavier gas, most probably argon. It is shown that this argon must be primordial, and that one can understand the evolution of Titan's atmosphere in terms of degassing of a mixed hydrate dominated by CH₄, N₂ and ³⁶Ar. This model satisfactorily explains the absence of neon and makes no special requirements on the satellite's surface temperature.