¹Erwin Dehouck,¹Scott M. McLennan,²Pierre-Yves Meslin,³Agnès Cousin
¹Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY, USA
²Institut de Recherche en Astrophysique et Planétologie, CNRS/Université Paul Sabatier, Toulouse, France
³Los Alamos National Laboratory, Los Alamos, NM, USA

X-ray diffraction patterns of the three samples analyzed by Curiosity’s CheMin instrument during the first year of the Mars Science Laboratory mission – the Rocknest sand; and the John Klein and Cumberland drill fines, both extracted from the Sheepbed mudstone – show evidence for a significant amorphous component of unclear origin. We developed a mass balance calculation program that determines the range of possible chemical compositions of the crystalline and amorphous components of these samples within the uncertainties of mineral abundances derived from CheMin data. In turn, the chemistry constrains the minimum abundance of amorphous component required to have realistic compositions (all oxides ≥0 wt%): 21—22 wt% for Rocknest and 15—20 wt% for Cumberland, in good agreement with estimates derived from the diffraction patterns (~27 and ~31 wt%, respectively). Despite obvious differences between the Rocknest sand and the Sheepbed mudstone, the amorphous components of the two sites are chemically very similar, having comparable concentrations of SiO2, TiO2, Al2O3, Cr2O3, FeOT, CaO, Na2O, K2O and P2O5. MgO tends to be lower in Rocknest, although it may also be comparable between the two samples depending on the exact composition of the smectite in Sheepbed. The only unambiguous difference is the SO3 content, which is always higher in Rocknest. The observed similarity suggests that the two amorphous components share a common origin or formation process. The individual phases possibly present within the amorphous components include: volcanic (or impact) glass, hisingerite (or silica + ferrihydrite), amorphous sulfates (or adsorbed SO42−) and nanophase ferric oxides.

Reference
Dehouck E, McLennan SM, Meslin P-Y, Cousin A (2014) Constraints on abundance, composition and nature of X-ray amorphous components of soils and rocks at Gale crater, Mars. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004716]

Published by arrangement with John Wiley&Sons

¹Svetoslav V. Georgiev, ^1,2Holly J. Stein, ^1,2Judith L. Hannah, ³Charles M. Henderson,
^4,5Thomas J. Algeo
¹AIRIE Program, Colorado State University, Fort Collins, 80523-1482 CO, USA
²CEED (Centre for Earth Evolution and Dynamics), University of Oslo, 0316 Oslo, Norway
³Department of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada
⁴Department of Geology, University of Cincinnati, Cincinnati, 45221-0013 OH, USA
⁵State Key Laboratories of BGEG and GPMR, China University of Geosciences, Wuhan 430074, China

The geochemical record for the Permian-Triassic boundary in northern latitudes is essential to evaluation of global changes associated with the most profound extinction of life on Earth. We present inorganic and organic geochemical data, and Re-Os isotope systematics in a critical stratigraphic interval of pre- and post-extinction Upper Permian-Lower Triassic sediments from Opal Creek, western Canada (paleolatitude of ∼30°N). We document significant and long-lived changes in Panthalassa seawater chemistry that were initiated during the first of four magmatic or meteoritic inputs to Late Permian seawater, evidenced by notable decreases of Os isotopic ratios upsection.

Geochemical signals indicate establishment of anoxic bottom waters shortly after regional transgression reinitiated sedimentation in the Late Permian. Euxinic signals are most prominent in the Upper Permian sediments with low organic carbon and high sulfur contents, and gradually wane in the Lower Triassic. The observed features may have been generated in a strongly euxinic ocean in which high bacterioplankton productivity sustained prolific microbial sulfate reduction in the sediment and/or water column, providing hydrogen sulfide to form pyrite. This scenario requires nearlycomplete anaerobic decomposition of predominantly labile marine organic matter (OM) without the necessity for a complete collapse of primary marine productivity. Similar geochemical variations could have been achieved by widespread oxidation of methane by sulfate reducers after a methanogenic burst in the Late Permian. Both scenarios could have provided similar kill mechanisms for the latest Permian mass extinction.

Despite the moderate thermal maturity of the section, OM in all studied samples is dominantly terrestrial and/or continentally derived, recycled and refractory ancient OM. We argue that, as such, the quantity of the OM in the section mainly reflects changes in terrestrial vegetation and/or weathering, and not in marine productivity. At Opal Creek, a pyrite layer and <20-cm-thick siltstones that are lean in OM mark dramatic and long-lived inorganic geochemical and stable isotope changes. Initial Os isotope ratios decline markedly toward values of ∼0.35 in the pyrite interval, indicating a mantle-sourced or meteoritic trigger for the intensification and expansion of latest Permian anoxia. Subsequent and stronger magmatic or meteoritic pulses recorded by low initial Os isotopes followed the main extinction.

Reference
Georgiev SV, Holly J. Stein HJ, Judith L. Hannah JL, Charles M. Henderson CM, Thomas J. Algeo TJ (2014)Enhanced recycling of organic matter and Os-isotopic evidence for multiple magmatic or meteoritic inputs to the Late Permian Panthalassic Ocean, Opal Creek, Canada. Geochimica et Cosmochimica (in Press)
Link to Article [doi:10.1016/j.gca.2014.11.019]

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