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Organic matter heterogeneities in 2.72 Ga stromatolites: Alteration versus preservation by sulfur incorporation |
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| Authors: | Kevin Lepot Karim Benzerara Marine Cotte Gordon E Brown Jr Pascal Philippot |
| Institution: | a Laboratoire Géobiosphère Actuelle et Primitive, Institut de Minéralogie et de Physique des Milieux Condensés, Institut de Physique du Globe de Paris, CNRS, Universités Paris 6 and 7, case 89, 4 place Jussieu, 75252 Paris, France b European Synchrotron Radiation Facility, 38043 Grenoble, France c Laboratoire du Centre de Recherche et de Restauration des Musées de France, CNRS, 14 Quai François Mitterand, 75001 Paris, France d Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA e Stanford Synchrotron Radiation Laboratory, SLAC, Menlo Park, CA 94025, USA |
| Abstract: | The stromatolites of the weakly metamorphosed 2.72 Ga Tumbiana Formation present abundant organic globules that resemble in size, shape and distribution the microorganisms observed in modern stromatolites. In order to evaluate the significance of these cell-like organic materials, we characterized organic matter in-situ down to the nanoscale using a combination of Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Raman microspectroscopy, scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM).These analyses revealed the occurrence of two distinct types of organic matter forming μm-scale textural and chemical heterogeneities distributed in distinct mineralogical laminae of the stromatolites. Type A organic matter, which is by far the most abundant, consists of sulfur-poor organic matter that is located in mud-type laminae at grain boundaries, mostly in association with silicate minerals. In contrast, Type B organic matter is rare and preserved as inclusions in the core of calcite grains forming laminates. It occurs as micrometer-sized cell-like globules containing variable amounts of organic sulfur likely in the form of thiophenes.Different scenarios may account for these compositional heterogeneities in the kerogen. Based on textural and compositional analogies with modern stromatolites, it is argued that Type B sulfur-rich globules may represent microbial cells protected by mineral encapsulation and selectively preserved through polymerization by early diagenetic sulfurization. In modern sediments, this reaction is fuelled by bacterial sulfate reduction (BSR). This metabolism has been widely considered as a major driver in modern stromatolites calcification and could thus have played an important role in the formation of the Tumbiana Formation stromatolites. In contrast, Type A sulfur-poor organic matter corresponds to either fossil extracellular polymer substances (EPS) or recondensed kerogen. This pool was likely not sulfurized due to either local and/or timely variations in the concentrations of H2S or adverse pyritization driven by the availability of iron. Our observations thus show the need to use spatially-resolved techniques to complement organic geochemistry analyses and provide a detailed analysis of the organic carbon pools composing Archean stromatolites. |
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