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Nanoscale petrographic and geochemical insights on the origin of the Palaeoproterozoic stromatolitic phosphorites from Aravalli Supergroup, India

Papineau, D; De Gregorio, B; Fearn, S; Kilcoyne, D; McMahon, G; Purohit, R; Fogel, M; (2016) Nanoscale petrographic and geochemical insights on the origin of the Palaeoproterozoic stromatolitic phosphorites from Aravalli Supergroup, India. Geobiology , 14 (1) pp. 3-32. 10.1111/gbi.12164. Green open access

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Abstract

Stromatolites composed of apatite occur in post-Lomagundi-Jatuli successions (late Palaeoproterozoic) and suggest the emergence of novel types of biomineralization at that time. The microscopic and nanoscopic petrology of organic matter in stromatolitic phosphorites might provide insights into the suite of diagenetic processes that formed these types of stromatolites. Correlated geochemical micro-analyses of the organic matter could also yield molecular, elemental and isotopic compositions and thus insights into the role of specific micro-organisms among these communities. Here, we report on the occurrence of nanoscopic disseminated organic matter in the Palaeoproterozoic stromatolitic phosphorite from the Aravalli Supergroup of north-west India. Organic petrography by micro-Raman and Transmission Electron Microscopy demonstrates syngeneity of the organic matter. Total organic carbon contents of these stromatolitic phosphorite columns are between 0.05 and 3.0 wt% and have a large range of δ(13) Corg values with an average of -18.5‰ (1σ = 4.5‰). δ(15) N values of decarbonated rock powders are between -1.2 and +2.7‰. These isotopic compositions point to the important role of biological N2 -fixation and CO2 -fixation by the pentose phosphate pathway consistent with a population of cyanobacteria. Microscopic spheroidal grains of apatite (MSGA) occur in association with calcite microspar in microbial mats from stromatolite columns and with chert in the core of diagenetic apatite rosettes. Organic matter extracted from the stromatolitic phosphorites contains a range of molecular functional group (e.g. carboxylic acid, alcohol, and aliphatic hydrocarbons) as well as nitrile and nitro groups as determined from C- and N-XANES spectra. The presence of organic nitrogen was independently confirmed by a CN(-) peak detected by ToF-SIMS. Nanoscale petrography and geochemistry allow for a refinement of the formation model for the accretion and phototrophic growth of stromatolites. The original microbial biomass is inferred to have been dominated by cyanobacteria, which might be an important contributor of organic matter in shallow-marine phosphorites.

Type: Article
Title: Nanoscale petrographic and geochemical insights on the origin of the Palaeoproterozoic stromatolitic phosphorites from Aravalli Supergroup, India
Location: England
Open access status: An open access version is available from UCL Discovery
DOI: 10.1111/gbi.12164
Publisher version: http://dx.doi.org/10.1111/gbi.12164
Language: English
Additional information: Copyright © 2015 John Wiley & Sons Ltd. This is the peer reviewed version of the following article: Gebiology, which has been published in final form at DOI:10.1111/gbi.12164. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.
UCL classification: UCL > School of BEAMS
UCL > School of BEAMS > Faculty of Maths and Physical Sciences
UCL > School of BEAMS > Faculty of Maths and Physical Sciences > London Centre for Nanotechnology
URI: http://discovery.ucl.ac.uk/id/eprint/1471041
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