Authigenesis of native sulphur and dolomite in a lacustrine evaporitic setting (Hellín basin, Late Miocene, SE Spain)

AbstractAbundant sulphur is present in the Late Miocene evaporitic sequence of the lacustrine Hellín basin in SE Spain. Weathering of Triassic evaporites controlled the chemical composition of the Miocene lake. The lacustrine deposits comprise gypsum, marlstones, diatomites and carbonate beds. Sulphur-bearing carbonate deposits predominantly consist of early diagenetic dolomite. Abundant dolomite crystals with a spheroidal habit are in accordance with an early formation and point to a microbial origin. The carbon isotopic composition of the dolomite (δ13C values between −10 and −4‰) indicates mixing of lake water carbonate and carbonate derived from the remineralization of organic matter by heterotrophic bacteria. Dolomite precipitated syngenetically under evaporitic conditions as indicated by high oxygen isotope values (δ18O between +6 and +11‰). Nodules of native sulphur are found in gypsum, carbonate beds and marlstone layers. Sulphur formed in the course of microbial sulphate reduction, as reflected by its strong depletion in34S (δ34S values as low as −17‰). Near to the surface many of the sulphur nodules were in part or completely substituted by secondary gypsum, which still reflects the sulphur isotopic composition of native sulphur (−18 to −10‰). This study exemplifies the role of bacterial sulphate reduction in the formation of dolomite and native sulphur in a semi-enclosed lacustrine basin during Late Miocene time.

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Role of Oxalic acid on Fractional Solubility of Aerosol Iron over Coastal Ocean: Evidence from compound-specific stable carbon isotopic composition and diagnostic mass ratios

10.5194/egusphere-egu2020-423 ◽

2020 ◽

Author(s):

Srinivas Bikkina ◽

Kimitaka Kawamura ◽

Manmohan Sarin ◽

Eri Tachibana

Keyword(s):

Oxalic Acid ◽

Organic Acids ◽

Isotopic Composition ◽

Linear Relationship ◽

Carbon Isotopic Composition ◽

Coastal Ocean ◽

Water Soluble ◽

Stable Carbon ◽

Carbon Isotopic

Atmospheric transport and the subsequent air-to-sea deposition of water-soluble iron (Fews), an essential micronutrient for the phytoplankton growth, have a profound influence on the biogeochemical cycles of carbon and nitrogen. Sources of Fews include contributions from poorly soluble natural mineral dust and highly soluble anthropogenic aerosols from biomass burning emissions and fossil-fuel combustion in the continental outflows. Apart from the source/emission contributions, atmospheric processing of aerosol iron (FeTot) by inorganic acidic species (e.g., non-sea-salt or nss-SO42- and NO3-) and/or organic acids also affect the supply of Fews to the surface waters that are downwind of pollution sources. Among these, the least understood process is the oxalic acid-mediated photochemical cycling of Fews. Laboratory studies have clearly demonstrated an enhancement in the fractional solubility of aerosol iron (i.e., Fews (%) = Fews/FeTot &#215;100) via the oxalic acid complexation with FeTot and subsequent photochemical reduction process. However, lacking support from the field measurements limits our ability to incorporate the proposed mechanism in the current biogeochemistry models. This study is designed with the overarching goal of investigating the role of oxalic acid on the Fews (%) over a coastal ocean (i.e., the Bay of Bengal: BoB) influenced by the atmospheric outflow from the Indo-Gangetic Plain (IGP) and South-east Asia (SEA) during the winter season. We analysed 31 PM2.5 samples for the mass concentrations of FeTot, Fews and other chemical composition including nss-SO42-, NO3-, oxalic acid and related polar compounds as well as stable carbon isotopic composition of oxalic acid (&#948;13Coxalic). Strong positive linear relationship of oxalic acid with FeTot and significant inverse linear relationship between &#948;13Coxalic and Fews over the BoB clearly emphasize the role of oxalic acid on the Fews (%). &#160;These findings comply with the notion that oxalic acid formed from the precursor water-soluble organic acids in the deliquescent aerosols, is complexed with aerosol-Fe and undergoes through successive photochemical reactions, contributing to an overall increase in the Fews (%).&#160;

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