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17O-Excess Traces Atmospheric Nitrate in Paleo Groundwater

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17O-Excess Traces Atmospheric Nitrate in Paleo Groundwater Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Biogeosciences Discuss., 10, 20079–20111, 2013 Open Access www.biogeosciences-discuss.net/10/20079/2013/ Biogeosciences BGD doi:10.5194/bgd-10-20079-2013 Discussions © Author(s) 2013. CC Attribution 3.0 License. 10, 20079–20111, 2013 This discussion paper is/has been under review for the journal Biogeosciences (BG). 17O-excess traces Please refer to the corresponding final paper in BG if available. atmospheric nitrate in paleo groundwater 17 O-excess traces atmospheric nitrate in M. Dietzel et al. paleo groundwater of the Saharan desert Title Page M. Dietzel1, A. Leis2, R. Abdalla1, J. Savarino3,4, S. Morin5, M. E. Böttcher6,*, and S. Köhler1,** Abstract Introduction 1Graz University of Technology, Institute of Applied Geosciences, Rechbauerstrasse 12, 8010 Conclusions References Graz, Austria Tables Figures 2Joanneum Research, Institute of Water Resources Management, Graz, Austria 3CNRS, Institut National des Sciences de l’Univers, France 4Laboratoire de Glaciologie et de Géophysique de l’Environnement, Université Joseph J I Fourier, Grenoble, France 5Météo-France – CNRS, CNRM-GAME URA 1357, CEN, Grenoble, France J I 6 Biogeochemistry Department, Max Planck Institute for Marine Microbiology, 28359 Bremen, Back Close Germany * now at: Leibniz Institute for Baltic Sea Research, Geochemistry & Isotope Geochemistry Full Screen / Esc Group, 18119 Warnemünde, Germany ** now at: Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Printer-friendly Version Sciences, Uppsala, Sweden Interactive Discussion 20079 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Received: 3 October 2013 – Accepted: 4 December 2013 – Published: 20 December 2013 Correspondence to: M. Dietzel ([email protected]) BGD Published by Copernicus Publications on behalf of the European Geosciences Union. 10, 20079–20111, 2013 17O-excess traces atmospheric nitrate in paleo groundwater M. Dietzel et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 20080 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract BGD Saharan paleo groundwater from the Hasouna area of Libya contains up to 1.8 mM of nitrate, the origin of which is still disputed. Herein we show that a positive 17O-excess in 10, 20079–20111, 2013 NO− (∆17O = δ17O − 0.52 δ18O ) is preserved in the paleo groundwater. The 3 NO3 NO3 NO3 17 − 17 5 O-excess traces O-excess provides an excellent tracer of atmospheric NO3 , which is caused by the interaction of ozone with NOx via photochemical reactions, coupled with a non-mass atmospheric nitrate dependent isotope fractionation. Our ∆17O data from 0.4 to 5.0 ‰ (n = 28) indicate in paleo groundwater NO3 x − − that up to [NO3 ]atm = 20 mol% of total dissolved NO3 originated from the Earth’s at- M. Dietzel et al. mosphere. High ∆17O values correspond to soils that are barren in dry periods, NO3 17 17 10 while low ∆ O values correspond to more fertile soils. Coupled high ∆ O NO3 NO3 − Title Page and high x[NO3 ]atm values are caused by a sudden wash out of dry deposition of − atmospheric NO3 on plant or soil surfaces within humid-wet cycles. The individual Abstract Introduction isotope and chemical composition of the Hasouna groundwater can be followed by a binary mixing approach using the lowest and highest mineralized groundwater as end- Conclusions References 34 18 15 members without considering evaporation. Using the δ S and δ O isotope SO4 SO4 Tables Figures signature of dissolved sulfate, no indication is found for a superimposition by denitrifi- cation, e.g. involving pyrite minerals within the aquifers. It is suggested that dissolved J I sulfate originates from the dissolution of calcium sulfate minerals during groundwater evolution. J I Back Close 20 1 Introduction Full Screen / Esc The accumulation of nitrate in groundwater is a well-known phenomenon occurring − Printer-friendly Version worldwide (Clark and Fritz, 1997; Kendall, 1998). Individual NO3 sources and mech- anisms for its accumulation depend strongly on the environmental conditions during − Interactive Discussion recharge, infiltration, and aquifer storage. High NO3 concentrations of paleo ground- − 25 water from Hasouna area (Libya) have been measured for decades, but the NO3 origin 20081 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | is still hotly debated (El-Baruni et al., 1985; Milne-Home and Sahli, 2007). Decipher- − ing the source of NO3 for Saharan groundwater in Libya is highly challenging, as the BGD present arid conditions preclude appreciable recharge (Edmunds, 2006). − 10, 20079–20111, 2013 The source of dissolved NO3 in groundwater is commonly attributed to (i) anthro- 5 pogenic origin, including leaching of fertilisers or animal waste, (ii) the dissolution of evaporite deposits, (iii) leaching of organic or inorganic nitrogen from soils, and (iv) at- 17O-excess traces mospheric deposition (e.g., Clark and Fritz, 1997; Saccon et al., 2013, and references atmospheric nitrate therein). Ancient Saharan groundwater of Hasouna (Libya) was recharged under envi- in paleo groundwater ronmental conditions that did not allow for an anthropogenic impact, but natural prove- − M. Dietzel et al. 10 nances of dissolved NO3 of the paleo groundwater have not yet been deciphered and quantified. Evidence for NO− origin is traditionally obtained by measuring 15N/14N and 18O/16O 3 Title Page ratios (Amberger and Schmidt, 1987; Kendall, 1998). From δ15N and δ18O data, NO3 NO3 − Abstract Introduction the NO3 provenances can be evaluated, and reaction mechanisms like denitrification − 15 may be deduced by considering changes in NO3 concentration and mass dependent Conclusions References isotope fractionation (MDF) (Kendall, 1998). However, throughout complex processes of nitrogen conversion in soils and within the aquifer δ15N and δ18O values of Tables Figures NO3 NO3 − dissolved NO3 can be strongly overprinted (e.g. by (de)nitrification; Xue et al., 2009). More recently, a triple stable isotope approach using 18O/16O and 17O/16O signals in J I − − 20 NO3 was successfully applied to identify exclusively NO3 of atmospheric origin. Atmo- J I − − 17 spheric NO (NO ,atm) is characterized by a O-excess according to the expression: 3 3 Back Close ∆17O = δ17O − 0.52 δ18O, (1) Full Screen / Esc where non-mass dependent isotope fractionation (NMDF) during the formation of − Printer-friendly Version 25 ozone, followed by photochemical reactions between atmospheric NOx and O3 to NO3 , results in non-zero ∆17O values (Michalski et al., 2003; Thiemens, 2006). Böhlke NO3 Interactive Discussion et al. (1997), for instance, found evidence from 15N/14N and 18O/16O isotope analyses − for the atmospheric source of NO3 for nitrate-rich salts in Atacama (Chile) and Mojave 20082 − Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Desert (California). The atmospheric origin of NO3 in the Atacama deposits has been 17 − BGD confirmed by a pronounced O-excess (Michalski et al., 2004a). Since then NO3 ,atm 17 − was quantified using the O-excess for various desert deposits and dissolved NO3 10, 20079–20111, 2013 in modern groundwater, lakes, and wet depositions (Darrouzet-Nardi et al., 2012; De- 5 jwakh et al., 2012; Li et al., 2010; Michalski et al., 2004b, 2005; Tsunogai et al., 2010, 17O-excess traces 2011; Nakagawa et al., 2013). atmospheric nitrate The aim of the present study is to trace the origin of NO− in Saharan paleo groundwa- 3 in paleo groundwater ter by using 15N/14N ratios and triple stable isotopes of oxygen. In addition, a reaction of nitrate with pyrite (FeS2) in the aquifers can act as a potential nitrate sink, which has M. Dietzel et al. 10 already been shown for European shallow aquifers (e.g., Böttcher et al., 1990; Zhang et al., 2012). This requires the analyses of the stable isotope signatures of dissolved sulfate. Furthermore, ancient recharge and climate conditions are reconstructed by Title Page using a multi-element and isotope approach. Abstract Introduction Conclusions References 2 Study area and sample sites Tables Figures 15 The Hasouna well field is located about 700 km south of Tripoli (Libya) within the Cambro-Ordovician Nubian Sandstone Aquifer System (Fig. 1). This system comprises J I one of the world’s largest paleo groundwater aquifers, which had been recharged at 14 about 30 ± 10 and 10 ± 3 kaBP based on C dating (Edmunds et al., 2003; Edmunds J I and Wright, 1979; Guendouz et al., 1998; Milne-Home and Sahli, 2007; Sonntag et al., Back Close 20 1978). Strong evidence exists for enhanced precipitation in north Africa during these time periods. However, individual epochs for humidity and groundwater recharge pe- Full Screen / Esc riods are still disputed, Pleistocene and Holocene humid periods in north Africa have been deduced from the records stored in lacustrine fresh-water sediments (Rognon, Printer-friendly Version 1987), wadi river systems (Kuper and Kröpalin, 2006), travertine (Carrara et al., 1998), Interactive Discussion 25 paleobotanic investigations (Doherty et al., 2000), rock art dating and varnish-coating of petroglyphs (Dietzel et al., 2008), dating of aragonitic mollusk shells (Blackwell et al., 20083 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2012), together with cyclicity of geochemical signals observed in marine sediments
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