ALBERT

Description: 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 NO3– (Δ17ONO3 = δ17ONO3 – 0.52 δ18ONO3) is preserved in the paleo groundwater. The 17O-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 dependent isotope fractionation. Our Δ17ONO3 data from 0.4 to 5.0‰ (n = 28) indicate that up to x [NO3–]atm = 20 mol % of total dissolved NO3– originated from the Earth's atmosphere. High Δ17ONO3 values correspond to soils that are barren in dry periods, while low Δ17ONO3 values correspond to more fertile soils. Coupled high Δ17ONO3 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 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-members without considering evaporation. Using the δ34SSO4 and δ18OSO4 isotope signature of dissolved sulfate, no indication is found for a superimposition by denitrification, e.g. involving pyrite minerals within the aquifers. It is suggested that dissolved sulfate originates from the dissolution of calcium sulfate minerals during groundwater evolution.

Description: The boundary conditions of saponite formation are generally considered to be well known, but significant gaps in our knowledge persist in respect to the influence of solution chemistry, temperature, and reaction time on the mineralogy, structure, stability, and chemical composition of laboratory-grown ferrous saponite. In the present study, ferrous saponite and Mgsaponite were synthesized in Teflon-lined, stainless steel autoclaves at 60, 120 and 180°C, alkaline pH, reducing conditions, and initial solutions with molar Si:Fe:Mg ratios of 4:0:2, 4:1:1, 4:1.5:0.5, 4:1.75:0.25, and 4:1.82:0.18. The experimental solutions were prepared by dissolution of sodium orthosilicate (Na4SiO4), iron(II)sulfate (FeSO4·6H2O) and magnesium chloride salts (MgCl2·6H2O with ≤ 0.005 mass% of K and Ca) in 50 mL ultrapure water that contained 0.05% sodium dithionite as the reducing agent. The precipitates obtained at two, five and seven days of reaction time were investigated by X-ray diffraction techniques, transmission electron microscopy analysis, infra-red spectroscopy, and thermo-analytical methods.The precipitates were composed mainly of trioctahedral ferrous saponite, with small admixtures of co-precipitated brucite, opal-CT, and 2-line ferrihydrite, and nontronite as the probable alteration product of ferrous saponite. The compositions of the obtained ferrous saponites were highly variable, (Na0.44−0.59K0.00−0.05Ca0.00−0.02) (Fe2+0.37−2.41Mg0.24−2.44Fe3+0.00−0.28)Σ2.65−2.85[(Fe3+0.00−0.37Si3.63−4.00)O10](OH)2, but show similarities with naturally occurring trioctahedral Fe and Mg end members, except for the Al content. This suggests that a complete solid solution may exist in the Fe-Mg-saponite series.A conceptual reaction sequence for the formation of ferrous saponite is developed based on the experimental solution and solid compositions. Initially, at pH ≥ 10.4, brucite-type octahedral template sheets are formed, where dissolved Si-O tetrahedra are condensed. Subsequent reorganization of the octahedra and tetrahedra via multiple dissolution-precipitation processes finally results in the formation of saponite structures, together with brucite and partly amorphous silica. The extent of Fe2+incorporation in the octahedral template sheets via isomorphic substitution is suggested to stabilize the saponite structure, explaining (i) the abundance of saponite enriched inVIFe2+at elevated Fe supply and (ii) the effect of structural Fe on controlling the net formation rates of ferrous saponite.

Description: Saharan paleo-groundwater from the Hasouna area of Libya contains up to 1.8 mM of nitrate, which exceeds the World Health Organization limit for drinking water, but the origin is still disputed. Herein we show that a positive 17O excess in NO3− (Δ17ONO3 = Δ17ONO3 − 0.52 δ18ONO3) is preserved in the paleo-groundwater. The 17O 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-dependent isotope fractionation. Our Δ17ONO3 data from 0.4 to 5.0 ‰ (n = 28) indicate that up to 20 mol % of total dissolved NO3- originated from the Earth's atmosphere (x[NO3−]atm), where the remaining NO3− refers to microbially induced nitrification in soils. High Δ17ONO3 values correspond to soils that are barren in dry periods, while low Δ17ONO3 values correspond to more fertile soils. Coupled high Δ17ONO3 and high x[NO3−]atm values are caused by a sudden wash-out of accumulated disposition of atmospheric NO3− on plants, soil surfaces and in vadose zones within humid–wet cycles. The individual isotope and chemical composition of the Hasouna groundwater can be followed by a binary mixing approach using the lowest and highest mineralised groundwater as end members without considering evaporation. Using the δ34SSO4 and δ18OSO4 isotope signature of dissolved SO42−, no indication is found for a superimposition by denitrification, e.g. involving pyrite minerals within the aquifers. It is suggested that dissolved SO42− originates from the dissolution of CaSO4 minerals during groundwater evolution.