<sup>17</sup>O excess traces atmospheric nitrate in paleo-groundwater of the Saharan desert

Abstract

Abstract. 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 17 O excess in NO

Abiotic versus biotic controls on soil nitrogen cycling in drylands along a 3200 km transect

“…5a, a pronounced trend (green arrow) toward higher δ 18 O and lower δ 15 N values is obvious for elevated NO − 3 concentrations in the arid-zone soils, which might be the result of mixed NO − 3 from both soil nitrification and atmospheric deposition. A similar result was observed in the groundwater of the Sahara (Dietzel et al, 2014). In the arid zone, extreme dryness and high alkalinity (an average pH of 8.3) might limit microbial activities, as suggested by the low gene abundance involving N transformations ( Fig.…”

“…After water, N availability is the most important limiting factor for plant productivity and microbial processes in dryland ecosystems (Collins et al, 2008;Hooper and Johnson, 1999). Despite low soil N mineralization rates, N losses are postulated to be higher relative to N pools in dryland ecosystems compared with mesic ecosystems (Austin, 2011;Austin et al, 2004;Dijkstra et al, 2012). However, we still lack a full understanding of the constraints on N losses in drylands because multiple processes contribute to N losses, and the response of those processes to changing climate is highly variable (Nielsen and Ball, 2015).…”

Oxygen-17 anomaly in soil nitrate: A new precipitation proxy for desert landscapes

“…Therefore, at sites absent of large-scale hydrological impacts, coupling the  17 O NO3soil evidences with the interpretation of other geological and geochemical information can help guarantee the feasibility of the  17 O NO3soil proxy for paleoprecipitation inference. In addition, groundwater in deserts is widely known to have high concentrations of NO that is mainly from surface NO 3 deposits (Dietzel et al, 2014). Since the recharge of desert groundwater is typically slow and not appreciable, ancient groundwater may also preserve paleoprecipitation information like paleosol, and the groundwater NO 3 - 17 O may then help constrain the paleoprecipitation.…”

“…found in the Cambro-Ordovician Nubian sandstone aquifer system in the Hasouna area (Libya) had  O values between 0.4 and 5‰ (Dietzel et al, 2014). The Nubian sandstone aquifer, one of the world's largest paleo-groundwater aquifers (>2,000,000 km 2 ), is situated in the Eastern end of the Sahara Desert and consists of fractured quartzitic sandstone that spans 500-1500m in thickness (see sites on Figure 1).…”

“…The Nubian sandstone aquifer, one of the world's largest paleo-groundwater aquifers (>2,000,000 km 2 ), is situated in the Eastern end of the Sahara Desert and consists of fractured quartzitic sandstone that spans 500-1500m in thickness (see sites on Figure 1). High contents of NO 3 -(0.47-1.87 mM) have been found in the Hasouna groundwater for decades, and stable isotope evidence suggested that this groundwater NO 3 was mainly soil NO 3 that were flushed from surface soil and not influenced by subsequent denitrification or mixing of in situ biotic NO 3 from nitrification in the aquifer owing to the limited organic matter and dissolved oxygen, respectively (Dietzel et al, 2014). 14 C dating indicates this paleo-aquifer was recently recharged at 10±3 ka, after which the recharge is slow under the present arid conditions (Edmunds and Wright, 1979).…”