High δ15N values as a tool for tracking ammonia degassing in past inland seas: constraining the environmental impact of large-scale NH3 volatilization.

Evaporitic and alkaline restricted environments (pH ≥ 9.25) are identified by high d¹⁵N values (>10 ‰) in geological times. The enriched isotopic values are commonly attributed to ammonia (NH₃) volatilization. The conversion of NH₄⁺ into H+ and NH₃ occur at pKa of 9.25 and preferentially releases volatile ¹⁴NH₃, a colorless toxic gas. The residual ¹⁵N-rich NH₄⁺ are stored in sediments and biomass, resulting in extremely enriched d¹⁵N values. Elevated d¹⁵N values are found in the sediments of a lower Permian inland sea (Irati-Whitehill sea). The observation of high d¹⁵N values suggests that NH₃ volatilization may have occurred. However, no constraints about the potential environmental impact of a large-scale ammonia degassing event have been traced. We suggest a new hypothesis that large-scale NH₃ volatilization may have caused global/regional atmospheric and biotic changes. New geochemical analyses (TOC, Rock-Eval, biomarkers, d¹³C and d¹⁵N, and XRD) have been performed on samples of sedimentary rocks from the Irati-Whitehill sea in Brazil (Paraná Basin) and South Africa (Karoo Basin). We show that, at ¹⁵N rich stages, the restriction of the water mass and negative hydrologic balance have led to the establishment of a stressful environment with the flourishment of halotolerant organisms. The predominance of regular isoprenoids (e.g., i-C₂₅ and i-C₃₀) was attributed to an environment rich in archaebacteria. Specifically, it has been shown that haloalkaliphilic archaebacteria have cell membranes containing large amounts of C₂₅ isoprenoid. In this scenario, the anomalous abundance of i-C₂₅ is suggestive of an alkaline environment. In addition, other studies report the occurrence of minerals such as analcime in the upper part of the Irati Fm., characterizing the alkalinity of these environments. Therefore, the elevated d¹⁵N values reported for the Irati Fm. and interpreted as enhancement in salinity/alkalinity are consistent with previous interpretations for Irati Fm. During the hypersaline phases (high pH and temperature) NH₃ volatilization from diagenetically released NH₄⁺ could have occurred through reactions that imprint large isotopic fractionation, as isotopically lighter NH₃ tends to escape to the atmosphere resulting in a heavier residual NH₄⁺ pool. Elevated d¹⁵N values (up to 12.49 ‰) are also observed in the African record of the Irati-Whitehill sea. Pulses of NH₃ volatilization probably have been favored by stratification of the water masses, suggested by the high abundance of the biomarker gammacerane. This condition may have led to the accumulation of a significant reservoir of NH₄ in bottom waters allowing the dissociation into ammonia followed by volatilization of NH₃ gas which, eventually, may have been released into the atmosphere. In the ancient record, NH₃ volatilization is suggested to explain the elevated d¹⁵N values for the early Jurassic Towaco Fm. (eastern USA). Nevertheless, a critical evaluation of the potential effect of the degassing of NH₃ on a large scale is still needed. Specifically, the evaluation of sustained pulses of NH₃ degassing in the huge lower Permian Irati-Whitehill inland sea and its impact on the atmosphere and the regional/global biota should be investigated further. Our current data show that in the northern area of the Paraná Basin (Irati Fm.) elevated d¹⁵N (up to 11.36 ‰) also occurs. Our organic geochemical data from the organic-rich mudstones from the Karoo Basin show that these rocks correlate with the Irati Fm. rocks, presenting elevated TOC values (up to 10.50%). We are performing d¹⁵N, d¹³C_org, biomarkers, and XRD analyses of the Whitehill Fm. samples and on new sites in the Paraná Basin (Irati Fm.) which will document the areal extent of the sea that would have emanated volatile NH_3. This may help to resolve to which extent released NH₃ impacted biota and ecosystems.