Rock-Sourced Nitrogen in Semi-Arid, Shale-Derived California Soils

Models suggest that rock-derived nitrogen (N) inputs are of global importance to ecosystem N budgets; however, field studies demonstrating the significance of rock N inputs are rare. We examined rock-derived N fluxes in soils derived from sedimentary rocks along a catena formed under a semi-arid climate. Our measurements demonstrate that there are distinct and traceable pools of N in the soil and bedrock and that the fraction of rock-derived N declines downslope along the catena. We used geochemical mass balance weathering flux measurements to estimate a rock-derived N flux of 0.145 to 0.896 kg ha–1 yr–1 at the ridgecrest. We also developed independent N flux estimates using a 15N-based isotope mixing model. While geochemical mass-balance-based estimates fell within the 95% confidence range derived from the isotope mixing model (−1.1 to 44.3 kg ha–1 yr–1), this range was large due to uncertainty in values for atmospheric 15N deposition. Along the catena, N isotopes suggest a diminishing effect of rock-derived N downslope. Overall, we found that despite relatively large N pools within the saprolite and bedrock, slow chemical weathering and landscape denudation limit the influence of rock-derived N, letting atmospheric N deposition (7.1 kg ha–1 yr–1) and N fixation (0.9–3.1 kg ha–1 yr–1) dominate N inputs to this grassland ecosystem.

Understanding 21Ne inventories in Precambrian basement below the Great Unconformity in Estonia

<p>Stable cosmogenic Ne isotopes are widely used to determine the erosion rate of slowly-eroding land surfaces through the Cenozoic. Constraining erosion and surface exposure back in Earth history remains a challenge largely due to the presence of Ne isotopes generated by natural decay processes over the lifetime of rocks.  Prospects are best when cosmogenic nuclide production has been significant and nucleogenic Ne production is low and can be quantified.  We have explored the limits of palaeo-cosmogenic Ne in one of the Earth’s most extensive erosion surfaces, the late Precambrian Great Unconformity in Estonia. Here deep kaolinitic saprolites formed on Baltica prior to the deposition of Late Ediacaran quartz sandstones. On the basis of geochemical mass balance the duration of saprolite development is estimated to be of the order of a few Myr.</p><p>Borehole F163 samples a section through still-buried weathered unconformity that includes a saprolite surface consistent with negligible erosion during the marine transgression. Samples from the unconformity have <sup>21</sup>Ne concentrations (>10<sup>8</sup> atoms/g) that are significantly higher than shielded samples from >20 m below the unconformity. This difference is borne out by Ne isotope composition, and leads to the tanatalising prospect that Precambrian cosmogenic Ne is present in the saprolite. Using modern <sup>21</sup>Ne production rates the palaeosols appear to record a few million years irradiation. This is broadly consistent with geochemical estimates of saprolite development.  Samples from the uppermost preserved part of the weathering profile in borehole F231 have low <sup>21</sup>Ne concentrations that are indistinguishable from deeper in the rock profile. This would require profile truncation or the redeposition of weathered material.  The borehole is located on the western flank of an uplifted basement block rising ca 130 m above the typical Precambrian basement level in the area and likely that the thick regolith contains material eroded from the uplifted basement units. Clearly these are early days and quantifying surface exposure in deep time will require effort in field as well as the lab.</p>

The influence of water–rock interactions on household well water in an area of high prevalence chronic kidney disease of unknown aetiology (CKDu)

Poor drinking water quality in household wells is hypothesised as being a potential contributor to the high prevalence of chronic kidney disease of uncertain aetiology (CKDu) among the farming communities of the Medawachchiya area, Anuradhapura, Sri Lanka. One of the natural processes that can affect water quality is the dissolution of minerals contained within an aquifer by water–rock interactions (WRIs). Here we present a comprehensive assessment of WRIs and their influence on the water chemistry in household wells and spring waters in the Medawachchiya area by combining measurements of environmental isotopes, such as strontium, lithium and stable carbon isotopes and inorganic chemistry parameters, and modelling geochemical mass balance reactions between rainfall and groundwater samples. Our results reveal the presence of strontium, dissolved from both silicate and carbonate minerals, with high isotopic (87Sr/86Sr) ratios of up to 0.7316. Geochemical mass balance modelling and prior 87Sr/86Sr studies on the Wanni Complex bedrock suggest these strontium values may be the result of biotite dissolution. We also identify lithium and uranium contributed from the dissolution of silicates, albeit at concentrations too low to constitute a known health risk. In contrast, the levels of magnesium and calcium in our samples are high and demonstrate that, despite the felsic bedrock, well water chemistry in the Medawachchiya area is dominated by carbonate dissolution.

The Origin of Major Ions of Groundwater in a Loess Aquifer

When groundwater represents most of the world’s reserves of unfrozen freshwaters, water quality is commonly a limiting factor in quantifying usable fresh groundwater storage, especially in arid and semi-arid areas. Tracing the origin of major ions is important to the understanding of hydrochemical evolution and water–rock interaction. Strontium (Sr) and calcium (Ca) are geochemically similar in terms of ionic radius and ionic charge and can substitute for each other in mineral lattices such as in carbonates and silicates. This allows the use of Sr as a proxy for Ca during water–rock interaction. Geochemical mass balance and carbon and strontium isotope techniques were employed to study the origin of each major ion (such as Na, Ca, Mg, Cl, and SO4) in a loess aquifer (there is 10% to 20% of carbonate in the loess). Geochemical mass balance between atmospheric deposition and groundwater shows that the Cl (average concentration of 5.5 mg/L) and SO4 (average concentration of 6.8 mg/L) in groundwater originated from atmospheric deposition. The dissolution of loess using acetic acid was used to analyze the 87Sr/86Sr ratio of carbonate (mainly deposited in continental environments). Groundwater 87Sr/86Sr ratios (0.710677 to 0.712319) are consistent with 87Sr/86Sr ratios in carbonate (0.710329 to 0.711085) but are significantly lower than the whole-rock (0.715136 to 0.717155) and residue (0.719091 to 0.720438), suggesting that Ca and Mg mainly originated from the dissolution of carbonate in the loess aquifer. However, Na originated from the dissolution of albite, suggesting saturation controls the ability of dissolution. There are cation exchanges between Ca + Mg and Na, resulting in Na concentration increases and Ca + Mg concentration decreases in groundwater. This study is important to the understand of the origin of the major ions in groundwater and the geochemical processes in silicate-carbonate aquifers.

Geochemical mass-balance, weathering and evolution of soils formed on a Quaternaryage basaltic toposequences

The purpose of this research is to assess the geochemical mass-balance and weathering intensity of Typic Haplustert and Lithic Ustorthent soils represented by four profiles that developed in a Quaternary-age basaltic toposequence under semi-humid conditions in the central Black Sea region of Turkey. The researchers employed mass-balance analysis with a view to measuring elemental gains and losses along with alterations concerning the soils formed on the basaltic parent material. For this end, geochemical properties, elemental mass-balance changes and certain physicochemical features were identified to benchmark the weathering levels of the profiles. Lithic Ustorthents are distinguished by having a rough texture along with a low organic substance ingredient, whereas Typic Haplusterts have a high clay texture with low bulk density and slickenside features. X-ray diffraction showed that smectites were the prevailing minerals inside the Typic Haplusterts, while a significant amount of kaolinite and illite was observed in the Lithic Ustorthents. Mass-balance computations indicated that massive mineral weathering resulted in substantial Si losses through leaching as well as an exchange of cations, such as Na+, K+, Ca2+ and Mg2+, particularly from the upper horizons. The study also took into account other features such as the pedogenic evolution of soils using weathering indices such as CIA, CIW, bases/R2O3, WIP, P and PIA. According to the results, CIA, CIW, PIA, P, WIP and bases/R2O3 index values of all soils varied between 42.33 to 73.83, 44.46 to 80.43, 37.53 to 65.63, 75.39 to 84.31 and 0.45 to 1.27 respectively, to solum depth. This result indicated that soils classified as Entisol and Vertisol have similar pedochemical properties. In spite of similar weathering rates, the soils were classified under different groups as a result of erosion. This showed that the conditions for soil development in the studied area had a far more impact on weathering and elemental loss than the parent material on the site.