Assessing the effects of oxidation of pyrite and reducing nitrogen fertilizer on chemical weathering and the carbon cycles in a karst river system by using multiple isotopes

<p>Pyrite is the most common sulfide mineral occurring in sedimentary and igneous rocks and globally contributes a greater flux of sulfate. Large quantity of reactive nitrogen as fertilizers for agricultural production has been released into the environment in China over recent decades. Sulfuric acid formed by oxidative weathering of pyrite (OWP) and nitric acid formed by oxidation of reducing nitrogen fertilizer (ONF) through neutralization with carbonate minerals can counteract CO<sub>2</sub> drawdown from chemical weathering. Here, we use the multiple isotopes (<sup>13</sup>C-DIC, <sup>34</sup>S and <sup>18</sup>O-SO<sub>4</sub><sup>2–</sup>, <sup>15</sup>N and<sup> 18</sup>O-NO<sub>3</sub><sup>–</sup>, and <sup>18</sup>O and D-H<sub>2</sub>O) and water chemistry, as well as historical hydrochemical data to assess the roles of strong acids in chemical weathering and the carbon cycle in a karst river system (Chishui River, southwestern China). The variations in alkalinity and the δ<sup>13</sup>C-DIC along with theoretical mixing models demonstrate the involvement of strong acids in carbonate weathering. However, the strong acid weathering flux determined by δ<sup>13</sup>C-DIC and mixing models is considered to be overestimated due to the effects of photosynthesis and degassing of CO<sub>2</sub> on δ<sup>13</sup>C-DIC signal. The protons liberated from OWP and ONF can be constrained by water chemistry and isotope techniques with the use of a Bayesian isotope mixing model. The strong acid weathering flux determined using proton information is higher that determined by δ<sup>13</sup>C-DIC and mixing models. This suggests that the additional protons derived from OWP and ONF might be consumed in other ways without affecting the δ<sup>13</sup>C-DIC signals, such as the neutralization of acidic waters. These results indicate that OWP and ONF coupled with carbonate dissolution significantly enhanced the coupling cycles of carbon, nitrogen and sulfur in this river system.</p>

Increased carbon capture by a silicate-treated forested watershed affected by acid deposition

Meeting internationally agreed-upon climate targets requires carbon dioxide removal (CDR) strategies coupled with an urgent phase-down of fossil fuel emissions. However, the efficacy and wider impacts of CDR are poorly understood. Enhanced rock weathering (ERW) is a land-based CDR strategy requiring large-scale field trials. Here we show that a low 3.44 t ha−1 wollastonite treatment in an 11.8 ha acid-rain-impacted forested watershed in New Hampshire, USA, led to cumulative carbon capture by carbonic acid weathering of 0.025–0.13 t CO2 ha−1 over 15 years. Despite a 0.8–2.4 t CO2 ha−1 logistical carbon penalty from mining, grinding, transportation, and spreading, by 2015 weathering together with increased forest productivity led to net CDR of 8.5–11.5 t CO2 ha−1. Our results demonstrate that ERW may be an effective, scalable CDR strategy for acid-impacted forests but at large scales requires sustainable sources of silicate rock dust.

A 3 b.y. record of a biotic influence on terrestrial weathering

Organic acids secreted by plants and microorganisms are ubiquitous in modern soils. These acids possess a particularly strong binding affinity for aluminum, accelerating its release from mineral surfaces and driving its preferential loss relative to immobile elements such as titanium. Aluminum-titanium (Al-Ti) decoupling in ancient soils may therefore serve as a tracer of a biotic influence on terrestrial weathering in Earth’s past. To explore this idea, we used a mass balance approach to quantify Al mobility in 33 definitive and chronologically well-constrained paleosols spanning the Archean to Miocene. We estimated expected Al release for a given weathering intensity under abiotic conditions using previously established experimental relationships between the relative losses of Al and magnesium (Mg), a mobile element with a much lower organic acid binding affinity. We report Al release likely attributable to organic acid weathering in all paleosols, with net loss in 13, net gain in 16, and a balance between loss and gain in four. This provides a new line of support for a significant terrestrial biosphere as far back as the mid-Archean. Interestingly, there is no statistically significant change in Al mobility across major transitions such as the Great Oxidation Event or the Paleozoic spread of vascular plants. This controversially suggests that localized organic acid weathering, at least within the surface environments captured by the paleosol record, may have been as common on early Earth as it is today.

Increased carbon capture by a silicate-treated forested watershed affected by acid deposition

Meeting internationally agreed-upon climate targets requires Carbon Dioxide Removal (CDR) strategies coupled with an urgent phase-down of fossil fuel emissions. However, the efficacy and wider impacts of CDR are poorly understood. Enhanced rock weathering (ERW) is a land-based CDR strategy requiring large-scale field trials. Here we show that a low 3.44 t ha−1 wollastonite treatment in an 11.8-ha acid-rain-impacted forested watershed in New Hampshire, USA led to cumulative carbon capture by carbonic acid weathering of 0.025–0.13 t CO2 ha−1 over 15 years. Despite a 0.8–2.4 t CO2 ha−1 logistical carbon penalty from mining, grinding, transportation and spreading, by 2015 weathering together with increased forest productivity led to net CDR of 8.5–11.5 t CO2 ha−1. Our results demonstrate that ERW may be an effective, scalable CDR strategy for acid-impacted forests but at large-scale requires sustainable sources of silicate rock dust.

Post-mining groundwater geochemistry evolution in on the Levikha sulfide deposits of the Middle Urals (Russia)

Mining of volcanogenic copper pyrite deposits of the Middle Urals leads to the rock disintegration, their grinding, increasing the interaction surface. Sulfide minerals are oxidized by interaction with oxygen-enriched meteoric water and secondary minerals are formed (crystallo-hydrates of sulphates, hydroxides). Here technogenic zone hypergene (sulfuric acid weathering crust) is being formed. Once the water pumping activities are finished, the piezometric level recovers its position decreasing the unsaturated zone created during the mining activity, this increases the water-rock ratio increasing the amount of suppliers of sulphates, iron, non-ferrous metals and other elements to groundwater. It defines a low rate of rehabilitation of mining areas after the completion of mining and cessation of dewatering. The regularities of acidic water formation after filling the depression cone are investigated on the abandoned copper mines of the Middle Urals (Russia). The longevity of formation of extremely acidic and polluted waters is determined by the degree of saturation of the supergene zone with secondary minerals and the intensity of water exchange. Seasonal decrease in the component’s concentrations t in the period of winter and summer low water was recorded. The unstable character of groundwater components changes with a half-life of more than 5 thousand days is established. The duration of rehabilitation and restoration of groundwater quality will be at least a hundred years.