Assessing hydrological controls on the lithium isotope weathering tracer

To investigate the impact of riverine discharge and weathering intensity on lithium isotopes (δ7Li) in a mono-lithological terrain, this study examines the dissolved load and leached suspended load (exchangeable, oxide, and clay fractions) from Icelandic rivers spanning a wide range of discharge, weathering rates, and weathering intensity. The δ7Lidissolved co-varies inversely with the discharge, confirming that water-rock interaction time is a primary control on the secondary mineral formation that fractionates Li isotopes. The “boomerang” shape observed in global rivers between the weathering intensity (i.e. W/D = weathering rate/denudation rate) and δ7Lidissolved also exists for these basaltic rivers at low to medium W/D. However, these rivers do not extend to such low δ7Lidissolved values as seen in the global compilation at low W/D, indicating that there is a lithological control on this relationship arising from the type of the lithology-specific secondary minerals forming and their precipitation rates. In addition, the Δ7Lix-dissolved between each leached solid phase and the dissolved load also co-varies with discharge. At low discharge (long water-rock interaction times), Δ7Lix-dissolved values agree with experimentally-determined equilibrium values, whereas less fractionated values are observed at higher discharge (shorter water-rock interaction times). As a result, there is a different relationship between W/D and Δ7Liclay-source in this basaltic terrain than previously reported from global multi-lithological river sediment samples, with clay leachates from Iceland more closely mimicking the boomerang shape of the dissolved load. However, the relationship between δ7Li and weathering processes is complicated because the fractionation between the clay fraction and the dissolved load is not constant but varies with both W/D and discharge. Overall, this study confirms the utility of Li isotopes as a tracer of modern and palaeo-weathering processes, and also has important implications for the specific interpretations of detrital δ7Li values, which may be more sensitive to weathering parameters than previously thought

Assessing hydrological controls on the lithium isotope weathering tracer

To investigate the impact of riverine discharge and weathering intensity on lithium isotopes (δ7Li) in a mono-lithological terrain, this study examines the dissolved load and leached suspended load (exchangeable, oxide, and clay fractions) from Icelandic rivers spanning a wide range of discharge, weathering rates, and weathering intensity. The δ7Lidissolved co-varies inversely with the discharge, confirming that water-rock interaction time is a primary control on the secondary mineral formation that fractionates Li isotopes. The “boomerang” shape observed in global rivers between the weathering intensity (i.e. W/D = weathering rate/denudation rate) and δ7Lidissolved also exists for these basaltic rivers at low to medium W/D. However, these rivers do not extend to such low δ7Lidissolved values as seen in the global compilation at low W/D, indicating that there is a lithological control on this relationship arising from the type of the lithology-specific secondary minerals forming and their precipitation rates. In addition, the Δ7Lix-dissolved between each leached solid phase and the dissolved load also co-varies with discharge. At low discharge (long water-rock interaction times), Δ7Lix-dissolved values agree with experimentally-determined equilibrium values, whereas less fractionated values are observed at higher discharge (shorter water-rock interaction times). As a result, there is a different relationship between W/D and Δ7Liclay-source in this basaltic terrain than previously reported from global multi-lithological river sediment samples, with clay leachates from Iceland more closely mimicking the boomerang shape of the dissolved load. However, the relationship between δ7Li and weathering processes is complicated because the fractionation between the clay fraction and the dissolved load is not constant but varies with both W/D and discharge. Overall, this study confirms the utility of Li isotopes as a tracer of modern and palaeo-weathering processes, and also has important implications for the specific interpretations of detrital δ7Li values, which may be more sensitive to weathering parameters than previously thought

Rapid releases of metal salts and nutrients following the deposition of volcanic ash into aqueous environments

Deposition of volcanic ash into aqueous environments leads to dissolution of adsorbed metal salts and aerosols, increasing the bioavailability of key nutrients. Volcanogenic fertilization events could increase marine primary productivity, leading to a drawdown of atmospheric CO2. Here we conduct flow-through experiments on unhydrated volcanic ash samples from a variety of locations and sources, measuring the concentrations and fluxes of elements into de-ionized water and two contrasting ocean surface waters. Comparisons of element fluxes show that dissolution of adsorbed surface salts and aerosols dominates over glass dissolution, even in sustained low pH conditions. These surface ash-leachates appear unstable, decaying in situ even if kept unhydrated. Volcanic ash from recent eruptions is shown to have a large fertilization potential in both fresh and saline water. Fluorine concentrations are integral to bulk dissolution rates and samples with high F concentrations display elevated fluxes of some nutrients, particularly Fe, Si, and P. Bio-limiting micronutrients are released in large quantities, suggesting that subsequent biological growth will be limited by macronutrient availability. Importantly, acidification of surface waters and high fluxes of toxic elements highlights the potential of volcanic ash-leachates to poison aqueous environments. In particular, large pH changes can cause undersaturation of CaCO3 polymorphs, damaging populations of calcifying organisms. Deposition of volcanic ash can both fertilize and/or poison aqueous environments, causing significant changes to surface water chemistry and biogeochemical cycles.<br/

Using Mg Isotopes to Estimate Natural Calcite Compositions and Precipitation Rates During the 2010 Eyjafjallajökull Eruption

Chemical weathering of silicate rocks is a key control on the long-term climate, via drawdown of atmospheric CO2. Magnesium isotopes are increasingly being used to trace weathering, but are often complicated by several coincident fractionating processes. Here we examine Mg isotope ratios of waters stemming from beneath lava flows from the 2010 Eyjafjallajökull eruption. Travertine calcite was observed directly precipitating from these high-TDS (total dissolved solids) waters, and were also sampled. This system therefore provides the opportunity to study natural Mg isotope fractionation by calcite. Riverine δ26Mg increase from −2.37 to +0.43% with flow distance, as isotopically light travertine precipitates (δ26Mg = −3.38 to −3.94%). The solution Mg isotope ratios also co-vary with pH, calcite saturation indices and Sr/Ca ratios, strongly indicating that they are dominantly controlled by carbonate precipitation. Using experimental isotopic fractionation factors and the measured δ26Mg values, we can predict the compositions of the precipitated travertines that are within uncertainty of the directly measured travertines. Hence, in some systems, Mg isotopes can be used to quantify carbonate precipitation

The lithium isotope response to the variable weathering of soils in Iceland

This study has analysed Li isotopes ratios from well-studied soil and pore water profiles from Iceland that have the same parent material but have experienced different degrees of chemical weathering. Thus, from least to most weathered, we have analysed vitrosols (V), gleyic andosols (GA), brown andosols (BA), Histosols (H) and Histic Andosols (HA). Although the most weathered H and HA soils have the highest content in clay-sized material, they have the least fractionated δ7Lipore water values. In contrast, the least weathered GA and BA pore waters are most fractionated for Li isotopes. Given that Li isotope ratios are fractionated by clay mineral formation, this appears counter-intuitive. A single trend for all samples of δ7Li as a function of Li/Na ratios suggests that they are all controlled by a process with a single fractionation factor, in this case likely the formation of poorly-crystalline allophane, which dominates in the “least weathered” soils. This rapidly forming secondary mineral dominates Li isotope fractionation over more slowly-forming crystalline clays. The fractionation along a single path shows that the key process here in controlling the Li isotope ratio of surface waters is the degree of Li uptake by secondary minerals. This does not necessarily correspond to the amount of clay minerals present in the soil, but to the amount of clay minerals that are being newly formed in a single passage of the pore water through the soil, or are in equilibrium with soil solutions at the time of sampling

Monitoring of jökulhlaups and element fluxes in proglacial Icelandic rivers using osmotic samplers

The quantification of volatile emissions from volcanoes is an integral part of understanding magmatic systems, with the exsolution and extent of volcanic degassing having a large impact on the nature of an eruption. Measurements of volatiles have traditionally focused on gas emissions into the atmosphere, but volatiles can also become dissolved in proximal water bodies en route to the surface. Thus the monitoring of rivers draining active volcanic areas can provide insights to identifying changes in activity. This process is particularly important for sub-glacial volcanoes in Iceland, where much of the volatile release is transported within glacial outbreak floods, termed jökulhlaups. Monitoring and characterising these phenomena is hampered by the dependence on spot sampling of stochastic events under challenging field conditions, which often leads to bias in the collected data. A recent technological advance is the osmotic sampler, an electricity-free pump that continuously collects water that can subsequently be divided into time-averaged samples. This technique allows for continued and unsupervised deployment of a sampler for weeks to months, representing a cost-efficient form of chemical monitoring. In this study we deployed osmotic samplers in two rivers in southern Iceland. Skálm is a proglacial river from Mýrdalsjökull glacier and Katla volcano, while Skaftá is a larger drainage system from the western part of Vatnajökull glacier. Both rivers are prone to jökulhlaups from geothermal and volcanic sources, and a small jökulhlaup of geothermal origin occurred during the second deployment in Skaftá in January 2014. The two deployments show that osmotic samplers are capable of delivering accurate chemical data in turbulent conditions for several key elements. Total dissolved fluxes for the deployment at Skaftá are calculated to be Na = 9.9 tonnes/day, Mg = 10.5 t/d, Si = 34.7 t/d, Cl = 11.0 t/d, Ca = 31.6 t/d, DIC = 50.8 t/d, and SO4 = 28.3 t/d, with significant elevations of element concentrations during the jökulhlaup. Dissolved fluxes vary considerably on temporal scales from days to seasons, so that spot sampling may miss pulses in concentrations. This is particularly important for elements such as Mn. The continuous geochemical records from the osmotic samplers make it possible to identify pulses of fluxes attributed to sea spray, groundwater, and subglacial sources. The samplers can also be combined with existing methods of river monitoring, such as conductivity and discharge, to accurately assess changes to fluvial chemistry due to volcanic inputs. Moreover, there is the potential to deploy osmotic samplers in a range of other affected water bodies (e.g. wells, springs, lakes) to gain further insights into volcanic processes

Quantifying the impact of riverine particulate dissolution in seawater on ocean chemistry

The quantification of the sources and sinks of elements to the oceans forms the basis of our understanding of global geochemical cycles and the chemical evolution of the Earth's surface. There is, however, a large imbalance in the current best estimates of the global fluxes to the oceans for many elements. In the case of strontium (Sr), balancing the input from rivers would require a much greater mantle-derived component than is possible from hydrothermal water flux estimates at mid-ocean ridges. Current estimates of riverine fluxes are based entirely on measurements of dissolved metal concentrations, and neglect the impact of riverine particulate dissolution in seawater. Here we present 87Sr/86Sr isotope data from an Icelandic estuary, which demonstrate rapid Sr release from the riverine particulates. We calculate that this Sr release is 1.1–7.5 times greater than the corresponding dissolved riverine flux. If such behaviour is typical of volcanic particulates worldwide, this release could account for 6–45% of the perceived marine Sr budget imbalance, with continued element release over longer timescales further reducing the deficit. Similar release from particulate material will greatly affect the marine budgets of many other elements, changing our understanding of coastal productivity, and anthropogenic effects such as soil erosion and the damming of rivers

Rapid CO2 mineralisation into calcite at the CarbFix storage site quantified using calcium isotopes

The engineered removal of atmospheric CO2 is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, in the case of Iceland’s CarbFix experiment, precipitates dissolved CO2 as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO2 injection waters to quantify the amount of carbonate precipitated, and hence CO2 stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO2 were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means

High reactivity of deep biota under anthropogenic CO2 injection into basalt

Basalts are recognized as one of the major habitats on Earth, harboring diverse and active microbial populations. Inconsistently, this living component is rarely considered in engineering operations carried out in these environments. This includes carbon capture and storage (CCS) technologies that seek to offset anthropogenic CO2 emissions into the atmosphere by burying this greenhouse gas in the subsurface. Here, we show that deep ecosystems respond quickly to field operations associated with CO2 injections based on a microbiological survey of a basaltic CCS site. Acidic CO2-charged groundwater results in a marked decrease (by ~ 2.5–4) in microbial richness despite observable blooms of lithoautotrophic iron-oxidizing Betaproteobacteria and degraders of aromatic compounds, which hence impact the aquifer redox state and the carbon fate. Host-basalt dissolution releases nutrients and energy sources, which sustain the growth of autotrophic and heterotrophic species whose activities may have consequences on mineral storage