Kinetic and Thermodynamic Controls of Divalent Metals Isotope Composition in Carbonate: Experimental Investigations and Applications

AbstractThe very contrasting steric and electronic properties of divalent metals (i.e. Ba, Ca, Mg, Sr, Zn, Cu, Cd, Mn, Co, Ni) dramatically affect the reactivity of their aqueous ions in solution and their partitioning between fluids and minerals including calcium carbonates. In this study we show that these contrasting properties result in very distinct kinetic and thermodynamic behaviors of their isotopic fractionation between aqueous fluids and carbonate minerals. For example, because of steric effects, the light isotopes of Ca, Mg, Sr, and Ba, are enriched in precipitated calcite but the extent of Mg isotopes fractionation decreases with increasing calcite growth rate whereas that of Ca, Sr and Ba increases with calcite growth rate. The distinct behavior of Mg stems from the reduced lability of water molecules in its coordination sphere compared to Ca, Ba and Sr. In contrast, the heavy isotopes of Zn (and probably Cu, Cd, Ni) are slightly enriched in precipitated calcite in accord with the great affinity of these metals for the solid (partition coefficient KD > 1), and the extent of their fractionation decreases with increasing calcite growth rates. Moreover, transition metals, especially Cu which is affected by the Jahn-Teller effect, exhibit a strong affinity for RO− ligands and thus a marked dependence of their equilibrium isotope distribution among aqueous fluids and calcite on solution pH, ΣCO2(aq) and the presence of aqueous inorganic and organic ligands. These observations provide new insights into the mechanisms controlling the incorporation of divalent metals into calcite as well as new tools to reconstruct paleo-environmental conditions from their isotope composition recorded in carbonate sediments

The surface area and reactivity of granitic soils: I. Dissolution rates of primary minerals as a function of depth and age deduced from field observations

Surface area-normalised dissolution rates of the primary minerals in two distinct granitic soils located in 1) the Dartmoor National Park, England and 2) Glen Dye, Scotland were determined as a function of depth. Each soil was sampled to a depth of ~ 1 m. The maximum soil ages based on 14C analysis of the humin fraction of the soil are 15,600 and 4400 years for the Dartmoor and Glen Dye soil profiles, respectively. The measured BET surface areas of the soil minerals are close to 5 m2/g in the B and C horizons, but decrease to less than 1 m2/g close to the surface. Retrieved geometric surface area normalised mineral dissolution rates are most rapid at the surface and at the bedrock–soil interface; this behaviour is interpreted to stem from a combination of the approach to equilibrium of the soil waters with depth and more rapid dissolution rates of fresh versus weathered surfaces. At the soil surface, the relative mineral dissolution rate order is found to be quartz > feldspar > mica, with quartz geometric surface area dissolution rates as fast as 2.6 to 4.1 × 10− 13 mol/m2/s. As observed in a number of past studies, field based rates obtained in this study are significantly slower than corresponding rates obtained from laboratory studies, suggesting that these latter rates may not accurately describe the reactivity of primary minerals in soils

Can Mg isotopes be used to trace cyanobacteria-mediated magnesium carbonate precipitation in alkaline lakes?

The fractionation of Mg isotopes was determined during the cyanobacterial mediated precipitation of hydrous magnesium carbonate precipitation in both natural environments and in the laboratory. Natural samples were obtained from Lake Salda (SE Turkey), one of the few modern environments on the Earth's surface where hydrous Mg-carbonates are the dominant precipitating minerals. This precipitation was associated with cyanobacterial stromatolites which were abundant in this aquatic ecosystem. Mg isotope analyses were performed on samples of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments. Laboratory Mg carbonate precipitation experiments were conducted in the presence of purified Synechococcus sp cyanobacteria that were isolated from the lake water and stromatolites. The hydrous magnesium carbonates nesquehonite (MgCO3·3H2O) and dypingite (Mg5(CO3)4(OH)25(H2O)) were precipitated in these batch reactor experiments from aqueous solutions containing either synthetic NaHCO3/MgCl2 mixtures or natural Lake Salda water, in the presence and absence of live photosynthesizing Synechococcus sp. Bulk precipitation rates were not to affected by the presence of bacteria when air was bubbled through the system. In the stirred non-bubbled reactors, conditions similar to natural settings, bacterial photosynthesis provoked nesquehonite precipitation, whilst no precipitation occurred in bacteria-free systems in the absence of air bubbling, despite the fluids achieving a similar or higher degree of supersaturation. The extent of Mg isotope fractionation (?26Mgsolid-solution) between the mineral and solution in the abiotic experiments was found to be identical, within uncertainty, to that measured in cyanobacteria-bearing experiments, and ranges from ?1.4 to ?0.7 ‰. This similarity refutes the use of Mg isotopes to validate microbial mediated precipitation of hydrous Mg carbonate

The rapid resetting of the Ca isotopic signatures of calcite at ambient temperature during its congruent dissolution, precipitation, and at equilibrium

This study provides direct experimental evidence of the resetting of the calcium (Ca) isotope signatures of calcite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over week-long timescales. Batch reactor experiments were performed at 25 °C in aqueous NaCl solutions; air or CO2-gas mixtures were bubbled through this fluid to fix pH. During congruent calcite dissolution, the fluid became enriched in isotopically heavy Ca, and the Ca isotope composition continued to become heavier after the fluid attained bulk chemical equilibrium with the mineral; the δ44/42Ca composition of the fluid was up to 0.8‰ higher than the dissolving calcite at the end of the dissolution experiments. Calcite precipitation was provoked by increasing the reactor fluid pH after chemical equilibrium had been attained via dissolution. Rayleigh isotope fractionation effects were observed immediately after the pH was increased and rapid calcite precipitation occurred. However, isotopic exchange continued after the system chemically equilibrated, eradicating this Rayleigh signal. Taken together, these observations 1) confirm dynamic mineral-fluid equilibrium (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium), and 2) indicate that isotopic compositions of calcite can readily equilibrate even when this mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests the preservation of paleo-environmental isotopic signatures in calcite may require a combination of the isolation of the fluid-mineral system from external chemical input and/or the existence of a yet to be defined calcite dissolution/precipitation inhibition mechanism

The efficient long-term inhibition of forsterite dissolution by common soil bacteria and fungi at earth surface conditions

San Carlos forsterite was dissolved in initially pure H2O in a batch reactor in contact with the atmosphere for five years. The reactive fluid aqueous pH remained relatively stable at pH 6.7 throughout the experiment. Aqueous Mg concentration maximized after approximately two years time at 3x10-5 mol/kg, whereas aqueous Si concentrations increased continuously with time, reaching 2x10-5 mol/kg after 5 years. Element release rates closely matched those determined on this same forsterite sample during short-term abiotic open system experiments for the first 10 days, then slowed substantially such that the Mg and Si release rates are approximately an order of magnitude slower than that calculated from the short-term abiotic experiments. Post-experiment analysis reveals that secondary hematite, a substantial biotic community, and minor amorphous silica formed on the dissolving forsterite during the experiment. The biotic community included bacteria, dominated by Rhizobiales (Alphaproteobacteria), and fungi, dominated by Trichocomaceae, that grew in a carbon and nutrient-limited media on the dissolving forsterite. The Mg isotope composition of the reactive fluid was near constant after 2 years but 0.25‰ heavier in δ26Mg than the dissolving forsterite. Together these results suggest long-term forsterite dissolution in natural Earth surface systems maybe substantially slower that estimated from short-term abiotic experiments due to the growth of biotic communities on their surfaces

The effect of the 2014-15 Bárðarbunga volcanic eruption on chemical denudation rates and the CO2 budget

Publisher's version (útgefin grein)Chemical denudation rates during the 2014–15 Bárðarbunga eruption, calculated using river chemical fluxes, increased substantially confirming that volcanic activity and its products such as fresh lava, and acidic volatiles accelerates these rates. Although the long-term net effect of the combined input of volcanic gases and basalt from the eruption appears to be the overall net drawdown of CO2, it is found that the rapid release of acid gases to surface waters once the basaltic lava comes in contact with surface waters will lead to a short-term release of CO2 from these waters.This study was funded by Ríkislögreglustjórinn Almannavarnadeild – The National Commissioner of the Icelandic Police, Jarðvísindastofnun Háskólans – Institute of Earth Sciences University of Iceland, Veðurstofa Íslands – IMO, and Rannsóknamiðstöð Íslands – The Icelandic Centre for Research RANNÍS (Grant # 163531-051 and 163531-052). The authors would like to thank to all of those who helped to collect the water samples. We also thank all the colleagues and co-workers from Institute of Earth Sciences and IMO for fruitful discussions during the time of the Bárðarbunga unrest.Peer reviewe

Carbon sequestration via enhanced weathering of peridotites and basalts in seawater

Enhanced weathering of mafic and ultramafic rocks has been suggested as a carbon sequestration strategy for the mitigation of climate change. This study was designed to assess the potential drawdown of CO 2 directly from the atmosphere by the enhanced weathering of peridotites and basalts in seawater. Pulverized, and ball milled dunite, harzburgite and olivine basalt were reacted in artificial seawater in batch reactor systems open to the atmosphere for two months. The results demonstrate that the ball-milled dunite and harzburgite changed dramatically the chemical composition of the seawater within a few hours, inducing CO 2 drawdown directly from the atmosphere and ultimately the precipitation of aragonite. In contrast, pulverized but unmilled rocks, and the ball-milled basalt, did not yield any significant changes in seawater composition during the two-month experiments. As much as 10 wt percent aragonite was precipitated during the experiment containing the finest-grained dunite. These results demonstrate that ball milling can substantially enhance the weathering rate of peridotites in marine environments, promoting the permanent storage of CO 2 as environmentally benign carbonate minerals through enhanced weathering. The precipitation of Mg-silicate clay minerals, however, could reduce the efficiency of this carbon sequestration approach over longer timescales

Stable and radiogenic strontium isotope fractionation during hydrothermal seawater-basalt interaction

The fluid-rock interactions occurring in hydrothermal systems at or near mid-oceanic ridges (MOR) were studied experimentally by reacting crystalline and glassy basalt with seawater at 250 °C and 290 °C while monitoring the liquid phase Sr isotopic evolution (87Sr/86Sr and δ88/86Sr). The results indicate that seawater Sr was incorporated into anhydrite during the early stages of seawater-basalt interaction. Liquid 87Sr/86Sr values trend towards the basaltic signature as non-stoichiometric basalt dissolution became the dominant process. This suggests that the interplay between fast Sr incorporation into secondary sulfates versus slow and continuous Sr liberation due to basalt dissolution at intermediate temperatures could partly explain previously identified discrepancies between MOR heat budget constraints and the marine 87Sr/86Sr budget. Late-stage anhydrite re-dissolution, likely caused by the liquid phase becoming more reducing through further basalt dissolution, as well as by quenching of the experiments, represents a potential explanation for the low amounts of anhydrite found in naturally altered oceanic basalt samples. Relatively strong decreases in liquid δ88/86Sr values in experiments with crystalline basalt suggest that isotopically light Sr was preferentially released due to non-stoichiometric dissolution. A slight preference of anhydrite for isotopically heavy Sr (‰εAnhydrite-Liquid88/86=0.034±0.019‰) is indicated by the data, suggesting that changes in MOR spreading rates and Sr removal could be recorded in the isotope compositions of authigenic, sedimentary Sr phases. Such insights will help to constrain the influence of hydrothermal systems on the oceanic stable Sr cycle