An emulation-based approach for interrogating reactive transport models

We present a new approach to understand the interactions among different chemical and biological processes modelled in environmental reactive transport models (RTMs) and explore how the parameterisation of these processes influences the results of multi-component RTMs. We utilize a previously published RTM consisting of 20 primary species, 20 secondary complexes, 17 mineral reactions and 2 biologically-mediated reactions which describes bio-stimulation using sediment from a contaminated aquifer. We choose a subset of the input parameters to vary over a range of values. The result is the construction of a new dataset that describes the model behaviour over a range of environmental conditions. Using this dataset to train a statistical model creates an emulator of the underlying RTM. This is a condensed representation of the original RTM that facilitates rapid exploration of a broad range of environmental conditions and sensitivities. As an illustration of this approach, we use the emulator to explore how varying the boundary conditions in the RTM describing the aquifer impacts the rates and volumes of mineral precipitation. A key result of this work is the recognition of an unanticipated dependency of pyrite precipitation on pCO2 in the injection fluid due to the stoichiometry of the microbially-mediated sulphate reduction reaction. This complex relationship was made apparent by the emulator, while the underlying RTM was not specifically constructed to create such a feedback. We argue that this emulation approach to sensitivity analysis for RTMs may be useful in discovering such new coupled sensitives in geochemical systems and for designing experiments to optimise environmental remediation. Finally, we demonstrate that this approach can maximise specific mineral precipitation or dissolution reactions by using the emulator to find local maxima, which can be widely applied in environmental systems.</p

Calcium isotope fractionation in sedimentary pore fluids from ODP Leg 175: Resolving carbonate recrystallization

We present calcium isotope data from pore fluids and solids from Ocean Drilling Program Leg 175: Sites 1081 and 1086 (off the coast of West Africa). These sites are similar with respect to geographic location, sediment age (from modern to 8 Myr), and water depth (800 m), but Site 1081 is carbonate-poor, whereas Site 1086 is carbonate-rich. Therefore, these sites are suited for the exploration of the influence of sediment type on carbonate dissolution, precipitation, and recrystallization. We use two numerical modelling approaches to explore the rates of carbonate dissolution and precipitation in the sediment column. The first is the standard diffusion-reaction approach, using the strontium concentration within the pore fluid to ascertain a dissolution rate for the carbonate, which is then applied to a second model of calcium isotopes within the pore fluid to calculate precipitation rates. Given the high sedimentation rates we also apply an advection-reaction model (Huber et al., 2017) which results in the same depth distribution of carbonate precipitation but significantly higher overall rates, which is discussed. Calcium isotope ratios in pore fluid calcium increase in zones where our model predicts carbonate precipitation, and approach isotopic equilibrium with the solid in zones where our model predicts equivalent rates of dissolution and precipitation, similar to previous findings. Contrary to previous findings in marine sediments, our model requires a calcium isotope fractionation on carbonate precipitation to fit the data, as there is an offset between the δ44Ca of the fluid and the solid. Using the zones of carbonate precipitation determined from the models and previously published carbon isotope profiles of the dissolved inorganic carbon from these sites, we suggest that the δ13C of the authigenic carbonate is uniformly lower than biogenic carbonate but by a wider range than was previously suggested

Diagenesis in salt dome roof strata: barite - calcite assemblage in Jebel Madar, Oman

Halokinesis causes a dynamic structural evolution with the development of faults and fractures, which can act as either preferential fluid pathways or barriers. Reconstructing reactive fluid flow in salt dome settings remains a challenge. This contribution presents for the first time a spatial distribution map of diagenetic phases in a salt dome in northern Oman. Our study establishes a clear link between structural evolution and fluid flow leading to the formation of diagenetic products (barite and calcite) in the salt dome roof strata. Extensive formation of diagenetic products occurs along NNE-SSW to NE-SW faults and fractures, which initiated during the Santonian (Late Cretaceous) and were reactivated in the Miocene, but not along the E-W fault, which was generated during Early Paleocene time. We propose that the diagenetic products formed by mixing of a warm (100 °C) saline (17 wt% NaCl eq.) 87Sr enriched (87Sr/86Sr: 0.71023) fluid with colder (35 °C) meteoric fluid during Miocene to Pleistocene. The stable sulphur and strontium isotope composition and fluid inclusion data indicate that a saline fluid, with sulfate source derived from the Ara Group evaporite and Haima Supergroup layers, is the source for barite formation at about 100 °C, predominantly at fault conjunctions and minor faults away from the main graben structure in the dome. In the Miocene, the saline fluid probably ascended along a halokinesis-related fault due to fluid overpressure (due to the rising salt and impermeable layers in the overlying stratigraphic sequence), and triggered the formation of barite due mixing with barium-rich fluids, accompanied by a drop in temperature. Subsequently, evolving salt doming with associated fault activity and erosion of the Jebel allows progressively more input of colder meteoric fluids, which mix with the saline warmer fluid, as derived from stable isotope data measured in the progressively younger barite-associated calcite, fault zone calcite and macro-columnar calcite. The reconstructed mixing model indicates a 50/50 to 90/10 meteoric/saline fluid mixing ratio for the formation of fault zone calcite, and a 10 times higher concentration of carbon in the saline fluid end member compared to the meteoric fluid end member. The presented mixing model of salt-derived fluids with meteoric fluids is suggested to be a general model applicable to structural diagenetic evolution of salt domes world wide

Calcium isotopes as a record of the marine calcium cycle versus carbonate diagenesis during the late Ediacaran

Calcium isotope ratios in ancient carbonate rocks can provide insight into the global marine calcium cycle as well as local conditions during carbonate mineral precipitation and diagenesis. We compare two extraction techniques for the separation of calcium from other ions before δ44Ca analysis, using an automated ion chromatograph and using manual gravity columns. The two techniques produce the same δ44Ca within error (2σ). We present 31 δ44Ca analyses of carbonate rocks from the Nama Group, Namibia, which record a negative shift in δ44Ca of 0.35‰ between ∼550 and ∼547 Ma, from −1.25‰ to −1.60‰, followed by persistently low δ44Ca (−1.48 ± 0.06‰) between ∼547 and 539 Ma. Very low δ44Ca (&lt;−1.5‰) are commonly interpreted to represent the preservation of local aragonite that has recrystallized to calcite under sediment-buffered conditions (where the composition of the diagenetic carbonate product is determined mainly by the original sediments). The shift in δ44Ca across the Nama Group could therefore represent a change from fluid-buffered diagenesis (where the composition of the diagenetic carbonate mineral is determined mainly by the fluid) to sediment-buffered diagenesis. However, this interpretation is not consistent with either potential geochemical indicators of diagenesis (e.g., δ18O), or changes in large-scale fluid-flow as predicted from sequence stratigraphy. We consider alternative interpretations for generating changes in the δ44Ca of ancient carbonate rocks including enhanced continental weathering, increases in evaporite deposition, and changes in the style of dolomitisation

The Calcium Isotope Systematics of the Late Quaternary Dead Sea Basin Lakes

We report the calcium isotopic composition (δ44Ca) of primary aragonite laminae, primary gypsum, and secondary gypsum in sediments deposited from Lake Lisan, the last glacial cycle of the Dead Sea (70–14.5 ka). The δ44Ca of primary gypsum varies between 0.17‰ and 0.71‰ versus bulk silicate earth, with an average of 0.29‰, whereas the aragonite δ44Ca varies between −0.68‰ and −0.16‰ with an average of −0.4‰. The secondary gypsum δ44Ca is close to the calcium isotope composition of the aragonite, averaging at −0.3‰. The aragonite δ44Ca shows small variations temporally in sync with lake level fluctuations, suggesting the aragonite δ44Ca reflects changes in the lake calcium balance, which in turn reflects changes in the local hydrological cycle. The secondary gypsum calcium isotope composition (−0.3‰) overlaps with that of coeval aragonite, suggesting the calcium for secondary gypsum was derived from the aragonite through quantitative, or near‐isotopic equilibrium, recrystallization of the aragonite to gypsum after the lake desiccation and exposure of sediments during the Holocene. A numerical box model is used to explore the effect of changing lake water levels on the calcium isotope composition of the aragonite and gypsum in the lake. The relatively low variability in the δ44Ca over the lake's history suggests that a high‐concentration calcium‐rich brine buffers the calcium cycle

Groundwater springs formed during glacial retreat are a large source of methane in the high Arctic

Permafrost and glaciers in the high Arctic form an impermeable ‘cryospheric cap’ that traps a large reservoir of subsurface methane, preventing it from reaching the atmosphere. Cryospheric vulnerability to climate warming is making releases of this methane possible. On Svalbard, where air temperatures are rising more than two times faster than the average for the Arctic, glaciers are retreating and leaving behind exposed forefields that enable rapid methane escape. Here we document how methane-rich groundwater springs have formed in recently revealed forefields of 78 land-terminating glaciers across central Svalbard, bringing deep-seated methane gas to the surface. Waters collected from these springs during February–May of 2021 and 2022 are supersaturated with methane up to 600,000 times greater than atmospheric equilibration. Spatial sampling reveals a geological dependency on the extent of methane supersaturation, with isotopic evidence of a thermogenic source. We estimate annual methane emissions from proglacial groundwaters to be up to 2.31 kt across the Svalbard archipelago. Further investigations into marine-terminating glaciers indicate future methane emission sources as these glaciers transition into fully land-based systems. Our findings reveal that climate-driven glacial retreat facilitates widespread release of methane, a positive feedback loop that is probably prevalent across other regions of the rapidly warming Arcti

The calcium isotopic composition of carbonate hardground cements: A new record of changes in ocean chemistry?

Reconstructing changes in the calcium isotopic composition (δ44Ca) of the ocean over Earth history has been challenging. This difficulty is due to the large range of calcium isotope fractionation factors during mineral precipitation and the potential for overwriting the initial δ44Ca of minerals during shallow marine diagenesis. We present a new δ44Ca record measured in carbonate hardground cements, an inorganic carbonate-mineral precipitate that rapidly forms at or near the sediment-water interface. The range in the δ44Ca for any particular carbonate hardground cements is between 0.05 and 0.56‰. In some cases, the progressive increase in the δ44Ca during precipitation can be observed, consistent with precipitation in a ‘closed-system’. Our data show an average calcium isotope fractionation during carbonate hardground cement precipitation that is −0.57 ± 0.27‰, similar to the calcium isotope fractionation factor for inorganic calcite precipitates in previous laboratory and modelling studies, and closer to what is considered a kinetic end member calcium isotope fractionation than growth at equilibrium. This is consistent with the rapid carbonate mineral precipitation expected for carbonate hardground cements. Our δ44Ca record over the Phanerozoic is similar to other calcium-bearing mineral records over the same time interval, with average δ44Ca becoming lower going back in time by about 0.5 to 0.7‰. Our results add further support for the evolution of seawater δ44Ca over time, and we discuss the possible causes of these changes with suggestions for future studies

Calcium isotope fractionation during microbially induced carbonate mineral precipitation

We report the calcium isotope fractionation during the microbially-induced precipitation of calcium carbonate minerals in pure cultures of the marine sulfate-reducing bacterium Desulfovibrio bizertensis. These data are used to explore how the calcium isotope fractionation factor during microbially-induced carbonate mineral precipitation differs from the better-constrained calcium isotope fractionation factors during biogenic or abiotic carbonate mineral precipitation. Bacterial growth was then modulated with antibiotics, and the evolution of δ44Ca in solution was monitored under different microbial growth rates. The faster the microbial growth rate, the larger the calcium isotope fractionation during carbonate mineral precipitation, with Δ44Ca(s-f) ranging from −1.07‰ to −0.48‰. The reported calcium isotope fractionation can help us understand the link between calcium isotope fractionation and microbial metabolism in carbonate minerals precipitated during sedimentary diagenesis

Modelling the Effects of Non-Steady State Transport Dynamics on the Sulfur and Oxygen Isotope Composition of Sulfate in Sedimentary Pore Fluids

We present the results of an isotope-enabled reactive transport model of a sediment column undergoing active microbial sulfate reduction to explore the response of the sulfur and oxygen isotopic composition of sulfate under perturbations to steady state. In particular, we test how perturbations to steady state influence the cross plot of δ34S and δ18O for sulfate. The slope of the apparent linear phase (SALP) in the cross plot of δ34S and δ18O for sulfate has been used to infer the mechanism, or metabolic rate, of microbial metabolism, making it important that we understand how transient changes might influence this slope. Tested perturbations include changes in boundary conditions and changes in the rate of microbial sulfate reduction in the sediment. Our results suggest that perturbations to steady state influence the pore fluid concentration of sulfate and the δ34S and δ18O of sulfate but have a minimal effect on SALP. Furthermore, we demonstrate that a constant advective flux in the sediment column has no measurable effect on SALP. We conclude that changes in the SALP after a perturbation are not analytically resolvable after the first 5% of the total equilibration time. This suggests that in sedimentary environments the SALP can be interpreted in terms of microbial metabolism and not in terms of environmental parameters

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis