Precipitation Of Secondary Carbonates Research Articles

A long-term perspective on coal combustion solid waste interacting with urban soil

Diverse wastes are produced during coal combustion and part of it is recycled worldwide in construction, agriculture, and other industries. As the waste becomes part of the environment, both intentionally and unintentionally, it is important to understand its long-term impact on surrounding soils. In this study, we characterized the situation where such waste, composed of mixed slag, fly ash, gangue, and coke material, was deposited 10–15 years ago as a layer within the urban soil. Two types of soil profiles were identified characterized by a different emplacement of the waste, which either occurred as a 10-cm thick layer or thin seams. Combined chemical and mineralogical analyses of the buried waste showed that slag fragments were affected by the dissolution of primary phases and precipitation of secondary carbonates, Fe-(hydr)oxides, and silica. The impact of the waste was detected in the underlying soils using ratios of trace elements enriched in the waste versus the uppermost soils (e.g., Ni/Pb, V/Zn, Ba/Pb) showing that it might be possible to recognize areas affected by similar deposited waste products. Combined leaching and mineralogical observations showed that V was released, but most probably as particles of unreactive V-rich sulfides, and Ba was preferentially leached from Ba-rich slag glass. Moreover, a combination of mineralogical observations, trace element ratios, and Pb isotopes showed that the thick layer of the waste may have acted not only as a source of potentially toxic elements but also as a trap (insulator) that stopped contamination coming from other sources (e.g., atmosphere).

Experimental investigation of multiple industrial wastes for carbon dioxide removal strategies

Industrial solid waste by-products are being increasingly employed for geochemical carbon dioxide removal (CDR) strategies due to their fine grain size, accessibility, and large annual production tonnages. Here, a range of such by-products has been tested experimentally for their reactivities with CO2 and water. Sample solutions were monitored for 100 h for changes in chemistry, and solid samples were characterised pre- and post-experiment. Samples rich in Ca- and Mg-bearing minerals, such as dunite, kimberlite, and ilmenite mine tailings, as well as marble quarry cuttings, were key cation sources. Ni sulphide, fluorite and borax tailings, coal-fired power plant fly ashes, and red mud samples showed high dissolution rates. The highest reaction rates were often observed during the initial few hours, and compared well to rates determined for rocks typically targeted for CDR purposes, such as basalt and gabbro. Several samples also showed secondary carbonate precipitation, suggesting opportunities for the development of single-step CDR technologies. Overall, the results of this study indicate that several industrial by-products can provide sufficient cations at favourable dissolution rates for geochemical CDR purposes. Any on-site or near-site conditions for reaction acceleration such as heat, concentrated CO2 or microbes, could further increase favourability for geochemical CDR opportunities.

The rare earth elements of sequentially leached phases in the loess-paleosol sequence at Weinan on the southeastern Chinese Loess Plateau

The rare earth elements (REEs) in loess sediments may preserve key information of paleoenvironmental changes in East Asia but remain poorly explored. Here we present the REE compositions of sequentially leached phases of loess sediments from a 16 m long loess-paleosol sequence collected from Weinan at the southern margin of the Chinese Loess Plateau (CLP). The REEs associated with carbonates, easily reducible oxides, reducible oxides, clay minerals, and the residual silicate minerals were sequentially leached with Na-acetate, hydroxylamine-HCl, Na-dithionite/Na-citrate, 12 mol L−1 HCl and concentrated HF-HNO3 solutions. The results show that the REE concentrations of different phases vary in the loess-paleosol layers and are most enriched in the easily reducible phase (except for the residual phase), which is likely explained by the association of REEs with Mn-oxides. Significant Eu anomalies can only be found in the carbonate and easily reducible phases, with the carbonate phase being characterized by appreciable positive Eu anomalies and the easily reducible phase being characterized by negative Eu anomalies, indicating the incorporation of Eu2+ into secondary carbonates that leaves the authigenic Mn-oxides with depleted Eu relative to other REEs. The carbonate phase shows significant negative Ce anomalies, which is likely due to the preferential removal of Ce4+ by Mn oxides before the formation of secondary carbonates. Nevertheless, appreciable positive Ce anomalies for the easily reducible phase can only be found at the bottom of the paleosol S1 while significant negative Ce anomalies were found within the paleosol S1, indicating the downward leaching of Ce4+ that shows stronger complexation with the organic matters than REE3+ in the soil solution. This may partly account for the negative Ce anomalies previously found in the paleosols. Overall, our results suggest a two-stage evolution of Eu and Ce anomalies of the carbonate phase, with the first stage controlled by the formation of both Mn-oxides and secondary carbonates and the second stage dominated by the precipitation of secondary carbonates. The comparison of carbonate Eu and Ce anomalies with other paleoclimatic proxies indicates that the combination of Eu and Ce anomalies of the carbonate fraction of loess may be applied as novel proxy records of climate change in East Asia.

Effects of Secondary Carbonate Precipitation and Dissolution on Changjiang (Yangtze) River Chemistry and Estimates of Silicate Weathering Rates

AbstractRiverine solute chemistry has long been used to infer modern weathering fluxes and processes by assuming that river‐dissolved loads are conservative mixtures of weathering products of different lithologies. Secondary carbonate precipitation and re‐dissolution are important processes that can alter the original weathering fluxes. However, it remains less well understood how secondary carbonates affect the solute chemistry of large rivers and the estimates of silicate weathering rates. Here we report the elemental and Sr‐Mg isotopic compositions of the Changjiang (Yangtze River) water collected biweekly from May 2010 to April 2011 and use this data set to explore the influence of secondary carbonate precipitation and re‐dissolution on the Changjiang river chemistry. Variable Ca/Sr and Ca/Mg ratios but stable 87Sr/86Sr and δ26Mg are observed, which can only be explained by the precipitation and dissolution of secondary carbonates. Across a range of possible Ca/Mg ratios of the primary weathering flux, we estimate that up to 30% of the dissolved Ca carried by Changjiang was controlled by secondary carbonate precipitation and re‐dissolution. Using inverse modeling, we show that secondary carbonate precipitation and re‐dissolution have a minor influence on the calculated silicate weathering rates. Our findings provide new insights into how secondary carbonates affect water chemistry in large river systems, highlighting the role of secondary carbonate precipitation and re‐dissolution in modulating riverine dissolved fluxes in the Changjiang basin.

Implications of the Riverine Response to Enhanced Weathering for CO2 removal in the UK

Enhanced silicate weathering, the application of crushed calcium and magnesium-rich rocks to arable cropland, has been proposed as a potential negative emissions technology for the drawdown of atmospheric CO2. Previous estimates have suggested that enhanced silicate weathering (EW) has the potential to remove significant quantities of CO2, as much as 6–30 Mt CO2 yr−1 for the UK. However, if secondary carbonates are precipitated during the riverine transport of the products of EW, a portion of this CO2 will be re-released, lowering the net carbon dioxide removal (CDR) potential of the mitigation strategy. Here, we assess the fluvial response to EW in the UK by calculating the expected riverine carbonate precipitation due to the dissolution of 10–50 t ha−1 yr−1 of silicate rock on available arable cropland in major UK catchments.Increases in calcite saturation due to the export of soluble cations sourced from EW from soil to rivers are simulated to cross thresholds for spontaneous carbonate precipitation in several major UK river catchments (e.g. the Great Ouse, Thames). Catchments most susceptible to potential secondary carbonate precipitation are those with a high ratio of cropland to water discharge. On average, carbonate precipitation for major UK catchments is expected to make EW 16% (10 t ha−1), 21% (20 t ha−1) or 27% (50 t ha−1) less effective at removing CO2 than that predicted in the absence of such precipitation. Furthermore, when placing strict silica limitations on weathering reactions within soils, the CDR potential of EW is reduced considerably, to 0.23 Mt CO2 yr−1.Although reducing the CDR potential of EW, we suggest that under rapid weathering conditions, carbonate precipitation in UK rivers will prevent pH increases over the safe level for freshwater ecosystems (pH > 9). Together, the simulations suggest that ambient hydrological conditions may make certain UK agricultural areas less effective for CDR by EW and call for methods to quantify secondary carbonate precipitation in response to EW treatments and to further investigate the role of silica saturation in suppressing weathering reactions.

Calcium isotopic geochemistry of geothermal systems in the tectonically active southeastern Tibetan Plateau

The global Calcium (Ca) cycle is closely coupled to the carbon cycle, and Ca isotopes have potential in tracing it. Even though groundwater is one of the main reservoirs of Ca at the Earth's surface, few data are available for groundwater, and the behavior of Ca and its isotopes in geothermal systems remains unknown. Here we analysed the stable Ca and radiogenic Sr isotope compositions of thermal waters distributed along the Jinsha and Yalong river valleys in the southeastern Tibetan Plateau. The Ca isotopic composition of the thermal water ranges from 0.45 to 2.16 ‰ (δ44/40Ca values relative to SRM 915a). The thermal waters collected from carbonate aquifers have higher δ44/40Ca values than bedrocks, which was attributed to secondary carbonate precipitation accompanied by CO2 degassing. In contrast, δ44/40Ca values in thermal waters collected from clastic and igneous rocks are similar to bedrock. Despite some thermal waters undergoing secondary silicates formation and CaNa ion exchange, such processes maybe not play a significant role in governing the Ca isotopic composition of these thermal waters. This suggests that Ca isotopes can be used to trace secondary carbonate precipitation driven by CO2 degassing (e.g. travertine) in geothermal systems located in tectonically active areas.

The efficacy of enhancing carbonate weathering for carbon dioxide sequestration

Enhanced weathering is a geoengineering strategy aiming to increase continental weathering rates, thereby increasing the delivery of atmospheric carbon (as HCO3-) to the oceans. Most enhanced weathering studies focus on the capacity of silicate rocks (e.g., basalt) and minerals (e.g., olivine, Mg2SiO4, or wollastonite CaSiO3) to remove atmospheric CO2. However, carbonate minerals (e.g., calcite, CaCO3) could provide an additional, rapid way to increase HCO3- export to the oceans. Recent studies suggest that 0.84 Gt C yr−1 could be removed from the atmosphere through the enhanced dissolution of calcite in soils, provided carbonic acid is the main dissolution agent. What is not clear is whether atmospheric CO2 dissolved in soils can be transported by rivers, which typically have lower [pCO2], to the oceans. This difference in calcite solubility between soils (where weathering occurs) and rivers (where HCO3- is transported) may lead to large amounts of secondary carbonate formation during transport, releasing the CO2 consumed through dissolution. Here, we present a modeling study comparing the estimated soil dissolution capacity (SDC) in 149 of Earth's largest river basins, to the potential transport capacity of carbon (PTCC) in corresponding rivers. We find the SDC can only be exported to the oceans, without secondary carbonate precipitation, if rivers are in disequilibrium with respect to calcite (i.e., SIc = 1). In this instance, 0.92 Gt C yr−1 may be sequestered above background weathering rates, which is ~20% of annual increases in atmospheric carbon. If rivers are at equilibrium with calcite (i.e., SIc = 0), approximately two-thirds of the carbon dissolved in soil waters are lost due to calcite precipitation in rivers, and just 0.26 Gt of additional atmospheric C yr−1 can be transported to the oceans. Overall, the efficacy of enhanced carbonate weathering is a function of the capacity rivers have for transporting the products from carbonate weathering to the oceans, rather than the dissolution capacity of soils. These findings have implications for the efficiency of enhancing silicate weathering for ocean alkalinity enhancement, as secondary carbonate precipitation during transport is not always considered.

Magnesium isotopic composition of rivers draining karst-dominated regions in Southwest China

Magnesium isotopic composition (δ26Mg) could constrain sources and processes of riverine Mg, thus providing new perspectives on budgeting the global Mg cycle. Carbonate lithology displaying karst covers about one-fifth of the Earth's ice-free continental area, however, Mg isotopic composition and associated fractionation processes in rivers draining carbonate are not well constrained. This study focused on tributaries of the mid-reaches of the Xijiang River, a carbonate dominated river in southwest China. The δ26Mg value of river water shows a large range of −2.23‰ to −0.61‰. The clay formation results in a maximum δ26Mg variation of ~0.3‰ and the influence of secondary carbonate precipitation on river δ26Mg is not observed. Lithology is the primary control of Mg isotopic composition of river water. The relations of Sr/Ca ratio and 87Sr/86Sr, and Mg/Ca ratio and δ26Mg value are suggestive of silicate, limestone, and dolomite weathering sources in the river. However, in some tributaries, the solute sources indicated by Sr and Mg isotopes are different, this could be due to the heterogeneous distribution of lateritic soil and weathering of reactive minerals with distinct Sr and Mg content. Compiling data of large rivers over the world, a relationship between the solute proportion of carbonate weathering and δ26Mg of river water is observed, suggesting that isotopic composition of carbonate weathering Mg flux is −2.3‰ at the continental scale. The result might provide links between riverine δ26Mg and chemical weathering flux in geological history.

Calcium isotopes tracing secondary mineral formation in the high-relief Yalong River Basin, Southeast Tibetan Plateau

Calcium is a critical element in the global carbon cycle due to its role in carbon sequestration via silicate weathering and carbonate formation. Here we apply calcium (δ44/40Ca) and strontium (87Sr/86Sr) isotopes to explore such chemical weathering processes in a river system draining a diverse range of geologic and climatic environments: the Yalong River, China, and its tributaries. This river originates on the Tibetan Plateau and represents one of the upper reaches of the Changjiang River, China. The Ca isotopic composition of the dissolved load of the Yalong River ranges from 0.60‰ to 1.02‰ (relative to the NIST standard SRM 915a). Higher δ44/40Ca values were found in the plateau and lowland rivers, with lower values in the mountainous rivers. Correlations between riverine dissolved δ44/40Ca values, Sr/Ca ratios, and calcite saturation index indicate that the precipitation of secondary carbonates governs the Ca isotopic composition and carbon transformation in most of this river system. However, such correlations are not seen in the lowland tributaries, where the relationship between δ44/40Ca and lithium (Li) isotopes instead suggests a control by topography and climate, via secondary clay mineral formation. Specifically, heavy rainfall in the lowland regions lowers the pH of the soil solution, which inhibits the precipitation of secondary carbonates. In addition, the flat terrain and thick soils increase the time for water-rock interaction, which favours the formation of secondary clay minerals that preferentially incorporate the lighter Ca isotopes. Overall, this study highlights the potential of stable Ca isotopes, when used in combination with other isotope systems (e.g. Sr and Li isotopes), to quantify secondary mineral formation processes in large river basins.

Conditions and processes controlling carbon mineral trapping in intraformational baffles

Carbon mineral trapping is commonly believed to be insignificant relative to other trapping mechanisms, yet, the role of cm-scale intraformational baffles is rarely considered in reservoir scale assessments. The present study aims to reveal conditions governing carbon mineral trapping and its quantification for a range of rock compositions in intraformational baffles. A series of 2-D multiphase reactive transport simulations were set-up to model the geochemical processes occurring at the interface of a baffle and a reservoir rock for a period of 1000 years. The study shows that the presence of baffles with certain rock properties within the reservoir rock can enhance or reduce carbon mineral trapping capacity relative to homogeneous reservoir rocks depending on clay and carbonate mineral content in baffles. The analysis shows that the precipitation of secondary carbonates occurs at the interface of the two rock types. Furthermore, it was found that higher clay and lower carbonate content of the baffle contribute towards enhancing mineral trapping while the mineralogy of reservoir rock does not have an important influence. The results are further utilized to derive averaged mineral composition for the integration of fine scale processes in coarser scale simulations.

Multiple stages of plant root calcification deciphered by chemical and micromorphological analyses.

Rhizoliths, that is, roots fossilized by secondary carbonates, have been known for ages and are increasingly used for paleoenvironmental reconstructions. However, knowledge about their formation mechanisms remains limited. This study reports the mineralogical and chemical characterization of rhizoliths at different stages of mineralization and fossilization in the Late Pleistocene loess-paleosol sequence of Nussloch (SW Germany). Scanning electron microscopy coupled with elemental mapping and 13 C solid-state nuclear magnetic resonance were used to concomitantly characterize the mineral and organic matter of the rhizoliths. These joint analyses showed for the first time that large rhizoliths are not necessarily remains of single large roots but consist of numerous microrhizoliths as remains of fine roots, formed mainly by calcium carbonates with only low amounts of Mg and Si. They further revealed that the precipitation of secondary carbonates occurs not only around, but also within the plant root and that fossilization leads to the selective preservation of recalcitrant root biopolymers-lignin and suberin. The precipitation of secondary carbonates was observed to occur first around fine roots, the epidermis acting as a first barrier, and then within the root, within the cortex cells, and even sometimes around the phloem and within the xylem. This study suggests that the calcification of plant roots starts during the lifetime of the plant and continues after its death. This has to be systematically investigated to understand the stratigraphic context before using (micro)rhizoliths for paleoenvironmental reconstructions in terrestrial sediments.

Early diagenesis of foraminiferal calcite under anoxic conditions: A case study from the Landsort Deep, Baltic Sea (IODP Site M0063)

The chemical composition of foraminiferal calcite is widely used for studying past environmental conditions and biogeochemistry. However, high rates of microbial organic matter degradation and abundant dissolved metal sources in sediments and pore waters may impede the application of foraminifera-based proxies due to formation of secondary carbonates or other authigenic minerals on and/or inside of foraminiferal tests. Secondary carbonate precipitation severely alters the bulk foraminiferal geochemistry and can be difficult to eliminate through standard foraminiferal trace element cleaning procedures. We present results showing the mineral composition and formation sequence of diagenetic coatings on foraminiferal tests formed under extreme anoxic conditions in the Baltic Sea's deepest basin (the Landsort Deep, IODP Exp. 347, Site M0063). Our study focuses primarily on the diagenetic carbonates present on and in the tests of the low-oxygen tolerant benthic foraminiferal species Elphidium selseyensis and Elphidium clavatum. We applied various geochemical and imaging methods to ascertain the diagenesis processes and the authigenic mineral formation sequence on foraminifera. The authigenic carbonates were enriched in Mn, Mg, Fe and Ba, depending on the environmental redox conditions when the authigenic carbonates were precipitated. Concentrations of redox-sensitive elements such as Mn and Fe were particularly elevated in bottom waters and sedimentary pore waters under oxygen-depleted conditions in the Landsort Deep, resulting in formation of carbonates with highly elevated Mn and Fe contents. In addition to Mn- and Fe carbonates, several other types of authigenic minerals also formed on and in the foraminiferal chambers, including authigenic calcite, and non-carbonate accessory minerals. The formation sequence reveals the redox sensitivities of different elements and the preferential sequence of oxidants used by the microbes during organic matter oxidation and secondary redox reactions. This study provides a case study of extreme early diagenesis of foraminiferal calcite and may serve as a valuable guide when interpreting foraminiferal trace element records from low oxygen environments.

Biogeochemical Controls on 13CDIC Signatures from Circum-Neutral pH Groundwater in Cu–W–F Skarn Tailings to Acidic Downstream Surface Waters

Regular features of ground and surface waters affected by drainage from mine waste include their acidity and elevated concentrations of dissolved metals, with their attendant negative effects on drinking water quality and aquatic life. One parameter that aids in buffering these waters against acidity and sustains aquatic life is dissolved inorganic carbon (DIC). In this study, the chemical and isotopic (δ13C) composition of primary calcite and DIC (δ13CDIC) in groundwater and surface waters within and downstream, respectively, of abandoned Cu–W–F skarn tailings at Yxsjöberg, Sweden, were used to trace the biogeochemical processes controlling their respective δ13CDIC signatures. In addition, the δ13C signatures of the inorganic (carbonate) fractions of the tailings were used to verify the formation of secondary carbonates within the tailings. Lower average δ13C values of the carbonate fractions (δ13Ccarb = −2.7‰) relative to those of the primary calcite (δ13C = +0.1‰) from the orebodies from which the tailings originated pointed to the precipitation of secondary carbonates. These lower δ13Ccarb signatures were assumed to represent mixed-source C signals involving isotopically light CO2 from the atmosphere, the degradation of organic matter in the upper part of the tailings and HCO3− from calcite dissolution. The groundwater δ13CDIC values (−12.6‰ to −4.4‰) were far lower than the hypothetical range of values (−4.6‰ to +0.7‰) for primary calcite and secondary carbonate dissolution. These signatures were attributed to carbonate (calcite and secondary carbonate) dissolution and the degradation of dissolved organic carbon (DOC) from various organic sources such as peat underneath the tailings and the surrounding forests. Downstream surface water samples collected in May had low δ13CDIC values (−16‰) and high DOC (14 mg C/L) compared to the groundwater samples. These signatures represented the oxidation of the DOC from the wash out of the mires and forests during the snowmelt and spring flood. The DOC and δ13CDIC values of the surface waters from June to September ranged from 6–15 mg·C/L and −25‰ to −8.6‰, respectively. These signatures were interpreted to reflect mixed C sources, including carbonate dehydration by acidity from Fe3+ hydrolysis due to the mixing of groundwater with surface waters and the subsequent diffusive loss of CO2 (g), aquatic photosynthesis, photooxidation, DOC degradation, as well as microbial respiration. Although the 13CDIC signatures of the downstream surface waters seemed to be seasonally controlled and influenced by variable groundwater contributions, the lack of data with respect to DIC concentrations, coupled with multiple potential biogeochemical processes that could influence the DIC pool and 13CDIC values, made it difficult to identify the major regulating process of the 13CDIC signatures. Therefore, other complimentary isotopes and elemental concentrations are recommended in order to decipher the dominant biogeochemical process.

Predicting the potential for mineral scale precipitation in unconventional reservoirs due to fluid-rock and fluid mixing geochemical reactions

Mineral precipitation within hydraulically fractured shale may affect fluid flow pathways and impact long-term hydrocarbon production. The ability to predict geochemical reactions causing problematic mineral precipitation will lead to active reservoir management strategies for improving production. Using the Marcellus Shale as a case study, laboratory experiments and reaction path modeling were applied to determine which reactions occur during hydraulic fracturing, shut-in, flowback, and production timeframes. Experimental results indicate that contact between fracturing fluid and shale will result in dissolution of primary minerals (carbonates, quartz, feldspars, kaolinite, chlorite, pyrite) and secondary mineral precipitation over time periods of less than one week. Precipitation of secondary carbonates, barite, iron oxides, feldspars, amorphous silica and clay is likely to occur within the reservoir during shut in and early flowback due to mixing between fracturing fluid and reservoir brine as based on modeling saturation indices using experimental fluid data. Reaction path modeling corroborates the dissolution and precipitation reactions observed experimentally. Comparison of the results to injected and produced waters from a Marcellus Shale well pad in Greene County, PA, USA, shows that the mineral reactions occur during the hydraulic fracturing, shut in, and early flowback periods. The results presented here demonstrate the value in applying experimental approaches to identify mineral precipitation/dissolution reactions that may significantly impact reservoir performance. The good agreement between geochemical models and experimental results provides confidence that numerical models can be applied to screen the potential fluid-mineral and fluid-mixing reactions in unconventional reservoirs that result in undesired mineral scale precipitation.

Ca isotope constraints on chemical weathering processes: Evidence from headwater in the Changjiang River, China

This study aims to clarify the relationship between chemical weathering of rocks and the carbon budget of rivers and better understand the weathering mechanisms of plateau watersheds. We chose to study the Jinsha River, which originates from the Tibetan Plateau and also is in the upper reaches of the Changjiang River. Analysis of hydrochemistry, radiogenic strontium isotope and stable calcium isotopes were conducted of the Jinsha River water samples, which were collected along its mainstream and main tributaries in the summer. The results show that the water chemistry of the mainstream waters is dominated by evaporite weathering, which have low 87Sr/86Sr values (0.7098–0.7108) and wide range of Sr contents (2.70–9.35μmol/L). In contrast, tributaries of the Jinsha River have higher 87Sr/86Sr (0.7090–0.7157) and lower Sr contents (∼1μmol/L). Moreover, the Ca isotopic compositions in the mainstream (0.87–1.11‰) are heavier than the tributaries (0.68–0.88‰) and could not be fully explained by the conventional mixing of different sources. We suggest that secondary carbonate precipitation fractionates Ca isotopes in the Jinsha River, and fractionation factors are between 0.99935 and 0.99963. At least 66% of Ca was removed in the mainstream of the Jinsha River through secondary mineral precipitation, and the average value is ∼35% in the tributaries. The results highlight that evaporite weathering results in more carbonate precipitation influencing Ca transportation and cycling in the riverine system constrained by stable Ca isotopic compositions and water chemistry.

Multi-elemental composition of authigenic carbonates in benthic foraminifera from the eastern Bering Sea continental margin (International Ocean Discovery Program Site U1343)

Bering Sea sediments represent exceptional archives, offering the potential to study past climates and biogeochemistry at a high resolution. However, abundant hydrocarbons of microbial origin, especially along the eastern Bering Sea continental margin, can hinder the applicability of palaeoceanographic proxies based on calcareous foraminifera, due to the formation of authigenic carbonates. Nonetheless, authigenic carbonates may also bear unique opportunities to reconstruct changes in the sedimentary redox environment.Here we use a suite of visual and geochemical evidence from single-specimens of the shallow infaunal benthic foraminiferal species Elphidium batialis Saidova (1961), recovered from International Ocean Discovery Program (IODP) Site U1343 in the eastern Bering Sea, to investigate the influence of authigenic carbonates on the foraminiferal trace metal composition. Our results demonstrate that foraminiferal calcite tests act as a nucleation template for secondary carbonate precipitation, altering their geochemistry where organoclastic sulphate reduction and anaerobic oxidation of methane cause the formation of low- and high-Mg calcite, respectively. The authigenic carbonates can occur as encrusting on the outside and/or inside of foraminiferal tests, in the form of recrystallization of the test wall, or as banding along natural laminations within the foraminiferal test walls. In addition to Mg, authigenic carbonates are enriched in U/Ca, Mn/Ca, Fe/Ca, and Sr/Ca, depending on the redox environment that they were formed in. Our results demonstrate that site-specific U/Ca thresholds are a promising tool to distinguish between diagenetically altered and pristine foraminiferal samples, important for palaeoceanographic reconstructions utilising the primary foraminiferal geochemistry. Consistent with previous studies, U/Mn ratios of foraminifera at IODP Site U1343 increase according to their degree of diagenetic alteration, suggesting a potential response of authigenic U/Mn to the microbial activity in turn linked to the sedimentary redox environment.

Stable isotopes of nonsulphide Zn–Pb ores in Britain and Ireland: fluid characteristics and palaeoclimatic variability

Stable isotope (C–O) analysis was applied to smithsonites and cerussites from Zn(Pb) nonsulphide ores from Britain and Ireland, to determine the nature of the fluids responsible for the precipitation of secondary carbonates, and any relationship with palaeoclimatic conditions. The carbon isotope compositions of the Irish smithsonites show a major contribution of organic carbon whereas those of both Zn- and Pb-carbonates in the UK deposits indicate contribution of organic carbon and host-rock carbonates. δ18OVSMOW values for cerussite from both areas are about 13‰ lower than those of smithsonites, UK smithsonite δ18OVSMOW values are more variable than those for cerussites and the Irish smithsonites have unusually high values. Isoscape maps indicate that δ18O values for both carbonates are higher in the west and decrease toward the east. This trend correlates with a general decrease in the oxygen isotope compositions of rainwater moving eastward, and suggests that oxidation of sulphides occurred at temperatures of about 15–20°C, under climatic conditions warmer than today. Supplementary material: A comprehensive overview of the investigated sites, detailed data for the samples and SEM images are available at https://doi.org/10.6084/m9.figshare.c.4579535

CaveCalc: A new model for speleothem chemistry & isotopes

CaveCalc is a PHREEQC-based numerical model for cave dripwater and speleothem chemistry, designed to aid in the interpretation of speleothem palaeoclimate records and cave monitoring datasets.CaveCalc forward-models dripwater and carbonate chemistry and isotopes through a variety of soil, karst and cave processes. Such processes include soil water & gas equilibration, carbonate bedrock dissolution, secondary carbonate precipitation and CO2 degassing. CaveCalc is able to quantitatively model bedrock dissolution under semi-open conditions — a feature necessary to accurately simulate the gas chemistry occurring in real cave environments.The model allows coupled modelling of multiple proxy systems, including δ18O, δ13C, a14C, δ44/40Ca and trace-elements (Mg/Ca, Sr/Ca, Ba/Ca), within a single framework. Additional proxy systems, chemical processes and calibration data may be added to the model as required.

Element mobilization and redistribution under extreme tropical weathering of basalts from the Hainan Island, South China

Chemical weathering of rocks has substantial influence on the global geochemical cycle. In this paper, the geochemical profile of a well-developed basalt weathering profile (>15 m thick, including soil, saprolite, semi-weathered rock and fresh basalt) on the Island of Hainan (South China) was presented. The soil and saprolite samples from this profile are characterized by high Al2O3 and Fe2O3 concentrations (up to 32.3% and 28.5%, respectively). The mineral assemblage is dominated by kaolinite, Fe-oxides/-hydroxides and gibbsite (or boehmite), indicating extensive desilicate and ferrallitic weathering. The acidic and organic-rich environment in the soil horizon may have promoted elemental remobilization and leaching.The strongest SiO2 depletion and Al2O3 enrichment at about 2.4 m deep indicate that the main kaolinite hydrolysis and gibbsite formation occurred near the soil-saprolite interface. The mild Sr reconcentration at about 3.9 m and 7.1 m deep may be attributed to secondary carbonate precipitation. Mn-oxides/-hydroxides precipitated at 6.1 m deep, accompanied by the strongest enrichment of Ba and Co. Uranium is mildly enriched in the middle part (about 7.1 m and 9.1 m deep) of the weathering profile, and the enrichment may have been caused by the decomposition of uranyl carbonates or the accumulation of zircon. Immobile element (i.e., Zr, Hf, Nb, Ta, Th and Ti) distributions at different depths are mainly controlled by secondary Fe-oxides/-hydroxides, and follow the stability sequence of Nb ≈ Ta ≈ Th > Zr ≈ Hf > Ti. The limited thickness (∼15 cm) of the semi-weathered basalt horizon at the rock-regolith interface (15.28 m deep) suggests that plagioclase and pyroxene are readily altered to kaolinite, smectite and Fe-oxides under tropical climate. The marked enrichment of transitional metals (such as Cu, Zn, Ni, and Sc) along the rock-regolith interface may have associated mainly with increasing pH values, as well as the dissolution of primary apatite and formation of secondary phosphates. Our findings highlight the importance of secondary phosphates in the redistribution of transition metals, and in the possible Mg, Cu, and Ni isotopic fractionation under extreme weathering of basalt in tropic climate.

A climosequence of chronosequences in southwestern Australia

SummaryTo examine how climate affects soil development and nutrient availability over long timescales, we studied a series of four long‐term chronosequences along a climate gradient in southwestern Australia. Annual rainfall ranged from 533 to 1185 mm (water balance from −900 to +52 mm) and each chronosequence included Holocene (≤ 6.5 ka), Middle Pleistocene (120–500 ka) and Early Pleistocene (∼2000 ka) dunes. Vegetation changed markedly along the climosequence, from shrubland at the driest site to Eucalyptus forest at the wettest. Soil pH was similar in the youngest soil of each chronosequence, although the carbonate and P contents of the parent sand declined from dry to wet along the climosequence, presumably linked to variation in offshore productivity. Despite this, soil development and associated nutrient status followed remarkably consistent patterns along the four chronosequences. Pedogenesis involved decalcification and secondary carbonate precipitation in Holocene soils and leaching of iron oxides from Middle Pleistocene soils, leading ultimately to bleached quartz sands in the oldest soils. Along all chronosequences soil pH and total P declined, whereas C:P and N:P ratios increased, which is consistent with the predicted change from N to P limitation of vegetation during ecosystem development. The expected unimodal pattern of leaf area index was most pronounced along wetter chronosequences, suggesting an effect of climate on the expression of retrogression. The four chronosequences do not appear to span a pedogenic climate threshold, defined as an abrupt change in soil properties across a relatively small change in climate, because exchangeable phosphate and base cations declined consistently during long‐term pedogenesis. However, the proportion of total P in organic form was greater along wetter chronosequences. We conclude that soil and nutrient availability on the coastal sand plains of southwestern Australia change consistently during long‐term pedogenesis, despite marked variation in modern vegetation and climate. The four chronosequences provide a rare soil‐age × climate framework within which to study long‐term ecosystem development.Highlights We describe four long‐term coastal dune chronosequences spanning a climate gradient in a global biodiversity hotspot. Pedogenesis involves depletion of phosphorus and cations linked to decalcification and subsequent podsolization. Climate has relatively little effect on patterns of nutrient availability during ecosystem development along the climosequence. The age by climate framework enables study of effect of edaphic change on above‐ and below‐ground communities.