Chemical Geology最新文献

Oceanic anoxia is regarded as the immediate cause of the Late Devonian Frasnian–Famennian (F–F) bio-crisis and Annulata bio-event. The oceanic anoxia has been linked to continental weathering and input. However, some previous findings indicate oxic marine environments during the F–F bio-crisis. Moreover, the relative impact of terrestrial input on the Late Devonian oceanic redox conditions, when comparing a bio-crisis and a bio-event, remains unknown . Here, we present a decoupled oceanic redox model suggesting the redox conditions were different between shallow and deep water columns during the F–F bio-crisis, i.e., intensified euxinia in the photic zone and a more oxygenated condition at the seafloor. Geochemical (biomarker and trace metal) and sedimentologic evidence from the Late Devonian Seaway, eastern U.S., indicate that the shallow-deep decoupled oceanic redox conditions were regulated by terrestrial weathering and input. During the F–F bio-crisis, land nutrient input intensified photic zone euxinia through eutrophication, while hyperpycnal flows oxygenated seafloors. After the F–F bio-crisis, the seafloor redox conditions shifted from oxygenation to silled euxinia, and the watermass became hydrographically restricted. During this post-F–F interval, the euxinia at the seafloor intensified due to land nutrient runoff during the Annulata bio-event. Importantly, the land weathering and input during the F–F bio-crisis were more prominent than during the Annulata bio-event. This resulted in widespread and intense photic zone euxinia during the F–F bio-crisis, relative to the seafloor euxinia that developed in restricted areas during the Annulata bio-event. Our decoupled oceanic redox model may explain the preferential decimation of shallow-water organisms during the F–F bio-crisis. The seafloor euxinia during the Annulata bio-event may provide a niche for the turnover of benthic faunas adapting to low oxygen levels. This study suggests that continental weathering and influxes played an important role in regulating oceanic oxygen evolution and impacting life evolution and bio-diversity.

The Late Ordovician and Early Silurian periods experienced glaciation and mass extinctions. However, debates exist regarding the factors that initiated the glaciation and the subsequent delay in biotic recovery. To uncover the relationships between paleoclimate, marine redox states, and biotic recovery in the Early Silurian, temporal variations in Zinc (Zn) isotopic compositions along with other multi-geochemical proxies were analyzed for a carbonate-dominated section in South China. A stratigraphic δ⁶⁶Zn trend was found in studied section, indicating a widespread disturbance to the marine Zn cycle. A significant negative shift in δ⁶⁶Zn by up to ∼0.7 ‰ was observed immediately following the Hirnantian Glaciation and HICE, which can be attributed to a combination of reduced burial efficiency of organic carbon, fast weathering of large igneous provinces and increased riverine Zn input. This was followed by a ∼ 0.4 ‰ rise in δ⁶⁶Zn values, alongside a sharp decrease in δ¹³C values, indicating an increase in sulfide precipitation (e.g., ZnS, FeS) due to expansions of euxinia. Subsequently, a notable rise in δ⁶⁶Zn values by up to ∼1.1 ‰ is interpreted as an increase in organic carbon burial, given the concurrent variation of δ⁶⁶Zn and δ¹³C during this period. The findings suggest that the increase in organic carbon burial, a reduction in reverse weathering, or a combination of them contributed to the glaciation in Silurian. Furthermore, the weathering-induced high primary productivity led to anoxic water conditions in the latest Hirnantian and Rhuddanian. This prolonged marine anoxia in seawater thus caused the deposition of organic-rich sediments and hindered biotic recovery in the Rhuddanian.

The stable carbon isotopic composition of speleothems is an important but incompletely understood parameter in paleoclimatological and paleoenvironmental reconstruction, recording changes in vegetation and hydrology. This study systematically assesses the influences of karst hydrology, cave environment, and carbon sources on the δ¹³C values of farmed calcite in Furong Cave, located in Chongqing, southwest China. Six drip sites (MP1-MP5) were monitored between 2009 and 2019, and Δ¹⁴C data (Δ¹⁴C is the difference in the ¹⁴C/¹²C ratio between a sample and a standard, expressed in permille, ‰) of soil organic matter, drip water and farmed calcite (calcite precipitated on a glass substrate) were obtained. A linear relationship was found between the seasonal variability of pCO₂ in cave air and δ¹³C_Cc of the farmed calcite in “Great Hall” characterised by a relatively stable microenvironment. The growth rate and δ¹³C_Cc values of the farmed calcite was not be affected by the drip rate unless the drip rate decreased to <1 drip/min for a long time, leading to a decrease in growth rate and higher δ¹³C_Cc values because of longer CO₂ degassing. MP2 and MP9 show faster drip rates, higher δ¹³C_DIC and lower Δ¹⁴C than MP1 and MP3, which suggests that the “old carbon” in MP9 is derived from the host rock. MP4 and MP5 are characterised by slower drip rates and lower δ¹³C_DIC and δ¹³C_Cc, and the Δ¹⁴C values of the farmed calcite are lower than those of MP9. The slow drip rate indicates that less infiltration water reaches MP4 and MP5, and soil CO₂ derived from the decomposition of “old” organic matter enters the fractures that lead to these drip sites. The fast drip rate of MP9 suggests that the fissures feeding this drip site are mostly water-saturated, limiting the exchange between soil CO₂ and CO₃²⁻ and allowing more “old carbon” from the bedrock to dissolve leading to higher δ¹³C_DIC. This study emphasizes that in addition to changes in the cave environment, the source(s) of the carbon may be a more important factor controlling the δ¹³C values of drip water (and speleothem) than drip rate.

In recent years, diffusion has gained increasing recognition as an important process for explaining the lithium (Li) isotope ratios (δ⁷Li) of magmatic systems. However, the role of diffusion in explaining the variability of mantle-derived basalts has not yet been investigated in detail. We have measured δ⁷Li values in a comprehensive suite of fresh, subglacial Icelandic basalt glasses and observe a wide range of values from 2.4‰ to 7.3‰, the largest range in Li isotope compositions identified at any ocean island. We find that δ⁷Li values do not correlate with lithophile element tracers, but do correlate with ³He/⁴He. One possibility is that the high diffusivities of both Li and He permit fractionation of Li isotope compositions that couple it to ³He/⁴He, and decouple it from other, slower-diffusing lithophile tracers. In this study, we explore this idea and model diffusive processes in crystal mushes and mantle melt channels, both with and without melt transport. Our modelling indicates that large (>3‰) fractionation of Li isotope compositions from MORB-like values can be generated by diffusion and that the coupling of Li isotope ratios and ³He/⁴He signatures can occur as a consequence of diffusive interactions within mantle melt channels. This suggests that the Li isotope compositions of melts cannot be used as a straightforward passive tracer of mantle heterogeneity, because they can be fractionated during magmatic processes. In contrast, we find that diffusive fractionation of ³He/⁴He is small (∼1 Ra) next to the variability in Icelandic basalts (∼8 to ∼35 Ra). Diffusive fractionation of Li isotope ratios should be most pronounced at OIB localities whose mantle melts have a wide range of [Li], such as Hawaii and Iceland. Although a mantle source can contain Li isotope heterogeneities as a result of crustal recycling, we show that a mantle that is homogenous in Li isotope composition can still generate melts that have variable δ⁷Li values via diffusion processes.

The same diffusion models can also be applied to other stable isotope systems such as H, Mg, Ca, and Fe. Indeed our models show that diffusion during magmatic transport within the mantle may be a significant source of “noise” in the δ⁷Li, δD, and possibly δ²⁶Mg isotopic systems.

The geochemistry of the Ría de Huelva estuary (Spain) has been widely studied owing to its unique conditions due to mixing of acid mine drainage (AMD), river water and seawater, which have been aggravated by the existence of an adjacent phosphate fertilizer plant and its associated phosphogypsum stack.

To better understand the complex geochemistry of the overall estuarine system, we make use of (1) measured concentrations of relevant trace elements (rare earth elements (REEs) and metal(loid)s) in the sediments and waters of the estuary and (2) geochemical modeling which combines mixing and adsorption processes.

Our study sheds light on the elemental distribution in the sediments and water in the water-mixing area. As seawater neutralizes the acidity of the river water (pH increases to 6.2) colloids of Fe- and Al-oxyhydroxysulphates (schwertmannite and basaluminite, respectively) precipitate as concentrations of aqueous Fe and Al increase. These phases have a high adsorption capacity and play a critical role in the geochemistry of the aquatic system by retaining Cu, Pb, Cr, As, P, REEs and smaller amounts of Zn, Co, Ni and Mn. The geochemical model reproduces the behaviour of the aqueous REEs, requiring high S/L ratios at pH ≥ 3.5 and participation of sediments and colloids in the reactions to match the field data.

The evaluation of the effect of the adjacent phosphogypsum stack on the estuarine geochemistry shows that the stack has a notorious impact on the geochemistry of the surrounding estuarine environment owing to the release of high quantities of phosphate.

The response of marine ammonia-oxidizing archaea (AOA) to environmental stress is reflected in changes in their membrane lipid composition, particularly the unique isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs). However, the influence of dissolved oxygen (DO) on the composition of iGDGTs in the ocean remains unclear. This study aims to investigate the link between DO levels and the fractional abundances of iGDGTs in the East China Sea (ECS) to establish a redox proxy. Results suggested that the absolute abundances of iGDGTs were influenced by Thaumarchaeota biomass when DO concentrations exceeded 2 mg/L. Vertical distributions of iGDGTs through the water column suggested their transport from bottom waters to sediments. Increasing proportions of iGDGT-0 and the sum of iGDGT-1, iGDGT-2 and iGDGT-3 from suspended particulate matters to surface sediments indicated their preferential preservation. DO concentrations (2–6 mg/L) in the water column showed a significant positive correlation with the relative abundance of crenarchaeol (cren%) but a negative correlation with iGDGT-0%, suggesting insufficient DO levels to promote AOA cyclization. However, bottom DO concentrations exhibited significant negative correlations with both cren% and iGDGT-0% in surface sediments, attributed to enhanced cyclization of iGDGTs and Euryarchaeota abundance within more reducing sediments, respectively. The consistent relationship between iGDGT-0% and DO in both water column and sediments enabled iGDGT-0% to be a potential redox proxy. Temporal variations in iGDGT-0% in the ECS in recent decades aligned well with in-situ DO monitoring data, further validating iGDGT-0% as a promising redox proxy.

Dissolution of biotite, the main Fe-bearing mineral in granitic bedrock, is of particular importance for the remediation of reducing conditions after the ingress of oxygen, such as after mining activities or the construction of deep repositories for toxic waste. This study investigated the leaching of biotite of size fraction 0.053–0.075 mm under anaerobic conditions at room temperature and pH 4 and 6.5 for a maximum of 160 days. The changes in the concentrations of the major elements in the leaching solutions were monitored. In addition, Fe(II) was analysed separately. pH-independent rate coefficients k_H+ were 4.8∙10⁻¹⁰, 6.9∙10⁻¹⁰, 6.3∙10⁻¹¹, and 1.0∙10⁻¹² mol^1-n m⁻² s^-1, for Fe, Fe(II), Mn, and Si, respectively. The corresponding proton reaction orders n_H+ were 0.61, 0.63, 0.33, and 0.09, respectively. The corresponding parameters for Al were not evaluated because of a suspected gibbsite precipitation at pH 6.5. The dissolution of biotite was found to be incongruent (non-stoichiometric) with respect to both the dissolving elements and the pH value. At pH 4, the dissolution was dominated by the octahedral layer element Fe, whereas at pH 6.5, the dissolution of the tetrahedral element Si dominated. There was no evidence of secondary phase formation, and the biotite leaching rates were consistent with those reported in previous studies conducted under aerobic conditions. In addition, the Fe(III)/Fe_tot ratio of biotite remained essentially unchanged before and after the experiment. This indicates that the anaerobic conditions alone have little effect on the rate and nature of biotite dissolution, although they may influence vermiculite formation. Therefore, biotite dissolution rates previously obtained under aerobic conditions may also be valid under anaerobic conditions.

The importance of light-stable isotopes and their mass-dependent fractionations in understanding past geological processes is enormous. The present research delivers precise, high-resolution, in-situ oxygen (^18/16O) and triple sulfur (^32/33/34S) isotope ratios from quartz and sulfide minerals found in the CuAu Kendadih deposit located along the Mesoproterozoic (∼1.6 Ga) Singhbhum Shear Zone (SSZ) in eastern India. Oxygen and sulfur isotope analyses were carried out on quartz and pyrite-chalcopyrite pairs that are in textural equilibrium, using the most advanced Large Geometry-Secondary Ion Mass Spectrometer (LG-SIMS; CAMECA IMS-1300HR³). The results show restricted ranges for δ¹⁸O_quartz (+6 to +7.7‰; average: 6.9 ± 0.9‰, n = 50) and δ³⁴S_{chalcopyrite-pyrite} (+11.4 to +11.9‰; average: 11.79 ± 0.2‰, n = 62). This isotopic homogeneity suggests a single, uniform fluid source for the ore-forming metals. The combined oxygen, sulfur isotope ratios, and fluid inclusion data are consistent with a hydrothermal fluid derived from an “I-type” granitic melt. This study incorporates existing fluid inclusion, stable, and radiogenic isotope data from temporally similar deposits within the shear zone, reevaluating their implications in light of the new findings. It proposes that, around 1.6 billion years ago, underplating the Singhbhum Craton by the Dalma Plume resulted in the remelting of the lower crust. This remelting is attributed to the plume's introduction of a significantly elevated thermal perturbation. The process is hypothesized to have led to the generation of second-order granitic melts. Upon emplacement in the lower crust, these granitic melts became the potential source for essential metals, ligands, and hydrothermal fluids, contributing to mineralization within the deep-seated Singhbhum Shear Zone. This research attempts to comprehensively describe the “source-to-sink” ore genesis model by proposing potential magma chamber processes that concentrate metals and ligands at the roof zone, followed by the separation of metal-rich hydrothermal fluids, fluid-rock interaction, and the physicochemical conditions governing the deposition of the extensive polymetallic deposit. This work offers novel insights into the Mesoproterozoic metallogenesis along the Singhbhum Shear Zone, challenging existing paradigms that favored evaporite or seawater brine sources. Furthermore, it sheds light on the Mesoproterozoic sub-crustal processes beneath the Indian Craton in this region.

In hydrothermal fluids, disulfur (S₂^•−) and trisulfur (S₃^•−) radical anions have been observed to coexist with the major hydrogen sulfide and sulfate species. These radical ions have potentially important effects on the solubility, transport and fractionation of metals and sulfur, with consequences for ore deposit formation and, more generally, for geochemical cycles of metals and volatiles. It is therefore essential to know the intrinsic isotopic properties of these important sulfur species in order to use sulfur isotopes for tracing different geological processes. Here, the theoretical equilibrium isotopic properties of the disulfur and trisulfur radical ions are computed and compared to the hydrogen sulfide (H₂S) and sulfate ion (SO₄²⁻), using, for the first time, a first-principles molecular dynamics (MD) approach. The isotopic properties are calculated directly from molecular dynamics trajectories using the vibrational density of states and the atomic kinetic energy, and then compared to the more established method based on sampling of several snapshots. This comparison allowed us to validate the new modelling method and to assess its advantages and limitations. The predicted equilibrium isotope fractionation in terms of ³⁴S/³²S between S₂^•− and S₃^•− is small, i.e. <1‰, with a slight enrichment in the heavier isotope for S₃^•−, over the temperature range 200–500 °C. Both radical ions are slightly depleted in the heavier isotope, by 1 to 2‰, relative to aqueous H₂S. Our results help tuning sulfur isotope fractionation models used for tracing the origin and evolution of hydrothermal fluids. Our method opens large perspectives for using the rapidly growing body of MD simulation data in geosciences on structure and stability of aqueous complexes to assess in parallel element isotope fractionations from MD-generated trajectories.