Chemical Geology

Journal: Chemical Geology

Abbreviation

Chem. geol.

Publisher

Elsevier

ISSN

0009-2541
1872-6836

Show all metadata

Search Results

Publications 1 - 10 of 229

Origin and dynamics of Fe and Mn sedimentary layers in Lake Baikal
Granina, Liba; Müller, Beat; Wehrli, Bernhard (2004)
Chemical Geology
Variable 222Rn emanation rates in an alluvial aquifer: Limits on using 222Rn as a tracer of surface water – Groundwater interactions
Peel, Morgan; Kipfer, Rolf; Hunkeler, Daniel; et al. (2022)
Chemical Geology
Radon-222 (222Rn) is routinely used as a tracer of surface water (SW) – groundwater (GW) interactions. However, methods that rely on this tracer to quantify SW-GW exchange fluxes, GW residence time, SW-GW mixing ratios, or GW flow velocities usually depend on the assumption of spatially constant 222Rn production within aquifers and/or hyporheic sediments. In the present study, we measure the natural variability of 222Rn emanation rates in a highly instrumented alluvial aquifer and underline some important issues in assuming homogenous 222Rn production in alluvial systems. A robust experimental setup was used to determine 222Rn emanation rates from sediments. High-resolution measurements reveal that 222Rn emanation rates are highest within the first few meters below the surface. These results have significant implications for the interpretation of 222Rn activities in GW, notably in the vicinity of infiltration zones, as changes in hydraulic conditions can lead to variations in 222Rn signatures that can only be resolved with proper knowledge of the spatial distribution of 222Rn production rates. In such contexts, the assumption of constant 222Rn production leads to an inadequate understanding of SW infiltration patterns, and biased 222Rn-based estimates of SW-GW exchange.
Contrasting sources and fates of sedimentary organic carbon in subtropical estuary-marginal sea systems
Hou, Pengfei; Yu, Meng; Eglinton, Timothy I.; et al. (2023)
Chemical Geology
Sediments deposited in subtropical deltas and continental shelves are the “recorders” of sources, transport processes, and accumulation of organic carbon (OC) at the land-ocean interface, but their role in the global carbon cycle remains poorly constrained. In this study, we determined the abundance and carbon isotopic composition (δ13C and Δ14C) of bulk OC, as well as characteristics of specific biomarker compounds (plant wax n-alkanes, n-fatty acids, and phytoplankton sterols) in surface sediments from the Pearl River estuary (PRE) and adjacent continental shelf of the northern South China Sea (SCS) in order to examine the sources and fates of OC delivered to a large subtropical river-marginal sea system. We observed marked spatial variability in the sources, hydrodynamic processes and accumulation of sedimentary OC in PRE and northern SCS shelf, with the emergence of two distinct regimes reflected in the bulk OC isotopic characteristics of surface sediments. In Pearl River impacted regions, relatively fresh terrestrial OC (OCterr) was subject to substantial degradation and aging during transport within the estuary and from the estuary to the shelf. In contrast, sedimentary OC in the Taiwanese river impacted region comprised a mixture of fluvially-derived refractory OCterr from the Choshui River and modern marine OC. Burial efficiencies of OCterr are also spatially heterogeneous as a result of these different OC sources and hydrodynamic processes. The overall OCterr burial efficiencies are relatively low in the Pearl River influenced region, with an average value of 27 ± 10% in the PRE and an average value of 17 ± 6% in the western shelf, in contrast to the higher values in the region impacted by Taiwanese rivers (55% ± 39%).These findings suggest both sources and hydrodynamic processes during transport exert strong influence on the fate of OC, with important implications for the role of river-dominated marginal seas in the global carbon cycle.
Controls on Mo isotope fractionations in a Mn-rich anoxic marine sediment, Gullmar Fjord, Sweden
Goldberg, Tatiana; Archer, Corey; Vance, Derek; et al. (2012)
Chemical Geology
Argon isotope constraints on modification of oxygen isotopes in Iceland Basalts by surficial processes
Burnard, Pete; Harrison, Darrell (2005)
Chemical Geology
Complex proterozoic to paleozoic history of the upper mantle recorded in the Urals lherzolite malssifs by Re-Os and Sm-Nd systematics
Tessalina, Svetlana G.; Bourdon, Bernard; Gannoun, Abdelmouhcine; et al. (2007)
Chemical Geology
Volatilities and diffusivities of Tl, Ag, Cu, Pb, Cd, Zn, Ga, and As from a Cl-bearing shoshonitic basalt and their application to volcanic degassing
Califano, Enrico; Mollo, Silvio; Sossi, Paolo A.; et al. (2025)
Chemical Geology
The emission of trace metals during volcanic eruptions is modulated by their diffusion rates through the silicate melt and their affinity for the gas phase. However, due to the multicomponent nature of natural magmas, the prevailing controls on emission rates remain poorly understood. To constrain how the presence of Cl affects the diffusivities and volatilities of trace metals (Cl, Tl, Ag, Cu, Cd, Zn, Pb, Ga and As) in a nominally anhydrous shoshonitic basalt, a series of degassing experiments was conducted at 1 atm and 1200, 1300 and 1400 °C, with initial Cl contents of ∼0.6 and ∼1.2 wt% Cl for durations of 1 and 4 h. The resulting concentration gradients perpendicular to the gas-melt interface attest to the diffusive transport of trace metals within the silicate framework in response to their evaporative loss. Diffusivities scale inversely with the ionic field strength, with monovalent cations diffusing at rates (∼10⁻¹⁰ m² s⁻¹) two orders of magnitude faster than trivalent cations (∼10⁻¹² m² s⁻¹). The presence of Cl causes a near-uniform increase in diffusivity of roughly 0.5 logarithmic units across all trace metals. Evaporation rates, defined as the rates at which volatile elements are lost from the melt surface to the coexisting vapor phase, are found to be fastest for Tl, Ag and Cd (∼10⁻⁹–10⁻⁸ m s⁻¹), whereas Ga and As (∼10⁻¹⁰–10⁻⁹ m s⁻¹) are the least volatile trace metals. Thermodynamic calculations indicate that all evaporating metal-bearing species are present as chlorides in the gas phase, except for As. A positive correlation is observed between evaporation mass transfer coefficients from this study and the gas-melt partition coefficients determined for volcanic gases, with Ga becoming relative more volatile, while As, Cd and Tl becoming less volatile in the experiments than observed in natural volcanic settings. Furthermore, modeling of bubble growth in magmas reveal that diffusive fractionation of slow- and fast-diffusing trace metals may substantially change the concentration ratio of the two species at the bubble-melt interface, with profound implications for the interpretation of volcanic gas compositions.
Carbonate metasomatism at mantle wedge conditions, evidence from trace element and stable isotope (Fe, Zn) signatures of orogenic peridotites
Debret, Baptiste; Sossi, Paolo A.; Malaspina, Nadia; et al. (2025)
Chemical Geology
The composition and redox state of the mantle wedge over geological time can be impacted by fluid transfer from the slab during subduction. Although arc magmas are oxidised and enriched in fluid-mobile elements relative to mid-ocean ridge basalts (MORB), the nature of the fluid phase (aqueous or melts) produced by the slab in mantle wedge remains debated. Here we compare the elemental and isotopic (Fe and Zn isotopes) composition of both unmetasomatised and metasomatized ultramafic rocks from the Western Alps, respectively the Balmuccia and Finero massifs, to identify and characterise the relative effects of subduction-related processes on mantle peridotite composition. The metasomatism of Finero massif is evidenced by Light Rare Earth Element (LREE), U and Th enrichment coupled with isotopically light Zn and Fe signatures and an increase in oxygen fugacity relative to the MORB mantle-like peridotites of the Balmuccia massif. Negative correlations between LREE/HREE and U/Th ratios in metasomatized samples suggest preferential transport of Th relative to U in the infiltrating phase. Based on experimental constraints on fluid/melt partitioning, these observations are most consistent with Th dissolution in slab-derived melts. On the other hand, the light Zn isotope signatures in the Finero peridotites relative to those of Balmuccia peridotites are inconsistent with metasomatism by silicate melts and melt extraction processes. Trace elements and Zn isotopes results can be reconciled through the metasomatism of the mantle wedge by carbonate bearing fluids or melts in an open system. This process favours the formation of isotopically light metasomatic minerals in the Finero peridotite and the subsequent release of isotopically heavy CO₂-rich fluids or melts, probably during massif exhumation.
Theoretical isotope fractionation of cadmium during complexation with organic ligands
Zhao, Yang; Li, Yongbing; Wiggenhauser, Matthias; et al. (2021)
Chemical Geology
Cadmium (Cd) isotopes are an important tool to better understand the inorganic and organic geochemistry of Cd. Organic ligands play a key role to control the toxicity and mobility of Cd in living organisms and also in terrestrial and aquatic environments. Therefore, knowledge of the equilibrium isotope fractionation of Cd with organic ligands is crucial to further advance Cd isotope source and process tracing in the field of biogeochemistry. In this study, we calculated reduced partition function ratios (103lnβ) of Cd isotopes in various organic Cd complexes by density functional theory. The calculations revealed that the 103lnβ of 114Cd/110Cd for these complexes decreased in the order of Cd(Hcit)(H2cit)− > Cd(cit)(H2O)3− > CdH(cit)(H2O)4 > CdEDTA > Cd(his)2H2O > Cd(cit)24− ≈ Cd(H2O)62+ > Cd(H2O)42+ > Cd(cys)(H2O)32+ > Cd(GS)2(H2O)22− > Cd(DMPS)(H2O)2− > Cd(DMPS)24− at 0–100 °C. In general, heavier Cd isotopes preferably bind to oxygen and nitrogen donor atoms while lighter Cd isotopes bind to sulfur donor atoms of organic ligands. Thus, the previously observed immobilization of light Cd isotopes in living organisms could be related to Cd detoxification processes with sulfur. The predicted equilibrium isotope fractionation will strengthen Cd isotopes as a process tracing tool in living organisms and will improve the understanding of Cd isotope cycling in aquatic and terrestrial systems.
Regional conditions cause contrasting behaviour in U-isotope fractionation in black shales: Constraints for global ocean palaeo-redox reconstructions
Gangl, Sophie K.; Stirling, Claudine H.; Jenkyns, Hugh C.; et al. (2023)
Chemical Geology
The U-isotope system is a well-established palaeo-redox proxy that potentially constrains the global extent of marine anoxia during average as well as extreme redox events throughout Earth's history. A typical archive that forms underneath a reducing water column and acts as an intense U sink is organic-rich black shale. However, the degree to which black shale archives reflect the marine U-isotope signature is not well understood because U-isotope fractionation between U(VI)-bearing seawater and U(IV)-bearing black shales may vary as a function of local environmental conditions. Here, we present a combination of U-isotope and elemental concentration datasets, supported by a complementary Mo-isotope record, for the Furlo sedimentary section in Marche–Umbria, Italy and interrogate the combined systematics to unravel the mechanisms controlling the U-isotope fractionation factor between black shales and ambient seawater. We examine black shales deposited before and during Oceanic Anoxic Event 2 (OAE 2), which was one of the most extreme climatic perturbations of the Mesozoic Era that took place around the Cenomanian–Turonian boundary (Late Cretaceous, c. 94 Ma). The results of this study show that the U-isotope signature in the black shales deposited before OAE 2 was controlled by different mechanisms than the U-isotope ratios recorded in black shales deposited during OAE 2, with both stratigraphic intervals likely influenced by local environmental conditions. Probable local environmental changes include increased U reduction associated with biomass at or above the sediment–water interface and varying dissolved hydrogen sulphide concentrations in the water column and sediment. The overall results of this study confirm that black shales are a highly complex archive for U-isotope studies of past oceanic redox conditions, due to the sensitivity of the U-isotope fractionation mechanism to local environmental conditions, which are difficult to constrain. We propose the application of a ∆238Ushale-seawater of 0.6 ± 0.1 ‰ to black shale records deposited under locally constant euxinic conditions at non-restricted settings.

Publications 1 - 10 of 229

Journal: Chemical Geology

Abbreviation

Publisher

Journal Volumes

ISSN

Description

Filters

Search Results