Pub Date : 2025-02-02DOI: 10.1016/j.gca.2025.01.043
Kun Wang, Zhen Tian, Megan Broussard, Mason Neuman, Piers Koefoed, Olga Pravdivtseva, Guillaume Avice, Richard V. Morris, Kees C. Welten, Randy L. Korotev, Bradley L. Jolliff
Space weathering has long been known to alter the chemical and physical properties of the surfaces of airless bodies such as the Moon. The isotopic compositions of moderately volatile elements in lunar regolith samples could serve as sensitive tracers for assessing the intensity and duration of space weathering. In this study, we develop a new quantitative tool to study space weathering and constrain surface exposure ages based on potassium isotopic compositions of lunar soils. We first report the K isotopic compositions of 13 bulk lunar soils and 20 interval soil samples from the Apollo 15 deep drill core (15004 – 15006). We observe significant K isotope fractionation in these lunar soil samples, ranging from 0.00 ‰ to + 11.77 ‰, compared to the bulk silicate Moon (–0.07 ± 0.09 ‰). Additionally, a strong correlation between soil maturity (Is/FeO) and K isotope fractionation is identified for the first time, consistent with other isotope systems of moderately volatile elements such as S, Cu, Zn, Se, Rb, and Cd. Subsequently, we conduct numerical modeling to better constrain the processes of volatile element depletion and isotope fractionation on the Moon and calculate a new K Isotope Model Exposure Age (KIMEA) through this model. We demonstrate that this KIMEA is most sensitive to samples with an exposure age lower than 1,000 Ma and becomes less effective for older samples. This novel K isotope tool can be utilized to evaluate the surface exposure ages of regolith samples on the Moon and potentially on other airless bodies if calibrated using other methods (e.g., cosmogenic noble gases) or experimental data.
{"title":"Potassium isotopic compositions and model exposure ages of lunar soils","authors":"Kun Wang, Zhen Tian, Megan Broussard, Mason Neuman, Piers Koefoed, Olga Pravdivtseva, Guillaume Avice, Richard V. Morris, Kees C. Welten, Randy L. Korotev, Bradley L. Jolliff","doi":"10.1016/j.gca.2025.01.043","DOIUrl":"https://doi.org/10.1016/j.gca.2025.01.043","url":null,"abstract":"Space weathering has long been known to alter the chemical and physical properties of the surfaces of airless bodies such as the Moon. The isotopic compositions of moderately volatile elements in lunar regolith samples could serve as sensitive tracers for assessing the intensity and duration of space weathering. In this study, we develop a new quantitative tool to study space weathering and constrain surface exposure ages based on potassium isotopic compositions of lunar soils. We first report the K isotopic compositions of 13 bulk lunar soils and 20 interval soil samples from the Apollo 15 deep drill core (15004 – 15006). We observe significant K isotope fractionation in these lunar soil samples, ranging from 0.00 ‰ to + 11.77 ‰, compared to the bulk silicate Moon (–0.07 ± 0.09 ‰). Additionally, a strong correlation between soil maturity (<ce:italic>I<ce:inf loc=\"post\">s</ce:inf></ce:italic>/FeO) and K isotope fractionation is identified for the first time, consistent with other isotope systems of moderately volatile elements such as S, Cu, Zn, Se, Rb, and Cd. Subsequently, we conduct numerical modeling to better constrain the processes of volatile element depletion and isotope fractionation on the Moon and calculate a new K Isotope Model Exposure Age (KIMEA) through this model. We demonstrate that this KIMEA is most sensitive to samples with an exposure age lower than 1,000 Ma and becomes less effective for older samples. This novel K isotope tool can be utilized to evaluate the surface exposure ages of regolith samples on the Moon and potentially on other airless bodies if calibrated using other methods (e.g., cosmogenic noble gases) or experimental data.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"52 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-02-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143462871","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.10.017
D. Gilliard , D.J. Janssen , N. Schuback , S.L. Jaccard
Over the last decades, the chromium (Cr) stable isotope system (referred to as δ53Cr) has emerged as a proxy to reconstruct past oxygenation changes in Earth’s atmosphere and oceans. Although Cr is a promising paleoproxy, uncertainties remain as to the modern marine Cr cycle, and limited data are yet available in large swaths of the ocean, including the Atlantic Ocean. Here we present dissolved seawater Cr concentrations ([Cr]) and δ53Cr along a meridional transect from the North to the South Atlantic (AMT 29). Chromium concentrations range from of 2.51 to 3.96 nmol kg−1 (n = 68) and δ53Cr values range from +0.86 ± 0.04 ‰ (2SEM) to +1.20 ± 0.02 ‰ (2SEM) (n = 68). In contrast to data from other ocean basins [Cr] and δ53Cr show only a weak correlation (δ53Cr vs. Ln([Cr]) R2 = 0.17), inconsistent with a closed-system Rayleigh distillation model. These results can mainly be explained by horizontal advection and water mass mixing, which our data demonstrate are the dominant processes controlling [Cr] and δ53Cr distributions throughout much of the Atlantic, while the impact of in situ biogeochemical cycling is comparatively minor. There is, indeed no clear impact of biological productivity nor of dysoxic environments in the (sub)tropical Atlantic on the cycling of Cr along the transect. This is likely explained by insufficiently depleted oxygen concentrations and relatively low biological productivity, resulting in these processes being of secondary importance relative to water mass mixing in shaping the distribution of Cr in the low- to mid-latitude Atlantic Ocean.
{"title":"Controls on the distribution of dissolved Cr in the upper water column of the Atlantic Basin","authors":"D. Gilliard , D.J. Janssen , N. Schuback , S.L. Jaccard","doi":"10.1016/j.gca.2024.10.017","DOIUrl":"10.1016/j.gca.2024.10.017","url":null,"abstract":"<div><div>Over the last decades, the chromium (Cr) stable isotope system (referred to as δ<sup>53</sup>Cr) has emerged as a proxy to reconstruct past oxygenation changes in Earth’s atmosphere and oceans. Although Cr is a promising paleoproxy, uncertainties remain as to the modern marine Cr cycle, and limited data are yet available in large swaths of the ocean, including the Atlantic Ocean. Here we present dissolved seawater Cr concentrations ([Cr]) and δ<sup>53</sup>Cr along a meridional transect from the North to the South Atlantic (AMT 29). Chromium concentrations range from of 2.51 to 3.96 nmol kg<sup>−1</sup> (n = 68) and δ<sup>53</sup>Cr values range from +0.86 ± 0.04 ‰ (2SEM) to +1.20 ± 0.02 ‰ (2SEM) (n = 68). In contrast to data from other ocean basins [Cr] and δ<sup>53</sup>Cr show only a weak correlation (δ<sup>53</sup>Cr vs. Ln([Cr]) R<sup>2</sup> = 0.17), inconsistent with a closed-system Rayleigh distillation model. These results can mainly be explained by horizontal advection and water mass mixing, which our data demonstrate are the dominant processes controlling [Cr] and δ<sup>53</sup>Cr distributions throughout much of the Atlantic, while the impact of in situ biogeochemical cycling is comparatively minor. There is, indeed no clear impact of biological productivity nor of dysoxic environments in the (sub)tropical Atlantic on the cycling of Cr along the transect. This is likely explained by insufficiently depleted oxygen concentrations and relatively low biological productivity, resulting in these processes being of secondary importance relative to water mass mixing in shaping the distribution of Cr in the low- to mid-latitude Atlantic Ocean.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 57-69"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673652","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.12.007
Christopher Emproto , Ryan Mathur , Mingguang Sun , Adam C. Simon , Linda Godfrey
Titanium (Ti) typically exhibits low mobility in geologic fluids due to the low aqueous solubility of common (Fe-)Ti oxide minerals. Consequently, Ti isotope variations (δ49/47Ti, given as δ49Ti) in geologic systems are primarily attributed to magmatic differentiation. Thus, the potential for fluid-mineral fractionation has received less attention. However, ligand-rich fluids are capable of mobilizing Ti as observed in natural systems and laboratory studies. As hydrothermal ore mineralization is commonly associated with ligand-rich brines capable of transporting significant quantities of metals, Ti isotopes may aid in understanding mineralization and alteration in complex hydrothermal systems. Here we present data from computational modeling of various Ti coordination complexes theorized to exist in geologic systems and/or under relevant experimental conditions as well as computed fractionation factors for various Ti-bearing crystalline phases to investigate the basic mechanics of equilibrium fluid-mineral Ti isotope fractionation. These results indicate that equilibrium fluid-mineral Ti isotope exchange between our modeled Ti complexes and phases with 6-coordinated Ti is predicted to generally lead to enrichment of heavy Ti isotopes in the fluid. Because minerals with 6-coordinated Ti (such as magnetite and ilmenite) are the most important reservoirs of Ti in the solid Earth, Ti isotope equilibration between terrestrial rocks and fluids can be generalized to enrich the fluid in heavy Ti isotopes. We also performed magnetite-ülvospinel leaching experiments to investigate fluid-mineral Ti isotope fractionation in this phase. Mineral leaching experiments varying acid strength, leaching temperature, and reaction time with HCl and HF qualitatively support the prediction that the fluid phase will become enriched in heavy Ti isotopes during fluid-mineral interactions that approach equilibrium with Ti-rich magnetite. Additionally, the leaching data also suggest that the fluid becomes slightly enriched in lighter Ti isotopes when Ti exchange is limited—potentially due to kinetic effects. Therefore, magnetite from natural systems may be depleted in heavy Ti isotopes during regenerative mineral replacement involving equilibration with fluids or may possibly become depleted in light Ti isotopes under a kinetic fractionation regime—leading to mineral δ49Ti values that are insufficiently explained by magmatic differentiation or inter-mineral fractionation. These results are a first look at fluid-mineral interactions that may affect Ti isotope fractionation in hydrothermal mineral systems, and Ti isotopes should be further studied as a potential method of understanding aqueous metal transport and tracing alteration in mineral deposits.
{"title":"Fluid-mineral titanium isotope fractionation: Computational and empirical results with implications for mineral deposits","authors":"Christopher Emproto , Ryan Mathur , Mingguang Sun , Adam C. Simon , Linda Godfrey","doi":"10.1016/j.gca.2024.12.007","DOIUrl":"10.1016/j.gca.2024.12.007","url":null,"abstract":"<div><div>Titanium (Ti) typically exhibits low mobility in geologic fluids due to the low aqueous solubility of common (Fe-)Ti oxide minerals. Consequently, Ti isotope variations (δ<sup>49/47</sup>Ti, given as δ<sup>49</sup>Ti) in geologic systems are primarily attributed to magmatic differentiation. Thus, the potential for fluid-mineral fractionation has received less attention. However, ligand-rich fluids are capable of mobilizing Ti as observed in natural systems and laboratory studies. As hydrothermal ore mineralization is commonly associated with ligand-rich brines capable of transporting significant quantities of metals, Ti isotopes may aid in understanding mineralization and alteration in complex hydrothermal systems. Here we present data from computational modeling of various Ti coordination complexes theorized to exist in geologic systems and/or under relevant experimental conditions as well as computed fractionation factors for various Ti-bearing crystalline phases to investigate the basic mechanics of equilibrium fluid-mineral Ti isotope fractionation. These results indicate that equilibrium fluid-mineral Ti isotope exchange between our modeled Ti complexes and phases with 6-coordinated Ti is predicted to generally lead to enrichment of heavy Ti isotopes in the fluid. Because minerals with 6-coordinated Ti (such as magnetite and ilmenite) are the most important reservoirs of Ti in the solid Earth, Ti isotope equilibration between terrestrial rocks and fluids can be generalized to enrich the fluid in heavy Ti isotopes. We also performed magnetite-ülvospinel leaching experiments to investigate fluid-mineral Ti isotope fractionation in this phase. Mineral leaching experiments varying acid strength, leaching temperature, and reaction time with HCl and HF qualitatively support the prediction that the fluid phase will become enriched in heavy Ti isotopes during fluid-mineral interactions that approach equilibrium with Ti-rich magnetite. Additionally, the leaching data also suggest that the fluid becomes slightly enriched in lighter Ti isotopes when Ti exchange is limited—potentially due to kinetic effects. Therefore, magnetite from natural systems may be depleted in heavy Ti isotopes during regenerative mineral replacement involving equilibration with fluids or may possibly become depleted in light Ti isotopes under a kinetic fractionation regime—leading to mineral δ<sup>49</sup>Ti values that are insufficiently explained by magmatic differentiation or inter-mineral fractionation. These results are a first look at fluid-mineral interactions that may affect Ti isotope fractionation in hydrothermal mineral systems, and Ti isotopes should be further studied as a potential method of understanding aqueous metal transport and tracing alteration in mineral deposits.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 251-263"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884585","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.12.002
S. Nemiah Ladd , Daniel B. Nelson , Blake Matthews , Shannon Dyer , Romana Limberger , Antonia Klatt , Anita Narwani , Nathalie Dubois , Carsten J. Schubert
Phytoplankton play a key role in biogeochemical cycles, impacting atmospheric and aquatic chemistry, food webs, and water quality. However, it remains challenging to reconstruct changes in algal community composition throughout the geologic past, as existing proxies are suitable only for a subset of taxa and/or influenced by degradation. Here, we investigate if compound-specific hydrogen isotope ratios (δ2H values) of common algal lipids can serve as (paleo)ecological indicators. First, we grew 20 species of algae – representing cyanobacteria, diatoms, dinoflagellates, green algae, and cryptomonads – in batch cultures under identical conditions and measured δ2H values of their lipids. Despite identical source water δ2H values, lipid δ2H values ranged from −455 ‰ to −52 ‰, incorporating variability associated with chemical compound classes and taxonomic groups. In particular, green algae synthesized fatty acids with higher δ2H values than other taxa, cyanobacteria synthesized phytol with relatively low δ2H values, and diatoms synthesized sterols with higher δ2H values than other eukaryotes. Second, we assessed how changes in algal community composition can affect net δ2H values of common algal lipids in 20 experimental outdoor ponds, which were manipulated via nutrient loading, and the addition of macrophytes and mussels. High algal biomass in the ponds, which was mainly caused by cyanobacterial and green algal blooms, was associated with higher δ2H values for generic fatty acids, relatively stable δ2H values for phytol and the dinoflagellate biomarker dinostanol, and lower δ2H values for the more cosmopolitan sterol stigmasterol. These results are consistent with expectations from our culture-based analyses, with both datasets indicating large taxon-specific changes that are unlikely to be driven by bacterial heterotrophy. This suggests that measuring δ2H values of multiple lipids from sediment and calculating 2H-offsets between them can resolve changes in algal community composition from changes in source water isotopes. With an appropriate availability of sedimentary lipids, this approach could permit the reconstruction of both taxonomic variability and hydroclimate from diverse sedimentary systems.
{"title":"Taxon-specific hydrogen isotope signals in cultures and mesocosms facilitate ecosystem and hydroclimate reconstruction","authors":"S. Nemiah Ladd , Daniel B. Nelson , Blake Matthews , Shannon Dyer , Romana Limberger , Antonia Klatt , Anita Narwani , Nathalie Dubois , Carsten J. Schubert","doi":"10.1016/j.gca.2024.12.002","DOIUrl":"10.1016/j.gca.2024.12.002","url":null,"abstract":"<div><div>Phytoplankton play a key role in biogeochemical cycles, impacting atmospheric and aquatic chemistry, food webs, and water quality. However, it remains challenging to reconstruct changes in algal community composition throughout the geologic past, as existing proxies are suitable only for a subset of taxa and/or influenced by degradation. Here, we investigate if compound-specific hydrogen isotope ratios (δ<sup>2</sup>H values) of common algal lipids can serve as (paleo)ecological indicators. First, we grew 20 species of algae – representing cyanobacteria, diatoms, dinoflagellates, green algae, and cryptomonads – in batch cultures under identical conditions and measured δ<sup>2</sup>H values of their lipids. Despite identical source water δ<sup>2</sup>H values, lipid δ<sup>2</sup>H values ranged from −455 ‰ to −52 ‰, incorporating variability associated with chemical compound classes and taxonomic groups. In particular, green algae synthesized fatty acids with higher δ<sup>2</sup>H values than other taxa, cyanobacteria synthesized phytol with relatively low δ<sup>2</sup>H values, and diatoms synthesized sterols with higher δ<sup>2</sup>H values than other eukaryotes. Second, we assessed how changes in algal community composition can affect net δ<sup>2</sup>H values of common algal lipids in 20 experimental outdoor ponds, which were manipulated via nutrient loading, and the addition of macrophytes and mussels. High algal biomass in the ponds, which was mainly caused by cyanobacterial and green algal blooms, was associated with higher δ<sup>2</sup>H values for generic fatty acids, relatively stable δ<sup>2</sup>H values for phytol and the dinoflagellate biomarker dinostanol, and lower δ<sup>2</sup>H values for the more cosmopolitan sterol stigmasterol. These results are consistent with expectations from our culture-based analyses, with both datasets indicating large taxon-specific changes that are unlikely to be driven by bacterial heterotrophy. This suggests that measuring δ<sup>2</sup>H values of multiple lipids from sediment and calculating <sup>2</sup>H-offsets between them can resolve changes in algal community composition from changes in source water isotopes. With an appropriate availability of sedimentary lipids, this approach could permit the reconstruction of both taxonomic variability and hydroclimate from diverse sedimentary systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 199-210"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884631","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The study of pristine chondrites provides insight into nebular processes that occurred prior to the accretion of small-sized parent bodies. The interchondrule matrix of the primitive chondrite Acfer 094 is characterized by the presence of submicron-sized anhydrous crystalline aggregates embedded in a silicate groundmass that is mostly amorphous. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS) were employed to investigate the matrix of Acfer 094 and its components.
The amorphous silicate groundmass is homogeneous in composition and exhibits a Mg depletion relative to the solar value. It embeds Fe-Ni nanosulfides, whose typical size is a few tens of nanometers. Nanometer-sized fibrous silicates are rare in the groundmass indicating a low degree of aqueous alteration. Submicron-sized Mg-rich crystalline silicates (olivine and pyroxene) occur either as isolated grains or as aggregates. These submicron-sized crystalline silicates occupy around 25 % of the matrix volume. The isolated grains display a wide range of shapes, from rounded to irregularly angular, and could have originated from the fragmentation of type I chondrules or from nebular condensation. The aggregates exhibit variable morphologies and grain sizes (typically a few tens of nm). They are chemically equilibrated, and likely formed by solid-state thermal annealing of amorphous precursors.
The Acfer 094 matrix contains a range of components that have undergone varying degrees of thermal modification. A significant proportion of a precursor material (i.e. nebular dust) resembling matrix is likely to have undergone one or more brief and intense thermal events, potentially associated with the process of chondrule formation. These events resulted in the formation of magnesium-rich anhydrous silicates (forsterite and enstatite) at high temperatures that were embedded in the matrix of Acfer 094 as isolated grains and crystalline aggregates.
{"title":"Submicron-sized anhydrous crystalline silicates and their relation to amorphous silicate in the matrix of Acfer 094","authors":"Hugues Leroux , Pierre-Marie Zanetta , Corentin Le Guillou , Maya Marinova","doi":"10.1016/j.gca.2024.11.028","DOIUrl":"10.1016/j.gca.2024.11.028","url":null,"abstract":"<div><div>The study of pristine chondrites provides insight into nebular processes that occurred prior to the accretion of small-sized parent bodies. The interchondrule matrix of the primitive chondrite Acfer 094 is characterized by the presence of submicron-sized anhydrous crystalline aggregates embedded in a silicate groundmass that is mostly amorphous. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS) were employed to investigate the matrix of Acfer 094 and its components.</div><div>The amorphous silicate groundmass is homogeneous in composition and exhibits a Mg depletion relative to the solar value. It embeds Fe-Ni nanosulfides, whose typical size is a few tens of nanometers. Nanometer-sized fibrous silicates are rare in the groundmass indicating a low degree of aqueous alteration. Submicron-sized Mg-rich crystalline silicates (olivine and pyroxene) occur either as isolated grains or as aggregates. These submicron-sized crystalline silicates occupy around 25 % of the matrix volume. The isolated grains display a wide range of shapes, from rounded to irregularly angular, and could have originated from the fragmentation of type I chondrules or from nebular condensation. The aggregates exhibit variable morphologies and grain sizes (typically a few tens of nm). They are chemically equilibrated, and likely formed by solid-state thermal annealing of amorphous precursors.</div><div>The Acfer 094 matrix contains a range of components that have undergone varying degrees of thermal modification. A significant proportion of a precursor material (i.e. nebular dust) resembling matrix is likely to have undergone one or more brief and intense thermal events, potentially associated with the process of chondrule formation. These events resulted in the formation of magnesium-rich anhydrous silicates (forsterite and enstatite) at high temperatures that were embedded in the matrix of Acfer 094 as isolated grains and crystalline aggregates.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 160-180"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142804468","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.12.026
K.E. Miller , D.I. Foustoukos , G.D. Cody , C.M. O’D. Alexander
Titan’s abundant atmospheric N2 and CH4 gases are notable characteristics of the moon that may help constrain its origin and evolution. Previous work suggests that atmospheric CH4 is lost on geologically short timescales and may be replenished from an interior source. Isotopic and noble gas constraints indicate that N2 may derive from a mixture of NH3 ice and heating of organic matter. Here, we report experimental results from hydrothermal alteration of insoluble organic matter from the Murchison meteorite and analog insoluble organic matter at temperatures and pressures that are relevant to Titan’s interior. Our results indicate both CH4 and CO2 are formed, with the ratio between the two depending on a multitude of factors, particularly temperature and, to a lesser degree, the dielectric constant of water and carbonyl abundance in the starting material. Sufficient CH4 is produced to source Titan’s atmospheric reservoir if temperatures are greater than 250 °C. Nitrogen is volatilized, primarily in the form of NH3, in sufficient abundances to source at least 50 % of Titan’s atmospheric N2. The isotopic characteristics of volatilized material relative to the starting organics are consistent with current constraints for the nature of the accreted complex organics and Titan’s evolved atmosphere.
{"title":"Experimental heating of complex organic matter at Titan’s interior conditions supports contributions to atmospheric N2 and CH4","authors":"K.E. Miller , D.I. Foustoukos , G.D. Cody , C.M. O’D. Alexander","doi":"10.1016/j.gca.2024.12.026","DOIUrl":"10.1016/j.gca.2024.12.026","url":null,"abstract":"<div><div>Titan’s abundant atmospheric N<sub>2</sub> and CH<sub>4</sub> gases are notable characteristics of the moon that may help constrain its origin and evolution. Previous work suggests that atmospheric CH<sub>4</sub> is lost on geologically short timescales and may be replenished from an interior source. Isotopic and noble gas constraints indicate that N<sub>2</sub> may derive from a mixture of NH<sub>3</sub> ice and heating of organic matter. Here, we report experimental results from hydrothermal alteration of insoluble organic matter from the Murchison meteorite and analog insoluble organic matter at temperatures and pressures that are relevant to Titan’s interior. Our results indicate both CH<sub>4</sub> and CO<sub>2</sub> are formed, with the ratio between the two depending on a multitude of factors, particularly temperature and, to a lesser degree, the dielectric constant of water and carbonyl abundance in the starting material. Sufficient CH<sub>4</sub> is produced to source Titan’s atmospheric reservoir if temperatures are greater than 250 °C. Nitrogen is volatilized, primarily in the form of NH<sub>3</sub>, in sufficient abundances to source at least 50 % of Titan’s atmospheric N<sub>2</sub>. The isotopic characteristics of volatilized material relative to the starting organics are consistent with current constraints for the nature of the accreted complex organics and Titan’s evolved atmosphere.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 38-56"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142936032","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.11.014
Ninna K. Jensen , Alexander A. Nemchin , Gavin Kenny , Martin J. Whitehouse , James N. Connelly , Takashi Mikouchi , Martin Bizzarro
Impact events were frequent in the early history of our Solar System, and the dynamics of planetary crust formation were, consequently, substantially different from the processes that dominate today. Mars, a planet with stagnant lid tectonics and a unique preservation of ancient surface terrains, provides an outstanding opportunity to investigate the early processes related to the formation and reshaping of the first crust. Northwest Africa (NWA) 7034 and paired meteorites (such as NWA 7533) are fragments of polymict, regolith breccia that provide a tangible record of the ancient, brecciated crust on Mars. Zircon and baddeleyite from NWA 7034/7533 record evidence for two events of intense crustal reworking at 4442 ± 17 and 4474 ± 10 million years ago (Ma) triggered by impacts, placing important constraints on the timing and the dynamics of early crust formation on Mars. To date, only few studies have focussed on the geochronology of the igneous clasts present within NWA 7034 and its pairs. Although these studies consistently report ancient ages (∼4.4 Ga) for basaltic, basaltic andesitic and monzonitic clasts, the associated precisions are generally too low to link the different lithologies with the two age peaks inferred from NWA 7034/7533 zircon and baddeleyite. Here, we conduct an isotopic and petrographic study of igneous clasts from NWA 7533 to shed further light on the timing and nature of crustal reworking in the early history of Mars. We show that six out of seven investigated igneous clasts, representing at least four distinct types, record undisturbed Lu-Hf isotope systematics that indicate contemporaneous formation. Together with two zircons hosted in basalt and basaltic andesite clasts, these igneous clasts yield an isochron age of 4440 ± 41 Ma (2SE, MSWD = 2.1). This isochron age is consistent with clast ages inferred from zircon U-Pb geochronology, and altogether the available age constraints for the lithic components in NWA 7533 indicate that they derive from the younger of the two peaks of intense crustal reworking on early Mars (4442 ± 17 Ma). The initial εHf values (the 176Hf/177Hf ratio in the sample normalised to that of the chondritic uniform reservoir at the time of crystallisation in parts per ten thousand) of the igneous clasts range between −2.07 and −0.74, consistent with crystallisation from enriched source melts deriving from impact-induced reworking of the crust. The mean Lu-Hf isotope composition of the igneous clasts constrains the timing of primordial crust formation and reveals planet formation and differentiation within the first 10 Myr of the history of the Solar System, in consistence with the conclusions in earlier reports. The results presented here suggest a 176Lu/177Hf ratio of ∼ 0.0135 or higher in the primordial martian crust.
{"title":"Timing of crustal reworking on Mars inferred from the Lu-Hf isotope systematics of igneous clasts in NWA 7533","authors":"Ninna K. Jensen , Alexander A. Nemchin , Gavin Kenny , Martin J. Whitehouse , James N. Connelly , Takashi Mikouchi , Martin Bizzarro","doi":"10.1016/j.gca.2024.11.014","DOIUrl":"10.1016/j.gca.2024.11.014","url":null,"abstract":"<div><div>Impact events were frequent in the early history of our Solar System, and the dynamics of planetary crust formation were, consequently, substantially different from the processes that dominate today. Mars, a planet with stagnant lid tectonics and a unique preservation of ancient surface terrains, provides an outstanding opportunity to investigate the early processes related to the formation and reshaping of the first crust. Northwest Africa (NWA) 7034 and paired meteorites (such as NWA 7533) are fragments of polymict, regolith breccia that provide a tangible record of the ancient, brecciated crust on Mars. Zircon and baddeleyite from NWA 7034/7533 record evidence for two events of intense crustal reworking at 4442 ± 17 and 4474 ± 10 million years ago (Ma) triggered by impacts, placing important constraints on the timing and the dynamics of early crust formation on Mars. To date, only few studies have focussed on the geochronology of the igneous clasts present within NWA 7034 and its pairs. Although these studies consistently report ancient ages (∼4.4 Ga) for basaltic, basaltic andesitic and monzonitic clasts, the associated precisions are generally too low to link the different lithologies with the two age peaks inferred from NWA 7034/7533 zircon and baddeleyite. Here, we conduct an isotopic and petrographic study of igneous clasts from NWA 7533 to shed further light on the timing and nature of crustal reworking in the early history of Mars. We show that six out of seven investigated igneous clasts, representing at least four distinct types, record undisturbed Lu-Hf isotope systematics that indicate contemporaneous formation. Together with two zircons hosted in basalt and basaltic andesite clasts, these igneous clasts yield an isochron age of 4440 ± 41 Ma (2SE, MSWD = 2.1). This isochron age is consistent with clast ages inferred from zircon U-Pb geochronology, and altogether the available age constraints for the lithic components in NWA 7533 indicate that they derive from the younger of the two peaks of intense crustal reworking on early Mars (4442 ± 17 Ma). The initial εHf values (the <sup>176</sup>Hf/<sup>177</sup>Hf ratio in the sample normalised to that of the chondritic uniform reservoir at the time of crystallisation in parts per ten thousand) of the igneous clasts range between −2.07 and −0.74, consistent with crystallisation from enriched source melts deriving from impact-induced reworking of the crust. The mean Lu-Hf isotope composition of the igneous clasts constrains the timing of primordial crust formation and reveals planet formation and differentiation within the first 10 Myr of the history of the Solar System, in consistence with the conclusions in earlier reports. The results presented here suggest a <sup>176</sup>Lu/<sup>177</sup>Hf ratio of ∼ 0.0135 or higher in the primordial martian crust.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 70-85"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841592","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.12.021
Devin L. Schrader , Zachary A. Torrano , Dionysis I. Foustoukos , Conel M.O’D. Alexander , Jan Render , Gregory A. Brennecka
We report a coordinated bulk Cr-Ti-H-C-N isotopic and compositional study of six carbonaceous chondrites from Antarctica that are often considered to be related and termed Yamato-like carbonaceous (CY) chondrites. These meteorites are known to have undergone extensive aqueous alteration followed by different degrees of thermal alteration, to be similar to one another in regard to mineralogy, and share affinities with both the Ivuna-like carbonaceous (CI) and Mighei-like carbonaceous (CM) chondrites. While mineralogically similar, a key difference among these samples is that chondrules have been found in some of these samples, but not in others. The aim of this study is to evaluate the relationship of these meteorites to one another, and investigate how they relate to the CI and CM chondrite groups.
We find that with the addition of the isotopic compositions of these ‘CY’ chondrites, there is now a continuum of isotopic compositions among the carbonaceous chondrites. The CI chondrites are no longer separate in O isotopic compositions from the other carbonaceous chondrite groups in plots of Cr-O and Ti-O. We also find that the ‘CY’ chondrites represent two distinct populations, which correlate with their heating stage. However, the peak temperatures experienced by each population can only explain the differences in H and C isotopes and abundances and N abundances between samples, and cannot have caused the differences in N, Cr, and Ti isotopes, or all the volatile element depletions of the sample’s bulk compositions. Instead, we conclude that the compositional and isotopic data of these two populations correlate with their known chondrule abundances, indicating distinct precursors for each population. We find it most likely that these samples originate from two distinct asteroids, implying that among the six samples studied here, there are not five related samples to constitute a new meteorite group. The chondrule-free heating Stage III samples are most likely heated CI chondrites, while the chondrule-bearing heating Stage IV samples could be heated CM chondrites.
{"title":"Reassessing the proposed “CY chondrites”: Evidence for multiple meteorite types and parent bodies from Cr-Ti-H-C-N isotopes and bulk elemental compositions","authors":"Devin L. Schrader , Zachary A. Torrano , Dionysis I. Foustoukos , Conel M.O’D. Alexander , Jan Render , Gregory A. Brennecka","doi":"10.1016/j.gca.2024.12.021","DOIUrl":"10.1016/j.gca.2024.12.021","url":null,"abstract":"<div><div>We report a coordinated bulk Cr-Ti-H-C-N isotopic and compositional study of six carbonaceous chondrites from Antarctica that are often considered to be related and termed Yamato-like carbonaceous (CY) chondrites. These meteorites are known to have undergone extensive aqueous alteration followed by different degrees of thermal alteration, to be similar to one another in regard to mineralogy, and share affinities with both the Ivuna-like carbonaceous (CI) and Mighei-like carbonaceous (CM) chondrites. While mineralogically similar, a key difference among these samples is that chondrules have been found in some of these samples, but not in others. The aim of this study is to evaluate the relationship of these meteorites to one another, and investigate how they relate to the CI and CM chondrite groups.</div><div>We find that with the addition of the isotopic compositions of these ‘CY’ chondrites, there is now a continuum of isotopic compositions among the carbonaceous chondrites. The CI chondrites are no longer separate in O isotopic compositions from the other carbonaceous chondrite groups in plots of Cr-O and Ti-O. We also find that the ‘CY’ chondrites represent two distinct populations, which correlate with their heating stage. However, the peak temperatures experienced by each population can only explain the differences in H and C isotopes and abundances and N abundances between samples, and cannot have caused the differences in N, Cr, and Ti isotopes, or all the volatile element depletions of the sample’s bulk compositions. Instead, we conclude that the compositional and isotopic data of these two populations correlate with their known chondrule abundances, indicating distinct precursors for each population. We find it most likely that these samples originate from two distinct asteroids, implying that among the six samples studied here, there are not five related samples to constitute a new meteorite group. The chondrule-free heating Stage III samples are most likely heated CI chondrites, while the chondrule-bearing heating Stage IV samples could be heated CM chondrites.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 24-37"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884584","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.12.018
Dongmei Tang , Marina Lazarov , Dachuan Wang , Noreen J. Evans , Kezhang Qin , Shengchao Xue , Ingo Horn , Stefan Weyer
The initial silicate melt related to magmatic Ni-Cu deposits located in orogenic settings in China (e.g., East Tianshan orogenic belt, East Kunlun orogenic belt) had a high fO2 that progressively decreased with continued magmatic evolution. It is still unknown if the sulfide melt that separated from the silicate melt inherited this high fO2, or even whether oxygen fugacity in sulfide plays an important role in the mineralization processes. In this work we undertook new in situ Fe, Cu, and Ni isotopic analyses of base metal sulfides from the Xiarihamu magmatic sulfide deposit (East Kunlun orogenic belt), and combined this new data with previously published Fe and Cu isotopic results from orogenic and cratonic magmatic sulfide deposits to assess changes in fO2 in sulfide during sulfide melt evolution, and the role of these processes in metal enrichment.
In the Xiarihamu deposit, pentlandite has a Fe/Ni ratio similar to high-temperature pentlandite (Fe4.95Ni4.08S7.96) found in the upper disseminated ores which host high-temperature maucherite inclusions. These findings indicate a high formation temperature for the sulfide in the upper disseminated ores. Atomic % Fe in pyrrhotite suggests that fO2 increased during the transition from disseminated mineralized ultramafic rocks (47.2–50.7), through net-textured + massive mineralized ultramafic rocks (47.1–48.1), to disseminated mineralized gabbros (46.9–47.3). Early crystallized, high temperature sulfides in disseminated ores do not display high oxygen fugacity characteristics (high Fe3+/ΣFe), whereas in the silicate melt, fO2 continued to decrease with progressed evolution. Orogenic magmatic sulfide deposits show consistent, uniquely lighter δ56Fe and δ65Cu in disseminated ores relative to the same sulfides from massive ores. This cannot be explained by crustal contamination and sulfide melt fractionation based on Fe and Cu isotopes. Uncoupled δ62Ni (insensitive to fO2 variations) and δ56Fe, as well as heavy δ56Fe and Co enrichment in late crystallized pentlandite (low temperature and high Fe3+/ΣFe) from the Xiarihamu and Kalatongke deposits (located in different orogenic belts), suggests that an increase in oxygen fugacity and related Fe3+/ΣFe ratios exert control on Co mineralization. Iron, Cu and Ni isotopes in sulfide can be used as indicators of Fe3+/ΣFe ratios in magmatic sulfide deposits in an orogenic environment, and changes in the Fe3+/ΣFe ratio play a critical role in Co enrichment.
{"title":"Response of Fe-Ni-Cu isotope systematics to sulfide liquid oxygen content: Implications for magmatic sulfide Co enrichment in orogenic settings","authors":"Dongmei Tang , Marina Lazarov , Dachuan Wang , Noreen J. Evans , Kezhang Qin , Shengchao Xue , Ingo Horn , Stefan Weyer","doi":"10.1016/j.gca.2024.12.018","DOIUrl":"10.1016/j.gca.2024.12.018","url":null,"abstract":"<div><div>The initial silicate melt related to magmatic Ni-Cu deposits located in orogenic settings in China (e.g., East Tianshan orogenic belt, East Kunlun orogenic belt) had a high <em>f</em>O<sub>2</sub> that progressively decreased with continued magmatic evolution. It is still unknown if the sulfide melt that separated from the silicate melt inherited this high <em>f</em>O<sub>2</sub>, or even whether oxygen fugacity in sulfide plays an important role in the mineralization processes. In this work we undertook new <em>in situ</em> Fe, Cu, and Ni isotopic analyses of base metal sulfides from the Xiarihamu magmatic sulfide deposit (East Kunlun orogenic belt), and combined this new data with previously published Fe and Cu isotopic results from orogenic and cratonic magmatic sulfide deposits to assess changes in <em>f</em>O<sub>2</sub> in sulfide during sulfide melt evolution, and the role of these processes in metal enrichment.</div><div>In the Xiarihamu deposit, pentlandite has a Fe/Ni ratio similar to high-temperature pentlandite (Fe<sub>4.95</sub>Ni<sub>4.08</sub>S<sub>7.96</sub>) found in the upper disseminated ores which host high-temperature maucherite inclusions. These findings indicate a high formation temperature for the sulfide in the upper disseminated ores. Atomic % Fe in pyrrhotite suggests that <em>f</em>O<sub>2</sub> increased during the transition from disseminated mineralized ultramafic rocks (47.2–50.7), through net-textured + massive mineralized ultramafic rocks (47.1–48.1), to disseminated mineralized gabbros (46.9–47.3). Early crystallized, high temperature sulfides in disseminated ores do not display high oxygen fugacity characteristics (high Fe<sup>3+</sup>/ΣFe), whereas in the silicate melt, <em>f</em>O<sub>2</sub> continued to decrease with progressed evolution. Orogenic magmatic sulfide deposits show consistent, uniquely lighter δ<sup>56</sup>Fe and δ<sup>65</sup>Cu in disseminated ores relative to the same sulfides from massive ores. This cannot be explained by crustal contamination and sulfide melt fractionation based on Fe and Cu isotopes. Uncoupled δ<sup>62</sup>Ni (insensitive to <em>f</em>O<sub>2</sub> variations) and δ<sup>56</sup>Fe, as well as heavy δ<sup>56</sup>Fe and Co enrichment in late crystallized pentlandite (low temperature and high Fe<sup>3+</sup>/ΣFe) from the Xiarihamu and Kalatongke deposits (located in different orogenic belts), suggests that an increase in oxygen fugacity and related Fe<sup>3+</sup>/ΣFe ratios exert control on Co mineralization. Iron, Cu and Ni isotopes in sulfide can be used as indicators of Fe<sup>3+</sup>/ΣFe ratios in magmatic sulfide deposits in an orogenic environment, and changes in the Fe<sup>3+</sup>/ΣFe ratio play a critical role in Co enrichment.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 1-23"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884629","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-02-01DOI: 10.1016/j.gca.2024.11.022
Hagar Hauzer , David Evans , Wolfgang Müller , Yair Rosenthal , Jonathan Erez
<div><div>The sodium-to-calcium ratio (Na/Ca) of biogenic CaCO<sub>3</sub> has recently been introduced as a proxy for past seawater Ca<sup>2+</sup> concentrations ([Ca<sup>2+</sup><sub>sw</sub>]) as demonstrated by a positive correlation between seawater and shell Na/Ca with a minor influence of salinity. In the present study, we investigate the effect of carbonate chemistry on the Na/Ca proxy by conducting a set of experiments in which pH and the concentration of dissolved inorganic carbon (DIC) were independently varied. In addition to Na<sup>+</sup>, the incorporation of Li<sup>+</sup>, Mg<sup>2+</sup>, and Sr<sup>2+</sup> into the shells of the large benthic high-Mg calcitic foraminifer <em>Operculina ammonoides</em> was assessed by culturing under constant DIC (∼2170 µmol kg<sup>−1</sup>) with varying pH (7.5–8.4 NBS scale), and under varying DIC (830–2470 µmol kg<sup>−1</sup>) with constant pH (∼7.9). Foraminiferal growth rate correlates linearly with calcite saturation state (Ω) of the experimental seawater (SW). The lowest pH and DIC experiments were characterized by low population growth rates, and some of these specimens died and their shells partially dissolved.</div><div>Na/Ca<sub>shell</sub> and Li/Ca<sub>shell</sub> in <em>O. ammonoides</em> are positively correlated with SW [CO<sub>3</sub><sup>2–</sup>] and Ω, whereas Sr/Ca<sub>shell</sub> and Mg/Ca<sub>shell</sub> are much less sensitive to these parameters. The relative sensitivity of Na/Ca<sub>shell</sub> to Ω in <em>O. ammonoides</em> is ∼ 4 % per Ω unit. However, given that past changes in surface water Ω were probably small relative to changes in [Ca<sup>2+</sup><sub>sw</sub>] the correction for this secondary effect over the Cenozoic is likely to be small. Therefore, we conclude that the sensitivity of <em>O. ammonoides</em> Na/Ca to the carbonate system is unlikely to compromise the use of this proxy to reconstruct past [Ca<sup>2+</sup><sub>sw</sub>]. In the case of the low-Mg planktic and benthic foraminifera, a data compilation exercise indicates that no resolvable carbonate chemistry effect exists on Na/Ca. Thus, the Na/Ca proxy in benthic nummulitid and planktic foraminifera can be utilized for past [Ca<sup>2+</sup><sub>sw</sub>] reconstructions. Furthermore, coupling this information with the distribution coefficients of other elemental and isotopic systems (e.g., Li<sup>+</sup>, Sr<sup>2+</sup>, Mg<sup>2+</sup>, K<sup>+</sup>, B, δ<sup>11</sup>B) may allow the reconstruction of wider aspects of past ocean chemistry. Finally, comparison of trace and minor element incorporation into low and high-Mg foraminiferal species, coccolithophores, inorganic calcite, and amorphous CaCO<sub>3</sub> (ACC), we propose a modified biomineralization model for hyaline foraminifera centered on SW vacuolization. Foraminiferal data can be explained by a biomineralization process in which high-Mg species utilize a precursor phase (ACC) to produce high-Mg calcite whereas low-Mg species acti
{"title":"The effect of carbonate chemistry on trace element incorporation in high-Mg calcitic foraminifera","authors":"Hagar Hauzer , David Evans , Wolfgang Müller , Yair Rosenthal , Jonathan Erez","doi":"10.1016/j.gca.2024.11.022","DOIUrl":"10.1016/j.gca.2024.11.022","url":null,"abstract":"<div><div>The sodium-to-calcium ratio (Na/Ca) of biogenic CaCO<sub>3</sub> has recently been introduced as a proxy for past seawater Ca<sup>2+</sup> concentrations ([Ca<sup>2+</sup><sub>sw</sub>]) as demonstrated by a positive correlation between seawater and shell Na/Ca with a minor influence of salinity. In the present study, we investigate the effect of carbonate chemistry on the Na/Ca proxy by conducting a set of experiments in which pH and the concentration of dissolved inorganic carbon (DIC) were independently varied. In addition to Na<sup>+</sup>, the incorporation of Li<sup>+</sup>, Mg<sup>2+</sup>, and Sr<sup>2+</sup> into the shells of the large benthic high-Mg calcitic foraminifer <em>Operculina ammonoides</em> was assessed by culturing under constant DIC (∼2170 µmol kg<sup>−1</sup>) with varying pH (7.5–8.4 NBS scale), and under varying DIC (830–2470 µmol kg<sup>−1</sup>) with constant pH (∼7.9). Foraminiferal growth rate correlates linearly with calcite saturation state (Ω) of the experimental seawater (SW). The lowest pH and DIC experiments were characterized by low population growth rates, and some of these specimens died and their shells partially dissolved.</div><div>Na/Ca<sub>shell</sub> and Li/Ca<sub>shell</sub> in <em>O. ammonoides</em> are positively correlated with SW [CO<sub>3</sub><sup>2–</sup>] and Ω, whereas Sr/Ca<sub>shell</sub> and Mg/Ca<sub>shell</sub> are much less sensitive to these parameters. The relative sensitivity of Na/Ca<sub>shell</sub> to Ω in <em>O. ammonoides</em> is ∼ 4 % per Ω unit. However, given that past changes in surface water Ω were probably small relative to changes in [Ca<sup>2+</sup><sub>sw</sub>] the correction for this secondary effect over the Cenozoic is likely to be small. Therefore, we conclude that the sensitivity of <em>O. ammonoides</em> Na/Ca to the carbonate system is unlikely to compromise the use of this proxy to reconstruct past [Ca<sup>2+</sup><sub>sw</sub>]. In the case of the low-Mg planktic and benthic foraminifera, a data compilation exercise indicates that no resolvable carbonate chemistry effect exists on Na/Ca. Thus, the Na/Ca proxy in benthic nummulitid and planktic foraminifera can be utilized for past [Ca<sup>2+</sup><sub>sw</sub>] reconstructions. Furthermore, coupling this information with the distribution coefficients of other elemental and isotopic systems (e.g., Li<sup>+</sup>, Sr<sup>2+</sup>, Mg<sup>2+</sup>, K<sup>+</sup>, B, δ<sup>11</sup>B) may allow the reconstruction of wider aspects of past ocean chemistry. Finally, comparison of trace and minor element incorporation into low and high-Mg foraminiferal species, coccolithophores, inorganic calcite, and amorphous CaCO<sub>3</sub> (ACC), we propose a modified biomineralization model for hyaline foraminifera centered on SW vacuolization. Foraminiferal data can be explained by a biomineralization process in which high-Mg species utilize a precursor phase (ACC) to produce high-Mg calcite whereas low-Mg species acti","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"390 ","pages":"Pages 105-116"},"PeriodicalIF":4.5,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142804470","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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