The Experts below are selected from a list of 132 Experts worldwide ranked by ideXlab platform
Jeanjacques Braun - One of the best experts on this subject based on the ideXlab platform.
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Limited iron isotope variations in recent lateritic soils from Nsimi, Cameroon: Implications for the global Fe Geochemical Cycle
Chemical Geology, 2020Co-Authors: Franck Poitrasson, Jerome Viers, Francois Martin, Jeanjacques BraunAbstract:International audienceLaterites are oxidized Fe-rich soils covering one third of the continents and are drained by half of the continental waters. They therefore represent a key component of the iron Geochemical Cycle at the Earth's surface, yet no iron isotope study has been conducted on recent laterites so far. Building on previous integrated morpho-pedological studies of soils located in an equatorial rainforest, Southern Cameroon, we have undertaken a mineralogical, elemental and isotopic study of iron in two lateritic profiles. One is a borehole 3620 cm deep going to the parent granodioritic rock, located at the top of a hill, whereas the other is 675 cm deep and is located downhill. Mossbauer spectroscopy reveals that typically >= 96% of the soil's iron is held in nanocrystalline hematite and goethite. Iron isotope measurements performed by plasma source mass spectrometry show that Fe isotopic equilibrium was rarely reached between these iron oxide and hydroxide despite their small size. Overall, it is found that most samples display iron isotope signatures very close to the mean crustal value, with a maximum range of 0.2 parts per thousand in delta Fe-57. Given that lateritic soils evolve over millions of years, show large variations in Fe concentrations and mineralogical abundances, this range is surprisingly small when compared to other Fe isotope studies of soils from different climatological contexts, that can easily show delta Fe-57 ranges in excess of 1 parts per thousand. at the bulk sample scale. The most likely explanation of this finding is that despite notable vertical and lateral Fe mobility in the studied laterites from Cameroon, as computed using the open-system mass fraction transport function, tau(Fe,w), iron remained mostly in the oxidized form as shown by Mossbauer spectroscopy. This probably results from the strong bioturbation and pedoturbation of these lateritic soils that lead them to remain porous over a large thickness and facilitated the flow of oxygenated waters from the surface down to the saprolite horizon. This study therefore reveals that soils that remained as an open system for iron do not necessarily show large Fe isotopic variations. It is suggested that the Fe drained from such lateritic soils should have delta Fe-57 values within similar to 0.1 parts per thousand. of that of the continental crust. This is in contrast with previous soil studies, notably from sites located at higher latitudes, that imply that these soils should release isotopically more variable Fe to surface waters. Such possible contrasted isotopic signatures from different surface waters will likely lead to an isotopically heterogeneous ocean given the short residence time of Fe in seawater
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limited iron isotope variations in recent lateritic soils from nsimi cameroon implications for the global fe Geochemical Cycle
Chemical Geology, 2008Co-Authors: Franck Poitrasson, Jerome Viers, Francois Martin, Jeanjacques BraunAbstract:Laterites are oxidized Fe-rich soils covering one third of the continents and are drained by half of the continental waters. They therefore represent a key component of the iron Geochemical Cycle at the Earth's surface, yet no iron isotope study has been conducted on recent laterites so far. Building on previous integrated morpho-pedological studies of soils located in an equatorial rainforest, Southern Cameroon, we have undertaken a mineralogical, elemental and isotopic study of iron in two lateritic profiles. One is a borehole 3620 cm deep going to the parent granodioritic rock, located at the top of a hill, whereas the other is 675 cm deep and is located downhill. Mossbauer spectroscopy reveals that typically $\geq 96%$ of the soil's iron is held in nanocrystalline hematite and goethite. Iron isotope measurements performed by plasma source mass spectrometry show that Fe isotopic equilibrium was rarely reached between these iron oxide and hydroxide despite their small size. Overall, it is found that most samples display iron isotope signatures very close to the mean crustal value, with a maximum range of 0.2% in $\delta^{57}Fe$. Given that lateritic soils evolve over millions of years, show large variations in Fe concentrations and mineralogical abundances, this range is surprisingly small when compared to other Fe isotope studies of soils from different climatological contexts, that can easily show $\delta^{57}F$e ranges in excess of 1% at the bulk sample scale. The most likely explanation of this finding is that despite notable vertical and lateral Fe mobility in the studied laterites from Cameroon, as computed using the open-system mass fraction transport function,$^{\tau}Fe,W$ iron remained mostly in the oxidized form as shown by Mossbauer pectroscopy. This probably results from the strong bioturbation and pedoturbation of these lateritic soils that lead them to remain porous over a large thickness and facilitated the flow of oxygenated waters from the surface down to the saprolite horizon. This study therefore reveals that soils that remained as an open system for iron do not necessarily show large Fe isotopic variations. It is suggested that the Fe drained from such lateritic soils should have $\delta^{57} Fe$ values within $\sim 0.1$ of that of the continental crust. This is in contrast with previous soil studies, notably from sites located at higher latitudes, that imply that these soils should release isotopically more variable Fe to surface waters. Such possible contrasted isotopic signatures from different surface waters will likely lead to an isotopically heterogeneous ocean given the short residence time of Fe in seawater.
Judith L. Hannah - One of the best experts on this subject based on the ideXlab platform.
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temporal record of osmium concentrations and 187os 188os in organic rich mudrocks implications for the osmium Geochemical Cycle and the use of osmium as a paleoceanographic tracer
Geochimica et Cosmochimica Acta, 2017Co-Authors: Xinze Lu, Holly J. Stein, Judith L. Hannah, Brian KendallAbstract:The final publication is available at Elsevier via https://doi.org/10.1016/j.gca.2017.06.046 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
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Temporal record of osmium concentrations and187Os/188Os in organic-rich mudrocks: Implications for the osmium Geochemical Cycle and the use of osmium as a paleoceanographic tracer
Geochimica et Cosmochimica Acta, 2017Co-Authors: Xinze Lu, Holly J. Stein, Brian Kendall, Judith L. HannahAbstract:We present a compilation of192Os concentrations (representing non-radiogenic Os) and initial187Os/188Os isotope ratios from organic-rich mudrocks (ORM) to explore the evolution of the Os Geochemical Cycle during the past three billion years. The initial187Os/188Os isotope ratio of a Re-Os isochron regression for ORM constrains the local paleo-seawater187Os/188Os, which is governed by the relative magnitudes of radiogenic Os (old continental crust) and unradiogenic Os (mantle, extraterrestrial, and juvenile/mafic/ultramafic crust) fluxes to seawater. A first-order increase in seawater187Os/188Os ratios occurs from the Archean to the Phanerozoic, and may reflect a combination of increasing atmosphere-ocean oxygenation and weathering of progressively more radiogenic continental crust due to in-growth of187Os from radioactive decay of187Re. Superimposed on this long-term trend are shorter-term fluctuations in seawater187Os/188Os ratios as a result of climate change, emplacement of large igneous provinces, bolide impacts, tectonic events, changes in seafloor spreading rates, and lithological changes in crustal terranes proximal to sites of ORM deposition. Ediacaran-Phanerozoic ORM have mildly higher192Os concentrations overall compared with pre-Ediacaran Proterozoic ORM based on the mean and 95% confidence interval of 10,000 median values derived using a bootstrap analysis for each time bin (insufficient Archean data exist for robust statistical comparisons). However, there are two groups with anomalously high192Os concentrations that are distinguished by their initial187Os/188Os isotope ratios. Ediacaran-Cambrian ORM from South China have radiogenic initial187Os/188Os, suggesting their high192Os concentrations reflect proximal Os-rich crustal source(s), ultraslow sedimentation rates, and/or other unusual depositional conditions. In contrast, the unradiogenic initial187Os/188Os and high192Os concentrations of some Mesozoic ORM can be tied to emplacement of large igneous provinces. Excluding these two anomalous groups and repeating the bootstrap analysis, we find that, overall, the192Os concentrations for the Ediacaran-Phanerozoic and pre-Ediacaran Proterozoic time bins are not significantly different. An improved understanding of Os Geochemical behavior in modern environments is required before our compilation can be fully used to constrain the temporal evolution of the seawater Os reservoir.
Laurent Simon - One of the best experts on this subject based on the ideXlab platform.
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Modelling the Geochemical Cycle of boron: Implications for the long-term δ11B evolution of seawater and oceanic crust
Chemical Geology, 2020Co-Authors: Laurent Simon, Christophe Lecuyer, Chloe Marechal, Nicolas ColticeAbstract:International audienceThe boron Geochemical Cycle has been simulated using a time-dependent Geochemical box model that was coupled to a onedimension model of seawater–oceanic crust interactions. Boron elemental and isotopic compositions of oceanic rocks as a function of depth were calculated by mass balance, using the temperature and porosity profiles of the crust as well as the available experimental and empirical distribution coefficients and fractionation factors between mineral and water. Ranges of boron elemental and isotopic variations of seawater were calculated for crust–seawater interactions that take place from the ridge-axis to the off-axis closure of the hydrothermal system. The present-day d11B of seawater (40‰) could represent a steady-state value. However, depending on crustal permeability, lifetime of water–rock interactions, and expansion rate of the oceanic ridge, the d11B of seawater may vary from 30‰ to 50‰ at the 10 million year scale. Some boron isotope compositions of Cretaceous biogenic carbonates and ophiolitic serpentinites from Oman are comparable to modern rock samples, suggesting that the d11B of Cretaceous seawater was close to the present-day value. Low d11B values of some biogenic carbonates cannot be attributed to low pH values of past seawater, but more probably to d11B variations of seawate or diagenetic alteration by crustal aqueous fluids. Boron isotope composition of hydrothermally altered serpentines could be considered as a promising proxy of the seawater composition
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modelling the Geochemical Cycle of boron implications for the long term δ11b evolution of seawater and oceanic crust
Chemical Geology, 2006Co-Authors: Laurent Simon, Christophe Lecuyer, Chloe Marechal, Nicolas ColticeAbstract:The boron Geochemical Cycle has been simulated using a time-dependent Geochemical box model that was coupled to a onedimension model of seawater–oceanic crust interactions. Boron elemental and isotopic compositions of oceanic rocks as a function of depth were calculated by mass balance, using the temperature and porosity profiles of the crust as well as the available experimental and empirical distribution coefficients and fractionation factors between mineral and water. Ranges of boron elemental and isotopic variations of seawater were calculated for crust–seawater interactions that take place from the ridge-axis to the off-axis closure of the hydrothermal system. The present-day d11B of seawater (40‰) could represent a steady-state value. However, depending on crustal permeability, lifetime of water–rock interactions, and expansion rate of the oceanic ridge, the d11B of seawater may vary from 30‰ to 50‰ at the 10 million year scale. Some boron isotope compositions of Cretaceous biogenic carbonates and ophiolitic serpentinites from Oman are comparable to modern rock samples, suggesting that the d11B of Cretaceous seawater was close to the present-day value. Low d11B values of some biogenic carbonates cannot be attributed to low pH values of past seawater, but more probably to d11B variations of seawate or diagenetic alteration by crustal aqueous fluids. Boron isotope composition of hydrothermally altered serpentines could be considered as a promising proxy of the seawater composition.
Holly J. Stein - One of the best experts on this subject based on the ideXlab platform.
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temporal record of osmium concentrations and 187os 188os in organic rich mudrocks implications for the osmium Geochemical Cycle and the use of osmium as a paleoceanographic tracer
Geochimica et Cosmochimica Acta, 2017Co-Authors: Xinze Lu, Holly J. Stein, Judith L. Hannah, Brian KendallAbstract:The final publication is available at Elsevier via https://doi.org/10.1016/j.gca.2017.06.046 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
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Temporal record of osmium concentrations and187Os/188Os in organic-rich mudrocks: Implications for the osmium Geochemical Cycle and the use of osmium as a paleoceanographic tracer
Geochimica et Cosmochimica Acta, 2017Co-Authors: Xinze Lu, Holly J. Stein, Brian Kendall, Judith L. HannahAbstract:We present a compilation of192Os concentrations (representing non-radiogenic Os) and initial187Os/188Os isotope ratios from organic-rich mudrocks (ORM) to explore the evolution of the Os Geochemical Cycle during the past three billion years. The initial187Os/188Os isotope ratio of a Re-Os isochron regression for ORM constrains the local paleo-seawater187Os/188Os, which is governed by the relative magnitudes of radiogenic Os (old continental crust) and unradiogenic Os (mantle, extraterrestrial, and juvenile/mafic/ultramafic crust) fluxes to seawater. A first-order increase in seawater187Os/188Os ratios occurs from the Archean to the Phanerozoic, and may reflect a combination of increasing atmosphere-ocean oxygenation and weathering of progressively more radiogenic continental crust due to in-growth of187Os from radioactive decay of187Re. Superimposed on this long-term trend are shorter-term fluctuations in seawater187Os/188Os ratios as a result of climate change, emplacement of large igneous provinces, bolide impacts, tectonic events, changes in seafloor spreading rates, and lithological changes in crustal terranes proximal to sites of ORM deposition. Ediacaran-Phanerozoic ORM have mildly higher192Os concentrations overall compared with pre-Ediacaran Proterozoic ORM based on the mean and 95% confidence interval of 10,000 median values derived using a bootstrap analysis for each time bin (insufficient Archean data exist for robust statistical comparisons). However, there are two groups with anomalously high192Os concentrations that are distinguished by their initial187Os/188Os isotope ratios. Ediacaran-Cambrian ORM from South China have radiogenic initial187Os/188Os, suggesting their high192Os concentrations reflect proximal Os-rich crustal source(s), ultraslow sedimentation rates, and/or other unusual depositional conditions. In contrast, the unradiogenic initial187Os/188Os and high192Os concentrations of some Mesozoic ORM can be tied to emplacement of large igneous provinces. Excluding these two anomalous groups and repeating the bootstrap analysis, we find that, overall, the192Os concentrations for the Ediacaran-Phanerozoic and pre-Ediacaran Proterozoic time bins are not significantly different. An improved understanding of Os Geochemical behavior in modern environments is required before our compilation can be fully used to constrain the temporal evolution of the seawater Os reservoir.
Xinze Lu - One of the best experts on this subject based on the ideXlab platform.
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temporal record of osmium concentrations and 187os 188os in organic rich mudrocks implications for the osmium Geochemical Cycle and the use of osmium as a paleoceanographic tracer
Geochimica et Cosmochimica Acta, 2017Co-Authors: Xinze Lu, Holly J. Stein, Judith L. Hannah, Brian KendallAbstract:The final publication is available at Elsevier via https://doi.org/10.1016/j.gca.2017.06.046 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
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Temporal record of osmium concentrations and187Os/188Os in organic-rich mudrocks: Implications for the osmium Geochemical Cycle and the use of osmium as a paleoceanographic tracer
Geochimica et Cosmochimica Acta, 2017Co-Authors: Xinze Lu, Holly J. Stein, Brian Kendall, Judith L. HannahAbstract:We present a compilation of192Os concentrations (representing non-radiogenic Os) and initial187Os/188Os isotope ratios from organic-rich mudrocks (ORM) to explore the evolution of the Os Geochemical Cycle during the past three billion years. The initial187Os/188Os isotope ratio of a Re-Os isochron regression for ORM constrains the local paleo-seawater187Os/188Os, which is governed by the relative magnitudes of radiogenic Os (old continental crust) and unradiogenic Os (mantle, extraterrestrial, and juvenile/mafic/ultramafic crust) fluxes to seawater. A first-order increase in seawater187Os/188Os ratios occurs from the Archean to the Phanerozoic, and may reflect a combination of increasing atmosphere-ocean oxygenation and weathering of progressively more radiogenic continental crust due to in-growth of187Os from radioactive decay of187Re. Superimposed on this long-term trend are shorter-term fluctuations in seawater187Os/188Os ratios as a result of climate change, emplacement of large igneous provinces, bolide impacts, tectonic events, changes in seafloor spreading rates, and lithological changes in crustal terranes proximal to sites of ORM deposition. Ediacaran-Phanerozoic ORM have mildly higher192Os concentrations overall compared with pre-Ediacaran Proterozoic ORM based on the mean and 95% confidence interval of 10,000 median values derived using a bootstrap analysis for each time bin (insufficient Archean data exist for robust statistical comparisons). However, there are two groups with anomalously high192Os concentrations that are distinguished by their initial187Os/188Os isotope ratios. Ediacaran-Cambrian ORM from South China have radiogenic initial187Os/188Os, suggesting their high192Os concentrations reflect proximal Os-rich crustal source(s), ultraslow sedimentation rates, and/or other unusual depositional conditions. In contrast, the unradiogenic initial187Os/188Os and high192Os concentrations of some Mesozoic ORM can be tied to emplacement of large igneous provinces. Excluding these two anomalous groups and repeating the bootstrap analysis, we find that, overall, the192Os concentrations for the Ediacaran-Phanerozoic and pre-Ediacaran Proterozoic time bins are not significantly different. An improved understanding of Os Geochemical behavior in modern environments is required before our compilation can be fully used to constrain the temporal evolution of the seawater Os reservoir.