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Robert C. Finkel - One of the best experts on this subject based on the ideXlab platform.

  • Cosmogenic nuclide methods for measuring long-term rates of physical erosion and Chemical Weathering
    Journal of Geochemical Exploration, 2005
    Co-Authors: James W. Kirchner, Clifford S. Riebe, Ken L. Ferrier, Robert C. Finkel
    Abstract:

    Understanding the evolution of geoChemical and geomorphic systems requires measurements of long-term rates of physical erosion and Chemical Weathering. Erosion and Weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and Weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and Chemical Weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and Chemical Weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and Chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and Chemical Weathering over 1000- to 10,000-year time scales.

  • Erosional and climatic effects on long-term Chemical Weathering rates in granitic landscapes spanning diverse climate regimes ☆
    Earth and Planetary Science Letters, 2004
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Robert C. Finkel
    Abstract:

    Abstract We used cosmogenic nuclide and geoChemical mass balance methods to measure long-term rates of Chemical Weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 °C, average precipitation ranging from 22 to 420 cm·year −1 , and denudation rates ranging from 23 to 755 t·km −2 ·year −1 . Long-term Chemical Weathering rates range from 0 to 173 t·km −2 year −1 , in several cases exceeding the highest granitic Weathering rates on record from previous work. Chemical Weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of Chemical Weathering and mineral supply from breakdown of rock. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89–95% of the variation in long-term Weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, Chemical Weathering rates increase proportionally with fresh-material supply rates. We refer to this as “supply-limited” Weathering, in which fresh material is Chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of Chemical Weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar Weathering and previous inter-comparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate Weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that Chemical Weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.

  • Sharp decrease in long-term Chemical Weathering rates along an altitudinal transect
    Earth and Planetary Science Letters, 2004
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Robert C. Finkel
    Abstract:

    We used cosmogenic nuclide and geoChemical mass balance methods to measure long-term rates of Chemical Weathering and physical erosion across a steep climatic gradient in the Santa Rosa Mountains, Nevada. Our study sites are distributed along a 2 km ridgeline transect that spans 2090 to 2750 m in altitude, and encompasses marked contrasts in both vegetative cover and snow depth, but is underlain by a single, roughly uniform, granodiorite bedrock. Cosmogenic nuclides in colluvial soils reveal that denudation rates vary by less than a factor of 1.4 (104–144 t/km2/yr) along this transect. Bulk elemental analyses indicate that, relative to the parent rock, soils are less intensively weathered with increasing altitude, and show little evidence of Weathering-related mass losses near the top of the ridge. Chemical Weathering rates decrease rapidly with increasing altitude, both in absolute terms (from 24 to 0 t/km2/yr) and as a fraction of total denudation rates (from 20 to 0%). Thus these results indicate an increasing dominance of physical erosion with altitude. The observed decrease in Chemical Weathering rates is greater than one would predict from the decrease in mean annual temperature using simple Weathering kinetics, suggesting that Weathering rates along our transect may also be affected by the progressive decline in vegetative cover and increase in snow depth with increasing altitude. These results, considered together with Weathering rate measurements for a wide range of climates in the Sierra Nevada, USA, suggest that Chemical Weathering rates may be particularly sensitive to differences in climate at higher-altitude sites. Consistent with this hypothesis, Chemical Weathering rates fall virtually to zero at the highest sites on our transect, suggesting that sparsely vegetated, high-altitude crystalline terrain may often be characterized by extremely slow silicate Weathering rates.

  • long term rates of Chemical Weathering and physical erosion from cosmogenic nuclides and geoChemical mass balance
    Geochimica et Cosmochimica Acta, 2003
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Robert C. Finkel
    Abstract:

    Quantifying long-term rates of Chemical Weathering and physical erosion is important for under- standing the long-term evolution of soils, landscapes, and Earth's climate. Here we describe how long-term Chemical Weathering rates can be measured for actively eroding landscapes using cosmogenic nuclides together with a geoChemical mass balance of weathered soil and parent rock. We tested this approach in the Rio Icacos watershed, Puerto Rico, where independent studies have estimated Weathering rates over both short and long timescales. Results from the cosmogenic/mass balance method are consistent with three independent sets of Weathering rate estimates, thus confirming that this approach yields realistic measurements of long-term Weathering rates. This approach can separately quantify Weathering rates from saprolite and from overlying soil as components of the total. At Rio Icacos, nearly 50% of Si Weathering occurs as rock is converted to saprolite; in contrast, nearly 100% of Al Weathering occurs in the soil. Physical erosion rates are measured as part of our mass balance approach, making it particularly useful for studying interrelationships between Chemical Weathering and physical erosion. Our data show that Chemical Weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and Chemical Weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. One can normalize for variations in physical erosion rates using the "Chemical depletion fraction," which measures the fraction of total denudation that is accounted for by Chemical Weathering. This measure of Chemical Weathering intensity increases with increasing average temperature and precipitation in data from climatically diverse granitic sites, including tropical Rio Icacos and six temperate sites in the Sierra Nevada, California. Hence, across a wide range of climate regimes, analysis of Chemical depletion fractions appears to effectively account for site-to-site differences in physical erosion rates, which would otherwise obscure climatic effects on Chemical Weathering rates. Our results show that by quantifying rates of physical erosion and Chemical Weathering together, our mass balance approach can be used to determine the relative importance of climatic and nonclimatic factors in regulating long-term Chemical Weathering rates. Copyright © 2003 Elsevier Ltd

  • Strong tectonic and weak climatic control of long-term Chemical Weathering rates
    Geology, 2001
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Darryl E. Granger, Robert C. Finkel
    Abstract:

    The relationships among climate, physical erosion, and Chemical Weathering have remained uncertain, because long-term Chemical Weathering rates have been difficult to measure. Here we show that long-term Chemical Weathering rates can be measured by combining physical erosion rates, inferred from cosmogenic nuclides, with dissolution losses, inferred from the rock-to-soil enrichment of insoluble elements. We used this method to measure Chemical Weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates. Chemical Weathering rates correlate strongly with physical erosion rates but only weakly with climate, implying that, by regulating erosion rates, tectonic uplift may significantly accelerate Chemical Weathering rates in granitic landscapes.

Zhengang Wang - One of the best experts on this subject based on the ideXlab platform.

  • Estimation of carbon sinks in Chemical Weathering in a humid subtropical mountainous basin
    Chinese Science Bulletin, 2011
    Co-Authors: Zhengang Wang, Shenghua Zhang, Xiaobing Ruan, Shuhong Li
    Abstract:

    The fluvial geochemistry of the mainstream and tributaries of the Zengjiang River was investigated, and the mass balance approach and deduction methods were used to estimate the uptake of atmospheric CO2 through rock Chemical Weathering. The results showed that the Chemical runoff mainly consisted of HCO3−, Ca2+, Na+, and dissolved Si, and that silicate mineral Weathering was significant, but carbonate mineral Weathering was a minor source of dissolved loads in the Zengjiang River basin because of the low amount of interlayered carbonate rock strata in the catchment. The ion composition indicated that atmospheric CO2 was the primary erosive agent for rock Chemical Weathering in the Zengjiang River basin. The CO2 consumption fluxes caused by rock Chemical Weathering were (3.50–3.81) × 105 mol km−2 a−1, which is just lower than that in tropical and subtropical basalt and carbonate regions, and is much higher than that in temperate and cold-temperate regions, indicating that surficial Chemical Weathering processes in the humid and hot monsoon current influencing the low-middle latitude zone of the Northern Hemisphere constitutes a significant carbon sink in the global biogeoChemical cycle.

  • Chemical Weathering and co2 consumption in the xijiang river basin south china
    Geomorphology, 2009
    Co-Authors: Xiakun Huang, Kefu Yu, Zhengang Wang
    Abstract:

    Monthly samples of riverine water were collected and analyzed for the concentrations of major ions (Ca2+, Mg2+, K+, Na+, HCO3-, SO42-, Cl-, NO3-), dissolved silicon, and total dissolved solids (TDS) at Wuzhou hydrological station located between the middle and lower reaches of the Xijiang River (XJR) from March 2005 to April 2006. More frequent sampling and analysis were carried out during the catastrophic flooding in June 2005. Stoichiometric analysis was applied for tracing sources of major ions and estimating CO2 consumption from the Chemical Weathering of rocks. The results demonstrate that the Chemical Weathering of carbonate and silicate rocks within the drainage basin is the main source of the dissolved Chemical substances in the XJR. Some 81.20% of the riverine cations originated from the Chemical Weathering processes induced by carbonic acid, 11.32% by sulfuric acid, and the other 7.48% from the dissolution of gypsum and precipitates of sea salts within the drainage basin. The CO2 flux consumed by the rock Chemical Weathering within the XJR basin is 2.37 x 10(11) mol y(-1), of which 0.64 x 10(11) mol y(-1) results from silicate rock Chemical Weathering, and 1.73 x 10(11) mol y(-1) results from carbonate rock Chemical Weathering. The CO2 consumption comprises 0.38 x 10(11) mol during the 9-d catastrophic flooding. The CO2 consumption from rock Chemical Weathering in humid subtropical zones regulates atmospheric CO2 level and constitutes a significant part of the global carbon budget. The carbon sink potential of rock Chemical Weathering processes in the humid subtropical zones deserves extra attention. (C) 2008 Elsevier B.V. All rights reserved.

Clifford S. Riebe - One of the best experts on this subject based on the ideXlab platform.

  • Cosmogenic nuclide methods for measuring long-term rates of physical erosion and Chemical Weathering
    Journal of Geochemical Exploration, 2005
    Co-Authors: James W. Kirchner, Clifford S. Riebe, Ken L. Ferrier, Robert C. Finkel
    Abstract:

    Understanding the evolution of geoChemical and geomorphic systems requires measurements of long-term rates of physical erosion and Chemical Weathering. Erosion and Weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and Weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and Chemical Weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and Chemical Weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and Chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and Chemical Weathering over 1000- to 10,000-year time scales.

  • Erosional and climatic effects on long-term Chemical Weathering rates in granitic landscapes spanning diverse climate regimes ☆
    Earth and Planetary Science Letters, 2004
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Robert C. Finkel
    Abstract:

    Abstract We used cosmogenic nuclide and geoChemical mass balance methods to measure long-term rates of Chemical Weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 °C, average precipitation ranging from 22 to 420 cm·year −1 , and denudation rates ranging from 23 to 755 t·km −2 ·year −1 . Long-term Chemical Weathering rates range from 0 to 173 t·km −2 year −1 , in several cases exceeding the highest granitic Weathering rates on record from previous work. Chemical Weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of Chemical Weathering and mineral supply from breakdown of rock. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89–95% of the variation in long-term Weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, Chemical Weathering rates increase proportionally with fresh-material supply rates. We refer to this as “supply-limited” Weathering, in which fresh material is Chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of Chemical Weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar Weathering and previous inter-comparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate Weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that Chemical Weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.

  • Sharp decrease in long-term Chemical Weathering rates along an altitudinal transect
    Earth and Planetary Science Letters, 2004
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Robert C. Finkel
    Abstract:

    We used cosmogenic nuclide and geoChemical mass balance methods to measure long-term rates of Chemical Weathering and physical erosion across a steep climatic gradient in the Santa Rosa Mountains, Nevada. Our study sites are distributed along a 2 km ridgeline transect that spans 2090 to 2750 m in altitude, and encompasses marked contrasts in both vegetative cover and snow depth, but is underlain by a single, roughly uniform, granodiorite bedrock. Cosmogenic nuclides in colluvial soils reveal that denudation rates vary by less than a factor of 1.4 (104–144 t/km2/yr) along this transect. Bulk elemental analyses indicate that, relative to the parent rock, soils are less intensively weathered with increasing altitude, and show little evidence of Weathering-related mass losses near the top of the ridge. Chemical Weathering rates decrease rapidly with increasing altitude, both in absolute terms (from 24 to 0 t/km2/yr) and as a fraction of total denudation rates (from 20 to 0%). Thus these results indicate an increasing dominance of physical erosion with altitude. The observed decrease in Chemical Weathering rates is greater than one would predict from the decrease in mean annual temperature using simple Weathering kinetics, suggesting that Weathering rates along our transect may also be affected by the progressive decline in vegetative cover and increase in snow depth with increasing altitude. These results, considered together with Weathering rate measurements for a wide range of climates in the Sierra Nevada, USA, suggest that Chemical Weathering rates may be particularly sensitive to differences in climate at higher-altitude sites. Consistent with this hypothesis, Chemical Weathering rates fall virtually to zero at the highest sites on our transect, suggesting that sparsely vegetated, high-altitude crystalline terrain may often be characterized by extremely slow silicate Weathering rates.

  • long term rates of Chemical Weathering and physical erosion from cosmogenic nuclides and geoChemical mass balance
    Geochimica et Cosmochimica Acta, 2003
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Robert C. Finkel
    Abstract:

    Quantifying long-term rates of Chemical Weathering and physical erosion is important for under- standing the long-term evolution of soils, landscapes, and Earth's climate. Here we describe how long-term Chemical Weathering rates can be measured for actively eroding landscapes using cosmogenic nuclides together with a geoChemical mass balance of weathered soil and parent rock. We tested this approach in the Rio Icacos watershed, Puerto Rico, where independent studies have estimated Weathering rates over both short and long timescales. Results from the cosmogenic/mass balance method are consistent with three independent sets of Weathering rate estimates, thus confirming that this approach yields realistic measurements of long-term Weathering rates. This approach can separately quantify Weathering rates from saprolite and from overlying soil as components of the total. At Rio Icacos, nearly 50% of Si Weathering occurs as rock is converted to saprolite; in contrast, nearly 100% of Al Weathering occurs in the soil. Physical erosion rates are measured as part of our mass balance approach, making it particularly useful for studying interrelationships between Chemical Weathering and physical erosion. Our data show that Chemical Weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and Chemical Weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. One can normalize for variations in physical erosion rates using the "Chemical depletion fraction," which measures the fraction of total denudation that is accounted for by Chemical Weathering. This measure of Chemical Weathering intensity increases with increasing average temperature and precipitation in data from climatically diverse granitic sites, including tropical Rio Icacos and six temperate sites in the Sierra Nevada, California. Hence, across a wide range of climate regimes, analysis of Chemical depletion fractions appears to effectively account for site-to-site differences in physical erosion rates, which would otherwise obscure climatic effects on Chemical Weathering rates. Our results show that by quantifying rates of physical erosion and Chemical Weathering together, our mass balance approach can be used to determine the relative importance of climatic and nonclimatic factors in regulating long-term Chemical Weathering rates. Copyright © 2003 Elsevier Ltd

  • Strong tectonic and weak climatic control of long-term Chemical Weathering rates
    Geology, 2001
    Co-Authors: Clifford S. Riebe, James W. Kirchner, Darryl E. Granger, Robert C. Finkel
    Abstract:

    The relationships among climate, physical erosion, and Chemical Weathering have remained uncertain, because long-term Chemical Weathering rates have been difficult to measure. Here we show that long-term Chemical Weathering rates can be measured by combining physical erosion rates, inferred from cosmogenic nuclides, with dissolution losses, inferred from the rock-to-soil enrichment of insoluble elements. We used this method to measure Chemical Weathering rates across 22 mountainous granitic catchments that span a wide range of erosion rates and climates. Chemical Weathering rates correlate strongly with physical erosion rates but only weakly with climate, implying that, by regulating erosion rates, tectonic uplift may significantly accelerate Chemical Weathering rates in granitic landscapes.

Thomas J Algeo - One of the best experts on this subject based on the ideXlab platform.

  • intensified Chemical Weathering during early triassic revealed by magnesium isotopes
    Geochimica et Cosmochimica Acta, 2020
    Co-Authors: Xinyang Chen, Fangzhen Teng, Kangjun Huang, Thomas J Algeo
    Abstract:

    Abstract Marine ecosystem recovery after the latest Permian mass extinction (LPME) was a protracted process during the Early Triassic (∼252–247 Ma) owing to repeated climatic and environmental perturbations. Chemical Weathering can supply nutrients to the ocean and may have played an important role in the Early Triassic carbon cycle and biological recovery. However, only limited geoChemical records of Chemical Weathering during the Early Triassic have been presented to date, and the relationship between changes in Weathering intensity and the slow recovery of marine ecosystems is effectively unknown. Here, we report magnesium (Mg) isotopic compositions of the siliciclastic components from shallow-marine carbonates in two well-studied Upper Permian-Lower Triassic sections in Iran and South China to track changes in Chemical Weathering intensity after the LPME. Both sections display a wide range of δ26Mg values (−2.09‰ to +1.10‰ at Zal, −2.30‰ to +0.33‰ at Zuodeng). We identified two distinct stages (I and II) in each section based on δ26Mg values and major elemental ratios. Variations of δ26Mg values in Stage I (Changhsingian to mid-Dienerian) are mainly controlled by mineralogical composition that can obscure Weathering signals. By contrast, δ26Mg variations in Stage II (upper Dienerian to upper Spathian) are independent of lithology and inferred to reflect control by Chemical Weathering intensity. The trends in Chemical Weathering intensity within Stage II correspond to first-order variations in climate and carbon cycling. Marine ecosystem recovery during the Early Triassic may have been linked to recurrent episodes of intense Chemical Weathering caused by CO2 degassing and climate warming. This study demonstrates the potential utility of the Mg isotopic compositions of the silicate fraction in marine carbonates as a proxy for Chemical Weathering intensity, laying the groundwork for general applications of this method to deep-time Earth systems.

Kangjun Huang - One of the best experts on this subject based on the ideXlab platform.

  • intensified Chemical Weathering during early triassic revealed by magnesium isotopes
    Geochimica et Cosmochimica Acta, 2020
    Co-Authors: Xinyang Chen, Fangzhen Teng, Kangjun Huang, Thomas J Algeo
    Abstract:

    Abstract Marine ecosystem recovery after the latest Permian mass extinction (LPME) was a protracted process during the Early Triassic (∼252–247 Ma) owing to repeated climatic and environmental perturbations. Chemical Weathering can supply nutrients to the ocean and may have played an important role in the Early Triassic carbon cycle and biological recovery. However, only limited geoChemical records of Chemical Weathering during the Early Triassic have been presented to date, and the relationship between changes in Weathering intensity and the slow recovery of marine ecosystems is effectively unknown. Here, we report magnesium (Mg) isotopic compositions of the siliciclastic components from shallow-marine carbonates in two well-studied Upper Permian-Lower Triassic sections in Iran and South China to track changes in Chemical Weathering intensity after the LPME. Both sections display a wide range of δ26Mg values (−2.09‰ to +1.10‰ at Zal, −2.30‰ to +0.33‰ at Zuodeng). We identified two distinct stages (I and II) in each section based on δ26Mg values and major elemental ratios. Variations of δ26Mg values in Stage I (Changhsingian to mid-Dienerian) are mainly controlled by mineralogical composition that can obscure Weathering signals. By contrast, δ26Mg variations in Stage II (upper Dienerian to upper Spathian) are independent of lithology and inferred to reflect control by Chemical Weathering intensity. The trends in Chemical Weathering intensity within Stage II correspond to first-order variations in climate and carbon cycling. Marine ecosystem recovery during the Early Triassic may have been linked to recurrent episodes of intense Chemical Weathering caused by CO2 degassing and climate warming. This study demonstrates the potential utility of the Mg isotopic compositions of the silicate fraction in marine carbonates as a proxy for Chemical Weathering intensity, laying the groundwork for general applications of this method to deep-time Earth systems.

  • episode of intense Chemical Weathering during the termination of the 635 ma marinoan glaciation
    Proceedings of the National Academy of Sciences of the United States of America, 2016
    Co-Authors: Fangzhen Teng, Kangjun Huang, Bing Shen, Shuhai Xiao, Xianguo Lang, Yong Fu, Yongbo Peng
    Abstract:

    Abstract Cryogenian (∼720–635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth’s history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense Chemical Weathering on continents. However, direct geoChemical evidence for the intense Chemical Weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)—indicative of an episode of intense Chemical Weathering—occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of Chemical Weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of Chemical Weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until Chemical Weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense Chemical Weathering at the onset of deglaciation but indicates that the maximum Weathering predated cap carbonate deposition.

  • Episode of intense Chemical Weathering during the termination of the 635 Ma Marinoan glaciation
    Proceedings of the National Academy of Sciences of the United States of America, 2016
    Co-Authors: Kangjun Huang, Fangzhen Teng, Bing Shen, Shuhai Xiao, Xianguo Lang, Yong Fu, Haoran Ma, Yongbo Peng
    Abstract:

    Abstract Cryogenian (∼720–635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth’s history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense Chemical Weathering on continents. However, direct geoChemical evidence for the intense Chemical Weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)—indicative of an episode of intense Chemical Weathering—occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (