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Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes.

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Title: Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes.
Authors: Dellinger, Mathieu1,2 mdelling@usc.edu, Gaillardet, Jerome2,3, Bouchez, Julien2, Calmels, Damien4, Louvat, Pascale2, Dosseto, Anthony5, Gorge, Caroline2, Alanoca, Lucia6, Maurice, Laurence6
Source: Geochimica et Cosmochimica Acta. Sep2015, Vol. 164, p71-93. 23p.
Subjects: Lithium isotopes, Weathering, Marine sediments, Magnesium isotopes, Shields (Geology)
Geographic Terms: Amazon River Watershed
Abstract: We report Li isotope composition (δ 7 Li) of river-borne dissolved and solid material in the largest River system on Earth, the Amazon River basin, to characterize Li isotope fractionation at a continental scale. The δ 7 Li in the dissolved load (+1.2‰ to +32‰) is fractionated toward heavy values compared to the inferred bedrock (−1‰ to 5‰) and the suspended sediments (−6.8‰ to −0.5‰) as a result of the preferential incorporation of 6 Li into secondary minerals during weathering. Despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share similar ranges of dissolved δ 7 Li (+1.2‰ to +18‰). Correlations between dissolved δ 7 Li and Li/Na and Li/Mg ratios suggest that the proportion of Li incorporated in secondary minerals during weathering act as the main control on the δ 7 Li diss across the entire Amazon basin. A “batch” steady-state fractionation model for Andean and lowland rivers satisfactorily reproduces these variations, with a fractionation factor between weathering products and dissolved load ( α sec - dis ) of 0.983 ± 0.002. Two types of supply-limited weathering regimes can be identified for the lowlands: “clearwaters” with dominant incorporation of Li in secondary minerals, and “black waters” (e.g., Rio Negro) where dissolution of secondary minerals enhanced by organic matter produces low δ 7 Li. Apart from the black waters, the δ 7 Li of Andean and lowland rivers is negatively correlated to the denudation rates with the lowest δ 7 Li corresponding to the rivers having the highest denudation rates. In contrast, the main tributaries draining both the Andes and the lowlands have higher δ 7 Li compared to other rivers. We propose that part of the dissolved Li derived from weathering in the Andes is re-incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved δ 7 Li along the course of these rivers. Unlike other rivers, the dissolved δ 7 Li in the main tributaries is best described by a Rayleigh fractionation model with a fractionation factor α sec - dis of 0.991. Altogether, the control imposed by residence time in the weathering zone and floodplain processes results in (i) a non-linear correlation between dissolved δ 7 Li and the weathering intensity (defined as W/D) and (ii) a positive relationship between the dissolved Li flux and the denudation rate. These results have important implications for the understanding of past ocean δ 7 Li and its use as a paleo weathering proxy. [ABSTRACT FROM AUTHOR]
Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes.
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  Data: <searchLink fieldCode="AR" term="%22Dellinger%2C+Mathieu%22">Dellinger, Mathieu</searchLink><relatesTo>1,2</relatesTo><i> mdelling@usc.edu</i><br /><searchLink fieldCode="AR" term="%22Gaillardet%2C+Jerome%22">Gaillardet, Jerome</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22Bouchez%2C+Julien%22">Bouchez, Julien</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Calmels%2C+Damien%22">Calmels, Damien</searchLink><relatesTo>4</relatesTo><br /><searchLink fieldCode="AR" term="%22Louvat%2C+Pascale%22">Louvat, Pascale</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Dosseto%2C+Anthony%22">Dosseto, Anthony</searchLink><relatesTo>5</relatesTo><br /><searchLink fieldCode="AR" term="%22Gorge%2C+Caroline%22">Gorge, Caroline</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Alanoca%2C+Lucia%22">Alanoca, Lucia</searchLink><relatesTo>6</relatesTo><br /><searchLink fieldCode="AR" term="%22Maurice%2C+Laurence%22">Maurice, Laurence</searchLink><relatesTo>6</relatesTo>
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  Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. Sep2015, Vol. 164, p71-93. 23p.
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  Data: <searchLink fieldCode="DE" term="%22Lithium+isotopes%22">Lithium isotopes</searchLink><br /><searchLink fieldCode="DE" term="%22Weathering%22">Weathering</searchLink><br /><searchLink fieldCode="DE" term="%22Marine+sediments%22">Marine sediments</searchLink><br /><searchLink fieldCode="DE" term="%22Magnesium+isotopes%22">Magnesium isotopes</searchLink><br /><searchLink fieldCode="DE" term="%22Shields+%28Geology%29%22">Shields (Geology)</searchLink>
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  Data: <searchLink fieldCode="DE" term="%22Amazon+River+Watershed%22">Amazon River Watershed</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: We report Li isotope composition (δ 7 Li) of river-borne dissolved and solid material in the largest River system on Earth, the Amazon River basin, to characterize Li isotope fractionation at a continental scale. The δ 7 Li in the dissolved load (+1.2‰ to +32‰) is fractionated toward heavy values compared to the inferred bedrock (−1‰ to 5‰) and the suspended sediments (−6.8‰ to −0.5‰) as a result of the preferential incorporation of 6 Li into secondary minerals during weathering. Despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share similar ranges of dissolved δ 7 Li (+1.2‰ to +18‰). Correlations between dissolved δ 7 Li and Li/Na and Li/Mg ratios suggest that the proportion of Li incorporated in secondary minerals during weathering act as the main control on the δ 7 Li diss across the entire Amazon basin. A “batch” steady-state fractionation model for Andean and lowland rivers satisfactorily reproduces these variations, with a fractionation factor between weathering products and dissolved load ( α sec - dis ) of 0.983 ± 0.002. Two types of supply-limited weathering regimes can be identified for the lowlands: “clearwaters” with dominant incorporation of Li in secondary minerals, and “black waters” (e.g., Rio Negro) where dissolution of secondary minerals enhanced by organic matter produces low δ 7 Li. Apart from the black waters, the δ 7 Li of Andean and lowland rivers is negatively correlated to the denudation rates with the lowest δ 7 Li corresponding to the rivers having the highest denudation rates. In contrast, the main tributaries draining both the Andes and the lowlands have higher δ 7 Li compared to other rivers. We propose that part of the dissolved Li derived from weathering in the Andes is re-incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved δ 7 Li along the course of these rivers. Unlike other rivers, the dissolved δ 7 Li in the main tributaries is best described by a Rayleigh fractionation model with a fractionation factor α sec - dis of 0.991. Altogether, the control imposed by residence time in the weathering zone and floodplain processes results in (i) a non-linear correlation between dissolved δ 7 Li and the weathering intensity (defined as W/D) and (ii) a positive relationship between the dissolved Li flux and the denudation rate. These results have important implications for the understanding of past ocean δ 7 Li and its use as a paleo weathering proxy. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
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  Data: <i>Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
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      – Type: doi
        Value: 10.1016/j.gca.2015.04.042
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      – Code: eng
        Text: English
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      Pagination:
        PageCount: 23
        StartPage: 71
    Subjects:
      – SubjectFull: Lithium isotopes
        Type: general
      – SubjectFull: Weathering
        Type: general
      – SubjectFull: Marine sediments
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      – SubjectFull: Magnesium isotopes
        Type: general
      – SubjectFull: Shields (Geology)
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      – SubjectFull: Amazon River Watershed
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      – TitleFull: Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes.
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