The Os isotopic composition of Himalayan river bedloads and bedrocks: importance of black shales
Identifieur interne : 001F06 ( Istex/Corpus ); précédent : 001F05; suivant : 001F07The Os isotopic composition of Himalayan river bedloads and bedrocks: importance of black shales
Auteurs : Anne-Catherine Pierson-Wickmann ; Laurie Reisberg ; Christian France-LanordSource :
- Earth and Planetary Science Letters [ 0012-821X ] ; 2000.
Abstract
Re–Os data are presented for bedrocks and bedloads of selected rivers from the Himalayan range, in Central Nepal and Bangladesh. These data demonstrate that the Os isotopic composition of most of the bedrocks is similar to that of the average continental crust. Notable exceptions to this are the black shales from the Lesser Himalaya (LH), which contain large quantities of radiogenic osmium. In the black shales, the Os is not concentrated in the sulfides, and may instead be associated with organic matter. Bedloads collected from rivers which include LH black shales in their catchment basins have notably high Os ratios (187Os/188Os≈3). This radiogenic signature persists into the Ganga river. In contrast, the Brahmaputra river is less radiogenic, suggesting an ophiolite input in the headwater region. Simple physical mixtures of LH black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high Os ratios observed at the outflow of the range. Assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture, respectively, this observation suggests that erosion may not be occurring at steady-state. In addition, mass balance calculations indicate that about 20% of the Os and 60% of the Re are carried in dissolved form, implying that chemical erosion plays an important role in the osmium budget. Finally, the contrast in Os isotope ratio between Ganga bedloads and bulk sediments in the Bengal Fan suggests that radiogenic Os carried by particulate matter and later remobilized may significantly influence the Os composition of seawater.
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DOI: 10.1016/S0012-821X(00)00003-0
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<front><div type="abstract" xml:lang="en">Re–Os data are presented for bedrocks and bedloads of selected rivers from the Himalayan range, in Central Nepal and Bangladesh. These data demonstrate that the Os isotopic composition of most of the bedrocks is similar to that of the average continental crust. Notable exceptions to this are the black shales from the Lesser Himalaya (LH), which contain large quantities of radiogenic osmium. In the black shales, the Os is not concentrated in the sulfides, and may instead be associated with organic matter. Bedloads collected from rivers which include LH black shales in their catchment basins have notably high Os ratios (187Os/188Os≈3). This radiogenic signature persists into the Ganga river. In contrast, the Brahmaputra river is less radiogenic, suggesting an ophiolite input in the headwater region. Simple physical mixtures of LH black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high Os ratios observed at the outflow of the range. Assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture, respectively, this observation suggests that erosion may not be occurring at steady-state. In addition, mass balance calculations indicate that about 20% of the Os and 60% of the Re are carried in dissolved form, implying that chemical erosion plays an important role in the osmium budget. Finally, the contrast in Os isotope ratio between Ganga bedloads and bulk sediments in the Bengal Fan suggests that radiogenic Os carried by particulate matter and later remobilized may significantly influence the Os composition of seawater.</div>
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<abstract>Re–Os data are presented for bedrocks and bedloads of selected rivers from the Himalayan range, in Central Nepal and Bangladesh. These data demonstrate that the Os isotopic composition of most of the bedrocks is similar to that of the average continental crust. Notable exceptions to this are the black shales from the Lesser Himalaya (LH), which contain large quantities of radiogenic osmium. In the black shales, the Os is not concentrated in the sulfides, and may instead be associated with organic matter. Bedloads collected from rivers which include LH black shales in their catchment basins have notably high Os ratios (187Os/188Os≈3). This radiogenic signature persists into the Ganga river. In contrast, the Brahmaputra river is less radiogenic, suggesting an ophiolite input in the headwater region. Simple physical mixtures of LH black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high Os ratios observed at the outflow of the range. Assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture, respectively, this observation suggests that erosion may not be occurring at steady-state. In addition, mass balance calculations indicate that about 20% of the Os and 60% of the Re are carried in dissolved form, implying that chemical erosion plays an important role in the osmium budget. Finally, the contrast in Os isotope ratio between Ganga bedloads and bulk sediments in the Bengal Fan suggests that radiogenic Os carried by particulate matter and later remobilized may significantly influence the Os composition of seawater.</abstract>
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<notesStmt><note type="content">Fig. 1: Map of the Ganga–Brahmaputra river system showing bedload sample locations and 187Os/188Os ratios (dots). Rectangle indicates map area of Fig. 2. MFT: Main Frontal Thrust, placing Mio–Pliocene Siwalik foreland basin sediments over the modern Gangetic Plain. MCT: Main Central Thrust, placing the HHC over the LH. STDZ: South Tibetan Detachment Zone, separating the TSS from the HHC.</note>
<note type="content">Fig. 2: Geological map of Narayani watershed showing bedload and bedrock sample locations and their 187Os/188Os ratios. Open squares represent bedrocks and black dots represent river bedloads. TSS: Tethyan Sedimentary Series, HHC: HH Cristallines, LH: Lesser Himalaya, HHL: High Himalaya Leucogranite, LHN: LH Nappe.</note>
<note type="content">Fig. 3: Os isotopic composition (187Os/188Os) of bedload and bedrock samples of the Himalayan range and Bangladesh versus 188Os abundance. 187Os/188Os ratios plot along two different trends: one at a roughly constant Os ratio (1.2) and the second towards radiogenic Os ratio with slightly increasing Os concentration. The Os ratios of bedloads at the outflow of the range, which are the most radiogenic, require the addition of LH black shales. Curves represent mixtures between LH black shales and an average of the HH (187Os/188Os=1.2 and [Os]=24 ppt). Curves (1), (2) and (3) were calculated using extreme and typical values of black shales from this study (Table 1: (1) NL 2, (2) AP 807, (3) AP 757). An average contribution of 4% of black shales is required to explain the radiogenic Os ratios of bedloads collected at the outflow of the range (black line on the figure represents 5%). This proportion is not sensitive to the particular black shale chosen as the mixing end-member.</note>
<note type="content">Fig. 4: Positive covariation of TOC (wt%) with bulk osmium concentration (ppt) of river bedloads and bedrocks. Note the scale change at 1% TOC. (b) This is an expanded view of the rectangle in (a).</note>
<note type="content">Fig. 5: εNd vs. 187Os/188Os of bedloads and bedrocks from Central Nepal and Bangladesh. Curve represents mixture between average HH-eroded material (187Os/188Os=1.21 and [Os]=24 ppt; εNd=−15 and [Nd]=30 ppm) and average LH sediments (white star). This average LH value represents a mixture of 99% non-radiogenic LH material (187Os/188Os=1.17, [Os]=13 pg/g and εNd=−25, [Nd]=30 ppm) and 1% LH black shale NL 2 (εNd=−25.5 and [Nd]=29 ppm; 187Os/188Os=5.75 and [Os]=505 ppt) (Os in Table 1). The vertical lines represent the proportion of LH material required by the Nd isotopes. This graph demonstrates that the LH component must contain much more than 1% black shale in order to match the data.</note>
<note type="content">Fig. 6: SiO2 versus [Os] and [Re] (pg/g). The intersection of the dashed lines represents the estimated average Os (35 ppt) and Re (570 ppt) abundances of suspended river load assuming a SiO2 content of 55% [25].</note>
<note type="content">Table 1: Os isotopic compositions and Os and Re concentrations of bedrocks from Central Nepal</note>
<note type="content">Table 2: Os isotopic compositions and Os and Re concentrations of bulk river bedloads from Central Nepal</note>
<note type="content">Table 3: Os isotopic compositions and Os and Re concentrations of bulk river bedloads from Bangladesh</note>
<note type="content">Table 4: Comparison of leachate and bulk Os of bedload samples</note>
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<author xml:id="author-1"><persName><forename type="first">Anne-Catherine</forename>
<surname>Pierson-Wickmann</surname>
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<email>annecath@crpg.cnrs-nancy.fr</email>
<note type="correspondence"><p>Corresponding author. Tel.: +33-38359-4226; Fax: +31-38351-1798</p>
</note>
<affiliation>Centre de Recherches Pétrographiques et Géochimiques, CNRS 15, rue Notre-Dame des Pauvres, P.O. Box 20, 54501 Vandœuvre-les-Nancy Cedex, France</affiliation>
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<author xml:id="author-2"><persName><forename type="first">Laurie</forename>
<surname>Reisberg</surname>
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<author xml:id="author-3"><persName><forename type="first">Christian</forename>
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<abstract xml:lang="en"><p>Re–Os data are presented for bedrocks and bedloads of selected rivers from the Himalayan range, in Central Nepal and Bangladesh. These data demonstrate that the Os isotopic composition of most of the bedrocks is similar to that of the average continental crust. Notable exceptions to this are the black shales from the Lesser Himalaya (LH), which contain large quantities of radiogenic osmium. In the black shales, the Os is not concentrated in the sulfides, and may instead be associated with organic matter. Bedloads collected from rivers which include LH black shales in their catchment basins have notably high Os ratios (187Os/188Os≈3). This radiogenic signature persists into the Ganga river. In contrast, the Brahmaputra river is less radiogenic, suggesting an ophiolite input in the headwater region. Simple physical mixtures of LH black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high Os ratios observed at the outflow of the range. Assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture, respectively, this observation suggests that erosion may not be occurring at steady-state. In addition, mass balance calculations indicate that about 20% of the Os and 60% of the Re are carried in dissolved form, implying that chemical erosion plays an important role in the osmium budget. Finally, the contrast in Os isotope ratio between Ganga bedloads and bulk sediments in the Bengal Fan suggests that radiogenic Os carried by particulate matter and later remobilized may significantly influence the Os composition of seawater.</p>
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<textClass><keywords scheme="keyword"><list><head>Keywords</head>
<item><term>erosion</term>
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<item><term>dissolved materials</term>
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<item><term>marine sediments</term>
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<item><term>osmium</term>
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<item><term>Himalayas</term>
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<head><ce:title>The Os isotopic composition of Himalayan river bedloads and bedrocks: importance of black shales</ce:title>
<ce:author-group><ce:author><ce:given-name>Anne-Catherine</ce:given-name>
<ce:surname>Pierson-Wickmann</ce:surname>
<ce:cross-ref refid="CORR1">*</ce:cross-ref>
<ce:e-address>annecath@crpg.cnrs-nancy.fr</ce:e-address>
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<ce:author><ce:given-name>Laurie</ce:given-name>
<ce:surname>Reisberg</ce:surname>
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<ce:author><ce:given-name>Christian</ce:given-name>
<ce:surname>France-Lanord</ce:surname>
</ce:author>
<ce:affiliation><ce:textfn>Centre de Recherches Pétrographiques et Géochimiques, CNRS 15, rue Notre-Dame des Pauvres, P.O. Box 20, 54501 Vandœuvre-les-Nancy Cedex, France</ce:textfn>
</ce:affiliation>
<ce:correspondence id="CORR1"><ce:label>*</ce:label>
<ce:text>Corresponding author. Tel.: +33-38359-4226; Fax: +31-38351-1798</ce:text>
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<ce:abstract-sec><ce:simple-para>Re–Os data are presented for bedrocks and bedloads of selected rivers from the Himalayan range, in Central Nepal and Bangladesh. These data demonstrate that the Os isotopic composition of most of the bedrocks is similar to that of the average continental crust. Notable exceptions to this are the black shales from the Lesser Himalaya (LH), which contain large quantities of radiogenic osmium. In the black shales, the Os is not concentrated in the sulfides, and may instead be associated with organic matter. Bedloads collected from rivers which include LH black shales in their catchment basins have notably high Os ratios (<ce:sup>187</ce:sup>
Os/<ce:sup>188</ce:sup>
Os≈3). This radiogenic signature persists into the Ganga river. In contrast, the Brahmaputra river is less radiogenic, suggesting an ophiolite input in the headwater region. Simple physical mixtures of LH black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high Os ratios observed at the outflow of the range. Assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture, respectively, this observation suggests that erosion may not be occurring at steady-state. In addition, mass balance calculations indicate that about 20% of the Os and 60% of the Re are carried in dissolved form, implying that chemical erosion plays an important role in the osmium budget. Finally, the contrast in Os isotope ratio between Ganga bedloads and bulk sediments in the Bengal Fan suggests that radiogenic Os carried by particulate matter and later remobilized may significantly influence the Os composition of seawater.</ce:simple-para>
</ce:abstract-sec>
</ce:abstract>
<ce:keywords class="keyword"><ce:section-title>Keywords</ce:section-title>
<ce:keyword><ce:text>erosion</ce:text>
</ce:keyword>
<ce:keyword><ce:text>dissolved materials</ce:text>
</ce:keyword>
<ce:keyword><ce:text>marine sediments</ce:text>
</ce:keyword>
<ce:keyword><ce:text>osmium</ce:text>
</ce:keyword>
<ce:keyword><ce:text>Himalayas</ce:text>
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<abstract lang="en">Re–Os data are presented for bedrocks and bedloads of selected rivers from the Himalayan range, in Central Nepal and Bangladesh. These data demonstrate that the Os isotopic composition of most of the bedrocks is similar to that of the average continental crust. Notable exceptions to this are the black shales from the Lesser Himalaya (LH), which contain large quantities of radiogenic osmium. In the black shales, the Os is not concentrated in the sulfides, and may instead be associated with organic matter. Bedloads collected from rivers which include LH black shales in their catchment basins have notably high Os ratios (187Os/188Os≈3). This radiogenic signature persists into the Ganga river. In contrast, the Brahmaputra river is less radiogenic, suggesting an ophiolite input in the headwater region. Simple physical mixtures of LH black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high Os ratios observed at the outflow of the range. Assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture, respectively, this observation suggests that erosion may not be occurring at steady-state. In addition, mass balance calculations indicate that about 20% of the Os and 60% of the Re are carried in dissolved form, implying that chemical erosion plays an important role in the osmium budget. Finally, the contrast in Os isotope ratio between Ganga bedloads and bulk sediments in the Bengal Fan suggests that radiogenic Os carried by particulate matter and later remobilized may significantly influence the Os composition of seawater.</abstract>
<note type="content">Fig. 1: Map of the Ganga–Brahmaputra river system showing bedload sample locations and 187Os/188Os ratios (dots). Rectangle indicates map area of Fig. 2. MFT: Main Frontal Thrust, placing Mio–Pliocene Siwalik foreland basin sediments over the modern Gangetic Plain. MCT: Main Central Thrust, placing the HHC over the LH. STDZ: South Tibetan Detachment Zone, separating the TSS from the HHC.</note>
<note type="content">Fig. 2: Geological map of Narayani watershed showing bedload and bedrock sample locations and their 187Os/188Os ratios. Open squares represent bedrocks and black dots represent river bedloads. TSS: Tethyan Sedimentary Series, HHC: HH Cristallines, LH: Lesser Himalaya, HHL: High Himalaya Leucogranite, LHN: LH Nappe.</note>
<note type="content">Fig. 3: Os isotopic composition (187Os/188Os) of bedload and bedrock samples of the Himalayan range and Bangladesh versus 188Os abundance. 187Os/188Os ratios plot along two different trends: one at a roughly constant Os ratio (1.2) and the second towards radiogenic Os ratio with slightly increasing Os concentration. The Os ratios of bedloads at the outflow of the range, which are the most radiogenic, require the addition of LH black shales. Curves represent mixtures between LH black shales and an average of the HH (187Os/188Os=1.2 and [Os]=24 ppt). Curves (1), (2) and (3) were calculated using extreme and typical values of black shales from this study (Table 1: (1) NL 2, (2) AP 807, (3) AP 757). An average contribution of 4% of black shales is required to explain the radiogenic Os ratios of bedloads collected at the outflow of the range (black line on the figure represents 5%). This proportion is not sensitive to the particular black shale chosen as the mixing end-member.</note>
<note type="content">Fig. 4: Positive covariation of TOC (wt%) with bulk osmium concentration (ppt) of river bedloads and bedrocks. Note the scale change at 1% TOC. (b) This is an expanded view of the rectangle in (a).</note>
<note type="content">Fig. 5: εNd vs. 187Os/188Os of bedloads and bedrocks from Central Nepal and Bangladesh. Curve represents mixture between average HH-eroded material (187Os/188Os=1.21 and [Os]=24 ppt; εNd=−15 and [Nd]=30 ppm) and average LH sediments (white star). This average LH value represents a mixture of 99% non-radiogenic LH material (187Os/188Os=1.17, [Os]=13 pg/g and εNd=−25, [Nd]=30 ppm) and 1% LH black shale NL 2 (εNd=−25.5 and [Nd]=29 ppm; 187Os/188Os=5.75 and [Os]=505 ppt) (Os in Table 1). The vertical lines represent the proportion of LH material required by the Nd isotopes. This graph demonstrates that the LH component must contain much more than 1% black shale in order to match the data.</note>
<note type="content">Fig. 6: SiO2 versus [Os] and [Re] (pg/g). The intersection of the dashed lines represents the estimated average Os (35 ppt) and Re (570 ppt) abundances of suspended river load assuming a SiO2 content of 55% [25].</note>
<note type="content">Table 1: Os isotopic compositions and Os and Re concentrations of bedrocks from Central Nepal</note>
<note type="content">Table 2: Os isotopic compositions and Os and Re concentrations of bulk river bedloads from Central Nepal</note>
<note type="content">Table 3: Os isotopic compositions and Os and Re concentrations of bulk river bedloads from Bangladesh</note>
<note type="content">Table 4: Comparison of leachate and bulk Os of bedload samples</note>
<subject><genre>Keywords</genre>
<topic>erosion</topic>
<topic>dissolved materials</topic>
<topic>marine sediments</topic>
<topic>osmium</topic>
<topic>Himalayas</topic>
</subject>
<relatedItem type="host"><titleInfo><title>Earth and Planetary Science Letters</title>
</titleInfo>
<titleInfo type="abbreviated"><title>EPSL</title>
</titleInfo>
<genre type="journal">journal</genre>
<originInfo><dateIssued encoding="w3cdtf">20000315</dateIssued>
</originInfo>
<identifier type="ISSN">0012-821X</identifier>
<identifier type="PII">S0012-821X(00)X0058-1</identifier>
<part><date>20000315</date>
<detail type="volume"><number>176</number>
<caption>vol.</caption>
</detail>
<detail type="issue"><number>2</number>
<caption>no.</caption>
</detail>
<extent unit="issue pages"><start>171</start>
<end>258</end>
</extent>
<extent unit="pages"><start>203</start>
<end>218</end>
</extent>
</part>
</relatedItem>
<identifier type="istex">57AB4F3213532B59E3D5A4AE59BFF56F4F05F473</identifier>
<identifier type="DOI">10.1016/S0012-821X(00)00003-0</identifier>
<identifier type="PII">S0012-821X(00)00003-0</identifier>
<accessCondition type="use and reproduction" contentType="copyright">©2000 Elsevier Science B.V.</accessCondition>
<recordInfo><recordContentSource>ELSEVIER</recordContentSource>
<recordOrigin>Elsevier Science B.V., ©2000</recordOrigin>
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