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 等待你給力的回復(fù)啊謝謝 啊Organic carbon burial forcing of the carbon cycle from Himalayan erosion Christian France-Lanord*† & Louis A. Derry† * Centre de Recherches Pe´trographiques et Ge´ochimiques, CNRS, BP20 54501 Vandoeuvre-les-Nancy, France † Cornell University, Department of Geological Sciences, Ithaca, New York 14853, USA Weathering and erosion can affect the long-term ocean–atmosphere budget of carbon dioxide both through the consumption of carbonic acid during silicate weathering and through changes in the weathering and burial rates of organic carbon1–4. Recent attention has focused on increased silicate weathering of tectonically uplifted areas in the India–Asia collision zone as a possible cause for falling atmospheric CO2 levels in the Cenozoic era5–7. The chemistry of Neogene sediments from the main locus of sedimentary deposition for Himalayan detritus, the Bengal Fan, can be used to estimate the sinks of CO2 from silicate weathering and from the weathering and burial of organic carbon resulting from Himalayan uplift. Here we show that Neogene CO2 consumption from the net burial of organic carbon during Himalayan sediment deposition was 2–3 times that resulting from the weathering of Himalayan silicates. Thus the dominant effect of Neogene Himalayan erosion on the carbon cycle is an increase in the amount of organic carbon in the sedimentary reservoir, not an increase in silicate weathering fluxes. Silicate weathering is typically incongruent, yielding both a solute and a secondary mineral phase, so direct evidence of chemical weathering can be found in the record of secondary minerals in sedimentary basins. The Bengal Fan and Ganges–Brahmaputra (GB) delta contain a huge volume of sediment derived from erosion of the India–Asia collision zone, with 6 3 106 km3 deposited in the past 20Myr (ref. 8). Isotopic data for Nd, Sr and O from Bengal Fan sediments show that the source for over 80% of the detritus since 20Myr ago has been the high-grade metasedimentary rocks of the High Himalayan crystalline (HHC) sequence9. Clastic and carbonate sediments of the Precambrian Lesser Himalaya (LH) and Palaeozoic–Mesozoic Tethyan Himalaya (TH) are the other important sources of sediment to the Bengal Fan during the Neogene. Carbon dioxide consumption from silicate weathering can be represented schematically by: where 1 mol of CO2 is sequestered as marine carbonate for each mol Ca (or Mg) derived from silicate dissolution.Weathering of Na and K silicates contributes a smaller fraction to the weathering CO2 sink because alkalis can exchange for Ca 6 Mg adsorbed on detrital clays in estuarine zones, and during alteration of the oceanic crust10,11.We estimate the CO2 consumption from silicate weathering by comparing the chemistry of the weathered sediments deposited in the Bengal Fan with the chemistry of their unaltered Himalayan source rocks. The comparison slightly overestimates CO2 consumption because any base cations released by weathering with strong acids (such as H2SO4) are still included in this CO2 consumption budget. To represent the source of Bengal Fan sediment we use a composite of 99 samples from outcrops in the HHC (Table 1). Adding samples from the LH and TH strata to the average value for Himalayan source rocks does not change the estimated weathering fluxes significantly, because the combined contribution from these units to the sediment flux in the Bengal Fan is ,20%, and all three (meta)sedimentary units are chemically similar. We analysed a subset of HHC samples from Central Nepal for total organic carbon (Corg) contents. Metamorphic rocks of the HHC average 0:05 6 0:03% Corg. Sediments of the Lesser Himalaya also have low Corg contents,<0.10%, except for rare black shale beds12. Sediments of the Tethyan Himalaya include both carbonates and Palaeozoic clastic sediments with low Corg values, and some Mesozoic shales with up to 1.5% Corg (ref. 13).We estimate a volume-weighted mean Corg content of 0:10 6 0:05% for the source rocks of Bengal Fan sediment. We sampled Himalayan-derived sediments from late Pleistocene to mid-Miocene age recovered from the distal Bengal Fan on Leg 116 of the Ocean Drilling Program14. The sediments were chosen to represent a range of weathering intensities based on clay mineralogy15. Before 7Myr and after 1Myr ago the clay mineral assemblage (,2 mm size fraction) in the Bengal Fan is dominantly illite plus chlorite (the IC assemblage). From 7 to 1Myr, clays in the Fan are dominantly pedogenic smectite and kaolinite (SK assemblage), reflecting more intense weathering in the GB floodplain16. The fine-grained, SK sediments from the Bengal Fan are the most intensely weathered Himalayan sediments, and are less abundant than the IC sediments. Before dissolution, whole rock samples were disaggregated in distilled H2O, then leached with 10% acetic acid to remove minor carbonates (diagenetic, biogenic and detrital). Leaching of diagenetic carbonate removes some Ca and Mg derived from alteration of detrital silicates,although Sr isotopic data on the carbonate fraction show that most cations are derived from sea water. Ion exchange of H+ for adsorbed cations on clays may also release some silicate-derived cations. Thus our technique probably causes us to overestimate the silicate-derived alkalinity flux. We normalized the major element concentrations of HHC source material and Bengal Fan sediment to their Al2O3 contents, on the assumption that during low-intensity weathering characteristic of the Himalayan drainage, aluminium is conservative. The global mean transport of dissolved Al is estimated to be ,0.1% of suspended load transport17, making this a reliable assumption. The differences in ratios of major cations to Al2O3 between the HHC and Bengal Fan sediments can result from weathering losses mineral sorting or residual enrichment of insoluble cations. Dissolved silica transport in the modern Ganges (from all sources) is about 0.3% of suspended load transport18, indicating that dissolution of quartz is minor. Low SiO2/Al2O3 in the sediments results from mineral sorting, as quartz is mechanically resistant and is transported less readily than finer-grained aluminosilicate minerals. If fine-grained silicates are preferentially transported to the distal Bengal Fan our estimate of alkalinity flux is too high, as the finegrained material contains a higher proportion of weathered clay minerals. However, the fine-grained material is also enriched in Mg, which tends to counteract this bias. Differences in the weathering intensity between the IC and SK sediments are apparent in the base cation/Al2O3 ratios. IC sediments have lost little or no K2O and MgO relative to theHHC source rocks, and about half of theirNa2O and CaO. These results are in good agreement with analyses of modern soil profiles developed on Himalayan gneisses19. SK sediments have lost some MgO, about half of their K2O and CaO, and much of theirNa2O. The mean Corg content of Bengal Fan sediments is 0.85% (n ¼ 155), with the mean for IC sediments being 0.4%, whereas the mean for SK sediments is 1.5%20. Data from fan sediments in the region near the delta yield similar values to the distal fan, with meanCorg ¼ 0:90% (ref. 21), suggesting that any sorting effect on Corg contents is minor. Consumption of CO2 due to Himalayan silicate weathering was calculated assuming that all Mg2+ and Ca2+ lost from the silicates forms marine carbonates. The fraction of K and Na involved in cation exchange reactions with sediments or the oceanic crust remains poorly known10,22. We conservatively estimate that 20% of K+ and 30% of Na+ in the global river flux exchanges for Ca 6 Mg and produces carbonates. Accounting for charge balance, CO2 consumption can be estimated from the base cation losses during Himalayan erosion We have ignored neoformation of marine clays (‘reverse weathering’), which may be a significant sink for base cations23 and would lower the estimated CO2 consumption. Consumption of CO2 by silicate weathering is 0.17 mol kg-1 for the less weathered IC sediments and 0.23 mol kg-1 for the SK sediments. Similarly, we estimate the CO2 consumption from organic carbon (Corg) burial by comparing the Corg content of the clastic sediments in the Bengal Fan with the Corg content of their Himalayan source rocks. For IC sediments, this sink is about 0.27 molCO2 kg-1; for SK sediments the sink is 1.1 molCO2 kg-1, 1.7 and 4.7 times their respective silicate weathering sinks (Fig. 1). The more intensely weathered SK sediments also have higher Corg contents, more than offsetting the greater cation losses. These averages are representative of the range of sediment organic and inorganic chemistries in the Bengal Fan.We emphasize that this conclusion is based on a probable overestimate of the magnitude of the silicate weathering flux for CO2 our sample preparation and data analysis tends to bias this estimate towards high values. The result is independent of the magnitude of sediment fluxes past or present, because we have measured CO2 consumption by inorganic and organic sinks in the same samples. The fluxes of CO2 implied by these estimates can be compared with independent global estimates. For a mean suspended load flux of 8 3 1011 kg yr21 (ref. 24) our data yield an average CO2 consumption by silicate weathering in the GB system of 0:17 3 1012 mol yr21 during the Neogene. This flux is 2.6% of the current global silicate weathering sink for CO2 (ref. 2). The modern GB contributes 2.7% of the global river water discharge and 2.1% of the global riverine SiO2 flux18,22. Thus our estimates for both the long-term discharge-weighted CO2 consumption via silicate weathering and the modern discharge-weighted SiO2 flux in the GB system are near the current world average. Despite the huge erosional flux from the Himalaya, the silicate weathering sink for CO2 is modest on the global scale. The extreme relief of the Himalaya and the monsoon climate result in very rapid physical denudation and fast transport of sediment to the ocean. One result is a strongly weathering-limited system in which the kinetics of chemical weathering are slow relative to the transport time of eroded rock to the sea. Furthermore, Ca silicates are not abundant in the Himalaya, and the silicate-derived alkalinity flux is largely in the form of Na and K cations which are inefficient sinks of CO2 The data above yield an average Neogene rate of net growth (burial2weathering . 0) of the sedimentary Corg reservoir of 0:58 3 1012 mol yr21 in the Himalayan–Bengal system. The net growth in the size of the global sedimentary Corg reservoir can be estimated from the marine carbon isotopic mass balance25. Our recent results from a d13C model26 yield a global average net flux to the sedimentary Corg reservoir of about 1:1 3 1012 mol yr over the past 15Myr. The results are not directly comparable because one value represents a regional flux, whereas the other represents a global flux. However, they are consistent, and suggest that Corg burial in excess of weathering in the Himalayan–Bengal system can contribute significantly to changes in the global Corg reservoir. Together with the Indus fan and the Indo-Gangetic plain, the Bengal Fan accounts for about 15% of the modern total burial flux of global Corg (ref. 20), so it is not surprising that any imbalance (burial2weathering Þ 0) in the Himalayan–Bengal Corg budget could have had a global impact. Up to 90% of Corg burial takes place in continental margin sediments27, so any process that increases continental margin sedimentation significantly, such as erosion of a major orogen, may be expected to increase Corg burial and possibly amplify imbalances in the Corg budget. Rapid erosion and the high suspended load of the GB system help drive Corg burial rates high enough to perturb the global carbon cycle significantly. Erosion of a major orogenic belt such as the Himalaya creates a large amount of mineral surface area, which is a strong control on organic carbon burial in continental margin settings Recent work on the evolution of the global climate during the Cenozoic era has focused almost exclusively on the possible perturbation of atmospheric CO2 levels resulting from weathering of silicates, especially in the Himalaya5,6,29,30. But Himalayan erosion produces very large Corg fluxes20,31,32. Although the hypothesized link between Himalayan silicate weathering and atmospheric CO2 levels remains poorly quantified, our results indicate that increased sedimentary Corg storage resulting from Neogene Himalayan erosion and weathering has had a significantly larger effect on the carbon cycle than silicate weathering, by a factor of 2–3. Both models of the net change in the global sedimentary Corg reservoir7,26 and data from the Himalayan–Bengal system are consistent with the hypothesis that an excess of Corg burial over weathering acted as a sink for atmospheric CO2 during the Neogene. |
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