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专业: 有机分子功能材料化学

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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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