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| Titel |
Silicon biogeochemical processes in a large river (Cauvery, India) |
| VerfasserIn |
Mangalaa Kameswari Rajasekaran, Arnaud Dapoigny, Jean Riotte, Sarma Vedula V. S. S., Nittala S. Sarma, Subramanian Sankaran, Gurumurthy Gundiga Puttojirao, Balakrishna Keshava, Damien Cardinal |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135055
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| Publikation (Nr.) |
 EGU/EGU2016-15852.pdf |
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| Zusammenfassung |
| Silicon (Si), one of the key nutrients for diatom growth in ocean, is principally released
during silicate weathering on continents and then exported by rivers. Phytoplankton
composition is determined by the availability of Si relative to other nutrients, mainly N and P,
which fluxes in estuarine and coastal systems are affected by eutrophication due to land use
and industrialization. In order to understand the biogeochemical cycle of Si and its supply to
the coastal ocean, we studied a tropical monsoonal river from Southern India (Cauvery) and
compare it with other large and small rivers. Cauvery is the 7th largest river in India
with a basin covering 85626 sq.km. The major part of the basin (∼66%) is covered
by agriculture and inhabited by more than 30 million inhabitants. There are 96
dams built across the basin. As a consequence, 80% of the historical discharge
is diverted, mainly for irrigation (Meunier et al. 2015). This makes the Cauvery
River a good example of current anthropogenic pressure on silicon biogeochemical
cycle.
We measured amorphous silica contents (ASi) and isotopic composition of dissolved
silicon (δ30Si-DSi) in the Cauvery estuary, including freshwater end-member and
groundwater as well as along a 670 km transect along the river course. Other Indian rivers
and estuaries have also been measured, including some less impacted by anthropogenic
pressure.
The average Cauvery δ30Si signature just upstream the estuary is 2.21±0.15 ‰ (n=3)
which is almost 1‰ heavier than the groundwater isotopic composition (1.38±0.03 ). The
δ30Si-DSi of Cauvery water is also almost 1‰ heavier than the world river supply to the
ocean estimated so far and 0.4‰ heavier than other large Indian rivers like Ganges (Frings et
al 2015) and Krishna. On the other hand, the smaller watersheds (Ponnaiyar, Vellar,
and Penna) adjacent to Cauvery also display heavy δ30Si-DSi. Unlike the effect of
silicate weathering, the heavy isotopic compositions in the river Cauvery may result
from the successive dams along the main course which are expected to favor the
retention of isotopically light Si isotopes in sediments via diatom uptake in reservoirs
and/or, Si uptake by vegetation. Both processes likely result in heavier δ30Si-DSi
downstream.
In the estuary, the average δ30Si-DSi is 2.20±0.17 ‰ (n=11). There is a significant
positive relationship between ASi contents and fucoxanthin (diatom marker pigment) (r=0.61,
p<0.05, n=11) suggesting a significant control of diatoms on ASi. However a dominant
mixing effect is observed in dissolved silicon with a strong positive relationship between
1/DSi and δ30Si-DSi (r=0.71, p<0.01, n=11).
A comparative study with a west-flowing river, the Netravathi (southwest India) will be
performed and presented during the session. We will also compare the Si isotopic signatures
in Cauvery along the transect with focus on seasonal variability and on upstream vs.
downstream large dams to strengthen interpretations. |
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