『Abstract
This study presents silicon isotope data for rivers and groundwater
from the Bengal Basin. Variations of Si isotope ratios were analyzed
by means of high-resolution MC-ICP-MS using a NuPlasma HR. The
rivers show typical positive δ30Si values between 1.3
and 1.7‰, whereas the groundwater samples show decreasing δ30Si
values from 1.3‰ in shallow groundwater to -0.2‰ in the deeper
groundwater. Beside a very distinctive isotope composition, the
concentration of dissolved Si in these groundwater sample is 2-3
times higher than in river samples taken during dry season. The
resulting Si flux by groundwater (9.3×1010 mol yr-1)
is on the order of the combined Ganges-Brahmaputra Si fluxes,
and equals 40% of the total (river + groundwater) annual Si flux
into the Bay of Bengal. Given the significant large flux and distinctive
isotope composition means that the overall isotopic input into
the ocean is different from riverine values. However, a sound
knowledge of all inputs into the ocean and of how these inputs
might vary throughout time is mandatory, in order to correctly
interpret palaeo-records of δ30Si variations as recorded
in sedimentary diatom opal from the last glacial maximum. We extrapolate
our results from the Bengal Basin onto a global scale and assess
the ocean's sensitivity to changes in inputs, as triggered by
large-scale events, such as glaciation periods, where the hydrological
cycle might be out of steady state due to the build-up of large
continental ice-shields. In such a glaciation scenario, riverine
vs. groundwater inputs can be shifted, favoring isotopically lighter
groundwater over heavier river inputs into the ocean. The model
proves impossible to change the biogenic output to a significant
degree, on time scales of a few thousand years, by just changing
the isotope inputs into the ocean.
Keywords: silicon isotopes; Si cycle; groundwater; ocean; isotope
balance; MC-ICP-MS』
1. Introduction
2. Sampling and analytical approaches
2.1. Sampling
2.2. Mass spectrometry
3. Results
3.1. Silicon concentration
3.2. Silicon isotope compositions
4. Discussion
4.1. Groundwater Si fluxes
4.2. Riverine Si isotope composition (δ30SiRiver)
4.3. Silicon isotope composition of groundwater (δ30SiGW)
4.4. Modeling the Si isotope composition of groundwater
4.5. Modeling the global perspective
6. Conclusions
Acknowledgements
References