『Abstract
In the northern part of the Indian sub-continent, the Ganga alluvial
plain (GAP) feeds its weathering products to the Ganga-Brahmaputra
River system, one of the world's largest fluvial systems. The
authors present a geochemical study of the GAP weathering products
transported by the Gomati River (the Ganga River tributary) to
understand weathering processes of an alluvial plain in a humid
sub-tropical climate. A total of 28 sediment samples were collected
during the monsoon season and were analysed by X-ray fluorescence
spectrometry for 25 major and trace elements. Bulk chemistry of
the channel, flood and suspended sediments mostly consists (＞90％,
＞80％ and ＞75％, respectively) of three elements; Al, Si and Fe.
Major element concentrations normalised with respect to upper
continental crust (UCC) show strong depletion of highly mobile
elements (Na, Ca) and enrichment of immobile elements (Ti, Si).
Silica enrichment in the sand fraction is probably caused by chemical
weathering of feldspar. Mineral sorting during fluvial transportation
acts as the single important factor that controls the geochemistry
of these weathering products and also strongly influences major
and trace element distribution in the individual sediment samples.
Trace element (Ba, Cr, Cu, Nb, Ni, Pb, V and Zn) concentrations
were strongly correlated with major element (Si, Al, Fe, Mn and
K) concentrations indicating that the abundance of trace elements
is controlled by the same processes that control the major element
distribution in these sediments.
The GAP weathering products were geochemically distinguished
as arkose to litharenite in rock classification. Chemical mobility,
normalised with respect to TiO2 in UCC, indicates
that Si, Na, Zr, Ba and Sr, mainly derived from feldspar, muscovite
and biotite, are lost during weathering. Iron and Zn remained
immobile during weathering and were strongly adsorbed by phyllosilicates
and concentrated in fine-grained sediment fractions. The chemical
index of alteration indicates that the GAP has experienced chemical
weathering of incipient to moderate intensity. The GAP weathering
products also demonstrated a progressive incomplete alteration
in the alluvial sequence made-up of the Himalayan-derived sediments.
A model has been proposed to better understand weathering processes
and products of the GAP in temporary storage of 〜50 ka in a humid
sub-tropical climate.』
1. Introduction
2. Study area
3. Sampling and analytical methods
4. Results
4.1. Major elements
4.1.1. Elemental ratio
4.1.2. Inter-element relationship
4.1.3. A-CN-K diagram and chemical index of alteration
4.1.4. A-CNK-FM and Si/10-CM-NK Diagrams
4.2. Trace elements
4.2.1. Elemental ratios
4.2.2. Inter-element relationship
4.3. Major and trace elements
4.3.1. Extended diagrams
4.3.2. Inter-element relationship
4.3.3. Chemical mobility
5. Weathering dynamics
6. Conclusions
Acknowledgements
References