H. M. Zakir Hossain

Analysis of sedimentary facies and depositional environments of the Permian Gondwana sequence in borehole GDH-45, Khalaspir Basin, Bangladesh

Lithofacies analysis of the Permian Gondwana sequence in borehole GDH-45 of the Khalaspir Basin was performed with a view to deduce the nature of depositional environments. On the basis of dominant lithofacies association, the sequence is divided into six lithostratigraphic units (units A to F). Five lithofacies (conglomerate, sandstone, siltstone, mudstone/shale and coal) are identified within these units. Several sub-lithofacies, such as massive, crudely stratified, cross-stratified, ripple and parallel laminated sandstones are also identified within these lithofacies. The sequence forms a fining-upward trend with a rare coarsening-upward unit. The generalised Gondwana sequence is characterised mainly by channel lags, pebbly massive to crudely cross-stratified sandstone, trough and planar cross-stratified sandstone, ripple laminated sandstone/siltstone, massive to parallel laminated siltstone, mudstone/shale and coal in ascending order. The facies associations represent several repeated fining-upward units and cycles, indicating various sub-environments (channel, floodplain, flood basin/backswamp) in fluvial regime. The conglomerates might have been deposited as debris flow or channel lag deposits. The sandstones were deposited mainly as multistoried channel and lateral bars in moderately braided and sinuous streams. The siltstone and mudstone lithofacies indicate bar top, natural levee or floodplain to flood basin environments. The coal lithofacies suggests deposition in low-lying, short to long persistent, moderately to well drained and sparse to densely vegetated backswamps in fluvial channel-flood-plain complex. The overall succession of the Gondwana borehole sediments suggests that the depositional basin became, with time, gentler in slope gradient, resulting in a more sinuous stream setting.

Spatial and Seasonal Variation in Surface Water pCO2 in the Ganges, Brahmaputra, and Meghna Rivers on the Indian Subcontinent

Recent studies have remarked on the importance of direct CO2 release from river water into the atmosphere on the global carbon cycle over a short timescale. In this study, we investigated carbonate systems, including spatial and seasonal variations of pCO2, in three major Himalayan rivers in Bangladesh: the Ganges, Brahmaputra, and Meghna Rivers, and their potential importance. Although pCO2 is known to be low in the upper reaches of these rivers, owing to active chemical weathering, we observed pCO2 values higher than the atmospheric pCO2 level along their lower reaches, where deep soils have developed and where high air temperatures promote active soil respiration. By a simple mixing calculation, we found that seasonal variations in these river water carbonate systems are controlled by subsurface water flows. In the rainy season, most of the lowlands are inundated, and the contribution of subsurface flow to river water carbonate systems increases, resulting in higher pCO2 values. In future research, more detailed spatial and seasonal investigations are required to clarify the role of terrestrial ecosystems, including rivers and the CO2 flux to the atmosphere, in the global carbon cycle and to examine how that role will change under global warming.

Downstream and seasonal changes of lithium isotope ratios in the Ganges-Brahmaputra river system

The Li isotope ratio (δ7Li) is expected to be a useful tracer of silicate weathering in river and groundwater systems, which is an important contributor to the seawater compositional changes that accompany the evolution of the Earths surface environment. To obtain accurate estimates of continental Li fluxes to the ocean, we determined δ7Li values of dissolved Li in the lower Ganges-Brahmaputra river system in both the dry and rainy seasons, and in deep groundwater in the Bengal basin. Dissolved Li and δ7Li values in the lower reaches of the rivers (0.04–0.66 µmol kg−1 and +19.1‰ to +34.2‰, respectively) were predominantly derived from silicate weathering, as is the case in the upper parts of these rivers. We observed large changes in δ7Li over a distance of more than 1000 km downstream that were due mainly to Rayleigh-type removal of Li from river water. Extremely high Li concentrations (1.15–1.67 µmol kg−1) and low δ7Li values (+5.1‰ to +11.6‰) in groundwater samples indicate congruent isotope leaching and dissolution of silicate minerals in the deep aquifer, where the water residence time is long. In the rainy season, Li concentrations and δ7Li values were lower than in the dry season, owing to the shorter residence time of river water and the substantial input of local subsurface flow through lowland alluvium. These results suggest that accurate estimation of continental Li fluxes to the ocean should take account of downstream and seasonal changes, as well as aquifer depth variations, in δ7Li values.