| dc.description.abstract | Chemical weathering of continental silicate rocks releases cations and anions which are eventually precipitated as carbonates on the ocean floor. This entire process transfers carbon from the atmosphere to the oceans leading to a net consumption of atmospheric carbon dioxide, thereby modulating the Earth’s climate, over millions of years. Although, basalts occupy only 3.5% of the Earth’s continental surface, due to their high weatherability, which is about eight times higher than that of granites, basalt weathering contributes to more than 30% of the global CO2 consumption flux by silicate weathering. Rivers transport the products of rock weathering to the oceans and play a significant role in terms of removal of atmospheric CO2. The present study aims to understand basalt weathering in a tropical climatic region by studying the water and sediment chemistry of the monsoon-fed Godavari River, the largest river in peninsular India, in the upper 700 km of its course where it drains only the Deccan basalts. River water samples from main channel of the Godavari River (n =104), and its tributaries (n = 24) were collected twice each during pre-monsoon, monsoon and post-monsoon seasons, from May 2017 to December 2018. These river water samples were analysed along with groundwater samples (n = 25), bedrocks (n = 15), and carbonate nodules (n = 12) from the Godavari Basin, as well as zeolites (n=7) and hydrothermal calcite (n=1) from the Deccan Traps. Specifically, this study utilizes the major and minor ion chemistry as well as Sr, Ca, Li and Mg isotopic compositions to constrain the main sources of ions to the Godavari River and understand the seasonal variability in the water chemistry in terms of varying sources and processes like congruent versus incongruent weathering, selective weathering of minerals and precipitation of carbonates.
The river water chemistry exhibits large spatial variability across the main channel and most of the chemical parameters show inter-seasonal as well as inter-annual variability at 5% level of significance. The river water composition, for example the high Na-normalized molar ratios of major ions in the upper reaches compared to the middle reaches of the river, reflects congruent weathering of basalts in a weathering-limited regime. The strontium isotopic composition (87Sr/86Sr) of river water samples in the upper reaches are relatively more radiogenic compared to that of the water samples in the middle reaches. This variability likely reflects the compositional variability of the Deccan basalts from the different sub-groups. Water samples from the upper reaches reflect contributions from the Jawahar/Igatpuri formations while those from the middle reaches reflect compositions of Poladpur and/or Khandala formations, which is consistent with the spatial distribution of these formations. The bedrock basalts collected as part of this study show 87Sr/86Sr which overlap with reported compositions of the Jawahar and Igatpuri formations. The 87Sr/86Sr of the tributaries overlap with that of main channel in the upper and middle reaches. The 87Sr/86Sr of carbonate nodules collected at upper reaches overlap with that of the river waters, but their elemental ratios suggest minimal contribution to the river water chemistry. The CO2 consumption flux due to silicate weathering in the upper Godavari River is 0.0854 x 1012 mol. C. yr-1. This study suggests that the CO2 consumption flux due to weathering of the entire Deccan basalts is 0.77x1012 mol. yr-1 which is ~6.6% of the global CO2 consumption flux by silicate weathering.
The compositions of the Godavari River water from Nashik, collected bi-weekly over a period of 14 months (n = 29), show significant variability in dissolved ion concentrations and major ion molar ratios but show limited variability in 87Sr/86Sr. These variations are explained by selective weathering of Ca- and Mg-bearing minerals (plagioclase and clinopyroxene) in basalts. Simple mass balance suggests that plagioclase weathering contributes 30-45% of the river water chemistry during post-monsoon and 25-40% during the monsoon season while clinopyroxene weathering contributes to 55-70% of the water chemistry during post-monsoon and 60-75% during monsoon. Additionally, the stable calcium isotopes (δ44/40Ca with reference to SRM915a) of selected river waters at Nashik show large variability (0.51 to 1.6‰), which is consistent with selective weathering of plagioclase versus clinopyroxene and removal of Ca from river waters by carbonate mineral precipitation.
Stable calcium isotopic compositions (δ44/40Ca) of the dissolved load of the Godavari River were measured in selected samples collected during the pre-monsoon (n=15) and monsoon (n=17) seasons. Additionally, selected samples of groundwaters, bedrock basalts, carbonate nodules, zeolites and hydrothermal calcite were analysed for stable Ca isotopes. The δ44/40Ca values of the river water samples show large spatial and seasonal variabilities with an overall range of ~1.1 ‰ along with large variations in Sr/Ca molar ratios, which is likely due to carbonate mineral precipitation from the river water during the dry seasons in the middle reaches. In the upper reaches of the river, the δ44/40Ca values (0.56 to 0.89‰) of water samples are in the same range as that of bedrock basalts (0.53 to 0.78‰), hence likely reflect congruent basalt weathering. Silicate weathering in the upper Godavari River Basin was further investigated using Li and Mg isotopes which were measured only in selected pre-monsoon river water samples. The Li isotopic compositions (δ7LiL-SVEC) of river water samples in the middle reaches of the Godavari River show large spatial variability of ~27 ‰ (+4.24 to +31.17 ‰) which is explained by nearly congruent weathering of basalts and clay mineral formation. In contrast, the Mg isotope compositions (δ26MgDSM3) measured in these water samples exhibit limited variability and range from -0.56 to -0.29‰ overlapping with reported compositions of basalts from other igneous provinces. The observed Mg isotope composition suggests congruent weathering dominated by weathering of Mg-bearing minerals in the Deccan basalts. | en_US |
