Large seasonal and spatial variability in δ44/40Ca values in the Godavari River, India: Implications for basalt weathering, groundwater interaction, and carbonate precipitation

Abstract

We present stable calcium isotopic compositions (δ44/40Ca) of the Godavari River, the largest river in peninsular India, from the upper 750 km of its course, where it primarily drains the Deccan basalts. In addition to river water samples collected during the pre-monsoon and monsoon seasons, we also present δ44/40Ca values of groundwater, bedrock Deccan basalts, suspended and bedload sediments, and carbonate nodules from the Godavari Basin as well as zeolites and hydrothermal calcite from several locations within the Deccan Traps. The δ44/40Ca values (reported relative to NIST SRM 915a) of the dissolved load of the Godavari River main channel display large variability during both the pre-monsoon (0.73–1.64 ‰) and the monsoon (0.56–1.28 ‰) seasons. In the upper reaches (within 160 km of the source), the river water samples during both seasons show δ44/40Ca values (0.56–0.89 ‰) and Sr/Ca (mmol/mol) ratios (1.47–2.86), which are in the same range as that of the bedrock Deccan basalts (0.53–0.78 ‰ and 1.1–5.19 mmol/mol, respectively), suggesting control of water chemistry by basalt weathering. In contrast, in the middle reaches of the river (beyond 160 km from the source), the riverine δ44/40Ca values (0.99–1.64 ‰) are much higher than δ44/40Ca of bedrock basalt, but similar to that of groundwater in the middle reaches. We conclude that groundwater discharge to the river during the low-flow period has a significant control on the observed high δ44/40Ca values of the Godavari River. Additionally, the positive correlation between δ44/40Ca values versus Sr/Ca ratios and weak positive correlation of δ44/40Ca values versus saturation index of calcite suggests that carbonate precipitation from the river water impacts the δ44/40Ca values of the Godavari River in the middle reaches; this conclusion is consistent with the low δ44/40Ca of acetic-acid leached carbonate nodules (0.52–0.56‰) collected from the Godavari riverbed.The δ44/40Ca values of zeolites exhibit a large range from −0.72‰ (heulandite) to 1.67‰ (apophyllite) while the δ44/40Ca values of leached hydrothermal calcite samples range from 0.75‰ to 0.92‰. These values indicate that weathering of these Ca-bearing minerals has limited contribution to the calcium isotopic composition of the Godavari River, which contrasts with observations from relatively smaller rivers draining basalt in Iceland. The δ44/40Ca values of the dissolved load of the basalt-draining Godavari River is significantly higher than the estimated global riverine input to the oceans (0.88‰); given the high weatherability of basalts, the high δ44/40Ca values of basalt-draining rivers suggests that the estimate of the δ44/40Ca value of the average continental flux to the oceans may have been underestimated. The results of this study also demonstrate distinct differences in geochemical and Ca isotopic compositions of basalt-draining rivers from Iceland and India, which reflects fundamental differences in the weathering process in very different climatic regions.