Spatial variability of Sr isotope of Gomati River Basin within Ganga Alluvial Plain: Implications for global seawater fluxioning
2020 年 54 巻 2 号 p. 57-70
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2020 年 54 巻 2 号 p. 57-70
The continuous increase of the 87Sr/86Sr isotopic ratio in the seawater since last 40 Ma has been correlated with the rise of the Himalaya. The Ganga-Brahmputra Fluvial System drains the Himalaya along with the Ganga Alluvial Plain (GAP) and the northern Indian Craton regions. Under the humid subtropical climatic condition, the rivers of the alluvial plain contribute a significant (~50%) in the water discharge. Previous studies have identified the Himalayan Rivers as a potential source for the steady increase of marine Sr budget; overlooking the contribution of alluvial rivers. We attempt to constrain the role of GAP as a source for Sr. The Gomati River, a 900 km-long tributary of the Ganga River, drains about 30,437 km2 of the GAP with 7,390 × 106 m3/a water discharge and provides an ideal opportunity to understand the role of GAP in contribution of the global 87Sr budget. A total of 44 river water, 33 groundwater, 6 rainwater, 3 lake water, and 13 alluvial sediment samples were analyzed for 87Sr/86Sr isotopic ratio to determine sources and mixing relationships of the rainwater, groundwater and river water within the GAP. In the Gomati River Basin, the average Sr isotopic ratio of the river water (0.7292) is higher than that of the average Ganga River water (0.7246) and much higher than that of world seawater (0.7119) and modern seawater (0.7092). The average Sr isotopic ratio of the shallow groundwater and rainwater was 0.7242 and 0.7139, respectively. The Gomati River drains the GAP having alluvial sediments with more radiogenic Sr isotopic ratio ranging from 0.7655 to 0.7244. Due to this, the river water displays strong seasonal variability with lower Sr isotopic ratio than groundwater during the monsoon season (0.7184). Our data indicate that the high water discharge contribution with reasonably higher Sr isotopic values from GAP river water makes it an important additional source of high radiogenic Sr in addition to the Himalayan source. The chemical weathering of alluvial sediments in GAP under the monsoon-controlled climatic condition is likely to make significant contributions to the evolution and budget of Sr isotope in the global sea.
