Abstract
Banni, located in the arid western India, is one of the largest tropical grasslands of the Asian continent. The net primary production in this grassland ecosystem is currently mediated by precipitation during the Indian summer monsoon (ISM). However, timing of the grassland expansion and its link to the intensity of monsoonal precipitation remains enigmatic due to the paucity of datasets. The major objective of this study is to understand the changes in monsoonal precipitation and vegetation for the last 4600 cal yr BP using hydrogen and carbon isotopic composition of n-alkanes (δDn-alkane and δ13Cn-alkane) measured from two core sediments (Chachi and Luna) in Banni region. The δ13CC29 and δ13CC31 values for Chachi core sediments vary from −30.9 ‰ to −27.2 ‰ and −34.4 ‰ to −25 ‰ respectively. The δ13Cn-alkane values from the core sediments are converted into %C4 plants based on a binary mixing model using the end-member δ13Cn-alkane values derived from the dominant modern vegetation in the Banni region. The prominent feature of the paleovegetation curve is the marked increase in the δ13Cn-alkane values after 2500 cal yr BP, which suggests proliferation of C4 grasses in this region. Similar changes after 2500 cal yr BP have also been observed in the δDn-alkane values. The δDC29 values are used to calculate δD value of paleoprecipitation that varied from 10 ‰ to −60.2 ‰. A significant increase in the δD values of paleoprecipitation (ca. 25 ‰) indicates a weakened ISM precipitation after ca. 2500 cal yr BP. The regional aridification and frequent fire events may have helped the expansion of C4 plant dominated grassland ecosystem in Banni region. Correlation between paleoclimatic records suggests that the southward migration of intertropical convergence zone and more frequent warm phases of El-Nino Southern Oscillation have triggered the weakening of monsoonal precipitation in the tropical region.
Highlights
Grasslands are globally important as they comprise nearly one-fifth of the world’s land surface and 80% of the total agriculturally productive land [1,2]
The low Paq values of Chachi core sediment indicate higher contribution of terrestrial biomass to the organic matter compared to aquatic productivity, and this observation is concordant with the Terrigenous versus aquatic ratio (TAR) values (Fig 2B and 2C)
Climate-driven changes in grassland ecosystems of the Indian Summer Monsoon realm have been investigated, for the first time using the isotopic composition of biomarkers in lake sediments
Summary
Grasslands are globally important as they comprise nearly one-fifth of the world’s land surface and 80% of the total agriculturally productive land [1,2]. Geographical distribution, community composition and net primary production of grassland ecosystems play crucial roles in global carbon budget [3] and influence regional climate by modulating the evapotranspiration flux [4]. Changes in CO2 level and fire events in the grassland of West Africa during the Late Holocene shifted the vegetation composition towards woody plant cover, despite the increased regional aridity [5]. The magnitude of the climatic factors and their effects on vegetation composition are globally variable, depending upon duration of the rainy season, precipitation amount and number of precipitation events [6]. Prolonged and recurrent droughts predicted in a warming environment, combined with human-induced changes in the natural hydrological regime, will likely have an adverse effect on biodiversity of the grasslands [7, 8]. The understanding of past climate-ecosystem coupling is vital to predict future ecosystem response to changing natural variables
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