Date Published: April 17, 2019
Publisher: Public Library of Science
Author(s): Sayak Basu, Prasanta Sanyal, Anusree A. S. Pillai, Anoop Ambili, Robert Hilton.
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.
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 not only play crucial roles in global carbon budget  but also influence regional climate by modulating the evapotranspiration flux . The vegetation composition of grassland ecosystems reflects the competing influence of precipitation, fire and atmospheric CO2 concentration. For instance, 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 . It is important to note that the interaction between above-mentioned forcing factors varies across the globe and thus produces a site-specific structure of the grassland ecosystem. Furthermore, the magnitude of the climatic factors and their effects on vegetation composition are also globally variable, depending upon duration of the rainy season, precipitation amount and number of precipitation events . 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 thus vital to predict future ecosystem response to changing natural variables.
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. After ca. 2500 cal yr BP, a gradual increase in the δ13Cn-alkane values indicates the dominance of C4 grasses at the expense of C3 plants. Difference between the δ13Cn-alkane values indicates that each n-alkane homologue had different C3-C4 proportional contributions. In the lake sediments, C29 and C31 homologues were mainly derived from C3 plants and C4 grasses respectively. The present study has used the δDC29 values as a proxy of the isotopic composition of paleoprecipitation. Meteorological and modeling observations show that δD values of modern precipitation in the Banni region are inversely related to the precipitation amount. This ‘amount effect’ based reconstruction suggests a marked decrease in the ISM precipitation after 2500 cal yr BP. The observed precipitation pattern in the climate record from Banni region corresponds well with published records. Coupled influence of southward migration of ITCZ and higher frequency of warm ENSO events led to the decrease in the ISM precipitation after 2500 cal yr BP and helped in establishing the grassland ecosystems.