Response of grassland ecosystem to monsoonal precipitation variability during the Mid-Late Holocene: Inferences based on molecular isotopic records from Banni grassland, western India

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.班尼（Banni）位于干旱的印度西部，是亚洲大陆最大的热带草原之一。该草原生态系统的净初级生产力目前受控于印度夏季风（Indian Summer Monsoon, ISM）带来的降水。然而，由于数据集匮乏，草原扩张的时间节点及其与季风降水强度的关联仍不明朗。本研究的核心目标为：依托班尼地区两处沉积物岩芯（Chachi与Luna）中正构烷烃（n-alkanes）的氢、碳同位素组成（δDn-alkane与δ13Cn-alkane），解析过去4600个日历年前（calibrated years before present, cal yr BP）的季风降水与植被变化。Chachi岩芯沉积物的δ13CC29与δ13CC31值分别介于-30.9‰至-27.2‰、-34.4‰至-25‰之间。研究团队基于班尼地区优势现代植被的端元正构烷烃碳同位素值，通过二元混合模型将岩芯沉积物的δ13Cn-alkane值转换为C4植物占比。古植被曲线的显著特征为：距今2500个日历年前后，δ13Cn-alkane值出现明显升高，指示该区域C4草本植物大量繁衍。距今2500个日历年前后，δDn-alkane值也呈现出类似变化趋势。研究利用δDC29值计算得到古降水δD值介于10‰至-60.2‰之间。古降水δD值显著升高（约25‰），表明距今约2500个日历年前后，印度夏季风降水有所减弱。区域气候干旱化与频发的火事件可能推动了班尼地区以C4植物为主的草原生态系统扩张。古气候记录的相关性分析表明：热带辐合带南移以及厄尔尼诺-南方涛动（El Niño Southern Oscillation, ENSO）暖相位频次增加，共同引发了热带地区季风降水的减弱。