A 2500 years deglacial record of paleo-vegetation over a cave of southern India as inferred from carbon isotopes of stalagmite

摘要

Carbon isotopes of stalagmites are influenced by various factors operating in a cave, like evaporation, vegetation, drip rate and changes in partial pressure of carbon-di-oxide (pCO$_{2}$) inside and around a cave system. Consequently, interpretation of $delta^{13}$C record becomes a bit complicated. However, the vegetation changes at a given cave location are generally considered as the dominating factor influencing $delta^{13}$C values of a stalagmite. The $delta^{13}$C records can provide useful information regarding changes in the class of vegetation over a cave due to dissimilar pathways of photosynthesis linking C$_{3}$ and C$_{4}$ vegetation. Here we present a high-resolution $delta^{13}$C record from a 180 mm long VSPM1 stalagmite collected from the Valmiki cave in Kurnool district of southern India. This study is mainly based on high-resolution $delta^{13}$C measurements of 263 subsamples. The data has been used to infer vegetation and climatic variations for the last deglacial period starting from 15,607 to 13,161 years BP. The StalAge modelling was employed on eight U–Th dates to reconstruct the age model of the stalagmite sample. The stalagmite grew at the rate of 0.07 mm per year with varying growth rate from around 0.03 to 0.8 mm per year. X-ray diffraction analysis reveals absolute aragonite mineralogy of the sample. The record exhibits a weak positive relationship between $delta^{13}$C and $delta^{18}$O values. The main factors influencing $delta^{13}$C values were associated with local hydroclimate. The $delta^{13}$C record suggests vital evidence of rapid alterations in vegetation changes from ${sim}$15,607 to 13,161 yr BP. A major shift in vegetation activity occurred from 15,607 to 15,105 yr BP with an episode of highly poor vegetation cover around 15,460 yr BP, followed by a gradual decline in vegetation conditions between 15,105 and 14,722 yr BP. $f{Highlights}$ $ullet$ This study reveals importance of stable carbon isotopes in stalagmites to understand regional hydroclimate and paleo-vegetation changes. $ullet$ The $delta^{13}$C record from a stalagmite sample of Valmiki cave located in South India indicates rapid vegetational changes controlled mainly by ISM variability during the last deglacial period. $ullet$ Three major phases have been observed; first a wet period from 15,607 to 15,105 yr BP, followed by a dry period between 15,105 and 14,722 yr BP and finally another wet event from 14,722 to 13,161 yr BP.