Climate Change and the Migration of a Pastoralist People c. 3500 cal. Years BP Inferred from Palaeofire and Lipid Biomarker Records in the Montane Western Ghats, India

Abstract

Human migration in response to climate change during the Holocene has been recorded in many regions of the world. The Todas are a pastoralist people who are believed to have colonized the higher elevations (>2000 m asl) of the Nilgiris in the Western Ghats, India, not earlier than about 2000 cal. yr BP. Vegetation shifts in response to changing climate in tropical montane forest-grassland mosaic of the Ghats have been well documented using stable carbon isotopes and pollen profiles; however, there have been no corresponding investigations of human presence and activity at the highest elevations. We used a number of other proxies to infer the human ecology of this montane region. Radiocarbon dated (~22,000 cal. BP to the present) peat samples from the Sandynallah basin (2200m asl, Nilgiri hills, Tamil Nadu State) were used to reconstruct fire history, animal abundance, and human presence since the Last Glacial Maximum (LGM). While the macro-charcoal record indicates fires at the LGM, macroand micro-charcoal counts indicate intense fire at ~3500 cal. yr BP, coprophilous fungal spores indicate a large population of herbivorous mammals, and steroid biomarkers indicate human faecal remains for the first time. This period is also characterized by dry arid conditions and dominant grassy vegetation as inferred from n-alkane signatures. We thus infer that a pastoralist people, most likely the Todas, migrated to the highest elevations of the Western Ghats along with their buffalo herds in response to prolonged or abrupt climate change in peninsular India, about 3500 cal. yr BP or at least 1500 years prior to what historical accounts assume.

Paleovegetation dynamics in an alternative stable states landscape in the montane Western Ghats, India

“…This could be due to gradually ameliorating hydrology (gradual, climate-initiated) or a disturbance to the shola leading to an unstable transition state (disturbance-initiated). The latter interpretation is supported by results from Kavil et al (2021) who studied sediments from a pit proximate to Core 1 and found charcoal at 22.1 ka. They also present Scanning Electron Microscopy images to show that the macrocharcoal has abundant woody and grass elements, an indicator that woody elements indeed were present at 22 ka (Figure 5 in Kavil et al (2021)).…”

“…Why would Cores 1 and 2 that are only ~170 m apart in the same valley show such a major difference in past vegetation composition? What role do disturbances such as the fires reported by Kavil et al (2021) play in maintaining these vegetation states? We explore the various reasons that might have resulted in these patterns within the alternative stable states framework.…”

“…Data from δ 13 C measurements on cellulose extracts from Cores 1 and 2 are provided in Supplemental Tables S1 and S2, available online, respectively. The δ 13 C values plotted against the median values from Bacon age-depth models are shown in Figure 4 (along with data on fires from Kavil et al (2021). The δ 13 C profile for Core 2 is different from that of Core 1, indicating different paleovegetation composition, which is very clear in the glacial period before 10 ka.…”

“…We use the information recorded in excavation samples from a pit in the Sandynallah valley close to Core 1 (Figure 1) reported by (Kavil et al, 2021). Fires are reported at ~22 ka (macrocharcoal) and ~3.5 ka (macro-, microcharcoal and charcoal/pollen ratio).…”

“…These studies have broadly demonstrated the response of the vegetation to wellknown global climatic events such as the Last Glacial Maximum, the Holocene Optimum and an additional phase of relatively arid and fluctuating monsoon (about 6-2 ka). Recently this valley has also been reported to preserve signatures of paleofires and human presence in this montane region at c. 3.5 ka (Kavil et al, 2021). Accordingly, Caner et al (2007) state that 'The Nilgiri highlands have long been an oasis for paleoenvironmental research within an otherwise unrewarding South Indian craton in terms of Quaternary environmental archives'.…”