A mid- to late-Holocene record of vegetation decline and erosion triggered by monsoon weakening and human adaptations in the south-east Indian Peninsula

Abstract

The mid- to late-Holocene monsoon decline led to aridification of the Indian Peninsula impacting the early agricultural practices in the region. Our analysis of organic carbon, mineral magnetic properties and AMS 14C dating of a 54.2-m-long sediment core (CY) from the Godavari Delta, India, showed changes in the organic carbon source and sediment provenance, which are linked to the changes in vegetation and soil/rock erosion caused by widespread aridification and associated human adaptation in central India. Our results show a decline in the concentration of ferrimagnetic minerals, indicating reduced input from the basalts of the Deccan Plateau after ~6.0 cal. ka BP in response to the weaker Indian monsoon. δ13C values show a distinct increase from ~4.9 cal. ka BP, indicating an increase in C4 plant sources under the continued weak monsoon phase, whereas a higher ferrimagnetic mineral concentration in the sediment suggested an increased Deccan basalt source. Abrupt increase in δ13C values and decrease in TOC content accompanied with a significant increase in ferrimagnetic mineral concentration from ~3.2 to 3.1 cal. ka BP reflected a shift of organic carbon and sediment source and a severe decline in vegetation coverage. Such phenomena indicate intensified deforestation and soil/rock erosion in the Deccan Plateau producing higher ferrimagnetic mineral inputs, which is in agreement with significant expansion of agricultural activities in the Deccan Chalcolithic cultural period. In addition, C3 plants recovered and magnetic concentration declined during the wet events (4.6 and 4.0 cal. ka BP) of Neolithic time, while both C3 plants and magnetic parameters increased during the wet events (3.1–2.8 and 2.1 cal. ka BP) of the Chalcolithic cultural period. This implies increased agricultural activity and the onset of human modification of the ecosystem.

Short communication: Massive erosion in monsoonal central India linked to late Holocene land cover degradation

“…2; see Supplement). Ferrimagnetic minerals interpreted as originating from the Deccan Plateau (Sangode et al, 2001;Kulkarni et al, 2014) also increase in late Holocene sediments in the Godavari Delta (Cui et al, 2017) and Bay of Bengal (Kulkarni et al, 2015), supporting our interpretation. Augmented Deccan input was suggested for the Godavari Delta even earlier after ∼ 6000 years ago (Cui et al, 2017), in step with the initial aridification.…”

“…Together these continental and oceanic records suggest that the CMZ aridification intensified in the latest Holocene via a series of short drier episodes . This interpretation is reinforced by speleothemderived records from central and northern India for the past thousand years (Sinha et al, 2011), and the overall evolution of the CMZ hydroclimate as seen from our core is supported by local reconstructions from the Lonar crater lake in central India (Prasad et al, 2014;Sarkar et al, 2015), Godavari Delta (Cui et al, 2017) and other records from the larger Indian monsoon domain (Gupta et al, 2003;Fleitmann et al, 2003;Prasad and Enzel, 2006;Berkelhammer et al, 2012;Dixit et al, 2014).…”

“…Once reaching the Bay of Bengal, sediment delivered by the Godavari has constructed a large Holocene delta (Rao et al, 2005;Cui et al, 2017). Offshore from the Godavari mouth, a persistent sediment plume extends over 300 km during the monsoon season, when over 90 % of the fluvial sediment is discharged (Sridhar et al, 2008).…”

Magnetic Mineralogical Approach for the Exploration of Gas Hydrates in the Bay of Bengal

“…A sharp magnetic susceptibility decrease between 570 and 600 °C suggests that the bulk magnetic mineralogy is dominated by magnetite (Dunlop et al, ; Figures a–l). The χ increase between 300 and 500 °C is likely due to conversion of paramagnetic minerals such as pyrite and iron‐bearing silicates, iron‐rich carbonates, siderite, and iron‐rich smectites into magnetite during heating (Cui et al, ; Hirt et al, ; Hirt & Gehring, ; Pan et al, ; Passier et al, ; Philips, ; Figures a–l).…”

On the Holocene evolution of the Ayeyawady megadelta

“…It is important to note that, like the Ayeyawady, many large river deltas developing under the Asian monsoon regime, such as the Mekong (Ta et al, 2002), Red River (Tanabe et al, 2003 or Godavari (Cui et al, 2017), started to form wave-built beach ridges between 5000 and 4000 years ago, changing from river-dominated morphologies to show stronger wave-influenced characteristics. Given that these deltas were at various stages of advance from within their incised valleys onto the shelf, it is more likely that their morphological evolution was climatically driven rather than controlled by local factors as previously proposed.…”