Signals of Holocene climate transition amplified by anthropogenic land-use changes in the westerly–Indian monsoon realm


The Indian summer monsoon (ISM) rainfall is the lifeline for people living on the Indian subcontinent today and was possibly the driver of the rise and fall of early agricultural societies in the past. The intensity and position of the ISM have shifted in response to orbitally forced thermal land–ocean contrasts. At the northwestern monsoon margins, interactions between the subtropical westerly jet (STWJ) and the ISM constitute a tipping element in the Earth's climate system because their non-linear interaction may be a first-order influence on rainfall. We reconstructed marine sea surface temperature (SST), supply of terrestrial material and vegetation changes from a very well-dated sediment core from the northern Arabian Sea to reconstruct the STWJ–ISM interaction. The Holocene record (from 11 000 years) shows a distinct, but gradual, southward displacement of the ISM in the Early to Mid-Holocene, increasingly punctuated by phases of intensified STWJ events that are coeval with interruptions of North Atlantic overturning circulation (Bond events). The effects of the non-linear interactions culminate between 4.6 and 3 ka BP, marking a climatic transition period during which the ISM shifted southwards and the influence of STWJ became prominent. The lithogenic matter input shows an up to 4-fold increase after this time period, probably related to the strengthened influence of agricultural activities of the Indus civilization, with enhanced erosion of soils. This anthropogenic land-use change is amplifying the impact of Bond events and adding to the marine sedimentation rates adjacent to the continent.
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