Monsoon source shifts during the drying mid-Holocene: Biomarker isotope based evidence from the core ‘monsoon zone’ (CMZ) of India

• Published in: Quaternary science reviews 2015-09, Vol.123, p.144-157
• Main Authors: Sarkar, Saswati, Prasad, Sushma, Wilkes, Heinz, Riedel, Nils, Stebich, Martina, Basavaiah, Nathani, Sachse, Dirk
• Format: Article
• Language: English
• Subjects: Compound-specific stable isotopic composition; Holocene; Indian Summer Monsoon; Lipid biomarkers; Lonar Lake
• ISSN: 0277-3791; 1873-457X
• DOI: 10.1016/j.quascirev.2015.06.020

A better understanding of past variations of the Indian Summer Monsoon (ISM), that plays a vital role for the still largely agro-based economy in India, can lead to a better assessment of its potential impact under global climate change scenarios. However, our knowledge of spatiotemporal patterns of ISM strength is limited due to the lack of high-resolution, continental paleohydrological records. Here, we reconstruct centennial-scale hydrological variability during the Holocene associated to changes in the intensity of the ISM based on a record of lipid biomarker abundances and compound-specific stable isotopic composition of a 10 m long sediment core from saline–alkaline Lonar Lake, situated in the core ‘monsoon zone’ of central India. We identified three main periods of distinct hydrology during the Holocene in central India. The period between 10.1 and 6 cal ka BP was likely the wettest during the Holocene. Lower average chain length (ACL) index values (29.4–28.6) and negative δ13Cwax values (−34.8‰ to −27.8‰) of leaf wax n-alkanes indicate the dominance of woody C3 vegetation in the catchment, and negative δDwax values (concentration weighted average) (−171‰ to −147‰) suggest a wet period due to an intensified monsoon. After 6 cal ka BP, a gradual shift to less negative δ13Cwax values (particularly for the grass derived n-C31) and appearance of the triterpene lipid tetrahymanol, generally considered as a marker for salinity and water column stratification, mark the onset of drier conditions. At 5.1 cal ka BP an increasing flux of leaf wax n-alkanes along with the highest flux of tetrahymanol indicate a major lowering of the lake level. Between 4.8 and 4 cal ka BP, we find evidence for a transition to arid conditions, indicated by high and strongly variable tetrahymanol flux. In addition, a pronounced shift to less negative δ13Cwax values, in particular for n-C31 (−25.2‰ to −22.8‰), during this period indicates a change of dominant vegetation to C4 grasses. In agreement with other proxy data, such as deposition of evaporite minerals, we interpret this period to reflect the driest conditions in the region during the last 10.1 ka. This transition led to protracted late Holocene arid conditions after 4 ka with the presence of a permanent saline lake, supported by the sustained presence of tetrahymanol and more positive average δDwax values (−122‰ to −141‰). A late Holocene peak of cyanobacterial biomarker input at 1.3 cal ka BP might represent an event of