Palaeoenvironmental implications of evaporative gaylussite crystals from Lonar Lake, central India A. ANOOP, 1,2 S. PRASAD, 1 * B. PLESSEN, 1 N. BASAVAIAH, 3 B. GAYE, 4 R. NAUMANN, 1 P. MENZEL, 4 S. WEISE 5 and A. BRAUER 1 1 German Research Center for Geosciences (GFZ), Section 5.2, Climate Dynamics and Landscape Evolution, Telegrafenberg, C 455D-14473, Potsdam, Germany 2 DFG Graduate School 1364, University of Potsdam, Germany 3 Indian Institute of Geomagnetism, Navi Mumbai, India 4 Universita ¨t Hamburg, Institute of Biogeochemistry and Marine Chemistry, Hamburg, Germany 5 UFZ Centre for Environmental Research, Department of Catchment Hydrology, Halle, Germany Received 23 March 2012; Revised 11 January 2013; Accepted 13 January 2013 ABSTRACT: We have undertaken petrographic, mineralogical, geochemical and isotopic investigations on carbonate minerals found within a 10-m-long core from Lonar Lake, central India, with the aim of evaluating their potential as palaeoenvironmental proxies. The core encompasses the entire Holocene and is the first well-dated high-resolution record from central India. While calcite and/or aragonite were found throughout the core, the mineral gaylussite was found only in two specific intervals (4630–3890 and 2040–560 cal a BP). Hydrochemical and isotope data from inflowing streams and lake waters indicate that evaporitic processes play a dominant role in the precipitation of carbonates within this lake. Isotopic (d 18 O and d 13 C) studies on the evaporative gaylussite crystals and residual bulk carbonates (calcite) from the long core show that evaporation is the major control on d 18 O enrichment in both the minerals. However, in case of d 13 C additional mechanisms, for example methanogenesis (gaylussite) and phytoplankton productivity (calcium carbonate), play an additional important role in some intervals. We also discuss the relevance of our investigation for palaeoclimate reconstruction and late Holocene monsoon variability. Copyright # 2013 John Wiley & Sons, Ltd. KEYWORDS: evaporites; gaylussite; isotopes; Lonar Lake; monsoon. Introduction The mineralogy and geochemistry of lacustrine carbonates are widely used to infer palaeoenvironmental changes (Stuiver, 1970; Drummond et al., 1995; Leng and Marshall, 2004; Mangili et al., 2010). Fluctuations in the isotope composition (d 18 O and d 13 C) of authigenic carbonate minerals can be used to infer long-term changes in the precipitation/evaporation (P/E) ratio, temperature, photosyn- thetic pathways, changes in seasonality, and amount and tracks of precipitation (Leng et al., 2006; Spo ¨tl et al., 2010). The fundamental assumption underlying such investigations is the primary, unaltered nature of the carbonates, as isotopic exchange with interstitial brines, re-precipitation and/or re- crystallization can result in the loss of primary signals (Pendall et al., 1994). In this study we present the results of our investigations on carbonates found in a 10-m-long sediment core, encompassing the Holocene, raised from the Lonar Lake in central India. Lonar, an impact crater lake (Fredriksson et al., 1973; Milton et al., 1975; Maloof et al., 2009), is located in the core monsoon zone (CMZ) of central India. The available evidence indicates that the Indian summer monsoon (ISM) remained the dominant precipitation source in this region during the Holocene (Prasad and Negendank, 2004). Central India is a climatically sensitive region as the interannual variability of the ISM in the CMZ is strongly correlated with the all India monsoon rainfall (Rajeevan et al., 2010) and is impacted by various teleconnections (El Ni~ no-Southern Oscillation, Indian Ocean Dipole, tropical mid-latitude interactions) (Krishnan et al., 2000; Gadgil, 2003; Ashok et al., 2004). Lonar Lake offers the rare possibility of undertaking a long-term regional palaeoclimate reconstruction as most of central India is covered by the Deccan basalts and lacks natural lakes. Investigations on Lonar sediment cores revealed abundant gaylussite crystals (Na 2 CO 3 · CaCO 3 .5H 2 O) in some sections – the first such discovery from the Indian subcontinent. The primary gaylussite mineral is mostly precipitated by the gradual evaporative concentration of the brine (Hardie and Eugster, 1970; Eugster and Hardie, 1978; Mees et al., 1991) at the sediment–water interface (Bischoff et al., 1991) in contrast to calcite and aragonite that form in surface waters (McConnaughey et al., 1994; Leng and Marshall, 2004; Reddy and Hoch, 2011). Gaylussite usually succeeds the precipitation of calcite and is the first sodium carbonate mineral to be precipitated when saline, alkaline waters are concentrated by evaporation (Rankama and Sahama, 1964; Renaut et al., 1986). Simulated evaporative concentration of brine after the precipitation of gaylussite eventually leads to the precipitation of trona (Na 3 H (CO 3 ) 2 .2H 2 O) (Hardie and Eugster, 1970; Eugster and Hardie, 1978). However, gaylus- site can also form by secondary processes when alkaline brine rich in sodium reacts with (i) primary calcite aragonite; (ii) waters containing Ca 2þ and HCO 3  ; or (iii) calcite-rich sediments (Eugster and Hardie, 1978). Due to its rare occurrence in nature (e.g. Eugster and Hardie, 1978; Renaut et al., 1986; Bischoff et al., 1991; Mees et al., 1991; Partridge et al., 1993), only one study (Mees et al., 1998) has as yet investigated the potential of isotope data from gaylussite for palaeoclimate reconstruction. The objectives of the present study were to: (i) examine the evidence for primary versus diagenetic origin of gaylussite crystals; (ii) investigate possible factors influencing the forma- tion of carbonates in Lonar Lake, namely stream water chemistry and/or salinity induced by changes in input (inflow þ precipitation)/evaporation; (iii) evaluate the feasibility of using carbonate mineralogy and isotopic composition of gaylussite  Correspondence: S. Prasad, as above. E-mail: sushma.prasad@gfz-potsdam.de Copyright # 2013 John Wiley & Sons, Ltd. JOURNAL OF QUATERNARY SCIENCE (2013) 28(4) 349–359 ISSN 0267-8179. DOI: 10.1002/jqs.2625