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Biomarker Seasonality Study in Lake Van, Turkey ⇑ C Organic Geochemistry 42 (2011) 1289–1298 Contents lists available at SciVerse ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem Biomarker seasonality study in Lake Van, Turkey ⇑ C. Huguet a, , S. Fietz a, M. Stockhecke b, M. Sturm b, F.S. Anselmetti b, A. Rosell-Melé a,c,d a Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Catalonia, Spain b Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Water Research and Management, Ueberlandstrasse 133, 8600 Dubendorf, Switzerland c Department of Geography, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Catalonia, Spain d ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Catalonia, Spain article info abstract Article history: The endorheic Lake Van in eastern Anatolia (Turkey) is the world’s largest soda lake and it is an important Received 29 April 2011 site in paleoclimate studies to understand past continental conditions in western Asia. In order to gain Received in revised form 16 September further insights into the biomarker signatures in Lake Van’s sediments we have analyzed particulate 2011 material in sediment traps deployed between August 2006 and July 2007. The biomarkers used were long Accepted 21 September 2011 chain alkenones (LCAs C –C , haptophyte lipids), isoprenoid glycerol dialkyl glycerol tetraethers Available online 2 October 2011 37 39 (GDGTs, Archaea membrane lipids) and pigments (chlorins and fucoxanthin). The biomarker fluxes indi- cate a strong seasonality in export primary productivity and the phytoplankton community structure. The highest total mass and organic carbon fluxes were found in summer, coupled to strong stratification while the lowest mass fluxes occurred in winter at the time of water column mixing. With increasing temperatures in early spring, phytoplankton export productivity grew, coupled with an increase of total mass flux and organic carbon, which might be associated to enhanced nutrient input from snowmelt run- off. The percentage of C37:4 shows some correspondence with observed seasonal changes in Lake Van’s stratification structure. We also evaluated the potential applicability of molecular temperature proxies derived from Archaea and haptophyte lipids. The use of the TEX86 proxy was precluded by low GDGT abundances. Estimated LCA temperatures were consistent with temperatures in the photic zone but no seasonality changes were observed despite the wide annual temperature range measured at Lake Van. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction environments, alkenone production is dominated by Emiliania hux- leyi and Gephyrocapsa oceanica, but lakes contain other species, like Understanding past changes in Earth’s climate requires the Isochrysis galbana and Chrysotila lamellosa (e.g. Marlowe et al., reconstruction of climatic patterns in both marine and continental 1984). Previously LCA distributions and specifically the C37/C38 ra- settings. Presently, continental climate records are often incom- tio have been used as indicators of haptophyte species identity (e.g. plete. However, lakes generally display higher production and sed- Chu et al., 2005; Liu et al., 2008; Toney et al., 2010), but a recent imentation rates than oceans and contain paleoclimatic and study revealed no clear correlation between LCA ratios and phylo- paleoenvironmental information on the local–regional continental genetic placement (Theroux et al., 2010). Thus, even though it is response to global changes (e.g. Meyers, 2003; Litt et al., 2009). known that lake species belong to the order Isochrysoidales, LCA Temperature is one of the key environmental factors to understand sources in specific lakes are still largely unknown (Theroux et al., climate changes, two molecular proxies initially developed in mar- 2010). ine environments also have specific lake calibrations, i.e. the long A marine sediment core study showed that the ratio of LCAs is K chain alkenones (LCAs) unsaturation index (U37) and the TEX86 in- sensitive to climatic changes (Brassell et al., 1986), and studies dex based on isoprenoidal glycerol dialkyl glycerol tetraethers using laboratory cultures of E. huxleyi proved that the extent of (GDGTs). unsaturation changed with growth temperatures (e.g., Marlowe K LCAs are a group of lipids comprising C37–C40 di-, tri- and tetra- et al., 1984). Based on this finding, the alkenone indices U37 and K0 unsaturated methyl and ethyl ketones, which are key biomarkers U37 were proposed and confirmed as sea surface temperature for haptophytes. They were originally found in marine environ- (SST) proxies by culture studies of E. huxleyi (Prahl and Wakeham, ments, but later discovered to be present also in lakes (e.g. Mar- 1987; Prahl et al., 1988) and global core-top studies (e.g. Müller lowe et al., 1984; Cranwell, 1988; Zink et al., 2001; Chu et al., et al., 1998; Conte et al., 2006). In contrast, the use of LCA ratios 2005; Pearson et al., 2008; Toney et al., 2010). In marine to reconstruct water temperatures in lacustrine settings is a more recent topic of study. Specific lake calibrations had to be developed ⇑ Corresponding author. Tel.: +34 93 5812488. (e.g. Chu et al., 2005; Toney et al., 2010) as LCA distribution in lakes E-mail address: [email protected] (C. Huguet). is different than the one found in marine settings, for instance 0146-6380/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2011.09.007 1290 C. Huguet et al. / Organic Geochemistry 42 (2011) 1289–1298 usually displaying a dominance of C37:4 and absence of ethyl alke- (e.g. Cranwell, 1988; Thiel et al., 1997; Meyers, 2003; Blaga et al., nones. In fact, until recently the appropriate use of LCA indices to 2009). In this paper we report data on chlorophyll a derivatives quantitatively reconstruct lake temperature was uncertain. In Lake as indicators of total export primary productivity (e.g. Villanueva Van, for example, Thiel et al. (1997) suggested that the LCA distri- et al., 1994; Leavitt and Hodgson, 2001; Walker and Keely, 2004), butions were unrelated to temperature, but the lake was later suc- fucoxanthin as indicators for fucoxanthin-containing algae such cesfully included in an Asian lake calibration study by Chu et al. as diatoms, chrysophytes and haptophytes (e.g. Jeffrey et al., (2005). 1997), LCAs as indicators for haptophytes (e.g. Marlowe et al., Another molecular temperature proxy that is also applied in 1984) and isoprenoid GDGTs as indicators for Archaea (e.g. lakes, is the TEX86 index based on archaeal isoprenoid GDGTs Schouten et al., 2002). We also evaluate the potential use of paleo- (e.g. Powers et al., 2004, 2010; Blaga et al., 2009). A number of Ar- temperature proxies derived from LCAs and GDGTs. chaea have been shown to change the relative proportion of GDGTs with growth temperature, i.e. higher temperatures result in an in- crease in the relative amounts of GDGTs with two or more cyclo- 2. Material and methods pentane moieties and thus higher TEX86 (Schouten et al., 2002; Wuchter et al., 2004). By measuring the TEX86, the temperature 2.1. Regional setting at which Archaea lived when they produced their membranes can be determined. Archaea ubiquitously occur in the marine Lake Van is situated in eastern Anatolia (Turkey) at 1648 m water column and are one of the dominant prokaryotic groups in above sea level and it has an area of 3570 km2, a maximum depth the modern oceans (Karner et al., 2001), but they are also found of 460 m, pH 9.5–9.9, 21–24‰ salinity and 155 m eq/l alkalinity in lakes (e.g. Schleper et al., 1997; Hicks, 2003; Casamayor and (Litt et al., 2009; Reimer et al., 2009; Kaden et al., 2010). The lake Borrego, 2009). Core-top studies have provided a linear regression receives water mainly through precipitation and snowmelt inflow 2 equation to convert TEX86 values into SSTs (Schouten et al., 2002; from a catchment area estimated to be 12,500 km and loses it Kim et al., 2010) or lake surface temperatures (Powers et al., through evaporation (Kadiog˘lu et al., 1997; Fig. 1). 2004, 2010). The climate in eastern Anatolia is strongly influenced by changes Although Lake Van has a relatively low sedimentation rate, the in the position of the westerly jet stream, the extension of the sub- varved deposition and its endhoreic nature make it an ideal loca- tropical low pressure belt and Siberian high pressure area (Wick tion to obtain paleoclimatic data (Litt et al., 2009). With the aim et al., 2003; Reimer et al., 2009). The study area experiences rela- of investigating continental paleoclimate, a record spanning multi- tively warm summers (above 20 °C) and cold winters (below 0 °C), ple glacial–interglacial cycles was recovered from Lake Van in but, due to the high salinity and the great depth of the lake, no ice 2010, within the framework of the International Scientific Conti- forms on the surface. Annual precipitation in the city of Van is about nental Drilling Program (ICDP) project PALEOVAN (Litt et al., 379 mm, with highest precipitation occurring in April and October. 2009; Fig. 1). Evaluating the environmental information stored in Surface water temperatures recorded during the study ranged from sedimentary records firstly depends on the knowledge about the 3.4–23 °C(Stockhecke, 2008). From May to October Lake Van is strat- ecosystem and understanding of present day link between envi- ified with surface temperatures of 11–23 °C(Table 2). In January, ronmental variability and lake response. Here we collected settling cooling of the surface water and impact of winds induces mixing of particles with sequential sediment traps between August 2006 and the upper 70 m of the water column, generating a homogeneously July 2007 to study the seasonal biomarker fluxes. Biomarkers in cold water column at 3.4 °C. Below 70 m the water temperature lakes have often been used to reconstruct the paleoenvironment ranges between 3.2 °C and 3.7 °C throughout the year (Stockhecke, surrounding the lake and the surface water column conditions 2008).
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