Distribution of Branched and Isoprenoid Tetraether Lipids in Lake Moncortés, Spain Min Cao1, Carme Huguet1, Valentí Rull2, Juan Pablo Corella3, Blas L. Valero-Garces4, Antoni Rosell-Melé1,5 1 Institut de Ciència i Tecnologia Ambientals (ICTA), Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Catalonia, Spain; 2 Institut de Botanic de Barcelona (CSIC), Passeig del Migdia s/n, 08038 Barcelona, Spain; 3 Museo Nacional de Ciencias Naturales, 28006 Madrid, Spain; 4 Instituto Pirenaico de Ecologıa (CSIC), Avda. Montañana 1005, 50059 Zaragoza, Spain; 5 Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Catalonia, Spain. . Results and discussion Introduction Glycerol dialkyl glycerol tetraethers (GDGTs), which are lipids found in the cell membrane of archaea and certain bacteria, can be a highly versatile tool in palaeolimnology (Sinninghe Damsté et al., 2012). One of the most successful applications is the TEX86 index (eq.1), which has been proposed as a proxy to estimate lake surface temperature (e.g. Powers et al., 2010; Blaga et al., 2009). Other applications rely on GDGTs from bacteria in soils and have been employed to infer relative terrigenous inputs (BIT, Hopmans et al., 2004), soil pH, and continental temperatures in records from aquatic settings (MBT/CBT, Weijers et al., 2007). Branched GDGTs (brGDGTs) are more abundant than isoprenoidal GDGTs (isoGDGTs), often contributing more than 60% to the abundance of the total GDGTs in the lake sediments (Fig.3). Thus, the BIT index (eq.3) varies from 0.7 to 1.0 as crenarchaeol concentration is one order of magnitude lower than brGDGTs. Downcore, both brGDGTs and isoGDGTs concentrations present similar trends. The abundance of all GDGTs marked the Little Ice Age (LIA, 1300-1850AD). Estimates of soil pH values (eq.6) vary between 7.5 and 8.2. A low-value period with higher lake levels was at the end of the Roman epoch (150-600AD) (Scussolini et al., 2011). Another period of low pH values occurred during the LIA. Estimates of mean annual air temperature (MAT) are derived from the MBT and CBT indices (eq.4, eq.5) (Weijers et al., 2007). From Fig.3, we see that MBT/CBT-derived MAT changes from -2.7 to 4.6 0C, whereas present mean annual air temperatures are around 12.3 0C (inferred from Corella et al., 2012). Even though, MAT does not change during the Medieval Climate Anomaly (MCA) it does during the LIA. Estimated TEX86 lake surface temperature in the record (LST, eq.2) vary between 12.6 and 33.9 ⁰C. In contrast, measured lake surface temperature vary from 4.9 to 23.9 ⁰C with a mean value of 12.5 ⁰C (Camps et al., 1976), However, given that the lake bottom can be anoxic, there is a possibility that the index may be biased by contributions from sedimentary methanogenic Archaea (Blaga et al., 2009). This can be attested by the ratio of GDGT-0 to crenarchaeol, which is higher than 2 in most samples, which suggests that GDGT-0 is predominantly derived from methanogens and that indeed the TEX86 values may not be just indicative of surface lake temperatures. The trends of estimated MAT and LST vary in parallel in certain sections of the record, but also display a number of apparent inconsistencies. For instance, the highest value of MAT occurred at 1881AD, while the highest LST value appeared in the MCA. It is clear that other factors apart from temperature can influence the distribution of GDGTs in the lake sediments. Work is underway to further constrain the application of GDGT proxies in lake Montcortès by comparing their values in recent varved sections with instrumental data. Indices and equations calculated from isoprenoid and branched GDGTs (Fig.1): TEX86 = (GDGT-2+GDGT-3+GDGT-4´)/(GDGT-1+GDGT-2+GDGT-3+GDGT-4´) (eq.1) LST=-14- 55.2×TEX86 (Powers et al., 2010) (eq.2) BIT= (Ia+IIa+IIIa)/(Ia+IIa+IIIa+GDGT4) (eq. 3) MBT = (I+Ib+Ic)/(I+Ib+Ic+II+IIb+IIc+III+IIIb+IIIc) (eq. 4) CBT = -Log ((Ib+IIb)/(I+II) (eq.5) pH=(3.33-CBT)/0.38(n=114, R2=0.7) (eq.6) MAT=(MBT-0.122-0.187*CBT)/0.02 These proxies are increasingly used in lake sediments to study palaeoenvironments and offer a unique way to reconstruct continental palaeotemperatures. This study aims to explore and interpret the distribution of GDGTs in a sediment record from lake Montcortès (southern pre-Pyrenees) spanning the last 2000 years. Figure1. Structures of branched and isoprenoidal GDGTs (modified from Weijers et al., 2011) Study area Lake Montcortès is located in the NE area of the Iberian peninsula at 1027m a.s.l. (Fig.2). Mean annual rainfall is 860 mm, mean monthly temperature ranges from 1.9 ⁰C in the coldest month (January) to 20.3 ⁰C in the warmest (July). The lake is ideal for high-resolution studies as the sediments are varved (Corella et al., 2011). Previous reconstructions using calcite layer thickness and pollen indicate that warm and cold conditions in Montcortès are coherent with most paleoclimatic, as well as vegetation and human activity changes in the NE Iberian Peninsula (e.g. Corella et al., 2011, 2012; Rull et al., 2011; Scussolini et al., 2011). Figure 3. Abundance of GDGTs and GDGT-based proxies in the Lake Montcortès sedimentary sequence Methods Two sediment cores (1A, 4A) were retrieved using a Kullenberg piston corer (Corella et al., 2011). Aliquots (1g) of freeze-dried samples were solvent extracted and to isolate the GDGT fraction. This was analyzed by liquid chromatography-mass spectrometry (HPLC-MS) to identify and quantify individual GDGTs. Conclusions ◎ Branched GDGTs are much more abundant than isoprenoidal GDGTS in lake Montcortès for last 2000 years. There is a possibility of significant contributions of methanogenic Archaea to the isoprenoidal GDGT sedimentary pool. ◎ Trends of GDGT indices do vary downcore during climatically significant episodes. However, estimates of temperatures from GDGT proxies are at odds with modern temperature ranges. Further work needs to be undertaken to appraise the reliability of the proxies in lake Montcortès. Acknowledgements: Funding was obtained from Spanish Ministry of Research (project TETRACLIM) China Scholarship Council for a scholarship to Min Cao Marie Curie Program , International Incoming fellowship to Carme Huguet References Blaga C. I. et al., 2009. Journal of Paleolimnology, 41, 523–540. Camps J. et al., 1976. Oecologia aquatica, 2, 99–110. Corella J.P. et al., 2011. Journal of Paleolimnology, 46, 351–367. Corella J.P. et al., 2012. Quaternary Rearch, 78, 323–332. Hopmans E. C. et al., 2004. Earth Planetary Science Letter, 224, 107–16. Powers L. et al., 2010. Organic Geochemistry, 41, 404–413. Rull V. et al., 2011. Journal of Paleolimnology, 46, 387–404. Scussolini P., et al., 2011. Journal of Paleolimnology, 46, 369–385. Sinninghe Damsté et al., 2012, Quaternary Science Reviews, 50, 43–54. Weijers J.W.H. et al., 2007. Geochimica et Cosmochimica Acta, 71, 703–713. Weijers J.W.H. et al., 2011. Organic Geochemistry, 42, 477–486. Figure 2. Map showing the location of Lake Montcortès within the Iberian Peninsula, selected core sites and a photo of the lake (taken by Santiago Giralt) )