PP13A-2263
Branched GDGTs in Lacustrine Environments: Tracing Allochthonous and Autochthonous Sources Using Compound-Specific Stable Carbon Isotope Analysis
Monday, 14 December 2015
Poster Hall (Moscone South)
Yuki Weber, University of Basel, Basel, Switzerland, Jaap S. Sinninghe Damsté, Utrecht University, Department of Earth Sciences, Utrecht, Netherlands; NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Organic Biogeochemistry, Texel, Netherlands, Moritz F Lehmann, University of Basel, Department of Environmental Sciences, Basel, Switzerland, Helge Niemann, University of Basel, Dept. of Environmental Sciences, Basel, Switzerland and Carsten Johnny Schubert, EAWAG Swiss Federal Institute of Aquatic Science and Technology, Duebendorf, Switzerland
Abstract:
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that are ubiquitous in soils and peat, as well as in sediments of lakes, rivers and coastal marine environments. It has been found that the distribution of brGDGTs changes systematically with ambient temperature and pH, attesting to their potential as proxy indicators for paleoclimatic reconstruction. In lacustrine sedimentary archives, brGDGTs can originate from two sources: (1) allochthonous soil organic matter and (2) autochthonous brGDGTs produced within the lake system, both of which display fairly distinct temperature-brGDGT relationships. Until now, disentangling the relative contribution of these sources was impossible, complicating the use of brGDGTs for quantitative paleotemperature reconstructions. BrGDGTs in soils display a narrow range with respect to their stable carbon isotope composition (δ13C), generally between ‑27 and ‑30 ‰, whereas we recently found contrasting δ13C values as low as ‑43 ‰ to ‑46 ‰ for brGDGTs in sediments of a small Alpine lake. To trace the origin of this distinct isotope signal, we determined the 13C content of brGDGTs in suspended particulate matter (SPM) from the water column of Lake Lugano (Switzerland). The δ13C of SPM-derived brGDGTs decreased systematically from ‑34 ‰ in the mixolimnion to ‑41 ‰ in the anoxic monimolimnion of Lake Lugano, providing evidence for aquatic in situ production of 13C-depleted brGDGT. In order to study whether the negative δ13C offset of water column- vs. soil-derived brGDGTs may serve as an indicator for lacustrine brGDGT production, we also analyzed surface sediments from 36 lakes across the Alpine Region. In most (~85 %) of the studied lake sediments, the δ13C of brGDGTs ranged between ‑34 ‰ and ‑45 ‰, indicating predominance or a substantial contribution of aquatically produced brGDGTs. However, in some lakes (~15 %) δ13C values between ‑27 ‰ and ‑30 ‰ suggest a mainly allochthonous (i.e., soil) source. Our data demonstrate the great potential of compound-specific C isotope analysis to constrain the origin of brGDGTs in lake sediments, possibly allowing the identification of freshwater environments that are particularly suited for brGDGT-based paleoenvironmental reconstructions.
