Distributions and sources of isoprenoidal GDGTs in Lake Lugano and other central European (peri-)alpine lakes: Lessons for their use as paleotemperature proxies

Isoprenoidal glycerol dialkyl tetraether (isoGDGT) lipids occur ubiquitously in freshwater and marine environments. Since their distribution varies with temperature, sedimentary isoGDGTs have been used as proxies for the reconstruction of past continental climate almost two decades. Yet, application lacustrine sediments is still not well constrained because niches isoGDGT-producing microorganisms lakes are often ill-defined. Here, we study hydroxy derivatives (OH-isoGDGTs) water column deep (288 m), meromictic northern basin Lake Lugano (Switzerland) using quantitative analysis various pools stable carbon isotopic composition isoGDGT-derived biphytanes. We provide strong evidence archaeal sources isoGDGTs, based on comparison lipid data microbial diversity determined by 16S rRNA next generation sequencing. find highest concentrations (i.e., 40 ng L?1) crenarchaeol, specific thaumarchaea, suspended particle matter (SPM) from deeper (30–100 m) waters below thermocline. This correlates thaumarchaeal gene abundances, comprised a single thaumarchaeote order Nitrosopumilales. The OH-isoGDGT 0–2 cyclopentane rings follow this profile, suggesting an identical source. In anoxic waters, community changes substantially was members Bathyarchaeota, Diapherotrites, Euryarchaeota, Woesearchaeota. change accompanied changing high contribution GDGT-0 polar head group, more negative ?13C value acyclic biphytane derived thereof. Comparison isoGDGT (distribution ?13C) surface sediment that sinking flux studied over 1 year at three depths indicates substantial downward transport to between thermocline hypolimnion, but (>100 waters. relatively low isoGDGT-based TEX86 (ca. 0.40) similar in-situ produced thermocline, consistent year-around temperatures (<6 °C). also analyzed 36 additional (peri-)alpine Central Europe; concentration show clear-cut difference lake size. large lakes, like North Basin, crenarchaeol much higher than small-sized shallow Nitrosopumilales spp. seemingly no niche shallower (0–?30 m depth), production, hence preservation record, (and other isoGDGTs) lower lakes. argue mid-latitudinal generally thrive cold where temperature largely affected fluctuations mean annual air temperature. Therefore, paleothermometer relative insensitive atmospheric these ponds can be present probably mainly Nitrososphaerales (either production or soil erosion). These thaumarchaea characterized content isomer same which may compromise accurate paleotemperature estimations small/shallow