Distribution of branched and isoprenoid tetraether lipids in an oligotrophic and a eutrophic Swiss lake: Insights into sources and GDGT-based proxies

Distributions of isoprenoid (isoGDGT) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) were measured in the water column and sediments of the eutrophic Lake Lugano and the oligotrophic Lake Brienz, Switzerland. Absolute concentrations of isoprenoid, i.e. archaeal GDGTs, were highest in the euphotic zone of both lakes, as well as in sediments deposited at times when lake eutrophication occurred. This indicates that GDGT concentrations may be used as indicators for primary productivity. Both lakes, including the anoxic bottom water of Lake Lugano, are characterised by GDGT distributions typical for group I Crenarchaeota with GDGT-0/crenarchaeol ratios of around 1. Comparison of the distribution of brGDGTs with isoGDGTs and other terrestrial biomarkers throughout the Lake Lugano water column, together with CBT/MBT-derived temperatures that resemble that of the lake, suggest significant in situ production. BIT index values for Lake Brienz sediments (ca. 0.4) were significantly higher than water column values (ca. 0.1), most probably because terrestrial run off events were not captured during water sampling. TEX86 – derived temperatures reflect surface water conditions to within a few degrees, while lower values obtained from deeper water layers suggest a contribution of in situ produced isoGDGTs. For both lake sediments, TEX86-derived temperatures could be matched reasonably with mean annual lake surface water temperature variation, albeit with a larger offset for Lake Lugano. This suggests that absolute temperatures can only be reconstructed from lake sediments for which a local calibration is known.