Controls on brGDGT distributions in the suspended particulate matter of the seasonally anoxic water column of Rotsee

Abstract. Developing reliable methods for quantifying past temperature changes is essential for understanding Earth's climate evolution and predicting future climatic shifts. The degree of methylation of branched tetraethers (MBT’5ME), of branched glycerol dialkyl glycerol tetraethers (brGDGTs), a group of bacterial membrane lipids, has become a widely accepted tool for lacustrine paleothermometry. To allow this, an empirical calibration was developed, based on MBT’5ME values of surface sediments across large spatial scales. As these sediments integrate variability across several years to decades, the sensitivity of MBT’5ME to seasonal and short-term environmental changes in the water column remains underexplored. Here, we present a record of brGDGTs in suspended particulate matter (SPM) from a monomictic, eutrophic temperate lake (Rotsee, Switzerland) over a 10-month period, examining both core lipids and intact polar lipids. Rotsee offers an ideal setting for this study due to its strong seasonal variations in temperature, conductivity, and dissolved oxygen caused by summer warming and associated stratification. In the oxic epilimnion, a minor increase in MBT'5ME during stratified summer months was caused by a rise in brGDGT Ia concentration. A similar increase in concentration of 6-methyl brGDGTs indicates a sensitivity to water temperature. In the seasonally anoxic hypolimnion, MBT'5ME correlated with water pH rather than temperature, suggesting that water chemistry influences this ratio, complicating its use as a temperature proxy. The production of intact polar lipid (IPL) tetraethers was observed exclusively in the anoxic hypolimnion during stratification, confirming anoxia as a key trigger for IPL tetraether production. Surface sediment samples along a depth gradient have a distinct depth-dependent distribution. Sediments below the oxic water column showed lower MBT'5ME values, likely due to the sedimentary production of brGDGTs IIa and IIIa. Sediments from seasonally anoxic areas reflected average epilimnion SPM values, suggesting the deposition of epilimnion brGDGTs into the sediments. This study of brGDGTs in Rotsee SPM and sediments thus indicates that temperature, pH and oxygen concentration impact GDGT distribution, with significant implications for using MBT'5ME as a temperature proxy in sediments from stratified lakes.