oa Branched Gdgts Emerge as a Novel Diagnostic Tool for Lake Thermal Stratification History

The MBT’5ME index of branched GDGTs, widely used to reconstruct mean annual air temperature (MAAT) in lakes, shows significant residual errors (2.0–2.7°C), suggesting additional environmental controls. This study investigates these influences by analyzing brGDGT distributions over seasonal (water column) and long-term (sediment core) timescales in Rotsee, a seasonally anoxic prealpine lake.

A year-long high-resolution sampling campaign collected suspended particulate matter (SPM) from the oxic epilimnion and anoxic hypolimnion of Rotsee, while sediment core spanning the last 14,000 years provided insight into past climate variability of the same lake. Results show that MBT’5ME reflects seasonal stratification and bottom-water mixing rather than MAAT. Specifically, brGDGT Ia increases with stratification, while brGDGT IIIa is linked to deep-water mixing. The isomer ratio (IR), traditionally not considered a temperature proxy, correlates better with MAAT in the epilimnion.

These findings suggest that MBT’5ME alone cannot reliably reconstruct past temperatures in lakes, as it captures both warming and lake mixing dynamics. Instead, a combined approach using MBT’5ME and IR can better distinguish thermal stratification from temperature trends. This study refines brGDGT-based paleoclimate reconstructions by proposing the novel application of multi-proxy approaches to improve temperature estimates, particularly in seasonally dynamic lakes.