Assessing tetraether lipids as a paleotemperature proxy on western edge of the Chinese Loess Plateau: A cautionary tale

The 130,000-year long, high-resolution paleotemperature record obtained from branched glycerol dialkyl glycerol tetraethers (brGDGTs) stored in a loess-paleosol sequence from Yuanbao on the western Chinese Loess Plateau (CLP) does not follow global temperature variations, whereas these are recorded by traditional loess proxy records from the same section. Specifically, reconstructed temperatures are lowest during Marine Isotope Stage 5 (MIS5), even indicating colder interstadials than stadials, which contradict brGDGT-based records from other sections on the CLP. The overall low temperatures during wet periods may be explained by an increase in soil moisture availability, which dampens land surface temperatures by increasing the soil heat capacity. However, comparison of brGDGT distributions at Yuanbao with those in modern surface soils from the CLP and the global soil calibration dataset reveals a lack of modern analogues during the interval from ∼73 to 63 ka. Notably, the Isomer Ratio (IR) rapidly increases from 0.2 to 0.6 (corresponding to a pH shift of ∼2 units) during this period, indicating a drastic shift in the composition of the bacterial community that in turn may have influenced the MBT′_5Me-temperature relationship. This community shift occurs after Chinese Stadial-20 around 73 ka, a weak monsoon event tentatively linked to oceanic reorganizations after the Toba eruption, suggesting that this drastic change in (hydro)climate might have impacted brGDGT producers. We thus recommend thoroughly assessing brGDGT distributions and understanding the environmental background prior to interpretating brGDGT-based temperatures in a paleoclimatic context.