Towards quantitative reconstruction of past East Asian monsoon precipitation based on lipid biomarkers in Chinese loess

Changes in past East Asian monsoon (EAM) climate have been inferred from records of speleothem δ¹⁸O and the magnetic susceptibility (MagSus) of loess sequences on the Chinese Loess Plateau (CLP). However, where the speleothem record is dominated by a 23-kyr cycle, loess MagSus primarily shows a 100-kyr cycle. Generating a long-term high-resolution record of plant wax hydrogen isotopes (δ²H_wax) from the Yuanbao section on the western CLP reveals that variations in δ²H_wax follow the precessional pattern of the speleothem record, as opposed to the glacial-interglacial changes in MagSus from the same loess section. We, therefore, propose that plant waxes mainly record the δ²H of precipitation during the growing season, whereas MagSus represents an annual climate signal, including precipitation and temperature. This implies that summer vs annual climate variability is driven by distinct orbital forcings.

Regardless, these proxies only allow for qualitative reconstructions of past EAM climate; a method to generate records of absolute monsoon precipitation changes and also leads to consistent results across the CLP is still lacking. We find that in the Yuanbao section, the degree of cyclisation (DC) of branched tetraether membrane lipids, so-called brGDGTs, which are membrane lipids of soil bacteria that are mostly known for their temperature-sensitivity, closely tracks the changes in δ²H_wax. The positive relationship of the DC with mean annual precipitation (MAP) in surface soils from the CLP enables us to quantify past MAP at Yuanbao, but also at other sites on the CLP with existing brGDGT datasets. This reveals a spatial gradient in MAP that is most pronounced during glacials, when the western CLP experiences relatively more arid conditions. Furthermore, the DC records provide independent support for precession as main forcing of precipitation on the CLP, as opposed to the 100-kyr cycle recorded by MagSus, which rather reflects the relative intensity of the EAM.