Differential timing of C4 plant decline and grassland retreat during the penultimate deglaciation

Compound-specific stable carbon isotope composition of terrestrial leaf wax n-alkyl lipids (δ13Calk) was measured in sediments from the northern South China Sea (SCS), allowing assessment of vegetation change in the source area within the penultimate glacial cycle during 60-160 kyr. Our δ13Calk record suggested a prominent vegetation contrast between the glacial and the interglacial, with more C4 plant developing during the glaciation and a rapid decline during the glacial termination. This deglacial change in δ13Calk was attributed to an increase in humidity caused by the enhancement of the East Asian Summer Monsoon (EASM). However, the inferred deglacial C4 decline lagged by ~ 4 kyr behind the grass retreat inferred from published pollen record. We argue that the time discrepancy in the two terrestrial plant proxies may have resulted from their differential responses to regional climate change, with grass community structure more sensitive to temperature rise and plant photosynthetic pathway more sensitive to humidity increase. Therefore, an earlier penultimate deglacial warming relative to humidity increase can be inferred from the lead of grass retreat relative to C4 decline, which implies that EASM may be delayed during the deglacial warming.