Land-snail eggs from Chinese L9 loess strata and seasonal cooling events during Marine Isotope Stages 24–22

Knowledge of the long-term evolution of seasonal cooling events contributes to our understanding of glacial-interglacial cycles. Recently, using land-snail eggs as a proxy, we have reconstructed the evolution of seasonal cooling events during glacial-interglacial cycles of the last 490 ka, corresponding to Marine Isotope Stages (MIS) 12–1. During this time interval, seasonal cooling events are strong during most climate cooling shifts. However, it is unclear how such events evolved under climate conditions significantly different from those during the last five climatic cycles. As the ending segment of the Mid-Pleistocene Transition (MPT), MIS 24–22 (∼940–870 ka) yields different climate conditions from those during the last five climatic cycles. Here we present two parallel land-snail egg records from the entire L9 loess and the upper part of the underlain S9 paleosol of the Chinese Loess Plateau (CLP), covering the time interval between 950 and 860 ka. Our results show that land-snail egg abundance documents four strong seasonal cooling events (SE-4 to SE-1). SE-4 and SE-2 occur during climate cooling transitions of MIS 25/24 and MIS 23/22, respectively; SE-3 and SE-1 occur near the glacial maxima of MIS 24 and MIS 22, respectively. This indicates that during the ending interval of the MPT, seasonal cooling events occurred during both climate cooling transitions and glacial maxima, but over the last five glacial-interglacial cycles, they mainly occurred during climate cooling transitions, as indicated by our previous results. The cooling-transition events, SE-4 and SE-2, are stronger than the glacial-maximum ones, SE-3 and SE-1. They do not seem to occur during deglacials and the warming maximum of MIS 23. These results lead us to infer that the occurrence of seasonal cooling events seems to be more closely related to ice-sheet growth at high northern latitudes. A certain amount of ice-sheet growth with low concentrations of ice-rafted debris at high northern latitudes could have strengthened seasonal cooling events over the CLP. However, during the last several glacial maxima, the Northern Hemisphere ice volumes could have been sufficiently large to diminish variability that does not favor the occurrences of seasonal cooling events in the CLP. Moreover, decreases in local spring insolation are also likely to have facilitated the occurrence of seasonal cooling events. Our study provides the first evidence to understand the evolution of seasonal cooling events during the ending segment of the MPT.