Response of mollusk assemblages from the Luochuan loess section to orbitalforcing since the last 250 ka

This note presents a new result of terrestrial mollusk study from the Luoch uan loess section since the last 250 ka. A total of 213 samples, taken at i ntervals of 10 cm in the So-L, portion, were analyzed for fossil mollusks. Generally, 150-600 individuals were counted in each sample. According to th e distribution of mollusk fossil assemblages in the loess section, 11 mollu sk fossil zones have been recognized, representing different climatic and e cological conditions. Three main ecological groups were identified accordin g to the temperature and moisture requirements of each taxon. The cold-arid iphilous group shows maxima at about 240-220, 190-182, 150-140 and 74 -66 k a BP. The thermo-humidiphilous set has high abundances for at least 6 times in the section at about 246-240, 220-216, 173-158, 92-86, 60-44 and 10 kaB P. Our results show that variations in mollusk ecological groups are relate d with changes in the Earth orbital parameters at the 41 and 20 ka frequenc ies. Maxima in thermo-humidiphilous taxa are in phase with accretion interv als of obliquity(41 ka period), reflecting the East Asian summer monsoon wi th 41 ka period in controlling variations in terrestrial mollusk ecological groups. In addition, maxima in thermo-humidiphilous taxa appearing at abou t 246-240, 220-216, 60 and 10 kaBP, are consistent with procession (20 ka p eriod) maxima, indicating that the procession period also plays an importan t role in adjusting the ecological pattern of mollusk groups.