The Study Of The Matuyama-brunhes Boundary Positional Discrepancy In China Loess Plateau

The youngest Matuyama-Brunhes reversal (M-B) occurred at0.78Ma. In mostmarine records the M-B boundary (M-BB) lies within interglacial marine isotopestage (MIS)19. However, in the Chinese loess-paleosol records it occurs within theloess layer L8deposited during the glacial MIS20, in the uppermost paleosol S8formed during the interglacial MIS21or around the boundary of loess L8andpaleosol S8. In order to clarity the reason of the M-BB positional discrepancy amongdifferent loess sections, as well as between Chinese loess-paleosol and marinesediments, a comparative investigation on magnetostratigraphy and climatestratigraphy has been performed in Chiness loess-paleosol sediments spanning theM-BB in Xifeng and Baoji sections. Three main conclusions can be drawn as follows:1. Magnetostratigraphic results further confirm that the M-BB occurs in themiddle-lower L8in the Baoji section, and it is located around the L8-S8boundaryin the Xifeng section.2. The East Asia winter monsoon paleoclimate records, which are constructed usingthe grain size, from Baoji and Xifeng sections show the same variation, and bothrecords are well consistent with the marine oxygen isotope (18O) records,indicating that the grain size of Chinese loess-palsosol sediments is a reliablypaleoclimatic proxy. The East Asia summer monsoon paleoclimate records, whichare constructed using the susceptibility, from Baoji and Xifeng sections revealquite different patterns, suggesting that susceptibility parameter is easily affectedby the region and/or local pedogenesis (climate). We thus conclude that thesusceptibility signal of Chinese loess-paleosol sediments is unsuitable as a reliablypaleoclimatic proxy for reconstructing the East Asia summer monsoon system oras an accurately chronostratigraphic time marker for correlating differentloess-paleosol sequences.3. Comparing magnetostratigraphy and climate stratigraphy from Xifeng and Baojiloess-paleosol sediments with those from marine sediments shows no a significantdiscrepancy of the M-BB position when correlating the Chinese loess-paleosolpaleoclimatic records constructed by the grain size with the marine paleoclimaticrecords constructed by the18O, but indicates quite different M-BB positions when comparing the Chinese loess-paleosol susceptibility records with the marine18O paleoclimatic records. We thus conclude that the M-BB positionaldiscrepancy among different loess sites, as well as between Chinese loess-paleosoland marine sediments, is not caused by the discrepancy of the remanence lock-indepth of loess-paleosol depositions, but due to regional and/or local pedogenesis(climate) variability across the Chinese Loess Plateau.