The Jaramillo subchron in Chinese loess-paleosol sequences

Abstract

The Chinese loess is one of the most important terrestrial records of the Quaternary climate changes. However, the positional discrepancy of geomagnetic polarity boundaries, such as Matuyama-Brunhes boundary (MBB), between Chinese loess and marine sediments remains controversial, hampering to establish an accurate chronological framework for the Chinese loess and its correlation with marine oxygen isotope records. The Jaramillo subchron within the Matuyama Chron is an important paleomagnetic age constraint for Quaternary magnetostratigraphic dating. Here, we present a high-resolution magnetostratigraphic, rock magnetism, and relative paleointensity (RPI) study of the stratigraphic interval between paleosols S9 and S18 in two parallel loess sections from the southeastern Chinese Loess Plateau, with an aim to explore the positions of the upper and lower boundaries of the Jaramillo in the Chinese loess. The results show that significant uncertainty and variability (more than one loess-paleosol cycle) for the positions of the measured upper and lower Jaramillo boundaries among different loess sections cannot be explained by the discrepancy of loess stratigraphy, sedimentation rate or regional climate. The measured upper and lower Jaramillo boundaries in the loess can be above or below their true positions, which cannot be explained by the lock-in effect. As the measured boundaries in the loess are mostly within the intervals of low-intensity geomagnetic field, we propose that the loess deposits may have acquired unstable primary remanent magnetization during the intervals of geomagnetic reversals or excursions with low-intensity geomagnetic field, and these deposits could be easily overprinted (remagnetizated) by the ensuing high-intensity field. This hypothesis is able to explain the phenomena that the measured MBB in the Eurasian loess is downward shifted by tens centimeters to several meters relative to the true boundary, and it can also explain the anomalistic polarity zones recorded in some loess sections.