Paleoclimatic implications of 14C age deviations in loess organic matter from Xinjiang, Northwest China

Abstract

This study explores climatic influences contributing to apparently unreliable 14C ages from loess organic materials. The relationship between anomalous 14C ages and climate suggests that young/old 14C age deviations correspond to wet/dry climate periods. Based on stepped-temperature combustion results, we found that the ratio of refractory (old)carbon liberated at high temperature (HT)(>900 °C) from loess deposited during a dry period can reach 70% of the total organic carbon (TOC). This high temperature fraction is known to contain aged carbon and can explain age reversals with respect to underlying sediments. Thus, after removing HT fractions, the 14C ages of LT fractions is relatively reasonable. In contrast, the ratio of young carbon from the low temperature fraction (LT) (<400 °C) is as high as 85% during a wet period. Mobile carbon that percolates downward in the sediments may result in younger TOC 14C ages. In our study region, the 14C ages of HT fractions are not absolutely reasonable after removing LT fractions. Once the 14C ages of TOC are used to establish age model, the influence of different climate types on the source of sediment organic matter should be taken into consideration. In summary, Climate and material are the main factors causing the 14C age deviations (unreliability) in loess from the Xinjiang region.