Abstract:

It is very important to determine how many sedimentary cycles are in the flood sediments underground in Kaifeng city. But traditional method dividing sedimentary cycle with granularity cannot be used in the sediments under the city because the original granularity sequence of the sediments were greatly affected by human activities, such as city reconstructions and treasures excavation. after flood events. Therefore, it is necessary to use other proxy indicators instead of granularity. Black carbon (BC) is a chemically heterogeneous class of carbon compounds formed during incomplete combustion of biomass or fossil fuels. It is widely distributed in environmental mediums, and can be well-preserved for quite a long time. In this paper, we use BC content in flood sediments as a new proxy indicator to divide sedimentary cycles of the sediments underground in Kaifeng. Two sedimentary cores of 25 m were drilled in May, 2012. One (ZK_sz) is at the central urban area of Kaifeng, and the other (ZK_jm) is at the western suburbs. The core section was split fresh in the field at a 10 cm interval and a total of 434 samples were acquired and stored in boxes. The different grain sizes less than 0.25 mm of the sediments were measured with Laser Particle Size Analyzer (LA-9200, HORIBA, Japan), and those larger than 0.25 mm were measured by sieving. BC contents in bulk sediments were measured using the method recommended by Lim in 1996 with minor modifications. Sedimentary cycles were divided based on the wavelet analysis with the help of MATLAB 7.0 software. The results show that most boundaries of the sedimentary cycles based on granularity is consistent with BC contents in the ZK_jm drill core, which was less disturbed by human activities, suggesting that BC contents can be used as another proxy indicator dividing sedimentary cycles. The boundaries of the granularity sedimentary cycles of the ZK_sz core are almost the same as BC sedimentary cycles from 8.8 m to 23.5 m. The upper core from 0.3 m to 8.8 m of the ZK_sz core that formed after the Song Dynasty can be divided into two granularity sedimentary cycles, but seven BC sedimentary cycles.

Key words: the Yellow River flood sediments, sedimentary cycle, black carbon, proxy indicator, Kaifeng