A new indicator for dividing sedimentary rhythms in alluvial deposits: A pollen-based method.

• The effect of Yellow River flood on characteristics of alluvial pollen are explored. • Pollen rhythms of two 25-m-long cores from suburban and urban areas were determined. • Pollen is not easy to be disturbed and can accurately identify sedimentary rhythms. • Pollen characteristics can indicate floods to some extent, even small-scale floods. Yellow River flooding has occurred many times near Kaifeng City resulting in the formation of a relatively complete sedimentary sequence, which provides a rare research carrier for exploring sedimentary rhythms in alluvium. In this paper, two 25-m-long cores were respectively obtained from suburban (JM core) and urban (SZ core) areas of Kaifeng City, China. Using detailed pollen data, grain size and our previous studies, we explore the effect of Yellow River flood events on the characteristics of alluvial pollen assemblages. Through cluster analysis of the pollen assemblages and pollen concentrations, we determined that there are 16 and at least 14 pollen zones/subzones (i.e. pollen rhythms) in the JM core and SZ core respectively. To aid an objective analysis we define a comprehensive rhythm from three other indicators, namely, grain size, black carbon and anthropogenic-biological elements. For the suburban JM core, less disturbed by human activity, each pollen rhythm has its corresponding comprehensive rhythm, and the upper and lower boundaries are basically the same. However, the number of pollen rhythms is found to be greater than the number of comprehensive rhythms, suggesting the precision of the pollen proxy is higher than the other three indicators. Similar phenomena were found in the SZ core, with significant human disturbance, which also suggests that the pollen proxy is less susceptible to human disturbance and can accurately identify sedimentary rhythms. Furthermore, the pollen concentration and its assemblage characteristics can indicate floods to some extent, even small-scale floods. This is thought to be related to circumstances where some Yellow River floods may have lasted for a long time, during which time there were several relatively short flood intervals due to seasonal and flood path changes; nevertheless, pollen rhythms can also be identified. Consequently, a pollen rhythm may not indicate a specific flood event. [ABSTRACT FROM AUTHOR]