Environmental evolution and carbon burial assessment of the west coast of Bohai Bay since Late Pleistocene

The quantitative assessment of carbon flux in soil or sediments of coastal wetlands has recently become a hotspot in carbon cycle research both at home and abroad. However, most of the depth of the sediment samples studied is less than 1 m (or a maximum no more than 3 meters), and there are few studies on carbon fluxes in deeper sediments or longer time scales, such as the millennium scale available. In order to reveal the carbon fluxes in deeper layers, a hole of 32.68 m deep (BHZK13) was drilled in the old Yellow River Delta on the west bank of Bohai Bay in 2016. Core samples are carefully described and tested for AMS¹⁴C and OSL dating, grain size analysis, foraminifera identification, and analysis of total carbon(TC), organic carbon (OC), and major elements (including nutrient elements) in addition to in-situ densities. The results show that since Late Pleistocene, the sedimentary environment of the old Yellow River Delta on the west coast of Bohai Bay can be subdivided into seven sub-environments, namely, the tidal flat in MIS5 (U1), floodplain (U2), river channel (U3), Holocene tidal flat (U4), Yellow River Delta phase one (U5, 5500～3600 cal.aBP), reconstruction layer (U6) and Yellow River Delta phase two (U7). The highest sedimentation rate is found in the deltaic front of the delta phase one (1.99 cm/a), while the lowest found in the tidal flat (0.014 cm/a). Correspondingly, the highest burial rate of organic carbon is found in the deltaic front of the Yellow River Delta phase one (134.56 g/(m²·a)), with the lowest found in river channel deposits. Correlation analysis suggests that the sedimentation rate is the main controlling factor on the burial rate of organic carbon in various sedimentary environments. TC and OC has a very significant correlation with each nutrient element. Although the content of organic carbon in the sediments of the Old Yellow River Delta is relatively low due to the high sedimentation rate of the Delta, the modern Yellow River Delta can still be considered as an excellent carbon sink also due to its high sedimentation rate.