Sources and influencing mechanisms of organic carbon in the western Bohai Sea over the past century

The controlling factors and sources of organic carbon in sediments of the western Bohai Sea over the past 100-year remain unclear. Taking the sediment samples of four stations (ZY009, ZY035, ZY045 and ZY054) in the western Bohai Sea as example, the grain size, organic carbon (TOC), total nitrogen (TN), and carbon, and oxygen isotopes (δ¹³C and δ¹⁵N) in the samples were analyzed and dated based on ²¹⁰Pb chronology. The source of organic carbon, burial flux, and controlling factors on the distribution of organic carbon were studied. Results show that the organic matter in the sediment cores of the Yellow River estuary and coastal zone of the Bohai Sea is essentially a mixture of terrestrial and marine organic matter. Specifically, in borehole ZY009 below 75cm is mainly terrestrial organic matter, and above 75cm is mainly marine dissolved carbon, while in borehole ZY054, organic carbon from C3 land plants of the Yellow River were dominant. The distribution of organic carbon is highly sensitive to the five migrations of the Yellow River estuary in the last 100 years. The northward migration of the Yellow River estuary led to the increase of organic carbon content in the study area, while the southward migration led to the decrease of organic carbon content. Since 1886, the organic carbon burial flux revealed in ZY009 core has been decreasing year by year due to the climatic change and reduction of the runoff and sediment transport in the Yellow River. As ZY009 is far from the estuary, the organic carbon burial flux is less disturbed by the river and human beings. Since 1886, the organic carbon burial flux in ZY054 core has been increasing, and began to decline since 1988 when the Yellow River was drying up. After 2002, due to artificial water and sediment control measures and increased human activities in the coastal zone, the primary productivity of the ocean has been increased, which in turn has led to an increase in the organic carbon burial flux in the area near the Yellow River estuary, resulting in the increase of organic carbon flux near the Yellow River estuary. In the past 100 years, the western Bohai Sea witnessed a complex interplay of factors on the sources of organic carbon in sediments. The characteristics of sediment grain size, the migration patterns of the Yellow River, and the effects of human activities each have exerted a notable impact on the distribution and burial flux of organic carbon in this region.