THE MIDDLE-AND LATE-HOLOCENE BURIAL HISTORY OF ORGANIC CARBON AND ITS INFLUENCING FACTORS IN THE YELLOW RIVER SUBAQUEOUS DELTA

Approximately 70 %~90 % of terrestrial organic carbon in the ocean is buried within coastal ocean areas, including river mouths, continental shelves, and particularly river deltas, which serve as crucial carbon sinks. As a globally significant mega-river, the Yellow River plays a central role in global source-to-sink systems, with its subaqueous delta serving as a key site for examining organic carbon burial patterns. However, earlier research on the subaqueous part of the Yellow River Delta(YRD) primarily focuses on modern to decadal sedimentary processes, understanding of carbon burial processes over millennial scales remains limited. To bridge this gap, this study examines the Middle-and Late-Holocene burial history of organic carbon in the subaqueous delta of the YRD. Building on previous seismic stratigraphy analyses of sub-bottom profiles in the Bohai Bay area, we analyzed comprehensively analyzed core BHB1 (38°24'50″N, 118°31'80″E, at water depth 15.8 m), collected in 2020, which penetrates well-defined seismic stratigraphy in the southern Bohai Sea. We conducted a comprehensive lithology analysis(upper 594.8 cm of BHB1), encompassing sediment granularity and other pertinent indicators, including 63 samples for grain size analysing at 8 cm intervals, 19 samples for TOC/TN analysing and 5 samples for AMS 14C dating, and conducted a comparative assessment of seismic and lithologic stratigraphy. The findings reveal the formation of a subaqueous deltaic succession since the Middle- and Late-Holocene in our study area, which can be divided into three distinct depositional units(DU1~3) from oldest to the youngest. We establish the chronology framework through AMS 14 C dating, and then we analyzed the proxies of total organic carbon(TOC), total carbon(TC), total nitrogen(TN), and stable carbon isotope(δ13C) values within core BHB1 to gain a comprehensive insights into evolutionary history of in the organic carbon burial pattern within the Yellow River subaqueous delta. We further discussed the sources of the burial organic carbon as well as the driving factors that influence its burial process. The results demonstrate a significant positive correlation between TOC and TN, with a positive intercept in the linear relationship, indicating consistent sources of organic carbon and nitrogen as well as the presence of inorganic nitrogen in the sediment. The δ13C values are mainly concentrated in -24.66 ‰ to -21.21 ‰, suggesting that the sediments stemmed from a mixed source of marine and terrestrial sources. According to the two-end member model, marine organic matter(53 %) dominates composition. DU1~3 roughly formed during 7~6 cal.ka B.P., during 6~3 cal.ka B.P., and since ca.1855 A.D.(corresponding to the modern YRD). Notably, sedimentation rate(SR) emerges as the most influential factor in shaping the organic carbon mass accumulation rate(OCMAR). DU3 registers the highest SR at 1.07 cm/a, resulting in the highest OCMAR at 33.22 g/m2·a, while DU2 exhibits the lowest SR at 0.08 cm/a, yielding the lowest OCMAR at 3.27 g/(m2 ·a). Since the Middle-and Late-Holocene, the average OCMAR of the subaqueous delta of the YRD(13.75 g/(m2 ·a)) is lower than that of the subaerial delta of the YRD(66.55 g/(m2 ·a)), which is speculated to be mainly affected by the sediment transport distance. In addition, the average OCMAR during 7~6 cal.ka B.P. is higher than that during 6~3 cal.ka B.P. for subaqueous delta sediments. The sediments have been characterized by decreasing the proportion of terrestrial organic matter(TOM %) since 4.5 cal.ka B.P., which is speculated to be related to two sharp weakening events of monsoon in the Middle-and Late-Holocene. Finally, the influence of human activities since 1855 A.D. has significantly shaped the evolution of the modern YRD, leading to an overall acceleration of organic carbon burial in the subaqueous delta of the Yellow River.