Changjiang (Yangtze River) Estuary has experienced severe hypoxia since the 1950s. In order to investigate potential ecological functions of key microorganisms in relation to hypoxia, we performed 16S rRNA‐based Illumina Miseq sequencing to explore the bacterial diversity in the surface sediments of the hypoxic zone near the Changjiang Estuary and in the East China Sea (ECS). The results showed that numerous Proteobacteria‐affiliated sequences in the sediments of the inner continental shelf were related to both sulfate‐reducing and sulfur‐oxidizing bacteria, suggesting an active sulfur cycle in this area. Many sequences retrieved from the hypoxic zone were also related to Planctomycetes from two marine upwelling systems, which may be involved in the initial breakdown of sulfated heteropolysaccharides. Bacteroidetes, which is expected to degrade high‐molecular‐weight organic matter, was abundant in all the studied stations except for station A8, which was the deepest and possessed the largest grain size. In addition, dissolved organic carbon, water depth, percentage ratio of clay to silt, salinity, and sedimentary grain size were environmental effectors that shaped the sedimentary microbial community structure. Our results showed that putative Gammaproteobacteria‐affiliated sulfur‐oxidizing bacteria may not only detoxify hydrogen sulfide produced by sulfate‐reducing prokaryotes, but also serve as the primary producers in the marine sediments. Specific groups of aerobic Bacteroidetes and Planctomycetes participated in degrading organic matter, which might contribute to the oxygen depletion in the hypoxic zones.

a b s t r a c tInsight into the content and composition of organic carbon (OC) in river systems contributes to our understanding of the global carbon cycle. The Changjiang (Yangtze River) plays a significant role in global carbon and hydrological cycles, as it is an important supplier of sediment, nutrients and OC to the East China Sea. To provide a preliminary insight into the source of OC transported by the Changjiang, we analyzed bulk (grain size, organic carbon content, d 13 C), and molecular (lignin phenols, branched and isoprenoid GDGTs) characteristics of organic matter in bed sediments at eight locations along the river. The d 13 C values and lignin phenol composition indicate that the OC originates from a mixture of primarily soils and plants. Comparison between branched GDGT (br GDGT) distributions in riverbed sediments and those in the different soil types in the drainage basin indicate that the br GDGT signal in the upper reaches is largely derived from soils from the Qinghai-Tibet Plateau. Downstream changes in br GDGT distributions can be linked to subsequent input of local soil material. The observed variation in br GDGT composition along the river implies that a portion of the OC may be oxidized or replaced by the local input during transit. Although the relationship between d 13 C and lignin phenol composition indicates that the contribution of phytoplankton is limited, comparison of GDGT ratios (GDGT-0:crenarchaeol, iso GDGTs:br GDGTs, BIT index) for the sediments and surrounding soils indicates that at least part of the iso GDGTs has an aquatic origin. Overall, the downstream evolution of br GDGTs in the Changjiang implies that their use as paleoclimate proxy in downcore applications will likely yield a local, rather than a basin-integrated continental climate record.

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Since the beginning of the twentieth century, the global ocean has undergone unprecedented changes caused by global warming (Gillett et al., 2021;Stocker et al., 2013). These changes are notably pronounced in the Arctic region due to polar amplification with major consequences for sea-ice cover and the thermohaline circulation (Cavalieri et al., 1997;Shindell & Faluvegi, 2009). The Arctic Ocean and its marginal seas are characterized by large seasonal sea-ice changes. Melting in summer and sea-ice formation in winter, including coastal polynyas, through the production of cold and saline waters, alter deep-water formation and subsequently the Arctic Ocean circulation (Arrigo, 2014;Cai et al., 2010;Overland & Wang, 2013;Xiao et al., 2013). At the current rate of decline (0.42% year −1 ), the Arctic Ocean may be ice-free in summer season in the next 50 or even 30 years (Comiso, 2012;Wang et al., 2019), which will profoundly impact the global climate and carbon cycle and further accelerate global warming causing damages to the polar ecosystems (Moline et al., 2008). In Arctic marginal seas changes in sea-ice cover and thickness involve different thermodynamic and dynamic factors (Polyakov et al., 2003). In the case of the East Siberian Sea (ESS), land run-off and atmospheric circulation are controlling factors of the sea-ice distribution (Park et al., 2020;Rigor & Wallace, 2004).Our knowledge on natural variability of Arctic sea ice and on-going changes are limited by the lack of long time series observations. Information on past sea-ice distribution can be obtained from micropaleontological fossils (