Studies of marine benthic archaeal communities are updating our view of their taxonomic composition and metabolic versatility. However, large knowledge gaps remain with regard to community assembly processes and inter taxa associations. Here, using 16S rRNA gene amplicon sequencing and qPCR, we investigated the spatiotemporal dynamics, assembly processes, and co-occurrence relationships of the archaeal community in 58 surface sediment samples collected in both summer and winter from across ∼1500 km of the eastern Chinese marginal seas. Clear patterns in spatiotemporal dynamics in the archaeal community structure were observed, with a more pronounced spatial rather than seasonal variation. Accompanying the geographic variation was a significant distance-decay pattern with varying contributions from different archaeal clades, determined by their relative abundance. In both seasons, dispersal limitation was the most important process, explaining ∼40% of the community variation, followed by homogeneous selection and ecological drift, that made an approximately equal contribution (∼30%). This meant that stochasticity rather than determinism had a greater impact on the archaeal community assembly. Furthermore, we observed seasonality in archaeal co-occurrence patterns: closer inter-taxa connections in winter than in summer, and unmatched geographic patterns between community composition and co-occurrence relationship. These results demonstrate that the benthic archaeal community was assembled under a seasonal-consistent mechanism but the co-occurrence relationships changed over the seasons, indicating complex archaeal dynamic patterns in coastal sediments of the eastern Chinese marginal seas.

Surface sediments from the Changjiang Estuary and adjacent shelf were analyzed using a variety of bulk and molecular techniques, including grain size composition, sediment surface area (SSA), elemental composition (C, N), stable carbon isotopic composition (δ¹³C), n‐alkanes, lignin phenols, and glycerol dialkyl glycerol tetraether lipids to obtain a more comprehensive understanding of the sources and fate of sedimentary organic carbon (SOC) in this dynamic region. Bulk N/C ratios of 0.09 to 0.15, δ¹³C of −24.4‰ to −21.1‰, branched/isoprenoid tetraether index of 0 to 0.74, n‐alkane content of 0.02 to 0.37 mg g⁻¹ organic carbon (OC), and lignin content (Λ₈) of 0.10 to 1.46 mg/100 mg OC and other related molecular indices in these samples indicate a mixed source of marine, soil, and terrestrial plant‐derived OC in the study area. A three‐end‐member mixing model using principal component analysis (PCA) factors as source markers and based on Monte Carlo (MC) simulation was constructed to estimate the relative contributions of OC from different sources. Compared with traditional mixing models, commonly based on a few variables, this newly developed PCA‐MC model supported bulk and biomarker data and yielded a higher‐resolution OC inputs to different subregions of this system. In particular, the results showed that the average contributions of marine, soil, and terrestrial OC in the study area were 35.3%, 47.0%, and 17.6%, and the highest contribution from each OC source was mainly observed in the shelf, inner estuary, and coastal region, respectively.

A regional synthesis of organic carbon (OC) burial was conducted using a comprehensive data set to reveal some of the key drivers and human multi‐stressors controlling OC burial and transport in the Eastern China Marginal Seas (ECMS). Both OC and Δ¹⁴C values of suspended particulate matter (SPM) in the Changjiang River, were significantly higher than estuarine mobile‐muds, suggesting selective decay of more labile younger OC from both marine and terrestrial sources and the accumulation of more recalcitrant older OC. Some of this decay is likely to be associated with iron‐redox cycling in mobile‐muds. In contrast, OC, δ¹³C, and Δ¹⁴C values increased along the Yellow River sediment dispersal pathway, indicating adding of young marine OC and less decay of terrestrial OC. OC burial efficiency in mud areas in the Bohai Sea (∼43%) was significantly higher than those in the Yellow (∼11%) and East China Seas (∼16%), owing to rapid deposition. Burial flux of biospheric OC in mud areas of the ECMS is 7.00 ± 0.79 Mt yr⁻¹, corresponding to atmospheric CO₂ drawdown by silicate weathering in major river drainage basins of mainland China. The burial flux of petrogenic OC was estimated to be 0.81 ± 0.25 Mt yr⁻¹, accounting for >1.9% of total burial in the global ocean. While the ECMS is an important OC sink, river damming has greatly reduced OC burial. Thus, the overall impact on anthropogenically altered river‐dominated marginal seas remains an important and rapidly changing component of the coastal ocean carbon budget.

To better understand the role of reactive Fe (Fe_R) in the preservation of sedimentary organic carbon (SOC) in estuarine sediments, we examined specific surface area, grain size composition, total OC (TOC), lignin phenols, Fe_R, Fe_R‐associated OC (Fe‐OC) and lignin phenols (Fe‐lignin), and δ¹³C of Fe_R‐associated OC (δ¹³C_Fe‐OC) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.4 ± 3.5% of the OC was directly bound with Fe_R in the Changjiang Estuary and adjacent shelf. Unusually low TOC/specific surface area loadings and Fe‐OC/Fe ratios in mobile muds suggest that frequent physical reworking may reduce Fe_R binding with OC, with selective loss of marine OC. More depleted ¹³C_Fe‐OC relative to ¹³C of TOC (¹³C_bulk) in deltaic regions and mobile muds showed that Fe_R was largely associated with terrestrial OC, derived from extensive riverine OC and Fe inputs. A higher proportion of hematite in the mobile muds compared to the offshore samples indicated that Fe oxides are likely subjected to selective sorting and/or become mature during long‐term sediment transport. When considering the percentage of Fe‐OC to SOC and SOC burial rates in different marine environments (e.g., nondeltaic shelf, anoxic basins, slope, and deep sea), our findings suggest that about 15.6 ± 6.5% of SOC is directly bound to Fe_R on a global scale, which is lower than the previous estimation (~21.5%). This work further supports the notion of a Rusty Sink where, in this case, Fe_R plays an important role in the preservation and potential transport of terrestrial OC in the marine environment.