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 seasonalconsistent mechanism but the co-occurrence relationships changed over the seasons, indicating complex archaeal dynamic patterns in coastal sediments of the eastern Chinese marginal seas.

This article presents a modified method for extraction of dissolved inorganic carbon (DIC) from seawater for radiocarbon measurement by accelerator mass spectrometry (AMS). Standard tests indicate that the extraction efficiencies of DIC are >96%, and the respective precisions of Δ14C-DIC and δ13C-DIC analyses are 6‰ and 0.1‰ or better. Using the method, we report Δ14C-DIC profiles collected from the shelf and slope in the East China Sea (ECS) of the northwest Pacific Ocean. Both the DIC concentration and Δ14C-DIC in the shelf and slope regions seem primarily affected by the Kuroshio Current. It is estimated that 54–65% of the bottom water in the shelf region could be from the intrusion of Kuroshio intermediate water, which carries a high concentration and low Δ14C values of DIC, and which influenced the DIC and its 14C signature on the shelf. Compared with the Δ14C-DIC profiles at other sites in the northwest Pacific reported previously, it appears that the Δ14C-DIC distributions are mainly controlled by the major oceanic currents in the region, and large variations in Δ14C-DIC occurred mostly in the upper 800 m of the water column. The similarity of Δ14C-DIC at depth suggests that the deep-water circulation patterns have been relatively stable in the northwest Pacific Ocean in the last 20 yr.

A combined carbon isotope (13C and 14C) study was carried out to investigate the sources and fate of organic carbon (OC) transported by the Yellow River and preserved in the sediments of the Bohai and Yellow Seas. In 2015, the Yellow River delivered 3.14 × 1010 g C and 4.12 × 1010 g C of dissolved organic carbon (DOC) and particulate organic carbon (POC) to the Bohai Sea. Carbon isotope signatures revealed that the Yellow River transports millennial‐aged DOC and POC during all seasons. The values of δ13C‐DOC ranged from −24.7‰ to −28.8‰ in the river basin, and −21.0‰ to −27.0‰ in the lower reach. The 14C ages of DOC were 415–1690 yr before present (BP) in the river basin, and they were relatively constant seasonally (1320–1690 yr BP) in the lower reach of the river. In comparison, POC δ13C values in the river were less variable (−22.8‰ to −25.0‰), but much older in both the river basin (4960 ± 1690 yr BP) and in the lower reach (4040 ± 1050 yr BP). Calculations using a dual‐isotopic three‐end member model revealed that biomass OC derived from C3 plants was the major source of riverine DOC, contributing 65% ± 8% and 52% ± 2% in the river basin and lower reach seasonally. Pre‐aged soil OC and fossil OC from weathering contributed 21–42% and 6–14% of the DOC, respectively. In contrast, pre‐aged soil OC and fossil OC contributed 60–70% and 17–27% of POC, and biomass OC contributed a minor fraction (13% ± 7%) of riverine POC. Our results further revealed that aged riverine POC had a major influence on OC preservation in the delta and coastal sediments of the Bohai and Yellow Seas. The age of OC in surface sediments varied widely (1610–8275 yr) due to the influence of Yellow River input. Pre‐aged soil OC and fossil OC each contributed 32% ± 8% and 22% ± 14% of OC preserved in the sediments. We estimate that about 0.27 Mt yr−1 and 0.07 Mt yr−1 of pre‐aged soil OC and fossil OC accumulate in the surface sediments from POC delivered by the modern Yellow River, and 0.013 Mt yr−1 and 0.002 Mt yr−1 of pre‐aged soil OC and fossil OC enters the coastal DOC cycle from riverine DOC. The millennial‐aged OC delivered to coastal seas by the Yellow River therefore has profound impacts not only on carbon cycling and the carbon budget in the marginal sea, but also on coastal ecosystems and biogeochemical processes.

The Changjiang (Yangtze River) and Huanghe (Yellow River) are the two largest rivers in China, and they transport large amounts of terrestrial carbon to the coastal waters of the East China Sea and the Bohai Sea. The sources and cycling of riverine carbon in these two large river estuaries, however, have not been well studied. In this article, we present the results of dual isotope (D 14 C and d 13 C) measurements of dissolved inorganic carbon (DIC) collected in the low reaches of the Changjiang and Huanghe and their estuaries during two cruises in 2014. Our results indicate that both the Changjiang and Huanghe carry very high concentrations of DIC ranging from 1384 lmol kg 21 to 1732 lmol kg 21 and 2711 lmol kg 21 to 4120 lmol kg 21 , respectively, and DIC levels varied with flow rates during high and low discharge periods. The cycling of DIC exhibited conservative behavior in both the Changjiang and Huanghe estuaries, suggesting DIC levels were controlled mainly by physical mixing processes.D 14 C-DIC values indicate that the Changjiang and Huanghe transport aged DIC (1060-1380 yr old). Both D 14 C-DIC and d 13 C-DIC values also showed conservative mixing in the two estuaries. Using a dual carbon isotopic model, we calculated that atmospheric CO 2 consumed mainly by silicate weathering was a major source, contributing 65.2 6 9.0% and 73.4 6 3.0% of DIC in the Changjiang and Huanghe, and 96.9-97.7% (by air-sea exchange) of DIC in the coastal waters of the East China Sea (ECS) and Bohai Sea, respectively. Our results indicate that carbonate dissolution was an important (12.3-17.4%) but not major process controlling the high DIC levels in both rivers, as suggested previously. Compared with the large Amazon River, respiration of riverine organic matter (OM) played a less important role, contributing only 15.4-17.2% of DIC in the two Asian rivers. Flux calculations indicate that the Changjiang and Huanghe discharged 1.46 3 10 13 g and 6.28 3 10 11 g DIC into the ECS and Bohai Sea in 2014, which were 9 and 17 times higher than the DOC fluxes in the two rivers. These large fluxes of riverine DIC, especially of aged DIC, could have significant impacts on primary production and carbon cycling in the ECS and Bohai Sea.

We present the carbon isotope ( 14 C and 13 C), dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC) concentration measurements in the South China Sea (SCS) to reveal the different sources and cycling time scales of the two major carbon pools in the SCS. The DIC concentrations ranged from 1,776 to 2,328 μmol kg −1 , and they were lower at the surface and increased with depth. Conversely, the DOC concentrations ranged from 38 to 95 μM, and they were higher on the surface and decreased rapidly in the upper 500-m water depth. The DIC Δ 14 C and DOC Δ 14 C values varied from −227‰ to 68‰ and −557‰ to −258‰, respectively, and both decreased with depth until 1,500 m and then remained relatively constant. DOC Δ 14 C values were −330‰ lower than DIC Δ 14 C, indicating that DOC has cycled for much longer than DIC in the SCS. The lower Δ 14 C-DIC and Δ 14 C-DOC values at depths shallower than 700 m were mainly influenced by intensified vertical mixing, which upwelled the deep water with low Δ 14 C-DIC and Δ 14 C-DOC values for thorough mixture with the upper layer water. Conversely, the small difference in the Δ 14 C signature in deep water (>1,500 m) between the SCS and the North Pacific confirmed the rapid water exchange through the Luzon Strait and rapid water mixing in the SCS basin, which plays an important role in controlling carbon cycling in the deep SCS.Plain Language Summary Dissolved inorganic carbon (DIC) is the largest carbon pool in the ocean and is closely linked to dissolved organic carbon (DOC), which is the largest exchangeable organic carbon pool in the ocean. Both DIC and DOC play important roles in the global carbon cycle, but their sources, distribution, and cycling time are different and controlled by different processes in the ocean. Here we report radiocarbon and stable carbon isotope measurements of DIC and DOC collected in the South China Sea (SCS) to reveal the sources and cycling time scales of the two major carbon pools in the SCS. The Δ 14 C values and 14 C ages indicate that DOC has cycled for much longer than DIC in the SCS. The rapid water exchange and mixing between the SCS and the Kuroshio Current in the Northwestern Pacific play important roles in controlling the distributions and cycling of DIC and DOC in the SCS.