Major rivers play important roles in transporting large amounts of terrestrial organic matter from land to the ocean each year, and the organic matter carried by rivers contains a significant fraction of black carbon (BC). A recent study estimated that 0.027 Gt of BC is transported in the dissolved phase by rivers each year, which accounts for ~10% of the global flux of dissolved organic carbon. The relative sources of this large amount of riverine dissolved black carbon (DBC) from biomass burning (young, modern 14C) and fossil fuel (old, 14C free) combustion are not known. We present radiocarbon measurements of BC in both dissolved and particulate phases transported by the Changjiang and Huanghe Rivers, the two largest rivers in China, during 2015. We show that two, distinct BC pools (young and old) were carried by the rivers. The DBC pool was much younger than the particulate BC (PBC) pool. Mass balance calculations indicate that most (78–85%) of the DBC in the Changjiang and Huanghe Rivers was derived from biomass burning, and only 15–22% was from fossil fuel combustion. In contrast, PBC from biomass burning and fossil fuel combustion were approximately equal in these two rivers. Export of PBC and DBC by the rivers are decoupled, and fluxes of PBC were 4.1 and 6.7 times higher than DBC in the Changjiang and Huanghe Rivers, respectively. The 14C age differences of the two BC pools suggest that BC derived from biomass burning and fossil fuel combustion are mobilized in different phases and on different time scales in these rivers.

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.

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