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Scientists elaborated upon carbon sink/source patterns of the Yellow Sea and East China Sea

Science China Press

The Yellow Sea (YS) and the East China Sea (ECS) locate in the western side of the Pacific Ocean, and are the most important continental marginal seas in our country. The carbon sink/source patterns in the YS and the ECS play vital roles in regulating the coastal ecosystems and the regional climate in China, which are widely concerned by oceanographic scientists from the world.

The Yellow Sea (YS) is a semi-enclosed marginal sea which is located between the China mainland and the Korean Peninsula (31°40'~39°50' N, 119°10'~126°50' E). Annually, the North YS could release 0.5×10 6 t C into the atmosphere. Sea surface temperature generally played a more important role in the annual cycle of air-sea CO2 exchanging than biological processes in most parts of the study area. Meanwhile, the intrusion of Yellow Sea Warm Current and the seasonal variation of thermocline also played important controlling roles in regulating the CO2 sink/source terms of NYS. The South YS could absorbed atmospheric CO2 only in spring, but released CO2 in summer, autumn, and winter. Seasonal large-scale field surveys estimated that net air-sea CO2 flux in the SYS was 7.4×10 6 t C, most of which occurred in winter when vertical mixing process was significant. Control of temperature on pCO2 was predominant in the offshore South YS; the non-temperature factors were predominant in the shallow nearshore area, especially in coast of Shandong Peninsula and the Jiangsu Shallow. The East China Sea (ECS, 23°00'~33°10' N, 117°11'-131°00' E) is a broad continental marginal sea surrounded by the China Mainland, Taiwan, South Korea, Kyushu, and Ryukyu Islands, 66% area of which is located on the continental shelf. Abundant data in recent decades estimate ECS is a net sink for atmospheric CO2 with the annual average CO2 exchanging flux of -2.6 mmol* m-2* d-1. In detail, the ECS could absorb CO2 during winter, spring, and summer, and release CO2 into atmosphere with the air-sea exchange flux of -5.8, -3.7, -3.7, and 2.8 mmol* m-2* d-1, respectively. As a whole, the ECS could absorb 8.5×10 6t C* yr-1annually.

Coastal regions of the YS and the ECS usually have high content of DIC, whereas DIC concentration in the outer shelf of the ECS is relative low. This distribution pattern indicates the profound effects of water mass and current systems on the regional variations of DIC and carbon sink/source. Not only that, the Huangpu River usually discharges abundant DIC into the Changjiang estuary, and this is the key reason why the inner estuary of Changjiang could release CO2 into the atmosphere significantly (15.5~34.2 mol* m-2* yr-1). It is estimated that the total storage of DIC in the YS and ECS is 425×10 6 t C and 1364×10 6 t C, respectively. Seasonal variation and regional discrepancy of DOC in the YS and the ECS are fairly significant. In general, the average content of DOC in autumn, summer, spring, winter is 1.99 mg* L-1, 1.66 mg* L-1, 1.56 mg* L-1, and 1.53 mg* L-1, respectively. Long-term (1997-2015) observation conducted in the SYS indicates there is a reducing trend for DOC content in recent years. Variation range of DOC in western part of ECS is 0.78-0.90 mg* L-1, whereas the DOC content in southern of ECS is basically higher than 1.02. The storages of DOC in the water column of YS and ECS are estimated to be 28.2×10 6 t C and 54.1×10 6 t C. These storages are higher than the magnitude of air-sea CO2 exchanging flux but lower than the reserves of DIC. POC contents in the YS and the ECS is high in spring (230.0 μg* L-1), followed by ummer (147.2 μg* L-1), winter (125.9 μg* L-1), and low in autumn (97.4 μg* L-1). Due to relative low water depth, the vertical profiles of POC in the YS are quite uniform in the function of vertical mixing. However, POC distributions in the Changjiang estuary show obvious discrepancy between the upper layer and the bottom layer. As for the ECS shelf, the combined action of phytoplankton activity and hydrological dynamics jointly control the variation of POC. It is estimated that the total reserves of POC in the YS and the ECS is about 10.6×10 6 t C (Table 3), which is at the same level of the air-sea CO2 exchanging flux. The evaluation about biological sequestration in Chinese marginal seas indicates that the amount of carbon fixed by phytoplankton in YS is about 60.42×10 6 t C* yr-1, and that in ECS is about 153.41×10 6 t C* yr-1. Most of the carbon is fixed during spring and summer. Fishing and mariculture can enhance the carbon sink strength efficiently. In recent years, the yield of large-size economic seaweeds in China marginal seas is about 1.20-1.50×10 6 t C, so the fixed carbon is about 4.0 ×10 5 t C.

Generally, organic carbon content in sediment is the most important index for carbon burial in the ocean. Concentration of organic carbon in surface sediment of YS and ECS range from 0.08~1.07% and 0.10~1.30%, respectively. The deposition flux of organic carbon in the YS and the ECS is calculated to be 4.75×10 6 t C* yr-1and 7.40×10 6 t C* yr-1, respectively, approximately accounts for 9% of the total deposition flux of organic carbon in the global ocean. Considering the area of YS and ECS cover only about 3% of the world continental marginal area (2.5×107 km2), the burial of carbon in YS and ECS is fairly significant. Summary about the data in recent research shows there are 1.72×10 6 t C* yr-1 of TOM and 3.03×10 6 t C* yr-1 of MOM depositing in the YS, whereas the TOM and MOM deposited in the ECS is 1.9×10 6 t C* yr-1and 5.5×10 6 t C* yr-1, respectively. Distribution terms of TOM in the YS and the ECS are controlled profoundly by riverine discharge and hydrological dynamics. The increase in nutrient supply caused by intensified human activities (e.g., fertilization and sewage discharge), along with a strengthened Kuroshio Current and East Asian Winter Monsoon (EAWM), led to an enhanced MOM contribution and increased sedimentary organic matter abundance in the ECS sediments over the past 100 years.

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This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA11020102)

See the article:

Song J, Qu B, Li X, Yuan H, Li N, Duan L. 2018. Carbon sinks/sources in the Yellow and East China Seas--Air-sea interface exchange, dissolution in seawater, and burial in sediments. Science China Earth Sciences, 61, https://doi.org/10.1007/s11430-017-9213-6

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