Spring carbonate chemistry and its stable isotope dynamics in the Yellow Sea: Effects of water mixing, metabolism and cold wave on interannual variations

What is it about?

The stable isotopic composition has been widely used for identifying potential carbon sources and verifying potential processes affecting carbonate dynamics in coastal seas. To better understand the dynamics of dissolved inorganic carbon (DIC) and its isotope composition (δ13C_DIC) in the Yellow Sea, five carbonate surveying cruises were carried out in March and April in 2022–2024. The characteristics and interannual variations in spring carbonate chemistry and δ13C_DIC were investigated. In the southern South Yellow Sea, water mixing among the coastal current, Changjiang Diluted Water and open-ocean waters dominated the distribution of carbonate chemistry and its stable isotope dynamics. In the proper of the South Yellow Sea (north of 33°N), however, a two-endmember water mixing scheme characterizing coastal current and the Yellow Sea Warm Current was applied in this study. The highest δ13C_DIC values (from −0.9 to 0.2‰) of this study were observed in the central South Yellow Sea in spring 2024, associated with the notable intrusion of the Yellow Sea Warm Current. In the central North Yellow Sea and the central South Yellow Sea, community metabolism controlled the variations in DIC and δ13C_DIC. Based on the two-endmember water mixing scheme and the mass balance of DIC and δ13C_DIC, the assumed uniform δ13C of oxygen-consuming organic matter in the central South Yellow Sea was estimated to be −20.3‰ (in April 2022); −21.7‰ (in April 2023); −21.1‰ (in April 2024), respectively, similar to the δ13C value of marine diatoms (approximately −21‰). In addition, the remarkably low yearly initial values of δ13CDIC in the central North Yellow Sea and the central South Yellow Sea in 2024 were attributed to the strong cold-wave events at the beginning of the year. Intense cooling process leads to a strong absorption of atmospheric CO2 into the ocean, together with depleted δ13C signals, resulting in a decrease in oceanic δ13C_DIC. This study outlines the use of stable isotopic carbon tracers to distinguish the sources and influences of waters in the Yellow Sea to provide further constraints on physical and biological processes and also points out the response of stable isotopic carbon to intense climate changes. The results will contribute to a further study on the carbon cycle and regulation mechanism of carbon isotope dynamics in coastal seas.

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