Factors Controlling Acidification in Intermediate and Deep/Bottom Layers of the Japan/East Sea.

Much effort has been devoted to studying the absorption of anthropogenic CO2 by the surface and subsurface ocean, where surface seawater acidifies rapidly while the deep ocean was thought to respond to CO2 intrusion at longer time scales. However, recent studies have revealed rapid acidification of the deep Japan/East Sea, implying that ongoing changes in ocean dynamics are accelerating acidification at depth, and the controlling mechanisms have not been fully addressed. Using a multibox model of ocean circulation and carbonate chemistry, we demonstrate that a combination of reduced deep/bottom water formation and an increase in organic matter remineralization can best explain the observed pattern of rapid acidification in deep waters. In contrast, acidification in the intermediate layer is controlled by increased particulate organic carbon remineralization accompanied by intensified intermediate ventilation. Variations of ventilation and remineralization of particulate organic carbon dominating acidification in different layers of the water column need to be considered in future investigations. Plain Language Summary: Increased oceanic uptake of anthropogenic CO2 from the atmosphere has caused serious environmental problems, such as ocean acidification. Typically, the rapid acidification occurs in the surface and subsurface sea waters only because CO2 intrusion into the deep ocean takes more time. Recent investigations in the Japan/East Sea indicate that its deep water is acidifying at an unexpected rate, though the controlling mechanisms have not been fully understood. Thus, we built a multibox of ocean circulation and carbonate chemistry to investigate possible controls. Our results indicate that the acidification in different layers of the water column is controlled by different factors. The reduced deep/bottom water formation related to global warming resulted in the rapid deep acidification, while the rapid acidification of intermediate water is related to increased remineralization of particulate organic carbon, fueled by intensified intermediate ventilation. The neutralization by dissolution of sedimentary calcium carbonate in the Japan/East Sea is nearly negligible so that fast deep acidification can take place. Thus, the Japan/East Sea has distinct features other than acidification in other global oceans in which acidification can be partially buffered by the dissolution of previously deposited calcium carbonate. Key Points: Factors controlling acidification of different water masses in Japan/East Sea are investigated using a multibox modelReduced deep ventilation and increased intermediate ventilation and remineralization control acidification in these two contrasting layersThe sedimentary calcium carbonate is insignificant in neutralizing the rapid deep acidification in the Japan/East Sea [ABSTRACT FROM AUTHOR]