Factors affecting the subsurface aragonite undersaturation layer in the Pacific Arctic region

This study evaluated interannual variation in the subsurface aragonite undersaturation zone (Ω_Ar<1 layer) in the Pacific Arctic Ocean, using data from the 2016-2019 period. The upper boundary (DEP_Ω<1^UB) of the Ω_Ar<1 layer generally formed at a depth where the contribution of corrosive Pacific water was approximately 98 %. The intensity of the Beaufort Gyre associated with freshwater accumulation mainly determined interannual variation in DEP_Ω<1^UB, but the direction of its effect was opposite west and east of ~166°W. The lower boundary (DEP_Ω<1^LB) of the Ω_Ar<1 layer was generally found at a depth range where equal contributions of Pacific and Atlantic water were expected. An Atlantic-origin cold saline water intrusion event in 2017 caused by an anomalous atmospheric circulation pattern significantly lifted the DEP_Ω<1^LB, thus the thickness of the Ω_Ar<1 layer decreased.

Carbon dioxide and methane fluxes from mariculture ponds: The potential of sediment improvers to reduce carbon emissions

The CO₂ and CH₄ fluxes across the water-air interface were determined in two groups of swimming crab (Portunus trituberculatus)-ridgetail white prawn (Exopalaemon carinicauda) polyculture ponds. One group of ponds with sediment improver application were referred to as SAPs, and the other group receiving no sediment improver were as NSPs. During the farming season, both the SAPs and NSPs acted as CO₂ sinks and CH₄ sources. The cumulative CO₂-C fluxes from the SAPs and NSPs were -26.78 and -23.49 g m^-2, respectively, and the cumulative CH₄-C emissions from the SAPs and NSPs were 0.24 and 0.28 g m^-2, respectively. CO₂ fluxes were significantly related to net primary production and water pH, and CH₄ fluxes were mainly regulated by water temperature during the farming season. The application of the oxidation-based sediment improver had a positive effect on reducing the CH₄ emissions across the water-air interface but had no effect on CO₂ fluxes. The sediment improver reduced the organic matter contents and improved the sediment pH and redox potential, which may have facilitated a decrease in CH₄ production in the sediment. The CO₂ produced through the oxidation of organic material in the sediment may have been absorbed by strong photosynthesis, resulting in a nonsignificant difference in CO₂ fluxes between the SAPs and NSPs. The results indicated that the application of sediment improvers in coastal polyculture ponds can reduce carbon emissions, especially CH₄ emissions, during the farming period and could help mitigate global warming with regard to the sustained-flux global warming potential (SGWP) and sustained-flux global cooling potential (SGCP) models over a 20-year time horizon. Future studies on the CO₂ and CH₄ production rates of the sediment and the related microbial community could improve our understanding of the effect mechanism of the application of sediment improvers on CO₂ and CH₄ emissions from mariculture ponds.

Vulnerability of polar oceans to anthropogenic acidification: comparison of arctic and antarctic seasonal cycles

Polar oceans are chemically sensitive to anthropogenic acidification due to their relatively low alkalinity and correspondingly weak carbonate buffering capacity. Here, we compare unique CO₂ system observations covering complete annual cycles at an Arctic (Amundsen Gulf) and Antarctic site (Prydz Bay). The Arctic site experiences greater seasonal warming (10 vs 3°C), and freshening (3 vs 2), has lower alkalinity (2220 vs 2320 μmol/kg), and lower summer pH (8.15 vs 8.5), than the Antarctic site. Despite a larger uptake of inorganic carbon by summer photosynthesis, the Arctic carbon system exhibits smaller seasonal changes than the more alkaline Antarctic system. In addition, the excess surface nutrients in the Antarctic may allow mitigation of acidification, via CO₂ removal by enhanced summer production driven by iron inputs from glacial and sea-ice melting. These differences suggest that the Arctic system is more vulnerable to anthropogenic change due to lower alkalinity, enhanced warming, and nutrient limitation.

Decrease in the CO2 uptake capacity in an ice-free Arctic Ocean basin

It has been predicted that the Arctic Ocean will sequester much greater amounts of carbon dioxide (CO2) from the atmosphere as a result of sea ice melt and increasing primary productivity. However, this prediction was made on the basis of observations from either highly productive ocean margins or ice-covered basins before the recent major ice retreat. We report here a high-resolution survey of sea-surface CO2 concentration across the Canada Basin, showing a great increase relative to earlier observations. Rapid CO2 invasion from the atmosphere and low biological CO2 drawdown are the main causes for the higher CO2, which also acts as a barrier to further CO2 invasion. Contrary to the current view, we predict that the Arctic Ocean basin will not become a large atmospheric CO2 sink under ice-free conditions.