Transit Time Distributions and Apparent Oxygen Utilization Rates in Northern South China Sea Using Chlorofluorocarbons and Sulfur Hexafluoride Data

Abstract

CFC‐12 and SF6 data were used in combination to estimate the mean age of water in the northern South China Sea (NSCS), to explore oceanographic processes related to “time,” including the transit time through the Luzon Strait (LS) with a three‐layer circulation structure and the apparent oxygen utilization rates (AOUR). Significant differences in mean ages of water were observed at the same density level in the water columns on both sides of the LS, presented as a westward flow in the upper layer, eastward flow in the intermediate layer, and westward flow in the deep layer with transit times of 8 ± 5, 39 ± 22, and 20 ± 18 yr, respectively. The AOUR was estimated to be 8.4 μmol kg−1 yr−1 at about 100 m and decreased to approximately 0.66 μmol kg−1 yr−1 at 1,500 m in the NSCS. The average organic carbon flux in the depth range of 100–1,500 m was 1.7 mol C m−2 yr−1 in the NSCS and 1.3 mol C m−2 yr−1 in the western Pacific Ocean (WP). The activation energy—derived using the Arrhenius equation—in the NSCS and WP (87.7–154.2 kJ mol−1) are close to those in the northern Pacific Ocean (60.8–133.5 kJ mol−1). These results suggest a conspicuous correlation between temperature and AOUR. The impact of climate change on the ocean and the feedback mechanism between ocean warming and oxygen consumption needs further investigation.

Epipelagic nitrous oxide production offsets carbon sequestration by the biological pump

“…Therefore, our estimates would probably fall at the lower end of the potential offset attributable to N 2 O production in this region. For example, ventilation times between 200 and 300 m in this region average 32 ± 5 years, with that age increasing to 50 years at 400 m and 100 years at 500 m 46 . Assuming the vertical attenuation of sinking particulate matter follows a power-law function 47 , we estimate ~57 ± 5% of the measured exported carbon would be remineralized above 500 m and could exchange with the atmosphere in <100 years, leading to less CO 2 sequestration and a higher N 2 O offset value (Supplementary Discussion 4).…”

“…Excess 45 N 2 O has also been observed in several studies conducted in the mid-latitude North Atlantic 37 , the western North Pacific 38 and the eastern tropical South Pacific 39,40 and has been interpreted as evidence of hybrid N 2 O production. However, isotope dilution of the tracer substrates, and ammonia oxidation coupled to NO 2 − reduction during the incubation, would also cause deviation of the measured 45 N 2 O: 46 N 2 O from the predicted ratio, and care should be taken in interpreting the observed excess 45 N 2 O (Supplementary Discussion 3). The presence of 46 N 2 O in both 15 NO 2 − and 15 NO 3 − labelling incubations suggests production of N 2 O via nitrifier denitrification and/or denitrification in micro-anoxic niches in the oxygenated ocean 11,12 .…”

“…However, isotope dilution of the tracer substrates, and ammonia oxidation coupled to NO 2 − reduction during the incubation, would also cause deviation of the measured 45 N 2 O: 46 N 2 O from the predicted ratio, and care should be taken in interpreting the observed excess 45 N 2 O (Supplementary Discussion 3). The presence of 46 N 2 O in both 15 NO 2 − and 15 NO 3 − labelling incubations suggests production of N 2 O via nitrifier denitrification and/or denitrification in micro-anoxic niches in the oxygenated ocean 11,12 . As nitrification is widely used as a key component for model parameterization to estimate N 2 O production in the oxygenated ocean 8,9,41 , our results strongly support the contribution of multiple precursors and pathways of N 2 O in the epipelagic ocean that need to be considered in biogeochemical models aiming to estimate marine N 2 O sources and air-sea flux.…”

“…N 2 O is mainly contributed by15 N 14 N16 O and 14 N 15 N 16 O (single labelled N 2 O) and production of 14 N 14 N 17 O during the incubation is negligible. Similarly, the accumulation of46 N 2 O is thus mainly contributed by 15 N 15 N 16 O (double labelled N 2 O), and the production of 14 N 15 N 17 O and 14 N 14 N 18 O are negligible compared with the double labelled N 2 O. Therefore, the rates of 45 N 2 O production and 46 N 2 O production can be derived using equations (6) and(7):R 45measure = 1 t × C N 2 O × (n t45 − n 045 ) × 24 (6) R 46measure = 1 t × C N 2 O × (n t46 − n 046 ) × 24 (7)Article https://doi.org/10.1038/s41561-022-01090-Depth profiles of N2O saturation, NO2− and NO3−/ Si at stations with distinctive shallow N2O accumulation in the epipelagic zone.…”

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

“…He et al (2018) used a D/G ratio of 1.2 in the North Pacific Ocean, and Sonnerup et al (2013) suggested that possible D/G ratios for directly ventilated waters in the North Pacific do not exceed 1.0 but are greater than 1.0, for denser waters that are ventilated by physical mixing. Waugh et al (2006) assumed a D/ G ratio of 1.0, which could accurately describe global tracer distributions, while Wang et al (2021) used a mean D/G ratio of 0.8 for the western North Pacific and the northern South China Sea. Based on the compilation of these values, we set the D/G ratio range to 0.8-1.2, in increments of 0.2.…”