
Citation: | Yangjun Chen, Jinxu Chen, Yi Wang, You Jiang, Minfang Zheng, Yusheng Qiu, Min Chen. Sources and transformations of nitrite in the Amundsen Sea in summer 2019 and 2020 as revealed by nitrogen and oxygen isotopes[J]. Acta Oceanologica Sinica, 2023, 42(4): 16-24. doi: 10.1007/s13131-022-2111-4 |
Nitrogen (N) is an essential nutrient needed for marine organisms to grow. Nitrate (
The Southern Ocean plays a pivotal role in oceanic nutrient cycling and climate change. For example, global cooling during the Late Pleistocene Ice Age was thought to have resulted from enhanced carbon storage in the Southern Ocean, thereby reducing atmospheric carbon dioxide (Chalk et al., 2017; Jaccard et al., 2013; Martínez-Garcia et al., 2011). One of the typical features in the Southern Ocean is that major nutrients such as
Current research on the N cycle in the Southern Ocean has focused more on
Seawater samples were collected through China’s 35th and 36th Antarctic Research Expeditions (CHINARE). During the 35th CHINARE conducted by R/V Xuelong, samples were collected from four sites at the Section A8 from January 16 to 19, 2019. During the 36th CHINARE conducted by R/V Xuelong, samples were collected from nine sites at the Sections A3 and RA2 from January 12 to February 5, 2020. Sections RA2, A3 and A8 are located in the western, central and eastern parts of the Amundsen Sea, respectively (Fig.1). Samples were collected from a 12 L Niskin bottle mounted on a Conductivity-Temperature-Depth (CTD) rosette. Only water samples above 150 m were collected for N and O isotopes analysis because the
Temperature and salinity were measured in situ by CTD probes on board (SBE 911plus, Sea Bird). The measurement accuracy of temperature and conductivity is 0.001℃ and 0.0003 S/m, respectively. Chl a concentration was measured and calibrated directly from the probe equipped on the CTD.
The concentrations of
An optimized azide reduction method was used to simultaneously measure the N and O isotopic compositions in
The sea surface temperature (SST) in the Amundsen Sea ranged from −1.56℃ to 0.38℃, with the largest variation in the Section A8 in the east (Fig. 2a). The temperature minimum (Tmin) and temperature maximum (Tmax) appeared at different depths in sequence, with Tmin at 50−200 m depth and Tmax at 300−500 m depth (except RA2-03 at 1000 m). Below Tmax, the temperature gradually decreased and gradually became stable (Fig. 2d, gray line). Tmin and Tmax are ubiquitous features in the Southern Ocean (Altabet and Francois, 2001; Bindoff et al., 2000; DiFiore et al., 2009, 2010; Kemeny et al., 2016; Fripiat et al., 2019; Smart et al., 2015). The Tmin reflects the characteristics of winter residual water retained in summer (Altabet and Francois, 2001; DiFiore et al., 2010), and the Tmax reflects the influence of Circumpolar Deep Water (Bindoff et al., 2000; DiFiore et al., 2009; Fripiat et al., 2019).
The sea surface salinity in the Amundsen Sea varied between 32.68 and 33.97. Unlike the distribution of SST, the surface salinity in the three regions was relatively uniform. The salinity gradually increased with the increase of depth and stabilized below 500 m (Fig. 2e).
The
The distribution of
In the Section RA2 in the western Amundsen Sea, due to the limitation of
δ18
In the Section A3 of the central Amundsen Sea, the δ15
The highest value of δ18
The variation ranges of δ15
In this study, the δ15
The theoretical range of δ15
Combining the influences of the above conventional N cycle processes, the potential variation range of δ15
As one of the sources of
Estimating the potential range of δ18
Combining the influences of the above conventional N cycle processes, the potential variation range of δ18
According to the above isotopic fractionation of the traditional N cycle processes, in the upper water of the Southern Ocean, the variation range of δ15
The existence of PNM has been observed in the three sections of the Amundsen Sea (Fig. 2c). Here, we analyze the source of
The formation of PNM requires that the source of
Our results show that the vast majority of δ15
The N and O isotopic compositions of
Measurements of N and O isotopic compositions of
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