OCeANiC

OCeANIC (nitrous Oxide and nitrogen Cycling in ANtarctic sea Ice Covered zone) is a Research action funded by the Belgian Federal Science Policy Office (2016-2020).

The network is composed of Université de Liège (Bruno Delille), Université Libre de Bruxelles (Jean-Louis Tison and François Fripiat), Vrije Universiteit Brussel (Frank Dehairs) and the Third Institute of Oceanography of China (Liyang Zhan).

Nitrous oxide (N2O) is a potent greenhouse gas with a high global warming potential (GWP) and is the dominant ozone-depleting substance emitted in the 21st Century. N2O is produced by nitrification, denitrification, and coupled nitrification-denitrification [Bange et al., 2007] and is therefore intimately linked to nitrogen (N) cycling. In this context, the Southern Ocean is considered as one of the dominant oceanic source of nitrous oxide for the atmosphere, due to the release of the N2O excess accumulated by organic matter remineralization along the deep oceanic pathway and ultimately ventilated in the surface waters of the Southern Ocean [Nevison et al., 1995, 2005; Suntharaling and and Sarmiento, 2000].

It is well recognized today that the Southern Ocean is playing a disproportionate role in setting the air-sea balance of CO2 and global primary productivity [Sarmiento et al., 2004; Gruber et al., 2009; Sigman et al., 2010]. Deep nutrient-rich waters ascend to the surface, mainly in the southern part of the Antarctic Circumpolar Current. They are either transported northward or southward to form the upper and lower limbs of the meridional overturning circulation, respectively. During their transport, the available nutrient pools are not fully utilized by phytoplankton [Sarmiento et al., 2004; Rintoul et al., 2009]. Such inability to strip nutrients out of the surface water (i.e., low biological pump efficiency) has been attributed to light and iron co-limitations [Martin et al., 1990; Mitchel et al., 1991] and represents (i) a missed opportunity to sequester more CO2 into the deep ocean [Sigman et al., 2010] and (ii) a nutrient leak toward low-latitude areas, sustaining up to 75% of the low-latitude primary productivity [Sarmiento et al., 2004]. A larger carbon sequestration in the deep Ocean is generally considered as the cause of lower atmospheric pCO2 during ice ages [Sigman et al., 2010], driven in the Southern Ocean by either an increase in the biological pump efficiency [Martin, 1990; Martinez-Garcia et al., 2014], lower ventilation in the Antarctic Zone [François et al., 1997; Toggweiler et al., 1999], or prevented evasion of CO2 by an extensive sea ice cover [Stephens and Keeling, 2000]. For the latter, it is assumed that sea ice is inert in terms of gas exchange. Our group has demonstrated that Antarctic sea ice, instead of being inert, actively exchanges CO2 with the atmosphere (i.e., contributing to ∼50% of the CO2 uptake south of 50°S) [Delille et al., 2014].

In the light of the above it appears important to investigate in parallel with the study of CO2 fluxes the impact of the biogeochemical processing of nitrogen in the water column and the sea ice, keeping in mind that emission of nitrous oxide, a by-product of the biological pump, will have an opposite effect on climate warming than CO2 sequestration.

Nitrogen and oxygen isotopes of fixed N compounds provide a means of studying both the input/output budget of oceanic fixed N, its cycling within the ocean, and of defining the biologically-mediated pathways leading to nitrous oxide production. Continental Antarctic shelves are the most productive areas in the Southern Ocean likely due to higher iron supply [Sedwick and DiTullio, 1998; Smith et al., 2000; Arrigo et al., 2008, 2015]. The lower cell of the overturning circulation is ventilated there, together with the polar Antarctic Zone, and therefore this general area exerts a significant influence on atmospheric greenhouse gases (CO2 and N2O) [Nevison et al., 1995; Nevison et al., 2005; Sigman et al., 2010]. In addition, deep water mass formation (Antarctic Bottom Water) occurs over the continental shelves through a combination of brine rejection during sea ice-growth and ice shelves-ocean interactions [Orsi et al., 1999]. Today, data documenting the distribution and isotopic compositions of fixed N compounds and N2O over the Antarctic continental shelves and in the Polar Antarctic zone in general are still scarce [DiFiore et al., 2009; Zhan and Chen, 2009; Zhan et al., 2015]. In order to bridge this gap, OCEANIC project will develop a new and strong collaboration between three Belgian universities (Vrije Universiteit Brussel, Université Libre de Bruxelles and Université de Liège) and the Second Institute of Oceanography and the Third Institute of Oceanography of China. OCEANIC aims to develop an innovative, comprehensive and integrated approach for the study of the nitrogen cycling and related air-sea fluxes of N2O in the Antarctic coastal area and the seasonal ice zone (SIZ).

OCEANIC will focus on the Prydz Bay area offering access to both, the Antarctica coastal area and the SIZ. Prydz Bay is the third largest embayment of Antarctica, and is an active area in terms of Antarctic Bottom Water formation (6-13% of the total). Prydz Bay is seasonally covered with sea ice and also hosts polynyas. We will detail the partitioning of fixed N compounds, N2O and their isotopic compositions between the water column and the sea ice cover, based on high resolution across-slope hydrographic sections and selected pack ice sampling stations (CHINARE oceanographic survey in Oct.-Nov. 2017). The data gathered will be complemented with results obtained earlier (end of 2015) for a fast-ice time-series station located in the vicinity of Davis Station, Prydz Bay. This would allow us:

(i) define and quantify routes of fixed N supply and mixing between the different water masses; (ii) assess the importance and distribution of the nitrification process in the water column and in sea ice, (iii) provide a reference for paleoceanographic studies based on N isotopes in the polar Antarctic Zone [Sigman et al., 1999; DiFiore et al., 2006; Fripiat et al., 2014, 2015a], (iv) resolve the role of sea ice covered area in air-sea N2O exchanges. For the latter, recent N2O measurements in coastal Antarctica appear somewhat paradoxical. Zhan et al. [2015] observed that Prydz Bay acts as a sink for atmospheric N2O and suggest that sea ice melt enhances this sink effect. In contrast, Delille et al. [in prep] observe high N2O concentrations in sea ice, likely due to nitrification [Fripiat et al., 2014], suggesting that sea ice might be a source of N2O.