Elsevier

Marine Chemistry

Volume 93, Issues 2–4, 15 January 2005, Pages 131-147
Marine Chemistry

The continental shelf pump for CO2 in the North Sea—evidence from summer observation

https://doi.org/10.1016/j.marchem.2004.07.006Get rights and content

Abstract

Data on the distribution of dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2) were obtained during a cruise in the North Sea during late summer 2001. A 1° by 1° grid of 97 stations was sampled for DIC while the pCO2 was measured continuously between the stations. The surface distributions of these two parameters show a clear boundary located around 54°N. South of this boundary the DIC and pCO2 range from 2070 to 2130 μmol kg−1 and 290 to 490 ppm, respectively, whereas in the northern North Sea, values range between 1970 and 2070 μmol kg−1 and 190 to 350 ppm, respectively. The vertical profiles measured in the two different areas show that the mixing regime of the water column is the major factor determining the surface distributions. The entirely mixed water column of the southern North Sea is heterotrophic, whereas the surface layer of the stratified water column in the northern North Sea is autotrophic. The application of different formulations for the calculation of the CO2 air–sea fluxes shows that the southern North Sea acts as a source of CO2 for the atmosphere within a range of +0.8 to +1.7 mmol m−2 day−1, whereas the northern North Sea absorbs CO2 within a range of −2.4 to −3.8 mmol m−2 day−1 in late summer. The North Sea as a whole acts as a sink of atmospheric CO2 of −1.5 to −2.2 mmol m−2 day−1 during late summer. Compared to the Baltic and the East China Seas at the same period of the year, the North Sea acts a weak sink of atmospheric CO2. The anticlockwise circulation and the short residence time of the water in the North Sea lead to a rapid transport of the atmospheric CO2 to the deeper layer of the North Atlantic Ocean. Thus, in late summer, the North Sea exports 2.2×1012 g C month−1 to the North Atlantic Ocean via the Norwegian trench, and, at the same period, absorbs from the atmosphere a quantity of CO2 (0.4 1012 g C month−1) equal to 15% of that export, which makes the North Sea a continental shelf pump of CO2.

Introduction

Human activities have released vast amounts of the greenhouse gas carbon dioxide (CO2) into the atmosphere by fossil fuel burning and deforestation, which corresponds to ∼5.4±0.3 petagrams of carbon per year (Pg C year−1) (1 Pg=1015 g) during the 1980s. It is well established that 3.3±0.1 Pg C year−1 remain in the atmosphere (IPCC, 2001). The world ocean behaves as a sink estimated as 1.9±0.6 Pg C year−1, and the terrestrial biosphere is assumed to trap the remaining 0.2±0.7 Pg C year−1 (IPCC, 2001). Since the land sink is usually considered as the closing term for the global budgeting, it is essential to reduce the uncertainties regarding the oceanic uptake of anthropogenic CO2. The estimates of this uptake still vary significantly (Lee et al., 1998, Gruber and Keeling, 2001, Orr et al., 2001, Thomas et al., 2001). One of the reasons for this uncertainty could be that all these assessments ignore the CO2 fluxes in the coastal oceans. Because of the small-scale variability observed in these regions, it has been difficult to consider coastal seas in global circulation models. Moreover, hitherto there has been a lack of field data on the spatial and temporal variability of dissolved inorganic carbon (DIC) and the partial pressure of CO2 (pCO2) for the coastal ocean.

The coastal ocean is known to house a disproportionately large fraction of the oceanic primary production of 15% to 30% (Walsh, 1991, Wollast, 1998), a contribution that is much larger than the contribution of coastal seas (7%) to the total ocean surface area. Thus, these regions strongly affect the global carbon cycle, however it has not been established yet whether they act as a sink or as a source of atmospheric CO2 (Walsh, 1991, Smith and Hollibaugh, 1993, Kempe, 1995, Gattuso et al., 1998, Mackenzie et al., 1998, Wollast, 1998).

The North Sea has been the subject of intense investigations for many decades by several institutions, making this area one of the best understood coastal seas of the world. Very recent results by Thomas et al. (2004) showed that the North Sea acts as a sink for atmospheric CO2 and export ≈93% of the absorbed CO2 to the North Atlantic Ocean, therefore acting like a continental Shelf pump as described by Tsunogai et al. (1999) for the East China Sea. Thomas et al., (2004) suggested that during the late summer situation, the differences between the two biogeochemical regions with regard to the air-sea exchange of CO2 are most prominent. A detailed investigation of the CO2 system in this period is provided here in order to gain insight in the control mechanisms of the CO2 air-sea exchange on the North Sea.

In the North Sea, previous investigations focused mainly on certain aspects related to the carbon cycle, e.g. primary production and the transport of organic matter within the North Sea (Postma and Rommets, 1984, Kempe et al., 1988). Pioneering investigations of the inorganic carbon cycle during summer 1986 had been done with potentiometric determination of DIC and alkalinity (Pegler and Kempe, 1988, Kempe and Pegler, 1991), upon which the alkalinity was used together with pH to calculate the pCO2. Moreover, many other regional studies were conducted (Hoppema, 1990, Hoppema, 1991, Kempe and Pegler, 1991, Bakker et al., 1996, Frankignoulle et al., 1996a, Frankignoulle et al., 1996b, Frankignoulle et al., 1996c, Frankignoulle et al., 1998, Borges and Frankignoulle, 1999, Brasse et al., 1999, Borges and Frankignoulle, 2002, Brasse et al., 2002). However, the currently available carbon data sets for the entire North Sea still are sparse and do not allow unequivocal conclusions about the carbon cycle.

The bottom topography of the North Sea is likely to constitute the major control for biogeochemical cycling, in particular for inorganic carbon parameters: The deeper northern part reveals depths of approximately 150 m on the shelf, of 400 m in the Norwegian Channel and of 700 m in the Skagerrak (Fig. 1). In the southern part (south of 54°N), depths are less than 50 m and even less than 20 m near the coasts (Eisma, 1987). Because of the shallow conditions in the south, the water column is mixed throughout the year, while thermal stratification occurs in the northern part during summer. The spatial distribution of the carbonate system is also affected by the inorganic and organic carbon inputs from the estuaries. The southern part receives most of the freshwater inputs, notably from the rivers Rhine, Scheldt, Thames and Elbe (Fig. 1). Haline stratification is an exception in the northern part and only occurs in the Norwegian Trench area and in the vicinity of river inlets with strong freshwater input.

The main aim of the present study is to verify the hypothesis of a “continental shelf pump” for the uptake of atmospheric CO2 with subsequent transport to the open ocean. Here we present a new comprehensive study in the late summer of 2001, with improved accuracy relying on now available certified DIC standards, and direct measurements of pCO2 which is calibrated versus certified gas mixtures. We also investigate the interactions between carbon, oxygen and nutrients in order to describe the “biological CO2 pump”. We can rely on a complete new data set with high spatial resolution for the carbonate system and related chemical, biological and physical parameters obtained during our cruise in the North Sea during late summer 2001.

In the present paper, we firstly describe the surface water distribution of the inorganic carbon system and for related parameters in the North Sea. In Section 4.1, we describe the different water masses, which govern these distributions. In 4.2 The CO, 4.3 The CO, we will focus on the differences in carbonate chemistry between the northern and the southern North Sea, respectively. In Section 4.4, we evaluate the monthly atmospheric CO2 fluxes in both regions and assess, whether the North Sea acts as a source or a sink for CO2 during late summer. Finally, in Section 4.5 we propose a mechanism of the continental shelf pump in the North Sea.

Section snippets

Material and methods

The data were obtained during a cruise in the North Sea (18.08.2001–13.09.2001), on board RV Pelagia. The North Sea was covered by an adapted 1° by 1° grid with 97 stations (Fig. 1). This grid was specifically designed to focus on the relevant regions for biogeochemical cycles such as the Shetland and English Channels (inflow of North Atlantic water), the Skagerrak area (inflow of Baltic Sea water) and the western Scandinavian coast (outflow to the North Atlantic). The stations were also denser

Temperature and salinity

The continuous measurements of salinity and temperature during the cruise allowed us to plot high-resolution maps of these parameters in the surface water of the North Sea. A clear gradient of decreasing temperature is present in the North Sea from the southeast to the northwest (Fig. 2A). Highest temperatures between 18 and 19.2 °C were observed in the shallow southern and southeastern area, i.e., in the coastal areas, which receive large freshwater inputs from the English, Belgian, Dutch,

Dissolved inorganic carbon versus salinity

The relationship between DIC and salinity is exploited in order to investigate the DIC water mass characteristics in the North Sea (Fig. 3). In the surface waters, the DIC/salinity relationship indicates the three main water masses in the North Sea, which govern the inorganic carbon distribution during late summer:

  • (1)

    The Central North Sea water, which has similar characteristics to the North Atlantic water of high salinity (≈35) and DIC concentrations between 2050 and 2070 μmol kg−1.

  • (2)

    The Southern

Summary

The distributions of DIC and pCO2 in the North Sea in late summer are mainly driven by the mixing regime of the water column influenced by the bottom topography of the North Sea. A clear boundary is evident in the distribution of DIC and pCO2 as well as for related parameters such as AOU, NO3/2 and PO4 at 54°N. South of this border, the strong tidal currents mix the entirely shallow water column down to the bottom. By consequence, the degradation of POM releases inorganic carbon into the

Acknowledgements

The excellent co-operation of the captains and the crews of RV Pelagia as well as the scientific crew is gratefully acknowledged. We thank DKRZ, DWD and Dr. Johannes Pätsch for making available the ECMWF wind data. We are grateful to Prof. Leif Anderson and Dr. Rik Wanninkhof for their helpful comments on a previous version of our manuscript. This study has been encouraged by and contributes to the LOICZ core project of the IGBP. It has been supported by the Netherlands Organisation for

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    Present address: Department of Oceanography, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia, Canada, B3H 4J1

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