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Enhanced Ocean Carbon Storage from Anaerobic Alkalinity Generation in Coastal Sediments

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Enhanced Ocean Carbon Storage from Anaerobic Alkalinity Generation in Coastal Sediments Biogeosciences, 6, 267–274, 2009 www.biogeosciences.net/6/267/2009/ Biogeosciences © Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License. Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments H. Thomas1,2, L.-S. Schiettecatte3, K. Suykens3, Y. J. M. Kone´3, E. H. Shadwick1, A. E. F. Prowe1,4, Y. Bozec5, H. J. W. de Baar2, and A. V. Borges3 1Dalhousie University, Dep. of Oceanography, Halifax, Canada 2Royal Netherlands Institute for Sea Research, Den Burg, Texel, The Netherlands 3University of Liege,` Chemical Oceanography Unit, Liege,` Belgium 4Leibniz-Institut fur¨ Meereswissenschaften, IFM-GEOMAR, Kiel, Germany 5Station Biologique de Roscoff, UMR 7144 CNRS et UPMC Univ. Paris 6, Equipe Chimie Marine, Roscoff, France Received: 28 July 2008 – Published in Biogeosciences Discuss.: 9 September 2008 Revised: 9 December 2008 – Accepted: 21 January 2009 – Published: 25 February 2009 Abstract. The coastal ocean is a crucial link between land, 1 Introduction the open ocean and the atmosphere. The shallowness of the water column permits close interactions between the sedi- Shelf and marginal seas constitute biogeochemically active mentary, aquatic and atmospheric compartments, which oth- environments linking fluxes of energy and matter between ∼ erwise are decoupled at long time scales (=1000 yr) in the land, the open ocean and the atmosphere. These areas host open oceans. Despite the prominent role of the coastal high biological activity, which is fuelled by nutrient inputs oceans in absorbing atmospheric CO2 and transferring it into from all three interfacing compartments (e.g. Wollast, 1998; the deep oceans via the continental shelf pump, the underly- Thomas et al., 2003; Patsch¨ and Kuhn,¨ 2008; Liu et al., ing mechanisms remain only partly understood. Evaluating 2009). Furthermore, shallow seas, in which the water col- observations from the North Sea, a NW European shelf sea, umn is mixed at the seasonal or annual time scale, establish we provide evidence that anaerobic degradation of organic a close link between surface sediments and the atmosphere. matter, fuelled from land and ocean, generates total alkalin- This permits relatively direct interactions between both the ity (AT) and increases the CO2 buffer capacity of seawater. sedimentary and atmospheric compartments, which are oth- At both the basin wide and annual scales anaerobic AT gen- erwise strictly separated at long time scales (=∼1000 yr) in the eration in the North Sea’s tidal mud flat area irreversibly fa- open oceans. It has been proposed that shelf and marginal cilitates 7–10%, or taking into consideration benthic denitri- seas, such as the East China Sea and North Sea, act as fication in the North Sea, 20–25% of the North Sea’s over- continental shelf pumps, transferring atmospheric CO2 into all CO2 uptake. At the global scale, anaerobic AT genera- deeper layers of the open ocean via physical or biological tion could be accountable for as much as 60% of the uptake processes, or a combination of the two (Tsunogai et al., 1999; of CO2 in shelf and marginal seas, making this process, the Thomas et al., 2004). However, as recently summarized anaerobic pump, a key player in the biological carbon pump. (Borges, 2005; Borges et al., 2005, 2006; Cai et al., 2006), a Under future high CO2 conditions oceanic CO2 storage via conclusive understanding has yet to be achieved on the role the anaerobic pump may even gain further relevance because of shelf and marginal seas as sinks or sources for atmospheric of stimulated ocean productivity. CO2 and of the underlying mechanisms. Recent investigations describe the North Sea, in the NW European Shelf, as a strong continental shelf pump, facili- tated through intense interaction between the deeper north- ern North Sea and the adjacent North Atlantic (Thomas et al., 2004, 2005a; Bozec et al., 2006). On the other hand, Correspondence to: H. Thomas the North Sea’s shallow southern part is strongly affected ([email protected]) by terrestrial influences such as riverine inputs (Borges and Published by Copernicus Publications on behalf of the European Geosciences Union. 268 H. Thomas et al.: Enhanced ocean carbon storage −1 ∼ AT and 1.5 µmol kg (=0.08%) for DIC. Surface water par- tial pressure of CO2 (pCO2) was determined continuously (Kortzinger¨ et al., 1996) with approximately 20 000 data points per cruise, and the uncertainty was estimated to be 1ppm (≈0.3%). Atmospheric pCO2 was measured hourly. The pH was calculated from DIC and pCO2 (Dickson and Millero, 1987). More detailed descriptions of the methods used have been reported elsewhere (e.g. Bozec et al., 2006). The variation of AT (1AT) in the southeastern bight of the North Sea (Fig. 1) was computed with the following equa- - tion: atmos. NO3 Wadden Sea AT water column 1AT = δAT(mix) + δAT(Riv) + δAT(Wadden Sea)+ (1) - D AT River AT − − − open NO3 δA (column NO ) + δA (riv. NO ) + δA (atm. NO ) - T 3 T 3 T 3 North Sea River NO3 North Sea Southern Bight where δAT (mix) represents the exchange between the south- Thomas et al., Enhanced ocean carbon storage from anaerobic alkalinity Fig. 1 eastern bight and the adjacent central North Sea. The δAT Fig. 1. Total Alkalinity budget of the southeastern bight of the North (mix) was computed by considering that the water in the Sea. The upper panel shows the investigation area. The lower panel southeastern bight is mixed at time scales of six weeks with ) reveals the major fluxes, contributing to the alkalinity (AT budget. water of the adjacent open North Sea (Lenhart et al., 1995). Direct AT transports are shown as normal font, while AT changes ◦ − The southeastern bight encompasses areas south of 57.5 N due to uptake/release of NO are shown in italics. 3 and east of 5◦ E (Figs. 1, 2). The concentrations of the ad- jacent Central North Sea were averaged from data obtained in an area between 2◦ E to 5◦ E and 55◦ N to 57.5◦ N. The Frankignoulle, 2002; Schiettecatte et al., 2006, 2007). Ear- A was linearly interpolated between the measurements ob- lier studies indicate that the Wadden Sea, a tidal mud flat T tained every 3 months. The δA (Riv) represents the riverine area, bordering the southeastern region of the North Sea, T inputs and was computed for the observational time period might influence carbon cycling in the Southern North Sea according to Patsch¨ and Lenhart (2004). The alkalinity in- (Hoppema, 1990; Brasse et al., 1999; Reimer et al., 1999). puts from the Wadden Sea (δA (WaddenSea)) are computed In the present study, we unravel the seasonal variability of T as closing term of Eq. (1). Furthermore, we consider the ef- total alkalinity (A ) and pH in the North Sea, assess A gen- T T fect of nitrate (NO−) uptake and release during new produc- eration in tidal mud flat areas, and evaluate the effects of this 3 tion and aerobic respiration of organic matter on A (Gold- A generation on CO uptake in the North Sea at basin wide, T T 2 man and Brewer, 1980). In the view of Dickson’s defini- annual scales. tion (Dickson, 1981), AT is altered by the uptake/release of + − H , paralleling the uptake/release of NO3 because of the re- − quired conservation of electrical charges. Sources of NO3 2 Methods − include the water column inventory (δAT (column NO3 )), − This study is based on an extensive field data set collected rivers (δAT (riv. NO3 )) and the atmosphere (δAT (atm. − − in 2001/2002 with spatial coverage over the whole North NO3 )). The NO3 water column inventories were obtained Sea region. The data set includes the full carbonate sys- from our observations (e.g. Bozec et al., 2006), while riverine tem and related parameters. The cruises were carried out and atmospheric inputs were computed according to Patsch¨ on R/V Pelagia during all four seasons consecutively in Au- and Kuhn¨ (2008). We assumed that δAT (Riv), δAT (column − − gust/September and November 2001 and February and May NO3 ) and δAT (riv. NO3 ) are constant over two mixing − 2002. The entire North Sea was sampled by an adapted periods. We considered δAT (atm. NO3 ) constant over the 1◦ by 1◦ grid of 97 identical stations resulting in high- full annual cycle. The magnitude of atmospheric deposition resolution data sets appropriate for assessing seasonal vari- of ammonia (Patsch¨ and Kuhn,¨ 2008) is even smaller than ability (e.g. Bozec et al., 2006). Approximately 750 samples that of nitrate and was neglected in the present evaluation − per cruise were analyzed for AT by potentiometric determi- (see Fig. 3 or Table 1). Fluxes of NO3 due to water mass nation and for dissolved inorganic carbon (DIC) by coulo- exchange between the Southern North Sea and the adjacent − metric determination (Johnson et al., 1993). Uncertainties open North Sea have been neglected, since NO3 observa- were estimated in the range of 2–3 µmol kg−1 (=∼0.1%) for tions are similar for both regions. Biogeosciences, 6, 267–274, 2009 www.biogeosciences.net/6/267/2009/ H. Thomas et al.: Enhanced ocean carbon storage 269 δA (WaddenSea) A 5˚W 0˚ 5˚E 10˚E 5˚W 0˚ 5˚E 10˚E T T δA (mix) δA (Riv) a) b) T T 60˚N 60˚N 60˚N 60˚N new prod. / resp. of Corg : - δA(riv.NO)T3 55˚N 55˚N 55˚N 55˚N - δAT3 (column NO ) - δA(atm.NO)T3 50˚N 50˚N 50˚N 50˚N 2,310 10 5˚W 0˚ 5˚E 10˚E 5˚W 0˚ 5˚E 10˚E -1 2,305 5 −80 −40 0 40 80 −80 −40 0 40 80 −1 −1 AT anomaly (Feb.) [μmol kg ] AT anomaly (May) [μmol kg ] 2,300 0 -1 -1 μ 5˚W 0˚ 5˚E 10˚E 5˚W 0˚ 5˚E 10˚E T A[molkg] 2,295 -5 c) d) μ 60˚N 60˚N 60˚N 60˚N T 2,290 -10 A [ mol kg 6weeks ] 55˚N 55˚N 55˚N 55˚N 0.0 3.0 6.0 9.0 12.0 Month 50˚N 50˚N 50˚N 50˚N 5˚W 0˚ 5˚E 10˚E 5˚W 0˚ 5˚E 10˚E Fig.
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