The impact of sedimentary alkalinity release on the water column CO<sub>2</sub> system in the North Sea

Abstract

Abstract. It has been previously proposed that alkalinity release from sediments can play an important role in the carbonate dynamics on continental shelves, lowering the pCO 2 of seawater and hence increasing the CO 2 uptake from the atmosphere. To test this hypothesis, sedimentary alkalinity generation was quantified within cohesive and permeable sediments across the North Sea during two cruises in September 2011 (basin-wide) and June 2012 (Dutch coastal zone). Benthic fluxes of oxygen (O 2 ), alkalinity (A T ) and dissolved inorganic carbon (DIC) were determined using shipboard closed sediment incubations. Our results show that sediments can form an important source of alkalinity for the overlying water, particularly in the shallow southern North Sea, where high A T and DIC fluxes were recorded in nearshore sediments of the Belgian, Dutch and German coastal zone. In contrast, fluxes of A T and DIC are substantially lower in the deeper, seasonally stratified, northern part of the North Sea. Based on the data collected, we performed a model analysis to constrain the main pathways of alkalinity generation in the sediment, and to quantify how sedimentary alkalinity drives atmospheric CO 2 uptake in the southern North Sea. Overall, our results show that sedimentary alkalinity generation should be regarded as a key component in the CO 2 dynamics of shallow coastal systems.

Structure and functioning of the acid–base system in the Baltic Sea

“…Generally, benthic processes linked to early diagenesis of organic matter can be an important source of A T to the water column (Brenner et al, 2016). This corresponds to the findings by Gustafsson et al (2014b), who, in a model study, found that external sinks and sources of A T in the Baltic Sea are imbalanced and cannot reproduce the observed A T inventory and that an internal A T source must exist in the Baltic Sea.…”

“…According to Eq. (2) this increases the seawater A T and thus raises the pH during biomass production (Brewer and Goldman, 1976). This assimilation also decreases the pCO 2 and therefore reinforces the drop in pCO 2 by biomass production.…”

“…Whereas the A T in surface water is mainly controlled by the mixing of different water masses, the deep-water A T distribution additionally depends on the organic-matter transformations by various redox processes (Brenner et al, 2016;Krumins et al, 2013;Thomas et al, 2009;Schulz and Zabel, 2006). A certain fraction of the organic matter produced in the euphotic zone is exported to deeper water layers and to surface sediments, where it undergoes mineralization, produces CO 2 and changes the alkalinity.…”

Long‐Term and Seasonal Trends in Estuarine and Coastal Carbonate Systems

“…Using the basin characteristics (Table 1), the A T increase in Ringkøbing Fjord from April to September corresponds to an average areal flux of approximately 7.5 mmol m À2 d À1 . In fact, measurements by Brenner et al (2016) at numerous sites in the Southern North Sea revealed sediment to water A T fluxes in a range 0-21-.4 mmol m À2 d À1 (mean: 6.6 mmol m À2 d À1 ). In a numerical experiment, including A T sources and sinks in both sediments and water column, Brenner et al (2016) calculated a net A T generation in the water column of 2.4 mmol m À2 d À1 .…”

“…In fact, measurements by Brenner et al (2016) at numerous sites in the Southern North Sea revealed sediment to water A T fluxes in a range 0-21-.4 mmol m À2 d À1 (mean: 6.6 mmol m À2 d À1 ). In a numerical experiment, including A T sources and sinks in both sediments and water column, Brenner et al (2016) calculated a net A T generation in the water column of 2.4 mmol m À2 d À1 . They identified the main A T sources as pelagic primary production and benthic CaCO 3 dissolution, denitrification, and sulfate reduction.…”