Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments

Abstract

Abstract. The coastal ocean is a crucial link between land, the open ocean and the atmosphere. The shallowness of the water column permits close interactions between the sedimentary, aquatic and atmospheric compartments, which otherwise are decoupled at long time scales ( ∼ =1000 yr) in the open oceans. Despite the prominent role of the coastal oceans in absorbing atmospheric CO 2 and transferring it into the deep oceans via the continental shelf pump, the underlying mechanisms remain only partly understood. Evaluating observations from the North Sea, a NW European shelf sea, we provide evidence that anaerobic degradation of organic matter, fuelled from land and ocean, generates total alkalinity (A T ) and increases the CO 2 buffer capacity of seawater. At both the basin wide and annual scales anaerobic A T generation in the North Sea's tidal mud flat area irreversibly facilitates 7-10%, or taking into consideration benthic denitrification in the North Sea, 20-25% of the North Sea's overall CO 2 uptake. At the global scale, anaerobic A T generation could be accountable for as much as 60% of the uptake of CO 2 in shelf and marginal seas, making this process, the anaerobic pump, a key player in the biological carbon pump. Under future high CO 2 conditions oceanic CO 2 storage via the anaerobic pump may even gain further relevance because of stimulated ocean productivity.

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

“…Thomas et al (2009) A T release from sediments has also been the subject of a number of studies on coastal systems other than the North Sea. Based on basin-wide budget calculations, Gustafsson et al (2014) estimated that sediments of the Baltic Sea generate A T with a mean rate of 2.4 mmol m −2 d −1 , which is on the same order as the A T input by rivers in that basin.…”

“…When coining this hypothesis, Thomas et al (2009) estimated that alkalinity generation in Wadden Sea sediments could be responsible for 20-25 % of the total CO 2 uptake of the entire North Sea. However, Moore et al (2011) estimated much smaller A T fluxes between the Wadden Sea and the North Sea based on activity measurements of radium isotopes in surface water.…”

“…However, Moore et al (2011) estimated much smaller A T fluxes between the Wadden Sea and the North Sea based on activity measurements of radium isotopes in surface water. While the strength of the A T flux between the Wadden Sea and the North Sea remains under debate, the Wadden Sea has been identified as a source of A T for the North Sea (Brasse et al, 1999;Thomas et al, 2009;Provoost et al, 2010;Moore et al, 2011). However, it remains an open question whether this A T export from the Wadden Sea drives any CO 2 uptake in the North Sea (maybe the A T -induced CO 2 uptake has already taken place in the Wadden Sea), and also, how important the sediments of the North Sea proper are in driving CO 2 uptake in the North Sea.…”

Impact of ocean acidification on benthic and water column ammonia oxidation

“…However, the impact of OA on nitrification within sediments has not been examined. It is conceivable that alkalinity generation in sediments [Thomas et al, 2009] and/or low-pH-adaptation by microorganisms may moderate the effects of decreasing pH on nitrification in sediments. Here we test the hypothesis that CO 2 -induced acidification causes a reduction in water-column and surface-sediment NH 3 oxidation using data generated from two controlled manipulative incubations and a set of field-measurements in the vicinity of natural CO 2 vents.…”

Key processes in the coupled carbon, nitrogen, and phosphorus cycling of the Baltic Sea

“…Anaerobic mineralization in general results in a TA generation (for example as a result of denitrification) that can enhance the buffering capacity for CO 2 in the water column and increase the absorption of atmospheric CO 2 (or decrease CO 2 outgassing) (e.g. Thomas et al 2009). …”