Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments

Abstract

Abstract. The coastal ocean constitutes the crucial link between land, the open ocean and the atmosphere. Furthermore, its shallow water column permits close interactions between the sedimentary and atmospheric compartments, which otherwise are decoupled at short time scales (<1000 yr) in the open oceans. Despite the prominent role of the coastal oceans in absorbing atmospheric CO2 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 alkalinity (AT) and increases the CO2 buffer capacity of seawater. At both the basin wide and annual scales anaerobic AT 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 CO2 uptake. At the global scale, anaerobic AT generation could be accountable for as much as 60% of the uptake of CO2 in shelf and marginal seas, making this process, the anaerobic pump, a key player in the biological carbon pump. Under future high CO2 conditions oceanic CO2 storage via the anaerobic pump may even gain further relevance because of stimulated ocean productivity.

Benthic pH gradients across a range of shelf sea sediment types linked to sediment characteristics and seasonal variability

“…As in this study, these studies reported that the pH minima approximated in depth with the OPD and was probably linked to intensified oxidation of reduced metabolites (Fe, Mn, HS) release from sub-oxic or anaerobic degradation (Boudreau and Canfield 1988 , S -,) while deeper down pH increases were observed in some sediments. These could be due to the isolated relocation of bottom water via macrofaunal bioirrigation or burrowing, or more generally due to anaerobic alkalinity generation (Thomas et al 2008). This interpretation links well with Fe and Mn pore-water profiles taken at Site A, which are presented in Figs.…”

Seasonal cycle of N:P:TA stoichiometry as a modulator of CO2 buffering in eastern boundary upwelling systems

“…However, in this study, we suggest that shelf production of TA has an important local and regional role through mitigation of low and high pCO 2 in naturally corrosive upwelling systems. Thomas et al [2009] observed the same effect in the North Sea and noted that this may increase the capacity of the continental shelf pump to export CO 2 . We propose the following hypothesis: nutrient trapping (POC/PON) and resulting anaerobic remineralization on the continental shelf of upwelling systems buffers pCO 2 and by means of enhanced TA flux within the sediments and the BBL.…”

“…Hu and Cai [2011] estimated that the net global production of TA in continental margins was 4-5 Tmol yr -1 -conservative compared to previous estimates (16-31 Tmol yr -1 ) [Thomas et al, 2009]. Thomas et al [2009] reported that buffering by anaerobic biogenic TA in the North Sea may account for up to 25% of CO 2 uptake.…”

“…The two aforementioned mechanisms are also responsible for production of total alkalinity (TA) that in turn has a buffering effect on pH, calcium carbonate saturation state ( ), and pCO 2 [Wolf-Gladrow et al, 2007]. Hu and Cai [2011] estimated that the net global production of TA in continental margins was 4-5 Tmol yr -1 -conservative compared to previous estimates (16-31 Tmol yr -1 ) [Thomas et al, 2009]. Thomas et al [2009] reported that buffering by anaerobic biogenic TA in the North Sea may account for up to 25% of CO 2 uptake.…”

Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere

“…The acid/base chemistry of estuaries and coastal ecosystems is particularly susceptible to global change because of additional water, solute, and sediment inputs from the continents and because the buffering capacities in such ecosystems are typically lower than those in the open ocean. Processes such as riverine freshwater inputs, acid rain, atmospheric dry deposition, sediment denitrification, anaerobic organic matter decomposition, and submarine groundwater discharge all have large impacts on the coastal inorganic C system (Doney et al 2007;Fennel et al 2008;Thomas et al 2009;Borges and Gypens 2010). Most of these processes are susceptible to anthropogenic forcing, and coastal ecosystems are particularly sensitive to higher pCO 2 .…”