Abstract
Continental shelf seas may have a significant role in oceanic uptake and storage of carbon dioxide (CO2) from the atmosphere through a ‘continental shelf pump’ mechanism. The northwest European continental shelf, in particular the Celtic Sea (50°N 8°W), was the target of extensive biogeochemical sampling from March 2014 to September 2015 as part of the UK Shelf Sea Biogeochemistry research programme (UK-SSB). Here, we use the UK-SSB carbonate chemistry and macronutrient measurements to investigate the biogeochemical seasonality in this temperate, seasonally stratified system. Following the onset of stratification, near-surface biological primary production during spring and summer removed dissolved inorganic carbon and nutrients, and a fraction of the sinking particulate organic matter was subsequently remineralised beneath the thermocline. Water column inventories of these variables throughout 1.5 seasonal cycles, corrected for air-sea CO2 exchange and sedimentary denitrification and anammox, isolated the combined effect of net community production (NCP) and remineralisation on the inorganic macronutrient inventories. Overall inorganic inventory changes suggested that a significant fraction (>50%) of the annual NCP of around 3 mol-C m–2 yr–1 appeared to be stored within a long-lived organic matter (OM) pool with a lifetime of several months or more. Moreover, transfers into and out of this pool appeared not to be in steady state over the one full seasonal cycle sampled. Accumulation of such a long-lived and potentially C-rich OM pool is suggested to be at least partially responsible for the estimated net air-to-sea CO2 flux of ∼1.3 mol-C m–2 yr–1 at our study site, while providing a mechanism through which a nutrient-conserving continental shelf pump for CO2 could potentially operate in this and other similar regions.
Highlights
The ocean is an important buffer for atmospheric carbon dioxide (CO2), naturally storing a large pool of dissolved inorganic carbon (CT), and absorbing about a quarter of annual anthropogenic CO2 emissions (Le Quéré et al, 2009) and thereby mitigating their climatic impacts (IPCC, 2013)
Our analysis indicated that the CCS site acted as a net atmospheric CO2 sink: the mean air-to-sea CO2 flux at CCS from 21st March 2014 to the same date in 2015 was 1.3 ± 0.3 mol m-2 yr-1 (Fig. 2). The magnitude of this flux is consistent with 330 previous Celtic Sea studies, it is towards their lower end
We suggest that the CT loss was a result of continued net community production (NCP) and organic matter (OM) formation, but that this was using regenerated dissolved inorganic nitrogen (DIN) within the surface layer as its N source rather than drawing from the inorganic pool, which was locked away by stratification in the deep layer, as has been observed in the North Sea (Bozec et al, 2006)
Summary
The ocean is an important buffer for atmospheric carbon dioxide (CO2), naturally storing a large pool of dissolved inorganic carbon (CT), and absorbing about a quarter of annual anthropogenic CO2 emissions (Le Quéré et al, 2009) and thereby mitigating their climatic impacts (IPCC, 2013). Transfer from the surface to the deep interior is a much slower rate-limiting step, which is performed by the solubility, soft tissue, carbonate and mixed-layer ‘pumps’ in the open ocean (Volk and Hoffert, 1985; Dall’Olmo et al., 2016). Tsunogai et al (1999) first proposed the existence of an additional ‘continental shelf pump’ (CSP) that facilitates CT transfer from shallow coastal surface waters into the interior ocean, and thereby enhances local oceanic CO2 uptake. In order to prevent the fixed CO2 from being quickly released back into the atmosphere, the carbon-enriched shelf waters need to be transported into the open ocean, beneath the seasonal thermocline (Tsunogai et al, 1999; Thomas et al, 2004)
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