Multidecadal accumulation of anthropogenic and remineralized dissolved inorganic carbon along the Extended Ellett Line in the northeast Atlantic Ocean

Abstract

Marine carbonate chemistry measurements have been carried out annually since 2009 during UK research cruises along the Extended Ellett Line (EEL), a hydrographic transect in the northeast Atlantic Ocean. The EEL intersects several water masses that are key to the global thermohaline circulation, and therefore the cruises sample a region in which it is critical to monitor secular physical and biogeochemical changes. We have combined results from these EEL cruises with existing quality-controlled observational data syntheses to produce a hydrographic time series for the EEL from 1981 to 2013. This reveals multidecadal increases in dissolved inorganic carbon (DIC) throughout the water column, with a near-surface maximum rate of 1.80 ± 0.45 μmol kg À1 yr À1 . Anthropogenic CO 2 accumulation was assessed, using simultaneous changes in apparent oxygen utilization (AOU) and total alkalinity (TA) as proxies for the biogeochemical processes that influence DIC. The stable carbon isotope composition of DIC (δ 13 C DIC ) was also determined and used as an independent test of our method. We calculated a volume-integrated anthropogenic CO 2 accumulation rate of 2.8 ± 0.4 mg C m À3 yr À1 along the EEL, which is about double the global mean. The anthropogenic CO 2 component accounts for only 31 ± 6% of the total DIC increase. The remainder is derived from increased organic matter remineralization, which we attribute to the lateral redistribution of water masses that accompanies subpolar gyre contraction. Output from a general circulation ecosystem model demonstrates that spatiotemporal heterogeneity in the observations has not significantly biased our multidecadal rate of change calculations and indicates that the EEL observations have been tracking distal changes in the surrounding North Atlantic and Nordic Seas.

Controls on Open‐Ocean North Atlantic ΔpCO2 at Seasonal and Interannual Time Scales Are Different

“…For example, in a model study Bennington et al () found that biological activity dominated the seasonal cycle of seawater p CO 2 , but not its interannual variability. On these longer time scales, the North Atlantic Oscillation (NAO), the dominant climate variability mode in the region, could affect oceanic CO 2 uptake (Gruber et al, ) and interior CO 2 storage (Humphreys et al, ). In a positive NAO phase, the North Atlantic Current increases in strength (Visbeck et al, ), bringing warm waters with relatively low DIC concentration into the subpolar northeast Atlantic.…”

Response to anonymous reviewer #2

“…Although the mean (±SD) residual for 58GS20030922 was greater ( 0.19 ± 0.16 ‰), it must firstly be considered that this depends on only three matching 13 C DIC measurements, and secondly that it is still within the range of the accuracy of 0.1-0.2 ‰ reported for its parent data set (Schmittner et al, 2013). These cruises and JR302 span a time interval of just over 20 years, so invasion of anthropogenic CO 2 could have modified the 13 C DIC through the Suess effect (Keeling, 1979), potentially inhibiting the use of cross-over anal- ysis. However, in the deeper part of the water column in this region, 13 C DIC has been observed to change at a rate of less than 0.01 ‰ yr 1 in recent decades .…”

“…Oceanic measurements during the past few decades (Quay et al, 2007) and over longer timescales in ice cores (Rubino et al, 2013) show that the rise in pCO 2 and DIC has been accompanied by a decline in the carbon-13 content of DIC, relative to carbon-12 (reported as 13 C, Eqs. 1 and 2), a phenomenon called the "Suess effect" (Keeling, 1979). This is caused by the lower 13 C of anthropogenic CO 2 relative to pre-industrial and present-day atmospheric CO 2 , and it provides another approach to constrain the spatial distribution and inventory of anthropogenic DIC (e.g.…”

Drivers of 21<sup>st</sup> Century carbon cycle variability in the North Atlantic Ocean

“…The Atlantic interior carbon content has been studied by repeatedly revisiting hydrographic sections over several years to decades. These studies find significant decadal biogeochemical variability associated with ocean circulation, and the anthropogenic increase is less obvious than at fixed observatories 30 (Wanninkhof et al, 2010;Humphreys et al, 2016).…”