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Abstract
The spatiotemporal variability of aragonite saturation state (ΩAr) has been studied in the eastern shelf of the Gulf of Cádiz (GoC) (SW Iberian Peninsula). The study was carried out during the years 2014 and 2016 aboard twelve oceanographic cruises, along three or five transects, located between Cape Trafalgar and the Guadiana River. The GoC exhibited oversaturated of calcium carbonate with ΩAr mean values of 2.68 ± 0.30 in surface and 2.05 ± 0.15 in deep waters. pH, total alkalinity (TA), calcium concentration (Ca2+) and ΩAr showed a high variability within the surface mixed layer (SML, z < 100 m). Biological activity seemed to be the main process to determine the ΩAr variability in the SML, revealing a greater importance than temperature, mixing or air-sea processes. The buffer factors of the CO2 system in the GoC have been estimated in the SML waters, and they are related with the biological activity and the temperature changes. A decrease of pH and ΩAr in this SML with depth was observed, due to the increase of the respiratory processes. In deep waters (z > 100 m), TA and Ca2+ concentration presented a conservative behaviour related to the distribution of the different water masses located in the GoC. The vertical variation of ΩAr also depends on the degree of mineralization of these water masses, obtaining the maximum values in the Subtropical North Atlantic Central Water (100–200 m), minimum values in the Subpolar North Atlantic Central Water (about 400 m), and intermediate values associated to the presence of the Mediterranean Water (>500 m). Results showed a significative acidification of the coastal areas, for those depths lower than 100 m from 2006 to 2016, with a mean decrease of pH and ΩAr of −0.0089 and −0.0552 yr−1, respectively.
Highlights
They were differentiated in surface, intermediate and deep intervals, this last water mass was only registered during the 2016 cruises
A high variability was observed in this surface layer (Fig. 2), due to the intense biological activity and continental inputs from the rivers in the area, which are composed of carbonated nature basins
The results obtained in this study reveal that the Gulf of Cádiz (GoC) is oversaturated of CaCO3 in the whole water column, and the highest values of ΩAr were found in the surface mixed layer (SML)
Summary
This CO2 increase is mainly due to the rise of anthropogenic activities, such as, fossil fuel combustions and land use changes. In consequence, this will lead to pronounced variations in our climate by the end of this century (Siegenthaler et al, 2005). The oceans act as the major sinks for this increasing atmospheric CO2 It has been estimated, based on the 2019 assessment of the Global Ocean Project, that over the decade 2009–2018 oceans have absorbed about 23 ± 5% (2.6 ± 0.5 PgC yr−1) of the total CO2 emissions (Hauck et al, 2020). The fundamental role of the oceans as buffers of natural and anthropogenic CO2 emissions, as well as, in mitigating climate change (Landschützer et al, 2019)
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