Abstract
The seasonal variability of the carbonate system in the eastern Mediterranean Sea (EMed) was investigated based on discrete total alkalinity (AT), total dissolved inorganic carbon (CT), and pH measurements collected during three cruises around Crete between June 2018 and March 2019. This study presents a detailed description of this new carbonate chemistry dataset in the eastern Mediterranean Sea. We show that the North Western Levantine Basin (NWLB) is unique in terms of range of AT variation vs. CT variation in the upper water column over an annual cycle. The reasons for this singularity of the NWLB can be explained by the interplay between strong evaporation and the concomitant consumption of CT by autotrophic processes. The high range of AT variations, combined to temperature changes, has a strong impact on the variability of the seawater pCO2 (pCO2SW). Based on Argo float data, an entire annual cycle for pCO2SW in the NWLB has been reconstructed in order to estimate the temporal sequence of the potential “source” and “sink” of atmospheric CO2. By combining this dataset with previous observations in the NWLB, this study shows a significant ocean acidification and a decrease in the oceanic surface pHT25 of −0.0024 ± 0.0004 pHT25 units.a–1. The changes in the carbonate system are driven by the increase of atmospheric CO2 but also by unexplained temporal changes in the surface AT content. If we consider that the EMed will, in the future, encounter longer, more intense and warmer summer seasons, this study proposes some perspectives on the carbonate system functioning of the “future” EMed.
Highlights
Since the beginning of the industrial era, the rise in atmospheric CO2 due to anthropogenic activities is considered to be the main factor responsible for current climate change (IPCC, 2018)
Several studies have reported a marked decline in the pH of the Mediterranean Sea (MedSea) over the last few decades (e.g., Touratier and Goyet, 2011; Hassoun et al, 2015b; Palmiéri et al, 2015; Flecha et al, 2019)
When compared to other oceanic areas, including the Western Mediterranean Sea (WMed), the oligotrophic Eastern Mediterranean Sea (EMed) (Pujo-Pay et al, 2011) is characterised by low primary production rates (Moutin and Raimbault, 2002). This low productivity reduces the vertical gradients of dissolved inorganic carbon, making the detection and understanding of decadal and seasonal changes in the carbonate system challenging in this area
Summary
Since the beginning of the industrial era, the rise in atmospheric CO2 due to anthropogenic activities is considered to be the main factor responsible for current climate change (IPCC, 2018). When compared to other oceanic areas, including the WMed, the oligotrophic EMed (Pujo-Pay et al, 2011) is characterised by low primary production rates (Moutin and Raimbault, 2002) This low productivity reduces the vertical gradients of dissolved inorganic carbon, making the detection and understanding of decadal and seasonal changes in the carbonate system challenging in this area. In the EMed, time-series measurements are scarce and mostly based in the Cretan Sea (Petihakis et al, 2018) or coastal sites such as the Lebanese coast (Hassoun et al, 2019) or the Israeli coast (SismaVentura et al, 2017), precluding a rigorous description of the temporal variability of the carbonate system in the open-ocean EMed. In the MedSea open-ocean, studies based on data derived from satellite observations have been conducted to decipher, over a seasonal and interannual scale, the variations in pCO2 (D’Ortenzio et al, 2008; Taillandier et al, 2012). Some hypotheses on the future of the carbonate system functioning of the EMed will be discussed
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