Seasonal and Interannual Variability of the CO2 System in the Eastern Mediterranean Sea: A Case Study in the North Western Levantine Basin

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.

High-Resolution Reanalysis of the Mediterranean Sea Biogeochemistry (1999–2019)

“…The simulated trend of acidification (0.0006-0.0012 pH units y −1 ; higher in the eastern subbasins) agrees with previously estimated global acidification trends (e.g., 0.0016 pH units y −1 ; Gehlen et al, 2020;Kwiatkowski et al, 2020), and is confirmed by the regional observations reported in Flecha et al (2015) for the Gibraltar Strait over the 2012-2015 period (i.e., an annual pH variation of −0.0044 ± 0.00006) and in Wimart-Rousseau et al (2021) for the north-western Levantine basin (i.e., an annual pH variation of −0.0024 ± 0.0004).…”

“…Focusing on limited areas and periods (given the length of in situ time series), changes in the deep oxygen concentration have been reported for the Levantine basin (Sisma-Ventura et al, 2021), Gulf of Lions (Coppola et al, 2018), southern Adriatic Sea (Lipizer et al, 2014), and southern Aegean Sea (Velaoras et al, 2019). Additionally, acidification tendency signals have been observed in the western (Gibraltar Strait;Flecha et al, 2015) and eastern Mediterranean Sea (Cretan basin; Wimart-Rousseau et al, 2021), whereas changes in carbonate system variables [e.g., an increase in alkalinity and dissolved inorganic carbon (DIC) concentrations] have been indirectly estimated in relation to climate change and anthropogenic pressure (Schneider et al, 2010;Touratier and Goyet, 2011;Álvarez et al, 2014;Wimart-Rousseau et al, 2021).…”

Ocean acidification research in the Mediterranean Sea: Status, trends and next steps

“…The disproportionality in flux direction and magnitude leads to an annual air-sea CO 2 outgassing of 845±270 mmol C m −2 y −1 (0.98 Tg C y −1 ), indicating that the South-Eastern Levantine is a significant net source of CO 2 to the atmosphere in contrast to other studied sub-basins in the Mediterranean that predominantly act as net sinks (Sisma-Ventura et al, 2017). The temporal succession of the sink/source status for atmospheric CO 2 of the adjacent North-Western Levantine sub-basin is ascribed to A T variations that regulate the seawater pCO 2 and the air-sea CO 2 exchanges, in addition to the thermodynamic and biological drivers (Wimart-Rousseau et al, 2021). Further, Canu et al (2015) indicates that CO 2 fluxes vary from year to year as a function of both the input from the Strait of Gibraltar and meteorological forcings, quantified fluxes at the country level, and proved that plankton biological activity is a key factor in increasing the dissolution fluxes (sink) that can switch a sub-basin's role from source to sink.…”

How subsurface and double-core anticyclones intensify the winter mixed-layer deepening in the Mediterranean Sea

“…4a-f and 5a-f). The Ierapetra eddy is a recurrent long-lived and intense anticyclone formed southeast of Crete (Theocharis et al, 1993;Lascaratos and Tsantilas, 1997;Ioannou et al, 2017) and recently surveyed by the PERLE 1 and 2 campaigns (Ioannou et al, 2019;Wimart-Rousseau, 2021). Similarly to Eratosthenes anticyclones previously shown, the density anomaly is mostly driven by a warm core, allowing the temperature profile to be used as a proxy for stratification (Ioannou et al, 2017).…”