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The most active deep convection area in the western Mediterranean Sea is located in the Gulf of Lions. Recent studies in this area provides some insights on the complexity of the physical dynamics of convective regions, but very little is known about their impacts on the biogeochemical properties. The CASCADE (CAscading, Surge, Convection, Advection and Downwelling Events) cruise, planed in winter 2011, give us the opportunity to compare vertical profiles of properties sampled either during stratified conditions or after/during a convection event. In the present study, we focus on the distributions of the carbonate system properties (mainly total alkalinity, A T ; and total dissolved inorganic carbon, C T ) because, in the context of the climate change, deep convection areas are suspected to significantly increase the sequestration of anthropogenic CO 2 (C ANT ). Given its limited size, the impact of the Mediterranean Sea on the global carbon budget is probably minor but this marginal sea can be used as a laboratory to better understand carbon sequestration and its transfer to the basin interior by deep convection processes. Distributions of A T and C T , both measured from bottle samples, and that of C ANT (estimated with the TrOCA approach) are first analyzed in the light of other key properties (salinity, temperature, and dissolved oxygen). An objective interpolation procedure is then applied to estimate C T and A T from CTD measured properties. With this procedure, the vertical resolution goes from a maximum of 32 samples per station to one property estimate every meter (more detailed distributions are obtained). Results provide arguments to conclude that C ANT is rapidly transferred to the deepest layer due to deep convection events. During deep convection events, the increase of C ANT in the water column is positively correlated to that of potential density and oxygen content. The challenge of quantifying the amount of sequestered carbon is however not resolved due to the complexity and the highly dynamical nature of the convective regions. Deep convection in the Gulf of Lions, in parallel with cascading along the continental slope, could thus potentially explain the very high levels of both C ANT and acidification estimated in the deep layers of the western Mediterranean Sea. [ABSTRACT FROM AUTHOR]

A compilation of data from several cruises between 1998 and 2013 was used to derive polynomial fits that estimate total alkalinity (AT) and total dissolved inorganic carbon (CT) from measurements of salinity and temperature in the Mediterranean Sea surface waters. The optimal equations were chosen based on the 10-fold cross-validation results and revealed that second- and third-order polynomials fit the AT and CT data respectively. The AT surface fit yielded a root mean square error (RMSE) of ±10.6 μmol kg-1, and salinity and temperature contribute to 96% of the variability. Furthermore, we present the first annual mean CT parameterization for the Mediterranean Sea surface waters with a RMSE of ±14.3 μmol kg-1. Excluding the marginal seas of the Adriatic and the Aegean, these equations can be used to estimate AT and CT in case of the lack of measurements. The identified empirical equations were applied on the 0.25° climatologies of temperature and salinity, available from the World Ocean Atlas 2013. The 7-year averages (2005-2012) showed that AT and CT have similar patterns with an increasing eastward gradient. The variability is influenced by the inflow of cold Atlantic waters through the Strait of Gibraltar and by the oligotrophic and thermohaline gradient that characterize the Mediterranean Sea. The summer-winter seasonality was also mapped and showed different patterns for AT and CT. During the winter, the AT and CT concentrations were higher in the western than in the eastern basin. The opposite was observed in the summer where the eastern basin was marked by higher AT and CT concentrations than in winter. The strong evaporation that takes place in this season along with the ultra-oligotrophy of the eastern basin determines the increase of both AT and CT concentrations. [ABSTRACT FROM AUTHOR]

We relate here the distributions of two carbonate system key properties (total alkalinity, AT; and total dissolved inorganic carbon, CT) measured along a section in the Mediterranean Sea, going from Marseille (France) to the south of the Cyprus Island, during the 2008 BOUM cruise. The three main objectives of the present study are (1) to draw and comment on the distributions of AT and CT in the light of others properties like salinity, temperature, and dissolved oxygen, (2) to estimate the distribution of the anthropogenic CO2 (CANT) in the intermediate and the deep waters, and (3) to calculate the resulting variation of pH (acidification) since the beginning of the industrial era. Since the calculation of CANT is always an intense subject of debate, we apply two radically different approaches to estimate CANT: the very simple method TrOCA and the MIX approach, the latter being more precise but also more diffucult to apply. A clear picture for the AT and the CT distributions is obtained: the mean concentration of A is higher in the oriental basin while that of CT is higher in the occidental basin of the Mediterranean Sea, fully coherent with the previous published works. Despite of the two very different approaches we use here (TrOCA and MIX), the estimated distributions of CANT are very similar. These distributions show that the minimum of CANT encountered during the BOUM cruise is higher than 46.3 µmol kg-1 (TrOCA) or 48.8 µmol kg-1 (MIX). All Mediterranean water masses (even the deepest) appear to be highly contaminated by CANT, as a result of the very intense advective processes that characterize the recent history of the Mediterranean circulation. As a consequence, unprecedented levels of acidification are reached with an estimated decrease of pH since the pre-industrial era of -0.148 to -0.061 pH unit, which places the Mediterranean Sea as one of the most acidified world marine ecosystem. [ABSTRACT FROM AUTHOR]

Abstract: In the Mediterranean Sea the carbon chemistry is poorly known. However, the impact of the regional and large-scale anthropogenic pressures on this fragile environment rapidly modifies the distribution of the carbonate system key properties like C T (total dissolved inorganic carbon), A T (total alkalinity), C ANT (anthropogenic CO2), and pH. This leads inexorably to the acidification of its waters. In order to improve our knowledge, we first develop interpolation procedures to estimate C T and A T from oxygen, salinity, and temperature data using all available data from the EU/MEDAR/MEDATLAS II database. The acceptable levels of precision obtained for these estimates (6.11μmol–kg−1 for C T and 6.08μmolkg−1 for A T) allow us to draw the distribution of C ANT (with an uncertainty of 6.75μmolkg−1) using the Tracer combining Oxygen, inorganic Carbon, and total Alkalinity (TrOCA) approach. The results indicate that: 1) all Mediterranean water bodies are contaminated by anthropogenic carbon; 2) the lowest concentration of C ANT is 37.5μmolkg−1; and 3) the western basin is more contaminated than the Eastern basin. After reconstructing the distribution of key properties (C T, A T, C ANT) for four periods of time (between 1986 and 2001) along a west–east section throughout the whole Mediterranean Sea, we analyze the impact of the Eastern Mediterranean Transient (EMT). Not only has the concentration of C ANT increased (especially in the intermediate and the bottom layers of the eastern basin, during and after the EMT), but also the distribution of all properties has been considerably perturbed. This is discussed in detail. For the first time, the level of acidification is estimated for the Mediterranean Sea. Our results indicate that for the year 2001 all waters (even the deepest) have been acidified by values ranging from −0.14 to −0.05 pH unit since the beginning of the industrial era, which is clearly higher than elsewhere in the open ocean. Given that the pH of seawater may affect a very large number of chemical and biological processes, our results stress the necessity to develop new programs of research to understand and then predict the evolution of the carbonate system properties in the Mediterranean Sea. [Copyright &y& Elsevier]

Abstract: Monthly observations accumulated over more than a decade at the DYFAMED time-series station allow us to estimate the temporal evolution of anthropogenic CO2 in the western Mediterranean Sea. This objective is reached by using recognized interpolation procedures to reconstruct the incomplete distributions of measured total dissolved inorganic carbon and total alkalinity. These reconstructed fields, associated with those available for dissolved oxygen and temperature, are used to estimate the distribution of anthropogenic CO2. This is done with the recently developed Tracer combining Oxygen, inorganic Carbon, and total alkalinity (TrOCA) approach. The main results indicate that (1) the concentrations of anthropogenic CO2 are much higher than those found in the Atlantic Ocean (the minimum concentration at the DYFAMED site is ∼50μmolkg−1), and (2) the temporal trend for anthropogenic CO2 is decreasing, especially in the intermediate and the deep layers of the water column at the DYFAMED site. This decrease in anthropogenic CO2 is significantly correlated with a decrease in the dissolved oxygen and with an increase in both salinity and temperature. These trends are discussed in the light of recent published works that propose explanations for the observed increases in salinity and temperature that occurred in the western basin since the 1950s. We conclude that the decrease in anthropogenic CO2 probably resulted from an invasion of old water masses. Different hypotheses on the origin of these water masses are considered and several arguments indicate that the eastern Mediterranean transient (EMT) could have played an important role in the observed decrease in anthropogenic CO2 concentrations at the DYFAMED site. [Copyright &y& Elsevier]

Factors controlling ocean acidification and its temporal variations were studied over the 1995–2011 period at the Dyfamed site at 10 m depth, in the North Mediterranean Sea. The results indicated a mean annual decrease of 0.003 ± 0.001 pH units on the seawater scale. The seasonal variability was characterized by a pH decrease during springtime and a strong pH increase in late fall. Anthropogenic CO 2 (C ANT ) absorption by the ocean was the key driver of seawater acidification in this region, accounting for about 70% of the observed drop in pH, followed by water temperature (about 30%). The total inorganic carbon (C T ) data showed a C T increase of 30.0 ± 1.0 μmol kg −1 per decade. This decadal increase is mainly due to the C ANT penetration (43.2 μmol kg −1 per decade) in surface waters, which is mitigated for by relatively small opposing changes in C T due to physical and biological processes. [ABSTRACT FROM AUTHOR]

This study presents an estimation of the anthropogenic CO 2 (C ANT ) concentrations and acidification (ΔpH=pH 2013 –pH pre-industrial ) in the Mediterranean Sea, based upon hydrographic and carbonate chemistry data collected during the May 2013 MedSeA cruise. The concentrations of C ANT were calculated using the composite tracer TrOCA. The C ANT distribution shows that the most invaded waters (>60 µmol kg −1 ) are those of the intermediate and deep layers in the Alboran, Liguro- and Algero-Provencal Sub-basins in the Western basin, and in the Adriatic Sub-basin in the Eastern basin. Whereas the areas containing the lowest C ANT concentrations are the deep layers of the Eastern basin, especially those of the Ionian Sub-basin, and those of the northern Tyrrhenian Sub-basin in the Western basin. The acidification level in the Mediterranean Sea reflects the excessive increase of atmospheric CO 2 and therefore the invasion of the sea by C ANT . This acidification varies between −0.055 and −0.156 pH unit and it indicates that all Mediterranean Sea waters are already acidified, especially those of the Western basin where ΔpH is rarely less than −0.1 pH unit. Both C ANT concentrations and acidification levels are closely linked to the presence and history of the different water masses in the intermediate and deep layers of the Mediterranean basins. Despite the high acidification levels, both Mediterranean basins are still highly supersaturated in calcium carbonate minerals. [ABSTRACT FROM AUTHOR]