Water column distribution of stable isotopes and carbonate properties in the South-eastern Levantine basin (Eastern Mediterranean): Vertical and temporal change

Highlights

• Water column distribution of stable isotopes and carbonate properties were studied in the Southeast Mediterranean.

• We observed δ¹³C_DIC temporal change, propagating to depth of about 700 m.

• The estimated anthropogenic CO₂ accumulation rate was 0.38 ± 0.12 mol C m^− 2 yr^− 1.

• We illustrate down-welling isotopically light C_ant during convection events.

Abstract

Water column distributions of the oxygen isotopic composition of sea-water (δ¹⁸O_SW) and the stable carbon isotope ratio of dissolved inorganic carbon (δ¹³C_DIC), total alkalinity (A_T) and the pH (total scale) at 25 °C (^25 °CpH_Total) were investigated along the Southeast Mediterranean (SE-Med) shelf and open water, during 2009–2010. While, the vertical profiles of δ¹⁸O_SW lacked a clear depth signature, those of δ¹³C_DIC were characterized by a structure that reflects the major water masses in the Levantine basin, with noticeable vertical gradients.

The δ¹³C_DIC Suess effect of the Levantine water column was estimated from the difference between the average profiles of 1988 and 2009–2010 (Δδ¹³C_DIC). We observed δ¹³C_DIC temporal change, which indicates propagation of anthropogenic CO₂ (C_ant) to depth of about 700 m. The Modified Atlantic Water (MAW; 0–200 m) and the Levantine Intermediate Water (LIW; 200–400 m) exhibited a depletion rate of − 0.13 ± 0.03 and − 0.11 ± 0.03‰ decade^− 1, respectively, representing ~ 50% of the atmospheric change, while the deep water of the Adriatic source (700–1300 m) did not change during this period. A Δδ¹³C_DIC depletion trend was also recognized below 1350 m, corresponding to the Aegean source deep water (EMDW_Aeg) and therefore associated to the Eastern Mediterranean Transient (EMT) event.

Anthropogenic CO₂ accumulation rate of 0.38 ± 0.12 mol C m^− 2 yr^− 1 for the upper 700 m of the SE-Med, over the last 22 yr, was estimated on the basis of mean depth-integrated δ¹³C_DIC Suess effect profile. Our results confirm lower accumulation rate than that of the subtropical North Atlantic, resulting due to the super-saturation with respect to CO₂ of the well-stratified Levantine surface water. High pCO₂ saturation during summer (+ 150 μatm), in oppose to a small degree of under-saturation in winter (− 30 μatm) was calculated from surface water A_T and ^25 °CpH_Total data. However, the δ¹³C_DIC depletion trend of the LIW and the EMDW_Aeg supports isotopically light C_ant penetrating into the Levantine interior during convection events, such as the EMT.

Introduction

The Eastern Mediterranean, in particular the Levantine basin, is one of the most evaporative marginal seas in the world — a crucial area in the larger framework of the Mediterranean conveyor belt (Millot and Taupier-Letage, 2005, Pinardi and Masetti, 2000, Tanhua et al., 2013). The water body in this region is both stratified and ultra-oligotrophic, due to the advection of low salinity, nutrients-poor Modified Atlantic Water (MAW) and limited freshwater input (Béthoux et al., 1999, Hecht., 1992). Hence, this region is an ideal location to investigate water column temporal and vertical changes, occurring on relative short time scales, annual to decadal.

The importance of the Levantine basin to the formation of mid-depth Levantine Intermediate Water (LIW) was described by Hecht et al. (1988). This water mass evolves from down-welling of saline surface water, the Levantine Surface Water (LSW), during the winter, and eventually circulates through the Gibraltar strait to high latitudes of the Atlantic ocean at depths around 1000 m, contributing heat, salt and mass to northward transport in the Atlantic (Bigg et al., 2003). Two deep water masses are currently observed below the LIW: the Eastern Mediterranean deep water (EMDW) from an Adriatic source (EMDW_Adr) and below it the EMDW from an Aegean source (EMDW_Aeg; Kress et al., 2014, Roether et al., 1996). Several studies have addressed in details the impact of the Eastern Mediterranean Transient (EMT) event on the hydrological properties of the Eastern Mediterranean (Klein et al., 1999, Roether et al., 1996, Roether et al., 2007). According to these studies, the current deep water stratigraphy reflects a switch in the deep water formation from the Adriatic to the Aegean that occurred during the early 1990's. Recent analysis of the oceanographic characteristics of the Eastern Mediterranean over the last three decades by Cardin et al. (2015) shows that the state of the EM deep water structure is still far from the pre-EMT conditions, as observed by the signature of the alternation of the two dense water sources, Adriatic and Aegean.

The Mediterranean Sea is considered a region capable of absorbing a considerable amount of anthropogenic CO₂ (C_ant) per unit area. Its high total alkalinity (Copin-Montégut, 1993, Cossarini et al., 2015, Schneider et al., 2007) gives it greater chemical capacity to take up C_ant (Álvarez et al., 2014). Furthermore, its deep waters are ventilated on relatively short time scales (ca. 100 yr; Schneider et al., 2014, Stöven and Tanhua, 2014), allowing penetration of C_ant (Palmiéri et al., 2015, Schneider et al., 2010). Anthropogenic CO₂ however, cannot be measured directly because the anthropogenic component cannot be distinguished from the much larger natural background (Palmiéri, et al., 2015). Instead, it has been estimated indirectly from observable physical and biogeochemical quantities (Schneider et al., 2010, Touratier and Goyet, 2009, Touratier and Goyet, 2011). Recent studies have reported that carbon of anthropogenic origin with a total of 1.0–1.7 Pg C had entered the Mediterranean water column, where 52% are from the air–sea flux and 48% are from the Atlantic water inflow (Palmiéri et al., 2015, Schneider et al., 2010). It has been proposed that the Mediterranean Sea will experience amplified acidification relative to the global average surface ocean, based on the TrOCA approach (El Rahman Hassoun et al., 2015, Touratier and Goyet, 2011) or acidification similar to the global-ocean average based on a thermodynamic model by Palmiéri et al. (2015). On a sub-basin scale more knowledge is still needed to fully understand the carbonate system distribution on decadal time resolution to better estimate the C_ant uptake of this region.

The oxygen isotopic composition of sea-water (δ¹⁸O_SW) and the stable carbon isotope ratio of dissolved inorganic carbon (δ¹³C_DIC) in sea-water have been widely used as tracers in oceanographic research (e.g., Craig and Gordon, 1965, Kroopnick, 1985, Rohling and Bigg, 1998). The traditional application of oxygen isotope ratios in process-oriented studies of hydrography, water masses mixing, and circulation patterns, is commonly based on the strong linear correlation with salinity (e.g., Schmidt, 1998, Rohling and Bigg, 1998, Bigg and Rohling, 2000, McConnell et al., 2009). Similar to the salinity of surface oceans, the oxygen isotope ratio of seawater is mainly controlled by the ratio between evaporation and freshwater input. Light isotopes are preferentially fractionated between the liquid and the vapor phases during evaporation, leaving the sea surface enriched in heavy isotopes. The opposite occurs during mixing with fresh water, which is generally depleted in heavy isotopes (Rohling and Bigg, 1998).

Spatial and depth distribution of δ¹⁸O_SW in the Mediterranean was studied during the late 1980's by Gat et al. (1996) and Pierre (1999). Based on measurements that covered the entire Mediterranean, Pierre (1999) formulated δ¹⁸O_SW/salinity relationship with a slope of 0.25 that differs significantly from the source water of the Atlantic. This behavior results from the excess evaporation over freshwater input along the Mediterranean west–east general circulation pathway. Seasonal observations of δ¹⁸O_SW/salinity enable better understanding of the processes involved in the formation of the Levantine saline surface layer, the Levantine Surface Water (LSW, upper 30 m water column). This surface layer imposes major effect over the Levantine basin water column stratigraphy, affecting both air–sea CO₂ gas exchange and down-welling C_ant into the basin interior (Touratier and Goyet, 2011). Furthermore, it is currently unclear if recent changes in deep water formation, such as the EMT events have affected the δ¹⁸O_SW variability along the Mediterranean water column. Study of the temporal changes in δ¹⁸O_SW provides additional information about the stability or mixing of water masses, since 1988.

Observations of δ¹³C_DIC in the oceans have received much attention over the last few decades, since these observations allow to estimate the re-distribution of emitted C_ant within the global carbon system, most importantly the uptake of C_ant by the oceans (e.g., Gruber et al., 1999, Keeling et al., 2001, Kortzinger et al., 2003, Lynch-Stieglitz et al., 1995, McNeil et al., 2001, Sonnerup and Quay, 2012, Quay et al., 2007). For most oceanic basins studying the δ¹³C_DIC temporal evolution depended mainly on repeated surveys of water column properties, providing the basis to investigate the DIC change of the oceans interior (e.g., Kroopnick, 1985, McNeil et al., 2001, Racapé et al., 2013a, Sonnerup et al., 1999, Sonnerup et al., 2000, Quay et al., 2003). Both biological production and air–sea gas exchange are primer forces, controlling the δ¹³C_DIC distribution in the oceans (e.g., Gruber et al., 1999, Lynch-Stieglitz et al., 1995). However, since air–sea gas exchange is slow in the modern oceans, the biological effect dominates spatial δ¹³C_DIC gradients both in the interior and at the surface (e.g., Tagliabue and Bopp, 2008, Schmittner et al., 2013). The SE-Med is an ultra-oligotrophic ecosystem, characterized by an extremely low primary production (Azov, 1991). δ¹³C_DIC variability from 1.0‰ to 1.35‰ was observed along the Levantine water column, driven mainly by the Mediterranean general thermo-haline circulation (Pierre, 1999). These data provide the basis to investigate the temporal and vertical evaluation of the Mediterranean δ¹³C_DIC Suess effect since the late 1980s, enabling estimation of C_ant uptake and accumulation.

This study aims to provide a detailed mapping of the vertical structure and seasonal variability of δ¹⁸O_SW, δ¹³C_DIC, total alkalinity (A_T) and pH (total scale) at 25 °C of the South-eastern Levantine basin. We use the data to investigate the temporal evolution of δ¹⁸O_SW and δ¹³C_DIC over the last 20 yr, by comparing the current depth distribution with that of pre-EMT time. We compare the δ¹³C_DIC change to the global atmospheric change to estimate the CO₂ uptake and the mean anthropogenic CO₂ accumulation rate for the upper 700 m of the Levantine basin.