# Assessing the role of submarine groundwater discharge as a source of Sr to the Mediterranean Sea

Giada Trezzi, Jordi Garcia-Orellana, Valenti Rodellas, Pere Masque, Ester Garcia-Solsona, Per S. Andersson

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## To cite this version:

Giada Trezzi, Jordi Garcia-Orellana, Valenti Rodellas, Pere Masque, Ester Garcia-Solsona, et al. "Assessing the role of submarine groundwater discharge as a source of Sr to the Mediterranean Sea." Geochimica et Cosmochimica Acta, 2017, 200, pp.42-54. 10.1016/j.gca.2016.12.005. hal-01765462

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Distributed under a Creative Commons Attribution 4.0 International License
Assessing the role of submarine groundwater discharge as a source of
Sr to the Mediterranean Sea

Giada Trezzi*¹, Jordi Garcia-Orellana*¹,², Valentí Rodellas¹,³, Pere Masqué¹,²,⁴,⁵,
Ester Garcia-Solsona⁶, Per S Andersson⁷.

¹Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona, 08193 Bellaterra, Catalunya, Spain

²Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Catalunya, Spain

³CEREGE, Aix-Marseille Université, 13545 Aix-en-Provence, France

⁴School of Natural Sciences and Centre for Marine Ecosystems Research, Edith Cowan University, 270 Joondalup Drive, Joondalup WA 6027, Australia

⁵Oceans Institute and School of Physics, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

⁶Departament de Dinàmica de la Terra i de l'Oceà, Universitat de Barcelona, C/Martí i Franqués, 08028 Barcelona, Catalunya, Spain

⁷Department of Geosciences, Swedish Museum of Natural History, 104 05, Stockholm, Sweden

*Corresponding authors: Giada Trezzi

Giada.Trezzi@uab.cat; giada.trezzi@gmail.com

Jordi Garcia-Orellana

Jordi.Garcia@uab.cat
Abstract

Submarine groundwater discharge (SGD) has been identified as an important
source of Sr to the ocean and the SGD-driven Sr flux to the global ocean has been
recently re-evaluated (Beck et al. 2013). However, the uncertainty of this value is still
high because of the uncertainties related to the determination of SGD flow rates and the
paucity of ⁸⁷Sr/⁸⁶Sr data in SGD end-members. As carbonates have high Sr
concentrations and are subjected to intense heightened weathering, they might
significantly influence the SGD input of Sr to the ocean. Here we present data on Sr
concentrations and ⁸⁷Sr/⁸⁶Sr ratios in three carbonate dominated sites of the western area
of the Mediterranean Sea, a semi-enclosed basin characterized by abundant coastal
carbonates. The ⁸⁷Sr/⁸⁶Sr ratios in groundwater were lower compared to modern
seawater (~ 0.70916), as expected for areas dominated by carbonate lithologies.
Concentrations of Sr and ⁸⁷Sr/⁸⁶Sr ratios in groundwater showed conservative mixing in
the studied subterranean estuaries. By using SGD flow rates reported in the literature for
the study areas, a meteoric SGD-driven Sr flux of (0.12 – 2.1)·10³ mol d⁻¹ km⁻¹ was
calculated for the region, with a fresh SGD end-member characterized by a Sr
concentration of 27 – 30 µM and a ⁸⁷Sr/⁸⁶Sr ratio of 0.708020 – 0.707834.

Integrating these Sr data with literature data (i.e. values of Sr concentration and ⁸⁷Sr/⁸⁶Sr ratio from other lithologies as well as SGD flow rates), we also calculated the fresh SGD-driven Sr flux to the entire Mediterranean Sea, obtaining a value of (0.34 – 0.83)·10⁹ mol y⁻¹, with a ⁸⁷Sr/⁸⁶Sr of 0.7086 – 0.7081. Thus, for the entire Mediterranean basin, SGD is globally a source of Sr less radiogenic compared to seawater. The SGD Sr flux to the Mediterranean Sea represents 5-6% of the SGD Sr flux to the global ocean and the Mediterranean SGD end-member has higher Sr
concentration (5.0 – 12 µM) than the global SGD end-member (2.9 µM). This confirms
the significant role of carbonate lithologies on SGD-driven Sr fluxes to seawater.
The fresh SGD-driven Sr flux to the Mediterranean Sea is about 20 – 50% of the
riverine Sr input and significantly higher than the input through atmospheric dust
deposition. Therefore SGD should be considered as an important continental source of
Sr to the basin.
# 1 Introduction

The $^{87}$Sr/$^{86}$Sr ratio in marine carbonates reflects the composition of seawater at the time of the carbonate deposition and thus can be used as a tool for studies on paleoclimatic reconstructions and stratigraphic correlations (Burke et al. 1982). The residence time of Sr in seawater (> 2.5 My) is much longer than the time of ocean mixing (~ 1500 y), so that the $^{87}$Sr/$^{86}$Sr ratio is homogeneous in seawater with a salinity of about 35, with a current value of ~ 0.70916 (Broecker and Peng 1982; Beck et al. 2013), but with large variation over geological time scale (Burke et al. 1982). However, the $^{87}$Sr/$^{86}$Sr ratio can be different in seawater with lower salinity in marginal and coastal seas (Andersson et al. 1992; Huang et al. 2011).

The $^{87}$Sr/$^{86}$Sr ratio of seawater is related to the isotope composition of the Sr inputs, including mainly rivers, hydrothermal systems, seafloor sediments and submarine groundwater discharge (SGD) (e.g. Albarède et al., 1981; Allègre et al., 2010; Beck et al., 2013; Davis et al., 2003; Martin and Moore, 2008; Palmer and Edmond, 1989). The different Sr isotope compositions of these sources of Sr reflect the Rb/Sr ratio of the minerals of the rocks interacting with the water. Generally, rocks composed of K-rich minerals (intrusive, metamorphic and siliciclastic sedimentary rocks) have higher $^{87}$Sr/$^{86}$Sr, as $^{87}$Sr is produced from the decay of $^{87}$Rb ($t_{1/2}$ = 4.9·10$^{10}$ y), which can substitute K in minerals. In contrast, rocks largely composed of Ca-rich minerals (carbonates, evaporites and volcanic rocks) have lower $^{87}$Sr/$^{86}$Sr, primarily due to a lower Rb/Sr ratio; these rocks are enriched in Sr, as Sr can substitute Ca in minerals (McNutt 2000).
Early calculations of the marine Sr isotope budget considering only rivers, hydrothermal systems and diagenesis of seafloor sediments, yield a significant imbalance, mainly because the worldwide fluvial transport of more radiogenic Sr exceeded the contribution of less radiogenic Sr from the other sources (Albarède et al. 1981; Palmer and Edmond 1989; Davis et al. 2003). The significance of SGD inputs of Sr has been evaluated only in recent years (Basu et al. 2001; Allègre et al. 2010; Rahaman and Singh 2012; Beck et al. 2013). SGD is the combination of meteoric groundwater and recirculated seawater, discharging into the coastal sea through coastal aquifers (Moore 1996; Burnett et al. 2003). This process is a significant source of chemical compounds (e.g., macronutrients, trace metals, carbon, REEs) to the coastal ocean (e.g. Cai et al. 2003; Slomp and Van Cappellen 2004; Windom et al. 2006; Johannesson et al. 2011). Beck et al. (2013) conducted the first calculation of the SGD contribution on the marine Sr isotope budget, concluding that SGD (fresh and brackish components) accounts for 13 - 31% of this budget and represents generally a source of Sr less radiogenic than modern seawater. This reduces the imbalance of the marine Sr isotope budget in the ocean.

Despite the significant improvements in estimating the marine Sr isotope budget, the uncertainty of the SGD-driven Sr flux is still large. This is primarily due to the uncertainties associated with the volumetric discharge of groundwater and the paucity of ⁸⁷Sr/⁸⁶Sr data in SGD. In particular, new data on carbonate SGD end-members may be essential to refine the global SGD estimation, as carbonates have major effects on SGD fluxes, being characterized by a high Sr concentration and alteration grade (Beck et al. 2013).
The Mediterranean Sea is an interesting site for the study of SGD-driven Sr fluxes primarily due to: i) the wide areas of carbonate bedrock in the drainage basin (Amiotte Suchet et al. 2003; Dürr et al. 2005); ii) the magnitude of SGD flow rates (Rodellas et al. 2015); iii) the significance of SGD as source of chemical compounds to this basin (e.g. Tovar-Sánchez et al., 2014; Rodellas et al., 2015; Trezzi et al., 2016a; Trezzi et al., 2016b).

In this work, we investigate the SGD-driven Sr fluxes in three different carbonate-
dominant coastal areas in the Western Mediterranean Sea: the Irta Range, the Peníscola
wetland and the Ebro Delta. The focus is primarily on the eastern coast of the Iberian
Peninsula, where SGD fluxes have been quantified recently (Garcia-Solsona et al. 2010;
Mejías et al. 2012; Rodellas et al. 2012, 2017) and the hydrogeological sites are well-
known. We also attempt to estimate the SGD-driven Sr flux to the entire Mediterranean
Sea and to compare it to the other sources of Sr to the basin.

## 2 Methodology

## 2.1 Area of study

The Mediterranean Sea is a semi-enclosed marginal basin that covers a surface of about 2.5·10⁶ km². It is connected to the Atlantic Ocean through the Strait of Gibraltar, with a width of 14 km at its narrowest point and a minimum depth of 290 m (Soto-Navarro et al. 2010). The inflow of Atlantic water into the Mediterranean Sea through the Strait of Gibraltar is around 26·10¹² m³ y⁻¹ and exceeds the outflow of
Mediterranean waters, which is around $25·10^{12} \text{ m}^3 \text{ y}^{-1}$ (Soto-Navarro et al. 2010).

Evaporation in the Mediterranean Sea is higher than freshwater inputs and this
difference exceeds the net inflow of oceanic waters, so that the salinity of the
Mediterranean Sea is higher than the salinity of the global oceans (Flecker et al. 2002).

The Mediterranean Sea is characterized by a drainage basin with predominant
sedimentary bedrock (Peucker-Ehrenbrink et al. 2010). Carbonate rocks occupy up to
20 % of the drainage basin, dominating in particular the northern catchment (Mesozoic
carbonates) (Dürr et al. 2005; Topper et al. 2011). In relative terms, the area covered by
carbonates is about two times larger than the area occupied by carbonates in the global
ocean drainage basin. Riverine discharge to the basin is currently around $0.3·10^{12} \text{ m}^3 \text{ y}^{-1}$
(Ludwig et al. 2009). Total SGD flow rate to the Mediterranean Sea ranges from
$0.3·10^{12}$ to $4.8·10^{12} \text{ m}^3 \text{ y}^{-1}$ (Rodellas et al. 2015), while the flow rate of fresh
groundwater is about $0.07·10^{12} \text{ m}^3 \text{ y}^{-1}$ (Zektser et al. 2007).

The eastern coast of the Iberian Peninsula is located in the Western Mediterranean
Sea and is characterized by outcrops of Mesozoic carbonate rocks (Instituto Geologico
Minero de España (IGME) 1986). In this area, SGD flow rates to the sea have been
quantified in several coastal locations in recent years (Garcia-Solsona et al. 2010;
Mejías et al. 2012; Rodellas et al. 2012, 2017). The riverine discharge along the eastern
coast of the Iberian Peninsula is dominated by the Ebro River, which is the 4th largest
river discharging into the Mediterranean Sea in terms of flow rate ($13·10^9 \text{ m}^3 \text{ y}^{-1}$)
(Ludwig et al. 2009).
In this work, we investigated in particular three locations, characterized by different common coastal environments of the Mediterranean area: the Irta Range area, the Peníscola wetland and the Ebro Delta area (Figure 1a).

*Irta Range.* The area is characterized by the presence of a coastal chain formed by Mesozoic carbonates subjected to karstification processes (Mejías et al. 2008). Brackish coastal springs have been identified along the shoreline in relation with karstified pathways for groundwater. The estimated SGD flow rate from the main coastal springs of the Irta Range is of (72 - 190)·10³ m³ d⁻¹ and it depends on the period of the year (Garcia-Solsona et al. 2010).

*Peníscola wetland.* The area is characterized by a shallow brackish-water marsh, located just north of the Irta Range. The marsh is artificially connected to the Mediterranean Sea through three channels converging into a single channel at the end of the wetland. It is fed mainly by shallow groundwater flows from the detrital aquifer, composed by porous material, and the local karstic aquifers (Vinarós-Penícola and Irta Range, respectively) (>80 %) and deep groundwater (~15 %) from the El Maestrat regional carbonate aquifer, while the contribution from seawater is small (<5 %) (Rodellas et al. 2012; Zarroca et al. 2014). Measured groundwater discharge flow rates through the artificial marsh outlet are in the range of (34 - 83)·10³ m³ d⁻¹ (Rodellas et al. 2012).

*Ebro Delta.* The Ebro Delta area is characterized by the presence of the Alfacs and El Fangar Bays, which are semi-enclosed bays, partially isolated from the Mediterranean Sea through a sandy barrier (Camp and Delgado 1987). The Alfacs Bay receives freshwater mainly from irrigation channels for rice fields located in the delta and groundwater from the carbonate Montsià Range (Bayó et al. 1993; Llebot et al. 2014). The irrigation channels connect the bay to the Ebro River, as well as to
groundwater accumulated in shallow ponds from large springs locally denominated "ullals" (Bayó et al. 1993). Estimated SGD flow rates to the bay from the Montsià Range coastal aquifer vary seasonally in the range of (40 - 310)·10³ m³ d⁻¹ (Rodellas et al. 2017).

## 2.2 Sample collection

In total six surveys were conducted to collect groundwater samples between December 2012 and November 2013. In the Irta Range area (Figure 1b), 3 coastal karstic springs (KS1, KS2, KS3) directly discharging into the sea were sampled and groundwater was also collected from a well located at the northern boundary of the Irta Range (W1), next to the sea. In the Peníscola wetland (Figure 1c), five groundwater samples (S1LL; S1C; S2LL; S2C; S3) were collected from piezometers at different depths (60, 25, 16, 12 and 4 m, respectively). Shallow groundwater of the detrital aquifer was collected from a channel in the northern part of the wetland (ST7). Two groundwater samples were collected from two large ponds (tens of meters in diameter) nourished by submerged springs (P1; P2). A channel characterized by a submerged spring (ST32) was sampled at three different water column depths: bottom depth (0.9 m), intermediate depth (0.4 m) and surface (ST32D, ST32M and ST32S, respectively). Groundwater from a deep well (W2) located at the west of the marsh, was collected. The outlet connecting marsh groundwater to the coastal sea (ST47) was also sampled, as well as seawater near the dock of Peníscola village (SW1). In the Ebro Delta area (Figure 1d), two coastal karstic springs (KS4, KS5) directly discharging into the sea and four irrigation channels connected to the sea (C1, C2, C3 and C4) were sampled. Groundwater was also collected from a well (W3) and from a shallow pond (P3), next
to the irrigation channels. Samples from the Ebro River (ER) and from the central part
of the bay (SW2) were also collected. Three seawater samples collected during the
Geotraces-GA04 - MedSea cruise (2nd May - 2nd June 2013) were included in our data
set (Figure 1a): i) surface seawater of the Catalano-Balearic basin, at 105 km offshore
from Barcelona (SW3), representative of the general Mediterranean seawater end-
member; ii) surface seawater at the Gibraltar Strait (SW4), representative of the Atlantic
inflow into the Mediterranean Sea; iii) 500 m-depth seawater at the Gibraltar Strait
(SW5), representative of the Mediterranean outflow.

For each sample, 125 mL of unfiltered water were collected following trace metal
clean techniques (Tovar-Sánchez 2012) and stored in acid-cleaned plastic bottles.
Samples were acidified to pH<2 with trace-metal grade HCl or HNO3. At each sampling
point, salinity was measured with a calibrated YSI 556 handheld probe. Samples SW3,
SW4 and SW5 were collected with a Niskin bottle and acidified to pH<2 with trace-
metal grade HNO3.

## 2.3 Analysis

Sr concentrations ($C_{Sr}$) and $^{87}$Sr/$^{86}$Sr ratios in water samples were determined using
thermal ionization mass spectrometry (TIMS). Between 0.2 and 2 mL of water were
used for the analysis, depending on the $C_{Sr}$ in the sample. An enriched $^{84}$Sr spike was
added to each sample, which was then passed through an ion-exchange resin following
the method described in Delmonte et al. (2008). About 400 ng of purified Sr mixed with
a Ta activator was load on a single Re filament and analysed in a Thermo Scientific
TRITON TIMS. Two hundred 8-second integrations were recorded in multi-collector
static mode, applying rotating gain compensation. Measured $^{87}$Sr intensities were
corrected for Rb interference using $^{87}$Rb/$^{85}$Rb = 0.38600 and ratios were reduced using
the exponential fractionation law and $^{88}$Sr/$^{86}$Sr = 8.375209. Repeated measurements of
prepared CIT #39 seawater provided a reproducibility of ±0.0000082 (n=21), which was
used as the uncertainty of the $^{87}$Sr/$^{86}$Sr measurements. The NBS 987 standard was also
measured regularly, with a reproducibility of ±0.000015 (n=12). The $^{87}$Sr/$^{86}$Sr ratio for
the NBS 987 standard was 0.710217 ± 0.000016 (n=5). $^{87}$Sr/$^{86}$Sr reported in this work
for the analysed samples are normalized to a value for NBS 987 of 0.710245 (Haley et
al. 2008). The external precision of the $C_{Sr}$ measured in the CIT #39 seawater was
estimated to be about 0.5%. The total procedural blank for Sr was < 20 pg, which is
several orders of magnitude lower than the $C_{Sr}$ measured in the samples, so that no
blank correction was necessary.

3 Results

Salinity, $C_{Sr}$ and $^{87}$Sr/$^{86}$Sr ratios for all samples are summarized in Table 1. The three seawater samples collected during the Geotraces-GA04 - MedSea cruise showed $^{87}$Sr/$^{86}$Sr ratios in agreement with the typical ocean $^{87}$Sr/$^{86}$Sr ratio of ~ 0.70916 (Broecker and Peng 1982; Beck et al. 2013). On the contrary, salinities and $C_{Sr}$ showed some difference: the surface seawater of the Catalano-Balearic Sea (SW3) had a salinity of 38.3 and a $C_{Sr}$ of 95 µM; the surface seawater of the Gibraltar Strait, representing the Atlantic inflow (SW4), had the lowest salinity and $C_{Sr}$ (36.3 and 90 µM, respectively); the 500 m-depth seawater of the Gibraltar Strait (Mediterranean outflow) had the highest salinity and $C_{Sr}$ (38.5 and 96 µM, respectively). Compared to surface seawater
SW3, seawater samples collected at Peníscola (SW1) and in the Ebro Delta area (SW2) had lower salinity (37.1 and 34.9, respectively), lower $C_{Sr}$ (92 µM and 89 µM, respectively) and comparable $^{87}$Sr/$^{86}$Sr ratio (0.709158). Figure 2 showed the conservative behaviour of $C_{Sr}$ in all the seawater samples analysed, excepting sample SW2, which plotted above the linear relationship between salinity and $C_{Sr}$. The deviation of seawater SW2 from this linear trend indicated that it is possible to detect deviations of Sr from conservative mixing at high salinity. Indeed, sample SW2, which is located in a semi-enclosed bay, could be influenced by the inputs of Sr through SGD, the irrigation channels or the Ebro River characterizing the Ebro Delta area.

In the Irta Range area, coastal karstic springs had salinities ranging from 5.8 to 8.7, $C_{Sr}$ from 19 to 28 µM (average: 24 ± 5 µM) and $^{87}$Sr/$^{86}$Sr ratios from 0.708904 to 0.708981 (average: 0.708946 ± 0.000039). The groundwater sample W1, which was the fresh groundwater end-member, had a salinity of 0.3, a $C_{Sr}$ of 6.0 µM and a $^{87}$Sr/$^{86}$Sr ratio of 0.707810. Taking the surface seawater sample from the Catalano-Balearic Sea (SW3) as the seawater end-member, the groundwater of the karstic springs showed a conservative behaviour of $C_{Sr}$ and Sr isotope ratios throughout the mixing between groundwater and seawater (Figure 3).

In the Peníscola wetland, $C_{Sr}$ and Sr isotope ratios showed conservative mixing patterns (Figure 4). Salinities of groundwater samples ranged from 0.3 to 20.3, $C_{Sr}$ ranged between 6.0 and 170 µM and $^{87}$Sr/$^{86}$Sr ratios ranged from 0.707810 to 0.708473. Sr data for the Peníscola wetland confirmed the results obtained by Rodellas et al. (2012) and Zarroca et al. (2014) (Figure 4): groundwater accumulated in the wetland was the result of mixing between deep groundwater from the regional carbonate aquifer
(S1LL) and shallow groundwater from the detrital aquifer (ST7) and/or the local karstic aquifer (sample W1, collected at the northern boundary of the Irta Range). On the contrary, the contribution from seawater to the wetland was not significant.

Groundwater collected from well W2 likely represented a mixing between detrital aquifer (ST7) and local karstic aquifer (W1) end-members. Water collected from the outlet connecting the marsh with the coastal sea (ST47) had a salinity of 5.3, a $C_{Sr}$ of 46 µM and a $^{87}$Sr/$^{86}$Sr of 0.708388. This sample was a natural average of waters in the Peníscola wetland and can be used to characterize groundwater discharging into the sea from this studied area (i.e. it can be used as the SGD end-member).

In the Ebro Delta area, groundwater collected from two karstic springs (KS4 and KS5) had salinities of 5.8 and 4.2, $C_{Sr}$ of 44 and 47 µM (average: 45.6 ± 2.4 µM) and $^{87}$Sr/$^{86}$Sr ratios of 0.708108 and 0.707949 (average: 0.708029 ± 0.000112).

Groundwater from the well W3 and from the shallow pond P3 had salinities of 2.2 and 1.2, $C_{Sr}$ of 30 µM and 12 µM and $^{87}$Sr/$^{86}$Sr ratios of 0.707923 and 0.708068, respectively. Water samples collected from the irrigation channels had salinities ranging from 0.9 to 31.2, $C_{Sr}$ ranging from 18 to 70 µM and $^{87}$Sr/$^{86}$Sr ratios between 0.708116 and 0.709090. The sample of the Ebro River had a salinity of 0.7, a $C_{Sr}$ of 22 µM and a $^{87}$Sr/$^{86}$Sr ratio of 0.708547. The Sr concentrations and the Sr isotope ratios measured in the Ebro Delta area showed conservative mixing behaviour (Figure 5). This was evident in the irrigation channels (C1 and C2), both for the mixing line between the Ebro River (ER) and seawater (SW3) and for the mixing line between the Ebro River (ER) and the shallow pond nourished with groundwater (P3). On the contrary, for the irrigation channel C3, the $C_{Sr}$ plotted below the conservative mixing line between the Ebro River (ER) and seawater (Figure 5a), indicating possible removal of Sr from the solution or
contribution from another source, while the ⁸⁷Sr/⁸⁶Sr follow the conservative mixing
trend (Figure 5b). It was not possible to establish whether Sr from groundwater in the
karstic springs KS4 and KS5 mixed conservatively with seawater or not, as the fresh
groundwater end-member of the system was not collected (Figure 5). Water from the
irrigation channel C4 and from the well W3 did not follow the two end-member mixing
lines plotted in Figure 5, indicating a mixing between a higher number of end-members.

**4 Discussion**

**4.1 Sr in the subterranean estuary**

The C<sub>Sr</sub> and the ⁸⁷Sr/⁸⁶Sr ratios show that Sr mixes conservatively in the investigated subterranean estuaries. This conservative mixing cannot be confirmed for the karstic springs of the Ebro Delta area, primarily because the fresh groundwater end-member of the system was not collected. However, the similarity of this hydrogeological context (groundwater flowing from a coastal karstic chain discharging into the sea through coastal brackish springs) with the Irta Range area, suggests that Sr would also mix conservatively in this subterranean estuary.

The conservative C<sub>Sr</sub>–salinity mixing are consistent with previous studies
conducted in other subterranean estuaries (Rahaman and Singh 2012; Beck et al. 2013).
The Sr concentrations in deep groundwater and in a submerged spring (ST32D) of the
Peníscola wetland were higher compared to seawater. This is probably due to the
dissolution of the evaporate-rich Keuper unit at the basement of the deep regional
carbonate aquifer (Zarroca et al. 2014). This Sr-enriched groundwater showed
conservative mixing in the wetland. The Keuper unit is also probably the source of the
high salinity (20.3) in the deep groundwater end-member of the wetland (S1LL).

Unlike Sr concentrations, the conservative Sr isotope ratio patterns in the studied subterranean estuaries are in contrast to observations from other locations, where non-conservative $^{87}$Sr/$^{86}$Sr trends in subterranean estuaries, associated with conservative Sr concentrations trends, were highlighted and attributed to isotope exchange between dissolved Sr in seawater and Sr adsorbed on aquifer solids (Beck et al. 2013). This two-way flux of Sr between solid and fluid phases has also been documented in the mixing between river water and seawater, in particular in the presence of volcanic riverine particulate material (Jones et al. 2012, 2014). The conservative mixing behaviour of Sr isotopes in the studied subterranean estuaries indicates minor impact of desorption/adsorption of Sr from/on the aquifer solids throughout the mixing of fresh and salty waters.

Despite the results on Sr isotopes obtained in the studied subterranean estuaries, it
is not possible to determine an unequivocal behaviour of Sr isotopes in subterranean
estuaries in carbonate terrains. Indeed, the trends of $^{87}$Sr/$^{86}$Sr in different subterranean
estuaries in carbonate sites may be different, as observed when comparing results from
this work and data reported by Beck et al. (2013) on the Yucatan Peninsula and on
Florida, which are characterized by carbonate lithologies.

## 4.2 SGD-driven Sr fluxes from the eastern coast of the Iberian Peninsula

Data on C<sub>Sr</sub> measured in this work can be used to quantify SGD-driven Sr fluxes
to the sea by multiplying C<sub>Sr</sub> of groundwater samples by the SGD flow rates reported in
the literature (Table 2).
In the Irta Range area, SGD flow rates of 72 - 190·10³ m³ d⁻¹ have been reported (Garcia-Solsona et al. 2010). Taking into account the average Cₛₚ measured in the three karstic samples KS1, KS2 and KS3, Sr fluxes are in the range of (1.7 - 4.4)·10³ mol d⁻¹. In the Peníscola marsh area, SGD flow rates of (34 - 83)·10³ m³ d⁻¹ have been measured in the channel connecting the wetland to the sea (Rodellas et al. 2012). Considering the Cₛₚ of ST47, an SGD-driven Sr flux of (1.6 - 3.9)·10³ mol d⁻¹ is obtained. For the Ebro Delta area, the SGD flow rate from the Montsià Range is (40 - 310)·10³ m³ d⁻¹ (Rodellas et al. 2017). Considering the average Cₛₚ of the two karstic springs KS4 and KS5 sampled in this work, Sr fluxes of (1.8 - 14)·10³ mol d⁻¹ are obtained. The SGD-driven Sr fluxes calculated for the Irta Range and Ebro Delta study areas include both the fresh groundwater and the recirculated seawater components of SGD. Since the Cₛₚ is generally higher in seawater compared to groundwater and Sr mixes conservatively, a portion of the Sr measured in the brackish mixture is derived from the seawater itself.

Assuming that the recirculation of seawater through the subterranean estuary do not represent a net source of Sr to the sea, the flux of Sr should be calculated only considering fresh groundwater end-members. The fresh groundwater flow rate in the Irta Range area is (51 - 160)·10³ m³ d⁻¹ (Garcia-Solsona et al. 2010). For the Ebro Delta area, we calculated the fresh groundwater flow rate considering the brackish flows and the average salinity of the springs, obtaining a value of (35 - 270)·10³ m³ d⁻¹. Water collected from well W1 is considered as the fresh groundwater end-member in the Irta Range system. For the Ebro Delta area, fresh groundwater was not collected, but the Cₛₚ of the fresh groundwater end-member can be calculated by extrapolating the Sr mixing line between the brackish springs and seawater SW3 to a salinity of zero (Figure 5a), obtaining a value of 38.2 ± 0.2 µM. Using this value of Cₛₚ, the ⁸⁷Sr/⁸⁶Sr ratio of this fresh groundwater end-member can be calculated from the mixing line between the
brackish springs and seawater SW3 in Figure 5b, obtaining a value of 0.707630 ± 0.000028. Fresh SGD Sr fluxes to the coastal sea are (0.31 - 0.96)·10³ and (1.3 - 10)·10³ mol d⁻¹ for the Irta Range and Ebro Delta areas, respectively.

In the case of the Peníscola wetland, it is not possible to calculate fresh SGD fluxes to the sea considering the salinity of the SGD end-members, as the groundwater end-member discharging into the sea is a mixing of fresh groundwater from the local detrital aquifer (ST7) and salty groundwater from the deep regional carbonate aquifer (see Section 4.1). However, as shown in Figure 4a and 4b, water flowing into the sea from the wetland is not particularly affected by seawater and thus the recirculation of seawater in the subterranean estuary is minimal or absent. Therefore, the brackish Sr water flux calculated above for the channel connecting the marsh to the sea can be considered a first approximation of the net Sr flux to the coastal sea.

From the fresh SGD end-members of the Irta Range and the Ebro Delta areas and the water of the outlet of the Peníscola wetland (ST47), we calculate a flow-weighted average $C_{Sr}$ and Sr isotope ratio, representative of the carbonate SGD end-member of the eastern coast of the Iberian Peninsula. Considering minimum and maximum fresh SGD flow rates of the study areas, ranges of 27 - 30 µM and 0.708020 - 0.707834 are obtained for the $C_{Sr}$ and the Sr isotope ratio, respectively. In accordance with the dominant carbonate lithology of the region, the flow-weighted $^{87}$Sr/$^{86}$Sr ratio of the SGD end-member is thus lower than in modern seawater, so that SGD is an input of less radiogenic Sr in this area of the Mediterranean Sea. The ranges in $C_{Sr}$ and Sr isotope ratios representative of the fresh SGD end-member can be compared with those of the Ebro River, the most important river in the region in terms of flow rate (36·10⁶ m³ d⁻¹) (Ludwig et al. 2009). The $C_{Sr}$ of the Ebro River at a salinity of 0.7 is 22 µM, which is
lower than the flow-weighted average $C_{Sr}$ of the fresh SGD end-members by a factor of 1.2 – 1.4. The $^{87}$Sr/$^{86}$Sr ratio of the Ebro River is 0.708547 and thus is higher than that of the SGD typical of the region, although it is lower than in modern seawater. Despite the lower $C_{Sr}$ in the Ebro River with respect to the fresh SGD end-member, the Sr flux from this source is $800 \cdot 10^3$ mol d$^{-1}$, which is at least four orders of magnitude higher than the SGD-driven Sr fluxes calculated for each study area.

## 4.3 SGD-driven Sr flux to the Mediterranean Sea

As Sr is generally more enriched in seawater compared to groundwater and it mixes conservatively in the subterranean estuaries, an approximation of the SGD-driven net Sr flux to the Mediterranean Sea could be obtained considering the fresh component of SGD.

We apply a simple model based on the combination of fresh SGD flow rates and Sr composition (concentration and isotope ratio) typical of groundwater from different types of aquifers in order to calculate the meteoric SGD Sr flux to the entire Mediterranean Sea. We use the volumetric discharges of meteoric groundwater obtained from regional hydrogeological assessments (Zektser et al., 2007), while the global lithological database GLIM (Hartmann and Moosdorf 2012) is used for the classification of the different types of aquifers and the evaluation of their relative percentages in the coastal areas of the Mediterranean Sea. This global lithological map presents 13 different primary categories of dominant lithologies, which have been assembled in 5 different groups of types of aquifers in this work, on the base of geological origin and/or predominant minerals (Table 3): carbonate, siliciclastic, granite, volcanic and complex groups. The most significant coastal lithologies of the
Mediterranean Sea are the siliciclastic and the carbonate groups, representing 47% and 46% of the coastal area, respectively. The complex group represents 5% of the coastal area, while granitic and volcanic lithologies have a minimal contribution (1% for each group).

For each lithological group, a value of $C_{Sr}$ and $^{87}$Sr/$^{86}$Sr ratio of the groundwater could be assigned. These values are established mainly following Beck et al. (2013) (Table 3). For the granite, the volcanic and the complex aquifer type groups, the same values used for the calculation of SGD-driven Sr flux to the global ocean are used. For the siliciclastic group, average values of the shale and the sandstone categories are used. For the carbonate category, $C_{Sr}$ of 27 - 30 µM and a respective Sr isotope ratio of 0.708020 – 0.707834 can be derived on the base of the data reported in this work (see Section 4.2). Compared to the values reported by Beck et al. (2013) for the carbonate end-member ($C_{Sr}$ of 10 µM and $^{87}$Sr/$^{86}$Sr of 0.708), the measured $C_{Sr}$ is up to 3 times higher. Since this difference is high, we use a range of 10 – 30 µM as representative of the $C_{Sr}$ of the carbonate SGD end-member in the calculation of the SGD-driven Sr flux to the entire Mediterranean Sea (Table 3).

For each SGD region delineated by Zektser et al. (2007), the average area and the concentration-weighted SGD end-member composition (concentration and isotope ratio) were calculated (Appendix A and B). The global SGD-driven Sr flux to the Mediterranean Sea is determined by combining the fresh SGD flow rate and the $C_{Sr}$ in the SGD end-member for each SGD region, while the Sr isotope ratio contribution is calculated for each region multiplying the SGD-driven Sr flux by the difference of $^{87}$Sr/$^{86}$Sr between SGD and the Mediterranean seawater (Appendix A and B). A value of (0.34 – 0.83)·10⁹ mol y⁻¹ is obtained for the Sr flux driven by SGD to the entire Mediterranean Sea, with an average discharge and concentration-weighted SGD end-
member composition of (5.0 – 12) µM for the Sr and (0.7086 – 0.7081) for the ⁸⁷Sr/⁸⁶Sr ratio. This flux represents 5 – 6 % of the Sr flux obtained for the global ocean (Beck et al. 2013). Considering a coastline of 64·10³ km (Stewart and Morhange 2009), a shore-normalized value of (5.3 – 13)·10³ mol y⁻¹ km⁻¹ is obtained for the Sr flux to the Mediterranean Sea. Whereas the shore-normalized fresh SGD flow rate in the Mediterranean Sea is 5 times lower than that in the global ocean, the shore normalized fresh SGD Sr flux to the Mediterranean Sea is only 2-3 times lower than the global ocean one. The SGD-driven Sr flux to the Mediterranean Sea associated with carbonate aquifers is 46 – 50% (Table 3) and the SGD end-member for the Mediterranean Sea is distinctly different from the SGD end-member of the global ocean, which is characterized by lower C<sub>Sr</sub> (2.9 µM) and higher ⁸⁷Sr/⁸⁶Sr ratio (0.7089) (Beck et al. 2013). Thus, the results demonstrated the significance of the carbonate end-member with respect to the others in the SGD contribution of Sr to seawater, as carbonates contain high concentration of Sr (exchangeable ion of Ca) and are easily dissolved by groundwater.

A non-conservative mixing of Sr isotopes has been reported in several subterranean estuaries corresponding to a wide range of lithologies, despite a conservative trend of Sr concentrations with salinity (Beck et al. 2013). Thus, it is possible that some subterranean estuaries in the Mediterranean basin show this non-conservative behaviour for Sr isotopes, contrary to the three studied estuaries. Although the saline component of SGD does not represent a net Sr flux, since a significant portion of Sr in brackish SGD is derived from the recirculated seawater itself (see Section 4.2), geochemical reactions in the fresh/saline interface of the subterranean estuaries may alter the ⁸⁷Sr/⁸⁶Sr ratio of groundwater discharging to the Mediterranean Sea. Therefore, additional studies on the behaviour of Sr isotopes in subterranean estuaries of the
Mediterranean Sea are needed in order to estimate the possible contribution of the
brackish SGD component to the SGD-driven Sr flux.

4.4 Sr fluxes into the Mediterranean Sea

The fresh SGD-driven Sr flux to the Mediterranean Sea calculated in Section 4.3
can be compared to other sources of Sr of the basin. The main continental sources of Sr
to seawater in the Mediterranean Sea are expected to be riverine discharge and also
atmospheric dust deposition, because of the proximity to the Saharan desert.

For the input of Sr through rivers, the estimation by Peucker-Ehrenbrink et al.
(2010), based on the geology of the drainage basin, yields a riverine dissolved Sr flux to
the Mediterranean Sea of 1.6·10⁹ mol y⁻¹ and a ⁸⁷Sr/⁸⁶Sr ratio of the river end-member
of 0.7077. Contrary to the global ocean, rivers are thus a source of less radiogenic Sr to
seawater in this area. Compared to the riverine Sr flux, the input of Sr through fresh
SGD, calculated in Section 4.3, is about 20 to 50% of it (Table 4), which is comparable
to the result obtained for the global ocean by Beck et al. (2013).

For the atmospheric dust deposition, an approximate Sr input can be calculated by
assuming that the continental crust has an average Sr concentration of 320 ng mg⁻¹
(Hawkesworth and Kemp 2006) and around 65% of this Sr is water-soluble in dust
particles (Hsu et al. 2010). Considering atmospheric mass fluxes of 8 – 35 g m⁻² y⁻¹ for
the Mediterranean basin (Guerzoni et al. 1999), a flux of water-soluble Sr of (48 –
210)·10⁶ mol y⁻¹ is obtained. We attribute to this flux a Sr isotope ratio of 0.709763 –
0.712092, as reported by Aarons et al. (2013) for the water soluble fraction of Saharan
dust. Thus, the atmospheric deposition is a source of Sr more radiogenic than seawater,
when there is an influence of Saharan dust. Compared to this atmospheric Sr flux, the
fresh SGD-driven Sr flux, calculated in Section 4.3, is significantly larger (Table 4).

Therefore, the comparison of the fresh SGD-driven Sr flux with riverine discharge
and atmospheric deposition clearly shows that SGD is a relevant continental source of
Sr to the Mediterranean Sea. However, at a basin scale the most important source of Sr
to the Mediterranean Sea is the inflow of Atlantic water through the Strait of Gibraltar.
Considering a flow rate of 25.5·10¹² m³ y⁻¹ (Soto-Navarro et al. 2010) and a Cₛₚ of 90
µM, as measured in sample SW4, the Sr flux into the Mediterranean Sea through the
Gibraltar Strait is 2.3·10¹² mol y⁻¹. This flux is 3 orders of magnitude higher than the
sum of the continental inputs of Sr to the basin (Table 4). Thus, considering also the
high residence time of Sr in seawater (>2.5 My) (Broecker and Peng 1982), it is clear
that any influence of the SGD-driven inputs and the other continental sources of Sr
cannot be noticed in the Sr isotope ratio of the Mediterranean seawater (the ⁸⁷Sr/⁸⁶Sr
ratio of the Atlantic inflow (SW4) is identical to that measured for the Mediterranean
Sea (SW3)). Nevertheless, the contribution of Sr to seawater from SGD (or from the
other continental inputs) in the Mediterranean Sea can be registered in the non-
conservative relation between Sr concentration and salinity in coastal seawater,
confirming that SGD should be considered as an important source of Sr to the
Mediterranean Sea.

# 5 Conclusion

The input of Sr to seawater through SGD was investigated in three subterranean
estuaries of the Western Mediterranean Sea dominated by carbonate matrix.
Concentrations of Sr and ⁸⁷Sr/⁸⁶Sr ratios showed conservative mixing patterns in these
subterranean estuaries, in contrast with the behaviour of Sr isotopes reported in the
literature for similar aquifer lithologies. This reveals that Sr isotopes do not have a
unique mixing behaviour, even in a given aquifer type.

The input of Sr to the entire Mediterranean Sea through SGD was also assessed,
obtaining a value of (0.34 – 0.83)·10⁹ mol y⁻¹ for the fresh SGD-driven Sr flux, which
represents the 5 – 6% of the Sr flux to the global ocean. SGD is thus a source of less
radiogenic Sr for the Mediterranean Sea. The Sr concentration of the global SGD end-
member for the Mediterranean Sea (5.0 – 12 µM) is higher than for the global ocean,
while the ⁸⁷Sr/⁸⁶Sr ratio is lower (0.7086 – 0.7081). This highlights the significance of
carbonates as a source of Sr to groundwater discharging into the Mediterranean Sea.

The fresh SGD Sr flux to the Mediterranean Sea is about 20 – 50% of the riverine flux
and significantly higher than the atmospheric flux, so that SGD should be considered as
a relevant continental source of Sr to the basin.

ACKNOWLEDGMENTS

We are grateful to Hans Schöberg and Karin Wallner for the great help with the
analyses of the samples. We also thank our colleagues at the Laboratori de
Radioactivitat Ambiental (Universitat Autònoma de Barcelona) and the crew of the B/O
Ángeles Alvariño (Geotraces-GA04 - Medsea Cruise) for their help with the sampling
activity. This project has been partially funded by the European Union Seventh
Framework Programme through the MetTrans Initial Training Network, Marie Curie
Actions, which also provide financial support through a PhD fellowship to GT (EU
FP7-People-2011-ITN-290336). Support provided by the Generalitat de Catalunya to
MERS (2014 SGR – 1356) is also acknowledged. VR acknowledges financial support
from the European Union’s FP7 (Marie Curie Actions PCOFUND-GA-2013-609102),
through the PRESTIGE programme coordinated by Campus France. The Spanish
Government (MINECO) is acknowledged for support to EGS through a post-doctoral
grant (FPDI-2013-15660) and support of ECOALFACS project (CTM2009-09581) to
the ICM-CSIC. PSA acknowledges support from the Swedish Research Council (VR
349-2012-6287). This research is also a contribution to the ANR @RAction chair
(ANR-14-ACHN-0007-01) and Labex OT-Med (ANR-11-LABEX-0061) funded by the
“Investissements d’Avenir” program through the A*MIDEX project (ANR-11-IDEX-
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Table 1

Sr concentrations (C<sub>Sr</sub>), Sr isotope ratio and salinity of water samples. Both internal and external precisions of <sup>87</sup>Sr/<sup>86</sup>Sr ratios are reported.

<table><thead><tr><th>Sample site</th><th>Sample ID</th><th>Sample type</th><th>Depth (m)</th><th>Salinity</th><th>Sr (µM)</th><th>Error 2σ Sr (µM)</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th>Int. prec. 2σm <sup>87</sup>Sr/<sup>86</sup>Sr</th><th>Ext. prec. 2σ <sup>87</sup>Sr/<sup>86</sup>Sr</th></tr></thead><tbody><tr><td rowspan="4">Irta Range</td><td>W1</td><td>Groundwater</td><td>n.r.</td><td>0.3</td><td>6.04</td><td>0.03</td><td>0.707810</td><td>0.000005</td><td>0.000008</td></tr><tr><td>KS1</td><td>Groundwater</td><td>surf</td><td>8.7</td><td>28.3</td><td>0.1</td><td>0.708953</td><td>0.000006</td><td>0.000008</td></tr><tr><td>KS2</td><td>Groundwater</td><td>surf</td><td>5.8</td><td>19.0</td><td>0.1</td><td>0.708904</td><td>0.000005</td><td>0.000008</td></tr><tr><td>KS3</td><td>Groundwater</td><td>surf</td><td>7.1</td><td>23.2</td><td>0.1</td><td>0.708981</td><td>0.000005</td><td>0.000008</td></tr><tr><td rowspan="12">Penícola wetland</td><td>S1LL</td><td>Groundwater</td><td>60</td><td>20.3</td><td>167</td><td>0.8</td><td>0.708471</td><td>0.000005</td><td>0.000008</td></tr><tr><td>S1C</td><td>Groundwater</td><td>25</td><td>19.4</td><td>167</td><td>0.8</td><td>0.708471</td><td>0.000004</td><td>0.000008</td></tr><tr><td>S2LL</td><td>Groundwater</td><td>16</td><td>17.6</td><td>154</td><td>0.8</td><td>0.708459</td><td>0.000006</td><td>0.000008</td></tr><tr><td>S2C</td><td>Groundwater</td><td>12</td><td>10.6</td><td>96.7</td><td>0.5</td><td>0.708455</td><td>0.000005</td><td>0.000008</td></tr><tr><td>S3</td><td>Groundwater</td><td>4</td><td>15.1</td><td>118</td><td>0.6</td><td>0.708385</td><td>0.000005</td><td>0.000008</td></tr><tr><td>ST7</td><td>Groundwater</td><td>surf</td><td>0.9</td><td>13.7</td><td>0.1</td><td>0.707947</td><td>0.000008</td><td>0.000008</td></tr><tr><td>ST32S</td><td>Groundwater</td><td>surf</td><td>3.3</td><td>31.9</td><td>0.2</td><td>0.708300</td><td>0.000005</td><td>0.000008</td></tr><tr><td>ST32M</td><td>Groundwater</td><td>0.4</td><td>6.9</td><td>57.9</td><td>0.3</td><td>0.708411</td><td>0.000005</td><td>0.000008</td></tr><tr><td>ST32D</td><td>Groundwater</td><td>0.9</td><td>13.0</td><td>107</td><td>0.5</td><td>0.708473</td><td>0.000005</td><td>0.000008</td></tr><tr><td>ST47</td><td>Groundwater</td><td>surf</td><td>5.3</td><td>46.5</td><td>0.2</td><td>0.708388</td><td>0.000004</td><td>0.000008</td></tr><tr><td>P1</td><td>Groundwater</td><td>surf</td><td>8.4</td><td>70.6</td><td>0.4</td><td>0.708441</td><td>0.000005</td><td>0.000008</td></tr><tr><td>P2</td><td>Groundwater</td><td>surf</td><td>6.7</td><td>57.0</td><td>0.3</td><td>0.708418</td><td>0.000005</td><td>0.000008</td></tr><tr><td>W2</td><td>Groundwater</td><td>n.r.</td><td>0.5</td><td>7.76</td><td>0.04</td><td>0.707930</td><td>0.000005</td><td>0.000008</td></tr><tr><td>SW1</td><td>Seawater</td><td>surf</td><td>37.1</td><td>92.5</td><td>0.5</td><td>0.709158</td><td>0.000005</td><td>0.000008</td></tr></tbody></table>
<table><tr><td rowspan="10">Ebro Delta</td><td>ER</td><td>River</td><td>surf</td><td>0.7</td><td>22.2</td><td>0.1</td><td>0.708547</td><td>0.000006</td><td>0.000008</td></tr><tr><td>C1</td><td>Channel</td><td>surf</td><td>0.9</td><td>19.4</td><td>0.1</td><td>0.708448</td><td>0.000004</td><td>0.000008</td></tr><tr><td>C2</td><td>Channel</td><td>surf</td><td>10.7</td><td>36.8</td><td>0.2</td><td>0.708846</td><td>0.000005</td><td>0.000008</td></tr><tr><td>C3</td><td>Channel</td><td>surf</td><td>31.2</td><td>70.1</td><td>0.4</td><td>0.709090</td><td>0.000005</td><td>0.000008</td></tr><tr><td>C4</td><td>Channel</td><td>surf</td><td>1.3</td><td>18.3</td><td>0.1</td><td>0.708116</td><td>0.000005</td><td>0.000008</td></tr><tr><td>KS4</td><td>Groundwater</td><td>surf</td><td>5.8</td><td>44.0</td><td>0.2</td><td>0.708108</td><td>0.000005</td><td>0.000008</td></tr><tr><td>KS5</td><td>Groundwater</td><td>surf</td><td>4.2</td><td>47.3</td><td>0.2</td><td>0.707949</td><td>0.000005</td><td>0.000008</td></tr><tr><td>W3</td><td>Groundwater</td><td>n.r.</td><td>2.2</td><td>30.0</td><td>0.1</td><td>0.707923</td><td>0.000005</td><td>0.000008</td></tr><tr><td>P3</td><td>Groundwater</td><td>surf</td><td>1.3</td><td>12.2</td><td>0.1</td><td>0.708068</td><td>0.000007</td><td>0.000008</td></tr><tr><td>SW2</td><td>Seawater</td><td>surf</td><td>34.9</td><td>89.4</td><td>0.5</td><td>0.709158</td><td>0.000004</td><td>0.000008</td></tr><tr><td>Catalano-Balearic Basin</td><td>SW3</td><td>Seawater</td><td>surf</td><td>38.3</td><td>95.2</td><td>0.5</td><td>0.709162</td><td>0.000005</td><td>0.000008</td></tr><tr><td>Atlantic inflow</td><td>SW4</td><td>Seawater</td><td>surf</td><td>36.3</td><td>90.4</td><td>0.5</td><td>0.709163</td><td>0.000004</td><td>0.000008</td></tr><tr><td>Mediterranean outflow</td><td>SW5</td><td>Seawater</td><td>surf</td><td>500</td><td>38.5</td><td>95.8</td><td>0.5</td><td>0.709159</td><td>0.000006</td><td>0.000008</td></tr></table>

n.r. = not registered

Surf = surface
Table 2

<table><caption>SGD-driven Sr fluxes from the eastern coast of the Iberian Peninsula and comparison with the riverine input.</caption><thead><tr><th rowspan="2">Study site</th><th colspan="3">Flow rates<br>(10<sup>3</sup> m<sup>3</sup> d<sup>-1</sup>)</th><th rowspan="2">Fresh end-members<br><sup>87</sup>Sr/<sup>86</sup>Sr</th><th rowspan="2">Net Sr flux<br>(10<sup>3</sup> mol d<sup>-1</sup>)</th></tr><tr><th>Brackish<br>SGD</th><th>Fresh SGD</th><th>Sr<br>(µM)</th></tr></thead><tbody><tr><td>Irta Range</td><td>72 - 190<sup>a</sup></td><td>51-160</td><td>6.0</td><td>0.707810</td><td>0.31 - 0.96</td></tr><tr><td>Peníscola</td><td>34 - 83<sup>b</sup></td><td>34-83</td><td>46</td><td>0.708388</td><td>1.6 - 3.9</td></tr><tr><td>Ebro delta</td><td>40 - 310<sup>c</sup></td><td>35 - 270</td><td>38</td><td>0.707604</td><td>1.3 - 10</td></tr><tr><td>E Iberian Peninsula</td><td colspan="3">Rivers (Ebro River)</td><td>27 - 30<br>0.708020 - 0.707834</td><td></td></tr><tr><td>E Iberian Peninsula</td><td colspan="3">36000<sup>d</sup></td><td>22</td><td>800</td></tr></tbody></table>

<footnote><sup>a</sup>Garcia-Solsona et al. (2010)</footnote>

<footnote><sup>b</sup>Rodellas et al. (2012)</footnote>

<footnote><sup>c</sup>Rodellas et al. (2017)</footnote>

<footnote><sup>d</sup>Ludwig et al. (2009).</footnote>
**Table 3**

Sr concentration of groundwater in aquifers of different lithology and the contribution of each lithology to the Sr flux into the Mediterranean Sea. The carbonate group includes only sedimentary carbonate rocks; the siliciclastic group is composed of sedimentary rocks dominated by silica-rich minerals and unconsolidated sediments, characterized by the absence of carbonate minerals; the granite group mainly consists of acid plutonic rocks, with a minor contribution of acid volcanic rocks; the volcanic group is composed principally by basic and intermediate volcanic rocks and secondly by basic plutonic rocks; the complex group includes sedimentary mixed rocks (both siliciclastic and carbonate rocks, which could not be separate at the map scale) and metamorphic rocks, for which it is difficult to establish the percentage of carbonate and silicate minerals.

<table><thead><tr><th rowspan="2">Lithological groups<sup>a</sup></th><th rowspan="2">% Coastal area</th><th colspan="2">End-member values<sup>b</sup></th><th rowspan="2">% Vol. flux<sup>c</sup></th><th rowspan="2">% SGD Sr flux<sup>d</sup></th><th rowspan="2">% Ratio contribution<sup>e</sup></th></tr><tr><th>Sr (µM)</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th></tr></thead><tbody><tr><td>Carbonate</td><td>46</td><td>10 - 30</td><td>0.708020 - 0.707834</td><td>36</td><td>46 - 50</td><td>56 - 65</td></tr><tr><td>Siliciclastic</td><td>47</td><td>2.5</td><td>0.7105</td><td>43</td><td>36 - 39</td><td>24 - 32</td></tr><tr><td>Granite</td><td>1</td><td>0.5</td><td>0.713</td><td>3</td><td>1</td><td>0</td></tr><tr><td>Volcanic</td><td>1</td><td>2</td><td>0.706</td><td>2</td><td>1</td><td>1</td></tr><tr><td>Complex</td><td>5</td><td>2</td><td>0.709</td><td>16</td><td>12 - 13</td><td>10 - 11</td></tr></tbody></table>

<footnote><sup>a</sup> Lithological groups are estimated starting from the GLIM database geological map (Hartmann and Moosdorf, 2012).</footnote>

<footnote><sup>b</sup> End-members values of Sr concentration and <sup>87</sup>Sr/<sup>86</sup>Sr are estimated following Beck et al., 2013 and employing Sr data of this study for the carbonate group.</footnote>

<footnote><sup>c, d, e</sup> Fluxes calculated as explained in Appendix C</footnote>
Table 4

Characterization of different sources of Sr to the Mediterranean Sea

<table><thead><tr><th>Source</th><th>Sr<br>(µM)</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th>Sr flux<br>(10<sup>6</sup> mol y<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Meteoric SGD</td><td>5.0 - 12</td><td>0.7081 - 0.7086</td><td>340 - 830</td></tr><tr><td>Rivers <sup>a</sup></td><td>4.0</td><td>0.7077</td><td>1600</td></tr><tr><td>Atmospheric deposition <sup>b</sup></td><td></td><td>0.709763 - 0.712092</td><td>47 - 210</td></tr><tr><td>Atlantic Ocean inflow <sup>c</sup></td><td>90</td><td>0.70916</td><td>2.3·10<sup>6</sup></td></tr></tbody></table>

<footnote><sup>a</sup> Peucker-Ehrenbrink et al., 2010</footnote>

<footnote><sup>b</sup> Calculated with data from Hawkesworth and Kemp (2006), Hsu et al. (2010), Guerzoni et al. (1999), Aarons et al. (2013)</footnote>

<footnote><sup>c</sup> Calculated with data from Soto-Navarro et al. (2010), Broecker and Peng (1982) and this study.</footnote>
**Figure 1**

a) Location of the areas of study in the Western Mediterranean Sea and of the seawater samples collected during the Geotraces-GA04 - MedSea cruise. b) c) d) Sampling stations in the Irta Mountain area, in the Peníscola wetland and in the Ebro delta zone, respectively. Sample W2 of the Peníscola wetland is out of scale in map b) but it is represented in map a).

**Figure 2**

Sr concentrations vs salinity in the analysed seawater samples. Dotted lines represent the conservative behaviour of Sr.

**Figure 3**

Irta Range area: a) $C_{Sr}$ vs salinity; b) $^{87}$Sr/$^{86}$Sr ratio vs 1/$C_{Sr}$. Black circles indicate water end-members of the system (well W1 and seawater SW3), while white triangles are the karstic spring samples (KS1, KS2, KS3). Dotted lines represent the conservative mixing behaviour. Error bars of $C_{Sr}$ and of Sr isotope ratios are smaller than the symbol size.

**Figure 4**

Peníscola wetland: a) $C_{Sr}$ vs salinity; b) $^{87}$Sr/$^{86}$Sr ratio vs 1/$C_{Sr}$. Black circles indicate water end-members of the system (channel ST7, piezometer S1LL, well W1, seawater SW1 and SW3). White triangles are the other groundwater samples of the marsh (piezometers S1C, S2LL, S2C and S3, channel ST32S, ST32M and ST32D, ponds P1 and P2). The grey diamond is the outlet of the marsh into the sea (ST47). Dotted lines
represent the conservative mixing behaviour. Error bars of C<sub>Sr</sub> and of Sr isotope ratios
are smaller than the symbol size.

**Figure 5**

Ebro delta area: a) C<sub>Sr</sub> vs salinity; b) <sup>87</sup>Sr/<sup>86</sup>Sr ratio vs 1/ C<sub>Sr</sub>. Black circles indicate water end-members of the system (Ebro River ER, pond P3, seawater SW2 and SW3), while white triangles are the karstic springs (KS4, KS5), the irrigation channels (C1, C2, C3, C4) and the well W3. Dotted lines represent the conservative mixing behaviours. Error bars of C<sub>Sr</sub> and of Sr isotope ratios are smaller than the symbol size.
<figure><img src="image_3.png" /></figure>

a

<figure><img src="image_4.png" /></figure>

b

<figure><img src="image_5.png" /></figure>

c

<figure><img src="image_6.png" /></figure>

d
<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>

R = 0.99594; p < 0.001; n = 5

<figure><img src="image_9.png" /></figure>

R = 0.99464; p < 0.01; n = 5
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>
# Appendix A

Calculations of SGD-driven Sr fluxes in each SGD region of the Mediterranean Sea (results reported in Appendix B).

- Average-weighted Sr concentration of the SGD end-member:

$$Sr_{reg} = \frac{\sum(Sr_{lith} \cdot \%_{lith})}{100} \quad (A.1)$$

- Average-weighted $^{87}$Sr/$^{86}$Sr concentration of the SGD end-member:

$$^{87}Sr / ^{86}Sr_{reg} = \frac{\sum(Sr_{lith} \cdot ^{87}Sr / ^{86}Sr_{lith} \cdot \%_{lith})}{Sr_{reg}} \quad (A.2)$$

- SGD-driven Sr flux:

$$SGD \ Sr \ flux_{reg} = Sr_{reg} \cdot SGD \ flow_{reg} \quad (A.3)$$

- SGD-driven Sr ratio flux:

$$SGD \ Sr \ ratio \ flux_{reg} = Sr_{reg} \cdot SGD \ flow_{reg} \cdot ^{87}Sr / ^{86}Sr_{reg} \quad (A.4)$$

where $Sr_{lith}$ and $^{87}$Sr/$^{86}$Sr$_{lith}$ are the Sr concentration and the $^{87}$Sr/$^{86}$Sr ratio assigned to each lithological group, respectively, $\%_{lith}$ is the relative percentage of each lithology in the SGD region and $SGD \ flow_{reg}$ is the SGD flow rate of the region as reported in Zektser et al. (2007).
Appendix B

<table><caption>Table B.1 Contribution of each SGD region to the Sr flux into the Mediterranean Sea.</caption><thead><tr><th>SGD region</th><th>Subsurface drainage area (10<sup>3</sup> km<sup>2</sup>)</th><th>SGD flow rate (km<sup>3</sup> y<sup>-1</sup>)</th><th>% of Med SGD</th><th>% carbonate</th><th>% siliciclastic</th><th>% granite</th><th>% volcanic</th><th>% complex</th><th>Sr<sup>a</sup> (µM)</th><th><sup>87</sup>Sr/<sup>86</sup>Sr<sup>b</sup></th><th>SGD Sr flux<sup>c</sup> (10<sup>6</sup> mol y<sup>-1</sup>)</th><th>Ratio contribution<sup>d</sup> (10<sup>3</sup>)</th></tr></thead><tbody><tr><td>Pyrenean Peninsula</td><td>44.5</td><td>2.37</td><td>3.5</td><td>5</td><td>55</td><td>5</td><td>0</td><td>35</td><td>3 - 4</td><td>0.7096 - 0.7091</td><td>6.2 - 8.5</td><td>+ (3.0 - 0.39)</td></tr><tr><td>South of France</td><td>24.4</td><td>4.88</td><td>7.2</td><td>25</td><td>65</td><td>0</td><td>0</td><td>10</td><td>4 - 9</td><td>0.7090 - 0.7083</td><td>21 - 46</td><td>- (3.4 - 38)</td></tr><tr><td>Apennine Peninsula</td><td>91.5</td><td>18.52</td><td>27</td><td>60</td><td>40</td><td>0</td><td>0</td><td>0</td><td>7 - 19</td><td>0.7084 - 0.7080</td><td>130 - 350</td><td>- (100 - 420)</td></tr><tr><td>Lombardic lowlands</td><td>16.0</td><td>3.2</td><td>4.6</td><td>0</td><td>100</td><td>0</td><td>0</td><td>0</td><td>3</td><td>0.7105</td><td>8.0</td><td>+ 11</td></tr><tr><td>Dinara Mountains</td><td>14.5</td><td>5.84</td><td>8.6</td><td>80</td><td>10</td><td>0</td><td>0</td><td>10</td><td>8 - 24</td><td>0.7081 - 0.7079</td><td>49 - 140</td><td>- (51 - 180)</td></tr><tr><td>Balkan Peninsula</td><td>78.5</td><td>13.84</td><td>20</td><td>25</td><td>30</td><td>0</td><td>0</td><td>45</td><td>4 - 9</td><td>0.7087 - 0.7082</td><td>57 - 130</td><td>- (28 - 130)</td></tr><tr><td>Lesser Asia</td><td>74.0</td><td>6.9</td><td>10</td><td>15</td><td>40</td><td>0</td><td>10</td><td>35</td><td>3 - 6</td><td>0.7088 - 0.7083</td><td>24 - 44</td><td>- (7.7 - 37)</td></tr><tr><td>Near east</td><td>34.1</td><td>1.42</td><td>2.1</td><td>35</td><td>55</td><td>0</td><td>5</td><td>5</td><td>5 - 12</td><td>0.7087 - 0.7081</td><td>7.2 - 17</td><td>- (3.5 - 18)</td></tr><tr><td>Lybian Desert</td><td>318.4</td><td>1.59</td><td>2.3</td><td>55</td><td>45</td><td>0</td><td>0</td><td>0</td><td>7 - 18</td><td>0.7084 - 0.7080</td><td>11 - 28</td><td>- (7.6 - 32)</td></tr><tr><td>Atlas Mountains</td><td>85.4</td><td>3.49</td><td>5.2</td><td>5</td><td>95</td><td>0</td><td>0</td><td>0</td><td>3 - 4</td><td>0.7101 - 0.7095</td><td>10 - 14</td><td>+ (9.1 - 4.2)</td></tr><tr><td>Sicily</td><td>24.9</td><td>1.25</td><td>1.8</td><td>70</td><td>20</td><td>0</td><td>10</td><td>0</td><td>8 - 22</td><td>0.7081 - 0.7079</td><td>9.6 - 27</td><td>- (9.9 - 35)</td></tr><tr><td>Sardinia</td><td>31.8</td><td>3.18</td><td>4.7</td><td>10</td><td>35</td><td>45</td><td>10</td><td>0</td><td>2 - 4</td><td>0.7093 - 0.7086</td><td>7.3 - 14</td><td>- (0.8 - 8.2)</td></tr><tr><td>Corsica</td><td>8.4</td><td>1.27</td><td>1.9</td><td>0</td><td>10</td><td>65</td><td>0</td><td>25</td><td>1</td><td>0.7106</td><td>1.4</td><td>+ 1.9</td></tr></tbody></table>

<footnote>a, b, c, d Calculated as explained in Appendix A</footnote>
# Appendix C

Calculation of SGD-driven Sr fluxes into the Mediterranean Sea from each aquifer type lithology (results reported in Table 3).

- Average-weighted SGD volume fluxes:

$$ \% volume\ flux_{lith} = \frac{\Sigma(% lith_{reg} \ SGD\ flow_{reg})}{\Sigma(SGD\ flow_{reg})} \quad (C.1) $$

- Average-weighted SGD-driven Sr flux:

$$ \%\ SGD\ Sr\ flux_{lith} = \frac{\Sigma(%\ lith_{reg} \ SGD\ flow_{reg} \cdot Sr_{reg})}{\Sigma(SGD\ flow_{reg} \cdot Sr_{reg})} \quad (C.2) $$

- Average-weighted SGD-driven Sr ratio flux:

$$ \%\ SGD\ Sr\ ratio\ flux_{lith} = \frac{\Sigma(%\ lith_{reg} \ SGD\ flow_{reg} \cdot Sr_{reg} \cdot (^{87}\text{Sr} / ^{86}\text{Sr}_{\text{reg}} - ^{87}\text{Sr} / ^{86}\text{Sr}_{\text{SW3}}))}{\Sigma(SGD\ flow_{reg} \cdot Sr_{reg} \cdot (^{87}\text{Sr} / ^{86}\text{Sr}_{\text{reg}} - ^{87}\text{Sr} / ^{86}\text{Sr}_{\text{SW3}}))} \quad (C.3) $$

where % lith<sub>reg</sub> is the percentage of the lithology in each SGD region, SGD flow<sub>reg</sub> is the SGD flow rate reported for the region in Zektser et al. (2007), Sr<sub>reg</sub> and <sup>87</sup>Sr/<sup>86</sup>Sr<sub>reg</sub> are the Sr concentration and the Sr isotope ratio calculated for each SGD region, respectively and <sup>87</sup>Sr/<sup>86</sup>Sr<sub>SW3</sub> is the <sup>87</sup>Sr/<sup>86</sup>Sr ratio of the Mediterranean Seawater.