Ca thermometry

Marine Geology 280 (2011) 195–204

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Diagenetic Mg-rich calcite in Mediterranean sediments: Quantification and impact on foraminiferal Mg/Ca thermometry S. Boussetta a,⁎, F. Bassinot b, A. Sabbatini c, N. Caillon b, J. Nouet d, N. Kallel a, H. Rebaubier b, G. Klinkhammer e, L. Labeyrie b a

University of Sfax, FSS, Lab. GEOGLOB, Route de Soukra, BP 802, 3028 Sfax, Tunisia LSCE (CEA/CNRS/UVSQ), Domaine du CNRS, Bat 12, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France Dipartimento di Scienze del Mare, Università Politecnica delle Marche, Via Brecche Bianche, 60131 Ancona, Italy d UMR IDES 8148, Université Paris-Sud 11, 91405 Orsay, France e College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA b c

a r t i c l e

i n f o

Article history: Received 3 March 2010 Received in revised form 28 December 2010 Accepted 28 December 2010 Available online 7 January 2011 Communicated by: G.J. de Lange Keywords: Mediterranean Sea planktonic foraminifera surface sediment Mg/Ca diagenesis Mg-rich calcite

a b s t r a c t Mg/Ca ratios in planktonic foraminifera have been developed into a powerful tool in paleoceanography to reconstruct past sea surface temperatures (SST). However, additional factors that might have an influence on Mg/Ca, like dissolution, salinity, diagenesis, and carbonate ion effects have come into focus. In this paper, the occurrence of diagenetic calcification and its potential effects on Mg/Ca have been studied using 20 wellpreserved core tops recovered from the Mediterranean Sea. Analyses were performed on two planktonic foraminifer species; Globigerinoides ruber and Globigerina bulloides using ICP-AES Mg/Ca measurements, SEM observations and X-ray diffractometry analyses. Foraminiferal Mg/Ca values are higher than those obtained in the open sea. The highest values were found in the Eastern Mediterranean basin. These anomalies cannot be simply explained by a salinity effect because Mg/ Ca ratios, when corrected for temperature influence, do not display any significant correlation with salinity. Our results seem to indicate that diagenesis can account for anomalously high Mg/Ca values. Indeed, SEM observations show aggregates of rhombohedral crystals that could be interpreted as secondary calcite overgrowths. We note also the occurrence of numerous coccoliths trapped in foraminifer walls and covered with a secondary mineral phase. X-ray diffraction diagrams of numerous foraminiferal samples exhibit peak of Mg-rich calcite (10–12%) associated with the usual foraminifer calcite peak. It has been demonstrated that this calcite highly rich in Mg, is compatible with an inorganic calcite precipitated directly from seawater. The deconvolution of the main XRD peak (104) shows that the percentage of Mg-rich calcite can reach up to ~ 21% of the total foraminiferal calcite in the Eastern basin whereas in the western Mediterranean Sea, proportions do not exceed 5%. In addition, we demonstrate that, the diagenetic process is very heterogeneous (even at the sample scale) and is not simply a reflection of ΔCO3 at the sea bottom. Thus, the high Mg/Ca ratios measured from Mediterranean Sea planktonic foraminifera does not appear to be caused by increased Mg uptake in a high salinity setting, but instead result chiefly from the presence of Mg rich calcite (10–12%) deposited during early diagenetic processes. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Mg/Ca in the calcite of foraminifers increases exponentially with temperature (Anand et al., 2003; Dekens et al., 2002; Elderfield and Ganssen, 2000; Nürnberg et al., 1996). This relationship is the backbone of Mg/Ca-paleothermometry reconstructions (Barker et al., 2005; de Garidel Thoron et al., 2005; Levi et al., 2007). Yet, recent studies pointed out several potential biases of Mg/Ca thermometry.

⁎ Corresponding author. Tel.: +216 22783336; fax: +216 74274437. E-mail address: [email protected] (S. Boussetta). 0025-3227/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2010.12.011

In the Mediterranean Sea, a semi-enclosed basin with high surface salinities ranging from 36.5 to 39 psu, a recent core-top calibration revealed a strong salinity effect on planktonic foraminiferal Mg/Ca, with a 29 ± 13% increase in Mg/Ca per psu for Globigerinoides ruber (Ferguson et al., 2008). This is about six times the Mg/Ca sensitivity to salinity suggested by culture experiments performed on planktonic foraminifera (Kısakürek et al., 2008; Lea et al., 1999). On the inside walls of foraminifera picked from an Eastern Mediterranean site, Ferguson et al. (2008) observed a thin coating (a few μm thick) of high-Mg inorganic calcite (≈10 wt.% Mg), which resisted their cleaning protocol. Regarding their other core tops, however, the authors concluded that the cleaning technique, which

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includes a leaching with a weak acid, essentially removed any diagenetic calcite coating that could have affected Mg/Ca analyses. Yet, one could not totally reject the possibility that the important scattering of the Mediterranean core top Mg/Ca values published by Ferguson et al. (2008), as well as the anomalously strong salinity effect they observed might result from an early diagenetic bias. The inorganic precipitation of Mg-rich calcite had been previously described by several authors working on Mediterranean bulk sediments (Ellis and Milliman, 1985; Mucci, 1987). No attempt has been made, however, to quantify the amount of Mg-rich, inorganic calcite coating on foraminiferal tests and estimate its potential effect on Mg/Ca thermometry. In the present paper, we will quantify the importance of Mg-rich inorganic precipitation deduced from well-preserved Mediterranean core tops and estimate its potential effect on Mg/Ca-T calibration based on ICP-AES Mg/Ca measurements, SEM observations and X-ray diffractometry analyses performed on two planktonic foraminifer species; G. ruber and Globigerina bulloides picked from well-preserved core tops. 2. Material and methods 2.1. Core top selection and planktonic species analysed For this study we selected 20 core tops. Radiometric datings, foraminiferal counts or isotopic stratigraphy, as defined in MARGO (Kucera et al., 2005), were used to control that core top sediments are Late Holocene (Table 1; Fig. 1). Since high-Mg calcite is more soluble than low Mg calcite (Brown and Elderfield, 1996; Davis et al., 2000;

Rushdi et al., 1998), the Mg/Ca ratio of foraminifera calcite diminishes with increasing in situ dissolution. We estimated, at each site, the bottom water departure from calcite saturation (ΔCO2− 3 ) based on the gridded bottom water [CO2− 3 ] atlas from Archer (1996) and using the revised empirical equation of [CO2− 3 ] at saturation versus water depth from Bassinot et al. (2004). At the location of our Mediterranean core tops (Fig. 1), bottom waters are highly supersaturated relative to calcite, with ΔCO2− values that always exceed +100 μmol kg− 1 3 (Table 1). This clearly suggests that the planktonic foraminifer Mg/Ca values we obtained are not affected by dissolution. We focused our study on two planktonic species, G.ruber (white) and G.bulloides, which are frequently used for reconstructing past sea surface Mg/Ca-temperatures (de Garidel Thoron et al., 2005; Elderfield and Ganssen, 2000; Lea et al., 2000; Levi et al., 2007; Mashiotta et al., 1999; Rickaby and Elderfield, 1999). G. ruber is a good indicator of tropical waters (Anand et al., 2003; Deuser, 1987; Field, 2004) where it dwells in the upper 50 m of the water column (Pujol and Grazzini, 1995). G. bulloides is a ubiquitous species that reflects subpolar surface water conditions and is also very abundant in tropical upwellings (Bé, 1977). In the Mediterranean, it is very common, especially in the western basin. But it shows a rather variable depth habitat (Pujol and Grazzini, 1995). G. ruber specimens were picked from the 250–315 μm size fraction and G.bulloides were picked from the 200–250 μm size fraction. 2.2. Analytical methods 2.2.1. Mg/Ca Analyses Trace element analyses were done at LSCE, a laboratory, which was involved in an interlaboratory comparison study of calibration

Table 1 Core top locationsa and stratigraphic controlb. Area

Core

G.ruber white (250–315 μm; habitat: Adriatic Sea MD90-917 Adriatic Sea KET82-16 Adriatic Sea MD90-916 Balearic basin MD90-901 Strait of Sicily KET80-37 Strait of Sicily MD04-2797 Ionian basin KET80-68 Levantine basin MD84-632 Levantine basin MD84-641 Levantine basin MD84-639 Tyrrhenian Sea KET80-22 Tyrrhenian Sea DED87-07 Tyrrhenian Sea KET80-03 Tyrrhenian Sea KET80-19 South of Sicily Site 560 Levantine basin Site 563 Levantine basin Site 564 Levantine basin Site 569

Latitude

Longitude

0–50 m; October–November) 41°17′N 17°37′E 41°31′N 17°59′E 41°30′3″N 17°58′2″E 39°56′84″N 1°33′52″E 36°57′N 11°39′E 36°57′N 11°40′E 38°06′N 17°17.5′E 32°47′3″N 34°22′8″E 33°02′N 32°38′E 33°40′N 32°42′E 40°35′N 11°42′5″E 39°41.22′N 13°34.51′E 38°49′2″N 14°29′5″E 40°33′N 13°21′E 35°51.27′N 14°06.20′E 33°43.08′N 23°30.07′E 32°59.97′N 23°37.61′E 33°27.18′N 32°34.51′E

Depth (m)

Bottom water CO32− (μmoles/kg) a

ΔCO32−

Age control MARGO

1010 1166 1150 1560 740 771 1578 1425 1375 870 2430 2970 1900 1920 989 1881 1496 1294

184.4 183.0 183.2 178.6 182.0 181.6 181.7 178.3 179.3 183.7 178.3 184.4 178.4 179.3 183.40 181.59 180.49 182.56

134.6 131.8 132.1 123.8 134.5 133.8 126.7 124.8 126.2 135.0 114.5 114.4 120.3 121.0 133.74 123.64 126.27 130.21

1 1 1 4 4 4 4 4 2 2 4 4 2 3 4 4 4 4

Siani et al. (2001) Fontugne et al. (1989) Fontugne et al. (1989) LSCE (unpblished data) Kallel et al. (1997) Essallami et al. (2007) LSCE Essallami et al. (2007) Fontugne and Calvert (1992) Fontugne et al. (1994) Paterne et al. (1986) Kallel et al. (2000) Paterne et al. (1986) Paterne et al. (1986) Sabbatini et al. (submitted) Sabbatini et al. (submitted) Sabbatini et al. (submitted) Sabbatini et al. (submitted)

178.6 182.0 181.6 181.7 176.3 177.2 178.3 184.4 178.4 179.3

123.8 134.5 133.8 126.7 113.7 117.0 114.5 114.4 120.3 121.0

4 4 4 4 1 1 4 4 2 3

LSCE Kallel et al. (1997) Essallami et al. (2007) LSCE IFREMER, Brest, France Melki et al. (2009) Paterne et al. (1986) Kallel et al. (2000) Paterne et al. (1986) Paterne et al. (1986)

G.bulloides (200–250 μm; habitat: variable depth (b200 m); April–May) Balearic basin MD90-901 39°56′84″N 1°33′52″E 1560 Strait of Sicily KET80-37 36°57′N 11°39′E 740 Strait of Sicily MD04-2797 36°57′N 11°40′E 771 Ionian basin KET80-68 38°06′N 17°17.5′E 1578 Gulf of Lion MD99-2344 42°02.61′N 4°09′04″E 2326 Gulf of Lion MD99-2346 41°59.84′N 4°50′14″E 2099 Tyrrhenian Sea KET 80-22 40°35′N 11°42′5″E 2430 Tyrrhenian Sea DED 87-07 39°41.22′N 13°34.51′E 2970 Tyrrhenian Sea KET80-03 38°49′2″N 14°29′5″E 1900 Tyrrhenian Sea KET80-19 40°33′N 13°21′E 1920

b

Referencesb

MARGO, Multiproxy Approach for the Reconstruction of the Glacial Ocean; LSCE, Laboratoire des Sciences du Climat et de l'Environnement. a Bottom Water CO32− values are extracted from Archer database (1996).The ΔCO2− was calculated using the revised empirical equation of [CO2− 3 3 ] at saturation versus water depth from Bassinot et al. (2004). b Chronostratigraphic quality levels go from 1 to 4 with different levels of uncertainty according to MARGO criteria: number 1 and 2 are for radiometric control within the interval 0–2 ka and 0–4 ka, respectively; number 3 is used for specific stratigraphic control (like percent Globorotalia hirsuta left coiling); and number 4 represents other stratigraphic constraints (Kucera et al., 2005).

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Fig. 1. Sample location map.

standards for foraminiferal Mg/Ca thermometry (Greaves et al., 2008). About 20–30 foraminifer tests per sample were picked for each Mg/Ca measurement. These shells were gently crushed to open the chambers. Samples were cleaned following the procedure of Barker et al. (2003). In this cleaning protocol, the reductive hydrazine step from the original works by Boyle (1981) and Boyle and Keigwin (1985) has been removed, being slightly corrosive for carbonates and reducing the foraminifer Mg/Ca (Rosenthal et al., 2004). Thus, Barker et al.'s (2003) cleaning protocol includes: (1) multiple, ultra-sonic cleanings in water and then alcohol to remove clays and other fine material; (2) the elimination of organic matter through a hydrogen peroxide treatment at 100 °C; (3) a rapid leaching with 0.001 M nitric acid in order to eliminate any adsorbed contaminants from the surface of the test fragments. Samples were finally dissolved in 400 μl of 0.1 M nitric acid, and centrifuged to remove any remaining insoluble particles. Samples were analysed with a Varian Vista Pro Inductively coupled Plasma Atomic Emission Spectrometer (ICP-AES) at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) following the method of de Villiers et al. (2002). On average, clay minerals contain between 1 and 10 weight% Mg (Deer et al., 1992). Clay contamination may cause, therefore, a significant bias in foraminifer Mg/Ca ratios. Barker et al. (2003) showed that the covariances of Fe/Ca and Mg/Ca, as well as Fe/Mg values exceeding 0.1 mol mol− 1 are indicators of clay contamination. All our cleaned samples showed a Fe/Mg ratio smaller than 0.1 mol mol− 1, and neither Fe/Ca nor Al/Ca correlate with Mg/Ca. This clearly shows the efficiency of the cleaning procedure in removing silicate contaminants. The mean external reproducibility of a standard solution of Mg/ Ca = 5.23 mmol mol− 1 is ±0.5% (RSD). We performed replicate analyses (2 per sample) on 8 samples of G.ruber. The mean reproducibility is ±10% (about ~0.49 mmol mol− 1). In this study, together with the conventional ICP-AES methodology, we obtained several Mg/Ca measurements from the flowthrough time resolved analysis method (FT-TRA; Haley and Klinkhammer, 2002) A total of 5 core-top samples, containing between 25 and 45 G. ruber (batch weights between 405 and 788 μg) were analysed at Oregon State University.. (See Appendix 1 for more details). 2.2.2. Scanning electron microscope observations On selected samples, 10 to 15 G. ruber and G. bulloides shells were picked and carefully cleaned in an ultrasonic bath with ethanol. They were mounted on a plot using a double-sided carbon tape and coated with a Polaron E5100 cool sputter coater equipped with a Au/Pd target. Secondary electron observations were carried out on a Philips

XL 30 scanning electron microscope operated at 25 Kev at UMR IDES (University Paris-Sud 11). 2.2.3. X-ray diffraction analyses G.ruber (white) and G.bulloides specimens were picked in the same size fraction as for ICP-AES minor element analyses (250–315 μm and 200–250 μm, respectively). In order to prepare powder for X-ray diffraction analyses, between 20 and 30 shells (about 250 μg) were soaked in ethanol, and gently grinded in an agate mortar. The powder was evenly distributed upon a zero-background sample holder within a drop of ethanol in order to facilitate the evenly distribution of particles. Analyses were performed at IDES (University Paris Sud 11) on an Xpert Pro X-ray diffractometer (Panalytical), equipped with a Cu Xray tube, a Ni filter that eliminates the cu Kβ X-ray, 0.02 rad soller slits and a real time multiple strip detector. 3. Results 3.1. Mg/Ca ratios Results for Mg/Ca are summarized in Table 2. As was observed in Ferguson et al. (2008) core top database, Mg/Ca values show a large variability with values ranging from 3.4 to 13 mmol mol− 1 (an exceptional high value of 35.5 mmol mol− 1 found to the South of Sicily; Site 560) and from 4.0 to 5.6 mmol mol− 1 for G.ruber and G. bulloides samples, respectively (Fig. 2). The G.ruber data from the Eastern Mediterranean basin show the highest values and the largest variability, with Mg/Ca ranging from ~6 to ~13 mmol mol− 1 in addition to the high value (35.5 mmol mol− 1) of site 560. In order to analyse the relationship between core top Mg/Ca and sea surface temperatures (SST), we used G. ruber and G. bulloides δ18O data (δ18Of) that had been obtained on the same core top material at LSCE during previous studies (Table 2). We calculated the calcification temperature (Tiso) of foraminifer species from these δ18Of measurements using the paleotemperature equation of Shackleton (1974):   18 18 Tiso = 16:9–4:38
δ Of + 0:27−δ Osw + 0:1  2 18 18
δ Of + 0:27−δ Osw where δ18Osw represents the oxygen isotopic ratio of seawater, whose values were extracted from a gridded atlas (LeGrande and Schmidt, 2006) at the appropriate depth habitats for each species in the Mediterranean Sea (Kallel et al., 1997; Pujol and Grazzini, 1995). We choose to correlate Mg/Ca to isotopic temperatures derived from the

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Table 2 Hydrographic parameters at the location of core top and geochemical data obtained on G.ruber (white) and G.bulloides. Core

SSS (psu)

δ O18a foram (PDB)

Referencesa

G.ruber white (250–315 μm; habitat: 0–50 m; October–November) MD90-917 38,5 0,64 Siani et al. (2001) KET82-16 38,5 0,71 Fontugne et al. (1989) MD90-916 38,5 0,87 Fontugne et al. (1989) MD90-901 37,4 0,54 LSCE (unpublished data) KET80-37 37,7 0,41 Kallel et al. (1997) MD04-2797 37,7 0,73 Essallami et al. (2007) KET80-68 38,3 0,48 LSCE MD84-632 39,3 0,66 Essallami et al. (2007) MD84-641 39,3 0,25 Fontugne and Calvert (1992) MD84-639 39,3 0,76 Fontugne et al. (1994) KET80-22 38,1 0,49 Paterne et al. (1986) DED87-07 38,0 0,63 Kallel et al. (2000) KET80-03 37,9 0,66 Paterne et al. (1986) KET80-19 38,0 0,84 Paterne et al. (1986) Site 560 37,7 0,93 Sabbatini et al. (submitted) Site 563 38,8 0,96 Sabbatini et al. (submitted) Site 564 38,8 0,69 Sabbatini et al. (submitted) Site 569 39,0 0,54 Sabbatini et al. (submitted) G.bulloides (200–250 μm; habitat: variable depth (b200 m); April–May) MD90-901 38,1 1,18 LSCE (unpublished data) KET80-37 37,7 MD04-2797 37,7 KET80-68 38,6 0,85 LSCE (unpublished data) MD99-2344 38,3 1,14 IFREMER, Brest, France MD99-2346 38,2 1,29 Melki et al. (2009) KET 80-22 38,2 1,36 Paterne et al. (1986) DED 87-07 38,1 1,06 Kallel et al. (2000) KET80-03 38,0 1,46 Paterne et al. (1986) KET80-19 38,1 1,14 Paterne et al. (1986)

δO18w (smow)

Mg/Ca (mmol mol− 1)

Tiso (°C)

T Mg/Ca (°C)

Mg/Ca expected (mmol mol− 1)

Δ Mg/Ca (mmol mol− 1)

1,55 1,55 1,55 1,14 1,28 1,28 1,51 1,70 1,69 1,69 1,45 1,40 1,34 1,39 1,27 1,60 1,60 1,63

3,4 4,3 3,9 3,3 3,8 4,8 4,9 5,6 12,4 10,0 4,3 3,9 4,0 4,5 35,5 17,0 17,8 19,6

19,7 19,4 18,7 18,4 19,6 18,1 20,3 20,4 22,2 19,8 20,0 19,1 18,7 18,1 17,2 18,5 19,7 20,6

22,5 25,1 24,0 22,3 23,8 26,3 26,5 28,1 36,8 34,4 25,2 24,2 24,4 25,6 48,6 40,4 40,9 42,0

2,7 2,6 2,4 2,3 2,6 2,3 2,8 2,8 3,3 2,7 2,7 2,5 2,4 2,3 2,1 2,4 2,7 2,9

0,8 1,7 1,5 1,0 1,2 2,5 2,1 2,8 9,1 7,3 1,6 1,4 1,6 2,2 33,4 14,6 15,1 16,7

1,46

4,0 4,0 5,6 4,4 5,3 4,8 5,1 4,3 4,6 4,9

17,0

19,8 19,7 23,9 20,8 23,1 22,0 22,7 20,7 21,4 22,3

3,2

0,8

3,7 3,3 3,1 3,0 3,3 2,9 3,2

0,6 2,0 1,7 2,1 1,0 1,7 1,7

1,56 1,49 1,48 1,47 1,47 1,45 1,45

18,9 17,3 16,5 16,2 17,5 15,7 17,1

Mg/Ca (mmol/mol)

SSS and δ18Osw values are extracted fromWOA05 (Antonov et al., 2006) and Legrande and Schmidt (2006) Atlas, respectively, for the appropriate depth habitats and season of peak abundance for each species. Mg/Ca was measured on G.ruber and G.bulloides and the corresponding temperatures (T Mg/Ca) calculated based on the appropriate calibration (from Anand et al. (2003) for G.ruber and from Elderfield and Ganssen (2000) for G.bulloides).Mg/Ca expected is the Mg/Ca ratio that would result by injecting Tiso in the appropriate temperature calibration used for each species. ΔMg/Ca is the difference between Mg/Ca measured and calculated Mg/Ca expected. a δ18Of (measured on G.ruber and G.bulloides) values was taken from the previous study and the isotopic temperature (Tiso) calculated using equation from Shackleton (1974).

36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 14

G.ruber G.bulloides

16

18

20

This study Fergusson et al. (2008) This study Ferguson et al. (2008)

22

24

Isotopic Temperature (°C) Fig. 2. Comparison of Mg/Ca data plotted versus the corresponding Tiso (δ 8O-derived temperature) of Globigerinoides ruber (symbolized by orange triangle) and Globigerina bulloides (symbolized by blues squares) from Mediterranean core tops (this study and Ferguson et al., 2008) with previous North Atlantic works on the same species. The orange continuous curve is the exponential regression of Anand et al. (2003) (Mg/ Ca = 0.449 e 0.09T) for G.ruber; The blue dashed curve is the exponential regression of Elderfield and Ganssen (2000) (Mg/Ca = 0.81 e 0.081T) for G.bulloides Graph showing the poor relationships between Mg/Ca values and the corresponding Tiso and, the high scattering Mg/Ca data.

equation of Shackleton (1974) to be fully consistent with the reference, Mg/Ca-T calibration exercise of Anand et al. (2003). The plot of Mg/Ca versus isotopic temperatures (Fig. 2) clearly shows that: 1) Mg/Ca values are usually higher than what can be expected from recent Mg/Ca-isotopic temperature empirical calibrations based on open-ocean sediment traps (Anand et al., 2003) or core tops (Elderfield and Ganssen, 2000); and 2) even when the highly anomalous value of Site 560 is not taken into account, the Mg/Ca values are highly scattered and show a non significant relationship to temperature (R2 = 0.17; n = 17 for G. ruber). The exponential correlation between G. ruber Mg/Ca and SST does not improve (R2 = 0.12; n = 17) if, instead of isotopic temperatures, one uses the seasonal sea surface temperatures derived from the World Ocean Atlas (Locarnini et al., 2006) at the species depth habitat and peak season (Kallel et al., 1997; Pujol and Grazzini, 1995). The scattering is still extremely high. In addition, the resulting, empirical regression equation between Mg/Ca and SST (Mg/Ca= 0.74exp 0.11⁎SST) is radically different from those previously obtained for this species (Anand et al., 2003; Dekens et al., 2002; Elderfield and Ganssen, 2000; Lea et al., 2000). For G. bulloides, there is clearly no relationship to atlas-derived SST (R2 = 0.04).

3.2. X-ray diffractometry (XRD) results The X-ray diffractograms confirmed that our core tops are neither contaminated by silicates nor by dolomite, which – because it contains 50% of Mg – could have constituted a strong contaminant (Cléroux et al., 2008).

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3.3. Scanning electron microscope observations Several foraminifer shells were examined with a scanning electron microscope. Foraminifer pores are usually closed. This is particularly visible on samples from the Eastern Mediterranean Sea (Fig. 4). On the outside surface of several foraminiferal chambers, we frequently observed coccoliths incrusted into a mineral gangue (Fig. 4a, b, and c). Aggregates of rhombohedral crystals are very common (Fig. 4d).

4. Discussion 4.1. Can a salinity bias explain planktonic foraminifera high Mg/Ca values, and scattering in Mediterranean core tops?

Fig. 3. X-ray diffraction peak obtained for G.ruber from Eastern Mediterranean sample (Site 560) showing the main foraminiferal calcite (104) peak (blue arrows) and, the occurrence of second calcite phase (red arrow).

However, all the diffractograms of G.ruber shells (Fig. 3) clearly reveal the presence of two calcitic phases. One set of diffraction peaks corresponds to a standard calcite diffraction pattern, while the other set shows calcite peaks that are shifted towards higher diffraction angles. This second type of calcite is particularly abundant in samples located in the eastern basin (i.e. MD 84-632; MD 84-639; MD 84-641; Site 560).

As shown above, Mg/Ca data obtained on G.ruber and G.bulloides from Mediterranean core tops are scattered and show anomalously high values that cannot be explained by Mg/Ca–SST relationships derived from open ocean traps (Anand et al., 2003) or core top material (Elderfield and Ganssen, 2000). Our Mg/Ca data are close to values obtained by Ferguson et al. (2008) (although the cleaning methods are different). However, in the Levantine basin, there is a strong discrepancy between our data and those of Ferguson et al. (2008) (Fig. 5). Fergusson et al. explained the higher values of Mediterranean planktonic foraminifer Mg/Ca by a strong imprint of salinity in this evaporative basin. We have also compared our results with those obtained by Mathien-Blard and Bassinot (2009) on well-preserved, open ocean core tops and several plankton tow samples to establish the dependence between salinity and foraminiferal Mg/Ca ratio (Fig. 5). Mathien-Blard and Bassinot (2009) showed that the difference (ΔMg/ Ca) between measured G. ruber Mg/Ca values, and Mg/Ca estimated from the oxygen isotopic temperatures using Anand et al. (2003) empirical equations, is a linear function of sea surface salinity (Fig. 5). For the Mediterranean core tops, we averaged World Ocean Atlas 2005 salinity data (Antonov et al., 2006) considering the appropriate depth habitats and peak season (October–November) for G.ruber in the Mediterranean Sea (Kallel et al., 1997).

34 32 This study G.ruber 30 Fergusson et al. (2008) 28 Mathien-Blard and Bassinot 26 (2009) 24 Mathien-Blard and Bassinot 22 (2009) 20 18 16 14 12 10 8 6 4 Mg/Ca = 0,67*SSS - 23, 97 2 R2 = 0,78 0 -2 -4 32 33 34 35 36 37

38

39

Sea Surface Salinity (psu)

Fig. 4. High resolution SEM image of the exterior of planktonic foraminifera shells (G. ruber) from an Eastern Mediterranean core (Site 569) showing (a) numerous, large rhombohedral mineral overgrowths; (b), (c) and, (d) coccoliths trapped on the foraminifera test.

Fig. 5. Residual Mg/Ca (ΔMg/Ca) = (Mg/Ca (measured) minus Mg/Ca expected) plotted against sea surface salinity for G.ruber. Mg/Ca expected was estimated from the oxygen isotopic temperatures using Anand et al. (2003) empirical equations. Mediterranean core tops of this study are symbolized by pink triangles; Mediterranean core tops of Ferguson's et al. (2008) study are symbolized by opened blue triangles; and Indo-Pacific and North Atlantic core tops and plankton tow samples analysed by Mathien-Blard and Bassinot (2009) are shown as red points. The black line materializes the linear regression equation obtained by Mathien-Blard and Bassinot (2009).

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We notice that the samples that depart the most from MathienBlard and Bassinot's empirical regression line, with the exception of Site 560, are those located in areas with salinity higher than 38.5 psu (Fig. 5). All these samples are located in the Levantine basin (S ~ 39 psu), and they show widely scattered values, with ΔMg/Ca ranging from 2.8 to 16.7 mmol mol− 1. It seems, therefore, that contrary to what was observed from open ocean core tops retrieved in high salinity waters (i.e. Arabian Sea or Red Sea; Mathien-Blard and Bassinot, 2009), the strong scattering and anomalously high Mg/Ca values measured on our Levantine basin core tops cannot be easily attributed to a sea surface salinity bias. 4.2. The deposit of diagenetic calcite within the Mediterranean Sea In numerous core tops, X-ray diffractometry of G.ruber and G. bulloides shells revealed the presence of two sets of calcite with different lattice parameters. The first set of calcite corresponds to calcite usually found in planktonic foraminifers (Nouet and Bassinot, 2007), while the other set shows diffraction peaks that are shifted towards higher angles. Since our cleaning protocol successively removed all traces of clays and other contaminant, it seems excluded that this Mg-rich calcite could correspond to a detrital contamination. The replacement of Ca by other cations with a different size results in the modification of the calcite lattice cell dimension and the corresponding angular displacement of diffraction peaks. Our ICP-AES results suggest that, among the common cations that can easily replace Ca in calcite (namely Sr, Fe, and Mg) only Mg is observed in noticeable proportions. The shift of the calcite peak according to Mg content was already reported by (Goldsmith et al., 1958) and recently applied to determining Mg/Ca ratio of fine Mg-calcite from the Caribbean Sea (Sepulcre et al., 2009). Using calcium carbonate material with %Mg content precisely measured from ICP-AES analyses and using the same analytical protocol (i.e. Xpert Pro X-ray diffractometer, zero-background sample holder), Nouet and Bassinot (2007) have derived a precise, empirical relationship between calcite main peak (104) angular position and %Mg content. They obtained the following linear relationship: %Mg = ð30:966
AÞ–910:66

  R2 = 0:99

with A the angular position of peak (104) in °2θ. According to this equation, the angular shift of the peaks corresponding to the second type of calcite in our Mediterranean samples translates into a 10–12 weight% of Mg (Table 3). It has been showed that bi-lamellar foraminifers have a heterogeneous Mg distribution, characterized by Mg-rich primary calcite layers (Eggins et al., 2003; Gehlen et al., 2004; Sadekov et al., 2009; Segev and Erez, 2006). Yet, the 10–12% of Mg we estimate from XRD analyses far exceed the highest Mg content that was reported for biologically precipitated, foraminifer calcite. The 10–12% Mg of the second calcitic phase is compatible with an inorganic calcite precipitated directly from seawater. Mucci (1987) summarized much of the data on the composition of magnesian calcite cements in different environments (such as Mediterranean Sea and Red Sea). He found that many of the shallow-water and deep-sea carbonate cements contain 10–15% magnesium (with a strong maximum in magnesium abundance at ~ 13%). Several authors (Aksu et al., 1995; Calvert and Fontugne, 2001; Milliman and Müller, 1973; Müller and Staesche, 1973) have shown that the Mediterranean sediments, especially those of eastern basin, are characterized by inorganic–diagenetic precipitation of high-magnesium calcite. Thus, overgrowths observed on foraminifer shells are likely related to the precipitation of high-magnesian calcite from interstitial waters close to the sediment/water interface. SEM observations of our Mediterranean core top foraminifers reveal rhombohedral, well-crystallised crystals that can be inter-

Table 3 Results of the deconvolution XRD (104) peak for G.ruber and quantification of its Mg amount. Core

Mg/Caa (mmol mol− 1)

(%) of highb Mg-calcite

% of Mg inc high Mg-calcite

KET 82-16 MD90-916 KET80-37 MD04-2797 MD84-632 MD84-641 MD84-639 KET80-22 MD90-901 Site 560 Site 563 Site 564 Site 569

4.3 3.9 3.8 4.8 5.6 12.4 10.0 4.3 3.3 35.5 17.0 17.8 19.6

0.8 0.9 3.7 2.5 7.8 9.0 9.4 2.0 4.9 4.9 11.0 12.8 20.6

14.0 12.1 11.0 10.5 11.8 12.4 10.9 11.8 11.6 12.4 11.5 11.5 11.8

a

Mg/Ca was measured by ICP-AES on G.ruber in 250–315 μm size fraction. The relative amount (in %) of the Mg-rich calcite (Mg-rich calcite/(Mg-rich + Mgpoor calcite)) has been estimated based on the areas of the two calcite phases in the XRD profiles. c The percentage of Mg in high Mg-calcite was calculated from the angular shift of the main calcite diffraction peak (104) based on Nouet and Bassinot's (2007) empirical relationship between calcite main peak (104) angular position and %Mg content. b

preted as secondary calcite overgrowths. They also show the occurrence of coccoliths incrusted in a secondary mineral phase that partially covers the foraminifer's walls. Put together with the XRD results, and in good accordance with several sedimentological studies (Aksu et al., 1995; Calvert and Fontugne, 2001; Milliman and Müller, 1973; Müller and Staesche, 1973), our SEM data clearly confirm that foraminiferal tests picked from Mediterranean core tops are frequently coated by a Mg-rich calcite (10–12% Mg), which likely precipitated during early diagenesis, when foraminifers where embedded in the sediments and could aggregate with coccoliths. 4.3. Deconvolution procedure and quantification of diagenetic calcite Overgrowths were observed by Crudeli et al. (2004, 2006), while studying nanofossils (Emiliania huxleyi). Crudeli et al. (2004) suggested that fluctuations in the abundance of nannofossil morphotypes in the Eastern Mediterranean do not necessarily reflect environmental conditions in surface waters (temperature, fertility, etc.), but could be due to diagenetic overprint during early burial. Based on scanning electron microscope (SEM) analyses of selected samples, these authors showed that E. huxleyi coccoliths are affected by atypical early carbonate diagenesis in Holocene–late Pleistocene Mediterranean sediments. Given the absence of strongly overgrown coccoliths in surface waters and their rarity in the water column, but their common occurrence in surface sediment samples, Crudeli et al. (2004) inferred that the alteration mainly occurs at the sediment water interface and likely within the first millimetres–centimetres of sediments. Similar diagenesis imprint has been reported in the Red Sea. Winter (1982a,b) observed E. huxleyi coccoliths, which were heavily affected by secondary calcite overgrowth and/or etching in late Quaternary sediments and proposed that this was likely due to early diagenetic processes. In order to quantify the amount of diagenetic coating on planktonic shells retrieved from Mediterranean Sea core tops, we deconvolved the main XRD peaks (104) of the two calcitic phases using the “Powder X-ray Data Analysis” software. The surface area of deconvolved XRD (104) peaks is proportional to the calcite abundance. This makes it possible to easily estimate the % of the diagenetic phase relative to the foraminifer calcite (Table 3). The geographical distribution of the relative abundance (%) of the diagenetic calcite is shown in Fig. 6A for G.ruber and 6B for G.bulloides. These maps show that the Mg-rich diagenetic calcite is present over the

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201

Fig. 6. a) map showing the geographic distribution of the relative amount (%) of Mg-rich, diagenetic calcite coating on G.ruber shells. b) map showing the geographic distribution of % Mg-rich calcite coating on G.bulloides shells.

whole Mediterranean Sea. However, its relative abundance decreases generally from East to West; the highest percentage of Mg-rich calcite being observed in the Eastern Mediterranean Sea with values reaching up to ~21%. In the western basin the relative abundance of Mg-rich calcite are lower, ranging from ~1 to 5% only for G.ruber and ~0.7 to 4% for G.bulloides. Waters of the Eastern Mediterranean Sea are more salty and warmer than those of the western basin; hence they should be more supersaturated with respect to calcium carbonate throughout the water column even at depth and, therefore, this high saturation state most likely triggers the precipitation of inorganic calcite on foraminiferal shells after they have finished their life cycle. Likewise, by studying record of the Mediterranean Sea, authors have shown that coccoliths, including E. huxleyi, with incipient syntaxial overgrowth are common in Holocene–late Pleistocene hardgrounds, which are widespread in the Eastern Mediterranean and less common in the western basin (Allouc, 1990). Data obtained on nearby core tops also clearly indicate that there is a strong heterogeneity at the local scale, even possibly at the sample scale. In the Levantine basin, for instance, the relative abundance of Mg-rich calcite deposited on G. ruber varies from ~ 8 to 21% in nearby core tops. In the same core tops, the percentage of diagenetic calcite is also different for G.ruber and G.bulloides (G.ruber usually showing a higher relative contribution of diagenetic calcite compared to G. bulloides with the exception of MD04-2797 core top). Similarly, Crudeli et al. (2004) have shown that E. huxleyi coccoliths are differentially affected by carbonate diagenesis. They observed that even within a single sample there were noticeable variations, from well-preserved E. huxleyi coccoliths to massively overgrown speci-

mens. They attributed the fact that overgrowths are usually more developed on E. huxleyi than on other coccoliths, to the more open disposition of their calcitic elements and to their crystallographic orientation. Most coccolith species are constituted of directly butting elements, leaving a smaller surface area for overgrowth to develop. It should be noted that the amount of diagenetic coating (4.9%) deposited on G. ruber retrieved from Site 560, cannot explain the very high Mg/Ca of this core top (35 mmol mol− 1). This 35 mmol mol− 1 Mg/ Ca value corresponds to the average of two independent analyses, which yielded similar, very high Mg/Ca values (30 and 40 mmol.mol− 1). The Fe/Ca and Al/Ca values (average 0.2 and 0.4 mmol mol− 1, respectively) measured during these analyses are slightly higher than what is usually observed for the other samples. Yet, such Fe and Al levels remain too small to suggest a significant contamination by clay minerals. In addition, XRD analysis did not reveal the presence of dolomite. Thus, at present, we have no clear explanation for the anomalously high Mg/ Ca value of Site 560. One possible hypothesis is that high-Mg coating is highly heterogeneous at the sample scale in this Site 560 sample, and that the foraminifer shells picked for XRD analyses were less affected by diagenetic overgrowths than those picked for trace elemental analyses. Unfortunately, there is not enough material left in this sample to duplicate the XRD analysis. 4.4. Does bottom water saturation control the amount of diagenetic calcite? In hypersaline basins like the Mediterranean, relatively high temperatures and salinities are expected to produce waters

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Table 4 Results of sequential leaching for G.ruber and G.bulloides when applying two successive acid leaching. Core

Species

Size fraction

Mg/Ca (mmol mol−)with 1 acid leach

Mg/Ca (mmol mol− 1)with 2 acid leach

Mg/Ca expected

MD99-2344 MD99-2346 MD84-639 MD84-641

G.bulloides G.bulloides G.ruber G.ruber

200–250 μm 200–250 μm 200–250 μm 200–250 μm

4.1 4.6 40.5 19.4

4.0 4.1 28.9 17.5

2.4 2.2 3.5 3.5

supersaturated with respect to calcium carbonate and, therefore, help the precipitation of inorganic calcite within bottom sediments or in the water column (Ellis and Milliman, 1985). As we mentioned previously, the bottom water departure from calcite saturation 2− 2− (ΔCO2− 3 = [CO3 ] bottom − [CO3 ] saturation ) of our Mediterranean core tops is high at all sites, exceeding 100 μmol kg− 1. We plotted the relative abundance of high-Mg calcite versus the corresponding, local bottom water (Δ CO2− 3 ) for our core tops. No correlation is observed (R2 = 0.01). The lack of relationship does not come as a surprise, giving the strong, local heterogeneity showed by the relative abundance of high-Mg calcite. Our data suggest that, even if super-saturation relative to calcite is probably requested for calcite precipitation to take place, the diagenetic process is not simply controlled by the sea bottom ΔCO3. The chemistry of pore fluids at the micro-scale level and the abundance of calcite-growth inhibitors (i.e. Mg, organic compounds; Morse, 1986; Hoch et al., 2000; Zhang and Dawe, 2000) play certainly a crucial role in the amount of Mg-rich, inorganic calcite that precipitates on foraminifera shells. The relative abundance of Mg-rich coating on G. ruber and G. bulloides shells from the same samples is different. One still needs to understand whether this is significant or not. This may well indicate that crystallography plays a major role in favouring overgrowth or dissolution of foraminifer shells. Crystalline factors (i.e. axis orientations, availability of specific crystal planes for secondary overgrowth) and/or morphologic factors (i.e. specific surface) might also play a significant role in the amount of diagenetic Mg-rich calcite that will ultimately coat the foraminifer shells.

4.5. Implication for Mg/Ca thermometry calibration in the Mediterranean Sea using conventional ICP-AES or ICP-MS measurements The deposition of secondary inorganic calcite containing 10–12% of Mg, even in small quantity, has the potential to change noticeably Mg/Ca values obtained on whole planktonic shells (which contain only a few mmol mol− 1 Mg/Ca; Sexton et al., 2006). A rapid calculation indicates that if a planktonic foraminifer shell containing 4.25 mmol mol− 1 of Mg (a ratio expected for SST around 25 °C; Anand et al., 2003), is coated with a Mg-rich calcite (12% Mg) that constitutes between 2 and 25% of the total calcite, the resulting Mg/Ca value measured from our ICP-AES method would reach 6.5 to 33 mmol mol− 1, respectively. One notices immediately that, even in the Levantine Basin (where the highest contribution of Mg-rich calcite was observed, 21%), we never obtained ICP-AES Mg/Ca values above 15 mmol mol− 1. This likely indicates that a large part of the Mg-rich diagenetic calcite is probably dissolved during the cleaning procedure (leaching phase), prior to ICP-AES Mg/Ca analyses. We could readily observe the effect of acid leaching on the total Mg/Ca of our core top samples by applying a sequential leaching for G. bulloides and G.ruber picked from four samples. Since we could not find enough G.ruber specimens left over in the 250–315 μm size fraction, the foraminifera picked for sequential leaching analyses were retrieved in the smaller size fraction (200–250 μm). The samples that have undergone two acid leaching clearly show lower Mg/Ca values than those that have undergone only one acid leaching (Table 4). Nevertheless, regarding SST over the Mediterranean Sea, Mg/Ca values that we obtained with two successive acid leaching are still significantly higher than what is expected based on published

Elderfield and Ganssen (2000) and Anand et al. (2003) temperature calibrations. Considering the results obtained using only one step of acid leaching, it is interesting to note that Mg/Ca values obtained on G. ruber picked in the small size fraction (200–250 μm; Table 4) are surprisingly higher than those measured on the larger size fraction (250–315 μm; Table 2). This inverse relationship between shell size and Mg/Ca level appears to be opposite to what has been usually described for planktonic foraminifer species (Elderfield et al., 2002; Ferguson et al., 2008). Our data suggest, therefore, that, in the Eastern Mediterranean Sea, carbonate diagenesis effect is more pronounced on small shells. 4.6. A solution for Mg/Ca thermometry in Mediterranean Sea: the flowthrough time resolved (FT-TRA) method The flow-through time resolved method (FT-TRA) to measure Mg/ Ca ratio was developed at Oregon State University (Klinkhammer et al., 2004). In this method, the sample is progressively dissolved and the Mg/Ca ratio of acid flow is measured all along the process (Hoogakker et al., 2009). This makes it possible to separate Mg contributions from various calcitic phases since Mg-rich phases will dissolve more rapidly than Mg-poor phases. This method can potentially separate the Mg signal of Mg-rich, diagenetic calcite from the more dissolution resistant, Mg-poor biogenic calcite. It can further resolve the biogenic components into primary and more resistant fractions that may reflect habitat or other changes during foraminifer ontogeny. In order to test the potential of this method for Mg/Ca thermometry in Mediterranean Sea, analyses were performed on G. ruber, in the same size fraction used in Mg/Ca analyses with ICP-AES. Mg/Ca values obtained with this method are significantly lower than those obtained by conventional ICP-AES analyses (Table 5) and show a much improved homogeneity. While the FT-TRA process applied here did not completely separate secondary phases from biogenic calcite, the separation was good enough to isolate a pristine ratio at some point during the dissolution process except for core MD 84-639 for which we report a maximum value (Table 5). Since the FT-TRA approach can remove the diagenetic calcite, the G.ruber temperature calculated from Mg/Ca (FT-TRA) using Anand et al. (2003) empirical equations fits relatively nicely to the calcification isotopic temperature (T.iso). Our data also show that the isotopic derived temperatures, with the exception of MD84-641 sample, are generally lower than the temperature calculated from Mg/Ca (FT-TRA). We suggest that recrystallization of Mediterranean planktonic foraminifer shells, which occurs at shallow burial depths and the addition of diagenetic Table 5 Comparison of Mg/Ca ratios measured by flow-through time resolved (FT-TRA) method and Mg/Ca ratios measured by ICP-AES. Core

Size fraction

Mg/Ca ICP-AES

SST Mg/Ca

Mg/Ca (FT-TRA)

SST Mg/Ca

Tiso

MD 84-641 MD 84-639 MD99-2346 MD04-2797 DED 87-07

250–315 μm 200–250 μm 250–315 μm 250–315 μm 250–315 μm

12.4 40.5 3.7 4.8 3.9

36.8 50.0 23.4 26.3 24.2

2.87 b 14 2.94 2.79 2.77

20.6 b38.2 20.9 20.3 20.2

22.2 19.84 19.14 18.10 19.1

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calcite might plausibly displace δ18O measurements to more positive values biasing the isotopic derived temperature towards cooler values than expected from present observations. A particular attention has been addressed to this process in other studies (Pearson et al., 2001). The comparison of a sequential flow-through time resolved diagram obtained for a sample from the western basin (See Appendix 2 Fig. A) and another sample from the eastern basin (more affected by diagenetic process; See Appendix 2 Fig. B) confirms that it is the presence of a diagenetic coating that is causing anomalously high planktonic foraminiferal Mg/Ca ratios, and that only by getting rid of this high Mg-coating we could get a reasonable Mg/Ca ratio.

5. Conclusions 1- Regarding SST values in the Mediterranean Sea, core top foraminiferal Mg/Ca appears anomalously high relative to those predicted using open ocean Mg/Ca-T calibrations. The highest Mg/ ca values are measured in the Levantine basin. 2- Our SEM and XRD results strongly support the hypothesis that these high and scattered Mg/Ca values result from diagenetic alteration and the post-depositional precipitation of inorganic, Mg-rich (10–12%) calcite on foraminiferal shells. This inorganic calcite coating can represent up to 21% of the total volume of calcite for G.ruber in the Eastern Mediterranean basin. 3- Although this diagenetic coating appears to be generally more important in the eastern basin, there is considerable local variability (even at the sample scale) and no relationship to bottom water (ΔCO2− 3 ) is observed. A variable part of this Mg-rich calcite is probably lost during cleaning procedure (acid leaching), adding to the ICP-AES Mg/Ca variability. 4- According to our results, and regardless of possible salinity effects on the Mg/Ca ratio (Ferguson et al., 2008), the Mg/Ca calibration in the Mediterranean Sea is not straightforward. All potential effects which may control the Mg incorporation into foraminiferal shells, including salinity, are masked in the Levantine basin by the early diagenesis processes that lead to the deposition of Mg-rich calcite coating. 5- Temperature interpretation of Mg/Ca obtained from conventional methods in Mediterranean sediments is, therefore, highly speculative and request that a careful observation of local diagenetic overprint is conducted. Our tests suggest that the flow-through time resolved analysis (FT-TRA), which can separate the Mg/Ca contribution of the foraminiferal calcite from the contribution of inorganic, Mg-rich calcite, is the best approach to develop Mg/Ca thermometry in the Mediterranean Sea.

Acknowledgements We are grateful to Andy Ungerer, for his assistance during the flow-through time resolved analyses. Pr. De Lange and two anonymous reviewers offered helpful suggestions that improved the manuscript. S. Boussetta and N. Kallel, gratefully acknowledge LSCE, University of Sfax, and « Ministère Tunisien de l'Enseignement Supérieur » for their technical and financial support. This work has been also supported by the French ANR-LAMA project.

Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.margeo.2010.12.011.

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