Marine Micropaleontology 78 (2011) 101–112

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Marine Micropaleontology

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Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloidesfrom Western Mediterranean plankton tow and core top samples

Ulrike Jannette van Raden a,⁎,1, Jeroen Groeneveld b,2, Markus Raitzsch b,3, Michal Kucera a

a IFG Tübingen, Eberhard-Karls Universität Tübingen, Sigwartstrasse 10, 72076 Tübingen, Germanyb MARUM, Universität Bremen, Leobener Strasse, 28359 Bremen, Germany

⁎ Corresponding author. Tel.: +41 44 632 07 06; fax:E-mail address: vanraden@erdw.ethz.ch (U.J. van Ra

1 Present address: Geological Institute, ETH Zürich, SSwitzerland.

2 Present address: Marum Excellence Cluster AlfredPolar and Marine Research, Columbussstrasse, 27568 Br

3 Present address: Lamont-Doherty EarthObservatory oof New York, 61 Route 9W, Palisades NY 10964, USA.

0377-8398/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.marmicro.2010.11.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 April 2010Received in revised form 23 November 2010Accepted 25 November 2010

Keywords:Mg/Ca paleothermometryPlanktonic foraminiferaPaleoceanographyMediterraneanRecentHabitat depthCalcite saturation state

Due to its strong gradient in salinity and small temperature gradient the Mediterranean provides an idealsetting to study the impact of salinity on the incorporation of Mg into foraminiferal tests. We haveinvestigated tests of Globorotalia inflata and Globigerina bulloides in plankton tow and core top samples fromthe Western Mediterranean using ICP-OES for bulk analyses and LA-ICP-MS for analyses of individualchambers in single specimens. Mg/Ca observed in G. inflata are consistent with existing calibrations, whereasG. bulloides had significantly higher Mg/Ca than predicted, particularly in core top samples from the easterlystations. Scanning Electron Microscopy and Laser Ablation ICP-MS revealed secondary overgrowths on sometests, which could explain the observed high core topMg/Ca.We suggest that theMediterranean intermediateand deep water supersaturated with respect to calcite cause these overgrowths and therefore increased bulkMg/Ca. However, the different species are influenced by diagenesis to different degrees probably due todifferent test morphologies. Our results provide new perspectives on reported anomalously high Mg/Ca insedimentary foraminifera and the applicability of the Mg/Ca paleothermometry in high salinity settings, byshowing that (1) part of the signal is generated by precipitation of inorganic calcite on the foraminifer test dueto increased calcite saturation state of the water and (2) species with high surface-to-volume shell surfacesare potentially more affected by secondary Mg-rich calcite encrustation.

+41 44 6321080.den).onneggstrasse 5, 8092 Zürich,

Wegener Institute (AWI) foremerhaven, Germany.f ColumbiaUniversity in theCity

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Mg/Ca in foraminiferal calcite is a widely applied proxy forreconstructing past ocean temperatures. This proxy relies on theobservation that bulk Mg/Ca of foraminiferal tests are mainlycontrolled by calcification temperature (Anand et al., 2003; Barkeret al., 2005; Dekens et al., 2002; Elderfield and Ganssen, 2000; Leaet al., 1999; Mashiotta et al., 1999; Nürnberg, 1995, 2000; Nürnberget al., 1996, 2000; Rosenthal et al., 1997; Russell et al., 2004). Anadvantage of thismethod is that the same biotic carrier can be used forboth Mg/Ca and oxygen isotope analyses, which assures a temporaland spatial conformity of the used samples. Since the δ18O record ofplanktonic foraminifera combines effects of sea surface temperatureand the isotopic composition of the ambient seawater (Rohling andCooke, 1999), Mg/Ca in the same biotic carrier can be used to subtract

the temperature effect on δ18O in order to gain information on pastsea water δ18O, which is directly related to variables like salinity andcontinental ice volume (Elderfield and Ganssen, 2000; Groeneveldet al., 2008; Lear et al., 2000; Rosenthal et al., 2000).

Secondary factors affecting shell Mg/Ca may include, (1) partialdissolution of Mg-rich shell components in waters under-saturatedwith respect to calcite (Brown and Elderfield, 1996; Dekens et al.,2002; Regenberg et al., 2007); (2) salinity, with sensitivitiesranging from 4±3% Mg/Ca per psu observed in culture studies(Kisakürek et al., 2008; Lea et al., 1999; Nürnberg et al., 1996) upto 15–59% per psu suggested by field studies on Mediterraneancore top samples (Ferguson et al., 2008); (3) pH (closely linked toCO3

2−), showing a 6% decrease in Mg/Ca per rising pH unit (Leaet al., 1999; Russell et al., 2004), (4) test size and weight, whichwas observed for some foraminiferal species (Anand et al., 2003;Elderfield et al., 2002; Chiessi et al. 2008, supplement). Presumably,the shell size/weight effect relates to the different Mg/Ca ratios ofthe outer layer (calcite crust) precipitated by many planktonicspecies and the inner layer (ontogenetic primary calcite) of thetests. Interestingly, some studies report lower Mg/Ca in the outerthan in the inner calcite layers of some planktonic species (Brownand Elderfield, 1996; Elderfield and Ganssen, 2000; Hathorne et al.,2003; Puechmaille, 1994), while other studies find higher Mg/Ca inthe outer than in the inner calcite layers (Allison and Austin, 2003;

102 U.J. van Raden et al. / Marine Micropaleontology 78 (2011) 101–112

Eggins et al., 2003, 2004; Gehlen et al., 2004; Nürnberg et al.,1996). It was suggested that migration through the water columnand thus calcification within different water masses could be areason for this phenomenon (Eggins et al., 2003; Lohmann, 1995;Lohmann and Schweitzer, 1990). Possible mechanisms which havebeen suggested are the precipitation of two distinct calcitecompositions via different calcification pathways (Erez, 2003),reservoir fractionation effects (Elderfield et al., 1996) and an activeregulation of the internal calcite saturation state under biologicalcontrol (Erez, 2003).

In this study, we aim to further constrain the potential impact ofvarying water mass properties (temperature and salinity) on theincorporation of Mg into foraminiferal tests. We analyzed Mg/Ca inthe planktonic foraminifera Globorotalia inflata and Globigerinoidesbulloides. These species were selected for this study since (1) bothspecies were abundant throughout all plankton tow and core topsamples and are commonly used in paleoceanographic reconstruc-tions (Chiessi et al., 2008; Cléroux et al., 2008; Nürnberg andGroeneveld, 2006), (2) both species are non-symbiont bearing, andhence we can exclude a possible additional influence of symbiontactivity on the geochemical signal of the foraminiferal tests, and(3) the species have different habitats which allows for thecomparison of Mg incorporation in surface dwelling species(G. bulloides) vs. species that calcify throughout the upper watercolumn and the thermocline (G. inflata). We used plankton tow andcore top samples from multi cores of the Western MediterraneanSea, which is characterized by a higher salinity and calcitesaturation state than the open-ocean. A simultaneous analysis ofplankton and sedimentary material allows a direct comparison ofMg incorporation at different water depths with measured watermass properties and with the Mg/Ca signal preserved in thesediment.

Fig. 1. a) Map with sampling locations (circles: core top locations, crosses: plankton tow locasection of the Western Mediterranean Sea with spring salinity (WOA01), sampling locatioMediterranean Intermediate Water, WMDW: Western Mediterranean Deep Water). (Schlit

2. Regional setting

The Mediterranean Sea is a semi-enclosed basin and has, due to itsposition and latitudinal dimension, a strong salinity gradient fromwest (~36 psu near the Strait of Gibraltar) to east (~40 psu) (Boyeret al., 2002; Lascaratos et al., 1999) accompanied with only a small seasurface temperature (SST) gradient (~4 °C) (Stephens et al., 2002)(Fig. 1). The increasing salinity towards the east reflects the dominantcirculation mode in the basin, which is controlled by the position ofthe only connection with open ocean waters in the west through theStrait of Gibraltar. Relatively low-saline Atlantic Water (AW) entersthe Mediterranean at the surface at the Strait of Gibraltar and thinsout towards the Strait of Sicily and into the EasternMediterranean Seabecoming warmer and more saline due to evaporation. Below thesurface, in about 200–500 m depth, the warm and high salineLevantine Intermediate Water (LIW) moves westward, mixes withthe Adriatic Deep Water (ADW) and flows through the Strait of Sicilyinto theWestern Mediterranean basin. In theWestern Mediterraneanthe Levantine Intermediate Water (LIW) mixes with WesternMediterranean Deep Water (WMDW), forms the MediterraneanIntermediate Water (MIW), and flows through the Strait of Gibraltarinto the Atlantic (Millot, 1999; Pinardi and Masetti, 2000; Rohling etal., 2007) (Fig. 1).

The large salinity gradient in the Mediterranean in the absence of alarge temperature shift, therefore, provides an ideal setting for testingthe influence of salinity on Mg/Ca in planktonic foraminifera. We notethat residence times for Mg (13 Myr) and Ca (1 Myr) are longcompared to the mixing time of only 70 y in the Mediterranean Sea,and that Mg and Ca concentrations in the basin can be expected to besimilar as in the open ocean and, hence, conservative (Broecker andPeng, 1982). The Mg/Ca ratio of Mediterranean seawater is thereforenot expected to bias Mg/Ca in foraminiferal tests.

tions) showing sea surface temperature (WOA01) in spring (April, May, June). b) Crossns and water masses (AW: Atlantic Water, LIW: Levantine Intermediate Water, MIW:zer, 2006).

103U.J. van Raden et al. / Marine Micropaleontology 78 (2011) 101–112

3. Materials and methods

The analyzed material was sampled in late March/early April 2006(RV Poseidon, cruise POS 334) in theWestern Mediterranean Sea. Thewesternmost Station POS 334–74 is situated at the border betweenthe Alboran Sea and the Balearic Abyssal Plane at a water depth of2033 m. Station POS 334–75 and POS 334–79 are located within theBalearic Abyssal Plane at water depths of 2703 m and 2996 m,respectively. Station POS 334–77 is at a shallower water depth of1104 m in the proximity of the Balearic Islands. The easternmostStation POS 334–81 has a water depth of 1226 m between Sardiniaand Sicily close to the Strait of Sicily (Table 1, Fig. 1).

For this study, plankton was sampled with a multinet (MSN) atstations POS-74 and 81 down to 700 m water depth in different

Table 1Sample locations, CTD data, mean spring and annual temperatures and salinities from Wor

Shipstation

Samplename

Location Samplingdate

Samplingdepth(m)

CTDmeanT (°C)

CTDmeanS (psu)

WOA01springT (°C)

POS334–74

MSN K17 B5

36°15`017 N;01°59`532 W

26.03.2006 0–20 15.5 36.53 17.55

MSN K17 B4

20–40 15.2 36.57 16.62

MSN K17 B3

40–60 15.0 36.63 15.09

MSN K17 B2

60–80 14.7 36.75 14.40

MSN K17 B1

80–100 14.6 36.82 14.20

MSN K16 B4

100–200 13.7 37.95 13.61

MSN K16 B3

200–300 13.3 38.39 13.34

MSN K16 B2

300–500 13.3 38.49 13.25

MSN K16 B1

500–700 13.3 38.52 13.10

MUC651

2033 – – 14.65a

POS334–75

MUC652

37°14`996 N;00°29`997 W

27.03.2006 2703 – – 14.32a

POS334–77

MUC653

38°25`004 N;01°31`098 E

28.03.2006 1104 – – 14.30a

POS334–79

MUC655

38°25`051 N;05°24`165 E

30.03.2006 2996 – – 14.61a

POS334–81

MSN K34 B5

38°45`166 N;11°00`633 E

01.04.2006 0–20 14.2 38.00 17.14

MSN K34 B4

20–40 13.8 38.03 16.08

MSN K34 B3

40–60 13.4 38.08 14.66

MSN K34 B2

60–80 13.3 38.13 14.14

MSN K34 B1

80–100 13.3 38.22 14.07

MSN K33 B4

100–200 13.8 38.53 13.98

MSN K33 B3

200–300 14.0 38.67 14.04

MSN K33 B2

300–500 13.9 38.70 13.96

MSN K33 B1

500–700 13.7 38.65 14.61

MUC657

1226 – – 14.61a

a Core top calcification temperature and salinity for G. bulloides: Average spring (April–Jub Core top calcification temperature and salinity for G. inflata: Average annual T and S inc Single measurements in () below the average w/o ().d G. bulloides only calcifies in the uppermost part of the water column. Hence, specimens f

most samples except for this one. Its value is more than 2 standard deviations away from thare present that this sample was contaminated, it was excluded from further consideration

intervals (Fig. 1; Table 1). The MSN with a mesh size of 100 μm and atop opening of 50×50 cm was heaved with 0.5 m/s. The planktonsamples from individual depths were buffered with hexamethylen-diamin to a pH of 8.5 and stored in a 4% formalin solution at 4 °C.Planktonic foraminiferawere then concentrated bywet sieving and allforaminifera were quantitatively picked from the air-dried concen-trates and counted on the basis of cytoplasm content and speciesattribution, using the taxonomy of Hemleben et al. (1989).

At all five stations (POS-334 74, 75, 77, 79, and 81) sedimentsamples of the upper few decimetres were recovered with amulticorer (MUC) (Fig. 1; Table 1). The top 5 mm sections werepreserved in Rose Bengal and alcohol and stored at 4 °C until furtherprocessing. For simplicity, these samples from the top of themulticorer tubes will be referred to as “core top samples” from here

ld Ocean Atlas (WOA, 2001), and ICP-OES Mg/Ca data.

WOA01springS (psu)

WOA01annualT (°C)

WOA01annualS (psu)

Bulk Mg/CaG.inflata(mmol/mol)

Bulk Mg/CaG.bulloides(mmol/mol)

G.inflatacontainingcytoplasm(tests/m3)

G.bulloidescontainingcytoplasm(tests/m3)

36.86 18.31 36.87 2.45 3.92 11.2 21.2

36.94 17.19 36.93 2.57 3.54 8 16

37.15 15.42 37.13 2.22 3.47 8.4 20.4

37.38 14.63 37.40 3.17 3.96 14 27.2

37.53 14.39 37.55 2.9 3.46 11.6 21.6

37.98 13.66 37.98 2.72 3.31 6 1.8

38.30 13.23 38.33 2.39 3.61 4.6 1

38.32 13.20 38.39 1.91 3.8 3.2 1.3

38.30 13.11 38.38 1.75 3.89 3.1 0.4

37.54a 14.33b 37.82b 2.44(2.8; 2.07)c

3.48(3.7; 3.26)c

– –

37.81a 14.11b 37.98b 2.13(2.08;2.18)c

4.46(4.25;4.66)c

– –

37.92a 14.29b 38.05b 3.12 6.1 – –

37.71a 14.61b 37.91b 2.06(1.89;2.22)c

3.97(3.34;4.59)c

– –

37.56 18.55 37.65 2.05 3.54 23.2 12

37.60 17.50 37.67 2.07 2.76 7.6 15.2

37.71 15.38 37.74 2.78 3.57 14 10.8

37.85 14.48 37.88 2.9 3.52 12 22

37.94 14.32 37.97 2.7 (5.4)d 7.2 12.4

38.25 14.07 38.29 2.39 3.04 3 0.5

38.59 14.06 38.61 2.28 3.56 0.1 0

38.65 13.96 38.66 2.22 3.14 1 0

38.61 13.70 38.63 1.55 3.25 1 0

37.97a 14.76b 38.23b 2.24(1.65;2.82)c

7.52 – –

ne) T and S in 20–200 m water depth.20–500 m water depth.

rom deeper tows are expected to be similar to the shallow ones. This is also indicated bye average G. bulloides Mg/Ca ratios for this tow location. Hence, although no indications.

104 U.J. van Raden et al. / Marine Micropaleontology 78 (2011) 101–112

on. For this study, the core top samples were cleaned from particlesb63 μm by wet-sieving, dried at 36 °C for 48 h and picked forforaminiferal Mg/Ca analyses.

3.1. Analytical methods

Only adult specimens of G. bulloides and G. inflata (both fromplankton tow and the core top samples disregarding the cytoplasmcontent) were prepared for geochemical analyses. We used the samesize fraction (150–250 μm in G. bulloides and 200–300 μm in G. inflata)in all samples for analyses to exclude size dependant Mg/Cavariations. All samples were cleaned according to the standardcleaning protocol for foraminiferal Mg/Ca analysis of Barker et al.(2003). This procedure was established for cleaning sedimentsamples, which typically show high contamination of clay mineralsbut low content of organic material. Since plankton tow samples arenot contaminated with clay but may still contain organic material, weslightly modified the method for these samples in that the steps forclay removal were minimized and an additional step for oxidizingorganic matter was included. 300–500 μg of uncleaned foraminiferalmaterial was used for analysis on the ICP-OES, which corresponds toabout 20 individuals of G. inflata and 30 individuals of G. bulloides.Each sample was first gently crushed between two clean glass platesin order to open all chambers and then transferred into an acid-cleaned tube. The tests were cleaned from clay particles by alternatingultrasonic treatment with washes in ultrapure water and methanol.Then, the sampleswere treated two to three timeswith 250 μL of a hot(97 °C) oxidizing 1% NaOH/H2O2 reagent for 10 min. Every 2.5 min,the solution was carefully agitated to release any gaseous build-ups.After 5 min, the samples were placed in an ultrasonic bath for a fewseconds to maintain the chemical reaction. The remaining oxidizingsolution was removed by three rinsing steps with ultrapure water.After transfer into clean vials a weak acid leach was performed byadding 0.001 QD M HNO3 and 30 s of ultrasonic treatment. After tworinses with ultrapure water and the removal of any remainingsolution, dissolution of the foraminiferal material was achieved byadding 500 μl QD 0.075 M HNO3 to the sample material. Then thesamples were centrifuged for 10 min (6000 rpm) to exclude anyremaining insoluble particles from the analyses. Finally, the sampleswere diluted with ultrapure water (Seralpur) to a Ca concentration of10–70 ppm.

Bulk trace element/Ca ratios were measured with an InductivelyCoupled Plasma Optical Emission Spectrometer (ICP-OES; Perkin ElmerOptima 3300RL with autosampler and Ultrasonic Nebulizer U-5000 AT(Cetac Technologies Inc.)) at the University of Bremen, Germany(Table 1). Mn/Ca, Fe/Ca, and Al/Ca were analysed to monitor thepresence of any remaining clays or Mn-coatings. Obviously, no claycontamination was present in the plankton tow samples. Ratios ofb0.10 mmol/mol in the core top samples indicate no significant amountof remaining contaminantswhich couldhavehadan influence onMg/Ca(Barker et al., 2003). Analytical precisionbasedupon three replicates peranalysis was 0.19% for G. inflata and 0.08% for G. bulloides. An in-housestandard solution with Mg/Ca of 2.93 mmol mol−1 was run after everyfive samples to monitor drift of the analyses (s=0.28%). Whenevertherewas enoughmaterialwe did replicatemeasurements to detect thenatural internal variability within one sample. Reproducibility of theresults based on replicate sampleswas 0.07 mmol/mol or 2.4% (n=11).The international ECRM752-1 standard (Greaves et al., 2008) wasadditionally analysed to validate the results.

For analyzing the internal shell structure and possible alterations,we used LA-ICP-MS on shells from G. inflata. A Nd:YAG laser(NewWave UP 193 nm), coupled to a Thermo-Finnigan Element 2Sector Field ICP-MS, was employed to analyze selected shells forpotential diagenetic overgrowths or sedimentary contaminants. Timeresolved data provide the possibility to analyze trace elementvariations throughout the test wall (ablation time~penetration

depth (Eggins et al., 2003)). From four core top and plankton towsamples (MUC 651, 653, 657 and MSN K 33 B1, which span the entiretransect) the final three chambers (f, f-1, f-2) of four to six randomlyselected specimens of G. inflata per sample were investigated forintra-shell trace element heterogeneities.We only used G. inflata sinceshells of G. bulloides were too thin and fragile to obtain significantlaser ablation profiles of sufficient length of time. Before analysis,uncrushed shells were cleaned following the protocol of Barker et al.(2003), but with omission of the ultrasonication treatment. Spot sizeswere set at 35–50 μm, energy density at ~0.14 GW/cm2, andrepetition rate was 5 Hz. Element concentrations were determinedon the isotopes 25 Mg, 27Al, 43Ca and 55Mn. Aluminum andmanganesewere monitored as indicators for clay fills and diagenetic over-growths, respectively. Calcium was handled as internal standard at aconcentration of 40 wt.%. A silicate standard NIST 612 (Pearce et al.,1997) and a carbonate standard JCt-1 (Inoue et al., 2004) for matrix-matched calibration were measured before and after five sampleanalyses as external standards. Analytical precision of laser ablationmeasurements was better than 97% with a standard error of 0.76% forMg/Ca.

We used scanning electron microscopy (SEM) to gain informationabout possible diagenetic overprints on selective shells. Several testfragments which were still large enough after cleaning were mountedon a carrier for scanning electron microscope (SEM) analysis with amagnification of up to 5000× (Zeiss Supra 40, Historical Geology andPalaeontology Group at the University of Bremen) as an additionaltool to detect contamination, dissolution or high-Mg-calcite over-growths that could lead to alteration of Mg/Ca ratios.

3.2. Hydrographic data and habitats of G. inflata and G. bulloides

In addition to plankton tow and core top samples, CTD (Conductivity–Temperature–Depth probe) was deployed in the upper water column ateach station (Schulz et al., 2006) to gain information on waterconductivity (salinity) and temperature at each specific location andwater depth (Fig. 2; Table 1). To determine the calcification depth ofG. bulloides andG. jinflata all individuals of theplankton tow sampleswerepicked and countedwith regard to their cytoplasm content (Fig. 2).Whilethe absence of cytoplasm in the inner test chambers indicates that theindividual was clearly deadwhen collected, the existence of cytoplasm inthe inner test chambers may indicate either living specimens orspecimens that died recently and are passively sinking trough the watercolumn.Nevertheless, thedominanceof shellswithcytoplasminaspecificdepth range is a reasonable way of deducing the preferred habitat of aspecies. The combination of determined calcification habitat of bothspecies and CTD data allows for the estimation of the calcificationconditions for the analyzed foraminifera.

3.3. Calculating calcite saturation state of sea water

The calcite saturation state of seawater (Ωcc) indicates if calcitetends to dissolve or precipitate and is given by:

Ωcc = Ca2+h i

T CO2−3

h i� �= Ksp ð1Þ

Where [Ca2+] and [CO32−] are the concentrations of calcium and

carbonate ions in sea water, respectively. Ksp is the solubility productof calcium and carbonate. WhenΩcc equals 1 equilibrium exists, whilesaturation (N1) and dissolution (b1) occur otherwise (Zeebe andWolf-Gladrow, 2001). As a first approximation of ambient Ωcc wecalculated the saturation state of sea water in the WesternMediterranean using the software package CO2sys (Pelletier et al.,2005). The different values for carbonate system parameters weretaken from the World Ocean Atlas (WOA, 2001). Dissociation

Fig. 2. CTD derived temperature and salinity and abundance of cytoplasm bearing tests of G. inflata und G. bulloides of the plankton tow locations at stations (a) POS 334–74 and(b) POS 334–81. Atlantic Water (AW), Levantine Intermediate Water (LIW) and Mediterranean Intermediate Water (MIW) are labeled.

105U.J. van Raden et al. / Marine Micropaleontology 78 (2011) 101–112

constants are from Mehrbach et al. (1973); (modified by Dickson andMillero (1987)) and used on a pH sea water scale.

4. Results

At the plankton tow stations POS 334–74 and 81 the abundancemaximum of both species was observed in the AtlanticWater. Between16 and 27 cytoplasm bearing G. bulloides/m3 and ~7 to 15 G. inflata/m3

were sampled at station POS 334–74 in the upper 100 m of the watercolumn, whereas below this level only 0.5 to 2 G. bulloides/m3 and~3.5 G. inflata/m3 were observed. At station POS 334–81 the samedecreasing abundance with water depth was found in G. bulloides,although overall counts where lower (0–22 specimens/m3). G. inflatashowed abundances up to24 specimens/m3 in the upper 100 mand lessthan 3 specimens/m3 below 100 m at station POS 334–81. For precisecomparison of water properties and the foraminiferal Mg/Ca in theplankton tow samples mean temperatures and salinities of eachsampling interval were derived from CTD data (Table 1, Fig. 2).

Previous studies from the Mediterranean (Barcena et al., 2004;Pujol and Vergnaud Grazzini, 1995; Schiebel et al., 1997) concludethat G. bulloides dominates in spring at an average depth of 20–200 m,whereas G. inflata calcifies throughout the year between 20–500 mwater depth. Accordingly, mean spring (April, May, June) and meanannual calcification temperatures and salinities of G. bulloides andG. inflata, respectively, were derived from World Ocean Atlas 2001data (WOA, 2001) for the respective habitat depths for the core topsamples (Table 1). Although the reproductive cycle of G. inflata hasnever been studied systematically, Hemleben et al. (1989) assume amonthly cycle. A short life cycle is consistent with the observation oflarge temporal fluctuations in its standing stocks as reported forexample by Pujol and Vergnaud Grazzini (1995).

The bulk geochemical analyses reveal that G. bulloides hasinherently higher Mg/Ca than G. inflata, which is in accordance withprevious studies (Anand et al., 2003; Cléroux et al., 2008; Elderfieldand Ganssen, 2000). Mg/Ca range from 2.76 to 3.96 mmol/mol(G. bulloides) and from 1.55 to 3.17 mmol/mol Mg/Ca (G. inflata) in

the plankton tow samples and from 3.34 to 7.52 mmol/mol(G. bulloides) and from 1.65 to 3.12 mmol/mol Mg/Ca (G. inflata) inthe core top samples (Table 1).

InG. inflata of the plankton tow samples we observemediumMg/Ca(2–2.5 mmol/mol) in the top 50 m, high Mg/Ca (up to 3.2 mmol/mol)between 50 and 100 m, and a clear decreasing trend down to waterdepths of 700 m.No such trend is apparent inG. bulloides (Fig. 3). Mg/Cain G. inflata from core top samples seems to average the Mg/Ca in theplankton tow samples (Fig. 3). Likewise, Mg/Ca in G. bulloides from thecore top samples at station POS 334–74 is within the range of Mg/Ca inthe plankton tow samples at this station (Fig. 3). In contrast, Mg/Cain G. bulloides from core top samples from the easternmost site(POS 334–81) do not match Mg/Ca found in plankton tow samplesfrom the upper water column, but exhibit much higher ratios of~7.5 mmol/mol (Fig. 3).

Laser ablation depth profiling in G. inflata shows that the internalshell structure consists of different layers with distinct Mg/Ca ratios(Fig. 4). In the plankton tow samples, the outer shell shows lowMg/Caof approximately 1 mmol/mol. The inner shell shows continuouslyincreasingMg/Ca of up to 6 mmol/mol towards the inner shell surface.It is striking that in contrast to the plankton tow samples, the outershell surfaces of the core top samples also exhibit highMg/Ca peaks ofup to ~8 mmol/mol, which are accompanied by increased Mn/Caratios (Fig. 4). In many cases of both plankton and core top samples,the between-chamber variability further demonstrates that Mg/Ca inthe final chamber (f) is significantly higher than in the chambers f-1and f-2. This difference in Mg/Ca between the final and the olderchambers may reach up to 35%.

5. Discussion

5.1. Mg/Ca in G. inflata

5.1.1. Plankton tow samplesG. inflata is a deep dwelling foraminifer which has been reported to

occur over a large depth range (Bé and Hutson, 1977; Wilke et al.,

Fig. 3. Bulk Mg/Ca in (a) G. inflata and (b) G. bulloides vs. water depth at both plankton tow stations POS 334–74 and POS 334–81. Gray and white areas mark depth ranges ofsampling intervals. Samples from the upper 700 m represent multi net samples, lower ones represent core top samples.

Fig. 4.Depth profiles for different chambers of G. inflata obtainedwith laser ablation ICP-MS for a) plankton tow samples and b) core top samples. Time resolved data show variationsof Mg/Ca and Mn/Ca over shell thickness. Shaded areas represent the within-spot reproducibility (~27% RSD) obtained from LA profiles through the carbonate standard JCt-1. Greyvertical bars indicate Mg/Ca peaks, which are accompanied by increased Mn/Ca ratios (N0.01 mmol/mol). Although the signal is likely a mixed signal from different layers, anincrease of Mg/Ca towards the inner shell is evident for the existence of different calcite layers. In contrast to the plankton tow samples (a), the core top samples (b) are probablyaffected by diagenetic Mn–Mg-rich overgrowths.

106 U.J. van Raden et al. / Marine Micropaleontology 78 (2011) 101–112

Fig. 5. Bulk Mg/Ca in a) G. inflata and b) in G. bulloides vs. temperature in comparison toexisting paleotemperature calibrations (Table 2).

107U.J. van Raden et al. / Marine Micropaleontology 78 (2011) 101–112

2006). Our plankton tow analyses show living G. inflata between 0 and700 m at the westernmost station POS 334–74, and between 0 and200 m at the easternmost station POS 334–81 (Fig. 2). Several studiesconcluded that calcification takes place over a large range between20–500 m water depth (Elderfield and Ganssen, 2000; Lončarić et al.,2006); Chiessi et al. 2008, supplement). Cléroux et al. (2008)concluded that G. inflatamainly calcifies at the bottom of the summerthermocline and only descends to greater water depth whentemperatures get warmer than 16 °C. The decreasing Mg/Ca trend inG. inflata with increasing depth in the plankton tow samples can thusbe readily explained by calcification continuing throughout the watercolumn, recording ever lower temperatures into their test calcite withdepth (Fig. 3). However, the observed decrease with depth in Mg/Caof 1.42 mmol/mol and 1.35 mmol/mol for the plankton tow sampleswould imply a temperature decrease of 7.9 and 8.3 °C, respectively(Chiessi et al. 2008, supplement) (Fig. 5, Table 2). This is significantlymore than the 2–2.5 °C change which has been measured by CTDduring sampling.

This apparent disagreement can probably be explained bybiomineralization changes during the life cycle of G. inflata. Althoughcare was taken that always the same size fraction was picked, theshallower collected specimens were usually slightly smaller andmostly uncrusted, whereas deeper collected specimens were slightlybigger and contained larger amounts of calcite crust. This indicatesthat the relative amount of calcite crust increases with depth. Forcomparison, a difference in reconstructed temperatures of up to 6 °Cbetween encrusted and uncrusted specimens of G. inflata was foundfor South Atlantic core top samples (Chiessi et al. 2008, supplement).It was also suggested that the temperature dependency of calcite crustis probably different from primary calcite (Erez, 2003). Therefore, thetemperature dependency of the shallow, less encrusted specimenscould be expected to be different from the dominantly encrusted deepspecimens. Since existing Mg/Ca paleotemperature calibrations forG. inflata are mostly based on a specific size fraction and encrustationstage (Chiessi et al. 2008, supplement), the estimated temperaturedecrease for plankton tow samples based on existing calibrations forsediment samples is most likely overestimated.

The differences in calcite composition of different calcite layerswithin one shell are further illustrated by laser ablation analyses. Weobserve a decreasing trend in Mg/Ca from the inner to the outer test,which shows the difference between primary calcite in the inner partof the test and calcite crust in the outer part (Fig. 4). Average Mg/Cafor the calcite crust are b1 mmol/mol (Fig. 4). This is consistent withplankton tow data, where the deepest samples, presumably contain-ing the highest percentage of calcite crust, show Mg/Ca of about1.5 mmol/mol. Also, Mg/Ca of the shallowest plankton tow samplesare 2.5–3 mmol/mol, similar to the inner calcite as shown by laserablation ICP-MS. These results are similar to laser ablation profiles forG. inflata from a North Atlantic sediment trap study (Hathorne et al.,2009).

A combination of bulk ICP-OES and LA-ICP-MS results suggeststhat the low-Mg/Ca calcite crust, which forms on the outer part of thetests, is the reason for the lower bulk Mg/Ca in the deeper samples(below ~300 m) (Figs. 4, 6). Thus, we see that the overall Mg/Ca trendseems to be in accordance with previous results. Taken together ourresults highlight the importance of calcification changes duringontogeny on the resulting bulk Mg/Ca signal in G. inflata.

5.1.2. Core top samplesBased on earlier studies (Elderfield and Ganssen, 2000; Loncaric

et al., 2006) and our own results we conclude that G. inflata in theWestern Mediterranean calcifies throughout the year within 20–500 mwater depth. Considering this inferred habitat and calcification depthrange of G. inflata, Mg/Ca of bulkG. inflata from the core top samples arecomparable with the average Mg/Ca within the plankton tow samples.G. inflataMg/Ca in core top samples show values similar to the plankton

tow samples from 20 to 500 m depth at the same location (Fig. 3).Although the plankton tow samples were taken during early spring, thecore top samples probably represent an annual average (Barcena et al.,2004; Pujol and Vergnaud Grazzini, 1995). Nevertheless, as the annualtemperature range at intermediate water depth at the samplinglocations is small (b1.6 °C) (Locarnini et al., 2006) the core top samplesseem to reliably register water mass temperature at intermediatedepths as was also shown for various locations in the Atlantic (Lončarićet al., 2006; Wilke et al., 2006; Chiessi et al. 2008, supplement). Threeadditional core top samples (at stations POS 334–75, 77, and 79)complete the transect through the Western Mediterranean. Althoughthe values show a relatively large spread they also record intermediatewater temperatures (Fig. 5). None of the values deviates substantiallyfrom theexpectedMg/Ca at the givenSST and salinity range andwe thusconclude that G. inflata shells in the Western Mediterranean contain aprimary signal reflecting their calcification habitat.

5.2. Mg/Ca in G. bulloides

5.2.1. Plankton tow samplesG. bulloides is a planktonic foraminifer that mainly lives in the upper

part of the water column without experiencing migration to greaterwater depths (Schiebel et al., 1997). Accordingly, calcification onlyshould take place down to a water depth of about 100 m (Ganssen andKroon, 2000; Schiebel et al., 1997). Our results, indeed, suggest thatG. bulloides calcifies very shallow (upper 200 m)asMg/Ca donot change

Table 2Mg/Ca-temperature calibrations for G. bulloides and G. inflata, source of the foraminifera, size fractions, and temperature range of the calibrations.

Species Source Size fraction(μm)

Aa Ba r2 Temperature range(°C)

Reference

G. inflata Surface samples n.a. 0.49 0.10 n.a. 7.5–15 Elderfield and Ganssen (2000)G. inflata Sediment trap 350–500 0.56 0.058 0.55 15–21 Anand et al. (2003)G. inflata Sediment trap 350–500 0.299 0.09 n.a. 15–21 Anand et al. (2003)G. inflata Surface samples 355–400 0.71 0.06 0.72 10.5–17.9 Cléroux et al. (2008)G. inflata Surface samples 350–500 0.831 0.066 0.78 3–16 Chiessi et al. (2008)G. bulloides Surface samples n.a. 0.56 0.1 n.a. 7.5–15 Elderfield and Ganssen (2000)G. bulloides Sediment trap 212–355 1.2 0.057 0.9 16–31 McConnell and Thunell (2005)G. bulloides Culture study n.a. 0.53 0.1 0.93 16–25 Lea et al. (1999)G. bulloides Culture and surface samples 250–350 0.474 0.107 0.98 9–25 Mashiotta et al. (1999)G. bulloides Culture study n.a. 0.528 0.102 0.93 9–25 Mashiotta et al. (1999)

a B is the temperature-sensitive component and A the y-axis intercept in the general exponential expression Mg/Ca=A*exp(B*temperature).

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with depth (Fig. 3). Specimens found in deeper water in our planktontow samples thus mostly represent sinking specimens which probablywere not living and calcifying, even if some small amount of tests stillseemed to contain remains of cytoplasm (Fig. 2). If these specimenswere calcifying far below 100 m,wewould expect a decreasing trend inMg/Ca caused by the temperature dependence of Mg uptake and colderwater temperatures (Fig. 3).

Fig. 6. a) Bulk Mg/Ca in G. inflata vs. water salinity. ΔMg/Ca is the difference betweenthe analyzed Mg/Ca ratio and the value expected from the calibration of Chiessi et al.2008, supplement. b) Bulk Mg/Ca in G. bulloides vs. water salinity. ΔMg/Ca is thedifference between the analyzed Mg/Ca ratio and the value expected from thecalibration of Mashiotta et al. (1999).

However, Mg/Ca of G. bulloides are significantly higher than inother regions where calibrations have been developed and applied totemperature reconstructions (Fig. 5, Table 2) (Anand et al., 2003;Elderfield and Ganssen, 2000; Lea et al., 1999; Mashiotta et al., 1999).Converted into temperature, G. bulloidesMg/Ca corresponds to valuesup to 20 °C (Lea et al., 1999; Mashiotta et al., 1999; McConnell andThunell, 2005), whereas in situ temperatures measured via CTDshowed values of 15.5 °C. Since the uppermost part of the watercolumn where G. bulloides is calcifying is formed by lower salineAtlantic inflow water rather than by the higher saline EasternMediterranean water (Fig. 1), it seems unlikely that salinity wasresponsible for these generally increased values.

5.2.2. Core top samplesIn the core top samples of the westernmost station POS 334–74,

Mg/Ca in G. bulloides is about the average of the plankton tow samplesfrom the same location. At the easternmost station POS 334–81,however, core top Mg/Ca are much higher than the plankton towsamples with values up to 7.5 mmol/mol (Fig. 3). Since the planktontow samples do not show these extreme values there must be asecondary factor influencing the sediment Mg/Ca. The core topsamples from station POS 334–77 have also highly increased Mg/Ca,while core top samples from POS 334–75 and 79 show only slightlyhigher values compared to the plankton tow samples (Fig. 5; Table 1).These results are similar to the pattern in the core top results forG. bulloides of Ferguson et al. (2008, supplement). The westernmostsamples are in agreement with the plankton tow samples, though ingeneral higher than expected.

5.3. Secondary influences on foraminiferal Mg/Ca

In G. inflata, the overall trend in Mg/Ca in both plankton tow andcore top samples is in accordance with previous results and isconsistent with vertical migration in the water column and ontoge-netic calcification changes. InG. bulloides, however, the absoluteMg/Cavalues in all plankton tow samples are higher than expected fromexisting calibrations, and someof the core top samples seem to containan additional imprint, leading to even higher Mg/Ca. These twoobservations imply that other processes than known relationshipsbetween Mg/Ca and SST have to be considered, as also suggested byWit et al. (2010). It also has to be explored why these factors seem tohave an influence on G. bulloides but not on G. inflata. Possible reasonscausing these unexpectedly high Mg/Ca are discussed in the followingsections.

5.3.1. SalinityRecently, several studies have investigated the influence of salinity

on Mg/Ca in planktonic foraminifera (Dueñas-Bohòrquez et al., 2009;Ferguson et al., 2008; Hoogakker et al., 2009; Kisakürek et al., 2008;

Fig. 7. SEM micrographs of G. bulloides of MUC 657 at station POS 334–81 showingpatchy diagenetical overgrowths of secondarily precipitated inorganic calcite.

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Mathien-Blard and Bassinot, 2009). Although the mechanism is notyet fully understood, it generally appears that higher salinitiesincrease the Mg content of foraminiferal tests. This was alreadyshown in the culturing studies by Nürnberg (1995) and Lea et al.(1999) who suggested a 3–10% increase in Mg/Ca per 1 psu increasein salinity. This slope was confirmed in a recent culturing experimentby Kisakürek et al. (2008) on G. ruber, whereas Dueñas-Bohòrquezet al. (2009) only found a dependency of ~2% on cultured G. sacculifer.Ferguson et al. (2008) used a core top collection from the EasternMediterranean to find apparent salinity dependencies for severalspecies varying from 19% to 59% increase in Mg/Ca per 1 psu increasein salinity. Recently, however, it was shown that anomalously highMg/Ca found in Red Sea samples are probably caused by diagenesisrather than calcification in high salinities (Hoogakker et al., 2009).

Although the Western Mediterranean is not characterized bysalinities as high as the Eastern Mediterranean or the Red Sea, salinityis still higher than in the open-ocean with values up to 38.5 psu(Fig. 1). Accordingly, we expect that some salinity effect on Mg/Cacould have taken place. This influence could explain the difference inMg/Ca between the shallowest plankton tow samples (0–50 m) ofG. inflata in comparison with the slightly deeper ones (~100 m)(Fig. 3). Inflowing Atlantic water (AW) into the Mediterranean with asalinity of only 37 psu occupies the upper 50 m of the water column.Below, saltier water (LIW,modified toMIW) originating in the EasternMediterranean is dominant. As temperature is not changing signifi-cantly in the upper part of the water column, it can be argued that theslightly higher Mg/Ca at 100 m for G. inflata than those at 0–50 m arecaused by calcification under different salinities. Since the AWmass isreaching slightly deeper in the west and thinning out towards the east(Figs. 1 and 2), we observe this increase in Mg/Ca slightly deeper atstation POS 334–74 than at station POS 334–81 (Fig. 3a). Assumingthat salinity is causing the observed differences in Mg/Ca of G. inflatain the upper 100 m of the water column, we conclude that G. bulloidesonly lives and calcifies in either one of the water masses since they donot show this difference within the upper water column. However, itis noteworthy, that in spring, salinity is only slightly increased in theWestern Mediterranean, where our plankton tow samples originateand therefore, these samples in this setting are not expected to show astrong salinity dependant change in Mg/Ca.

To assess the potential salinity effect, we corrected Mg/Ca for theknown temperature dependency. We calculated the expected Mg/Caratio for the samples by applying the SST calibration of Chiessi et al.2008, supplement for G. inflata and Mashiotta et al. (1999) forG. bulloides and subtracted this from the analyzed values. The residualΔMg/Ca was then compared with salinity (Fig. 6). Since the apparentrelationship of ΔMg/Ca and salinity in G. bulloides core top samples isonly caused by two high values and shows a slope much higher thanany known salinity effect on planktonic Mg/Ca, it is rather unlikelythat the two extreme values really result from calcification underincreased salinity. Apart from these two extreme values, unlike theobservations by Ferguson et al. (2008), we do not observe any clearcorrelation between salinity and plankton tow or core top sampleΔMg/Ca. Consequently, beside a possible salinity effect on G. inflata atthe sea surface, the anomalous Mg/Ca values in plankton tow and inparticular in core top samples of G. bulloides cannot be explained bychanging salinities.

5.3.2. Diagenetic influence on foraminiferal Mg/CaDiagenetic alteration of foraminifer tests can influence or even

completely overprint the geochemical signature of the primary calciteprecipitated in the surface ocean (Hover et al., 2001; Walter andMorse, 1984). In particular, Mg/Ca is easily altered by the influence ofinorganic carbonate phases on the foraminifer tests, which are oftennot removed by standard cleaning techniques (Groeneveld et al.,2008; Regenberg et al., 2007). We investigated several samples withScanning Electron Microscopy (SEM) to determine the possible

presence of secondary carbonate phases (Fig. 7). Foraminifera usedfor SEM analysis all underwent the normal cleaning process to removecontaminants like clays and coccoliths, both from plankton tow andcore top samples. Tests from the plankton tow samples were found tobe free of any secondary phases. In the core top samples, however,secondary overgrowths were found, but interestingly only on tests ofG. bulloides (Fig. 7). Similar overgrowths were also found onforaminifera from the Caribbean, Red Sea, and Eastern Mediterranean(Ferguson et al., 2008; Groeneveld et al., 2008; Regenberg et al.,2007). SEM did not reveal any obvious crystal overgrowths on shellsof G. inflata.

Indications for diagenesis such as anomalous Mg/Ca ratios andcrystalline overgrowths visible via SEM analysis only occur inG. bulloides in the more easterly core top samples of the transect(stations POS 334–77, –79, and –81). A possible reason for this couldbe the change in the calcite saturation state Ωcc of the water column.Deepwater coming from the EasternMediterranean is both warm andmore saline (Boyer et al., 2002; Stephens et al., 2002) and, hence, has avery high saturation state (Ωcc). With an increase in salinity thenumber of ions in seawater increases resulting in a higher ionicstrength. Also, the activity of ions in seawater with a higher salinitydecreases. As the activity for the different ions in the carbonate systemis not changing equally, a new equilibrium between them is formed inwhich the concentration of CO3

2− is higher than at lower salinity. Asthe concentration of CO3

2− mainly determines the saturation state ofseawater, an increase in salinity leads to an increase in saturation state(Zeebe and Wolf-Gladrow, 2001).

It was recently shown during cultivating experiments that anextremely high calcite saturation state causes inorganic calciteprecipitation on foraminifer tests (Raitzsch et al., 2010). Theconsequence of these highly saturated conditions is that precipitationof inorganic calcite can occur on any available surface. Therefore, thesurface of foraminiferal tests, as soon as they are formed and especiallyafter being deposited, become available as crystal nuclei for the growth

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of inorganic calcite crystals. This would also provide an alternativeexplanation for the high core topMg/Cadata fromFerguson et al. (2008)from the EasternMediterranean. Although a correlation betweenMg/Caand salinity was found, it seems more likely that the high calcitesaturation state of the Eastern Mediterranean led to increasedprecipitation of inorganic calcite on the foraminifer tests. On the otherhand, deep water in the westernmost part of the Mediterranean is amixture from the Eastern Mediterranean and the Gulf of Lyons deepwater, which has a substantially lower saturation state than pureEastern Mediterranean water (Ωcc=5, Stommel, 1972). Therefore,samples from the westernmost part of the transect are not affected bydiagenesis linked to saturation state of bottom waters.

In addition to this increase ofΩcc fromwest to east there is a decreasein Ωcc from the surface towards the bottom, in line with an increase inpressure with depth has been observed (Zeebe and Wolf-Gladrow,2001). Since the solubility of calcium carbonate increaseswith pressure,the concentration at which CO3

2− is saturated increases with waterdepth while the in situ CO3

2− concentration decreases leading to adecrease in saturation state.

These two trends (increasing Ωcc from west to east and decreasingΩcc with water depth) lead to overall supersaturated waters in theMediterranean, from the surface (Ωcc=4–6) to the bottom (Ωcc=2–3)(Millero et al., 1979; Schneider et al., 2007). Especially the increasedΩcc

in thedeeperwaters is unusual as in the openoceanmost bottomwatersare either undersaturated (dissolution occurs) or only slightly super-saturated (Ωcc just over 1). The core top samples of G. bulloides showthese two trends in theirMg/Ca, which reflect an increased potential fordiagenesis due to increased Ωcc towards the east. Since Ωcc is higher atshallowwaterdepths, the shallowcore top samples at stationsPOS334–77 and –81 (~1000 mwater depth) are more diagenetically altered andthus show higher Mg/Ca than foraminifera at POS 334–79 at 2996 mdepth.

This observation is in accordance with the data presented byFerguson et al. (2008) where G. bulloides show the highest Mg/Ca incore top T87 65B (Fig. 5), which is located in the eastern part of theWestern Mediterranean at a water depth of 904 m, thus at a locationand depth with highly elevated in Ωcc. All other core top samples ofG. bulloides reported by Ferguson et al. (2008) show lower Mg/Ca,which is consistent with their greater sampling depth and, therefore,less increased Ωcc. One core top sample from above 300 m waterdepth in the Alboran Sea also shows low Mg/Ca, which is in this casedue to the fact that the sample location is mainly influenced byinflowing Atlantic Water with a lower saturation state.

A possible reason why G. bulloides but not G. inflata would besignificantly affected by diagenesis might be due to the different waystheir shells are built. Tests of G. inflata form a thick calcite crust duringtheir life cycle, which is very homogeneous and massive (Sadekovet al., 2009). Conversely, tests of G. bulloides are lightly built withmany pores and a relatively open crystal structure. This means thatthe reactive area available for diagenesis is much larger for G. bulloidesthan for G. inflata (Sadekov et al., 2009; Walter and Morse, 1984).Consequently, G. bulloides can be expected to be much moresusceptible to diagenesis than G. inflata. A similar difference betweenthese two species has been shown in terms of dissolution suscepti-bility. Berger (1975) showed his dissolution index for planktonicforaminifera that G. inflata is among the most resistant species whileG. bulloides is very susceptible to dissolution.

Laser Ablation analyses (Fig. 4) show that the tests of G. inflatacontain a thin coating of calcite with elevatedMg concentration on theouter side of the tests which was not clearly visible by SEM. Thiscoating is not present in specimens from the plankton tow sampleseven in the deepest layers and, hence, must have been formed afterthe specimens descended onto the sediment surface. As Mn/Ca is alsoincreased, the coatingmost likely contains aMn-carbonate phasewithslightly elevated Mg/Ca. As can be seen in Fig. 4, the coating isvolumetrically small compared to the bulk of the shell calcite and

given the range of bulk Mg/Ca for shells from core top samples ofG. inflata, the coating does not seem to have a significant influence onthe bulk Mg/Ca. However, as the total mass of a G. bulloides test ismuch less than that of G. inflata, the same amount of diagenesis wouldhave a larger impact on shell chemistry in the former species.

Possible diagenetic overgrowths can explain the large difference inMg/Ca between plankton tow and core top samples in G. bulloides, butthey cannot explain why also the plankton tow samples, which areobviously unaffected by diagenesis, show consistently higher Mg/Cain G. bulloides than expected from known calibrations (Fig. 5). A directeffect of the calcite saturation state on Mg/Ca of G. bulloides can beexcluded, as it cannot be expected to lead to higher than normalratios. Several cultivating experiments on planktonic foraminifera,including G. bulloides, have shown that less Mg rather thanmoreMg isincorporated into the tests during calcification at higher saturationstates (Kisakürek et al., 2008; Lea et al., 1999; Russell et al., 2004). Onefactor to be considered is the possibility that the Mediterraneangenotypes of G. bulloides are distinct from those used in previouscalibrations. Previously, it has been shown that differentmorphotypesof Globigerinoides ruber, which are most likely representative ofdifferent genotypes (Aurahs et al., 2009; Kuroyanagi et al., 2008), havedistinct Mg/Ca (Steinke et al., 2005). So far, only a sequence ofG. bulloides Type Ib has been identified in the Mediterranean bydeVargas et al. (1997). This genetic type is abundant in the north-eastern Atlantic and together with the Indopacific Type Ia constitutesthe warm-water lineage within this morphospecies (Darling andWade, 2008; Kucera and Darling, 2002). The fact that this genetic typeis not restricted to the Mediterranean makes it unlikely that theobserved anomalously high Mg/Ca is due to genotype-specific vitaleffects.

6. Conclusion

We have investigated Mg/Ca of tests of Globorotalia inflata andGlobigerina bulloides from theWestern Mediterranean from a series ofplankton tow and core top samples. Our results show that:

1. The decreasing Mg/Ca trend with increasing water depth withinthe plankton tow samples in G. inflata can be explained byprecipitation of low-Mg calcite crust in deeper and colder watermasses and the observed trend reflects the life cycle and habitat ofthis deep dwelling species. The homogeneous Mg/Ca ratios inG. bulloides throughout the upper 700 m of the water column atboth stations reflect the origin of the Mg/Ca signal in this species atthe surface within the same water mass. They also imply thatspecimens with cytoplasm from lower levels in the water columnwere no longer calcifying or that they represented sinking, deadindividuals.

2. Scanning electron microscopy analyses and Laser Ablation ICP-MSof core top samples suggest secondary precipitation of inorganiccalcite on the foraminifer tests. This diagenetic overprint seen inincreased Mg/Ca is mainly present in foraminifera sampled at theeastern locations with water depth less than 1500 m. This is inaccordance with two observed trends in the Mediterranean:(1) increasing calcite saturation state from west to east and(2) decreasing calcite saturation state with water depth. Thesesuper-saturated waters can cause the precipitation of inorganiccalcite on the foraminifer test after their lifecycle has ended.

3. Diagenetic effects alter the test chemistry of G. bulloidesmore thanthat of G. inflata due to the differences in test structure, i.e. thin,porous test for G. bulloides, and encrusted, massive tests forG. inflata. The core top samples of G. inflata might be slightlyinfluenced by diagenesis, as suggested by laser ablation ICP-MSanalyses. But the volumetric contribution to the bulk signal is small(up to 5%, if the shell wall thickness is considered).

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4. The reason for the consistently higher Mg/Ca in plankton towsamples of G. bulloides of the Western Mediterranean cannot beexplained by a direct influence of salinity or calcite saturation stateon the incorporation of Mg into the test and this phenomenonrequires further investigation.

Acknowledgements

We thank Hartmut Schulz and the captain and the crew of RVPoseidon POS 334 for the samples used in this study. Silvana Pape andMartin Kölling are thanked for laboratory assistance; Stephan Steinke,Ed Hathorne, and Prof. Christoph Hemleben for discussion; PetraWitte for assistance with SEM analyses. JG would like to thank theMARUM for a MARUM fellowship.

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