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Geochimica et Cosmochimica Acta 73 (2009) 6134–6146

Towards a better understanding of magnesium-isotope ratiosfrom marine skeletal carbonates

Dorothee Hippler a,*, Dieter Buhl b, Rob Witbaard c, Detlev K. Richter b,Adrian Immenhauser b

a Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlandsb Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Universitatsstrasse 150, 44801 Bochum, Germany

c Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands

Received 13 January 2009; accepted in revised form 20 July 2009; available online 3 August 2009

Abstract

This study presents magnesium stable-isotope compositions of various biogenic carbonates of several marine calcifyingorganisms and an algae species, seawater samples collected from the western Dutch Wadden Sea, and reference materials.The aim of this study is to explore the influence of mineralogy, taxonomy and environmental factors (e.g., seawater isotopiccomposition, temperature, salinity) on magnesium-isotopic (d26Mg) ratios of skeletal carbonates. Using high-precision multi-collector inductively coupled plasma mass spectrometry, we observed that the magnesium-isotopic composition of seawaterfrom the semi-enclosed Dutch Wadden Sea is identical to that of open marine seawater. We further found that a considerablecomponent of the observed variability in d26Mg values of marine skeletal carbonates can be attributed to differences in min-eralogy. Furthermore, magnesium-isotope fractionation is species-dependent, with all skeletal carbonates being isotopicallylighter than seawater. While d26Mg values of skeletal aragonite and high-magnesium calcite of coralline red algae indicatethe absence or negligibility of metabolic influences, the d26Mg values of echinoids, brachiopods and bivalves likely result froma taxon-specific level of control on Mg-isotope incorporation during biocalcification. Moreover, no resolvable salinity andtemperature effect were observed for coralline red algae and echinoids. In contrast, Mg-isotope data of bivalves yield ambig-uous results, which require further validation. The data presented here, point to a limited use of Mg isotopes as temperatureproxy, but highlight the method’s potential as tracer of seawater chemistry through Earth’s history.� 2009 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Skeletal carbonates of marine organisms are sensitive ar-chives recording the past physical and chemical environ-ment, such as ocean temperature, salinity, alkalinity, pH,ocean circulation, paleo-productivity, or seawater isotopicratios often with high temporal resolution (Khim et al.,2000; Vander Putten et al., 2000; Henderson, 2002; Rosaleset al., 2004; Steuber et al., 2005; Armendariz et al., 2008;

0016-7037/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2009.07.031

* Corresponding author. Present address: Institute of AppliedGeosciences, Technical University Berlin, Ackerstrasse 76, 13355Berlin, Germany. Tel.: +31 20 598 7365; fax: +31 20 598 9941.

E-mail address: dorothee.hippler@falw.vu.nl (D. Hippler).

Foster et al., 2008). Carbonate hardparts being composedof aragonite or high-magnesium calcite commonly (butnot always; Lecuyer and Bucher, 2006) undergo diageneticalteration within some thousands of years (Al-Aasm andVeizer, 1982, 1986; Gagan et al., 2000). In contrast, low-magnesium calcite shells of some bivalves and brachiopodshave the potential to preserve their primary geochemistrythroughout the Phanerozoic (Veizer et al., 1999; Wierzbow-ski, 2004; Immenhauser et al., 2005, 2008).

In an attempt to extract relevant environmental infor-mation from these archives, most workers analyze the shellcarbon and oxygen-isotopic or elemental ratios such as Mg/Ca, Sr/Ca or Mn/Ca (e.g., Henderson, 2002; Rosenheimet al., 2004; Caroll et al., 2006; Freitas et al., 2006). Yet,

Magnesium isotopes in skeletal carbonates 6135

none of these proxies is unambiguous. Many proxies, bothlight stable isotopes and elemental ratios often respond tomore than one environmental parameter. Oxygen-isotopefractionation in carbonates is temperature dependent butalso affected by seawater salinity or pH (McCrea, 1950;Usdowski et al., 1991; Zeebe, 1999; Zeebe, 2005; Brandet al., 2003). Furthermore, so-called ‘‘vital effects” (Ureyet al., 1951) might complicate the interpretation of geo-chemical proxies. Investigating the elemental ratios of mar-ine and freshwater bivalves, Gillikin et al. (2005) and Geistet al. (2005) have been able to show that the elemental ra-tios reflect biological (metabolic), as well as physico-chem-ical parameters.

A reasonable approach is the application of multi-proxydata sets with different proxies being affected by differentparameters (Carpenter et al., 1991; Klein et al., 1996;Immenhauser et al., 2005). Particularly, the introductionof new proxies can complement these approaches. One ofthe more recently explored isotope systems is magnesium(Galy et al., 2001). Magnesium is a major element in theoceans, and plays an important role in hydrological andbiogeochemical cycles. Magnesium has three naturallyoccurring stable isotopes (24Mg: 78.992%; 25Mg: 10.003%;26Mg: 11.005%) and their distribution may provide new in-sights into these cycles.

Here, we report the Mg-isotopic composition (d26Mg) ofvarious carbonate reference materials, seawater and skeletalcarbonates of five marine calcifiers. Our aims are (1) to as-sess the factors controlling Mg-isotope fractionation inmodern biogenic carbonates, such as mineralogy, (2) toinvestigate and compare Mg-isotope records between andwithin taxonomic groups, and (3) to explore the influenceof environmental and biological factors, such as ambientseawater temperature, salinity and growth rate on Mg-iso-tope fractionation. This approach might therefore providenew insights into the methods’ applicability as environmen-tal proxy or tracer of paleo-seawater chemistry, and Mg cy-cling during biomineralization.

2. MATERIALS AND METHODS

2.1. Sampling material

Three different sample sets were incorporated in thisstudy. First, in order to compare data from different labo-

Table 1Magnesium isotope ratios of various reference materials.

Standard Material d25Mg(& DSM3)

±2r d26Mg(& DS

JCp-1 Aragonitic coral(Porites sp.)

�1.03 0.02 �1.96

IAPSO Seawater �0.42 0.02 �0.80Cambridge 1 Mono-elemental

Mg-solution�1.34 0.02 �2.58

DSM3 Mono-elementalMg-solution

0.02 0.05 0.06

N, number of analyses, with five runs making an analysis (cf. Section 2)* Measured between January and July 2008.

ratories and to assess the quality of the data, a number ofreference materials (Table 1) were included. The carbonatereference material, JCp-1 was prepared by the GeologicalSurvey of Japan from an aragonitic Porites sp. coral skele-ton (Inoue et al., 2003). Furthermore, IAPSO (Interna-tional Association for the Physical Sciences of theOceans) was used as reference seawater, which is a seawatersalinity standard provided by the ‘‘Ocean Scientific Interna-tional”, Southampton. The salinity is stated to be34.998 psu. Mono-elemental Mg-solutions, Cambridge 1and DSM3, which have been developed in the Departmentof Earth Sciences, University of Cambridge (Galy et al.,2003; Tipper et al., 2006) complete the set of referencematerials.

The second sample set is composed of carbonate-precip-itating marine organisms and algae, collected at differentlocalities along the coasts of the North Atlantic, the NorthSea, the Kattegat (strait between the North Sea and theBaltic Sea) and the Mediterranean (Fig. 1, Table 2). Bulklow-Mg calcite (LMC), high-Mg calcite (HMC) and arago-nite samples were drilled from modern brachiopods, endo-benthic echinoids (Echinocyamus pusillus), articulatecoralline red algae (Corallina officinalis), and scaphopods(Antalis costatum), respectively. According to Richter andBruckschen (1998) and Richter (1984), echinoid and coral-line red algae HMC is made up of 10–14 mol% MgCO3 and10–17 mol% MgCO3, respectively.

The third sample set was collected from experimentallygrown shells of the blue mussel Mytilus edulis. Specimenswere collected in 2005 and cultured subsequently in fieldexperiments in the western Dutch Wadden Sea close tothe Royal Netherlands Institute for Sea Research (Fig. 1,Table 3) under naturally variable environmental conditions.During the time of the experiment, data on sea-surface tem-perature (SST) and salinity (SSS) were monitored at thesampling site (see Table in Electronic Annex EA-1). In or-der to get continuous time series of SST and SSS, auto-mated conductivity-temperature (CT) salinity loggers(StarOddi�) were applied. Temperature and salinity weremeasured with an accuracy of 0.1 �C and 0.75 psu, respec-tively. Individual shells were marked with tags and shelllength of the specimens was (sub-)monthly measured withdigital calipers to obtain continuous time series of changingindividual growth. Subsequent to seasonal sample collec-tion, soft tissue was removed, and shells were cleaned and

M3)±2r d25Mg0

(& DSM3)d26Mg0

(& DSM3)D25Mg0 N

0.04 �1.03 �1.96 0.00 6

0.05 �0.42 �0.80 0.00 100.04 �1.34 �2.58 0.01 56*

0.07 0.02 0.06 �0.01 5

.

Fig. 1. Simplified map of sampling sites (stars) in the Mediterranean Sea (1) Crete, Greece, (6) Gulf of Corinth, Greece, (7) Marseille, France,(12) Peloponnese, Greece, (13) Karpathos, Greece; in the Cantabrian Sea: (8) Asturias, Spain; in the North Atlantic: (2) Isle of Skye, UnitedKingdom, (3) NW-Island, (4) Tromsø, Norway, (9) Bretagne, France; in the North Sea: (5) Helgoland, Germany and the along the Danishcoast of the Kattegat (10, 11). Field-culturing experiments on Mytilus edulis were performed in a coastal marine setting of the island of Texelin the Western Dutch Wadden Sea (14). More details on the sites are given in Tables 2–4 and Electronic Annex EA-1.

6136 D. Hippler et al. / Geochimica et Cosmochimica Acta 73 (2009) 6134–6146

air dried. Shell samples were further oven dried at 40 �Covernight. The periostracum was removed by grinding pa-per along the ventral margin of the shell. The shell of M.

edulis consists of two layers, an inner aragonitic layer,and an outer calcitic layer. The aragonitic layer lags the cal-citic layer substantially, thus all new growth is calcitic. Sam-ples of this recently precipitated shell calcite, which couldbe related to the corresponding temporal phase, and there-fore to the environmental conditions, under which the car-bonate had been deposited, were hand-drilled along theventral margin of the shell.

The latter sample set was complemented by four seawa-ter samples, which were collected during high tide at theexperimental site at different calendar dates in 2007 (Table4). Seawater temperature, ranging from 7.5 to 19.0 �C andsalinity, ranging from 28.0 to 30.5 psu at the time of collec-

tion are provided in Table 4. The seawater samples were fil-tered and acidified with 6 M quartz-distilled HNO3, andkept in the refrigerator prior to geochemical analysis. In or-der to obtain the Ca and Mg concentrations of these seawa-ter samples (Table 4), one batch of each sample wasanalyzed by ICP-OES in the laboratories of the Christian-Albrechts University, Kiel (Germany). Ca concentrationswere between 340 and 370 mg/l, and Mg concentrations be-tween 1050 and 1150 mg/l, respectively.

2.2. Purification of magnesium/ion-exchange

chromatographic procedure

Magnesium was purified by ion chromatography. Car-bonate samples, representing ±50 lg Mg, were dissolvedin supra-pure 6 M HCl and subsequently evaporated to

Table 2Magnesium isotope ratios of modern marine calcifiers.

Sampled species Sampling site (numbersaccording to Fig. 1)

Min. ØTA

(�C)ØSALA

(psu)d25Mg(& DSM3)

±2r d26Mg(& DSM3)

±2r d25Mg0

(& DSM3)d26Mg0

(& DSM3)D25Mg0 N

ScaphopodAntalis costatum 1. Mediterranean Sea, Crete (GR) Arag 20.0 39.0 �1.10 0.02 �2.07 0.05 �1.10 �2.07 �0.02 1

Red algaeCorallina officinalis 2. North Atlantic, Isle of Skye (UK) HMC 10.0 34.5 �1.67 0.03 �3.22 0.06 �1.67 �3.22 0.01 1Corallina officinalis 3. North Atlantic, NW-Island (IS) HMC 6.5 34.5 �1.69 0.01 �3.24 0.04 �1.69 �3.24 0.00 1Corallina officinalis 4. North Atlantic,Troms0 (N) HMC 6.0 34.0 �1.68 0.03 �3.24 0.02 �1.68 �3.24 0.01 1Corallina officinalis 5. North Sea, Helgoland (D) HMC 11.0 31.5 �1.54 0.02 �2.97 0.06 �1.54 �2.97 0.00 1Corallina officinalis 6. Mediterranean Sea, Gulf of Corinth (GR) HMC 19.0 38.0 �1.58 0.02 �3.06 0.08 �1.58 �3.06 0.01 1Corallina officinalis 7. Mediterranean Sea, Marseille (F) HMC 16.0 37.5 �1.60 0.02 �3.08 0.01 �1.60 �3.09 0.01 2

EchinoidsEchinocyamus pusillus 8. Cantabrian Sea (N. Atl.), Asturias (E) HMC 14.5 34.5 �1.36 0.00 �2.61 0.02 �1.36 �2.61 0.00 2Echinocyamus pusillus 9. North Atlantic, Bretagne (F) HMC 12.5 34.5 �1.35 0.02 �2.62 0.04 �1.35 �2.62 0.01 1Echinocyamus pusillus 5. North Sea, Helgoland (D) HMC 11.0 31.5 �1.42 0.03 �2.75 0.05 �1.42 �2.76 0.01 1Echinocyamus pusillus 10. Kattegat (DK) HMC 9.5 25.0 �1.37 0.03 �2.65 0.07 �1.37 �2.65 0.01 1Echinocyamus pusillus 11. Kattegat (DK) HMC 9.5 22.0 �1.31 0.04 �2.56 0.05 �1.31 �2.56 0.02 1Echinocyamus pusillus 12. Mediterranean Sea, Peloponnese (GR) HMC 19.0 38.0 �1.38 0.03 �2.65 0.04 �1.38 �2.65 0.00 1Echinocyamus pusillus 13. Mediterranean Sea, Karpathos (GR) HMC 19.5 38.5 �1.34 0.01 �2.58 0.07 �1.34 �2.58 0.00 2

BrachiopodsTerebratula sp. 9. North Atlantic, Bretagne (F) LMC 12.5 34.5 �1.19 0.02 �2.29 0.06 �1.19 �2.29 0.00 1Terebratula sp. 5. North Sea, Helgoland (D) LMC 11.0 31.5 �0.97 0.02 �1.88 0.10 �0.97 �1.88 0.01 1

Min., mineralogy; ØTA (�C), mean annual sea-surface temperature; ØSALA (&), mean annual sea-surface salinity; N, number of analyses, with five runs making an analysis (cf. Section 2).

Magn

esium

isoto

pes

insk

eletalcarb

on

ates6137

Table 3Magnesium isotope ratios of selected calcite shell edges of Mytilus edulis.

Sample Min. Growthperiod

Days IS(mm)

S1

(mm)S2

(mm)ØGR(mm/day)

ØT

(�C)ØSAL d25Mg

(& DSM3)±2r d26Mg

(& DSM3)±2r d25Mg0

(& DSM3)d26Mg0

(& DSM3)D25Mg0 N

B133 LMC 19/12/05–14/03/06 85 16.5* 22.7 24.4 0.020 3.5 29.1 �2.04 0.03 �3.92 0.08 �2.04 �3.92 0.01 1B202 LMC 19/12/05–14/03/06 85 33.5 33.9 36.0 0.024 3.5 29.1 �2.43 0.02 �4.69 0.04 �2.43 �4.70 0.02 1B234 LMC 19/12/05–14/03/06 85 29.7 29.8 32.2 0.028 3.5 29.1 �2.45 0.02 �4.62 0.04 �2.45 �4.63 �0.04 1B240 LMC 03/11/05–19/12/05 38 30.2 30.2 32.1 0.051 8.4 29.3 �2.30 0.02 �4.40 0.04 �2.30 �4.41 0.00 1B188 LMC 03/11/05–19/12/05 38 26.6 26.6 27.8 0.033 8.4 29.3 �1.80 0.02 �3.37 0.06 �1.80 �3.37 �0.05 1B182 LMC 03/11/05–19/12/05 38 29.3 29.3 30.8 0.040 8.4 29.3 �1.96 0.03 �3.67 0.07 �1.96 �3.68 �0.05 1B193 LMC 03/11/05–19/12/05 38 34.8 34.8 35.8 0.028 8.4 29.3 �1.85 0.03 �3.47 0.03 �1.85 �3.48 �0.04 1B132 LMC 20/04/06–04/05/06 14 15.7* 21.6 22.7 0.079 10.7 27.4 �2.40 0.05 �4.54 0.07 �2.40 �4.55 �0.03 1B179 LMC 20/04/06–04/05/06 14 25.1 29.9 30.9 0.071 10.7 27.4 �2.44 0.02 �4.66 0.02 �2.44 �4.67 0.00 1B128 LMC 24/08/05–01/11/05 69 17.5* 17.5 25.0 0.109 16.5 28.5 �2.33 0.02 �4.46 0.03 �2.33 �4.46 �0.01 1B137 LMC 24/08/05–01/11/05 69 15.2* 15.2 20.5 0.076 16.5 28.5 �2.43 0.04 �4.64 0.03 �2.43 �4.65 0.00 1B196 LMC 05/07/06–24/07/06 19 31.0 43.4 44.5 0.055 18.8 28.5 �2.48 0.04 �4.80 0.07 �2.48 �4.81 0.02 1B130 LMC 05/07/06–24/07/06 19 13.4* 29.7 31.9 0.116 18.8 28.5 �2.47 0.04 �4.79 0.10 �2.48 �4.81 0.03 1B165 LMC 05/07/06–24/07/06 19 38.7 50.3 52.0 0.089 18.8 28.5 �2.53 0.02 �4.88 0.06 �2.54 �4.90 0.01 1B178 LMC 05/07/06–24/07/06 19 24.6 37.2 39.0 0.091 18.8 28.5 �2.61 0.03 �5.07 0.07 �2.62 �5.08 0.03 1

Min., mineralogy; growth period related to sampled calcite shell edges; growth period related to sampled calcite shell edges (in days); IS, initial size after shell collection and prior to start of theexperiment; *, labelled on the 24/08/2005, without * labelled on the 03/11/2005; S1, size 1 corresponds to the start of the respective growth period; S2, size 2 corresponds to the end of the respectivegrowth period; ØGR, mean growth rate over the respective growth period; ØT (�C), mean sea-surface temperature over the respective growth period; ØSAL (&), mean sea-surface salinity over therespective growth period; N, number of analyses, with five runs making an analysis (cf. Section 2).

6138D

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73(2009)

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Table 4Magnesium-isotope ratios of seawater from the western Dutch Wadden Sea and IAPSO reference seawater.

Sample Samplingdate

T

(�C)SAL(psu)

Ca(mg/l)

Mg(mg/l)

d25Mg(& DSM3)

±2r d26Mg(& DSM3)

±2r d25Mg0

(& DSM3)d26Mg0

(& DSM3)D25Mg0 N

DWS 08-03-07 HT 08.03.2007 7.6 28.0 343.8 1055 �0.42 0.04 �0.80 0.02 �0.42 �0.80 0.00 1DWS 25-04-07 HT 25.04.2007 13.5 29.5 367.1 1147 �0.41 0.01 �0.77 0.02 �0.41 �0.77 0.00 1DWS 23-05-07 HT 23.05.2007 15.0 30.5 365.4 1146 �0.42 0.01 �0.80 0.03 �0.42 �0.80 0.00 1DWS 21-08-07 HT 21.08.2007 19.0 28.7 – – �0.42 0.02 �0.79 0.03 �0.42 �0.79 �0.01 1IAPSO 35.0 418.3 1370.2 �0.42 0.02 �0.80 0.05 �0.42 �0.80 0.00 *

T (�C), sea-surface temperature during high-tide measured at the time of sampling; SAL (psu), sea-surface salinity during high-tide measuredat the time of sampling; Ca concentration in (mg/l) obtained by ICP-OES, Christian-Albrechts-University, Kiel (Germany), RSD% = 0.8(obtained for IAPSO); Mg concentration in (mg/l) obtained by ICP-OES, Christian-Albrechts-University, Kiel (Germany), RSD% = 0.2(obtained for IAPSO); N, number of analyses, with five runs making an analysis (cf. Section 2).* Values given for IAPSO are mean values based on nine (Ca- and Mg-concentrations) and 10 (Mg isotopes) analyses, respectively.

Magnesium isotopes in skeletal carbonates 6139

dryness. In order to destroy organic compounds, aliquotswere treated with 100 ll of HClO4 (later a H2O2:HNO3

mixture replaced the HClO4) minimizing potential interfer-ences related to complexation of cations. Seawater samples(0.5 ml) were first evaporated, than the dry residue was dis-solved in supra-pure 6 M HCl and subsequently evaporatedto dryness. After evaporation, all samples (carbonates andseawater) were re-dissolved in 2.5 M HCl and loaded onquartz glass columns. The Mg fraction was recovered usingBioRad ion exchange resin AG50W-X12 (200–400 mesh).The column yield for Mg was >98%, which was verifiedby inductively coupled plasma optical emission spectrome-try (ICP-OES). After final evaporation, the Mg fractionwas taken up in 3.5% HNO3 and diluted to produce a500 ppb solution for Mg-isotope measurement. Total pro-cedural blanks are typical in the range of <10 ng Mg corre-sponding to a blank to sample ratio of 2 � 10�4.

2.3. Analyses of the magnesium-isotopic composition

Mg-isotope measurements were carried out on a Ther-moElectron Neptune multi-collector inductively coupledplasma mass spectrometer (MC-ICP-MS) in the isotopegeochemistry laboratory of the Ruhr-University in Bo-chum, Germany applying the standard bracketing tech-nique. Mg concentration of the standard and sample waskept within a 25% limit, which proved to minimize potentialmatrices effects (Galy et al., 2001). The samples were intro-duced into the plasma via a combination of two desolvatingsystems, an ApexIR (ESI) and Aridus (Cetac) enhancingthe signal intensity and stability. Mg-isotope ratios were re-ported as per mil deviations from the isotope referencestandards: dxMg = [(xMg/24Mg)sample/(

xMg/24Mg)standard

� 1] � 1000; where x is either mass 25 or 26. As referencematerial the DSM3 was chosen because of the publishedMg-isotope heterogeneity of the NIST SRM980 (Galyet al., 2003). The internal precision on both d25Mg andd26Mg is generally 0.03–0.10& (2r). In the protocol usedhere, all d-values are based on a sequence of five repetitionsmeasured from the same solution. Each repetition com-prises 45 measured isotope ratios. The average of these fiverepetitions constitutes what is referred to as a single deltavalue for a sample. The averages and ±2r errors of thesemeasurements are reported in Tables 1 to 4. The long-term

reproducibility (or external precision) of Mg isotope ratios,as determined by repeated analyses of mono-elementalstandards Cambridge 1 solution vs. DSM3 is ±0.02& (2rfor d25Mg and ±0.07& (2r for d26Mg (Table 1). In princi-ple, both d25Mg and d26Mg can be used to describe thefractionation behavior of the samples without interferencefrom analytical artifacts. We will thereafter only refer tod26Mg.

3. RESULTS

The measured Mg-isotopic composition of seawater andskeletal carbonates indicate mass-dependent behavior withall samples within uncertainty of the equilibrium fraction-ation line (Fig. 2). The Mg-isotope data, where delta valueshave been converted to d25Mg0 and d26Mg0 to describe a lin-ear regression (Young and Galy, 2004), define a single massfractionation curve on an Mg0 three-isotope plot with aslope of 0.521 ± 0.004 (r = 0.999, N = 31, p < 0.0001;PAST: Paleontological Statistics Software Package byHammer et al. (2001)), which is thus indistinguishable tothe theoretical equilibrium gradient of 0.521 (Younget al., 2002). For Mg isotopes, deviation from the equilib-rium mass fractionation line is expressed as D25Mg0.

3.1. Magnesium isotopes in reference materials and seawater

The Mg-isotopic composition of the mono-elementalMg-standard, standard seawater and seawater collectedfrom the western Dutch Wadden Sea is displayed in Tables1 and 2. The long-term average (January to July 2008) ofCambridge 1 against DSM3 was calculated to be�2.58 ± 0.04& (N = 56). This value is in overall agreementwith Galy et al. (2003; d26Mg: �2.60 ± 0.14&; N = 35),Black et al. (2006; d26Mg: �2.59 ± 0.15&; N = 17), Tipperet al. (2008; d26Mg: �2.592 ± 0.087&; N = 67) and Bolou-Bi et al. (2009; d26Mg: �2.62 ± 0.14&; N = 18). The coralJCp-1 yields a mean d26Mg value of �1.96 ± 0.04& (N =6), which is in agreement with Wombacher et al. (2009;d26Mg: �2.01 ± 0.22&; N = 37). Analyses of IAPSO stan-dard seawater (salinity = 34.998 psu) result in a d26Mg va-lue of �0.80 ± 0.05& (N = 10), similar to what waspostulated by Ra and Kitagawa (2007, d26Mg: �074&;N = 4). Samples of the western Dutch Wadden Sea yield

6140 D. Hippler et al. / Geochimica et Cosmochimica Acta 73 (2009) 6134–6146

d26Mg values around �0.79 ± 0.03& (N = 4). Note thatwithin the method’s uncertainty, the Mg-isotopic composi-tion of the individual seawater samples from the DutchWadden Sea collected in March, April, May and August2007, which are characterized by different seasonal temper-atures (7.5–19.0 �C) and salinities (28.0–30.5 psu), are indis-tinguishable from each other (Table 4). Furthermore, theirmean d26Mg value is favorably to seawater data from otherlaboratories averaging d26Mg values of �0.82& (e.g.,Chang et al., 2004; de Villiers et al., 2005; Tipper et al.,2008). Thus, within the range of studied salinities, salinityhas most likely no impact on the Mg-isotopic compositionof seawater. It was, however, beyond the scope of this studyto examine seawater samples from estuarine to brackishenvironments. Recently, Pogge von Strandmann et al.(2008) presented U, Li, and Mg element and isotope datafor both dissolved and suspended material from two estuar-ies in the predominantly basaltic islands of Iceland andAzores, in order to illustrate the effects of estuarine mixingon the isotopic composition of the dissolved load. Thed26Mg value of the dissolved phase was explained in theterms of simple mixing between a river endmember, whichhas relatively high d26Mg and low [Mg], and a seawater(high [Cl]) endmember, which has relatively low d26Mgand high [Mg]. Furthermore, negligible Mg-isotope varia-tions were reported for seawater Mg concentrations aslow as 11 mM (Pogge von Strandmann et al., 2008).

3.2. Magnesium isotopes in modern marine biogenic

carbonates

The Mg-isotopic composition obtained from modernscaphopod, coralline red algae, echinoids and brachiopodssampled in low resolution as bulk biogenic carbonates islisted in Table 2. Notably, the skeletons of all studied marinecalcifiers are enriched in the light isotope, relative to the Mg-isotopic composition of seawater. The Mg-isotopic composi-tion (d26Mg) of all these samples varies between �1.88 ±0.10& and �3.24 ± 0.04&. The scaphopod sample exhibitsa d26Mg of �2.07 ± 0.05&. The brachiopod samples, col-lected at two different localities, yield slightly differentd26Mg values of �2.29 ± 0.06& and �1.88 ± 0.10&. Allechinoid and coralline red algae samples measured, beingof the same species, but from different localities, show similarMg-isotope ratios with average values of�2.63 ± 0.13& and�3.13 ± 0.23& for d26Mg, respectively.

3.3. Magnesium isotopes of the calcite shell layer of M. edulis

The M. edulis samples have the lightest Mg-isotope com-position within the sample set. Their Mg-isotopic composi-tion varies between �3.37 ± 0.06& and �5.07 ± 0.07&

(Table 4). In contrast to the echinoid and coralline red algaesamples, the overall M. edulis data set show a larger vari-ability of 1.70&, which amounts to half the overall varia-tion (considering all measured samples) in d26Mg of3.19&. Note, however, that the calcite shell layer of M. edu-

lis was not sampled as bulk biogenic calcite. The chosensampling strategy, milling only recently grown calcite (seeSection 2) allowed us to achieve a higher temporal resolu-

tion assigning recently precipitated calcite to the respectivegrowth season. Thus, differences in Mg-isotopic composi-tion can easily be studied in terms of growth and ambientseasonal environmental parameters at the time of calciteprecipitation.

Summarizing, replicate analyses of reference materialshighlight the quality and reliability of the Mg-isotope datapresented here. The observed variations in d26Mg ratios ofbiogenic carbonates considerably exceed the error of ana-lytical precision and clearly demonstrate the heterogeneityof marine biogenic Mg-isotope records precipitated in mod-ern oceans.

4. DISCUSSION

4.1. Potential factors influencing magnesium-isotope

fractionation

Large overall variations in d26Mg of more than 3& arefound in the present data set. The d-values of all modernskeletal carbonates are more negative than that of dissolvedMg in seawater (Figs. 2 and 3). This suggests (1) Mg-iso-tope fractionation between an aqueous solution and theprecipitating carbonate at low temperatures and (2) thatbiological processing preferentially utilizes the lighter iso-topes of magnesium. Given the observation that seawaterd26Mg is constant, irrespective of depth or geographic loca-tion (cf. western Dutch Wadden Sea; this study, and NorthAtlantic Ocean, Pacific Ocean, and Mediterranean Sea;Chang et al., 2004; de Villiers et al., 2005; Tipper et al.,2006, 2008) as it is expected from its long residence time(s � 13 Myr; Broecker and Peng, 1982) the overall varia-tions observed in this study do not reflect regional or tem-poral differences in seawater Mg-isotopic composition.Concerning echinoid and coralline red algae samples thisfact becomes even more important since these samples werecollected at different localities, which are characterized bydifferent mean annual sea-surface temperatures and salini-ties. Large variations in d26Mg of approximately 4& havefurthermore been reported for terrestrial systems, particu-larly for speleothems (Galy et al., 2002; Young and Galy,2004; Buhl et al., 2007), demonstrating that the marineand terrestrial isotope composition of Mg is not unique.To date, only a small number of peer-reviewed articles havebeen published on the processes and factors potentiallyinfluencing the distribution of Mg isotope in marine skeletalcarbonates. Therefore, a number of previously publishedabstracts complement the discussion.

4.1.1. Mineralogy

A considerable component of the observed variability ind26Mg values of marine skeletal carbonates can be attrib-uted to differences in mineralogy. Specific d26Mg valueswere observed for aragonite, high and low-Mg calcite(HMC and LMC) shells (Fig. 3). Investigating bulk marineand terrestrial carbonates, Galy et al. (2002) also suggesteda strong mineralogical control. They observed that thed26Mg of dolostones is approximately 2–3& higher thanthe d26Mg of limestone, and that the d26Mg of speleothemscontaining dolomite is also around 2& higher then the

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±2σ

Fig. 2. Three-isotope plot (d26Mg0 vs. d25Mg0) for standardsolution (DSM 3), seawater (this study, Chang et al., 2004, 2003;Tipper et al., 2008), modern marine calcifiers and Mytilus edulis

(this study) analyzed. The slope of 0.521 ± 0.004 (r = 0.999,N = 31, p < 0.0001) calculated by standard regression (Hammeret al., 2001) is based on the data of all marine calcifiers of this studyand indicates mass-dependent behavior of Mg-isotope fraction-ation. The error bars in the right corner represent the 2r errors ofthe analyses.

Magnesium isotopes in skeletal carbonates 6141

d26Mg of calcitic speleothems. To date, only a small num-ber of aragonitic samples have been measured. Availabledata on Mg isotopes from coral and sclerosponge aragonite(Chang et al., 2004; Dessert et al., 2005; Wombacher et al.,2006), however, are very similar to that of the JCp-1 coraland the scaphopod sample. Both aragonitic samples yieldedrelatively heavy Mg-isotopic compositions in comparisonto calcitic skeletons. The latter could be separated in theHMC skeletons of echinoids and coralline red algae show-

Mixed calcifiers I - Aragonite

Scaphopod - Aragonite

Echinoids - HMC

Red algae - HMC

Mytilus edulis - LMC

Brachiopods - LMC

Foraminifera - LMC

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Mixed calcifiers II - HMC

Inorganic calcite(sclerosponge, deep-sea corals, red algae)

(corals, sclerosponges)

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Fig. 3. Bar chart summarizing d26Mg values relative to the respectisclerosponges and scaphopods (this study, Chang et al. 2004; Wombacheralgae (this study), mixed high-Mg calcifiers (Wombacher et al., 2006; PoggMg calcite (LMC) of bivalves and brachiopods (this study), and foramStrandmann, 2008), respectively. See text for further details.

ing intermediate d26Mg values whereas LMC skeletons ofbivalves (M. edulis, this study) and planktic foraminifera(Chang et al., 2004; Wombacher et al., 2006; Pogge vonStrandmann, 2008) display lowest d26Mg values. Based onthese findings, a strong mineralogical control was suggestedto describe the Mg-isotope compositions of skeletalcarbonates.

4.1.2. Taxonomic differences

The data presented here demonstrate that considerablevariations of Mg-isotope composition exist both betweenmarine carbonate-precipitating organisms and algae, aswell as within a taxonomic group (Figs. 2 and 3). The uni-form Mg-isotope composition of aragonitic skeletons (JCp-1 coral, scaphopod) is consistent with published Mg isotoperecords of biogenic aragonite (corals, sclerosponges; Changet al., 2004; Wombacher et al., 2006), supporting thehypothesis of Wombacher et al. (2006) that metabolic influ-ences are likely absent. Small but considerable variability ind26Mg (<0.5&) has been found for HMC skeletons (coral-line red algae, endobenthic echinoids) and LMC skeletons(brachiopods), with echinoids approximately 1.8&, red al-gae 2.1–2.4&, and brachiopods 1.0–1.5& lighter than sea-water. The d26Mg values of HMC coralline algae (thisstudy) are very similar to the uniform HMC data (sclero-sponges, a calcitic coral, a red algae; �2.5 ± 0.2&) ob-tained by Wombacher et al. (2006). These values arefurther close to the assumed inorganic value of approxi-mately �2.7& that has been found between calcitic speleo-thems and corresponding dripwater (Galy et al., 2002). Asalready proposed for aragonitic skeletons, this finding indi-cates the absence or negligibility of metabolic influences.Data for HMC echinoid and LMC brachiopod samples ob-

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[‰]

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ve growth solution (seawater, dripwater), for aragonitic corals,et al., 2006), high-Mg calcite (HMC) of echinoids and coralline rede von Strandmann, 2008), inorganic calcite (Galy et al., 2002), low-inifera (Chang et al., 2004; Wombacher et al., 2006; Pogge von

6142 D. Hippler et al. / Geochimica et Cosmochimica Acta 73 (2009) 6134–6146

tained in this study yield Mg-isotope values that are verysimilar to those shown in the dataset from the taxonomicgroups in Wombacher et al. (2006). Finally, the LMC ofM. edulis exhibits variable Mg-isotope compositions (vari-ability in d26Mg values of 61.7&) being 2.5–4.2& lighterthan seawater. In contrast to the coralline algae data shownhere, data for echinoids, brachiopods and bivalves yieldMg-isotope fractionations that are considerably smaller(echinoids, brachiopods) or larger (M. edulis) than those de-fined by most HMC data (red algae, sclerosponges, calciticcoral; this study, Wombacher et al., 2006). Furthermore,their isotopic fractionation is considerably different fromthe assumed inorganic value. This suggests that metabolicprocesses during echinoid and brachiopod shell formationmore strongly favor the incorporation of heavy Mg iso-topes than inorganic precipitation or than most HMC cal-cifiers do. By contrast, M. edulis preferentially incorporateslight Mg isotopes in their calcitic outer shell relative to inor-ganic calcite and HMC calcifiers. Thus, there must be tax-on-specific level of control on Mg-isotope incorporation, asMg is transported from seawater to the site of calcification.For bivalves, the latter hypothesis is supported by recentfindings illustrating that skeletogenesis, and hence theresulting geochemical composition, of mollusk exoskeletontakes place under very strong biological influence (Weinerand Dove, 2003).

4.1.3. Biomineralization

Calcium carbonates of biogenic origin are often organic/inorganic composites and are produced by the organismthrough biological control and mediation (e.g., Lowenstamand Weiner, 1989). For many organisms, however, thecomplex mechanisms during biomineralization are not yetfully understood and are subjected to ongoing research.As outlined above, Mg-isotope fractionation in echinoids,brachiopods and bivalves appears to be affected by meta-bolic influences, which might mirror the organism’sadopted calcification strategies. Therefore, the current the-ory of biologically controlled calcification processes (cf.Weiner and Dove, 2003) for these species should be brieflyoutlined.

Shells of bivalves and brachiopods are believed to min-eralize primarily by biologically controlled extracellularprocesses (e.g., Crenshaw, 1980; Falini et al., 1996; Gotlivet al., 2003; Gaspard et al., 2007). The main control is amacromolecular matrix outside the cell in an area that willbecome the site of mineralization. The structure and com-position of these organic templates are genetically pro-grammed to mediate the calcification process by allowingcrystal nucleation and growth in specific orientations. Inthe case of bivalves, shell formation takes place in an iso-lated space, which is called the extrapallial cavity, situatedbetween the growth surface of the shell and the secretoryepithelium of the mantle (Crenshaw, 1980). This volumeis filled with the extrapallial fluid (EPF), which containsseveral macromolecules (e.g., proteins, polysaccharides orglycoproteins), supplying a three-dimensional template formineral formation (Addadi et al., 2006). The EPF acts asa dynamic physiological medium, regulating the pH withinthe animal (Simkiss and Wilbur, 1989). Ambient seawater,

ingested particulate matter or metabolic products of respi-ration provide the shell-forming components (Ca2+,Mg2+, Sr2+, CO3

2�). In order to reach the site of calcifica-tion, it is suggested that these elements are transportedacross the epithelium via inter- and/or intra-cellular path-ways (Watabe and Kingsley, 1989). Cations are either ac-tively pumped across the cell membrane or move bypassive diffusion through extracellular fluids to the site ofcalcification (Weiner and Dove, 2003). Echinoids or seaurchins, on the contrary, favor a biologically controlledintracellular mineralization strategy. Their mineralized tis-sues (e.g., plates) are formed within a vesicle that is theproduct of a thin organic membrane (Markel et al., 1986;Wang, 1998). The mineralized unit is secreted to the envi-ronment, only if and when the membrane is degraded (Mar-kel et al., 1986). Previous studies revealed that the firstspicular structures of sea urchin larvae begin as amorphouscalcium carbonate, passing finally into monocrystals of cal-cite (e.g., Lowenstam and Weiner, 1989).

As presented in the previous paragraph, organic mem-branes and/or templates (matrices) play an important roleduring biocalcification. Therefore, a possible implicationof these findings is that Mg-isotope fractionation occursduring the passage of Mg through the epithelial layer orby adsorption to organic matrices. Conclusive statementson the origin of Mg-isotope fractionation during biominer-alization, however, could only be achieved by investigatingthe isotopic composition of the organism’s soft tissue andfluids, which are involved when Mg is transported from sea-water to the site of calcification, an issue that has not beenthe topic of this study.

4.1.4. Salinity, temperature and/or growth rate

The Mg-isotope composition of most samples was alsoevaluated in the context of key environmental factors, suchas salinity and temperature, and growth rate. The latter fac-tor was only assessed for the M. edulis data, for whichgrowth rate data was available. Echinoids and corallinered algae cover a wide range of mean annual salinities, withsalinity differences of 16.5 and 6.5 psu, respectively. How-ever, no obvious salinity dependence of the Mg-isotopefractionation in echinoids and coralline red algae was ob-served (Fig. 4a). By comparison, d26Mg values of M. edulis

appear to increase with increasing salinity. The weak posi-tive correlation (r = 0.548, N = 15, p = 0.035) of ambientsalinity and d26Mg (Fig. 4b) covers, however, a salinityrange (27.0–29.5 psu), which is rather small, particularlyin view of the error bar related to the salinity data set(±0.75 psu). Thus, in order to validate the insusceptibilityof d26Mg values to seawater salinity in general, future workshould include multi-taxa data covering a wide range ofsalinities.

The second important environmental factor that hasbeen evaluated, namely temperature, yielded contrasting re-sults. Although temperature dependence is to be expected ina thermodynamic system (Bigeleisen and Meyer, 1947;Urey, 1947), a significant sensitivity of temperature onMg-isotope composition has not yet been observed in ter-restrial and marine systems in previous studies. Galyet al. (2002) pointed out that temperature-dependent Mg-

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Fig. 4. Environmental factors (seawater salinity and seawater temperature) vs. Mg-isotope composition. (A) Seawater salinity has clearly noeffect on the d26Mg of echinoids (black squares) and red algae (grey squares). (B) d26Mg values of M. edulis (open diamonds) appear to berelated to seawater salinity. However, the salinity range is rather small. (C) d26Mg values of echinoids and red algae are not affected byseawater temperature (D) d26Mg values of M. edulis appear to be inversely correlated to ambient seawater temperature. Size of the symbols(squares and diamonds) incorporates the uncertainty (2r) on d26Mg. Error bars for the salinity and temperature data are shown in the leftbottom corner.

Magnesium isotopes in skeletal carbonates 6143

isotope fractionation between a fluid and the correspondingcarbonate is difficult to predict on a theoretical base, sinceMg bonds are mainly ionic in character. This study also re-ported a minor increase of the Mg-isotope fractionationwith increasing temperatures during speleothem formation.According to these authors, the temperature effect amountsto less than 0.02&/AMU/�C, which means that only rela-tively large temperature difference could be resolved usinghigh-precision analytical techniques. Investigating severalplanktonic foraminifers, spanning temperature ranges ofapproximately 10 �C, Chang et al. (2004) and Pogge vonStrandmann (2008) found no resolvable temperature-dependent fractionation of Mg isotopes. The work pre-sented here reports Mg-isotope data for echinoids andcoralline red algae, which span a measured seawatertemperature range of 10 and 13 �C, respectively (Fig. 4c).In line with the last mentioned studies, d26Mg values ofechinoids and coralline red algae show no resolvable tem-perature dependence (Fig. 4c). This finding is furthermorein agreement with Wombacher et al. (2006), who found,across a seawater temperature range of approximately20 �C, no correlation with temperature for aragonite andHMC data (including data on red algae). In contrast tothe findings on coralline red algae and echinoids, thed26Mg values of M. edulis appear inversely correlated toambient sea-surface temperatures (r = 0.530, N = 15, p =0.042; Fig. 4d), which is illustrated in slightly decreasingd26Mg ratios with increasing temperatures. The tempera-

ture sensitivity found for M. edulis is in line with Wanget al. (2006), reporting high-resolution d26Mg records fromcoral aragonite. Distinct annual cycles in d26Mg have beenfound within two coral colonies that were in concert withannual sea-surface temperature records at the site of collec-tion. Their empirically calibrated fractionation factor had alinear relationship to inverse absolute temperature, corre-sponding to a temperature sensitivity of about 0.11& per1 �C. On the basis of these findings, we suggest that a de-tailed sampling strategy (cf. M. edulis) or high-resolutionsampling of single growth increments (cf. Wang et al.,2006), precipitated either annually or sub-annually depend-ing on the organism, may provide better insights in howseasonally changing environmental factors affect isotopefractionation.

Magnesium-isotope data of M. edulis were also evalu-ated in terms of growth rate. Studies on trace elemental ra-tios (Sr/Ca) of bivalve calcite and aragonite have shownthat growth rate might induce kinetic fractionation (Gilli-kin et al., 2005; Lorrain et al., 2005). Calculating meangrowth rates for the respective sampled time interval alongthe axis of maximum growth for each M. edulis specimen, anegative correlation between Mg-isotope composition andgrowth rate has been found (r = 0.593, N = 15, p = 0.020;Fig. 5a). Slow growing individuals (<0.05 mm/day) seemto preferentially incorporate the heavier isotopes relativeto faster growing individuals, and are closer to the assumedinorganic fractionation of calcite (Galy et al., 2002). This

6144 D. Hippler et al. / Geochimica et Cosmochimica Acta 73 (2009) 6134–6146

finding is, however, in disagreement with Mg-isotope datafrom four different size fractions of planktonic foraminiferaGlobigerinoides sacculifer (Pogge von Strandmann, 2008).In the latter study, different foraminiferal test sizes are as-sumed to reflect differences in growth rate. While Mg/Caratios were found to increase with test size, Mg-isotope ra-tios remained constant, suggesting that foraminiferalgrowth rate has no influence on Mg-isotope ratios of thisspecies (Pogge von Strandmann, 2008). Nevertheless, awide spectrum of factors influences growth rates of bi-valves. Amongst these, food supply and/or temperatureare considered to be of major importance (Widmann andRhodes, 1991; Seed and Suchanek, 1992). For the time ofthe experiment, data on nutrition availability were not yetavailable. In contrast, the relationship between temperatureand growth rate was assessed for the M. edulis data set.Testing of both parameters reveals a conspicuously positiverelationship over the whole temperature range (3–20 �C),with only one outlier (B196 in Fig. 5b). This indicates, thatindividual mean seasonal growth rates are related to therespective mean seasonal temperatures, which in turn im-plies that ambient seawater temperature is possibly re-corded in the calcite shells of M. edulis in an indirectmanner via growth rate. An important consequence of this

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Fig. 5. Mg-isotope composition and seawater temperature inrelation to growth rate. Mean growth rates are calculated for eachof the sampled calcite shell edges of M. edulis for the respectivegrowth interval (A) d26Mg values of M. edulis decrease withincreasing mean growth rates. (B) For samples analyzed, meangrowth rates are related to mean ambient seawater temperatures ofthe respective growth interval.

observation is that Mg-isotope data from bivalve calcitehave limited significance as proxy for sea-surfacetemperatures.

5. CONCLUSION

Systematic Mg-isotope analyses of seawater and skeletalcarbonates of five marine organisms reveal that the d26Mgratios in the marine environment are highly variable. Allskeletal carbonates are isotopically lighter than seawater.Skeletal aragonite displays near uniform d26Mg ratios sug-gesting that metabolic influences are most likely negligibleor absent. Negligible metabolic effects can also be postu-lated for Mg-isotope ratios of high-Mg calcitic corallinered algae. Their d26Mg ratios are close to the assumed inor-ganic value. Furthermore, these data are similar to themean d26Mg ratios of HMC mixed calcifiers, including scle-rosponges, calcitic coral, and red algae (Wombacher et al.,2006). In contrast, echinoids, brachiopods and bivalves areenriched or, conversely, depleted in d26Mg compared toHMC mixed calcifiers and inorganic calcite. From this itconcludes that during biocalcification, taxon-specific pro-cesses – being either weaker or stronger than those experi-enced during inorganic calcification – are likelyinfluencing Mg-isotope incorporation in these biominerals.Whether Mg-isotope fractionation occurs during the pas-sage of Mg through certain cell membranes or by adsorp-tion to organic matrices could not be conclusivelyanswered at that point. Hence, species-specific studies mustbe undertaken in order to better understand Mg-isotopepathways and thus fractionation in biogenic systems.

In agreement with Mg-isotope data of inorganic precip-itates (Galy et al., 2003; Buhl et al., 2007) and foraminifera(Chang et al., 2003; Pogge von Strandmann et al., 2008),endobenthic echinoids and coralline red algae show neitherresolvable salinity nor temperature effects. This insensitivityto environmental factors suggests that echinoids and coral-line red algae are potential proxy carriers of past seawaterMg-isotopic composition, provided a reliable determinationof the fractionation factor exists. In contrast, the bivalve M.

edulis exhibit weak salinity, temperature and growth rate ef-fects, which require further validation. In bivalves, how-ever, the dependency and/or interaction of environmentaland biological factors are likely to limit the use of d26Mgas salinity or temperature proxy.

ACKNOWLEDGMENTS

This is a contribution to the EuroClimate project 04 ECLIMFP08 CASIOPEIA and was supported by the Dutch ResearchCouncil (NWO). Thanks to F. Plonchon (Royal Museum forCentral Africa, Tervuren, Belgium) and F. Wombacher (FreeUniversity Berlin, Germany) for important feedback on the Mg-isotope data. The last named and A. Kolevica (IfM-GEOMAR,Kiel, Germany) are kindly acknowledged for providing ICP-MSreference material JCp-1. Thanks to D. Garbe-Schonberg andK. Kissling (Christian-Albrechts-University, Kiel, Germany) forproviding elemental data of seawater samples measured by ICP-OES, and H.v. Aken (Royal Netherlands Institute for SeaResearch, Netherlands) for providing long-term sea-surface tem-perature and salinity data.

Magnesium isotopes in skeletal carbonates 6145

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.gca.2009.07.031.

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