Towards a better understanding of magnesium-isotope ratios from marine skeletal carbonates

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Geochimica et Cosmochimica Acta 73 (2009) 6134–6146 www.elsevier.com/locate/gca

Towards a better understanding of magnesium-isotope ratios from marine skeletal carbonates Dorothee Hippler a,*, Dieter Buhl b, Rob Witbaard c, Detlev K. Richter b, Adrian Immenhauser b a b

Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Universita¨tsstrasse 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 calcifying organisms 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 isotopic composition, temperature, salinity) on magnesium-isotopic (d26Mg) ratios of skeletal carbonates. Using high-precision multicollector inductively coupled plasma mass spectrometry, we observed that the magnesium-isotopic composition of seawater from the semi-enclosed Dutch Wadden Sea is identical to that of open marine seawater. We further found that a considerable component of the observed variability in d26Mg values of marine skeletal carbonates can be attributed to differences in mineralogy. Furthermore, magnesium-isotope fractionation is species-dependent, with all skeletal carbonates being isotopically lighter than seawater. While d26Mg values of skeletal aragonite and high-magnesium calcite of coralline red algae indicate the absence or negligibility of metabolic influences, the d26Mg values of echinoids, brachiopods and bivalves likely result from a taxon-specific level of control on Mg-isotope incorporation during biocalcification. Moreover, no resolvable salinity and temperature effect were observed for coralline red algae and echinoids. In contrast, Mg-isotope data of bivalves yield ambiguous results, which require further validation. The data presented here, point to a limited use of Mg isotopes as temperature proxy, but highlight the method’s potential as tracer of seawater chemistry through Earth’s history. Ó 2009 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Skeletal carbonates of marine organisms are sensitive archives recording the past physical and chemical environment, such as ocean temperature, salinity, alkalinity, pH, ocean circulation, paleo-productivity, or seawater isotopic ratios often with high temporal resolution (Khim et al., 2000; Vander Putten et al., 2000; Henderson, 2002; Rosales et al., 2004; Steuber et al., 2005; Armenda´riz et al., 2008; *

Corresponding author. Present address: Institute of Applied Geosciences, Technical University Berlin, Ackerstrasse 76, 13355 Berlin, Germany. Tel.: +31 20 598 7365; fax: +31 20 598 9941. E-mail address: [email protected] (D. Hippler). 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.07.031

Foster et al., 2008). Carbonate hardparts being composed of aragonite or high-magnesium calcite commonly (but not always; Le´cuyer and Bucher, 2006) undergo diagenetic alteration within some thousands of years (Al-Aasm and Veizer, 1982, 1986; Gagan et al., 2000). In contrast, lowmagnesium calcite shells of some bivalves and brachiopods have the potential to preserve their primary geochemistry throughout the Phanerozoic (Veizer et al., 1999; Wierzbowski, 2004; Immenhauser et al., 2005, 2008). In an attempt to extract relevant environmental information from these archives, most workers analyze the shell carbon and oxygen-isotopic or elemental ratios such as Mg/ Ca, Sr/Ca or Mn/Ca (e.g., Henderson, 2002; Rosenheim et al., 2004; Caroll et al., 2006; Freitas et al., 2006). Yet,

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ratories and to assess the quality of the data, a number of reference materials (Table 1) were included. The carbonate reference material, JCp-1 was prepared by the Geological Survey of Japan from an aragonitic Porites sp. coral skeleton (Inoue et al., 2003). Furthermore, IAPSO (International Association for the Physical Sciences of the Oceans) was used as reference seawater, which is a seawater salinity standard provided by the ‘‘Ocean Scientific International”, Southampton. The salinity is stated to be 34.998 psu. Mono-elemental Mg-solutions, Cambridge 1 and DSM3, which have been developed in the Department of Earth Sciences, University of Cambridge (Galy et al., 2003; Tipper et al., 2006) complete the set of reference materials. The second sample set is composed of carbonate-precipitating marine organisms and algae, collected at different localities along the coasts of the North Atlantic, the North Sea, the Kattegat (strait between the North Sea and the Baltic Sea) and the Mediterranean (Fig. 1, Table 2). Bulk low-Mg calcite (LMC), high-Mg calcite (HMC) and aragonite samples were drilled from modern brachiopods, endobenthic echinoids (Echinocyamus pusillus), articulate coralline red algae (Corallina officinalis), and scaphopods (Antalis costatum), respectively. According to Richter and Bruckschen (1998) and Richter (1984), echinoid and coralline red algae HMC is made up of 10–14 mol% MgCO3 and 10–17 mol% MgCO3, respectively. The third sample set was collected from experimentally grown shells of the blue mussel Mytilus edulis. Specimens were collected in 2005 and cultured subsequently in field experiments in the western Dutch Wadden Sea close to the 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 temperature (SST) and salinity (SSS) were monitored at the sampling site (see Table in Electronic Annex EA-1). In order to get continuous time series of SST and SSS, automated conductivity-temperature (CT) salinity loggers (StarOddiÒ) were applied. Temperature and salinity were measured with an accuracy of 0.1 °C and 0.75 psu, respectively. Individual shells were marked with tags and shell length of the specimens was (sub-)monthly measured with digital calipers to obtain continuous time series of changing individual growth. Subsequent to seasonal sample collection, soft tissue was removed, and shells were cleaned and

none of these proxies is unambiguous. Many proxies, both light stable isotopes and elemental ratios often respond to more than one environmental parameter. Oxygen-isotope fractionation in carbonates is temperature dependent but also affected by seawater salinity or pH (McCrea, 1950; Usdowski et al., 1991; Zeebe, 1999; Zeebe, 2005; Brand et al., 2003). Furthermore, so-called ‘‘vital effects” (Urey et al., 1951) might complicate the interpretation of geochemical proxies. Investigating the elemental ratios of marine and freshwater bivalves, Gillikin et al. (2005) and Geist et al. (2005) have been able to show that the elemental ratios reflect biological (metabolic), as well as physico-chemical parameters. A reasonable approach is the application of multi-proxy data sets with different proxies being affected by different parameters (Carpenter et al., 1991; Klein et al., 1996; Immenhauser et al., 2005). Particularly, the introduction of new proxies can complement these approaches. One of the more recently explored isotope systems is magnesium (Galy et al., 2001). Magnesium is a major element in the oceans, and plays an important role in hydrological and biogeochemical cycles. Magnesium has three naturally occurring stable isotopes (24Mg: 78.992%; 25Mg: 10.003%; 26 Mg: 11.005%) and their distribution may provide new insights into these cycles. Here, we report the Mg-isotopic composition (d26Mg) of various carbonate reference materials, seawater and skeletal carbonates of five marine calcifiers. Our aims are (1) to assess the factors controlling Mg-isotope fractionation in modern biogenic carbonates, such as mineralogy, (2) to investigate and compare Mg-isotope records between and within taxonomic groups, and (3) to explore the influence of environmental and biological factors, such as ambient seawater temperature, salinity and growth rate on Mg-isotope fractionation. This approach might therefore provide new insights into the methods’ applicability as environmental proxy or tracer of paleo-seawater chemistry, and Mg cycling during biomineralization. 2. MATERIALS AND METHODS 2.1. Sampling material Three different sample sets were incorporated in this study. First, in order to compare data from different labo-

Table 1 Magnesium isotope ratios of various reference materials. Standard

Material

d25Mg (& DSM3)

±2r

d26Mg (& DSM3)

±2r

d25Mg0 (& DSM3)

d26Mg0 (& DSM3)

JCp-1

Aragonitic coral (Porites sp.) Seawater Mono-elemental Mg-solution Mono-elemental Mg-solution

1.03

0.02

1.96

0.04

1.03

0.42 1.34

0.02 0.02

0.80 2.58

0.05 0.04

0.02

0.05

0.06

0.07

IAPSO Cambridge 1 DSM3

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

D25Mg0

N

1.96

0.00

6

0.42 1.34

0.80 2.58

0.00 0.01

10 56*

0.02

0.06

0.01

5

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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, United Kingdom, (3) NW-Island, (4) Tromsø, Norway, (9) Bretagne, France; in the North Sea: (5) Helgoland, Germany and the along the Danish coast of the Kattegat (10, 11). Field-culturing experiments on Mytilus edulis were performed in a coastal marine setting of the island of Texel in the Western Dutch Wadden Sea (14). More details on the sites are given in Tables 2–4 and Electronic Annex EA-1.

air dried. Shell samples were further oven dried at 40 °C overnight. The periostracum was removed by grinding paper 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 calcitic layer substantially, thus all new growth is calcitic. Samples of this recently precipitated shell calcite, which could be related to the corresponding temporal phase, and therefore to the environmental conditions, under which the carbonate had been deposited, were hand-drilled along the ventral margin of the shell. The latter sample set was complemented by four seawater samples, which were collected during high tide at the experimental site at different calendar dates in 2007 (Table 4). Seawater temperature, ranging from 7.5 to 19.0 °C and salinity, ranging from 28.0 to 30.5 psu at the time of collec-

tion are provided in Table 4. The seawater samples were filtered and acidified with 6 M quartz-distilled HNO3, and kept in the refrigerator prior to geochemical analysis. In order to obtain the Ca and Mg concentrations of these seawater samples (Table 4), one batch of each sample was analyzed by ICP-OES in the laboratories of the ChristianAlbrechts University, Kiel (Germany). Ca concentrations were between 340 and 370 mg/l, and Mg concentrations between 1050 and 1150 mg/l, respectively. 2.2. Purification of magnesium/ion-exchange chromatographic procedure Magnesium was purified by ion chromatography. Carbonate samples, representing ±50 lg Mg, were dissolved in supra-pure 6 M HCl and subsequently evaporated to

Table 2 Magnesium isotope ratios of modern marine calcifiers. Min.

ØTA (°C)

ØSALA (psu)

d25Mg (& DSM3)

±2r

d26Mg (& DSM3)

±2r

d25Mg0 (& DSM3)

d26Mg0 (& DSM3)

D25Mg0

N

Scaphopod Antalis 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 algae Corallina officinalis Corallina officinalis Corallina officinalis Corallina officinalis Corallina officinalis Corallina officinalis

2. 3. 4. 5. 6. 7.

HMC HMC HMC HMC HMC HMC

10.0 6.5 6.0 11.0 19.0 16.0

34.5 34.5 34.0 31.5 38.0 37.5

1.67 1.69 1.68 1.54 1.58 1.60

0.03 0.01 0.03 0.02 0.02 0.02

3.22 3.24 3.24 2.97 3.06 3.08

0.06 0.04 0.02 0.06 0.08 0.01

1.67 1.69 1.68 1.54 1.58 1.60

3.22 3.24 3.24 2.97 3.06 3.09

0.01 0.00 0.01 0.00 0.01 0.01

1 1 1 1 1 2

Echinoids Echinocyamus Echinocyamus Echinocyamus Echinocyamus Echinocyamus Echinocyamus Echinocyamus

8. Cantabrian Sea (N. Atl.), Asturias (E) 9. North Atlantic, Bretagne (F) 5. North Sea, Helgoland (D) 10. Kattegat (DK) 11. Kattegat (DK) 12. Mediterranean Sea, Peloponnese (GR) 13. Mediterranean Sea, Karpathos (GR)

HMC HMC HMC HMC HMC HMC HMC

14.5 12.5 11.0 9.5 9.5 19.0 19.5

34.5 34.5 31.5 25.0 22.0 38.0 38.5

1.36 1.35 1.42 1.37 1.31 1.38 1.34

0.00 0.02 0.03 0.03 0.04 0.03 0.01

2.61 2.62 2.75 2.65 2.56 2.65 2.58

0.02 0.04 0.05 0.07 0.05 0.04 0.07

1.36 1.35 1.42 1.37 1.31 1.38 1.34

2.61 2.62 2.76 2.65 2.56 2.65 2.58

0.00 0.01 0.01 0.01 0.02 0.00 0.00

2 1 1 1 1 1 2

9. North Atlantic, Bretagne (F) 5. North Sea, Helgoland (D)

LMC LMC

12.5 11.0

34.5 31.5

1.19 0.97

0.02 0.02

2.29 1.88

0.06 0.10

1.19 0.97

2.29 1.88

0.00 0.01

1 1

pusillus pusillus pusillus pusillus pusillus pusillus pusillus

Brachiopods Terebratula sp. Terebratula sp.

North Atlantic, Isle of Skye (UK) North Atlantic, NW-Island (IS) North Atlantic,Troms0 (N) North Sea, Helgoland (D) Mediterranean Sea, Gulf of Corinth (GR) Mediterranean Sea, Marseille (F)

Magnesium isotopes in skeletal carbonates

Sampling site (numbers according to Fig. 1)

Sampled species

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).

6137

6138

Sample

Min.

Growth period

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 B202 B234 B240 B188 B182 B193 B132 B179 B128 B137 B196 B130 B165 B178

LMC LMC LMC LMC LMC LMC LMC LMC LMC LMC LMC LMC LMC LMC LMC

19/12/05–14/03/06 19/12/05–14/03/06 19/12/05–14/03/06 03/11/05–19/12/05 03/11/05–19/12/05 03/11/05–19/12/05 03/11/05–19/12/05 20/04/06–04/05/06 20/04/06–04/05/06 24/08/05–01/11/05 24/08/05–01/11/05 05/07/06–24/07/06 05/07/06–24/07/06 05/07/06–24/07/06 05/07/06–24/07/06

85 85 85 38 38 38 38 14 14 69 69 19 19 19 19

16.5* 33.5 29.7 30.2 26.6 29.3 34.8 15.7* 25.1 17.5* 15.2* 31.0 13.4* 38.7 24.6

22.7 33.9 29.8 30.2 26.6 29.3 34.8 21.6 29.9 17.5 15.2 43.4 29.7 50.3 37.2

24.4 36.0 32.2 32.1 27.8 30.8 35.8 22.7 30.9 25.0 20.5 44.5 31.9 52.0 39.0

0.020 0.024 0.028 0.051 0.033 0.040 0.028 0.079 0.071 0.109 0.076 0.055 0.116 0.089 0.091

3.5 3.5 3.5 8.4 8.4 8.4 8.4 10.7 10.7 16.5 16.5 18.8 18.8 18.8 18.8

29.1 29.1 29.1 29.3 29.3 29.3 29.3 27.4 27.4 28.5 28.5 28.5 28.5 28.5 28.5

2.04 2.43 2.45 2.30 1.80 1.96 1.85 2.40 2.44 2.33 2.43 2.48 2.47 2.53 2.61

0.03 0.02 0.02 0.02 0.02 0.03 0.03 0.05 0.02 0.02 0.04 0.04 0.04 0.02 0.03

3.92 4.69 4.62 4.40 3.37 3.67 3.47 4.54 4.66 4.46 4.64 4.80 4.79 4.88 5.07

0.08 0.04 0.04 0.04 0.06 0.07 0.03 0.07 0.02 0.03 0.03 0.07 0.10 0.06 0.07

2.04 2.43 2.45 2.30 1.80 1.96 1.85 2.40 2.44 2.33 2.43 2.48 2.48 2.54 2.62

3.92 4.70 4.63 4.41 3.37 3.68 3.48 4.55 4.67 4.46 4.65 4.81 4.81 4.90 5.08

0.01 0.02 0.04 0.00 0.05 0.05 0.04 0.03 0.00 0.01 0.00 0.02 0.03 0.01 0.03

1 1 1 1 1 1 1 1 1 1 1 1 1 1 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 the experiment; *, 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 respective growth 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 the respective growth period; N, number of analyses, with five runs making an analysis (cf. Section 2).

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

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Table 4 Magnesium-isotope ratios of seawater from the western Dutch Wadden Sea and IAPSO reference seawater. Sample DWS 08-03-07 DWS 25-04-07 DWS 23-05-07 DWS 21-08-07 IAPSO

Sampling date HT HT HT HT

d26Mg0 D25Mg0 N ±2r d26Mg ±2r d25Mg0 T SAL Ca Mg d25Mg (& DSM3) (& DSM3) (& DSM3) (°C) (psu) (mg/l) (mg/l) (& DSM3)

08.03.2007 7.6 28.0 25.04.2007 13.5 29.5 23.05.2007 15.0 30.5 21.08.2007 19.0 28.7 35.0

343.8 367.1 365.4 – 418.3

1055 1147 1146 – 1370.2

0.42 0.41 0.42 0.42 0.42

0.04 0.01 0.01 0.02 0.02

0.80 0.77 0.80 0.79 0.80

0.02 0.02 0.03 0.03 0.05

0.42 0.41 0.42 0.42 0.42

0.80 0.77 0.80 0.79 0.80

0.00 0.00 0.00 0.01 0.00

1 1 1 1 *

T (°C), sea-surface temperature during high-tide measured at the time of sampling; SAL (psu), sea-surface salinity during high-tide measured at 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.

dryness. In order to destroy organic compounds, aliquots were treated with 100 ll of HClO4 (later a H2O2:HNO3 mixture replaced the HClO4) minimizing potential interferences related to complexation of cations. Seawater samples (0.5 ml) were first evaporated, than the dry residue was dissolved in supra-pure 6 M HCl and subsequently evaporated to dryness. After evaporation, all samples (carbonates and seawater) were re-dissolved in 2.5 M HCl and loaded on quartz glass columns. The Mg fraction was recovered using BioRad ion exchange resin AG50W-X12 (200–400 mesh). The column yield for Mg was >98%, which was verified by inductively coupled plasma optical emission spectrometry (ICP-OES). After final evaporation, the Mg fraction was taken up in 3.5% HNO3 and diluted to produce a 500 ppb solution for Mg-isotope measurement. Total procedural blanks are typical in the range of <10 ng Mg corresponding to a blank to sample ratio of 2  104. 2.3. Analyses of the magnesium-isotopic composition Mg-isotope measurements were carried out on a ThermoElectron Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) in the isotope geochemistry laboratory of the Ruhr-University in Bochum, Germany applying the standard bracketing technique. Mg concentration of the standard and sample was kept within a 25% limit, which proved to minimize potential matrices effects (Galy et al., 2001). The samples were introduced into the plasma via a combination of two desolvating systems, an ApexIR (ESI) and Aridus (Cetac) enhancing the signal intensity and stability. Mg-isotope ratios were reported as per mil deviations from the isotope reference standards: dxMg = [(xMg/24Mg)sample/(xMg/24Mg)standard  1]  1000; where x is either mass 25 or 26. As reference material the DSM3 was chosen because of the published Mg-isotope heterogeneity of the NIST SRM980 (Galy et al., 2003). The internal precision on both d25Mg and d26Mg is generally 0.03–0.10& (2r). In the protocol used here, all d-values are based on a sequence of five repetitions measured from the same solution. Each repetition comprises 45 measured isotope ratios. The average of these five repetitions constitutes what is referred to as a single delta value for a sample. The averages and ±2r errors of these measurements 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-elemental standards Cambridge 1 solution vs. DSM3 is ±0.02& (2r for d25Mg and ±0.07& (2r for d26Mg (Table 1). In principle, both d25Mg and d26Mg can be used to describe the fractionation behavior of the samples without interference from analytical artifacts. We will thereafter only refer to d26Mg. 3. RESULTS The measured Mg-isotopic composition of seawater and skeletal carbonates indicate mass-dependent behavior with all samples within uncertainty of the equilibrium fractionation line (Fig. 2). The Mg-isotope data, where delta values have been converted to d25Mg0 and d26Mg0 to describe a linear regression (Young and Galy, 2004), define a single mass fractionation curve on an Mg0 three-isotope plot with a slope of 0.521 ± 0.004 (r = 0.999, N = 31, p < 0.0001; PAST: Paleontological Statistics Software Package by Hammer et al. (2001)), which is thus indistinguishable to the theoretical equilibrium gradient of 0.521 (Young et al., 2002). For Mg isotopes, deviation from the equilibrium mass fractionation line is expressed as D25Mg0 . 3.1. Magnesium isotopes in reference materials and seawater The Mg-isotopic composition of the mono-elemental Mg-standard, standard seawater and seawater collected from the western Dutch Wadden Sea is displayed in Tables 1 and 2. The long-term average (January to July 2008) of Cambridge 1 against DSM3 was calculated to be 2.58 ± 0.04& (N = 56). This value is in overall agreement with Galy et al. (2003; d26Mg: 2.60 ± 0.14&; N = 35), Black et al. (2006; d26Mg: 2.59 ± 0.15&; N = 17), Tipper et al. (2008; d26Mg: 2.592 ± 0.087&; N = 67) and BolouBi et al. (2009; d26Mg: 2.62 ± 0.14&; N = 18). The coral JCp-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 standard seawater (salinity = 34.998 psu) result in a d26Mg value of 0.80 ± 0.05& (N = 10), similar to what was postulated by Ra and Kitagawa (2007, d26Mg: 074&; N = 4). Samples of the western Dutch Wadden Sea yield

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d26Mg values around 0.79 ± 0.03& (N = 4). Note that within the method’s uncertainty, the Mg-isotopic composition of the individual seawater samples from the Dutch Wadden Sea collected in March, April, May and August 2007, which are characterized by different seasonal temperatures (7.5–19.0 °C) and salinities (28.0–30.5 psu), are indistinguishable from each other (Table 4). Furthermore, their mean d26Mg value is favorably to seawater data from other laboratories 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, salinity has most likely no impact on the Mg-isotopic composition of seawater. It was, however, beyond the scope of this study to examine seawater samples from estuarine to brackish environments. Recently, Pogge von Strandmann et al. (2008) presented U, Li, and Mg element and isotope data for both dissolved and suspended material from two estuaries in the predominantly basaltic islands of Iceland and Azores, in order to illustrate the effects of estuarine mixing on the isotopic composition of the dissolved load. The d26Mg value of the dissolved phase was explained in the terms of simple mixing between a river endmember, which has relatively high d26Mg and low [Mg], and a seawater (high [Cl]) endmember, which has relatively low d26Mg and high [Mg]. Furthermore, negligible Mg-isotope variations were reported for seawater Mg concentrations as low as 11 mM (Pogge von Strandmann et al., 2008).

tion assigning recently precipitated calcite to the respective growth season. Thus, differences in Mg-isotopic composition can easily be studied in terms of growth and ambient seasonal environmental parameters at the time of calcite precipitation. Summarizing, replicate analyses of reference materials highlight the quality and reliability of the Mg-isotope data presented here. The observed variations in d26Mg ratios of biogenic carbonates considerably exceed the error of analytical precision and clearly demonstrate the heterogeneity of marine biogenic Mg-isotope records precipitated in modern oceans. 4. DISCUSSION 4.1. Potential factors influencing magnesium-isotope fractionation

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

Large overall variations in d26Mg of more than 3& are found in the present data set. The d-values of all modern skeletal carbonates are more negative than that of dissolved Mg in seawater (Figs. 2 and 3). This suggests (1) Mg-isotope fractionation between an aqueous solution and the precipitating carbonate at low temperatures and (2) that biological processing preferentially utilizes the lighter isotopes of magnesium. Given the observation that seawater d26Mg is constant, irrespective of depth or geographic location (cf. western Dutch Wadden Sea; this study, and North Atlantic 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 variations observed in this study do not reflect regional or temporal differences in seawater Mg-isotopic composition. Concerning echinoid and coralline red algae samples this fact becomes even more important since these samples were collected at different localities, which are characterized by different mean annual sea-surface temperatures and salinities. Large variations in d26Mg of approximately 4& have furthermore been reported for terrestrial systems, particularly for speleothems (Galy et al., 2002; Young and Galy, 2004; Buhl et al., 2007), demonstrating that the marine and terrestrial isotope composition of Mg is not unique. To date, only a small number of peer-reviewed articles have been published on the processes and factors potentially influencing the distribution of Mg isotope in marine skeletal carbonates. Therefore, a number of previously published abstracts complement the discussion.

The M. edulis samples have the lightest Mg-isotope composition within the sample set. Their Mg-isotopic composition varies between 3.37 ± 0.06& and 5.07 ± 0.07& (Table 4). In contrast to the echinoid and coralline red algae samples, the overall M. edulis data set show a larger variability of 1.70&, which amounts to half the overall variation (considering all measured samples) in d26Mg of 3.19&. Note, however, that the calcite shell layer of M. edulis was not sampled as bulk biogenic calcite. The chosen sampling strategy, milling only recently grown calcite (see Section 2) allowed us to achieve a higher temporal resolu-

4.1.1. Mineralogy A considerable component of the observed variability in d26Mg values of marine skeletal carbonates can be attributed to differences in mineralogy. Specific d26Mg values were observed for aragonite, high and low-Mg calcite (HMC and LMC) shells (Fig. 3). Investigating bulk marine and terrestrial carbonates, Galy et al. (2002) also suggested a strong mineralogical control. They observed that the d26Mg of dolostones is approximately 2–3& higher than the d26Mg of limestone, and that the d26Mg of speleothems containing dolomite is also around 2& higher then the

3.2. Magnesium isotopes in modern marine biogenic carbonates The Mg-isotopic composition obtained from modern scaphopod, coralline red algae, echinoids and brachiopods sampled in low resolution as bulk biogenic carbonates is listed in Table 2. Notably, the skeletons of all studied marine calcifiers are enriched in the light isotope, relative to the Mgisotopic composition of seawater. The Mg-isotopic composition (d26Mg) of all these samples varies between 1.88 ± 0.10& and 3.24 ± 0.04&. The scaphopod sample exhibits a d26Mg of 2.07 ± 0.05&. The brachiopod samples, collected at two different localities, yield slightly different d26Mg values of 2.29 ± 0.06& and 1.88 ± 0.10&. All echinoid and coralline red algae samples measured, being of the same species, but from different localities, show similar Mg-isotope ratios with average values of 2.63 ± 0.13& and 3.13 ± 0.23& for d26Mg, respectively.

Magnesium isotopes in skeletal carbonates

ing intermediate d26Mg values whereas LMC skeletons of bivalves (M. edulis, this study) and planktic foraminifera (Chang et al., 2004; Wombacher et al., 2006; Pogge von Strandmann, 2008) display lowest d26Mg values. Based on these findings, a strong mineralogical control was suggested to describe the Mg-isotope compositions of skeletal carbonates.

1.0

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Mytilus edulis Red algae Echinoids Brachiopods Scaphopod Seawater DSM 3

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4.1.2. Taxonomic differences The data presented here demonstrate that considerable variations of Mg-isotope composition exist both between marine carbonate-precipitating organisms and algae, as well as within a taxonomic group (Figs. 2 and 3). The uniform Mg-isotope composition of aragonitic skeletons (JCp1 coral, scaphopod) is consistent with published Mg isotope records of biogenic aragonite (corals, sclerosponges; Chang et al., 2004; Wombacher et al., 2006), supporting the hypothesis of Wombacher et al. (2006) that metabolic influences are likely absent. Small but considerable variability in d26Mg (<0.5&) has been found for HMC skeletons (coralline red algae, endobenthic echinoids) and LMC skeletons (brachiopods), with echinoids approximately 1.8&, red algae 2.1–2.4&, and brachiopods 1.0–1.5& lighter than seawater. The d26Mg values of HMC coralline algae (this study) are very similar to the uniform HMC data (sclerosponges, a calcitic coral, a red algae; 2.5 ± 0.2&) obtained by Wombacher et al. (2006). These values are further close to the assumed inorganic value of approximately 2.7& that has been found between calcitic speleothems and corresponding dripwater (Galy et al., 2002). As already proposed for aragonitic skeletons, this finding indicates the absence or negligibility of metabolic influences. Data for HMC echinoid and LMC brachiopod samples ob-

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±2σ

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0.0

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δ26Mg' [‰ DSM3]

Fig. 2. Three-isotope plot (d26Mg0 vs. d25Mg0 ) for standard solution (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 (Hammer et al., 2001) is based on the data of all marine calcifiers of this study and indicates mass-dependent behavior of Mg-isotope fractionation. The error bars in the right corner represent the 2r errors of the analyses.

d26Mg of calcitic speleothems. To date, only a small number of aragonitic samples have been measured. Available data 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 coral and the scaphopod sample. Both aragonitic samples yielded relatively heavy Mg-isotopic compositions in comparison to calcitic skeletons. The latter could be separated in the HMC skeletons of echinoids and coralline red algae show-

26

δ Mg 0.0

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carbonate-solution

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Mixed calcifiers I - Aragonite (corals, sclerosponges)

Scaphopod - Aragonite Echinoids - HMC Red algae - HMC Mixed calcifiers II - HMC (sclerosponge, deep-sea corals, red algae)

Inorganic calcite Mytilus edulis - LMC Brachiopods - LMC Foraminifera - LMC

Fig. 3. Bar chart summarizing d26Mg values relative to the respective growth solution (seawater, dripwater), for aragonitic corals, sclerosponges and scaphopods (this study, Chang et al. 2004; Wombacher et al., 2006), high-Mg calcite (HMC) of echinoids and coralline red algae (this study), mixed high-Mg calcifiers (Wombacher et al., 2006; Pogge von Strandmann, 2008), inorganic calcite (Galy et al., 2002), lowMg calcite (LMC) of bivalves and brachiopods (this study), and foraminifera (Chang et al., 2004; Wombacher et al., 2006; Pogge von Strandmann, 2008), respectively. See text for further details.

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tained in this study yield Mg-isotope values that are very similar to those shown in the dataset from the taxonomic groups in Wombacher et al. (2006). Finally, the LMC of M. edulis exhibits variable Mg-isotope compositions (variability in d26Mg values of 61.7&) being 2.5–4.2& lighter than seawater. In contrast to the coralline algae data shown here, data for echinoids, brachiopods and bivalves yield Mg-isotope fractionations that are considerably smaller (echinoids, brachiopods) or larger (M. edulis) than those defined by most HMC data (red algae, sclerosponges, calcitic coral; this study, Wombacher et al., 2006). Furthermore, their isotopic fractionation is considerably different from the assumed inorganic value. This suggests that metabolic processes during echinoid and brachiopod shell formation more strongly favor the incorporation of heavy Mg isotopes than inorganic precipitation or than most HMC calcifiers do. By contrast, M. edulis preferentially incorporates light Mg isotopes in their calcitic outer shell relative to inorganic calcite and HMC calcifiers. Thus, there must be taxon-specific level of control on Mg-isotope incorporation, as Mg is transported from seawater to the site of calcification. For bivalves, the latter hypothesis is supported by recent findings illustrating that skeletogenesis, and hence the resulting geochemical composition, of mollusk exoskeleton takes place under very strong biological influence (Weiner and Dove, 2003). 4.1.3. Biomineralization Calcium carbonates of biogenic origin are often organic/ inorganic composites and are produced by the organism through biological control and mediation (e.g., Lowenstam and Weiner, 1989). For many organisms, however, the complex mechanisms during biomineralization are not yet fully understood and are subjected to ongoing research. As outlined above, Mg-isotope fractionation in echinoids, brachiopods and bivalves appears to be affected by metabolic influences, which might mirror the organism’s adopted calcification strategies. Therefore, the current theory of biologically controlled calcification processes (cf. Weiner and Dove, 2003) for these species should be briefly outlined. Shells of bivalves and brachiopods are believed to mineralize primarily by biologically controlled extracellular processes (e.g., Crenshaw, 1980; Falini et al., 1996; Gotliv et al., 2003; Gaspard et al., 2007). The main control is a macromolecular matrix outside the cell in an area that will become the site of mineralization. The structure and composition of these organic templates are genetically programmed to mediate the calcification process by allowing crystal nucleation and growth in specific orientations. In the case of bivalves, shell formation takes place in an isolated space, which is called the extrapallial cavity, situated between the growth surface of the shell and the secretory epithelium of the mantle (Crenshaw, 1980). This volume is filled with the extrapallial fluid (EPF), which contains several macromolecules (e.g., proteins, polysaccharides or glycoproteins), supplying a three-dimensional template for mineral formation (Addadi et al., 2006). The EPF acts as a dynamic physiological medium, regulating the pH within the animal (Simkiss and Wilbur, 1989). Ambient seawater,

ingested particulate matter or metabolic products of respiration provide the shell-forming components (Ca2+, Mg2+, Sr2+, CO32). In order to reach the site of calcification, it is suggested that these elements are transported across the epithelium via inter- and/or intra-cellular pathways (Watabe and Kingsley, 1989). Cations are either actively pumped across the cell membrane or move by passive diffusion through extracellular fluids to the site of calcification (Weiner and Dove, 2003). Echinoids or sea urchins, on the contrary, favor a biologically controlled intracellular mineralization strategy. Their mineralized tissues (e.g., plates) are formed within a vesicle that is the product of a thin organic membrane (Ma¨rkel et al., 1986; Wang, 1998). The mineralized unit is secreted to the environment, only if and when the membrane is degraded (Ma¨rkel et al., 1986). Previous studies revealed that the first spicular structures of sea urchin larvae begin as amorphous calcium carbonate, passing finally into monocrystals of calcite (e.g., Lowenstam and Weiner, 1989). As presented in the previous paragraph, organic membranes and/or templates (matrices) play an important role during biocalcification. Therefore, a possible implication of these findings is that Mg-isotope fractionation occurs during the passage of Mg through the epithelial layer or by adsorption to organic matrices. Conclusive statements on the origin of Mg-isotope fractionation during biomineralization, however, could only be achieved by investigating the isotopic composition of the organism’s soft tissue and fluids, which are involved when Mg is transported from seawater to the site of calcification, an issue that has not been the topic of this study. 4.1.4. Salinity, temperature and/or growth rate The Mg-isotope composition of most samples was also evaluated in the context of key environmental factors, such as salinity and temperature, and growth rate. The latter factor was only assessed for the M. edulis data, for which growth rate data was available. Echinoids and coralline red algae cover a wide range of mean annual salinities, with salinity differences of 16.5 and 6.5 psu, respectively. However, no obvious salinity dependence of the Mg-isotope fractionation in echinoids and coralline red algae was observed (Fig. 4a). By comparison, d26Mg values of M. edulis appear to increase with increasing salinity. The weak positive correlation (r = 0.548, N = 15, p = 0.035) of ambient salinity and d26Mg (Fig. 4b) covers, however, a salinity range (27.0–29.5 psu), which is rather small, particularly in view of the error bar related to the salinity data set (±0.75 psu). Thus, in order to validate the insusceptibility of d26Mg values to seawater salinity in general, future work should include multi-taxa data covering a wide range of salinities. The second important environmental factor that has been evaluated, namely temperature, yielded contrasting results. Although temperature dependence is to be expected in a thermodynamic system (Bigeleisen and Meyer, 1947; Urey, 1947), a significant sensitivity of temperature on Mg-isotope composition has not yet been observed in terrestrial and marine systems in previous studies. Galy et 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 no effect on the d26Mg of echinoids (black squares) and red algae (grey squares). (B) d26Mg values of M. edulis (open diamonds) appear to be related to seawater salinity. However, the salinity range is rather small. (C) d26Mg values of echinoids and red algae are not affected by seawater 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 left bottom corner.

isotope fractionation between a fluid and the corresponding carbonate is difficult to predict on a theoretical base, since Mg bonds are mainly ionic in character. This study also reported a minor increase of the Mg-isotope fractionation with increasing temperatures during speleothem formation. According to these authors, the temperature effect amounts to less than 0.02&/AMU/°C, which means that only relatively large temperature difference could be resolved using high-precision analytical techniques. Investigating several planktonic foraminifers, spanning temperature ranges of approximately 10 °C, Chang et al. (2004) and Pogge von Strandmann (2008) found no resolvable temperaturedependent fractionation of Mg isotopes. The work presented here reports Mg-isotope data for echinoids and coralline red algae, which span a measured seawater temperature range of 10 and 13 °C, respectively (Fig. 4c). In line with the last mentioned studies, d26Mg values of echinoids and coralline red algae show no resolvable temperature dependence (Fig. 4c). This finding is furthermore in agreement with Wombacher et al. (2006), who found, across a seawater temperature range of approximately 20 °C, no correlation with temperature for aragonite and HMC data (including data on red algae). In contrast to the findings on coralline red algae and echinoids, the d26Mg values of M. edulis appear inversely correlated to ambient sea-surface temperatures (r = 0.530, N = 15, p = 0.042; Fig. 4d), which is illustrated in slightly decreasing d26Mg ratios with increasing temperatures. The tempera-

ture sensitivity found for M. edulis is in line with Wang et al. (2006), reporting high-resolution d26Mg records from coral aragonite. Distinct annual cycles in d26Mg have been found within two coral colonies that were in concert with annual sea-surface temperature records at the site of collection. Their empirically calibrated fractionation factor had a linear relationship to inverse absolute temperature, corresponding to a temperature sensitivity of about 0.11& per 1 °C. On the basis of these findings, we suggest that a detailed sampling strategy (cf. M. edulis) or high-resolution sampling of single growth increments (cf. Wang et al., 2006), precipitated either annually or sub-annually depending on the organism, may provide better insights in how seasonally changing environmental factors affect isotope fractionation. Magnesium-isotope data of M. edulis were also evaluated in terms of growth rate. Studies on trace elemental ratios (Sr/Ca) of bivalve calcite and aragonite have shown that growth rate might induce kinetic fractionation (Gillikin et al., 2005; Lorrain et al., 2005). Calculating mean growth rates for the respective sampled time interval along the axis of maximum growth for each M. edulis specimen, a negative correlation between Mg-isotope composition and growth rate has been found (r = 0.593, N = 15, p = 0.020; Fig. 5a). Slow growing individuals (<0.05 mm/day) seem to preferentially incorporate the heavier isotopes relative to faster growing individuals, and are closer to the assumed inorganic fractionation of calcite (Galy et al., 2002). This

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finding is, however, in disagreement with Mg-isotope data from four different size fractions of planktonic foraminifera Globigerinoides sacculifer (Pogge von Strandmann, 2008). In the latter study, different foraminiferal test sizes are assumed to reflect differences in growth rate. While Mg/Ca ratios were found to increase with test size, Mg-isotope ratios remained constant, suggesting that foraminiferal growth rate has no influence on Mg-isotope ratios of this species (Pogge von Strandmann, 2008). Nevertheless, a wide spectrum of factors influences growth rates of bivalves. Amongst these, food supply and/or temperature are considered to be of major importance (Widmann and Rhodes, 1991; Seed and Suchanek, 1992). For the time of the experiment, data on nutrition availability were not yet available. In contrast, the relationship between temperature and growth rate was assessed for the M. edulis data set. Testing of both parameters reveals a conspicuously positive relationship over the whole temperature range (3–20 °C), with only one outlier (B196 in Fig. 5b). This indicates, that individual mean seasonal growth rates are related to the respective mean seasonal temperatures, which in turn implies that ambient seawater temperature is possibly recorded in the calcite shells of M. edulis in an indirect manner via growth rate. An important consequence of this

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5. CONCLUSION Systematic Mg-isotope analyses of seawater and skeletal carbonates of five marine organisms reveal that the d26Mg ratios in the marine environment are highly variable. All skeletal carbonates are isotopically lighter than seawater. Skeletal aragonite displays near uniform d26Mg ratios suggesting that metabolic influences are most likely negligible or absent. Negligible metabolic effects can also be postulated for Mg-isotope ratios of high-Mg calcitic coralline red algae. Their d26Mg ratios are close to the assumed inorganic value. Furthermore, these data are similar to the mean d26Mg ratios of HMC mixed calcifiers, including sclerosponges, calcitic coral, and red algae (Wombacher et al., 2006). In contrast, echinoids, brachiopods and bivalves are enriched or, conversely, depleted in d26Mg compared to HMC mixed calcifiers and inorganic calcite. From this it concludes that during biocalcification, taxon-specific processes – being either weaker or stronger than those experienced during inorganic calcification – are likely influencing Mg-isotope incorporation in these biominerals. Whether Mg-isotope fractionation occurs during the passage of Mg through certain cell membranes or by adsorption to organic matrices could not be conclusively answered at that point. Hence, species-specific studies must be undertaken in order to better understand Mg-isotope pathways and thus fractionation in biogenic systems. In agreement with Mg-isotope data of inorganic precipitates (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 neither resolvable salinity nor temperature effects. This insensitivity to environmental factors suggests that echinoids and coralline red algae are potential proxy carriers of past seawater Mg-isotopic composition, provided a reliable determination of the fractionation factor exists. In contrast, the bivalve M. edulis exhibit weak salinity, temperature and growth rate effects, which require further validation. In bivalves, however, the dependency and/or interaction of environmental and biological factors are likely to limit the use of d26Mg as salinity or temperature proxy. ACKNOWLEDGMENTS

0.04 0.02 0.00 -5

observation is that Mg-isotope data from bivalve calcite have limited significance as proxy for sea-surface temperatures.

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Fig. 5. Mg-isotope composition and seawater temperature in relation to growth rate. Mean growth rates are calculated for each of the sampled calcite shell edges of M. edulis for the respective growth interval (A) d26Mg values of M. edulis decrease with increasing mean growth rates. (B) For samples analyzed, mean growth rates are related to mean ambient seawater temperatures of the respective growth interval.

This is a contribution to the EuroClimate project 04 ECLIM FP08 CASIOPEIA and was supported by the Dutch Research Council (NWO). Thanks to F. Plonchon (Royal Museum for Central Africa, Tervuren, Belgium) and F. Wombacher (Free University Berlin, Germany) for important feedback on the Mgisotope data. The last named and A. Kolevica (IfM-GEOMAR, Kiel, Germany) are kindly acknowledged for providing ICP-MS reference material JCp-1. Thanks to D. Garbe-Scho¨nberg and K. Kissling (Christian-Albrechts-University, Kiel, Germany) for providing elemental data of seawater samples measured by ICPOES, and H.v. Aken (Royal Netherlands Institute for Sea Research, Netherlands) for providing long-term sea-surface temperature and salinity data.

Magnesium isotopes in skeletal carbonates

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