Calcium isotope fractionation in ikaite and vaterite

Chemical Geology 285 (2011) 194–202

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

Calcium isotope fractionation in ikaite and vaterite Nikolaus Gussone a,⁎, Gernot Nehrke b, Barbara M.A. Teichert c a b c

Institut für Mineralogie, Universität Münster, Corrensstr. 24, 48149 Münster, Germany Alfred Wegener Institut für Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany Institut für Geologie und Paläontologie, Universität Münster, Corrensstr. 24, 48149 Münster, Germany

a r t i c l e

i n f o

Article history: Received 28 July 2010 Received in revised form 15 January 2011 Accepted 5 April 2011 Available online 12 April 2011 Editor: J.D. Blum Keywords: Vaterite Ikaite Calcium carbonate polymorph Calcium isotopes

a b s t r a c t We investigated the isotopic composition of calcium in the metastable calcium carbonate polymorphs vaterite and ikaite. The synthetic vaterite precipitates show a smaller degree of Ca isotope fractionation than calcite or aragonite at comparable temperatures and revealed no significant temperature sensitivity between 10 and 50 °C. Ikaite, the water-bearing Ca carbonate modification (CaCO3·6H2O) was precipitated at 2 °C in controlled laboratory experiments and in addition recovered from marine sediments and sea-ice. Synthetic and natural ikaites show similar values of Ca isotope fractionation. In general, ikaite is less depleted in heavy isotopes compared to calcite or aragonite at low temperatures. The observed offset of vaterite and ikaite to calcite or aragonite precipitated at similar temperatures is presumably mainly related to differences in precipitation rates and regimes, which seem however to be typical for the growth of these carbonate minerals. Small differences in Ca isotope ratios of naturally formed ikaites from different locations seem to be related to different geochemical conditions in the respective sediment settings. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The isotopic composition of calcium (Ca) in various kinds of archives (e.g. biogenic minerals, porewaters, etc.) provides information about geological and geochemical processes as well as past environmental conditions. Paleoceanographic applications of Ca isotope ratios include reconstructions of relations in paleo-foodwebs (cf. Skulan et al., 1997; Clementz et al., 2003), reconstruction of the oceanic Ca-budget (cf. De La Rocha and DePaolo, 2000; Heuser et al., 2005) and sea surface temperature (cf. Nägler et al., 2000; Hippler et al., 2006; Gussone et al., 2004). Shells of marine calcifiers are used as proxy archives in most of these studies (cf. De La Rocha and DePaolo, 2000; Nägler et al., 2000; Hippler et al., 2006; Heuser et al., 2005). However, complex fractionation patterns found in shells of foraminifera (cf. Gussone et al., 2009; Gussone and Filipsson, 2010), a group of organisms which tests are dominantly used as paleoceanographic archives, emphasise the importance of a solid understanding of Ca isotope fractionation processes during both, transport to the site of calcification and precipitation. Several studies addressed the fractionation behaviour of Ca isotopes in synthetically precipitated calcium carbonates (cf. Gussone et al., 2003, 2005; Lemarchand et al., 2004; Marriott et al., 2004; Tang et al., 2008). These studies found temperature and rate-dependent fractionation effects, which are, however, partly discrepant. Calcium isotope fractionation in carbonates is presently not fully understood and its basic mechanisms are

⁎ Corresponding author. E-mail address: [email protected] (N. Gussone). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.04.002

still under debate (cf. Eisenhauer et al., 2009). Different models that were proposed to explain the respective observations include kinetic isotope fractionation effects (cf. Gussone et al., 2003), equilibrium fractionation (Marriott et al., 2004), disequilibrium fractionation (Lemarchand et al., 2004), non-equilibrium fractionation (surface entrapment) (Tang et al., 2008) and a surface kinetic model (DePaolo, in press). The discrepant dependency of Ca isotope fractionation on precipitation rate found by Lemarchand et al. (2004) and Tang et al. (2008) was recently suggested to be related to the respective experimental setups and regimes of crystal growth (DePaolo, 2011), but it is unclear which experiments mirror better precipitation in natural environments, including calcifying spaces in organisms. Research on Ca isotope fractionation so far focussed on the most abundant CaCO3 polymorphs calcite and aragonite and revealed a comparable temperature dependence of about 0.015‰/°C in both and an offset between both polymorphs of approximately 0.5‰ (cf. Marriott et al., 2004; Gussone et al., 2003, 2005), which was suggested to be related to the surface effects and/or different coordinations of Ca in calcite (Ca[6]CO3) and aragonite (Ca[9]CO3) during crystal growth. Besides the most common calcium carbonate polymorphs in the environment calcite and aragonite, two less abundant ones are the minerals vaterite and ikaite (both with 8-fold coordinated Ca), whose Ca isotope fractionation is yet unconstrained. Vaterite is a rare CaCO3 polymorph found in nature for instance as biogenic mineral, as spicule in ascidians (Lowenstam and Abbott, 1975), in the inner ear of some fish-species (Mann, 2001), in snail shells (Hasse et al., 2000) or as bacterially mediated mineralisation in soils (Rodriguez-Navarro et al., 2007). It can also occur in weathering crusts, as alteration product of calc-silicate rocks or metamorphosed marls (McConnell, 1960). Since

N. Gussone et al. / Chemical Geology 285 (2011) 194–202

vaterite is metastable under environmental conditions (below 400 °C), it is generally not preserved in the sedimentary record. Nevertheless, the characterisation of Ca isotope fractionation of vaterite is of great interest, because it is used by some organisms for Ca storage and shell build-up and might therefore contribute to the observed apparent Ca isotope fractionation in biominerals. Furthermore, vaterite often occurs as a precursor in CaCO3 precipitation and transformation experiments and may alter the apparent Ca isotope fractionation by its temporary existence. Ikaite (CaCO3·6H2O) is a Ca carbonate mineral that contains 6 water molecules per molecule CaCO3. In the marine environment, three main settings of formation are known: formation of ikaite tufa columns due to mixing of seawater and alkaline spring water (Buchardt et al., 1997), precipitation within the sediments near the seafloor (e.g. Suess et al., 1982; Schubert et al., 1997; Zabel and Schulz, 2001) and in Antarctic sea-ice (Dieckmann et al., 2008, 2010). Ikaite occurrences are also known from the terrestrial realm, where they occur at hypersaline, alkaline lakes (e.g. Council and Bennett, 1993) and cold spring discharges (e.g. Ito, 1998; Omelon et al., 2001). For ikaite formation within marine sediments, high bicarbonate and orthophosphate concentrations as well as low temperatures seem to favour ikaite precipitation over calcite (Bischoff et al., 1993). Such conditions are met in organic matter rich polar or deep-marine settings. Ikaite is not preserved over longer geologic times and transforms to calcite during diagenesis. The calcite pseudomorph after ikaite is called glendonite and has been described throughout earth history (cf. James et al., 2005). Since Ca isotope fractionation was proposed to be related to the dehydration of Ca-aquocomplexes and related changes in the coordination of Ca ions (Marriott et al., 2004; Gussone et al., 2006; Langer et al., 2007), the characterisation of Ca isotope ratios of ikaite might also contribute to a better understanding of Ca incorporation into the carbonate lattice and associated fractionation processes. We conducted precipitation experiments of vaterite and ikaite under controlled laboratory conditions, in order to improve our understanding of Ca isotope fractionation mechanisms during inorganic crystal growth. We also analysed the Ca isotopic composition of natural ikaite samples from different locations, to test if the conditions of our precipitation experiments apply to observations from natural environments.

2. Materials and methods 2.1. Precipitation experiments If a substance like calcium carbonate has numerous polymorphs, it is challenging to experimentally precipitate one of the more soluble phases exclusively. In order to do so, the solubility product of the more stable phases has to be exceeded without its precipitation. To prevent precipitation of the more stable phases three different approaches are possible. Precipitation can be inhibited by the addition of inhibitors (e.g. Mg ions inhibit the precipitation of calcite), the physico-chemical

195

parameters are chosen to match the thermodynamic temperature field (e.g. temperatures below 4 °C for ikaite), or supersaturation can be increased very fast. In the latter case the more soluble phases often precipitate first (Ostwald rule of stages). However, precipitation experiments which require a high degree of supersaturation, result in relatively high precipitation rates. In such an experimental setup, it is not possible to precipitate the more soluble polymorphs slowly, and the determination of precipitation rates is associated with relatively large uncertainties.

2.1.1. Preparation of vaterite Vaterite was produced as described in Nehrke and Van Cappellen (2006) where 500 ml of a 1 M CaCl2·H2O and 2 M NH3 solution was placed in a wide-mouthed glass Erlenmeyer flask (all chemicals were from MERCK®, in the quality per analysis). The Erlenmeyer flask was placed in a temperature controlled water bath (accuracy ±0.5 °C) to perform precipitation experiments at different temperatures (Table 2). Pure CO2 was bubbled slowly through the solution using a fritted glass bubbler. After some time (between 5 and 30 min, depending on temperature), the solution became cloudy, indicating nucleation. After an additional period of 1 to 5 min (also depending on temperature), the suspension was filtrated through pre-weighted paper filters (no. 42 Whatman®) using a Buchner funnel. The filtrate was washed with ethanol (80%) and dried for 20 min under vacuum, before being placed in a dessicator containing silica gel. The amount precipitated was determined by weighing after the precipitate dried for one week in the dessicator. The amount of Ca in the precipitated solid was subtracted from the total mass of Ca which could theoretical be precipitated to calculate the reservoir Ca consumption (see Table 2). The precipitates were identified as vaterite by X-ray diffraction analysis as described by Nehrke and Van Cappellen (2006). The determination of the growth rate is not a straightforward task in experiments in which unseeded precipitation occurs, since nucleation and growth proceed simultaneously. Furthermore the short time in which the vaterite precipitation occurred (few minutes) does not allow to accurately follow changes in crystal morphology/size. Therefore, the growth rates given in Fig. 4 are estimated based on the final size of the crystallites (determined by scanning electron microscopy) as follows. First the size of a vaterite crystal and its density (2.54 g/cm2) was used to calculate the mass of one crystal (the volume of one crystal was calculated assuming spherical geometry). Growth rates are then calculated from the mass of one crystal, the molar weight of CaCO3 (100 g/mol), its surface area (assuming spherical geometry), and the time between nucleation and the end of the experiment (it was assumed that the number of nuclei did not change after the solution became cloudy, and growth of the nuclei proceeded from this time on until the end of the precipitation experiment). The error given for the growth rate is associated with the uncertainty in the determination of the crystallite size. It has to be noted, that the surface area will change during growth (starting with a nuclei for which a geometric surface area is nearly impossible to calculate). However, since it was not possible to

Table 1 Sample locations of natural ikaite. Site

Region

Latitude

Longitude

Water depth (mbsl)a

Bottom water temperature (°C)

Cruise

GeoB 4914–2 GeoB 1401-4 GeoB 2809-4 GeoB 6308-4 PS69ANTXXIII/7 SL 131

Southern Congo-fan Southern Congo-fan Uruguay continental margin Uruguay continental margin Antarctica Sumatra

− 6.933 − 6.926 − 36.332 − 39.302

9.000 9.006 − 51.522 − 53.965

3972 3952 3539 3620 0

2.563

96.757

~ 2.3 ~ 2.3 ~2 ~2 ~0 ~6

M41/1! M16/1# M29/2$ M46/3§ ANTXXIII/7+ SO189/2&

!: Schulz and cruise participants (1998), #: Wefer and cruise participants (1991), $: Bleil and cruise participants (1994), §: Bleil and cruise participants (2001), +: Dieckmann et al. (2008), &: Wiedicke-Hombach et al. (2006). a mbsl: meters below sealevel.

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Table 2 Calcium isotope ratios and isotope fractionation in synthetic vaterite. Sample ID

Mineralogy

T °C

Ca isotope ratios δ44/40Ca ‰

±2σm

Apparent fractionation factors δmuCa ppm/amu

±2σm

n

Δ44/40Ca ‰

±2σm

ΔmuCa ppm/amu

Reservoir effect correction ± 2σm

Ca consumption

1000lnα

106lnαmu /amu

ppm

1.1 1.3 all 1.4 1.4b 1.5 3 4 001(v2) 002(v2) 003(v2) 004(v2) 005(v2) Fluid

Vaterite Vaterite/amorph Vaterite Vaterite Vaterite/amorph Vaterite Vaterite Vaterite Vaterite Vaterite Vaterite Vaterite

20 38 20 20 10 30 48 20 10 30 40 30

− 1.10 − 1.03 − 1.08 − 1.13 − 1.08 − 1.02 − 0.85 − 1.02 − 1.00 − 0.98 − 1.02 − 1.08 − 0.67

0.10 0.09 0.09 0.06 0.04 0.04 0.06 0.00 0.07 0.05 0.02 0.04 0.05

− 296 − 278 − 290 − 304 − 291 − 274 − 227 − 275 − 269 − 264 − 274 − 292 − 179

28 24 24 17 9 11 17 1 19 14 4 11 14

3 5 4 2 3 5 5 2 2 3 2 3 7

− 0.43 − 0.37 − 0.41 − 0.46 − 0.42 − 0.35 − 0.18 − 0.35 − 0.33 − 0.31 − 0.35 − 0.42

0.11 0.10 0.10 0.08 0.06 0.07 0.08 0.05 0.09 0.07 0.05 0.06

− 117 − 98 − 111 − 125 − 112 − 95 − 48 − 95 − 89 − 84 − 94 − 112

31 28 27 22 17 18 22 14 24 19 14 17

− 0.46 − 0.37 − 0.44 − 0.49 − 0.44 − 0.51 − 0.26

10% 1% 10% 10% 10% 50% 50% b5% b5% b5% b5% b5%

− 123 − 99 − 117 − 132 − 118 − 137 − 69

δ44/40Ca and δmuCa relative to NIST SRM 915a. For calculation of δmuCa and 106 lnαmu see Gussone et al. (2005).

determine the change in surface area with time due to the short experimental period (few minutes), the growth rates are calculated based on the final size of the crystallites.

189-2 (Wiedicke-Hombach et al., 2006). These ikaites experienced the first stage of pseudomorph formation and are now present as small uncemented calcite grains.

2.1.2. Precipitation of ikaite Ikaite was precipitated from solution and identified by means of synchrotron based X-ray powder diffraction as described in Dieckmann et al. (2008). Precipitation was performed by dispensing 100 ml 0.3 M CaCl2 and 0.3 M K2CO3 simultaneously to 600 ml of 0.045 M NaOH using a two channel peristaltic pump (Rickaby et al., 2006). All chemicals were from MERCK®, in the quality per analysis, and the complete setup was placed in a temperature controlled room (2± 1 °C). Since the precipitation of ikaite requires a temperature below 4 °C it was not possible to perform the precipitation experiments over a range of temperatures. The growth rate given in Fig. 4 was estimated as described above for vaterite (using the density of ikaite (1.8 g/cm3) and cubic geometry).

2.3. Sample preparation and Ca isotope analysis

2.2. Natural ikaite samples Natural ikaite was recovered from Antarctic sea-ice during Polarstern PS69 ANTXXIII/7. An exact description of the material used can be found in Dieckmann et al. (2008). Further sample sets of natural ikaites were retrieved from sediments of the southern Congo-fan during Meteor cruises M 16/1 and M 41/1 (Schulz and cruise participants, 1998; Wefer and cruise participants, 1991) and from the Uruguay continental margin during Meteor cruises M 29/2 and M 46/3 (see Table 1; Bleil and cruise participants, 1994, 2001) in sediment depths between 4 and 13 mbsf (metres below seafloor) at a water depth of about 3500 mbsl (meters below sealevel). Calcium carbonate crystals that represent an early stage of conversion from ikaite to calcite were found during Sonne cruise SO

Vaterite and ikaite samples were digested in 2.5 N HCl. An aliquot of the sample solution (~400 ng Ca) was mixed with a Ca-doublespike to correct for isotope fractionation during data acquisition in the mass-spectrometer. Vaterite samples were analysed on a Finnigan Triton T1 thermal ionization mass spectrometer at the IFM-GEOMAR (Kiel) using a 43Ca–48Ca-doublespike; the applied method resembles the descriptions in Heuser et al. (2002) with modifications in respect to doublespike composition and cup configuration (cf. Gussone et al., 2007). Calcium isotope ratios of the synthetic as well as natural ikaite samples were determined on a Finnigan Triton T1 thermal ionization mass spectrometer at the ‘Zentral Labor für Geochronologie’ (Institut für Mineralogie - Universität Münster) using a 42Ca–43Ca-doublespike (cf. Holmden, 2005; Gopalan et al., 2006; Arning et al., 2009). The data reduction follows the description in Heuser et al. (2002) based on the iterative approach of Compston and Oversby (1969). Measurements of calcium isotope ratios were performed in static mode, simultaneously measuring masses 40Ca, 41K, 42Ca, 43Ca and 44Ca with collectors L3, L1, C, H1 and H2, respectively. Potential interference of 40K on 40Ca was monitored on 41K, but the applied interference correction was always negligible. Intensities on 40Ca ranged normally between 12 and 15 V (using 1011 Ω resistors). The measured Ca isotope ratios are expressed as δ44/40Ca values relative to the NIST SRM 915a standard (δ44/40Ca [‰] = ((44Ca/40Ca)sample/(44Ca/40Ca)SRM 915a − 1) · 1000) and as δmuCa

Table 3 Calcium isotope ratios and isotope fractionation in synthetic ikaite. Sample

Mineralogy

T °C

δ44/40Ca ‰

± 2σm

δmuCa

I 26 I 27 I 28 I 29 I 30 I 31 I 32 I 33 Fluid

Ikaite Ikaite Ikaite Ikaite Ikaite Ikaite Ikaite Ikaite

2 2 2 2 2 2 2 2

0.55 0.33 0.54 0.58 0.65 0.54 0.66 0.40 1.08

0.03 0.03 0.05 0.05 0.01 0.02 0.04 0.01 0.02

147 88 144 155 174 146 177 108 289

δ44/40Ca and δmuCa relative to NIST SRM 915a. For calculation of δmuCa and 106 lnαmu see Gussone et al. (2005).

ppm

/amu

± 2σm

n

1000lnα

± 2σm

106lnαmu

7 8 13 12 3 5 10 2 6

2 2 3 3 2 2 2 2 4

− 0.53 − 0.75 − 0.54 − 0.50 − 0.43 − 0.53 − 0.42 − 0.67

0.03 0.04 0.05 0.05 0.02 0.03 0.04 0.02

− 142 − 201 − 145 − 134 − 115 − 143 − 112 − 181

ppm

/amu

±2σm

Ca consumption

9 10 14 14 6 7 11 6

b 5% b 5% b 5% b 5% b 5% b 5% b 5% b 5%

N. Gussone et al. / Chemical Geology 285 (2011) 194–202

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Table 4 Calcium isotope ratios and isotope fractionation in natural ikaite. Sample I-7 I-8 I-3 I-4 I-5 I-2 I-11 I-12 I-13 I-14 I-1 I-6 I-9 I-10 I-15 S-1 S-2 S-3 S-4 S-7a S-7b S-8 S-10 S-11 S-12 S-13 S-14 S-15 S-16 S-17 S-18 S-19 Seawater

Site GeoB 4914-2 GeoB 4914-2 GeoB 1401-4 GeoB 1401-4 GeoB 1401-4 GeoB 1401-4 GeoB 2809-4 GeoB 2809-4 GeoB 2809-4 GeoB 2809-4 GeoB 6308-4 GeoB 6308-4 GeoB 6308-4 GeoB 6308-4 PS69 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131 SL 131

Depth (mbsf)

δ44/40Ca ‰

11.25 11.25 13.67 13.67 13.67 13.57 9.4 9.4 9.4 9.4 10.13 10.13 4.9 4.9 0 3.48 3.48 3.48 3.48 3.6 3.6 3.6 3.6 3.6 3.46 3.46 3.46 3.46 3.46 3.5 3.5 3.5

1.18 1.23 1.10 1.04 1.15 1.18 1.09 1.13 1.27 1.14 1.16 1.13 1.07 1.18 1.26 1.36 1.32 1.29 1.35 1.37 1.41 1.32 1.46 1.32 1.47 1.29 1.25 1.29 1.33 1.34 1.39 1.36 1.88

2 S.E. ‰

N

δmuCa /amu

2 S.E. ppm /amu

Δ44/40Ca* ‰

316 330 296 279 309 317 292 303 341 306 312 303 287 316 337 365 355 347 364 368 380 356 393 355 396 346 337 347 358 361 374 367 505

3 5 11 2 6 14 12 17 0 10 6 5 2 39 0 4 13 9 27 18 11 9 25 3 5 2 17 14 2 10 8 12

− 0.70 − 0.65 − 0.78 − 0.84 − 0.73 − 0.70 − 0.79 − 0.75 − 0.61 − 0.74 − 0.72 − 0.75 − 0.81 − 0.70 − 0.62 − 0.52 − 0.56 − 0.59 − 0.53 − 0.51 − 0.47 − 0.56 − 0.42 − 0.56 − 0.41 − 0.59 − 0.63 − 0.59 − 0.55 − 0.54 − 0.49 − 0.52

ppm

0.01 0.02 0.04 0.01 0.02 0.05 0.04 0.06 0.00 0.04 0.02 0.02 0.01 0.14 0.00 0.01 0.05 0.03 0.10 0.07 0.04 0.03 0.09 0.01 0.02 0.01 0.06 0.05 0.01 0.04 0.03 0.05

2 2 2 2 2 5 2 3 2 2 2 2 2 5 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

ΔmuCa* /amu

Cruise

− 189 − 175 − 209 − 227 − 196 − 188 − 213 − 202 − 164 − 200 − 193 − 202 − 218 − 189 − 168 − 141 − 150 − 158 − 141 − 137 − 125 − 150 − 112 − 150 − 109 − 160 − 168 − 158 − 147 − 144 − 131 − 138

M 41/1 ! M 41/1 ! M 16/1 # M 16/1 # M 16/1 # M 16/1 # M 29/2 $ M 29/2 $ M 29/2 $ M 29/2 $ M 46/3 § M 46/3 § M 46/3 § M 46/3 § ANTXXIII/7 SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2& SO 189/2&

ppm

+

δ44/40Ca and δmuCa relative to NIST SRM 915a; calculation of δmuCa and 106 lnαmu see Gussone et al. (2005). *: Assuming seawater Ca isotope composition of the fluid. !: Schulz and cruise participants (1998), #: Wefer and cruise participants (1991), $: Bleil and cruise participants (1994), §: Bleil and cruise participants (2001), +: Dieckmann et al. (2008), &: Wiedicke-Hombach et al. (2006).

as α (α = (44Ca/40Ca)solid/(44Ca/40Ca)fluid) and αmu = αa/((b + 1)·(a − b)) with αa/b = (aCa/bCasolid)/(aCa/bCafluid) (for details see Gussone et al., 2005). For experiments during which more than 5% of the dissolved Ca from solution was precipitated, we express the results as apparent

(δmuCa [ppm/amu] = 268.3·δ44/40Ca) assuming equilibrium isotope fractionation (Gussone et al., 2005). Samples were normalised to the standard measurements of the respective measuring campaign. The Ca isotope fractionation between solution and solid is expressed 2.0

ikaite (synthetic) 500 ikaite (natural) vaterite

seawater

200 0.5 100 0

0.0

-100 -0.5

mu

Ca-fluid (ikaite)

δ

δ44/40Ca‰ SRM 915a

300 1.0

Ca (ppm/amu) SRM 915a

400

1.5

Ca-fluid (vaterite)

-200 -1.0 -300 0

10

20

30

40

50

o

T ( C) Fig. 1. Calcium isotope ratios of synthetic vaterite and synthetic and natural ikaite as a function of temperature: vaterite and ikaite are isotopically lighter compared to the solutions they precipitated from (horizontal lines). Error bars and confidence bands represent 2σm uncertainties.

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fractionation Δ44/40Ca (=δ44/40Caprecipitate − δ44/40Cafluid) and ΔmuCa (=268.3·Δ44/40Ca). The average 2 S.D. of the vaterite samples was typically about 0.10 to 0.13‰ (27–35 ppm/amu) and ranged typically between 0.04 and 0.10‰ (10–27 ppm/amu) for the ikaites (see Tables 2 to 4).

fractionation factors (Δ44/40Ca) for this reservoir effect assuming a Raleigh type fractionation and obtained Ca isotope fractionation factors between −0.50 and −0.26‰ (1000 lnα) or −137 and −69 ppm/amu (106lnαmu). Calcium isotope fractionation in the synthetically precipitated vaterite is not significantly dependent on the surrounding temperature (Eq. (1)).

3. Results The synthetic vaterite and ikaite crystals are depleted in 44Ca compared to the solution from which they precipitated (Fig. 1 and Tables 2–4). The measured δ44/40Ca values of the vaterite samples range between −1.13 and −0.85‰ relative to the SRM 915a standard (−304 to −227 ppm/amu) with a δ44/40Cafluid of −0.67± 0.05‰ (−179 ± 14 ppm/amu). The apparent Ca isotope fractionation between vaterite and solution ranges between −0.46 and −0.18‰ (Δ44/40Ca) or -125 and −48 ppm/amu (ΔmuCa) (Table 2, Fig. 2). In some experiments, a significant amount (up to 50%) of Ca was consumed during the precipitation of vaterite (Table 1). The calculated apparent fractionation factors are biased by a successive depletion of 40Ca relative to 44Ca in the solution (Teichert et al., 2005). We therefore corrected the apparent Ca isotope

1000lnα =−0:47F0:11 + ð0:003F0:003Þ ·Tð°CÞ; R2 =0:19; n=12; P N F : 0:16 6

ð1Þ

10 lnαmu = −126F30 + ð0:8F0:8Þ·Tð°CÞ

The Ca isotope ratios of synthetic ikaite samples cover a range of δ44/40Ca values from 0.33 to 0.66‰ (88–177 ppm/amu) (Fig. 1, Table 3); together with an isotope composition of the fluid of 1.08±0.02‰ (289± 6ppm/amu) this reveals a Ca isotope fractionation of −0.75 to −0.42‰ (1000 lnα) or −201 to −112 ppm/amu (106lnαmu) (Fig. 2, Table 3). The natural ikaite samples range from 1.04 to 1.47‰ (280– 395 ppm/amu) (Fig. 2B, Table 4). The Ca isotope composition of the natural ikaite sample retrieved from sea-ice is 1.26‰ (337 ppm/amu). Assuming natural seawater (1.88‰; 505 ppm/amu) as Ca source from

-0.1

A -50

-0.2

-0.3

1000lnα

-0.5 -150

106 ln(αmu)

-100 -0.4

-0.6

-0.7 -200 -0.8

ikaite (synthetic) vaterite (synthetic)

0

10

20

30

40

50

T(oC) -0.1 -0.2

B

Congo-fan

Uruguay margin

Antarctica

Sumatra

-50

-0.5

synthetic ikaite

Δ44/40Ca (‰)

-100 -0.4

-0.6

-150

ΔmuCa(ppm/amu)

-0.3

-0.7 -200 -0.8

5

S1 S2 S3 SS- 4 7 S- a 7 S- b S- 8 1 S- 0 1 S- 1 1 S- 2 1 S- 3 1 S- 4 1 S- 5 1 S- 6 1 S- 7 1 S- 8 19

I-1

I-1 I-11 I-12 I-13 4 I-1 I-6 I-9 I-1 0

I-7 I-8 I-3 I-4 I-5 I-2

-0.9

Fig. 2. A) Temperature dependent Ca isotope fractionation of vaterite and ikaite: synthetic vaterite and natural and synthetic ikaite are enriched in 40Ca compared to the fluid. Error bars and confidence band represent 2σm uncertainties. B) Apparent calcium isotope fractionation of natural sedimentary ikaite from the Congo-fan, Uruguay, Sumatra and Antarctica relative to present day seawater isotope composition. No systematic difference in the Ca isotope fractionation of natural and synthetic ikaite is found, but Sumatra ikaites are about 0.2‰ heavier compared to the samples of the South Atlantic.

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which the ikaite precipitated, an apparent Ca isotope fractionation of −0.62‰ (Δ44/40Ca) or 168 ppm/amu (ΔmuCa) is revealed. The δ44/40Ca values of the ikaites from the southern Congo-fan range from 1.04 to 1.23‰ (280–331 ppm/amu). Assuming seawater as Ca source of the Congo-fan-samples reveals Δ44/40Ca values of −0.65 to −0.84‰ (−175 to −226 ppm/amu). The δ44/40Ca values of the ikaites from the Uruguay continental margin have similar values, ranging from 1.07 to 1.27‰ (288–341 ppm/amu), with an apparent fractionation relative to seawater (Δ44/40Ca) of −0.61 to −0.81‰ (−164 to −218 ppm/amu). The former ikaites from Sumatra show δ44/40Ca values of 1.25–1.47‰ (336– 395 ppm/amu), with an apparent fractionation to seawater (Δ44/40Ca) of −0.41 to −0.61‰ (-110 to −164 ppm/amu). In general, the apparent Ca isotope fractionation of the marine ikaites, relative to present seawater, covers a range similar to the values revealed by the ikaite growth experiments, with the ikaites derived from sea-ice and the sediments off Sumatra being roughly 0.2‰ heavier compared to the samples from the southern Congo-fan and Uruguay continental margin. 4. Discussion 4.1. Influence of mineralogy, temperature and precipitation rate on calcium isotope fractionation in synthetic vaterite and ikaite The vaterite and ikaite crystals precipitated in this study are depleted in 44Ca relative to the fluid, in agreement to previous experiments with synthetic calcite and aragonite (cf. Gussone et al., 2003; Marriott et al., 2004; Lemarchand et al., 2004; Tang et al., 2008). For both investigated minerals, Ca isotope fractionation is smaller than the fractionation of calcite (Marriott et al., 2004) or aragonite (Gussone et al., 2003, 2005) at comparable temperatures (Fig. 3). The offsets between vaterite and calcite as well as aragonite decrease with increasing temperature, due to the insignificant temperature sensitivity found in the vaterite precipitates. The synthetic ikaite grown at a temperature of 2 °C is considerably less depleted in heavy isotopes compared to calcite and aragonite precipitated at comparably low temperatures. Because of the limited stability field of ikaite (Marland, 1975), we did not investigate its temperature dependence. Besides temperature and crystallographic structure of a mineral, the precipitation rate is an equally important factor that influences the

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Ca isotope fractionation in carbonates (Lemarchand et al., 2004; Fantle and DePaolo, 2007; Tang et al., 2008; DePaolo, 2011). The range of Ca isotope fractionation of vaterite as well as ikaite falls close to the rate dependent Ca isotope fractionation array of unstirred calcite of Lemarchand et al. (2004) (Fig. 4), who found decreasing isotope fractionation with increasing precipitation rates. In contrast, Tang et al. (2008) found increasing Ca isotope fractionation in calcite with increasing precipitation rates. The different rate dependencies observed by Tang et al. (2008) and Lemarchand et al. (2004) were recently explained by DePaolo (2011) by calcite precipitation under different growth regimes and it was suggested that calcite crystals grown in unstirred precipitation experiments (Lemarchand et al., 2004) might represent growth under transport limitation while those of Tang et al. (2008) might mainly be controlled by surface reaction processes. Our vaterite grains exhibit spherical shapes, which are indicative of transport limited growth; their growth regime should therefore resemble more strongly that of the unstirred samples of Lemarchand et al. (2004). Since the vaterite falls close to the transport-limited fractionation trend of calcite we suggest that the rate dependent Ca isotope fractionation under such a growth regime is very similar for different CaCO3 polymorphs. The small and insignificant temperature dependence of Ca isotope fractionation of our vaterite grown at relatively high growth rates under transport limited growth is generally consistent with the fractionation model of Lemarchand et al. (2004), but differs from the observations of Tang et al. (2008) on calcite grown in a regime of surface reaction control, who found increasing temperature sensitivities with increasing growth rates. Previously described fractionation between different CaCO3 polymorphs as well as significant temperature dependent fractionation might both be related to surface reaction processes (see DePaolo, 2011). The observation that Ca isotope fractionation during ikaite growth falls into the range of the water free CaCO3 modification calcite and vaterite (Fig. 3) might shed additional light on fractionation processes during mineral growth, since the dissolved Ca in the growth solution is only partly dehydrated during incorporation into ikaite (cf. Rickaby et al., 2006), while it is completely dehydrated in the anhydrous minerals. In particular, the similarity in rate dependent Ca isotope fractionation of ikaite and the water free CaCO3-modifications grown in a transport limited regime (Fig. 4) suggests that the dehydration of

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T( C) Fig. 3. Temperature dependent Ca isotope fractionation of different CaCO3 polymorphs: synthetic vaterite and ikaite in comparison with published calcite and aragonite. At given temperatures, inorganic vaterite and ikaite are less depleted in 44Ca compared to calcite or aragonite. Error bars and confidence bands represent 2σm uncertainties.

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0.2 0.0

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synthetic aragonite (10-50°C)

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log R (µmol·m-2·h-1) Fig. 4. Rate dependent Ca isotope fractionation of different CaCO3 polymorphs. Calcium isotope fractionation of vaterite and ikaite in comparison to synthetic calcite (Lemarchand et al., 2004; Tang et al., 2008) and aragonite (Gussone et al., 2003, 2005). Error bars are 2σm uncertainties.

the Caaq, does not play a dominant role for Ca isotope fractionation in this growth regime, while it is presumably of importance under pure surface reaction controlled growth conditions. In general, the good agreement of Ca isotope fractionation in synthetic and natural ikaite, indicates that our experiments seem to adequately characterise the fractionation behaviour of naturally occurring ikaite.

4.2. Natural variation of ikaite and preservation of isotopic signatures The range of δ44/40Ca values of the natural ikaite samples is in agreement with the fractionation found in the synthetic ikaite samples. There is a systematic difference of about 0.2‰ (54 ppm/amu) between the samples from the southern Congo-fan and the Uruguay continental margin on one hand and the Sumatra samples (that were recovered as calcite) on the other hand. The sea ice sample shows an intermediate value. The samples from offshore Sumatra were found in shallow sediment depth (at about 3 mbsf, and were formed presumably at ~2 mbsf), in a porefluid with presumably a seawater-like Ca isotope composition. The ikaites from the Uruguay continental margin and southern Congo-fan were found in greater sediment depth of 4–13 mbsf (Table 3). The formation depth of these ikaites is not exactly known, but Zabel and Schulz (2001) suggest that they were formed in similar depth. Since the δ44/40Ca of sedimentary porewaters is generally getting lighter with increasing depth for the upper tens of meters (Fantle and DePaolo, 2007; Teichert et al., 2009), the observed offset might be related to a different formation depth. Furthermore, the process of bicarbonate formation differs between the Atlantic and the Sumatra sites. At the cold seep site offshore Sumatra, methanotrophic sulphate reduction takes place, leading to a shallow sulphate–methane-interface (SMI), where considerable amounts of bicarbonate are released and carbonate precipitation is triggered. For the precipitation of carbonate minerals in sedimentary porewater environments a growth regime affected by surface reaction processes as well as transport limitation (DePaolo, 2011) appears to be more likely than surface reaction processes alone. The high bicarbonate production at the Sumatra seep site is assumed to lead to comparatively fast ikaite growth. Considering a rate-dependent Ca isotope fractionation under a transport limited regime (DePaolo, 2011) – similar to the observations on the unstirred calcite by (Lemarchand et al., 2004) – we would expect a relative small Ca isotope fractionation. In contrast, at the Atlantic sites, bicarbonate is

produced by organotrophic sulphate reduction (Hensen et al., 2003; Riedinger et al., 2005). The lower bicarbonate concentrations will presumably induce slower growth rates; a larger Ca isotope fractionation compared to the Sumatra setting is therefore likely. Since the Ca isotope values of the Sumatra samples agree well with the experimentally determined fractionation factors, we do not have an indication that Ca isotope fractionation takes place during the conversion from ikaite to calcite. We rather suggest that the 0.2‰ offset is not related to the dissolution–reprecipitation process, but rather a primary feature related to the different geochemical growth environments and that the Ca isotope composition does not change when ikaite recrystallises to calcite. This concept is in general agreement to experiments that revealed no Ca isotope fractionation during pseudomorphic replacement of aragonite by apatite (Kasioptas et al., 2011). 5. Summary and conclusions – At a given temperature, synthetic vaterite and ikaite are less depleted in 44Ca than calcite and aragonite grown from an aqueous solution. This offset is mainly caused by the typically higher precipitation rates of the higher soluble polymorphs. – A similar rate dependent Ca isotope fractionation for different CaCO3 polymorphs grown under transport limited conditions is suggested by the similarity of our highly soluble polymorphs with the calcite crystals grown in unstirred precipitation experiments (Lemarchand et al., 2004). At a given precipitation rate, there is no major difference in Ca isotope fractionation of hydrated and waterfree Ca carbonates. This suggests that the dehydration of the Caaquocomplex does not play an important role during transport limited Ca isotope fractionation. Instead, Caaq dehydration and mineralogy-specific Ca isotope fractionation are rather related to surface reaction controlled fractionation processes. – The Ca isotope fractionation obtained from synthetic ikaite agrees with natural ikaites. The precipitation experiments seem to be suited to characterise the fractionation behaviour of natural ikaite. – Small differences in Ca isotope ratios of naturally formed ikaites from different locations seem to be related to different geochemical conditions in the respective sediment settings. – Precipitation and decomposition of ikaite in sedimentary porewaters might significantly contribute to the development of porewater Ca isotope profiles.

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Acknowledgements We would like to thank Horst D. Schulz and Matthias Zabel (Universität Bremen) for providing natural ikaite samples from the southern Congo fan and the Uruguay continental margin and Gerhard Dieckmann (Alfred-Wegener-Institut, Bremerhaven) for providing natural ikaite samples from Antarctic sea-ice. We are grateful to Anton Eisenhauer for making the TIMS in Kiel available for the vaterite measurements. We thank captains and crews from the research vessels Polarstern, Sonne and Meteor. We greatly appreciate the constructive and helpful comments of Joel D. Blum, Thomas Bullen and Anton Eisenhauer. This research was supported by the DFG by grant GU 1035/1-1 and NE 1564/1-1 (SPP 1158).

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