Data Brief Volume 11, Number 8 18 August 2010 Q08015, doi:10.1029/2010GC003201 ISSN: 1525‐2027
Boron and magnesium isotopic composition of seawater G. L. Foster Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Now at School of Ocean and Earth Sciences, National Oceanography Centre, Southampton, University of Southampton, European Way, Southampton SO14 3ZH, UK (
[email protected])
P. A. E. Pogge von Strandmann and J. W. B. Rae Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK [1] The isotopic systems of boron and magnesium are increasingly being used as proxies for a number of
environmental variables and processes. The isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard. Given the long residence times of both elements (∼107 years) it is commonly assumed that seawater is isotopically homogenous for these systems, yet no systematic studies currently exist. Here we present the B and Mg isotopic composition of 26–28 seawater samples from a number of ocean basins that encompass a significant range in salinity (32 to 38 psu), temperature (−0.3 to +25.9°C) and water depth (0 to 1240 m). We find no significant or systematic variation for either system in accordance with their long residence times. We recommend that the mean values we report (d 11B = 39.61 ± 0.04 ‰ (2 s.e.; n = 28), d 25Mg = −0.43 ± 0.01 ‰ (2 s.e.; n = 26), d 26Mg = −0.82 ± 0.01 ‰ (2 s.e.; n = 26)) be used in future studies involving Mg and B isotopes. Components: 5700 words, 3 figures, 3 tables. Keywords: seawater; boron isotopes; magnesium isotopes; MC‐ICPMS. Index Terms: 1094 Geochemistry: Instruments and techniques; 4825 Oceanography: Biological and Chemical: Geochemistry; 4870 Oceanography: Biological and Chemical: Stable isotopes (0454, 1041). Received 30 April 2010; Revised 17 June 2010; Accepted 23 June 2010; Published 18 August 2010. Foster, G. L., P. A. E. Pogge von Strandmann, and J. W. B. Rae (2010), Boron and magnesium isotopic composition of seawater, Geochem. Geophys. Geosyst., 11, Q08015, doi:10.1029/2010GC003201.
1. Introduction [2] Boron and magnesium both have a residence time in seawater of ∼107 years [Lemarchand et al., 2002; Berner and Berner, 1996]. Since this is much greater than the mixing time of the oceans (∼103 years) they should be well mixed and behave conservatively. Consequently, B and Mg concentra-
Copyright 2010 by the American Geophysical Union
tions are salinity dependant and are 432.6 mmol/kg and 52.8 mmol/kg of seawater at 35 psu, respectively [Lee et al., 2010; Carpenter and Manella, 1973]. The concentration of Mg in biogenic marine carbonates has become a widely applied proxy for paleo‐temperature [Nürnberg et al., 1996; Lear et al., 2000] and, more recently, the concentration of B in marine carbonates has also begun to receive attention as it is thought to be a 1 of 10
Geochemistry Geophysics Geosystems
3
G
proxy for some component of the marine carbonate system [Ni et al., 2007; Yu et al., 2007; Foster, 2008; Yu and Elderfield, 2007]. The isotopic composition of these elements has a number of potential applications [e.g., Vengosh, 1998] with the boron isotope proxy for paleo‐pH being relatively well established [e.g., Sanyal et al., 1995; Hönisch and Hemming, 2005; Foster, 2008; Hönisch et al., 2009]. Mg isotopes have less potential as an environmental proxy [Pogge von Strandmann, 2008] but can provide valuable insights into weathering processes and dolomitization in the geological past [e.g., Galy et al., 2002; de Villiers et al., 2005; Tipper et al., 2006a, 2008a; Pogge von Strandmann et al., 2008a, 2008b]. The isotopic composition of seawater for both B and Mg has an important bearing on these applications. What is more, as analyses become more precise, the uncertainty on this datum becomes more significant. Also, isotopic studies of both these elements commonly analyze seawater as an interlaboratory standard in order to compare results between laboratories and to assess accuracy, yet to date there has been no systematic study of seawater to determine its isotopic composition for either element. To address this shortcoming we present the B and Mg isotope composition of a number of samples (28 for B and 26 for Mg) of seawater from the Atlantic, Pacific, Southern Ocean and the Mediterranean Sea that encompass a range in salinity, temperature and depth.
1.1. Boron Isotopic Systematics [3] Boron has two stable isotopes (10B and
11
B=10 Bsample
11 B=10 B
! 1
also dependent on pH [Hemming and Hanson, 1992; Sanyal et al., 1996, 2000]. This can be used to reconstruct paleo‐seawater pH to a relatively high precision (e.g., ±0.02 pH units [Foster, 2008]) and importantly, with a number of assumptions, the concentration of atmospheric CO2 in the past [e.g., Sanyal et al., 1995; Hönisch and Hemming, 2005; Foster, 2008; Pearson et al., 2009; Seki et al., 2010]. A crucial constraint for the d11B pH proxy is that the isotopic composition of seawater (d 11Bsw) must be known. Due to boron’s long residence time d 11Bsw is not expected to change significantly in the past ∼5 million years, but over the course of the Cenozoic geochemical models suggest fluctuations in d 11Bsw of several per mil [Lemarchand et al., 2002]. [4] The commonly used value for d 11Bsw is 39.5 ‰.
While this value is supported by a compilation of published estimates (Table 1), reported values range from 37.7 to 40.4 ‰ with a mean of 39.46 ± 1.45 ‰ (n = 26; 2 s.d.). Given the long residence time it is unlikely that this range reflects real variations in d11Bsw and instead is probably due to analytical uncertainties, which for boron can be relatively large [see Gonfiantini et al., 2003; Aggarwal et al., 2009]. An accurate determination of d11Bsw is critical for accurate reconstructions of paleo‐pH as calibration of the proxy is dependent on it. Also, seawater is commonly used as an interlaboratory standard [e.g., Gonfiantini et al., 2003; Hönisch and Hemming, 2005; Palmer et al., 1998]; a more precise definition of d 11Bsw is therefore desirable.
11
B) and variations in isotope ratio are expressed as parts per thousand deviation from the 11B/10B of NIST SRM boric acid standard 951 as follows: 11 B ¼
10.1029/2010GC003201
FOSTER ET AL.: DATA BRIEF
1000
1.2. Mg Isotope Systematics [5] Mg has three stable isotopes (24Mg, 25Mg, 26Mg)
and is reported in delta notation referenced to a standard as follows:
std
where 11 B/ 10 B std = 4.04367 [Catanzaro et al., 1970]. Boron predominantly exists in seawater as two aqueous species, B(OH)3 and B(OH)−4 , the abundance of which is dependent on pH [Dickson, 1990]. There is a pronounced isotopic fractionation between these species, with B(OH)−4 enriched in 10 B by 27.2 ± 0.6 ‰ [Klochko et al., 2006]. Because the abundance of each species changes with pH the isotopic composition of each species is also pH dependent. It is thought that the B(OH)−4 species is predominantly incorporated into CaCO3 [Hemming and Hanson, 1992], and consequently the isotopic composition of marine carbonate is
25
Mg ¼
26 Mg ¼
25
Mg=24 Mgsample
25 Mg=24 Mg std
26
Mg=24 Mgsample
26 Mg=24 Mg std
! 1
1000
! 1
1000
where 25Mg/24Mgstd and 26Mg/24Mgstd is the Mg isotope composition of the Mg standard DSM‐3 [Galy et al., 2003]. Due to the relatively large mass contrast between the isotopes of Mg they are fractionated by numerous low‐temperature processes [e.g., Young and Galy, 2004; Tipper et al., 2006a; Chang et al., 2004; Black et al., 2006] and most 2 of 10
Geochemistry Geophysics Geosystems
3
G
Table 1.
10.1029/2010GC003201
FOSTER ET AL.: DATA BRIEF
Compilation of Published Estimates of the Boron Isotopic Composition of Seawater
Vengosh et al. [1991] Vengosh et al. [1992] Vengosh et al. [1989] Barth [1997] Barth [1997] Barth [1997] Spivack and Edmond [1987] Spivack and Edmond [1987] Spivack and Edmond [1987] Spivack and You [1997] Palmer et al. [1998] Gaillardet and Allegre [1995] Gaillardet and Allegre [1995] Gaillardet and Allegre [1995] Lemarchand et al. [2002] Hemming and Hanson [1992] Hemming and Hanson [1992] Hönisch et al. [2003] Hönisch and Hemming [2004] Aggarwal et al. [2003] Lécuyer et al. [2002] Vengosh [1998] Foster et al. [2006] Foster et al. [2006] Gonfiantini et al. [2003] Gonfiantini et al. [2003] Average
d 11B
2sigmaa
Methodb
37.70 39.00 38.40 39.70 39.50 40.30 39.37 39.57 39.63 39.10 39.70 39.77 40.40 40.25 39.60 39.90 40.10 39.58 39.70 39.45 40.26 40.30 39.20 37.70 38.60 39.09 39.46
3.0 3.0 2.2 0.6 0.5 0.6 0.2 0.2 0.2 1.5 0.3 0.3 0.3 0.3 0.3 0.6 0.6 0.3 0.3 0.4 0.6 2.0 0.8 1.2 1.7 0.4 1.45
NTIMS NTIMS NTIMS NTIMS NTIMS NTIMS PTIMS PTIMS PTIMS NTIMS NTIMS PTIMS PTIMS PTIMS PTIMS NTIMS NTIMS NTIMS NTIMS MC‐ICPMS MC‐ICPMS NTIMS TE‐NTIMS TE‐NTIMS PTIMS NTIMS
Depthc (m)
Locationd Mediterranean Sea
2000 10 2500 10 10 1300 10 10 10 30
150 10 0 10 10
Jarvis Bay Eastern Australia Pacific (SW of Easter Island, Chile) Atlantic (Florida Bay) North Sea (Germany) Pacific (21°N East Pacific Rise) Atlantic (Sargasso Sea) Pacific (Panama Basin) North Atlantic (Bermuda, NAAS‐3) North Atlantic (Nova Scotia, CAAS‐2) South Atlantic (Pointe Indienne, Congo) North Atlantic (Nova Scotia, CAAS‐2) Atlantic (Tavernier Cay, Florida) Atlantic (Long Island Sound) Pacific (Cook Straits, NZ) Mediterranean Mediterranean IAEA‐B1 (Mediterranean) Southern Ocean (Bransfield Strait) IAEA‐B1 (Mediterranean) IAEA‐B1 (Mediterranean)
a This is the uncertainty quoted in each publication and is either 2 standard deviations or 2 standard errors (2sd/n0.5) of repeat measurements of in house standards. b Analytical methodology: NTIMS (negative ion thermal ionization mass spectrometry), PTIMS (positive ion thermal ionization mass spectrometry), MC‐ICPMS (multicollector inductively coupled plasma mass spectrometry), TE‐NTIMS (total evaporation NTIMS). c Reported water depth of collection. No entry indicates no depth was reported in the manuscript. d Reported location of collection. No entry indicates no location was listed in the manuscript.
attention is currently focusing on the insights that Mg isotopes can provide as a tracer of surface processes such as weathering [de Villiers et al., 2005; Tipper et al., 2006a, 2006b, 2008a; Pogge von Strandmann et al., 2008a] and dolomite formation [Galy et al., 2002; Tipper et al., 2006b]. In contrast to B, the Mg isotopic composition of seawater has only been determined in the last decade (Table 2) with reported d25Mg and d 26Mg values ranging from −0.40 to −0.77 ‰ and from −0.69 to −1.47 ‰, respectively. Most recent studies report a narrower range (d 26Mg = −0.69 to −0.89 ‰), and the values reported by de Villiers et al. [2005] (d26Mg ≈ −1.37) are clear outliers probably because an isotopically heterogeneous standard was used for normalization [Galy et al., 2003]. Excluding this study, mean values are d25Mg = −0.41 ± 0.06 (n = 22; 2 s.d.) and d 26Mg = of −0.81 ± 0.11 ‰ (n = 22; 2 s.d.). In general, there is a better agreement between laboratories for this isotope system compared to boron, probably as a consequence of the similarity in measurement technique used by the smaller number of active groups
(Table 2). It should be noted, however, that purification methods vary and one study reports significantly different results (Table 2). Also, few studies report measurements of seawater d 25Mg and d26Mg from more than three locations and there is clearly a need for a more comprehensive investigation into the variability of d25Mg and d 26Mg in seawater. Mg isotopes behave conservatively in estuarine environments [Pogge von Strandmann et al., 2008b] but are significantly fractionated by biologic activity, where plant material preferentially takes up the light isotopes, driving residual solutions isotopically heavy [Black et al., 2006; Ra and Kitagawa, 2007]. Hence, despite its long residence time the utilization of Mg by marine organisms could potentially cause local Mg isotope variations in seawater. In addition, the present range in published seawater values currently makes it difficult to distinguish seawater d 26Mg from that of average continental runoff (d 26Mg = −1.09 ± 0.05‰ [Tipper et al., 2006b]), thus complicating the debate about whether Mg in the present‐day 3 of 10
Geochemistry Geophysics Geosystems
3
G
Table 2.
10.1029/2010GC003201
FOSTER ET AL.: DATA BRIEF
Compilation of Published Estimates of the Mg Isotopic Composition of Seawatera d 25Mg
2 sigmab
d 26Mg
2sigmab
Methodc
Depthd (m)
Locatione
Chang et al. [2003] Chang et al. [2004]
−0.42 −0.40
0.08 0.04
−0.82 −0.75
0.04 0.12
MC‐ICPMS MC‐ICPMS
0 600 1000 2600 5800 0 50
North Atlantic (n = 4) North Atlantic, Med. Atlantic, 3 samples Mediterranean Sea Atlantic (SMOW) Mediterranean surface water East Pacific Rise East Pacific Rise East Pacific Rise East Pacific Rise North Pacific Mediterranean Sea Mediterranean Sea
Young and Galy [2004] Young and Galy [2004] Young and Galy [2004]
−0.43 −0.41 −0.42
0.10 0.01 0.12
−0.86 −0.80 −0.85
0.12 0.04 0.24
MC‐ICPMS MC‐ICPMS MC‐ICPMS
de Villiers et al. [2005] de Villiers et al. [2005] de Villiers et al. [2005] de Villiers et al. [2005] de Villiers et al. [2005] de Villiers et al. [2005] de Villiers et al. [2005] Tipper et al. [2006a] Ra and Kitagawa [2007] Ra and Kitagawa [2007] Ra and Kitagawa [2007] Ra and Kitagawa [2007] Ra and Kitagawa [2007] Ra and Kitagawa [2007] Pogge von Strandmann et al. [2008b] Pogge von Strandmann [2008] Tipper et al. [2008b] Bolou‐Bi et al. [2009] Hippler et al. [2009] Hippler et al. [2009] Wombacher et al. [2009] Wombacher et al. [2009] Chakrabarti and Jacobsen [2010] Chakrabarti and Jacobsen [2010] Average Average (minus de Villiers et al. [2005])
−0.77 −0.73 −0.74 −0.69 −0.75 −0.70 −0.66 −0.43 −0.37 −0.46 −0.38 −0.39 −0.34 −0.4 −0.46
0.06 0.18 0.12 0.04 0.04 0.09 0.06 0.15 0.04 0.12 0.13 0.16 0.12 0.14 0.14
−1.47 −1.39 −1.39 −1.34 −1.41 −1.40 −1.28 −0.84 −0.74 −0.90 −0.69 −0.77 −0.86 −0.73 −0.89
0.14 0.40 0.22 0.08 0.16 0.07 0.08 0.13 0.07 0.20 0.22 0.40 0.30 0.29 0.18
MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS
−0.44
0.08
−0.83
0.09
MC‐ICPMS
North Atlantic (n = 11)
−0.41 −0.47 −0.42 −0.42 −0.41 −0.43 −0.40
0.04 0.06 0.01 0.02 0.06 0.07 0.04
−0.80 −0.89 −0.79 −0.80 −0.79 −0.84 −0.79
0.06 0.10 0.03 0.05 0.10 0.16 0.07
MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS MC‐ICPMS
North Sea Dutch Wadden Sea IAPSO std. IAPSO std. NASS‐5 IAPSO std. (n = 14)
−0.41
0.05
−0.82
0.08
MC‐ICPMS
Bermuda (n = 13)
−0.49 −0.41
0.27 0.06
−0.95 −0.81
0.51 0.11
0 1000 2000 3000 4000
IAPSO std. North Pacific North Pacific North Pacific North Pacific North Pacific IAPSO std. (n = 20)
a
Relative to DSM‐3. This is the uncertainty quoted in each publication and is either 2 standard deviations or 2 standard errors (2sd/n0.5) of repeat measurements of in house standards. c Analytical methodology: MC‐ICPMS (multicollector inductively coupled plasma mass spectrometry). d Reported water depth of collection. No entry indicates no depth was reported in the manuscript. e Reported location of collection. No entry indicates no location was listed in the manuscript. b
oceans is at steady state [Holland, 2005; Vance et al., 2009].
2. Sample Descriptions and Analytical Methodology 2.1. Sample Description [6] The locations of the seawater samples used in this study are shown in Figure 1 and listed in Table 3. Temperature and salinity data we use are not measured on these same samples but are mean annual values from the World Ocean Atlas (2005)
from nearby sites. All samples were filtered to 10 times background level of 23Na) data were discarded. This was necessary in only one instance. Effective recovery was also monitored for every sample by checking the B concentration of an additional 90 ml elution following boron collection, in all cases this tail represented 99.9% [Pogge von Strandmann, 2008] ensuring minimal isotopic fractionation is likely to be induced by this chemical separation. Total procedural blanks are typically around 0.2 to 0.3 ng and can be considered insignificant. Analyses were performed as described in Pogge von Strandmann [2008], except that an Apex‐Q rather than a quartz spray chamber was used. We found that this “moist” plasma yields a higher Mg intensity compared to wet plasma, but with a similar lack of interferences (e.g.,
