High-precision radiogenic strontium isotope measurements of the modern and glacial ocean: Limits on glacial–interglacial variations in continental weathering

Earth and Planetary Science Letters 415 (2015) 111–120

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High-precision radiogenic strontium isotope measurements of the modern and glacial ocean: Limits on glacial–interglacial variations in continental weathering Fatima Mokadem a , Ian J. Parkinson a,b , Ed C. Hathorne a,c , Pallavi Anand a , John T. Allen d , Kevin W. Burton a,e,∗ a

Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Clifton BS8 1RJ, United Kingdom c GEOMAR, Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, D-24148 Kiel, Germany d School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL, United Kingdom e Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, United Kingdom b

a r t i c l e

i n f o

Article history: Received 8 May 2014 Received in revised form 24 January 2015 Accepted 27 January 2015 Available online xxxx Editor: G.M. Henderson Keywords: radiogenic strontium isotopes 87 Sr/86 Sr seawater foraminifera glacial–interglacial climate change weathering and erosion

a b s t r a c t Existing strontium radiogenic isotope (87 Sr/86 Sr) measurements for foraminifera over Quaternary glacial– interglacial climate cycles provide no evidence for variations in the isotope composition of seawater at the ±9–13 ppm level of precision. However, modelling suggests that even within this level of uncertainty significant (up to 30%) variations in chemical weathering of the continents are permitted, accounting for the longer-term rise in 87 Sr/86 Sr over the Quaternary, and the apparent imbalance of Sr in the oceans at the present-day. This study presents very high-precision 87 Sr/86 Sr isotope data for modern seawater from each of the major oceans, and a glacial–interglacial seawater record preserved by planktic foraminifera from Ocean Drilling Program (ODP) Site 758 in the north-east Indian ocean. Strontium isotope 87 Sr/86 Sr measurements for modern seawater from the Atlantic, Pacific and Indian Oceans are indistinguishable from one another (87 Sr/86 Sr = 0.7091792 ± 0.0000021, n = 17) at the level of precision obtained in this study (±4.9 ppm 2σ ). This observation is consistent with the very long residence time of Sr in seawater, and underpins the utility of this element for high precision isotope stratigraphy. The 87 Sr/86 Sr seawater record preserved by planktic foraminifera shows no resolvable glacial–interglacial variation (87 Sr/86 Sr = 0.7091784 ± 0.0000035, n = 10), and limits the response of seawater to variations in the chemical weathering flux and/or composition to ±4.9 ppm or less. Calculations suggest that a variation of ±12% around the steady-state weathering flux can be accommodated by the uncertainties obtained here. The new data cannot accommodate a short-term weathering pulse during de-glaciation, although a more a diffuse weathering pulse accompanying protracted ice retreat is permissible. However, these results still indicate that modern weathering fluxes are potentially higher than average over the Quaternary, and such variations through glacial cycles can also account for the longer-term rise in 87 Sr/86 Sr over this time interval. The very high-precision measurements made for the marine 87 Sr/86 Sr record in this study place clear limits on the magnitude and timing of changes in the chemical weathering flux during glacial–interglacial cycles. Further, constraints must be sought from even higher precision measurement or elements with shorter residence times in the ocean, such as osmium (Os), that have the capacity to respond to short-term variations in input. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

1. Introduction

*

Corresponding author at: Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, United Kingdom. Tel.: +44 (0)191 334 4298; fax: +44 (0)191 334 2301. E-mail address: [email protected] (K.W. Burton). http://dx.doi.org/10.1016/j.epsl.2015.01.036 0012-821X/Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

Chemical weathering of silicates exerts a control on global climate, through its effect on atmospheric CO2 levels, which in turn can drive temperature changes by modifying greenhouse warming. On million-year timescales silicate weathering leads to the drawdown of CO2 (e.g. Walker et al., 1981) and on millennial timescales

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changes in the flux of alkalinity affects the calcium carbonate saturation state of the oceans, and hence their uptake of CO2 (e.g. Archer et al., 2000). Many natural radiogenic isotopes in seawater are sensitive to changes in the balance of input from continental weathering (via rivers, groundwaters and aeolian input) against that from hydrothermal exchange at mid-ocean ridges. Of these, the rubidium–strontium (87 Rb–86 Sr) radiogenic isotope system is widely used and variations in seawater 87 Sr/86 Sr ratios on both long and short timescales are thought to reflect changing continental input through time (e.g. Brass, 1976; Hess et al., 1986; Hoddell et al., 1990; Capo and DePaolo, 1990; Hodell et al., 2007). Felsic continental rocks, such as granites, often possess relatively high Rb/Sr (parent/daughter) ratios and evolve to radiogenic 87 Sr/86 Sr compositions over time (due to the growth of 87 Sr produced by the decay of 87 Rb). In contrast, mafic and ultramafic rocks (such as basalts and abyssal peridotites) generally possess low Rb/Sr ratios and evolve to relatively unradiogenic 87 Sr/86 Sr ratios. Consequently, the Sr isotope signal from the weathering of continental rocks is generally characterised by a relatively radiogenic 87 Sr/86 Sr isotope composition compared to that derived from hydrothermal exchange at mid-ocean ridges (e.g. Palmer and Edmond, 1989) and variations in the 87 Sr/86 Sr isotope composition of seawater over time reflect changes the balance of those inputs, along with lithology. Amongst the many controls on silicate weathering rates (e.g. White and Brantley, 1995; White and Blum, 1995; Bluth and Kump, 1994; Gaillardet et al., 1999) there are a number that are likely to be affected by global climate change. For example, many studies have shown that mineral dissolution rates are enhanced at higher temperatures (e.g. White and Brantley, 1995; White and Blum, 1995; Dessert et al., 2003), and chemical weathering rates also increase with runoff (Bluth and Kump, 1994; Gaillardet et al., 1999; Gislason et al., 2009). Therefore, it might be expected that during glacial intervals cooler temperatures and reduced continental runoff will result in lower rates of chemical weathering. However, it has been suggested that the decrease in the weathering flux from the continents to the oceans during glacial intervals is likely to be partly balanced by the exposure of readily weathered shelf carbonates accompanying a fall in sea level (Gibbs and Kump, 1994; Stoll and Shrag, 1998). During deglaciation freshly exposed mineral surfaces in fine-grained glacial sediments, left behind by the retreating ice sheets, are likely to have been highly susceptible to weathering (Blum and Erel, 1995). On the timescale of an individual glacial–interglacial cycle (i.e. ∼100 kyr over the past ∼900 kyr), the very long residence time of Sr (2–3 Ma or longer) (Hodell et al., 1990; Richter and Turekian, 1993) in the oceans necessitates significant changes in the flux or composition of material delivered by continental weathering in order to cause a measurable change in the composition of seawater (Richter and Turekian, 1993). Early studies suggested that there may have been variation in the 87 Sr/86 Sr composition of seawater over recent glacial cycles (Dia et al., 1991; Clemens et al., 1993). However, subsequent work involving high-precision measurements on foraminifera by Henderson et al. (1994) and more recently Ando et al. (2010) has found no resolvable variation in the 87 Sr/86 Sr ratio at the ±13 ppm and ±9 ppm (2σ ) level of precision, respectively, of those studies. At first sight, this might appear to place severe constraints on any changes in chemical weathering accompanying glacial cycles. However, as a result of the long residence time this is not the case, and the data of Henderson et al. (1994) allow for a ∼30% variation in the flux of continental material delivered to the oceans. More recently it has been argued that the riverine flux to the oceans is not in steady state and that a transient (10–30 ka) post-glacial weathering pulse some 70% greater than the steady state weathering flux can account for both the long term increase in 87 Sr/86 Sr over the Quaternary and the present-day imbalance of

this element in the oceans (Vance et al., 2009) and is permitted by the same high precision 87 Sr/86 Sr record. Over the past decade there have been significant advances in thermal-ionisation mass spectrometry allowing isotope ratios to be determined with a long-term external precision better than 5 ppm for Sr or Nd (e.g. Caro et al., 2003, 2006). This raises the possibility of detecting variations in the Sr isotope composition of seawater that would have been unresolved in previous work. This study uses those advances in mass spectrometry to obtain very high precision 87 Sr/86 Sr isotope data for modern seawater from each of the major oceans, and a seawater record preserved by planktic foraminifera through the last glacial maximum (LGM) (45,000 yr) from Ocean Drilling Program (ODP) Site 758 in the north-east Indian ocean. This site is of particular interest because it preserves not only a global record of variations in seawater composition accompanying climate change, but also the local effects of erosion from the Himalaya–Tibet region that are seen in other radiogenic isotope systems such as Nd and Os (Burton and Vance, 2000; Burton et al., 2010). Moreover, much of the previous work on 87 Sr/86 Sr variations over the Quaternary has been undertaken on samples from this site (Dia et al., 1991; Clemens et al., 1993; Henderson et al., 1994), enabling a direct comparison with the data from those studies. The new data from this study show that there are no resolvable variations in the 87 Sr/86 Sr isotope composition of modern seawater at the external precision of this study, neither are there any detectable variations through the last glacial maximum. These results can then be used to place limits on any variations in continental weathering through the LGM, allowing the behaviour of Sr compared with that of other natural long-lived radiogenic isotopes over the same time interval. 2. Sample localities and previous work Modern seawater samples were analysed from each of the major oceans (Atlantic, Indian and Pacific) and from the Labrador Sea. For the samples from the north Atlantic (62◦ 00 N; 20◦ 00 W; 1800 m water depth) a complete depth profile was measured, and for the Indian ocean samples from the Madagascar basin (32◦ 39 05 S; 48◦ 28 16 E; 3753 water depth) samples from both the thermocline and deep waters were analysed (samples from Thomas et al., 2006). For the Pacific Ocean (9◦ 46 00 N; 104◦ 11 00 W; 10 km east of the East pacific rise ridge axis, sample suite of Woodhouse et al., 1999) and Labrador Sea (66◦ 56 45.4 N; 53◦ 42 26.2 W; close to the coastal town of Sisimiut) surface seawater samples were analysed. The foraminifera were taken from sediment samples from ODP Site 758 which is located at the northern end of the Ninetyeast ridge (5◦ 23 N, 90◦ 21 E; 2925 m water depth), in the North East Indian Ocean, and lies about 1000 m above the Bengal fan (Fig. 1). The sediment is primarily biogenic pelagic CaCO3 (35 to 65% CaCO3 ) but fine-grained terrigenous silts and clays predominantly sourced by rivers in the north and east of the Bay of Bengal, and volcanic tephra from Toba, in northern Sumatra, are also present (Shipboard Scientific Party, Site 758, 1989). General circulation models indicate that during the last glacial maximum (LGM) the presence of Northern hemisphere ice sheets caused a migration of the Inter-Tropical Convergence Zone (ITCZ) (Chiang and Bitz, 2005; Broccoli et al., 2006), reducing monsoonal rainfall and runoff over the Indian subcontinent, which resulted in a decrease in the input of freshwater from the Ganges– Brahmaputra (Cullen, 1981; Duplessy, 1982) and increasing precipitation over the Irrawaddy drainage basin and the Andaman Sea. Therefore, since the LGM both local variations in riverine input to the Bay of Bengal, and global variations in weathering and erosion, are likely to have had a marked impact on a number of radiogenic isotopes that serve as tracers of continental

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ing, and those in the Himalayas might be large enough to cause short-term fluctuations in the seawater 87 Sr/86 Sr about the longer term increasing trend over the Quaternary. Short-term variations in seawater 87 Sr/86 Sr compositions preserved by foraminifera from Site 758 were reported in two studies (Dia et al., 1991; Clemens et al., 1993); however, these data were controversial because the reported variation was very close to the analytical uncertainties in each case. Indeed, subsequent work on samples from the same site found no resolvable variation in seawater 87 Sr/86 Sr accompanying glacial–interglacial climate change (Henderson et al., 1994), at the level of precision reported by that work (±13 ppm) which was significantly better than that of the previous studies. 3. Analytical methods

Fig. 1. Map showing the location of ODP Site 758.

weathering. For Nd, both dissolved and suspended material carried by the Ganges and Brahmaputra are characterised by relatively unradiogenic isotope compositions (e.g. Goldstein et al., 1984; Goldstein and Jacobsen, 1987; Allègre et al., 2010), resulting in a north–south gradient in the surface waters of the Bay of Bengal (Amakawa et al., 2000). The Nd isotope variations preserved in planktic foraminifera from Site 758 demonstrate the existence of significant variations on glacial–interglacial timescales, that show a close correspondence with the oxygen isotope record (Farrell and Janacek, 1991) suggesting a process controlling the Nd isotopes that responds in phase with global climate cycles (Burton and Vance, 2000). These variations were originally attributed to a change in the flux or composition of Nd from the Ganges– Brahmaputra into the Bay of Bengal (Burton and Vance, 2000), although more recent work with sampling across the Bay has attributed the Nd isotope variations to a shift from the modern dominance of unradiogenic Nd from the Ganges–Brahmaputra to the more radiogenic Nd sources of Arakan coastal rivers and Irrawaddy during the LGM (Stoll et al., 2007). The Os isotope composition of the Brahmaputra is not particularly radiogenic (Sharma et al., 1999) however, the Ganges possesses 187 Os/188 Os ratios that range from 1.6 to 2.9 (Sharma et al., 1999; Levasseur et al., 1999), significantly more radiogenic than estimates for global average riverine input (187 Os/188 Os ≈ 1.38; e.g. Levasseur et al., 1999; Peucker-Ehrenbrink, 2002) or modern seawater itself (187 Os/188 Os = 1.07 ± 0.07; Levasseur et al., 1998; Woodhouse et al., 1999). This raises the possibility that any change in the riverine flux into the Bay of Bengal during the LGM may have had an effect on the Os isotope composition of seawater. Recent Os isotope data for planktic foraminifera from Site 758 does indeed show covariations with the oxygen and Nd isotope records, suggesting a response that is linked to climate change (Burton et al., 2010). The Sr isotope compositions of Himalayan rivers, particularly those of the Ganges–Brahmaputra system, are extremely radiogenic (e.g. Krishnaswami et al., 1992; Galy et al., 1999). Over recent glacial–interglacial cycles it has been shown that glacial sediments are systematically characterised by more radiogenic 87 Sr/86 Sr compositions than those seen in interglacial sediments (Colin et al., 1999, 2006). This has been attributed to an increase in physical erosion in the highlands and efficient transport in the floodplains during glacial stages, due to an increase in the extent of mountain glaciers and a fall in sea level (Colin et al., 2006). In this way, Rb-rich detrital phases (such as biotite) reach the Bay of Bengal without experiencing strong weathering. Therefore, it can be envisaged that changes in global weather-

Planktic foraminifera (N. dutertrei and G. conglobatus) were separated from disaggregated sediments and then cleaned following techniques described elsewhere (Barker et al., 2003; modified after Boyle, 1981; Boyle and Keigwin, 1985). Using this cleaning technique Al/Ca and Mn/Ca ratios were below 20 and 130 μmol/mol, respectively, indicating the effective removal of clays, Fe–Mn oxides, and Mn carbonates from the samples (e.g. Barker et al., 2003, and references therein). Chemical separation of Sr from seawater and foraminifera followed established methods (e.g. Deniel and Pin, 2001). The purified Sr was then dried and loaded on single Re filaments (out-gassed at 4.2 A for 20 min). Approximately 0.7 μL of TaF5 activator was first dried down on the filament and the purified Sr was loaded in 1 μL of 16N HNO3 (cf. Charlier et al., 2006). For the high-precision measurements between 225 and 450 ng of Sr was used for analysis, as it was found that ∼225 ng is the smallest amount of Sr that can ensure a signal of 8–10 V of 88 Sr for 5–6 h. This amount of Sr equates to about 0.4 mg of foraminifera, although ∼1 mg of material was initial separated for analyses. All of the Sr isotope data obtained in this study were measured on a Thermo Scientific Triton thermal ionisation mass spectrometer (TIMS). The virtual amplifier system on the Triton reduces gain differences between different amplifiers by sequentially rotating the amplifier connected to each cup. Under normal circumstances gain differences are of the order 2 ppm, which theoretically limits isotope ratios based on three isotope measurements such as 87 Sr/86 Sr (which needs 86 Sr, 87 Sr and 88 Sr), to an external precision of 7 ppm. However, Nd isotope studies using the Triton have measured 142 Nd/144 Nd ratios with an external precision of 2.4 ppm, by utilising the virtual amplifier system (e.g. Caro et al., 2003, 2006). The precision of the isotope measurement is then largely dictated by ion counting statistics (shot noise) and the effects of instrumental background (Johnson noise from the amplifiers and 1011  resistor) (Birck, 2001). Strontium has four isotopes, with highly variable abundances. For example NBS 987 comprises 84 Sr (0.56%), 86 Sr (9.86%), 87 Sr (7.00%) and 88 Sr (82.58%). The relatively low abundances of three of the isotopes means that a large number of ions need to be collected in order to obtain high precision Sr isotope measurements. This is best met by a combination of long-counting times and relatively high beam intensities so that background effects are reduced on 86 Sr and 87 Sr. It is straightforward to model the likely precision for any strontium isotope ratio, by calculating the precision for each Sr isotope (in volts) (e.g. after Ludwig, 1997). The precision of the Sr isotope ratio can then be calculated using standard error propagation equations (e.g. Powell et al., 1998). Calculations indicate that 540 ratios each measured for 16.894 s (a default value on the Triton), will yield 87 Sr/86 Sr data with an internal precision of ∼3 ppm and an external precision of <5 ppm. In practice, the data for 225 to 450 ng of the standard NBS 987 gave a value of 0.7102557 ± 0.0000035 (2σ (2 standard devia-

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Table 1 87 Sr/86 Sr isotope data for modern seawater.

±2σm b

ppm 2σ

0.7091820 0.7091802 0.7091769 0.7091791 0.7091766 0.7091761 0.7091750 0.7091772 0.7091769 0.7091786

2.43E−06 2.23E−06 2.42E−06 2.40E−06 2.52E−06 2.50E−06 2.65E−06 3.30E−06 2.37E−06 2.62E−06

3.4 3.1 3.4 3.4 3.6 3.5 3.7 4.7 3.3 3.7

8.56 8.43 8.13 7.94 7.50 6.46 4.42 3.66 3.17 2.84

35.20 35.20 35.20 35.20 35.19 35.13 35.01 34.97 34.96 34.98

Indian Ocean WIND-3c 125 0.7091835 450 0.7091806 1200 0.7091835 2750 0.7091828 3753 0.7091786

5.07E−06 2.56E−06 4.99E−06 3.75E−06 3.06E−06

7.2 3.6 7.0 5.3 4.3

17.00 12.86 4.14 2.04 1.09

35.59 35.23 34.38 34.76 34.72

Water depth (m) Atlantic Ocean IB17 25 50 100 200 500 800 1200 1400 1600 1787

Pacific Ocean 3-6Td 105

87

Sr/86 Sra

T (◦ C)

Salinity

0.7091769

2.99E−06

4.2

13.23

34.80

Labrador Sea GR-EST-3e 0−5 0.7091824

2.45E−06

3.5

6 .0

32.40

Average Reproducibility

0.7091792 7.9 ppm

Sr/86 Sr normalised to 86 Sr/88 Sr = 0.1194. Errors represent the internal precision of each measurements, 2 standard errors (2σm ). c Samples from Thomas et al. (2006). d Sample from Woodhouse et al. (1999). e This sample yields a 143 Nd/144 Nd = 0.511316 ± 18 (2σm ), where 143 Nd/144 Nd is normalised to 146 Nd/144 Nd = 0.7219, and the La Jolla Nd standard (20 ng) gave a value of 0.511840 ± 13 (n = 5). Nd measured following techniques described in Burton and Vance (2000). a 87

b

Fig. 2. Water-column data, showing 87 Sr/86 Sr against water depth for samples from the Atlantic, Indian and Pacific oceans, and the Labrador Sea. There is no evidence for any variation outside of analytical uncertainty in present-day seawater compositions, either between oceans or at different water depths within the same ocean (see text). All errors are 2 standard deviations from the mean. The grey line indicates the mean value of all seawater samples.

species. Thus, these data indicate that there has been no resolvable variation in 87 Sr/86 Sr over the past 50 ka, entirely consistent with the data of Henderson et al. (1994) and Ando et al. (2010) when normalised to their given value for modern seawater. However, it can be seen that at the level of measurement uncertainty obtained in the present study this now constrains any variations that might exist to ±4.9 ppm or less. 5. Discussion

tions); n = 23, see appendix 1) or an external precision ±4.9 ppm, close that predicted from model calculations. No adjustment of the sample data to standard values has been made and all analyses are reported as measured. 4. Results 4.1. Modern seawater The 87 Sr/86 Sr isotope, temperature and salinity data for modern seawater samples are given in Table 1. Samples from the North Atlantic show no resolvable variation in 87 Sr/86 Sr with water depth (Fig. 2). The mean 87 Sr/86 Sr isotope composition for these samples is 0.7091778 ± 0.0000021 (3 ppm; n = 10). Similarly, those samples from different depths at the Indian Ocean site also possess an indistinguishable 87 Sr/86 Sr value of 0.7091818 ± 0.0000021 (3 ppm; n = 5), as do the samples from the Pacific and Labrador Sea (Fig. 2). Combining all of these seawater measurements gives an average 87 Sr/86 Sr isotope composition of 0.7091792 ± 0.0000021 (7.9 ppm; n = 17) for the modern ocean (relative to the value obtained here for NBS 987). Therefore, there is no evidence for any variation outside of analytical uncertainty in present-day seawater compositions. 4.2. Glacial interglacial foraminiferal record The 87 Sr/86 Sr isotope data for planktic foraminifera are given in Table 2 and shown in Fig. 3. The 87 Sr/86 Sr data are indistinguishable from the value for modern seawater of 0.7091784 ± 0.0000035 (4.9 ppm; n = 11) and there is no variation with foraminiferal

5.1. Modern seawater Despite the very different Sr isotope compositions being input by rivers to the oceans at the present-day (e.g. Palmer and Edmond, 1989; Allegre et al., 2010; Peucker-Ehrenbrink et al., 2010) the 87 Sr/86 Sr data for the modern seawater shows no resolvable variation outside of present analytical uncertainty, at least not for the samples analysed here. Of particular interest here is the sample from the Labrador Sea, as these waters possess an extremely unradiogenic Nd isotope composition (Stordal and Wasserburg, 1986) consistent with proximal river sources from the very old, adjacent, continental land-masses. This seawater sample yields a 143 Nd/144 Nd isotope composition of 0.511316 ± 18 (Nd = −25.8) (unpublished data; see Table 1), similar to the composition obtained for other surface samples in this area (Stordal and Wasserburg, 1986) and corresponding to a Nd model age of 2.45 Ga. As age is one of the principal controls on the 87 Sr/86 Sr of rocks in such continental shield terrains, it might be anticipated that local sources of Sr to seawater would possess a radiogenic 87 Sr/86 Sr composition, but no difference is observed in the Labrador Sea sample relative to the major oceans. Indeed, the uncertainty for all these data (±7.9 ppm, 2σ ; n = 17) although slightly greater than the long-term reproducibility of the NBS 987 standard (±4.9 ppm, 2σ ; n = 23) is not outside analytical uncertainty. Such isotope homogeneity for Sr in the oceans is entirely consistent with the long residence time of this element in seawater (e.g. Hoddell et al., 1990; Richter and Turekian, 1993) and is a fundamental prerequisite for the application of high-precision Sr isotope stratigraphy (e.g. McArthur and Howarth, 2004).

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Table 2 87 Sr/86 Sr isotope data for foraminifera from ODP Site 758A. Core, section, interval (cm)

Agea (ka)

Species

87

121-758A1H-1, 4–6 1H-1, 12–13 1H-1, 21–23 1H-1, 31–33 1H-1, 42–43 1H-1, 42–43 1H-1, 51–53 1H-1, 60–62 1H-1, 81–83 1H-1, 81–83 1H-1, 81–83

2.7 6.4 10.8 15.1 19.7 19.7 24.3 28.7 38.9 38.9 43.8

N. dutertrei N. dutertrei N. dutertrei N. dutertrei G. conglobatus N. dutertrei N. dutertrei N. dutertrei G. conglobatus N. dutertrei N. dutertrei

0.7091802 0.7091795 0.7091762 0.7091801 0.7091779 0.7091762 0.7091783 0.7091766 0.7091798 0.7091806 0.7091772

Average Reproducibility

Sr/86 Srb

±2σm b

ppm ±2σm c

87 Sr ppmd

2.56E−06 2.74E−06 2.52E−06 2.35E−06 2.46E−06 2.40E−06 3.14E−06 2.35E−06 2.51E−06 2.37E−06 2.38E−06

3.6 3.9 3.6 3.3 3.5 3.4 4.4 3.3 3.5 3.4 3.4

1.5 0.4 −4.4 1.3 −1.9 −4.4 −1.3 −3.8 0.8 1.9 −2.9

0.7091784 4.9 ppm

a The chronostratigraphy used here is based on the correlation of the Site 758 oxygen isotope record from the planktic foraminifera Globigerinoides sacculifer (Farrell and Jancek, 1991), to the global average oxygen isotope record, the SPECMAP stack (Imbrie et al., 1984). This chronostratigraphy is consistent with biostratigraphic data (Shipboard Scientific Party, Site 758, 1989) and the disappearance of the pink variety of Globigerinoides ruber at 120 kyr.

Sr/86 Sr normalised to 86 Sr/88 Sr = 0.1194. Errors represent the internal precision of each measurements, 2 standard errors (2σm ). 87 Sr (ppm) = ((87 Sr/86 Srsample /87 Sr/86 Srseawater ) − 1) × 106 .

b 87 c d

87

Fig. 3. 87 Sr/86 Sr isotope data for planktic foraminifera (N. dutertrei and G. conglobatus) shown against age (ka). The 87 Sr/86 Sr data are indistinguishable from the value for modern seawater (Fig. 2) and there is no variation between foraminiferal species. Thus, these data indicate that there has been no resolvable variation in 87 Sr/86 Sr over the past 40 ka, consistent with the data of Henderson et al. (1994) and Ando et al. (2010). However, at the level of measurement uncertainty obtained in the here this now constrains any variations that might exist to ±4.9 ppm or less. (87 Sr (ppm) = ((87 Sr/86 Srsample )/ (87 Sr/86 Srseawater ) − 1) × 106 .)

5.2. Glacial–interglacial variations in continental weathering The 87 Sr/86 Sr isotope data for planktic foraminifera from ODP Site 758 show no resolvable variation over the past ∼45 kyr (Fig. 3). However, at this site other long-lived natural radiogenic isotopes, such as Os and Nd, with shorter residence times in the oceans do show variations that are thought to reflect changes in the flux or composition of continental material delivered to the oceans (Burton and Vance, 2000; Stoll et al., 2007; Burton et al., 2010). Moreover, there are a number of lines of evidence to suggest that the flux and composition of Sr to the oceans will also change over Quaternary glacial–interglacial cycles, even if a seawater response cannot be detected, such as changes in dissolution and weathering rates associated with the changing climate (see introduction). Rock type also influences chemical weathering by determining the presence of minerals with differing weathering rates (e.g. Bluth and Kump, 1994), and rock type and age will also influence the Sr isotope composition. For example, felsic rocks (such as granites) often possess relatively high Rb/Sr ratios and evolve to radiogenic

Sr/86 Sr compositions over time whereas mafic and ultramafic rocks (such as basalt and abyssal peridotites) generally possess low Rb/Sr ratios and evolve to relatively unradiogenic 87 Sr/86 Sr ratios. During glacial intervals much of the continental crust was covered with large ice sheets decreasing the surface area of continental crust available for weathering (about 25% of the continental surface (Kump and Alley, 1994)). Moreover much of that continental surface comprised ancient shield rocks (of a felsic composition) exposed at high-latitudes, often possessing radiogenic Sr isotope compositions. Therefore, it might also be anticipated that during glacial intervals the signal from continental weathering would be less radiogenic than present-day because such radiogenic shieldrocks were isolated from weathering by ice-sheet cover. A more radiogenic weathering signal would return as the ice-sheet retreated. It has also been suggested that during glacial cycles preferential weathering of minerals themselves may play a role in changing the flux and composition of Sr delivered by continental weathering (Blum and Erel, 1995). During deglaciation the fine-grained, little weathered, glacial till left behind by retreating ice sheets would be highly susceptible to weathering, particularly in the presence of large quantities of melt water and increasingly warmer temperatures (Blum and Erel, 1995). Data suggests that the initial weathering of this fine-grained soil-parent material will release highly radiogenic Sr, due to the preferential breakdown of biotite (and its alteration product vermiculite) both of which possess very high (parent/daughter) Rb/Sr ratios (Blum and Erel, 1995). As the soils age over the interglacial the amount of weathering would decrease, and there would be a gradual shift to less radiogenic Sr being released. Blum and Erel (1995) suggest that the enhanced flux and radiogenic isotope composition released by the deglacial weathering of fine-grained glacial till could account for an increase in the 87 Sr/86 Sr of 0.002 during periods of glacial–interglacial cycling, and that this, in turn, could account for the longer term change in 87 Sr/86 Sr of 0.05 ppm kyr−1 over the past 2.5 Myr. Field and laboratory observations also suggest that freshly exposed fine-grained material is highly reactive when initially exposed chemical weathering, but that rates decrease dramatically with time (Taylor and Blum, 1995; White and Brantley, 1995; Porder et al., 2007). The slope of the power law relationship between weathering rate and exposure time for post-glacial soils is similar to that obtained from laboratory experiments on both granite and basalt. Using the relationship between soil age and silicate

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weathering rate, it has been estimated that if such a pulse of rapid weathering was initiated during the last deglaciation (∼18 ka ago) then it will not yet have decayed away and weathering rates at the present-day will be about twice the average for an entire late Quaternary glacial cycle (Vance et al., 2009). On this basis it has been suggested that a post-glacial weathering pulse not only accounts for the long-term rise in 87 Sr/86 Sr over the Quaternary, but also the apparent imbalance of Sr inputs from estimates of riverine flux and hydrothermal exchange at the present-day (Davis et al., 2003; Vance et al., 2009). However, more recently it has been argued that this apparent imbalance may simply indicate that the riverine and groundwater input from basalt terrains, in particular, basaltic islands (that are highly susceptible to weathering) has not been properly accounted for in estimates of the global river flux (Allègre et al., 2010; Jones et al., 2012). 5.3. Limits on glacial–interglacial weathering variations from seawater Sr During the Cenozoic, one of the periods of steepest rise in Sr/86 Sr has been over the past 2.5 Myr (Hodell et al., 1990; Capo and DePaolo, 1990). The intensification of Northern hemisphere glaciation around 2.5 Myr ago (e.g. Shackleton et al., 1984; Tiedemann et al., 1994), was accompanied by an increase in marine 87 Sr/86 Sr at a rate of 0.05 ppm kyr−1 , taken by many as an indication that glaciation must play a role in the marine Sr cycle (Hodell et al., 1990; Capo and DePaolo, 1990; Blum and Erel, 1995). Fig. 4b shows the data obtained here and that of Henderson et al. (1994) and Ando et al. (2010) illustrating the long-term variations in marine 87 Sr/86 Sr over the past 400 kyr. The 87 Sr/86 Sr isotope data for planktic foraminifera obtained here show no resolvable variation through the last glacial cycle (Fig. 3). Glacial– interglacial variations in the weathering flux or composition may still exist, but the new data constrains the maximum amplitude of the seawater response to ±4.9 ppm. If our new data are representative of Sr isotope variation during glacial–interglacial events, then we might expect no resolvable fluctuation around the longer-term rise in 87 Sr/86 Sr over the past 400 kyr (Fig. 4b), or at least no variation that is capable of generating shifts in 87 Sr/86 Sr greater than ±4.9 ppm, although this clearly needs to be tested with a longer high-precision Sr isotope record. The contours on Fig. 4b (of 3, 6 and 9 ppm) illustrate the permissible extent of those fluctuations about the long-term mean over the past 400 kyr. Some limit on the variations in silicate chemical weathering rates permitted by the data obtained in this study can be made using the modelling approach outlined by Vance et al. (2009) (see also Blum and Erel, 1995). In this model it is assumed that about 25% of the currently ice-free continental area was covered by large ice-sheets during Quaternary glacial periods, including the last glacial maximum 18 ka ago. Chemical weathering rates beneath the ice were considered to have been negligible, although physical weathering rates will have been high. Therefore, the retreat of the continental ice-sheets would leave large amounts of fresh finely grained minerals in soils and glacial tills that are highly susceptible to chemical weathering. The proportion of the glaciated regions covered by ice at any time is assumed to vary linearly with δ 18 O, and the retreat of the ice sheets is simulated with a 1 ka resolution. The average chemical weathering for the area exposed to weathering is calculated using the relationship between soil age and silicate weathering rate (Taylor and Blum, 1995; White and Brantley, 1995; Porder et al., 2007) (see also Fig. 3 in Vance et al., 2009 and supplementary material). An assessment of the magnitude of the variations in the 87 Sr/86 Sr composition of the riverine input can be made from a comparison of rivers draining glaciated and non-glaciated terrains (see compila87

Fig. 4. (a) Change in river flux assuming a post-glacial weathering pulse. The two curves plotted illustrate the difference in having constant weathering of shelf carbonate (red curve) or shelf weathering linked to sea level changes (blue curve) (flux variations calculated after the model of Vance et al., 2009; also see supplementary data). (b) Modelled seawater 87 Sr/86 Sr response to a post-glacial pulse in weathering for the flux variations given in panel a. Also shown are the 87 Sr/86 Sr isotope data for foraminifera from this study, Henderson et al. (1994) and Ando et al. (2010) for the past 400 ka. The contours for variations of 3, 6 and 9 ppm about the longterm increase over the Quaternary are indicated in green, light blue and dark blue respectively. Note that the new data obtained here constrain variations about the long-term mean to a maximum amplitude of ±4.9 ppm which is much smaller than the shift indicated by the model calculations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

tion in Vance et al., 2009). These data suggest that rivers draining glaciated terrains possess an average 87 Sr/86 Sr composition of 0.71177 compared to those draining non-glaciated terrains that have a value of 0.71134. By comparison, leaching experiments indicate that incongruent weathering of glacial tills and soils derived from granites and gneisses (typical of continental shield rocks) can release waters with an 87 Sr/86 Sr composition as high as 0.794667 (some 9% higher than the 87 Sr/86 Sr for the bulk soil) (Blum and Erel, 1995). We have performed a series of model calculations for the evolution of seawater 87 Sr/86 Sr that take into account change in weathering rate (see supplementary information). These model results indicate that such a large spike in weathering rate during initial ice retreat results in 87 Sr/86 Sr variations of ∼15 ppm over an individual glacial cycle, with much of the variation in the first 20 ka; such variations cannot be accommodated by the new high-precision data from this study (see Fig. 4). It has been suggested that sealevel low stands during glacial intervals lead to an increased flux of Sr to the oceans through the dissolution of shelf carbonate and the recrystallisation of aragonite to calcite (the latter having a lower concentration of Sr) (Stoll and Shrag, 1998). A shelf flux is an intrinsic part of the models of Vance et al. (2009) as it provides an unradiogenic source to replace the reduction in the hydrothermal

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flux suggested by Davis et al. (2003), but a lack of shelf flux during ice-retreat also enhances the rate of change of 87 Sr/86 Sr during interglacial periods (see Fig. 4) resulting in even greater shifts that are not observed in the data. It was argued by Krabbenhöft et al. (2010) that because of this shelf flux the Sr stable isotope composition, 88 Sr/86 Sr, of the weathering signal would have been 70 ppm lighter during glacial intervals than the riverine signal at the present day. However, we note here that the postulated shift of 70 ppm is for the input of Sr to seawater, not seawater itself. Such a variation in input is small and will have little impact on the overall 88 Sr/86 Sr composition of seawater (<3 ppm) on glacial timescales, which, in turn, translates to <1.5 ppm on the 87 Sr/86 Sr isotope composition. The flux and composition estimates of Vance et al. (2009) were based on a present-day riverine input of 3.37 × 1010 mole per year yielding a steady-state residence time of ∼2.8 Myr. A more recent estimate of river inputs of 4.7 × 1010 mole per year, but with a lower 87 Sr/86 Sr ratio of 0.7111 (Peucker-Ehrenbrink et al., 2010), gives a residence time of ∼2.1 Myr; modelling of seawater 87 Sr/86 Sr evolution with these values yields curves with a similar structure to those illustrated in Fig. 4, but with a slightly larger amplitude of ∼6 ppm (see supplementary information). Thus, at first sight the new data obtained here does not support the model of Vance et al. (2009). However, other changes in both the weathering flux and composition cannot be ruled out. Glacial and geological observations indicate that substantial recession of the Laurentide ice sheet did not occur until about 14 ka ago, and not until 10–11 ka in the High Arctic, but in any case was a gradual process that continued until ∼7 kyr ago (e.g. Dyke et al., 2002; Clark et al., 2001; Carlson and Clark, 2012). Modelling of northern hemisphere ice sheet behaviour does indeed indicate a notable pulse in the freshwater flux at 14.3 ka, attributed to increases in surface runoff from the ice sheet during deglaciation (Zweck and Huybrechts, 2005). However, the same models also indicate a sustained and continuous freshwater flux between 12 and 9 ka (∼50% higher than modern values), attributed to the gradual disintegration of the North American ice sheet (Zweck and Huybrechts, 2005). Other estimates of runoff from models of ice sheet melting, taking into account net moisture changes, suggest that runoff from the Laurentide ice sheet remained more or less constant throughout deglaciation until the final demise of the ice sheet at ∼7 ka, at which time the flux decreased by ∼30% to modern values (see Clark et al., 2001). By comparison, actual sea-level changes during deglaciation based on coral reef records in Barbados have been taken to indicate a meltwater pulse at 14 ka followed by a slower rise during the Younger Dryas event, a further pulse at 11.3 ka and a systematic rise in sea-level thereafter (Peltier and Fairbanks, 2006). Although other coral records preserve no evidence for the 11.3 ka meltwater pulse (e.g. Cutler et al., 2003; Bard et al., 2010) they do show the same broad changes in sealevel that point to systematic rises in sea-level during deglaciation, rather than a singular meltwater pulse. Taken together, these geological observations, paleoceanographic data and model results indicate that ice sheet recession was a protracted process, as were changes in runoff. Consequently, it seems likely that changes in the flux and composition of weathered material exposed by ice sheet retreat will have also occurred gradually during deglaciation, consistent with the variations in Os seen at ODP Site 758 (and elsewhere) which closely follow the δ 18 O record (Burton et al., 2010). In this scenario, chemical weathering rates will have been high throughout deglaciation as freshly weathered material is continuously exposed, rather than simply being linked to a singular pulse in the weathering rate. In order to model this scenario we have taken two approaches (see supplementary data) and in both cases the models produce seawater 87 Sr/86 Sr curves that are accommodated by the new sea-

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Fig. 5. (a) Changes in river flux assuming that weathering changes closely followed the δ 18 O record through Quaternary glacial cycles (blue curve) or a more diffuse post-glacial weathering pulse (red curve) assuming a constant weathering of shelf carbonate (see text). (b) Modelled seawater 87 Sr/86 Sr in response to changes in the weathering flux and composition that follow the δ 18 O record and diffuse postglacial weathering pulse. Note that both models are permissible with the available data (see text). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

water data through the last glacial cycle (i.e. ±4.9 ppm). The first model adapts that of Vance et al. (2009), but produces a more diffuse weathering pulse, consistent with the glacial and geological observations outlined above, but only having maximum shifts in riverine flux of <±15% from the mean Quaternary value (Fig. 5). The second model utilises the δ 18 O record as a proxy for ice-volume and therefore riverine flux, and the high-precision 87 Sr/86 Sr curve allows a variation in riverine flux of ±12% around a mean Quaternary value (Fig. 5). This particular modelled 87 Sr/86 Sr record additionally suggests that there may be some structure in the 87 Sr/86 Sr record at the ±6 ppm level including elevated values between 70–110 ka (consistent with the data of Ando et al., 2010) and 200–290 ka, which could be tested with further high-precision analyses. Further modifications of the model were also assessed. These included, adding a short-weathering pulse to the model using the δ 18 O record, and using an ice sheet model (Gregoire et al., 2012) rather than the δ 18 O record to mimic ice volume. The first model produces much larger variations in the 87 Sr/86 Sr record for the last 45 ka than our new data can accommodate, whereas the latter model produces a reasonable fit the our data but larger amplitude variations in the longer term Sr isotope curve (see supplementary data). There are some clear conclusions that can be derived from these results. Our modelling limits the variation in riverine flux to <±15% over the last 50 kyr. If there is a post-glacial weathering pulse it must be accommodated within the variation of the riverine flux; this is most readily done if the weathering pulse is a diffuse rather than a short event. However, all of the models allow the current riverine flux to be higher than the Quaternary average (e.g. Vance et al., 2009). Moreover, this non-steady steady-

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state behaviour can account for the long-term rise in 87 Sr/86 Sr over the Quaternary (Fig. 5) yielding a long-term seawater evolution indistinguishable from that of models that include a post-glacial weathering pulse. Alternatively, if the present-day flux to the oceans is much closer to steady-state due to the flux from basaltic islands (Allègre et al., 2010) then significant changes in the weathering rate are not permitted. This is primarily because the basaltic island flux is assumed to be the missing unradiogenic Sr isotopic source to the oceans (see supplementary information) and any additional large variations in silicate flux are difficult to accommodate, because they would drive the Sr seawater curve to higher values than is currently observed. Modelling using estimates of the flux from basaltic islands from Li and Elderfield (2013) restricts the variations in the chemical weathering flux to be less than 10%, a conclusion consistent with a simple scaling of the modelling of Henderson et al. (1994) and taking into account the increased level of precision in this study. Any variations that did exist would more likely reflect changes in the composition of the continental signal due to changes in the balance of continental rock types being weathered (i.e. basaltic vs. felsic rocks) rather than a change in the flux itself. Finally, any model that tries to include both the flux from basaltic islands and a shelf-weathering flux produces some interesting model observations. Mass balance of these unradiogenic sources requires the average Quaternary riverine flux to be higher than the present day flux. This goes against the model of Vance et al. (2009) and it seems unlikely that the riverine flux would be higher during glacial periods than the present day. A more likely scenario is that the estimates of the basaltic island and shelfweathering flux are too large and need to be reassessed. To conclude, the very high-precision measurements made in this study for the marine 87 Sr/86 Sr record provide some constraint on both the timing and magnitude of changes in the chemical weathering flux during glacial–interglacial cycles, although with some important caveats. These constraints can be quantified further when there are better estimates for Sr flux from carbonates on the shelf and the role of basalt weathering on ocean islands is better understood (e.g. Jones et al., 2012, 2014), as both these sources are major reservoirs of unradiogenic Sr that affect the residence time of Sr and the balance of 87 Sr/86 Sr in the oceans. Critically, the shelf and/or ocean island weathering flux keep the residence time of Sr in the range of 2–3 Ma, which in turn limits the range of weathering flux to ±15%, a value that could be greater if the residence time was significantly longer. It is interesting to note that an independent estimate of glacial–interglacial variations in silicate weathering may be derived from global-weathering models because they require an estimate of the silicate weathering flux to model bicarbonate fluxes. Munhoven (2002) produced estimates using a number of global-weathering models and global-circulation models and concluded that a limited but statistically significant variation of 5–10% in silicate weathering flux over glacial–interglacial timescales was compatible with these models; a value similar to that obtained here. 5.4. Strontium, osmium and neodymium isotope records through the last glacial cycle at Site 758 The marine 87 Sr/86 Sr record obtained here can be compared with the 143 Nd/144 Nd and 187 Os/188 Os from foraminifera from the same time interval from the same site (Fig. 6). This comparison illustrates the differential response of these isotopes in seawater to variations in continental weathering, principally as a function of elemental residence time in the oceans. For Nd (Fig. 6a), with a relatively short residence time of around 500 yr (e.g. Tachikawa et al., 2003), variations in 143 Nd/144 Nd closely follow the δ 18 O, indicating that the process controlling Nd

Fig. 6. (a) 143 Nd/144 Nd in G. menardii (Burton and Vance, 2000). (b) 187 Os/188 Os in mixed planktic foraminifera (Burton et al., 2010). (c) 87 Sr/86 Sr in N. dutertrei and G. conglobatus (this study) from Site 758 pelagic carbonates shown against age (ka). Data for modern seawater for 143 Nd/144 Nd in the Bay of Bengal (Amakawa et al., 2000; Singh et al., 2014); 187 Os/188 Os in the west Indian ocean and Bay of Bengal (Levasseur et al., 1998; Sharma et al., 2007, respectively); 87 Sr/86 Sr in modern seawater (this study).

isotopes is in phase with global climate change. These Nd variations can be most readily accounted for by a change in the flux or composition of riverine Nd delivered to the nearby Bay of Bengal, principally the Ganges, Brahmaputra and Irrawaddy, accompanying climate change (Burton and Vance, 2000; Stoll et al., 2007). For Os (Fig. 6b), with an intermediate residence time of ∼37 kyr, 187 Os/188 Os variations also closely follow the δ 18 O record, again indicating a response closely linked to global climate change, although weathering of lithologies with distinct Os isotope compositions may also play a role in this correlation (Georg et al., 2013). Finally, for Sr (Fig. 6c), with a residence time of ∼2.1 Myr or longer, 87 Sr/86 Sr variations cannot be resolved at the ±4.9 ppm level, although it is reasonable to assume that variations in both the flux and composition of weathered material must have accompanied glacial climate change (see discussion above). 6. Conclusions The results presented here for modern seawater from each of the major oceans indicate no resolvable variation outside of the present analytical uncertainties (±4.9 ppm 2σ ) despite the very

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large compositional and geographical differences in continental input (from rivers, groundwaters and aeolian input). Such a result is entirely consistent with the very long residence time of Sr in the oceans, and underpins the utility of this element for high-precision isotope stratigraphy. The 87 Sr/86 Sr results for planktic foraminifera through the last glacial maximum also show no resolvable variation within the level of analytical uncertainty of this study, and limit the response of seawater to variations in silicate chemical weathering flux (or composition) to ±4.9 ppm or less. The level of precision accommodates recently proposed models of changing flux and composition during Quaternary glacial–interglacial cycles, but requires either a more diffuse postglacial spike in the chemical weathering flux or a model where the flux and composition simply varies with oxygen isotopes, notwithstanding the necessity for a process that can account for the longer-term rise in 87 Sr/86 Sr over the past 2.5 Myr. Acknowledgements The samples for this study were provided by the Ocean Drilling Program (ODP Leg 121 Site 758). We would like to thank Alex Thomas for the provision of seawater samples from the Indian Ocean and Greg Ravizza for the Pacific seawater sample. We would like to thank the two anonymous reviewers who provided constructive comments on the manuscript and the editorial input of Gideon Henderson. This work was supported by the Natural Environment Research Council (NERC ref. NE/B502701/1). Appendix A. Supplementary material Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2015.01.036. References Allègre, C.J., Louvat, P., Gaillardet, J., Meynadier, L., Rad, S., Capmas, F., 2010. The fundamental role of island arc weathering in the oceanic Sr isotope budget. Earth Planet. Sci. Lett. 48, 148–154. Amakawa, H., Alibo, D.S., Nozaki, Y., 2000. Nd isotopic composition and REE pattern in the surface waters of the eastern Indian Ocean and its adjacent seas. Geochim. Cosmochim. Acta 64, 1715–1727. Ando, A., Nakano, T., Kawahata, H., Yokoyama, Y., Khim, B.-H., 2010. Testing seawater Sr isotopic variability on a glacial–interglacial timescale: an application of latest high-precision thermal ionization mass spectrometry. Geochem. J. 44, 347–357. Archer, D., Winguth, A., Lea, D., Mahowald, N., 2000. What causes the glacial/interglacial atmospheric pCO2 cycles? Rev. Geophys. 38, 159–189. Bard, E., Hamelin, B., Delanghe-Sabatier, D., 2010. Deglacial meltwater Pulse 1B and Younger Dryas sea levels revisited with boreholes from Tahiti. Science 327, 1235–1237. Barker, S., Greaves, M., Elderfield, H., 2003. A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry. Geochem. Geophys. Geosyst. 8407. http://dx.doi.org/10.1029/2003GC000559. Birck, J.-L., 2001. The precision and sensitivity of thermal ionisation mass spectrometry (TIMS): an overview of the present status. Geostand. Newsl. 25, 253–259. Blum, J.D., Erel, Y., 1995. A silicate weathering mechanism linking increase in marine 87 Sr/86 Sr with global glaciation. Nature 373, 415–418. Bluth, G., Kump, L., 1994. Lithologic and climatologic controls of river chemistry. Geochim. Cosmochim. Acta 58, 2341–2359. Boyle, E.A., 1981. Cadmium, zinc, copper and barium in foraminifera tests. Earth Planet. Sci. Lett. 53, 11–35. Boyle, E.A., Keigwin, L.D., 1985. Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: changes in deep ocean circulation and chemical inventories. Earth Planet. Sci. Lett. 76, 135–150. Brass, G.W., 1976. The variation of the marine 87 Sr/86 Sr ratio during Phanerozoic time: interpretation using a flux model. Geochim. Cosmochim. Acta 40, 720–730. Broccoli, A.J., Dahl, K.A., Stouffer, R.J., 2006. Response of the ITCZ to northern hemisphere cooling. Geophys. Res. Lett. 33, L0172. http://dx.doi.org/10.1029/ 2005GL024546. Burton, K.W., Vance, D., 2000. Glacial–interglacial variations in the neodymium isotope composition of seawater in the Bay of Bengal recorded by planktonic foraminifera. Earth Planet. Sci. Lett. 176, 425–441.

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