A continuous ice-core 10Be record from Mongolian mid-latitudes: Influences of solar variability and local climate

Earth and Planetary Science Letters 437 (2016) 47–56

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Earth and Planetary Science Letters www.elsevier.com/locate/epsl

A continuous ice-core 10 Be record from Mongolian mid-latitudes: Influences of solar variability and local climate F. Inceoglu a,b,∗ , M.F. Knudsen b , J. Olsen c , C. Karoff a,b , P.-A. Herren d,e , M. Schwikowski d,e , A. Aldahan g,f , G. Possnert h a

Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark Department of Geoscience, Aarhus University, Aarhus, Denmark AMS, 14 C Dating Centre, Department of Physics, Aarhus University, Aarhus, Denmark d Paul Scherrer Institut, Villigen, Switzerland e Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland f Department of Earth Sciences, Uppsala University, Sweden g Department of Geology, United Arab Emirates University, Al Ain, United Arab Emirates h Tandem Laboratory, Uppsala University, Sweden b c

a r t i c l e

i n f o

Article history: Received 29 October 2014 Received in revised form 6 January 2016 Accepted 6 January 2016 Available online 18 January 2016 Editor: G.M. Henderson Keywords: cosmogenic nuclide beryllium 10 mid-latitude past solar activity climate

a b s t r a c t High-resolution 10 Be records used for studies of detailed changes in atmospheric 10 Be production rates predominantly derive from polar ice cores. In this study, we present the first 10 Be record from a midlatitude ice core. The ice core derives from the Tsambagarav mountain range located in the Mongolian Altai region. The new 10 Be concentration record spans the period from AD 1550 to 2009, while the flux record extends from AD 1816 to 2009. The 10 Be concentration in the Tsambagarav ice core ranges between ∼1.5 × 104 and ∼10 × 104 atoms g−1 , whereas the 10 Be flux changes from ∼0.02 to ∼0.15 atoms cm−2 s−1 . The average 10 Be flux at Tsambagarav is four times higher than the average 10 Be flux recorded in the NGRIP and Dome Fuji ice cores, which is in accordance with model predictions. In general, the long-term trends observed in the Tsambagarav 10 Be concentration and flux records are reasonably similar to those observed in the NGRIP ice core. A comparison between the Tsambagarav 10 Be record, group sunspot numbers (GSNs), and solar modulation potentials based on 14 C in tree rings suggests that the Maunder Minimum was associated with a prolonged maximum in 10 Be concentrations at Tsambagarav, whereas the Dalton Minimum was associated with a minor increase in the 10 Be concentration and flux that was delayed relative to the primary minimum in GSNs. The sulphate record from Tsambagarav shows that large positive anomalies in the sulphate concentration are associated with negative anomalies in the 10 Be concentration. A concurrent positive sulphate anomaly may explain why the main phase of the Dalton Minimum is subdued in the 10 Be record from Tsambagarav. Spectral analysis indicates that the 11-yr solar-cycle signal may have influenced the new 10 Be record, but the evidence supporting a direct link is ambiguous. Local and regional climatic changes, such as cyclonic versus anticyclonic conditions and related storm tracks, most likely played a significant role for the 10 Be deposition in the Tsambagarav region. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Past production rates of cosmogenic nuclides recovered from terrestrial archives, such as 10 Be in ice cores and 14 C in tree rings, are widely used to acquire information on past solar variations during the pre-telescopic era prior to 1610 AD (Bard and Frank, 2006; Muscheler et al., 2007; Steinhilber et al., 2012;

*

Corresponding author. E-mail address: [email protected] (F. Inceoglu).

http://dx.doi.org/10.1016/j.epsl.2016.01.006 0012-821X/© 2016 Elsevier B.V. All rights reserved.

Usoskin, 2013; Inceoglu et al., 2014). Changes in solar activity have additionally been identified in 10 Be and 14 C records from lake sediments (Staff et al., 2011; Berggren et al., 2013). Cosmogenic nuclides are produced mainly in the lower stratosphere and the upper troposphere by interactions of galactic cosmic ray (GCR) particles from space with atmospheric elements, such as N and O (Lal and Peters, 1967; Masarik and Beer, 1999, 2009). Their production rates are inversely correlated with the solar magnetic activity and the geomagnetic field intensity due to the nonlinear shielding effect of the solar magnetic field and the geomagnetic dipole

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Fig. 1. Location of the Tsambagarav coring site in Mongolian Altai, red star, 4130 m asl 48◦ 3 N90◦ 51 E (http://www.geomapapp.org). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

field (Lal and Peters, 1967). A strengthening of the solar magnetic and geomagnetic fields thus results in a lower production rate of cosmogenic nuclides and vice versa (Masarik and Beer, 1999). About two-thirds of the total atmospheric 10 Be production takes place in the stratosphere, whereas the remaining one-third is produced in the troposphere (Lal and Peters, 1967). Masarik and Beer (1999) estimated these values as 56% and 44%, respectively, while a more recent study by Heikkilä et al. (2009) suggests that 65% of the total atmospheric 10 Be is produced in the stratosphere and the remaining 35% is produced in the troposphere. Following its production, 10 Be is rapidly adsorbed mainly onto atmospheric sulphate aerosols. After a residence time of one to two years in the lower stratosphere (Raisbeck et al., 1981; Heikkilä et al., 2013), the aerosols are transported into the lower troposphere by air mass exchanges taking place between the troposphere and stratosphere at mid-latitudes (Koch and Rind, 1998). It has also been suggested that direct stratosphere–troposphere exchange of 10 Be takes place over the Antarctic regions, but evidence for such a link in the Arctic is absent (Pedro et al., 2011). Following a residence time around three weeks in the troposphere (Heikkilä et al., 2013), the aerosols are eventually deposited at the surface by both dry and wet deposition. The concentration of 10 Be in terrestrial archives is therefore not only influenced by changes in production rate caused by variable solar activity, but also by transport and deposition processes, atmospheric mixing, scavenging efficiency, and snow accumulation rates at the coring site (Heikkilä et al., 2008b; Baroni et al., 2011; Pedro et al., 2012). As the atmospheric exchange and transport processes play an important role for the 10 Be flux at the Earth’s surface, there is a maximum in 10 Be deposition at mid-latitudes (Field et al., 2006). By contrast, all the currently available high-resolution 10 Be records derive from the polar regions associated with Antarctica and Greenland. Our current understanding of past changes in 10 Be production and deposition at mid-latitudes is therefore very limited. In this study, we present the first continuous, high-resolution 10 Be measurements from a mid-latitude mountain glacier ice core. The primary objective is to map past changes in mid-latitude 10 Be deposition, and assess whether the average 10 Be deposition in this

mid-latitude region is higher compared to polar regions, as suggested by Field et al. (2006). This study also aims to investigate the degree to which solar variability and local climate influenced 10 Be deposition at this location over the past five centuries, i.e. across the two grand solar minima known as the Maunder Minimum (1645–1715 AD) and Dalton Minimum (1795–1825 AD). Finally, a new continuous 10 Be record from a mid-latitude ice core may potentially improve our understanding of past changes in the global 10 Be production rate, which may contribute to a better understanding of past solar activity, including the amplitude of grand solar maxima and minima. 2. Location and age model The Tsambagarav ice core was collected in 2009 in the Tsambagarav mountain range located in the Mongolian Altai (Fig. 1, red star, 4130 m asl, 48◦ 39 N90◦ 51 E). The climate over the Altai mountain region can generally be characterised by cold and dry winters associated with the Siberian Anticyclone (SA), which is a high-pressure system that develops from October to March (Sahsamanoglou et al., 1991; Klinge et al., 2003). The area is also characterised by relatively warm summers, when most of the precipitation occurs in association with stationary cyclones that transport moisture from the Pacific Ocean to the Altai region (Aizen et al., 2005). 2.1. Ice-core chronology and accumulation rates The Tsambagarav ice core extends back to 6000 BP but the deeper parts of the core are strongly influenced by thinning and deformation of the ice (see Herren et al., 2013, for details). Dating of the Tsambagarav ice core was performed using a combination of different techniques, including identification of reference horizons based on 3 H and major volcanic eruptions, annual layer counting (ALC), radioactive dating using 210 Pb and 14 C, and glacier flow modelling (see Herren et al., 2013, for details). Strong thinning of the ice impeded ACL and identification of volcanic ash horizons in the period before 1815 AD. The uncertainty of the ACL in the

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period between 1815–2009 AD is estimated to ±1 yr within one decade of the identified horizons, with the uncertainty increasing to ±2–3 yr outside these ranges, i.e. 1974–1998, 1944–1952, 1865–1902, and 1826–1843 (Herren et al., 2013). For the period 1700–1815 AD, the uncertainty of the age-depth scale of the Tsambagarav ice core is ±17 yr, while it is ±55 yr for the period 1500–1700 AD (Herren, 2013). The average time resolution of the new 10 Be record from the Tsambagarav ice core is 3.5 yr, varying between annual to biannual resolution in the uppermost section to roughly decadal resolution in the lowermost section. The accumulation rates has been reconstructed for the Tsambagarav ice core in the period between 1815–2009 AD (Fig. 2(b)), using the ratio of the measured annual layer thickness to the modelled thickness multiplied by the surface accumulation rate. Measured annual layer thicknesses and modelled thicknesses were obtained by ACL and glacier flow models, respectively (Herren et al., 2013). 3. Methods In order to extract the 10 Be signal stored in the Tsambagarav ice core during the period from 1550 to 2009 AD, we collected 131 continuous ice samples with weights ranging from ∼110 to ∼450 g and an average weight of ∼250 g. The samples were then melted and filtered. Prior to melting the samples, 100 μl of 9 Be carrier (Scharlau BE03450100, 1000 mg/l) was added to each sample. The 9 Be carrier was not acidified and no acid was added to the samples at this stage to avoid release of 10 Be from dust. The pore size of the filter is important as the 10 Be atoms can be transported and re-deposited by dust particles. Previous studies of ice cores from Greenland show that the dust-associated component of 10 Be retained by 0.45 μm filters was negligible (<5%) during the Holocene period (last ∼11.7 kyr), whereas up to ∼50% of the total 10 Be concentrations were associated with dust during the last glacial period, when the continental dust concentrations in polar ice cores were significantly higher (Baumgartner et al., 1997; Yiou et al., 1997; Finkel and Nishiizumi, 1997). Since it is expected that a mid-latitude ice core contains more dust than polar ice cores, it is likely that 0.45 μm filters will remove a significant fraction of the total 10 Be content in the samples from Tsambagarav. The application of filters with a fine pore size is needed, however, for the purpose of investigating past 10 Be production rates, because the influence of dust-borne 10 Be, in particular 10 Be adhered to remobilised dust, should be minimised. Importantly, the use of 0.45 μm filters also makes it easier to compare 10 Be data from Tsambagarav with existing 10 Be records from polar regions. Consequently, filters with a pore size of 0.45 μm were applied to all 131 samples in this study, i.e. a filtering procedure that is identical to that used for the 10 Be extraction of the GRIP (Greenland Ice Core Project) (Yiou et al., 1997) and the GISP2 (Greenland Ice Sheet Projects 2) (Finkel and Nishiizumi, 1997) ice cores. The geochemical sample preparations were carried out at the Paul Scherrer Institute (PSI), Switzerland, and at the Department of Geosciences, Aarhus University, following the procedures used by Berggren et al. (2009). Subsequently, the 10 Be measurements were performed using the Uppsala University 5 MV AMS system and the NIST SRM 4325 standard (10 Be/9 Be = 3.03 × 10−11 , internally standardised value) at machine and statistical errors of <15%. At the Uppsala Tandem laboratory, the NIST value of 3.03 × 10−11 has been obtained through calibration with an internally irradiated 10 Be standard. Blanks were prepared following the sample procedure of Berggren et al. (2009) and the full chemistry blank 10 Be/9 Be ratio was ∼6 × 10−14 , i.e. smaller than the ice-sample average of ∼2 × 10−12 .

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4. Results 4.1.

10

Be concentrations and fluxes

The 10 Be concentration in the Tsambagarav ice core ranges between ∼1.5 × 104 and ∼10 × 104 atoms g−1 , with a mean concentration of ∼4.6 × 104 atoms g−1 (Fig. 2(a)), while the 10 Be fluxes vary between ∼0.02 and ∼0.15 atoms cm−2 s−1 with an average of ∼0.04 atoms cm−2 s−1 (Fig. 3(a)). The Tsambagarav 10 Be fluxes were calculated for the period ranging between 1816 and 2009 AD, since the accumulation rates are only known with sufficient resolution during this period (Herren et al., 2013). The Tsambagarav 10 Be concentrations and fluxes were smoothed using the lowess (locally weighted scatter plot smooth) method with a span of 10% and subsequently standardised according to their individual mean and standard deviation values (Fig. 4(a) and 4(b)). The Tsambagarav 10 Be concentrations show a clear peak coinciding with the low activity level of the Sun associated with the Maunder Minimum, and a slight increase towards the end of the Dalton Minimum (∼1825) (Fig. 4(a)) as defined by changes in the GSNs. Other distinct anomalies are observed at 1719 AD and 1760 AD, as well as during the interval 1936–1950 AD. Similar to the 10 Be concentrations, the Tsambagarav 10 Be fluxes (Fig. 4(b)) show an increase towards the end of the Dalton Minimum (∼1825), as defined by changes in the GSNs. The most notable flux anomaly, however, is found in the period 1936–1950 AD. To quantitatively assess whether the Tsambagarav 10 Be concentrations were significantly different during the periods associated with the Maunder and Dalton Minima, we applied a Wilcoxon rank-sum test to the concentration data. This nonparametric method tests the null hypothesis that the two independent data sets are samples from continuous distributions with equal medians against the alternative hypothesis that they are not. For this analysis, we first isolated 10 Be concentrations associated with the Maunder and Dalton Minima and tested them against the concentrations outside these periods (hereafter termed as normal activity periods). The results suggest that there is no significant difference between in the Tsambagarav 10 Be concentrations during the solar minima periods and normal activity periods at the 95% significance level. However, this result may be caused by the peaks observed in 1719, 1760, and the significant increase in 10 Be concentrations between 1936–1950 AD. Furthermore, this result is based on the Maunder Minimum period spanning from 1645 to 1715 AD, which is defined as the period with virtually no GSNs. Due to the fact that the galactic cosmic rays in the heliosphere are modulated by the open solar magnetic field rather than the closed solar magnetic field, which is responsible for the formation of the sunspots (Lean et al., 2002; Wang, 2004), we also used an alternative definition of the Maunder Minimum period based on the solar modulation potential (SMP) reconstruction by Muscheler et al. (2007) (Fig. 2(e)). The Maunder Minimum period in the SMP reconstruction is significantly broader, i.e. between 1632–1760 AD, than the one based on GSNs. To test whether the Tsambagarav 10 Be concentrations show a significant increase during the Maunder Minimum as defined by the SMP reconstruction, we performed the Wilcoxon rank-sum test using the Tsamgabarav 10 Be concentrations between 1632–1760 AD as representing the Maunder Minimum. Following this modification, the Wilcoxon rank-sum test suggests that the Tsambagarav 10 Be concentrations are higher during the Maunder Minimum than during normal activity periods at the 95% significance level.

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Fig. 2. (a) The 10 Be concentrations measured in the Tsambagarav, (b) accumulation rates of the Tsambagarav ice core, (c) the NGRIP and (d) the Dome Fuji 10 Be concentrations, and (e) GSNs (black) and solar modulation potential (red). The purple and the blue bands show the Maunder Minimum (1645–1715) and the Dalton Minimum (1790–1830), respectively. The light purple band represents the extended Maunder Minimum (1632–1760) defined based on the solar modulation potential reconstruction (see text). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2. Power spectrum analysis of the Tsambagarav 10 Be records To investigate if the Tsambagarav 10 Be concentrations and flux records contain significant periodicities that potentially may be associated with changes in solar activity, such as the 11-yr solar cycle, or periodic climate variability, we apply the publicly available REDFIT algorithm (Schulz and Mudelsee, 2002) to the data sets (N Sim = 1000, n50 = 1, ofac = 10, hifac = 1 and rectangular window). Prior to the analyses, we standardised the Tsambagarav 10 Be flux and concentration records using their individual mean and standard deviation values. The resulting bias-corrected power spectrum of the Tsambagarav 10 Be flux record (Fig. 5) shows that periodicities around 62.3, 6.4 and 5.7 yr exceed the 95% red-

noise false-alarm level (∼confidence levels). Other periodicities are centred around 96.6, 22.5, 12.4, and 13.5 yr, although these are less significant when compared to red-noise false-alarm levels. As for the Tsambagarav 10 Be concentrations (Fig. 6), periodicities of 207.6, 89.5 and 22.5 yr exceed the 95% red-noise false-alarm level, whereas periodicities of 13.5 and 11.5 yr exceed the 90% red-noise false-alarm level. 5. Discussion To investigate past changes in 10 Be deposition rate in the Mongolian Altai region in relation to polar deposition rates, we compare the Tsambagarav 10 Be concentrations and fluxes with the

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Fig. 3. (a) The calculated 10 Be fluxes of the Tsambagarav, (b) the NGRIP and (c) the Dome Fuji 10 Be fluxes, and (d) GSNs (black) and solar modulation potential (red). The blue band show the Dalton Minimum (1790–1830). The vertical dashed line at 1815 AD shows the response of the Tsambagarav 10 Be fluxes to the decrease in the SMP. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. (a) The smoothed 10 Be concentrations measured in the Tsambagarav (magenta), the NGRIP (blue) and the Dome Fuji (green) ice cores, (b) the 10 Be fluxes of the three ice cores, and (c) the smoothed observed GSNs and solar modulation potential. The purple and the blue bands show the Maunder Minimum and the Dalton Minimum, respectively. The light purple band represents the extended Maunder Minimum (1632–1760) defined based on the solar modulation potential reconstruction (see text). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

NGRIP (Berggren et al., 2009) and Dome Fuji (Horiuchi et al., 2008) 10 Be concentration and flux records. The 10 Be records from NGRIP and Dome Fuji have annual and decadal time resolutions, respectively. We also compare the Tsambagarav records with observed

annual group sunspot numbers (GSNs), ranging from 1610 to the present (Hoyt and Schatten, 1998), and the annually resolved solar modulation potential (SMP) reconstruction based on 14 C in tree rings (1511–1950) and ionisation chamber data (1937–2001),

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Fig. 5. Lomb Scargle periodogram of the Tsambagarav 10 Be fluxes, together with the averaged theoretical red noise (blue curve) and the mean of the 1000 simulated red noise spectra (red curve), and 80%, 90% and 95% red-noise false-alarm levels. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the NGRIP ice core seems insensitive to changes in solar activity during this period (Fig. 2), but the smoothed NGRIP 10 Be flux record shows a clear increase that coincides with the maximum decrease in the solar activity level during the Dalton Minimum (Fig. 4(b)). The 10 Be flux record from Tsambagarav covers only the latter half of the Dalton Minimum period, where it shows a peak preceded by a small minimum. A similar change from lower to higher 10 Be flux around 1825 AD is observed in the flux record from NRGIP, but not in the flux record from Dome Fuji, which has a lower time resolution than the other records. A very similar change is observed in the SMP reconstruction, where an increase in the SMP precedes a secondary minimum around 1825 AD (Figs. 2(d) and 3d). The Maunder Minimum period is characterised by higher 10 Be concentrations in the Tsambagarav and NGRIP records, whereas the record from Dome Fuji indicates a small minimum in 10 Be concentration during this interval. The Tsambagarav and NGRIP 10 Be concentration anomalies are wider than the period traditionally defined as the Maunder Minimum (1645–1715 AD) based on changes in GSNs, both showing an increase prior to 1645 AD. This is in agreement with changes in the solar modulation potential (SMP) based on 14 C in tree rings (Fig. 2(e)) showing that the decrease in solar magnetic activity started earlier than 1645 AD and continued after 1715 AD, possibly until 1755–1760 AD. This relatively long-lasting minimum in solar magnetic activity is also observed in the NGRIP 10 Be concentration record, where the peak associated with the Maunder Minimum extends to ∼1750 AD. 5.1. Periodicities in the Tsambagarav 10 Be records

Fig. 6. Lomb Scargle periodogram of the Tsambagarav 10 Be concentrations, together with the averaged theoretical red noise (blue curve) and the mean of the 1000 simulated red noise spectra (red curve), and 80%, 90% and 95% red-noise false-alarm levels. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

which spans the period 1511 to 2001 AD (Muscheler et al., 2007). The solar modulation potential is a proxy for the solar magnetic activity. The average 10 Be concentration in the Tsambagarav ice core is ∼4.6 × 104 atoms g−1 , while it is ∼1.8 × 104 atoms g−1 and ∼9.4 × 104 atoms g−1 in the NGRIP and the Dome Fuji ice cores, respectively (Fig. 2). The average 10 Be flux calculated for the Tsambagarav ice core is ∼0.04 atoms cm−2 s−1 , while it is ∼0.01 atoms cm−2 s−1 for the Dome Fuji and the NGRIP ice cores (Fig. 3). These results support the model work by Field et al. (2006), who suggest that maximum 10 Be deposition takes place at mid-latitudes, because the high precipitation rates within the mid-latitude storm tracks together with 10 Be-rich stratospheric air injection to the troposphere may enhance the 10 Be deposition flux (Field et al., 2006; Heikkilä et al., 2008a). Even though there is a general agreement between parts of the concentration and flux records from the three ice cores, there are some notable discrepancies among the records (Figs. 2, 3 and 4). The Tsambagarav 10 Be concentrations exhibit a slight increase around the end of the Dalton Minimum (∼1825), whereas it is more pronounced and spanning the full duration of the Dalton Minimum in the Dome Fuji record. The 10 Be concentration in

The power spectrum analysis of the Tsambagarav 10 Be flux and concentration records revealed a number of periodicities, some of which can be associated with known solar cycles. The 11-yr solar cycle, which in reality has a period that varies between 9 and 14 yr (Friis-Christensen and Lassen, 1991), is identified as significant at the 90% false-alarm level in the 10 Be concentration record, whereas it is present but less significant in the 10 Be flux record. Periodicities similar to the 88-yr Gleissberg cycle are also present in both the flux and concentration records, most prominently in the 10 Be concentration record as a highly significant ∼90-yr period. Caution should be taken when interpreting the ∼97-yr periodicity in 10 Be flux record because the record is too short to constrain periodicities with such a long period, and because the spectrum is likely dominated by the peak in 10 Be flux around 1936–1950 AD, which is difficult to associate with changes in solar activity. The ∼208-yr period identified in the 10 Be concentration record is very similar to the 210-yr Suess solar cycle, also known as the de Vries cycle. However, the 10 Be concentration record from Tsambagarav is too short to firmly conclude that this period has a solar origin. Some of the identified periodicities cannot be readily associated with known solar cycles, including the significant 5–6-yr periodicities in the 10 Be flux. These periodicities are dominated by the fluctuations observed across the interval characterised by high 10 Be flux levels around 1936–1950 AD. To compare the general behaviour of the Tsambagarav and the NGRIP 10 Be flux records in relation to the 11-yr solar cycle signal, we compare band-pass filtered versions of the 10 Be flux record from Tsambagarav (Fig. 7(b)) with band-pass filtered versions of the GSN record and 10 Be flux record from the NGRIP (Fig. 7(a)). We band-pass filtered the Tsambagarav 10 Be and the NGRIP fluxes, and the GSN record using a Butterworth filter of degree 10 within the frequency band (1/8)–(1/15) yr−1 . For the band-pass filtering, we used standardised Tsambagarav 10 Be fluxes that were linearly interpolated at 1-yr intervals, the NGRIP 10 Be fluxes, and the GSN record. Cross-correlation analysis shows that the highest correlation between the band-pass filtered GSN record and the Tsamba-

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Fig. 7. (a) The standardised and band-pass filtered 10 Be fluxes of the NGRIP (blue) ice core together with GSNs (black), and (b) standardised and band-pass filtered 10 Be fluxes calculated for the Tsambagarav (magenta) ice core and GSNs (black). The bandpass filtering performed using a Butterworth filter of degree 10 within a frequency range of (1/8)–(1/15) yr−1 . The blue band shows the Dalton Minimum (see text). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ity found in this frequency band of the Tsambagarav 10 Be flux to some extent reflects the 11-yr solar cycle. It is clear, however, that high-frequency climatic changes also affected the deposition and flux of 10 Be at Tsambagarav. 5.2. Influences of mineral dust, sulphate, and regional climate

Fig. 8. Cross-correlation analysis performed between (a) the Tsambagarav 10 Be fluxes and GSNs, and (b) between the NGRIP 10 Be fluxes and GSNs (b) for the overlapping period between 1816–1994.

garav 10 Be record is found for a lag of two years (see Fig. 8). The cross-correlation coefficient of −0.15 is low, because the phasing of the two records vary from anti-phase, as expected, to in-phase. The maximum/minimum correlation coefficient of −0.55 between the band-pass filtered NRGIP flux data and the GSN data is found for a lag of one year, reflecting the fact that the band-pass filtered NGRIP data is mostly in anti-phase with the GSN data over the past 200 yr. Given a residence time of 10 Be in the stratosphere of one to two years (Raisbeck et al., 1981; Heikkilä et al., 2013), these results are in line with expectations because a large fraction of the 10 Be atoms deposited at mid-latitudes are produced in the stratosphere. A phase lag of two years at mid-latitudes and a smaller time lag at polar latitudes is therefore consistent with model simulations of 10 Be production and transport in the atmosphere. This, in turn, may suggest that, when averaged over 15 solar cycles, the variabil-

Comparisons among the Tsambagarav 10 Be records (Fig. 9) and the available Tsambagarav Ca2+ and SO24− d ion records may throw some light on the possible influences of remobilised dust and changes in local climate. The Ca2+ ion record is considered a tracer of mineral dust, whereas various sources contribute to the total SO24− concentration, including natural emissions of mineral dust, volcanic eruptions, and anthropogenic sulphur dioxide emissions (Herren et al., 2013). The anthropogenic sulphate dominates the SO24− concentrations between 1941–2009 AD, preventing unambiguous identification of volcanic horizons in this interval (Herren et al., 2013). However, the major increase in anthropogenic sulphate occurs around 1960 AD and lasts until ∼2000 AD (Fig. 9(b)). Changes in mineral dust inferred from the Ca2+ record are relatively small, except for large peaks around 1974 AD and 1688 AD. Both ion records have annual resolution between 1790–2008 AD, while the resolution is decadal during the period 1568–1778 AD (Figs. 9(b) and (c)). Neither of the two ion records correlate particularly well with changes in the 10 Be concentration or 10 Be flux throughout the entire timespan of the records, and multiple linear regression analyses indicate that the two ion records only explain a small, statistically insignificant, fraction of the total 10 Be variation. It is clear, nevertheless, that increases in the SO24− concentration are associated with low concentrations of 10 Be (Fig. 9(a)) and low 10 Be fluxes (Fig. 9(d)). The wide peak in SO24− concentrations between 1960–2000 AD coincides with very low concentrations of 10 Be and the abrupt increase in sulphate in 1950 AD coincides with a low concentration of 10 Be. The correlation between the 10 Be concentrations and the SO24− ion record after

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the years with anomalously high sulphate concentrations, i.e. that large fractions of the total 10 Be were adhered to sulphate particles during these times. Given the absence of a relationship between Ca2+ and 10 Be, however, it seems that the filters effectively removed the potential influence of (10 Be released from) remobilised dust. The relative roles of dry versus wet deposition may influence the total deposition of 10 Be. Changes in the strength of the Siberian anti-cyclone (SA) is likely to have influenced the source region of the snow and dust deposited at Tsambagarav, including the relative proportions of dry versus wet deposition. Sahsamanoglou et al. (1991) showed that the strength of the SA during the winter season changed from a maximum around 1935 AD to a minimum peaking around 1945 AD. This may potentially explain the peak in 10 Be concentration and flux in the period 1936–1950 AD, because it coincides with a maximum in the strength of the Siberian Anticyclone. The injection of stratospheric air into the upper layers of the troposphere occurs during cyclonic conditions, causing stratosphere-to-troposphere airmass exchange, which will introduce stratospheric aerosols into the upper troposphere (Zanis et al., 2003). However, the air from the upper troposphere is transported into the lower layers of the troposphere under anticyclonic conditions, resulting in higher concentrations of 10 Be in the lower troposphere (Zanis et al., 2003; Yamagata et al., 2010). We suggest that the change from cyclonic to anti-cyclonic conditions around 1936 AD may have resulted in a temporary enrichment of the 10 Be concentrations in the Tsambagarav ice core, and that the 5–6-yr periodicity in the 10 Be flux (Fig. 5) reflect semi-periodic changes in the storm tracks associated with the SA. 6. Conclusions

Fig. 9. (a) The Tsambagarav 10 Be concentrations, (b) the Tsambagarav SO24− and (c) Ca2+ records, and (d) the Tsambagarav 10 Be fluxes. The red colour in (b) and (c) shows the annual ion data (2008–1790), while the blue colour shows the decadal data (1778–1568). The light purple band represents the extended Maunder Minimum (1632–1760) defined based on the solar modulation potential reconstruction (see text). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

∼1911 AD is r = −0.24 (p = 0.22) (Fig. 10(a)), whereas the correlation with changes in the Ca2+ record is r = −0.04 (p = 0.82) (Fig. 10(b)). To avoid undesired effects related to interpolation of the data, these correlation coefficients are calculated for the period 1911–2008 AD, relying only on ages with overlapping data. The major sulphate peak associated with the Tambora volcanic eruption in 1815 AD coincides with a low concentration of 10 Be during the Dalton Minimum, where a positive 10 Be anomaly is expected. Finally, the relatively low concentrations of 10 Be around 1700 AD, which occur between peaks of high 10 Be concentration during the Maunder Minimum, coincide with high concentrations of SO24− associated with an increase of mineral dust, as inferred from the Ca2+ record (Fig. 9(c)). This inverse relationship between SO24− and 10 Be at Tsambagarav is in contrast to the positive relation between sulphate peaks associated with the Agung and Pinatubo volcanic eruptions and the 10 Be observed in the Vostok ice core (Baroni et al., 2011). Although the importance of dust and sulphate aerosols for the deposition of 10 Be is uncertain, this could suggest that relatively large fractions of the total 10 Be were removed with the 0.45 μm filters for

The Tsambagarav record provides the first continuous, longterm 10 Be record from a mid-latitude ice core. The average 10 Be flux recorded in the Tsambagarav ice core is higher than the flux recorded in polar ice cores, supporting the model work by Field et al. (2006). The solar influences on the 10 Be records from Tsambagarav include: significantly higher 10 Be concentrations during the extended Maunder Minimum and the presence of an ∼11-yr cycle in concentrations significant at the 90% false-alarm level. Changes in sulphate concentrations associated with natural emissions of mineral dust, volcanic eruptions, and anthropogenic sulphur dioxide emissions influenced the 10 Be deposition via adsorption of 10 Be onto sulfate particles during certain discrete intervals, but the overall influence of mineral dust appears to have been modest. The new mid-latitude 10 Be record from Tsambagarav in the Mongolian Altai thus reflects a complex interplay between solar variability, atmospheric sulphate concentrations, and changes in regional climatic conditions, such as the strength of the Siberian Anticyclone and associated storm tracks. The influence of regional climatic phenomena are not likely to be constant across the mid-latitudes and more long-term, high-resolution 10 Be records from mid-latitudes are needed to disentangle and better understand these regional climatic effects, and to achieve a global perspective on past 10 Be production rates. Acknowledgements FI is grateful to Ann-Marie Berggren and Anna Sturevik Storm for help during the 10 Be chemistry work at Uppsala University. The work was funded by the Danish Council for Independent Research, Natural Sciences and the Villum Foundation. Funding for the Stellar Astrophysics Centre is provided by the Danish National Research Foundation (Grant agreement No. DNRF106). MFK, CK and JO acknowledge support from the Carlsberg Foundation and Villum Foundation.

F. Inceoglu et al. / Earth and Planetary Science Letters 437 (2016) 47–56

Fig. 10. Correlation between (a) the Tsambagarav 10 Be and SO24− concentrations, and (b) between the Tsambagarav scatter plots only include data points that overlap in time for the period 1911–2008 AD.

Appendix A. Supplementary material Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2016.01.006.

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Be and Ca2+ concentrations. To avoid interpolation, the

Horiuchi, K., Uchida, T., Sakamoto, Y., Ohta, A., Matsuzaki, H., Shibata, Y., Motoyama, H., 2008. Ice core record of 10 Be over the past millennium from Dome Fuji, Antarctica: a new proxy record of past solar activity and a powerful tool for stratigraphic dating. Quat. Geochronol. 3 (3), 253–261. Hoyt, D.V., Schatten, K.H., 1998. Group sunspot numbers: a new solar activity reconstruction. Sol. Phys. 181, 491–512.

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