10Be is produced in the atmosphere by the interaction between high-energy cosmic ray particles (galactic cosmic rays – GCRs) and atmospheric atoms. About 65 % of atmospheric 10Be nuclides are produced in the stratosphere (Heikkilä et al., 2009, 2013; Masarik and Beer, 1999), where they can be assumed to be well mixed due to an average residence time of 1–2 years (Heikkilä et al., 2008). Upon binding to sulfate aerosols (e.g., Igarashi et al., 1998), 10Be is removed from the atmosphere by wet and dry deposition and can be measured in environmental archives, such as ice cores from Greenland and Antarctica. Using the ECHAM5 general circulation model coupled with the aerosol module HAM, Heikkilä et al. (2009) showed that the dominant component of 10Be in polar ice cores is stratospheric 10Be, making up to about two-thirds of the signal preserved in ice cores from Greenland and Antarctica. Consequently, they argued that deposition fluxes of 10Be reflect well the changes in the global average 10Be atmospheric production rate.

The flux of GCRs reaching the Earth is modulated by the magnetic fields of Sun and Earth. The production of 10Be thus anticorrelates with the strength of the helio- and geomagnetic fields allowing us to use 10Be from ice cores to reconstruct solar activity (e.g., Beer et al., 1990; McCracken, 2004; Muscheler et al., 2007; Steinhilber et al., 2012; Vonmoos et al., 2006), and the variations in the geomagnetic field dipole moment (e.g., Muscheler et al., 2005; Raisbeck et al., 1985, 2006).

In the last decade, it has also been shown that solar storms can leave a significant imprint in 10Be from Greenland and Antarctica. These events can, in fact, lead to short-lived peaks in the radionuclide concentrations (Mekhaldi et al., 2015; Miyake et al., 2015, 2019; O'Hare et al., 2019; Paleari et al., 2022). Mekhaldi et al. (2021) have modeled the theoretically expected globally averaged annual production rate of 10Be since the 1950s, using neutron monitor data, and the theoretical production induced by ground level enhancements (GLEs); i.e., solar energetic particle (SEP) events that cause sudden increases in ground-based neutron monitor count rates. The strongest directly observed GLE – GLE no. 05 from 1956 – caused, for instance, an increase of only about 5 % in the annual 10Be production rate. Considering that the 11-year solar cycle is estimated to cause a variability of ±15 % to ±35 % (e.g., Baroni et al., 2011; Paleari et al., 2022; Pedro et al., 2012; Mekhaldi et al., 2021), the signal of modern events cannot be unequivocally distinguished from the variability caused by the 11-year cycle and climate and local weather influences (Mekhaldi et al., 2021). For the instrumental period, no unambiguous SEP signal could be detected in ice cores so far, not even in seasonal 10Be data (e.g., Pedro et al., 2011a, b; Zheng et al., 2020).

Moreover, as proposed by Baroni et al. (2011, 2019), peaks in 10Be concentrations in ice cores may sometimes be linked to stratospheric volcanic eruptions. The Agung and Pinatubo eruptions, for instance, are estimated to have caused an increase in 10Be deposition of 66 % and 35 %, relative to the baseline radionuclide concentration in the Vostok ice core from Antarctica (Baroni et al., 2011).

Especially for detecting the short-lived spikes caused by SEPs in the past, high-resolution measurements are required. These are often labor-intense and limited by the availability of sufficient ice core sample material. In this study, we aim at testing the use of excess meltwater samples usually discarded from continuous flow analysis (CFA) for the measurement of cosmogenic 10Be. This method could provide the potential for high-resolution and cost-efficient 10Be sampling without the need for competing for the valuable ice. We collected the meltwater from the CFA system at the Institute for Climate and Environmental Physics at the University of Bern (Switzerland). The system is used to measure impurities in ice cores (e.g., Erhardt et al., 2022), which requires the use of pristine ice. The system is equipped with a melt head designed to only capture the inner part of the ice core for the measurement of impurities, preventing the ice from being possibly contaminated by the surrounding atmosphere (Erhardt et al., 2019). The outer part can in theory be used for cosmogenic radionuclide measurements, as they are less prone to contamination. This technique has already been used to collect a short record of about 40 years from the EGRIP (East Greenland Ice core Project) ice core to study an extreme SEP event that hit Earth 9125 yr BP (Paleari et al., 2022), which is estimated to be 2 orders of magnitude larger than modern GLEs. While suitable for the assessment of the existence of the peak in 10Be concentrations, it was not possible to analyze in depth and ultimately quantify the uncertainties related to this method. That is because the record was only compared to ice core records from other locations from Greenland and Antarctica and therefore characterized by different climate and weather conditions. Here, we present a new 10Be record from the S6 firn core from EGRIP (see Fig. 1 for the location), covering the depth from about 4 to 26 m and corresponding approximately to the period between 1900 and 2008 CE. To assess the suitability of the CFA samples to measure cosmogenic radionuclides and retrieve a solar signal, 10Be records obtained from CFA and discrete firn samples from the EGRIP S6 core (Zheng et al., 2023) are compared. The use of two 10Be records from the same core allows us to directly assess the quality and possible differences between the sampling methods.

In addition, we investigate the causes of short-term and non-GCR-related 10Be variability in the 10Be records from EGRIP S6 and other available records from Greenland and Antarctica. More specifically, we aim to investigate whether we can detect the signal of large SEP events (or GLEs) and volcanic eruptions. Furthermore, we investigate the preservation of the 11-year solar cycle in the 10Be records.

The 10Be record from EGRIP S6 CFA excess water samples is shown in pink in Fig. 2a. The 10Be concentrations were measured in firn from about 4 to 26 m depth, encompassing the period 1900–2008, according to the chronology by Zheng et al. (2023). The 10Be concentrations have an average of 1.43(±0.31)×104 atoms g-1. Figure 2b shows the 10Be concentrations from discrete firn samples from the same core. The average 10Be concentration of the discrete record is 1.42(±0.32)×104 atoms g-1 (Zheng et al., 2023). The meltwater samples have, on average, a measurement uncertainty of 5 %, similar to the 10Be measurements carried out on the discrete firn samples (Zheng et al., 2023), despite the smaller size (100 g for CFA samples versus ca. 140 g for discrete samples).

As the sampling rates of the two S6 records are different, the 10Be records need to be virtually resampled to be directly compared. The 10Be concentrations from CFA samples and from discrete samples were interpolated linearly at steps of 0.1 year, and the yearly 10Be concentrations were calculated by averaging over 10 consecutive data points. The two yearly resampled 10Be records agree well (r=0.77; p<0.01 – computed using a t test). If we resample the discrete record to match the resolution of the CFA 10Be record, then we obtain a correlation coefficient of 0.71 (p<0.01). It can, however, be pointed out that the first ∼8 m of the CFA excess water record (between ∼4 and ∼12 m depth) is seemingly more autocorrelated compared to the deeper part of the record, and relative to the discrete 10Be record, the latter also smoother in the first ∼8 m of the record compared to the deeper part. The sampling method and rate can likely affect the degree to which the two records agree. 10Be deposition is known to be highly seasonal, with maxima closely related to the enhanced stratosphere–troposphere exchanges (STEs), causing an increased descent of 10Be from the stratosphere, where most of the production takes place, to the troposphere (Stohl et al., 2003; Zheng et al., 2020; Spiegl et al., 2022; Heikkilä and Smith, 2013; Pedro et al., 2011a). Moreover, one of the main complications of dealing with CFA systems is the possible smoothing of the signal locked in the ice (Erhardt et al., 2022; Kaufmann et al., 2008; Mekhaldi et al., 2017; Sigg et al., 1994). 10Be ions may attach to the walls of the plastic tube connecting the melt head to the centrifuge tubes, as 10Be precipitates in a basic environment. Therefore, it is possible that some atoms may precipitate on the plastic walls and be remobilized at a later stage. Loss of 10Be to the centrifuge tube walls is considered unlikely, since the 9Be carrier (slightly acidic) was added to the centrifuge tubes before collecting the samples, and the 10Be/9Be ratio is, in theory, preserved. It is, however, most likely that the smoothing in the shallower part of the CFA 10Be record is caused by analytical issues related to the upward wicking of meltwater in the very porous snow during melting, which affects the shallow most parts of the core.

To investigate whether the CFA excess water samples are suitable to preserve the 10Be production rate signal as robustly as discrete firn samples, the 10Be records from EGRIP S6 are compared to other available 10Be records from Greenland and Antarctica. We compare the 10Be record from CFA samples to 10Be concentration records from the NGRIP (Berggren et al., 2009), NEEM (Zheng et al., 2021), Dye 3 (Beer et al., 1990), Renland (Aldahan et al., 1998), Das2 (Pedro et al., 2012), DML (Aldahan et al., 1998), DSS (Pedro et al., 2012), Vostok (Baroni et al., 2011), and Concordia (Baroni et al., 2011) cores. The NEEM record is available at sub-annual resolution, and thus, we calculated mass-weighted means of each year. Figure 3 shows the comparison between the CFA 10Be record (gray line) and the other records from Greenland and Antarctica starting in 1900 CE. The degree to which the CFA 10Be record agrees with the other 10Be records can be seen in the correlation matrix in Fig. 4. For this analysis, we used the CFA and discrete 10Be records resampled at 1-year resolution to match the timescale of the other records. To calculate the correlation coefficients, the records were normalized to their mean for the period 1949–1985, thus representing the time span shared by all records.

In general, the EGRIP CFA 10Be record shows similar but somewhat higher correlation coefficients with the other records from Greenland and Antarctica, compared with the discrete EGRIP 10Be record. While the higher correlation coefficients can likely be attributed to the higher autocorrelation inherent to the smoothing effect of CFA sampling in the shallower part of the record, our results indicate that the signal measured in the CFA samples is reproducing the common radionuclide signal in Greenland and Antarctica, as well as the discrete firn samples. The EGRIP S6 records from CFA and discrete firn samples agree well with the 10Be records from Greenland, except for Das2 (see Fig. 4). This could be explained by the very high 10Be concentrations in the Das2 record during the 1960s, corresponding to a pronounced minimum in accumulation rate (Pedro et al., 2012). Moreover, the differences between the EGRIP S6 records and the Das2 record in the early 1990s likely affect the correlation coefficient. We obtain correlation coefficients of 0.54 (p<0.01) and 0.39 (p<0.01) between the stack of 10Be records from Greenland (excluding the EGRIP S6 records) and the S6 records from CFA and discrete samples, respectively. We obtain correlation coefficients of 0.39 (p<0.01) and 0.29 (p<0.01) between the stack of records from Antarctica and the S6 records from CFA and discrete samples, respectively.

Differences between the records could arise from non-production-related biases such as measurement and sampling uncertainty, meteorological influences on the transport and deposition of 10Be en route and on site, and local re-distribution of snow by surface winds. For instance, Heikkilä and Smith (2013) found a strong correlation between 10Be deposition and the North Atlantic Oscillation (NAO) in Greenland, with a positive correlation on the east coast and negative on the west coast.

CFA and discrete 10Be records are then compared with the stack of the normalized 10Be records from Greenland (excluding EGRIP S6 records) and Antarctica (Fig. 5). The calculation of a stack with several records from different sites allows us to reduce the noise by isolating their common signal, which is expected to more closely reflect the atmospheric production signal. The CFA 10Be record has a correlation coefficient of 0.60 (p<0.01) with the global 10Be stack, while the 10Be record from discrete firn samples has a correlation coefficient of 0.42 (p<0.01).

To assess the suitability of CFA meltwater samples to reconstruct solar activity, we investigate the presence of the 11-year solar cycle in the data. The wiggles of the 11-year solar cycles are estimated to cause a variability of about ±15 % to ±35 % in the 10Be records from Greenland and Antarctica (e.g., Baroni et al., 2011; Heikkilä et al., 2009; Mekhaldi et al., 2021; Paleari et al., 2022). To investigate the presence of the 11-year cycle in the data, we performed continuous wavelet transform (Grinsted et al., 2004) on the 10Be records from CFA and discrete samples and on the global 10Be stack, calculated using the records from NGRIP, NEEM, Dye 3, Renland, Das2, DML, DSS, Vostok, and Concordia. The continuous wavelet power spectrums are shown in Fig. 9.

Figure 9 shows that the 11-year solar cycle is present in the EGRIP S6 records. In particular, the figure highlights that the 11-year solar cycle is preserved in the CFA and discrete records spanning the period 1900–1960, especially so in the CFA record. Conversely, it is not observed in the more recent part of the record, likely due to higher background noise. As expected from the increased signal-to-noise ratio, Fig. 9 also confirms that the 11-year cycle is well preserved throughout the global stack. Moreover, Fig. 9 highlights that the discrete 10Be record is characterized by more prominence of higher-frequency noise (period < 4 years), as opposed to the CFA 10Be record, likely due to the smoothing caused by analytical issues inherent to the CFA system (Erhardt et al., 2022; Kaufmann et al., 2008; Mekhaldi et al., 2017; Sigg et al., 1994).

There also exists a 22-year cycle, whereby the Sun reverses polarity after every 11-year cycle. This is evidenced in neutron monitor measurements that show that the GCR flux at Earth has a polarity-dependent shape. More specifically, 11-year cycles take the form of a peak under negative polarity and the form of a plateau under positive polarity (e.g., Poopakun et al., 2022; Lockwood et al., 2001). Bearing this difference in mind, we separate our analysis between periods characterized by negative and positive polarity (Fig. 10a and b, respectively). Because we are now interested in the shape of the cycles, as seen in neutron monitor data that are anticorrelated with solar activity, we now consider solar maxima as the onset of solar cycles to investigate the peak shape in the radionuclide data. The 10Be records from Greenland (except EGRIP S6) and Antarctica were split into time windows for each period and normalized to their respective mean and therefore used to create a global stack (blue line). The blue envelopes denote the standard deviation of the records. The stacks obtained from the modeled 10Be production rate from Mekhaldi et al. (2021) are shown as a dashed line for comparison. The global stack is compared to the CFA stack (magenta) and discrete firn samples stack (green) from EGRIP S6.

The global and modeled stacks agree very well in both shape and amplitude for both periods of negative (Fig. 10a) and positive (Fig. 10b) polarity. Similarly, the stacks from discrete and CFA samples agree very well (r=0.74 for negative polarity (Fig. 10a) and r=0.87 for positive polarity (Fig. 10b); p<0.01), although the S6 CFA stacks are smoother than the discrete stacks. However, CFA and discrete stacks agree well, showing that the two records show a very similar signal. This result is promising, as it shows the suitability of CFA samples for solar activity reconstructions.

In this study, we present the 10Be record obtained from excess water from CFA measurements of the EGRIP S6 firn core. The record, spanning from 1900 to 2008, has been compared to another 10Be record obtained from discrete firn samples from the same core. Although some smoothing due to analytical issues inherent to the CFA system may be present in the shallower part of the core, the CFA 10Be record agrees well with the 10Be record from discrete firn samples and with other 10Be records from Greenland.

By subtracting the global 10Be production rate variability caused by the 11-year solar cycle, we investigate the possible causes of excursions in the EGRIP S6 10Be records and in the stacks of Greenlandic and Antarctic records. We find that values exceeding 1σ in the standardized residuals are coeval with the timing of some of the major volcanic eruptions that are also detected in sulfate records (Zielinski et al., 1997; Sigl et a., 2015), such as the Agung (1963) and Pinatubo (1991) eruptions. Moreover, values exceeding 1σ in the residuals in 1957 may be consistent with the occurrence of GLE no. 05. However, an increase in sulfate concentrations was detected in records from Greenland, according to Sigl et al. (2013), illustrating the challenges to separate the causes for small radionuclide excursions. We find an increase of 16 % in the stack from Greenland and in 4.3 % the stack from Antarctica in 1957. This study shows that, presently, volcanic eruptions and GLEs influences cannot be unambiguously distinguished from the internal variability in annually resolved 10Be records, in agreement with previous studies (e.g., Pedro et al., 2012; Mekhaldi et al., 2021).

Finally, we analyzed how the 11-year solar cycles are preserved in the discussed records. Our results show that the 11-year solar cycles are well preserved in CFA and discrete 10Be records from EGRIP S6. By stacking periods of different polarity of the Sun's magnetic field separately, it is possible to see their different shape in 10Be data, as expected according to the flux of the GCRs at Earth. The stacks from CFA 10Be samples agree well with the stacks from discrete 10Be samples from EGRIP S6, thus showing that CFA excess water samples are suitable for the reconstruction of solar activity. These results open the possibility of collecting continuous and high-resolution 10Be records with a more time-efficient sampling and preparation, while saving an important portion of valuable ice for other measurements.