Synchronizing 10 Be in two varved lake sediment records to IntCal 13 14 C during three grand solar minima

Timescale uncertainties between paleoclimate reconstructions often inhibit studying the exact timing, spatial expression and driving mechanisms of climate variations. Detecting and aligning the globally common cosmogenic radionuclide production signal via a curve fitting method provides a tool for the quasi-continuous synchronization of paleoclimate archives. In this study, we apply this approach to synchronize 10Be records from varved sediments of Tiefer See and Lake Czechowskie covering the Maunder, Homeric and 5500 a BP grand solar minima with 14C production rates inferred from the IntCal13 calibration curve. Our analyses indicate best fits with 14C production rates when the 10Be records from Tiefer See were shifted for 8 (−12/+4) (Maunder Minimum), 31 (−16/+ 12) (Homeric Minimum) and 86 (−22/+18) years (5500 a BP grand solar minimum) towards the past. The best fit between the Lake Czechowskie 10Be record for the 5500 a BP grand solar minimum and 14C production was obtained when the 10Be time series was shifted 29 (−8/+ 7) years towards present. No significant fits were detected between the Lake Czechowskie 10Be records for the Maunder and Homeric minima and 14C production, likely due to intensified in-lake sediment resuspension since about 2800 a BP, transporting “old” 10Be to the coring location. Our results provide a proof of concept for facilitating 10Be in varved lake sediments as a novel synchronization tool required for investigating leads and lags of proxy responses to climate variability. However, they also point to some limitations of 10Be in these archives, mainly connected to in-lake sediment resuspension processes.


Introduction
Paleoclimate archives provide unique insights into the dynamics of the climate system under various forcing conditions (Adolphi et al., 2014;Brauer et al., 2008;Neugebauer et al., 2016).Particularly the timing and spatial expression of climate variations can provide valuable information about the underlying driving mechanisms (Czymzik et al., 2016b, c;Lane et al., 2013;Rach et al., 2014).However, timescale uncertainties between different paleoclimate records often inhibit the investigation of such climate variations.Climateindependent synchronization tools offer the possibility for synchronizing individual paleoclimate archives and, thereby, robust studies of leads and lags in the climate system.In addition to volcanic tephra layers (Lane et al., 2013), atmospheric trace gases (Pedro et al., 2011) and paleomagnetism (Stanton et al., 2010), cosmogenic radionuclides like 10 Be and 14 C provide such a synchronization tool (Adolphi et al., 2017;Adolphi and Muscheler, 2016).The isotopes are produced mainly in the stratosphere through cascades of nuclear reactions triggered by incident high-energy galactic cosmic rays (Lal and Peters, 1967).The flux of these galactic cosmic rays into the atmosphere is, in turn, modulated on up to multi-centennial scales mainly by solar activity changes (Stuiver and Braziunas, 1989).At > 500-year intervals, further cosmogenic radionuclide production changes induced by the varying geomagnetic field become increasingly important (Lal and Peters, 1967;Snowball and Muscheler, 2007;Simon et al., 2016).Detecting and aligning the externally forced cosmogenic radionuclide production signal via a curve fitting method enables the quasi-continuous synchronization of natural environmental archives (Adolphi and Muscheler, 2016;Muscheler et al., 2014).
One challenge with this approach is the unequivocal detection of the cosmogenic radionuclide production signal because of transport and deposition processes.Subsequent to production, 14 C oxidizes to 14 CO 2 and enters the global carbon cycle.Varying exchange rates between Earth's carbon reservoirs add non-production variability to the atmospheric 14 C record (Muscheler et al., 2004).This uncertainty can theoretically be accounted for by calculating 14 C production rates using a carbon cycle model.However, changes in Earth's carbon reservoirs are difficult to assess (Köhler et al., 2006). 10Be in midlatitude regions is nearly exclusively scavenged from the atmosphere by precipitation (Heikkilä et al., 2013).Varying atmospheric circulation and scavenging during the about 1 month long tropospheric residence time (about 1-year stratospheric residence time) result in spatially nonuniform 10 Be deposition patterns (Aldahan et al., 2008;Raisbeck et al., 1981).Despite these non-production effects, common changes in 10 Be and 14 C records are considered to reflect the cosmogenic radionuclide production signal, due to their common production mechanism and different chemical behavior (Lal and Peters, 1967;Muscheler et al., 2016).
To date, synchronization studies based on cosmogenic radionuclides are mainly limited to 14 C records from trees and 10 Be time series from Arctic and Antarctic ice cores (Raisbeck et al., 2017;Muscheler et al., 2014).For example, Adolphi and Muscheler (2016) synchronized the Greenland ice core and IntCal13 timescales for the last 11 000 years.Synchronizing 10 Be records in sedimentary archives opens the opportunity for the synchronization of paleoclimate records around the globe.Thereby, the temporal resolution of this approach is limited by the lowest-resolution record involved.First studies underline the potential of varved lake sediments for recording the 10 Be production signal, down to annual resolution (Berggren et al., 2010(Berggren et al., , 2013;;Czymzik et al., 2015Czymzik et al., , 2016a;;Martin-Puertas et al., 2012).
In the following, we attempt to synchronize 10 Be records from varved sediments of Tiefer See (TSK) and Lake Czechowskie (JC) covering the grand solar minima at 250 (Maunder Minimum), 2700 (Homeric Minimum) and 5500 a BP with 14 C production rates inferred from the Int-Cal13 calibration curve (Muscheler et al., 2014;Reimer et al., 2013).Annual 10 Be time series from both lake sediment archives yield the broad preservation of the 10 Be production Figure 2. 10 Be concentrations ( 10 Be con ) in Tiefer See (TSK) sediments around the Maunder, Homeric and 5500 a BP grand solar minima and corresponding proxy time series from the same archive. 10Be con compared with sediment accumulation rates (SARs), total organic carbon (TOC), Ti, Ca and Si.Correlation coefficients were calculated for the complete time series covering all three grand solar minima.Significance levels of correlations were calculated using 10 000 iterations of a nonparametric random phase test taking into account trend and autocorrelation present in the time series (Ebisuzaki, 1997).Error bars indicate AMS measurement uncertainties.Non-varved intervals in TSK sediments are indicated by bars.
The targeted three grand solar minima comprise among the lowest solar activity levels throughout the last 6000 years (Steinhilber et al., 2012).
2 Study sites TSK (53 • 35 N, 12 • 31 E, 62 m a. s. l.) and JC (53 • 52 N, 18 • 14 E, 108 m a. s. l.) are situated within the Pomeranian Terminal Moraine in the southern Baltic lowlands (Fig. 1) (Dräger et al., 2017;Ott et al., 2016;Słowiński et al., 2017).The lake basins are part of subglacial channel systems formed at the end of the last glaciation and had no major inflows during the Holocene (Dräger et al., 2017;Ott et al., 2016).Both lakes are of similar size (TSK: 0.75 km 2 ; JC: 0.73 km 2 ), but the catchment of JC (19.7 km 2 ) is about 4 times larger than that of TSK (5.5 km 2 ) (Fig. 1).TSK sediments during the investigated grand solar minima are composed of alternating intervals of organic, calcite and rhodochrosite varves as well as intercalated non-varved sections (Dräger et al., 2017).JC sediments for these time windows comprise endogenic calcite varves with couplets of calcite and organic/diatom sub-layers, and an additional layer of resuspended littoral material since about 2800 a BP (Ott et al., 2016;Wulf et al., 2013).TSK and JC are located at the interface of maritime westerly and continental airflow.Mean annual precipitation is similar at both sites: 640 mm a −1 at TSK and 680 mm a −1 at JC (Czymzik et al., 2015).

Sediment subsampling and proxy records
Continuous series of sediment samples at a ∼ 20-year resolution (about 20 mm sediment) were extracted for 10 Be measurements from sediment cores TSK11 and JC-M2015, based on varve chronologies (Dräger et al., 2017;Ott et al., 2016).Complementary sediment accumulation rate (SAR), geochemical X-ray fluorescence (µ-XRF) and total organic carbon (TOC) time series were constructed using existing high-resolution datasets from the same sediment cores by calculating 10 Be sample averages (Dräger et al., 2017;Ott et al., 2016;Wulf et al., 2016).Measured µ-XRF data (cps) were normalized by dividing by the sum of all elements, to reduce the effects of varying sediment properties (Weltje and Tjallingii, 2008).
www.clim-past.net/14/687/2018/Clim.Past, 14, 687-696, 2018 Figure 3. 10 Be concentrations ( 10 Be con ) in Lake Czechowskie (JC) sediments around the Maunder, Homeric and 5500 a BP grand solar minima and corresponding proxy time series from the same archive. 10Be con compared with sediment accumulation rates (SARs), total organic carbon (TOC), Ti, Ca and Si.Correlation coefficients were calculated for the complete time series covering all three grand solar minima.Significance levels of correlations were calculated using 10 000 iterations of a nonparametric random phase test taking into account trend and autocorrelation present in the time series (Ebisuzaki, 1997).Error bars indicate AMS measurement uncertainties.

10 Be extraction and accelerator mass spectrometry (AMS) measurements
After spiking with 0.5 mg 9 Be carrier, authigenic Be was leached from 0.2 g ground sediment samples overnight with 8 M HCl at 60 • C (Berggren et al., 2010).The resulting solutions were filtered to separate the undissolved fractions.Further addition of NH 3 and H 2 SO 4 caused the precipitation of metal hydroxides and silicates, which were again removed by filtering.The remaining solutions were treated with EDTA to separate other metals and, then, passed through hydrogen form ion exchange columns in which Be was retained.Be was extracted from the columns using 4 M HCl and Be(OH) 2 precipitated through the addition of NH 3 at pH 10.The samples were washed and dehydrated three times by centrifuging and oxidized to BeO at 600 • C in a muffle furnace.After mixing with Nb, AMS measurements of BeO were performed at the Tandem Laboratory of Uppsala University.Final 10 Be concentrations were calculated from measured 10 Be / 9 Be ratios, normalized to the NIST SRM 4325 reference standard ( 10 Be / 9 Be = 2.68 × 10 −11 ) (Berggren et al., 2010).

Original chronologies
The age models for TSK and JC sediments were constructed using a multiple-dating approach.Microscopic varve counts were carried out for both lake sediments.Non-varved intervals in TSK sediments were bridged based on varve thickness measurements in neighboring well-varved sediment sections.Independent age control for the TSK and JC varve chronologies was provided by radiocarbon dating and tephrochronology (for details see: Dräger et al., 2016;Ott et al., 2016Ott et al., , 2017;;Wulf et al., 2013).Resulting chronological uncertainties are ±17 (TSK) and ±4 years (JC) for the Maunder Minimum, ±139 (TSK) and ±29 years (JC) for the Homeric Minimum as well as ±74 (TSK) and ±56 years (JC) for the 5500 a BP grand solar minimum (see Fig. 6).

Timescale synchronization
Lag-correlation analyses were applied to determine best fits between the 10 Be records from TSK and JC for the Maunder, Homeric and 5500 a BP grand solar minima and 14 C production rates inferred from the IntCal13 calibration curve (Muscheler et al., 2014;Reimer et al., 2013).Before the correlation, all time series were 75-to 500-year band-pass filtered and normalized by dividing by the mean, to reduce  .Tiefer See (TSK) and Lake Czechowskie (JC) 10 Be concentration ( 10 Be con ), corrected 10 Be ( 10 Be environment ) and 10 Be composite ( 10 Be comp ) time series around the Maunder, Homeric and 5500 a BP grand solar minima.All time series are resampled to a 20-year resolution and normalized by dividing by the mean.A 75-year low pass filtered was applied to reduce noise.Uncertainty ranges of 10 Be comp (gray shadings) are expressed as the differences between the 10 Be con and 10 Be environment time series.Significance levels of correlations between 10 Be con and 10 Be comp were calculated using a random phase test (Ebisuzaki, 1997).
Significance levels for all correlation coefficients were calculated using 10 000 iterations of a nonparametric random phase test, taking into account autocorrelation and trend present in the time series (Ebisuzaki, 1997).Chronological uncertainty ranges were reported as the time spans in which significances of correlations are below the given significant level.Before the analyses, all time series were resampled to a 20-year resolution.

10 Be production signal in TSK and JC sediments
Environment and catchment conditions can add nonproduction variations to 10 Be con records from varved lake sediments (Berggren et al., 2010;Czymzik et al., 2015).In the following chapter we will, first, describe our approach used for detecting and correcting possible non-production features in our 10 Be time series and, then, discuss possible mechanisms behind the statistically inferred connections.
To detect and reduce non-production effects in our 10 Be time series, we perform a three-step statistical procedure following Czymzik et al. (2016a), with a slight modification.First, multi-linear regressions were calculated between the 10 Be con records and TOC, SAR, Ca, Si, and Ti proxy time series from TSK and JC, reflecting changes in sediment accumulation and composition (Dräger et al., 2017;Ott et al., 2016;Wulf et al., 2016) Group sunspot number (reversed axis)

Lake Czechowskie
Figure 5. 10 Be composites ( 10 Be comp ) from Tiefer See (TSK) and Lake Czechowskie (JC) compared with group sunspot numbers back to 340 a BP (Svalgaard and Schatten, 2016).Time windows of the Maunder and Dalton solar minima are highlighted (Eddy, 1976;Frick et al., 1997).Time series are shown at 20-year resolution (thin lines) and with a 75-year low-pass filter, to reduce noise (thick lines). 10Be comp records were normalized by dividing by the mean.
with significant contributions (p < 0.1) for TSK and JC were included in the final multi-regressions.Subsequently, the resulting 10 Be bias time series from TSK and JC sediments were subtracted from the original 10 Be con records in an attempt to construct an environment-corrected version of the 10 Be record ( 10 Be environment ).However, this statistical approach also removes variability in the 10 Be con records only coincident with variations in proxy time series but without a mechanistic linkage, potentially resulting in an overcorrection.Such coinciding variability can be introduced by solar activity variations causing 10 Be production changes and climate variations imprinted in the proxy time series.Therefore, final 10 Be composite records ( 10 Be comp ) were calculated by averaging the 10 Be con and 10 Be environment records from each site.
To enhance the robustness of the corrections, the procedure was performed on the complete 10 Be con records from TSK and JC covering all three grand solar minima.Uncertainty ranges of the calculated 10 Be comp records are expressed as the differences between the 10 Be con and 10 Be environment time series (Fig. 4).Calculated 10 Be comp time series from TSK and JC sediments yield modified trends but similar multi-decadal variability as the original 10 Be con records during the Maunder (TSK: r = 0.84, p < 0.01; JC: r = 0.91; p < 0.01), Home-ric (TSK: r = 0.81, p < 0.01; JC: r = 0.74; p < 0.01) and 5500 a BP grand solar minimum (TSK: r = 0.89, p < 0.01; JC: r = 0.68; p < 0.01) (Fig. 4).These linkages suggest that our correction procedure predominantly reduced trends in the 10 Be con records introduced by varying sedimentary TOC and Ca contents but largely preserved multi-decadal variations connected with varying 10 Be production (Figs. 2,3 and 4).Comparable linkages between measured and corrected 10 Be records (based on a similar approach) were found in Meerfelder Maar sediments covering the Lateglacial-Holocene transition as well as in recent TSK and JC sediments (Czymzik et al., 2015(Czymzik et al., , 2016a)).
The statistical connections to TOC and Ca for TSK and JC might point to depositional mechanisms of 10 Be in lake sediment records.Significant contributions to the multiregression as well as significant positive correlations for TSK (r = 0.62, p < 0.01) and JC (r = 0.77, p < 0.01) suggest a preferential binding of 10 Be to organic material (Figs. 2 and  3).This result is supported by significant positive correlations of 10 Be with TOC in two annually resolved time series from varved sediments of TSK and JC spanning solar cycles 22 and 23 and in Meerfelder Maar sediments covering the Lateglacial-Holocene transition (Czymzik et al., 2015(Czymzik et al., , 2016a)).Additional sub-layer of littoral material Figure 6.Synchronization of 10 Be composites ( 10 Be comp ) from Tiefer See (TSK) and Lake Czechowskie (JC) for the Maunder, Homeric and 5500 a BP grand solar minima with 14 C production rates from the IntCal13 calibration curve (Muscheler et al., 2014).Best fits between the records were calculated using lag correlation, based on given chronological uncertainties (Dräger et al., 2017;Ott et al., 2016).Significance levels of the correlations were calculated using 10 000 iterations of a nonparametric random phase test taking into account autocorrelation and trend present in the time series (Ebisuzaki, 1997).Uncertainties are given as the time spans in which the significances of the correlations are below their respective significant levels. 10Be comp is shown on its original timescale and, if applicable, synchronized with IntCal13 14 C production rates.An interval with an additional sub-layer of resuspended littoral material in JC varves is indicated.Arrows mark the peaks of the targeted three grand solar minima.
Significant contributions of Ca to the multi-regressions as well as significant negative correlations with 10 Be for TSK (r = −0.68,p < 0.01) and JC (r = −0.62,p < 0.01) might point to a reduced affinity of 10 Be for Ca (Figs. 2 and 3).A similar behavior was detected in studies about 10 Be scavenging from the marine realm (Aldahan and Possnert, 1998;Chase et al., 2002, Simon et al., 2016).

10 Be comp and group sunspot numbers
To evaluate the preservation of the cosmogenic radionuclide production signal based on observational data, the 10 Be comp time series from TSK and JC were compared with a group sunspot number record reaching back to 340 a BP (AD 1610) (Svalgaard and Schatten, 2016) (Fig. 5).Since sunspot and cosmogenic radionuclide records reflect different components of the heliomagnetic field (closed and open magnetic flux), no perfect correlation is expected (Muscheler et al., 2016).Nevertheless, a comparison of a 14 C based solar activity reconstruction with group sunspot data points to a largely linear relationship between both types of data (Muscheler et al., 2016).
Variations in the 10 Be comp records from TSK and JC resemble multi-decadal to centennial variability in the group sunspot number time series with the highest values around the Maunder Minimum (Fig. 5).Secondary 10 Be comp maxima in TSK and JC sediments broadly coincide with the Dalton Minimum and solar activity minimum around 30 a BP (AD 1920) (Fig. 5).In JC sediments, the 10 Be comp excursion from −50 to 0 a BP (AD 2000(AD -1950) ) without an expression in the group sunspot number record as well as the about 20-year delayed Maunder Minimum response could be explained by transport of "old" 10 Be from the littoral to the coring site (see details on the sub-layer of resuspended littoral sediments in JC varves back to 2800 a BP in Sect.5.3) and/or spatially inhomogeneous 10 Be deposition patterns (Fig. 5).

5.3
Synchronizing TSK and JC 10 Be with IntCal13 14 C Shared variance of 10 Be and 14 C records can be interpreted in terms common changes in cosmogenic radionuclide production (Czymzik et al., 2016a;Muscheler et al., 2014).Moreover, it provides the opportunity to synchronize cosmogenic radionuclide records from different archives (Adolphi and Muscheler, 2016).Lag-correlation analyses were performed to synchronize the TSK and JC 10 Be comp records covering the Maunder, Homeric and 5500 a BP grand solar minima with 14 C production rates from the IntCal13 calibration curve (Muscheler et al., 2014).
The best fit between the 10 Be comp record from JC sediments during the 5500 a BP grand solar minimum and Int-Cal13 14 C production rates (r = 0.81, p < 0.01) was determined when the 10 Be record was shifted for 29 −8/ + 7 years towards present (within the given chronological uncertainty of ±56 years) (Fig. 6).No significant correlations between the 10 Be comp records from JC and 14 C production rates were obtained for the Maunder and Homeric minima, within the respective varve counting uncertainties of ±4 and ±29 years (Fig. 6).This lack of significant correlation might be explained by a change in sedimentation at about 2800 a BP (Fig. 6).Since that time JC varves include an additional sublayer of littoral calcite and diatoms transported to the profundal by wave driven water turbulences in fall.Presumably, the resuspended material also contains "old" 10 Be inhibiting the clear detection of the expected 10 Be production signal (Fig. 6).Since the 10 Be signal present in the resuspended sediments is unknown, this uncertainty is difficult to correct for.Comparable influences of sediment resuspension were also found in a sample from an annually resolved 10 Be record from JC sediments covering the period AD 2009-1988(Czymzik et al., 2015).A varve with an exceptionally thick (3.7 mm) layer of resuspended littoral diatoms and calcite deposited in fall 2003 reveals anomalous 10 Be concentrations (Czymzik et al., 2015).

Conclusions
Detecting and aligning the common cosmogenic radionuclide production variations allows the synchronization of 10 Be time series from TSK sediments covering the Maunder, Homeric and 5500 a BP grand solar minima and JC sediments for the 5500 a BP grand solar minimum to IntCal13 14 C production rates.These synchronizations provide a novel type of time marker for varved lake sediment archives enabling improved chronologies and robust investigations of proxy responses to climate variations.Mismatches between 10 Be in JC sediments and 14 C production rates during the Maunder and Homeric minima are likely associated with inlake resuspension of "old" 10 Be, altering the expected 10 Be production signal.German Science Foundation Research Fellowship (DFG grants CZ 227/1-1 and CZ 227/1-2).Further financial support was provided through an Endowment of the Royal Physiographic Society in Lund, a Linnaeus grant to Lund University (LUCCI) and the Swedish Research Council (Dnr: 2013-8421).Florian Adolphi is supported by the Swedish Research Council (VR grant: 4.1-2016-00218).Ala Aldahan thanks the UAEU for support through UPAR funding.This study is a contribution to the Virtual Institute of Integrated Climate and Landscape Evolution Analyses (ICLEA) grant number VH-VI-415, the "BaltRap" network of the Leibniz association (The Baltic Sea and its southern lowlands: Proxy-environment interactions in times of rapid changes) and the climate initiative REKLIM Topic 8 "Abrupt climate change derived from proxy data" of the Helmholtz Association.We thank Inger Påhlsson for the extraction of 10 Be from sediment samples. 10Be data from TSK and JC are available at the PANGAEA data library (www.pangaea.de).Quentin Simon and an anonymous reviewer are acknowledged for their constructive comments which helped to improve the manuscript.
The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
Edited by: Zhengtang Guo Reviewed by: Quentin Simon and one anonymous referee

Figure 1 .
Figure 1.Settings of Tiefer See (TSK) and Lake Czechowskie (JC).(a) Location of TSK and JC in the southern Baltic lowlands.(b) Bathymetric map of TSK with position of sediment core TSK11 and lake-catchment sketch.(c) Bathymetric map of JC with position of sediment core JC-M2015 and lake-catchment sketch.

Figure 4
Figure 4. Tiefer See (TSK) and Lake Czechowskie (JC)10 Be concentration ( 10 Be con ), corrected 10 Be ( 10 Be environment ) and 10 Be composite ( 10 Be comp ) time series around the Maunder, Homeric and 5500 a BP grand solar minima.All time series are resampled to a 20-year resolution and normalized by dividing by the mean.A 75-year low pass filtered was applied to reduce noise.Uncertainty ranges of 10 Be comp (gray shadings) are expressed as the differences between the 10 Be con and 10 Be environment time series.Significance levels of correlations between 10 Be con and 10 Be comp were calculated using a random phase test(Ebisuzaki, 1997).