Introduction
The 2013 International Calibration Curve (IntCal13; Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards and Friedrich2013a) is the “gold standard” for geochronological 14C dating over the past ~50,000 years. Based on 14C measurements from tree rings, plant macrofossils, speleothems, corals, and marine sediments, the IntCal dataset is regularly updated towards higher resolution and precision (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards and Friedrich2013a). Accounting for 14C/12C variations in various proxy archives of the Holocene (e.g., tree rings) and even well into the Pleistocene (e.g., plant macrofossils, speleothems, and corals), reservoir standardization and intercomparisons between hemispheres and archives are key factors for the establishment and maintenance of IntCal (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards and Friedrich2013b).
Due to their resolution and independent cross-dating (Büntgen et al. Reference Büntgen, Wacker, Galván, Arnold, Arseneault, Baillie, Beer, Bernabei, Bleicher and Boswijk2018), tree-ring-based, annual to subdecadal 14C measurements form the backbone of the calibration curve throughout the Holocene. The “Holocene Oak Chronology” by Becker (Reference Becker1993), together with the “Preboreal Pine Chronology” from Central Europe represent the longest absolutely dated tree-ring record securely extending back to 12,325 cal BP (PPC; Spurk et al. Reference Spurk, Friedrich, Hofmann, Remmele, Frenzel, Leuschner and Kromer1998; Friedrich et al. Reference Friedrich, Kromer, Spurk, Hofmann and Kaiser1999, Reference Friedrich, Remmele, Kromer, Hofmann, Spurk, Kaiser, Orcel and Küppers2004; Reinig et al. Reference Reinig, Nievergelt, Esper, Friedrich, Helle, Hellmann, Kromer, Morganti, Pauly and Sookdeo2018a). A lack of subfossil wood from across the Northern Hemisphere (Reinig et al. Reference Reinig, Nievergelt, Esper, Friedrich, Helle, Hellmann, Kromer, Morganti, Pauly and Sookdeo2018a), however, challenges IntCal precision during the preceding Younger Dryas (YD), so that the “Swiss Late Glacial Master Chronology” covering the Allerød and Bølling is still floating (SWILM; Kaiser et al. Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub, Boeren, Remmele, Talamo, Guibal and Sivan2012). Although the SWILM positioning has recently been improved by high-resolution 14C measurements from New Zealand’s subfossil Kauri wood (Agathis australis; Hogg et al. Reference Hogg, Southon, Turney, Palmer, Ramsey, Fenwick, Boswijk, Büntgen, Friedrich and Helle2016), interhemispheric wiggle-matching (Bronk Ramsey Reference Bronk Ramsey2001) remains challenging (Muscheler et al. Reference Muscheler, Adolphi and Knudsen2014).
In order to better understand IntCal during the Younger Dryas, this study details the dendrochronological characteristics of 1448 new 14C dates between ~11,950 and 13,160 cal BP from 13 subfossil Scots pine trees (Pinus sylvestris L.) that were growing in Zurich, Switzerland. The exceptional quality (resolution) and quantity (replication) of this new LG 14C compilation significantly improves 14C wiggle-matching and dendrochronological cross-dating during the YD.
Material and Methods
At various construction sites in Zurich more than 400 subfossil pines were discovered and excavated between 1973 and 2013 (Kaiser et al. Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub, Boeren, Remmele, Talamo, Guibal and Sivan2012; Reinig et al. Reference Reinig, Nievergelt, Esper, Friedrich, Helle, Hellmann, Kromer, Morganti, Pauly and Sookdeo2018a). Cellulose-based 14C measurements supported tree-ring width (TRW) dating of the material between ~14,000–11,500 cal BP (Figure 1; Reinig et al. Reference Reinig, Nievergelt, Esper, Friedrich, Helle, Hellmann, Kromer, Morganti, Pauly and Sookdeo2018a). After sanding down the samples with up to 400 grain size sandpaper, at least two TRW radii per disc were measured and cross-dated using a LINTAB device (precision of 0.01 mm) and TSAP-Win software (both RINNTECH, Heidelberg). The TRW measurements where visually and statistically cross-dated considering t-values (tBP) and Gleichläufigkeit (Glk) indices (Baillie and Pilcher Reference Baillie and Pilcher1973) in TSAP-Win. Chronologies were established and checked with COFECHA (Holmes Reference Holmes1983).
Figure 1 Temporal distribution of the Swiss YD tree-ring records and corresponding 1448 14C measurements (Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020). High-resolution 14C measurements of the Northern Hemisphere GAENALCH (light blue dots), YD-A (dark blue) and ZHYD-1 (orange dots) chronology and single tentative linked trees (yellow dots) wiggle-matched against the SH 14C Kauri record (black dots, Hogg et al. Reference Hogg, Southon, Turney, Palmer, Ramsey, Fenwick, Boswijk, Büntgen, Friedrich and Helle2016) now bridge the YD (a). An absolute dendrochronological link between the corresponding 14C-measured trees (dashed bars) and Swiss TRW chronologies (filled bars) throughout the YD has not yet been established (b). However, the overall potential window for a temporal shift of the TRW records is minimal (± 8 yr, 2σ). Dashed lines indicate the Northern Hemispheric TRW gap between the absolutely dated PPC (Friedrich et al. Reference Friedrich, Remmele, Kromer, Hofmann, Spurk, Kaiser, Orcel and Küppers2004) and floating SWILM chronology (Kaiser et al. Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub, Boeren, Remmele, Talamo, Guibal and Sivan2012). Sample replication (nT) and total amount of 14C dates (n14C) of each chronology is provided in parentheses. (c) Table of high-resolution 14C-measured trees, outlining the trees’ total number of rings and 14C dates. (Please see electronic version for color figure.)
A subset of 13 YD trees from the Zurich sites Gaenziloo, Krankenheim Wiedikon, and Binz (Reinig et al. Reference Reinig, Nievergelt, Esper, Friedrich, Helle, Hellmann, Kromer, Morganti, Pauly and Sookdeo2018a) was selected and individual tree rings separated (Figure 1b). High-precision 14C AMS measurements were performed at the Laboratory of Ion Beam Physics, ETH-Zurich, on a “MIniCArbonDatingSystem” (MICADAS, Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020). Wood of 2–4 consecutive rings was pooled, if material from a single ring was <20 mg. Holocellulose was extracted by the base-acid-base-acid method (Nemec et al. Reference Nĕmec, Wacker and Gäggeler2010), bleached and graphitized with an AGE system (Wacker et al. Reference Wacker, Bonani, Friedrich, Hajdas, Kromer, Nĕmec, Ruff, Suter, Synal and Vockenhuber2010). The 14C blanks, standards and references were continuously cross-checked in accordance with the ETH quality protocol (Sookdeo et al. Reference Sookdeo, Kromer, Büntgen, Friedrich, Friedrich, Helle, Pauly, Nievergelt, Reinig, Treydte, Synal and Wacker2019), ensuring the accuracy and comparability of the resulting 1448 14C dates (Figure 1a). The floating 14C records derived from the individual trees were wiggle-matched to the Southern Hemisphere (SH) 14C Kauri record (Hogg et al. Reference Hogg, Southon, Turney, Palmer, Ramsey, Fenwick, Boswijk, Büntgen, Friedrich and Helle2016) using a χ2 test (Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020).
Results and Discussion
The combination of TRW cross-dating and high-resolution 14C wiggle-matching of 13 trees from Zurich closes the geochronological gap in the YD (Figure 1b). The samples are grouped into three distinct YD periods from ~11,950 to 13,160 cal BP, during which good agreement is found within and between the TRW and 14C measurements. Low sample size, juvenile growth disturbance and enhanced wood decay in the outer section of the samples (Reinig et al. Reference Reinig, Gärtner, Crivellaro, Nievergelt, Pauly, Schweingruber, Sookdeo, Wacker and Büntgen2018b), however, still challenges the establishment of a continuous, absolutely dated tree-ring chronology throughout the entire YD. The youngest trees are securely linked by dendrochronology and 14C to the absolute chronology (12,314–11,863 cal BP), whereas the older trees yet remain floating.
Link between YD Swiss Trees and the PPC
The 14C record of PPC was recently extended back to 12,049 cal BP by including previously undated trees from Breitenthal in southern Germany (PPC-Brei, Sookdeo et al. Reference Sookdeo, Kromer, Büntgen, Friedrich, Friedrich, Helle, Pauly, Nievergelt, Reinig, Treydte, Synal and Wacker2019), which allows the tentative cross-dating of two trees from the Zurich collection against the now improved German record. These two Swiss trees indicate good comparative t-values (tBP = 4.6, GLK = 66; OVL = 231) and synchronicity in both high frequency and long-term trends (see SM2). The 14C measured tree “LAND0062” cross-dates with an overlap (OVL) of 187 years with PPC-Brei at tBP = 3.1 (Glk = 59), while BINZ0087 cross-dates to PPC-Brei at tBP = 2.9 (Glk = 57) over 99 years (Table 1). The YD-D ring width chronology, compiled from three trees from the site Krankenheim-Wiedikon in Zurich, including 14C data of tree KHWI0028 (Reinig et al. Reference Reinig, Nievergelt, Esper, Friedrich, Helle, Hellmann, Kromer, Morganti, Pauly and Sookdeo2018a; Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020), shows good visual and statistical coherence with BINZ0087 (Table 1; SM2). Cross-correlation of BINZ0087 improves to tBP = 4.8, when excluding the 77 outermost and disturbed rings from YD-D (YD-Dcut). LAND0004, the oldest 14C dated tree of this absolutely dated chronology, is securely connected to YD-D (see SM2) and extends the chronology back to 12,314 cal BP. All established dendrochronological placements were found to be in accordance with the overlapping 389 high-resolution 14C dates of the three trees (Figure 1c; Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020). While the dendrochronological link between the Swiss and German pines is tentative, as the underlying Swiss sample size is low (n = 6), the good agreement with 14C wiggle-matching (± 8 years (2σ); Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020) securely links these tree-ring chronologies. This Swiss record represents with its 14C support an important YD anchor to the PPC, for both 14C wiggle-matching and future dendrochronological cross-dating.
Table 1 Cross-dating results of all high-resolution 14C dated Swiss trees and BINZ0087 in the obtained three dendrochronological YD periods (SM5).
Floating Swiss YD Chronologies
Two floating TRW chronologies and corresponding high-resolution 14C measurements have been produced for the YD. ZHYD-1 is a chronology of 15 trees from the Zurich sites Gaenziloo (n = 4), Krankenheim Wiedikon (n = 3) and Binz (n = 8), reaching a cumulative length of 384 years. The six trees selected for 14C measurements indicate good agreement to establish a robust mean curve, independent of their site of origin (see SM3). Interseries correlations between these trees range from tBP = 3.1 to tBP = 7.0, and all trees overlap by at least 131 years within the ZHYD-1 chronology (Table 1). The TRW-based dating is reinforced by 635 high-resolution 14C measurements (Figure 1c; Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020), supporting the chronology particularly during periods of relatively weak replication. When developing ZHYD-1, two trees from Birmensdorf, originally included in the YD-B chronology (Kaiser et al. Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub, Boeren, Remmele, Talamo, Guibal and Sivan2012), were removed, as their revised TRW measurements and additional 14C dates proved their dating to be incorrect. Instead, the Binz material discovered in 2013 serves as the link between the oldest Gaenziloo and youngest Krankenheim Wiedikon trees in the chronology. The 14C dates from ZHYD-1, wiggle-matched against the SH Kauri 14C record (Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020), suggested an TRW overlap of ~20 years to LAND004 from the absolutely dated Swiss trees (Figure 1a,b). However, no valid dendrochronological link could be established as decay in the outermost rings of KHWI0030 and KHWI0031, as well as juvenile growth disturbances in LAND0004, challenge the extension of the absolute record.
The revised YD-A chronology from Kaiser et al. (Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub, Boeren, Remmele, Talamo, Guibal and Sivan2012), excluding decayed and disturbed TRW sections, represents the second floating chronology from Switzerland in the YD comprising of a total of eight trees and reaching a length of 199 years. The chronology is well-replicated over the first 140 years, but sample depth rapidly decreases thereafter. The youngest 35 years are represented by only one tree (GAEN0008). Due to the low replication, the TRW chronology displays increased variability over the most recent 60 years, limiting a secure cross-dating to ZHYD-1 and leading the obtained dendrochronological link to remain tentative. However, within YD-A the two high-resolution 14C dated trees show good agreement to the YD-A mean curve (Table 1; see SM3). GAEN0007 (28 14C dates) cross-dates at tBP = 3.4 to the YD-A mean (Glk = 65; OVL = 130), while GAEN0008 (132 14C dates) is securely integrated in this chronology (tBP = 6.6, Glk = 71; OVL = 152). These placements are coherent with the 14C wiggle-matching results (Figure 1c; Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020). Based on 14C wiggle-matching of the individual Swiss chronologies to the SH 14C Kauri record and their obtained dendrochronological linkage, the combined record was shifted by seven decades, compared to the former YD-B position obtained through calibration using IntCal13 (Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020). Nonetheless, additional dendrochronological analyses are necessary to conclusively verify the link between YD-A and ZHYD-1. The improved sample size in the YD may allow the cross-dating of additional 14C dated YD tree-ring samples from Switzerland and Central Europe, facilitating temporal and spatial extensions of the record. The investigation of potential climate signals throughout the YD will hopefully become feasible.
Younger Dryas Transition
GAEN0005, with 245 rings, is the youngest tree included in the Gaenziloo chronology (GAENALCH; n = 55; Kaiser et al. Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub, Boeren, Remmele, Talamo, Guibal and Sivan2012) and represents the final portion of SWILM (Kaiser et al. Reference Kaiser, Friedrich, Miramont, Kromer, Sgier, Schaub, Boeren, Remmele, Talamo, Guibal and Sivan2012). With an overlap of 207 years, GAEN0005 is securely cross-dated into GAENLACH (tBP = 6.3; Glk = 64, Table 1; see SM4), and the now new 164 14C measurements (Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020) cover the transition from the Allerød into the YD (Figure 1c). The striking 14C increase of GAEN0005 is confirmed by high-resolution 14C measurements on LG pines from southern France (Capano et al. Reference Capano, Miramont, Guibal, Kromer, Tuna, Fagault and Bard2018). Nonetheless, no dendrochronological link could be established between the southern and northern Alpine sites. The distance between the sites, the varying climatic forcing, and the changing micro-site growth conditions might all contribute to their disagreement in TRW.
The long overlap (436 years), and high correlation (tBP = 11.1; Glk = 68; Table 1), demonstrate a secure link between GAEN0071 and the well-replicated GAENALCH chronology (see SM4). The now newly obtained 100 additional 14C dates at 3-yr resolution from GAEN0071 (Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020) will further improve the calibration towards the end of the Allerød (Figure 1c). In addition, the confirmed decreasing 14C structure between ~12,800–12,550 cal BP serves as an important period for wiggle-matching LG 14C records globally. Their alignment along this distinct 14C feature, obtained through the new high-resolution data, will support the correction of dating discrepancies among various 14C records. Accordingly, as the SWILM chronology is now shifted by 35 ± 8 years (2σ) after wiggle-matching to the SH Kauri 14C record (Sookdeo et al. Reference Sookdeo, Kromer, Adolphi, Beer, Brehm, Büntgen, Christl, Eglinton, Friedrich, Guidobaldi, Helle, Muscheler, Nievergelt, Pauly, Reinig, Tegel, Treydte, Turney, Synal and Wacker2020), it can serve as a basis for additional high-resolution 14C measurements back to ~14,226 cal BP. In comparison to the dating precision of IntCal13 (± 20, 1σ), this marks a significant improvement. Cross-dating and 14C wiggle-matching of additional European and global tree-ring records can now be performed at higher temporal precision, which will eventually also improve dating accuracy throughout the Bølling and Allerød.
Conclusions
This study provides dendrochronological insight into 1448 new 14C dates from 13 pine trees that were growing in Switzerland between ~11,950 and 13,160 cal BP. Coherency between TRW cross-dating and 14C wiggle-matching substantially improves the dating accuracy during the transition from the LG into the Early Holocene. Compared to their previous placement in IntCal13, the Swiss YD chronologies and the SWILM were shifted older by 70 and 35 years, respectively.
Acknowledgments
F. Reinig, K. Treydte, A. Sookdeo, L. Wacker, and U. Büntgen received funding from the Swiss National Science Foundation (SNF Grant 200021L_157187/1), and U. Büntgen received further support from the Ministry of Education, Youth and Sports of Czech Republic within the National Sustainability Program I (NPU I grant number LO1415), respectively. The German Research Foundation funded Jan Esper (Inst 247/665-1 FUGG and ES 161/9-1), M. Friedrich, and B. Kromer (DFG grant KR 726/10-1), W. Tegel (DFG grant TE 613/2-1), G. Helle and M. Pauly (DFG grant HE3089/9-1). Author contributions: UB, LW, AS, and FR initiated the study, and all authors contributed to discussion. AS, GG, and LW performed radiocarbon measurements and interpreted the results together with BK, while DN, MF, and FR carried out TRW measurements. FR and UB wrote the paper with input from all authors.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2020.15
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