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Allerød interstadial
Quaternary Science Reviews 30 (2011) 1825e1828
Contents lists available at ScienceDirect
Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev
Rapid Communication
Spring-season changes during the Late Pleniglacial and Bølling/Allerød interstadial Friederike Wagner-Cremer*, André F. Lotter Palaeoecology, Institute of Environmental Biology, Utrecht University, Budapestlaan 4, NL-3584 CD Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2010 Received in revised form 4 April 2011 Accepted 6 May 2011 Available online 2 June 2011
Earlier spring onset and the associated extension of the growing season in high latitudes belong to the most obvious consequences of global warming. The natural dynamics of growing-season properties during past climate shifts however, are extremely difficult to reconstruct since temperature reconstructions are hardly ever seasonally resolved and the applied proxies such as chirinomid or pollen analysis are mainly sensitive to summer temperatures. Here we apply a newly developed leaf cuticlebased proxy to reconstruct growing degree-days (GDD) in a quantitative way and to estimate changes in the timing of spring onset over the last deglaciation. Cuticle analyses of fossil birch leaves preserved in lake sediments from southern Germany reveal extremely low GDD values during the Late Pleniglacial, which are rapidly increasing at the onset of the Bølling/Allerød interstadial. While temperature and GDDs show a simultaneous warming during deglaciation, a GDD decline precedes lowering of summer temperatures during the Older Dryas cooling. Later bud-burst dates support the hypothesis of a shortening the growing season during this cool pulse. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Last termination Growing season Cuticle analysis Palaeoclimatology
1. Context and aim Seasonality has been identified to be a major player in climate change, not only at present, where the significant lengthening of the growing season is a likely consequence of ongoing global warming (IPCC, 2007), but also during natural climate swings of the past. At the end of the last glacial, about 14,700 years ago, temperature shifts of 10 C or more pushed the climate system within a few years to decades into the warm Bølling/Allerød interstadial (Steffensen et al., 2008). It is well accepted that these estimates, based on Greenland ice-core records, have to be separated into high amplitude changes during wintertime, and only moderate alterations of the summer temperatures (Isarin and Renssen, 1999; Denton et al., 2005, 2010; Wu et al., 2007). Greenlandic ice-core stable isotope records with seasonal resolution clearly demonstrate the importance of resolving subannual signals to understand mechanisms of climate change (Denton et al., 2005; Steffensen et al., 2008; Vinther et al., 2010). Outside Greenland, however, well-constrained quantification of seasonality remains a major challenge for palaeoclimatologists (Caseldine and Turney, 2010) because most climate proxies are usually responding to maximum summer or, to a lesser extent, minimum winter temperatures. Just recently, a new palaeobotanical approach
* Corresponding author. Tel.: þ31 30 2532636. E-mail address: [email protected] (F. Wagner-Cremer). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.05.003
has been introduced, using epidermal cell characteristics of dwarf birch leaves (Betula nana) to determine the amount of growing degree-days (GDD) (Wagner-Cremer et al., 2010). GDD, being the cumulative sum of degrees Celsius above a certain threshold value, are a combined measure for the total duration of the growing season as well as the mean daily temperature during the growing season. In today’s subarctic biomes, the thermal properties of the growing season determine the duration of leaf growth, which produces detectable imprints on the leaf morphology. The high correlation between GDD5 (cumulative temperature above 5 C, the standard threshold temperature for subarctic biomes; growing season defined as May through September) and the shape of the epidermal cells allows to systematically infer GDD5 values from fossil B. nana leaf remains often abundantly preserved in Quaternary sediments. Moreover, a significant correlation between cell shape and the date of bud-burst also allows for the very first time concomitant estimates of the timing of the phenological spring onset (Wagner-Cremer et al., 2010). 2. Evidence from modern and fossil leaves A distinct correlation between epidermal cell morphology of modern B. nana and GDD5 as well as bud-burst date has been described from annually-collected B. nana leaves from Kevo, Northern Finland (69.74 N, 27.14 E). Under subarctic conditions, spring onset in combination with GDD5 determines the period available for leaf growth. Early spring onset, thus prolonged growth
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F. Wagner-Cremer, A.F. Lotter / Quaternary Science Reviews 30 (2011) 1825e1828
periods, stimulates epidermal cell wall undulation during leaf maturation, which can be expressed as the undulation index (UI) (Kürschner, 1997). The distinct imprint of GDD5 on the cuticle morphology has been validated by studying B. nana leaf fragments from young peat deposits at the same site. Based on meteorological data from Kevo the modern and sub-fossil leaf material covers a GDD5 range from 450 to 850 C . This modern training set was used to develop inference models for GDD5 and bud-burst date from UI values (Table 1). The response of the B. nana UI to GDD5 from additional peat sections in Sweden and Finland has been validated by comparison with historical instrumental data from Scandinavia for the past 150 years (Wagner-Cremer et al., 2010). In the present study we measured the UI of B. nana leaves preserved in 15.5e13.5 kyr BP old sediments from Schleinsee (Fig. 1), spanning the termination of the last glacial and the onset of the interstadial (i.e. the Bølling/Allerød warm phases). The ageassessment (Table 2) for the 70 cm Schleinsee sediment section presented here is based on the correlation of the bulk carbonate stable oxygen isotope (d18O) record with that of the dated Greenland NGRIP d18O record (Andersen et al., 2004; Rasmussen et al., 2006). Moreover, an Accelerator Mass Spectronomy (AMS) 14 C date on Dryas otopetala leaves, calibrated with CALIB04 (Reimer et al., 2009) provided the basal age of the section. The chronology used for correlation of terrestrial and Greenlandic ice-core correlation is based on the INTIMATE event stratigraphy (Lowe et al., 2008). The high abundance and generally good preservation of B. nana macro remains in these calcareous lacustrine sediments have initially been described by Lang (1952), who tentatively ascribed the larger size of leaves from the interstadial to improved growing conditions. Systematic UI analysis performed on leaf fragments from new Schleinsee sediment cores (Fig. 2a) allow the quantification of both GDD5 (Fig. 2b) and the date of bud-burst (Fig. 2c) on a (multi-)decadal scale. 3. Growing season changes during the Pleniglacial/Bølling-Allerød Lowest UI values (Fig. 2a) are recorded during the late Pleniglacial (GS-2) that translate into GDD5 of w500 C (Fig. 2b). At the transition to the lateglacial interstadial GDD5 start to increase, reaching maximum values of 750 C in the early interstadial (GI-1e, Bølling). GDD5 are constantly high for w300 years before a decrease after 14.3 kyr BP indicates a prolonged phase of low GDD5, lasting until 13.8 kyr BP. The youngest part of the record, from 13.8 to 13.6 kyr BP (GI-1c, Allerød) is again characterized by GDD5 values of w700 C, but not reaching early interstadial (GI-1e) maximum levels any more. Inferred bud-burst dates (Fig. 2c) indicate an advance in spring onset by w18 days, from day-of-year 167 in the late Pleniglacial (GS-2) to 149 in the early interstadial (GI-1e, Bølling), subsequently becoming successively later again with a second peak at 13.9 kyr BP, with inferred bud-burst at dayof-year 161. Table 1 Correlation points and radiocarbon date for the Schleinsee chronology. Core SCH05 II sample depth
NGRIP d18O age tie-points (b2k ¼ years before A.D. 2000)
Age year BP
AMS14C age
Calibrated age year BP (1 sigma)
491e490 cm POZ32706 470e469 cm
e
e
GS-2/GI-1: 14,700 Onset GI-1d: 14,000
14,650
13,020 100 BP e
15,215 e15,959 BP e
13,950
e
e
435e434 cm
Fig. 1. Site map. Location of Betula nana leaf fragments-bearing Schleinsee (SCH) site in southern Germany (47.61 N, 9.63 E, 475 m a.s.l.) and Greenland NGRIP ice-core drill site (75.10 N, 42.43 W).
By correlating the d18O record of the mid-latitude Schleinsee site in Germany to the Greenland NGRIP d18O record, the GDD5 and date of bud-burst data are directly comparable to the air temperature changes detected in the Greenland ice. The extremely low GDD5 values recorded in the Schleinsee leaf material during the Late Pleniglacial parallel the most negative d18O values, thus the coldest temperatures. The transition towards peak GDD5 starts at 14.7 kyr BP, mirroring the transition from GS-2 to GI1e. Due to the lack of B. nana leaves between 14.7 and 14.6 kyr BP the dynamics of the warming, however, are difficult to determine. Vegetation model-based estimates for the region north of the Alps predict 1500e2500 C lower GDD5 during the Last Glacial Maximum compared to present-day (Wu et al., 2007). In our reconstruction, the difference in GDD5 between 15.4 kyr BP and present-day is approximately 1600 C (2100 C MJJAS GDD5 for 2008, reference site Friedrichshafen, Germany), which agrees well with the model results. A striking divergence between the GDD5 and the NGRIP d18O record occurs during the second half of the Bølling (GI-1e), where a rapid decrease in GDD5 values in combination with delayed budTable 2 GDD5 and bud-burst inference models. Inference models for GDD5 and bud-burst based on modern training sets of B. nana UI values from Kevo, climate station data and phenological records from the same site (Wagner-Cremer et al., 2010). GDD5 inference model
Bud-burst inference model
GDD5 ¼ 10(2.4355þ5.5996(logUIfossil)), r2 ¼ 0.63, p¼<0.001, RMSE ¼ 63 GDD5 day-of-year ¼ 10(2.309(1.745(logUIfossil)), r2 ¼ 0.64, p ¼ 0.002, RMSE ¼ 4.3 days
F. Wagner-Cremer, A.F. Lotter / Quaternary Science Reviews 30 (2011) 1825e1828
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Fig. 2. Reconstructed GDD5 and bud-burst dates correlated to NGRIP d18O. a. Undulation Index (UI) data (crosses) and mean values per analyzed horizon (black dots) versus d18O measured on Schleinsee sediment bulk carbonates. b. UI inferred GDD5 values (black line, dashed indicates gap in dataset). Grey envelope is the RMSE (63 C) for GDD5. d18O NGRIP record plotted in red. c. UI inferred ordinal date (day-of-year and date) of bustburst (solid black line, dashed indicates gap in dataset). Grey envelope is the RMSE (4.2 days) for budburst date. The chronology is based on a correlation between the d18O Schleinsee record and the Greenland NGRIP record. Ages are given in calibrated years BP. Vertical dashed lines show the d18O tie-points. Arrow indicates the position of the AMS 14C date. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
burst dates indicates a shortening of the thermal growing season, with a later spring onset from 14.3 kyr BP onwards. It culminates around 13.95 kyr BP, which corresponds to the Older Dryas/ Aegelsee Oscillation (Lotter et al., 1992) (GI-1d). The rapid warming marking the onset of the Allerød (GI-1c) is synchronous again in both records. The divergence of the two records points to an asynchronous development of seasonal temperature regimes during the cool pulse of the Older Dyras/Aegelsee Oscillation (GI-1d). From our data, a scenario arises where a significant shortening of the growing
season, with delayed spring onset precedes the slow mean annual temperature decline as reflected in both, the NGRIP and Schleinsee d18O record. The synchronicity of GDD5, bud-burst date, and d18O during the termination of GS-2 (Late Pleniglacial) and the onset of GI-1e (Bølling) on the other hand, indicates contemporaneous rapid warming and increasing length of the thermal growing season. An ultra-high resolution multi-proxy study at Gerzensee (Switzerland) combining bulk sediment d18O analysis with pollen and chironomid analysis (Lotter et al., 2010) shows a rapid July temperature increase at c. 14.7 kyr BP reaching maximum
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temperatures during the early interstadial (Bølling). In the continuation of the record, pollen and chironomid summer temperature records diverge, with chironomids predicting a continuous temperature increase, even over the Aegelsee Oscillation (GI-1d) into the Allerød (GI-1c). At Gerzensee, pollen predict constantly high temperatures until the Aegelsee Oscillation (GI-1d), where a significant lowering in July temperatures is evident. It has been suggested that aquatic biota such as chironomids are following mainly summer temperatures, whereas the terrestrial vegetation combines summer temperature with a seasonality signal (Lotter et al., 2010). Our GDD5 data during this period show a slightly earlier shortening of the thermal growing season than the polleninferred decrease in July temperature. Whether the pollen record indeed holds a seasonality signal is difficult to determine from the comparison with our data, but the overall agreement supports the assumption of a seasonality component in the pollen record that is not reflected in the chironomid data. Stressing the issue of potential biases in established temperature reconstructions caused by unresolved seasonality changes, we provide for the first time evidence from a biological proxy for changes in the crucial spring onset dates and temperatures over the last termination. Recent re-analysis of Greenlandic d18O records for the past 1500 years has demonstrated that large parts of the signal are reflecting changes in November through April temperatures that are dominated by the dynamics of the North Atlantic Oscillation (NAO) (Denton et al., 2005; Vinther et al., 2010). Although rapid atmospheric circulation reorganization is a likely cause for climate change over the last deglaciation (Steffensen et al., 2008), the role of NAO especially during the cold stages is not clear (Pausata et al., 2009). Nonetheless, NAO-like sea level pressure dynamics exert complex spatial variability over the continental North Atlantic realm, which impedes uncertainties in the proxy-based reconstructions of past temperature where the seasonality of surface climate variability is not taken into account (Lotter et al., 2010). Pinning down seasonality through time by a geographicallygridded application of the new UI proxy may enable a sound mapping of seasonal variability that places apparent discrepancies between continental temperature records into a logical context. The determination of spatial patterns in seasonality changes during natural climate shifts may contribute to a better assessment of the consequences resulting from ongoing, human induced warming. Acknowledgements A. van Rosmalen produced parts of data. This research was funded by the Utrecht University. References Andersen, K.K., Azuma, N., Barnola, J.M., Bigler, M., Biscaye, P., Caillon, N., Chappellaz, J., Clausen, H.B., DahlJensen, D., Fischer, H., Fluckiger, J., Fritzsche, D., Fujii, Y., Goto-Azuma, K., Gronvold, K., Gundestrup, N.S., Hansson, M., Huber, C.,
Hvidberg, C.S., Johnsen, S.J., Jonsell, U., Jouzel, J., Kipfstuhl, S., Landais, A., Leuenberger, M., Lorrain, R., Masson-Delmotte, V., Miller, H., Motoyama, H., Narita, H., Popp, T., Rasmussen, S.O., Raynaud, D., Rothlisberger, R., Ruth, U., Samyn, D., Schwander, J., Shoji, H., Siggard-Andersen, M.L., Steffensen, J.P., Stocker, T., Sveinbjornsdottir, A.E., Svensson, A., Takata, M., Tison, J.L., Thorsteinsson, T., Watanabe, O., Wilhelms, F., White, J.W.C., Project, N.G.I.C., 2004. High-resolution record of Northern Hemisphere climate extending into the Last Interglacial period. Nature 431 (7005), 147e151. Caseldine, C.J., Turney, C., 2010. The bigger picture: towards integrating Palaeoclimate and environmental data with the history of societal change. Journal of Quaternary Science 25, 88e93. Denton, G.H., Alley, R.B., Comer, G.C., Broecker, W.S., 2005. The role of seasonality in abrupt climate change. Quaternary Science Reviews 24 (10e11), 1159e1182. Denton, G.H., Anderson, R.F., Toggweiler, J.R., Edwards, R.L., Schaefer, J.M., Putnam, A.E., 2010. The last glacial termination. Science 328 (5986), 1652e1656. IPCC, 2007. IPCC, 2007: climate change: synthesis report. In: Pachauri, R.K., Reisinger, A. (Eds.), IPCC. IPCC, Geneva, Switzerland, p. 104. Isarin, R.F.B., Renssen, H., 1999. Reconstructing and modelling late Weichselian climates: the younger Dryas in Europe as a case study. Earth-Science Reviews 48 (1e2), 1e38. Kürschner, W.M., 1997. The anatomical diversity of recent and fossil leaves of the durmast oak (Quercus petraea Lieblein/Q. pseudocastanea Goeppert) e Implications for their use as biosensors of palaeoatmospheric CO2 levels. Review of Palaeobotany and Palynology 96, 1e30. Lang, G., 1952. Späteiszeitliche Pflanzenreste in Südwestdeutschland. Beiträge zur natukundlichen Forschung in Südwestdeutschland XI (2). Lotter, A.F., Birks, H.J.B., Eicher, U., Siegenthaler, U., 1992. Late-glacial climatic oscillations as recorded in Swiss lake sediments. Journal of Quaternary Science 7, 187e204. Lotter, A., Heiri, O., Brooks, S., van Leeuwen, J., Eicher, U., Ammann, B., 2010. Rapid summer temperature changes during Termination 1a: high-resolution multiproxy climate reconstructions from Gerzensee (Switzerland). Quaternary Science Reviews. doi:10.1016/j.quascirev.2010.06.022. Lowe, J.J., Rasmussen, S.O., Bjorck, S., Hoek, W.Z., Steffensen, J.P., Walker, M.J.C., Yu, Z.C., Grp, I., 2008. Synchronisation of palaeoenvironmental events in the North Atlantic region during the last termination: a revised protocol recommended by the INTIMATE group. Quaternary Science Reviews 27 (1e2), 6e17. Pausata, F.S.R., Li, C., Wettstein, J.J., Nisancioglu, K.H., Battisti, D.S., 2009. Changes in atmospheric variability in a glacial climate and the impacts on proxy data: a model intercomparison. Climate of the Past 5 (3), 489e502. Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.L., Johnsen, S.J., Larsen, L.B., DahlJensen, D., Bigler, M., Rothlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E., Ruth, U., 2006. A new Greenland ice core chronology for the last glacial termination. Journal of Geophysical Research-Atmospheres 111 (D6). Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Ramsey, C.B., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeye, C.E., 2009. INTCAL09 and MARINE09 radiocarbon age calibration curves, 0e50,000 years cal BP. Radiocarbon 51 (4), 1111e1150. Steffensen, J.P., Andersen, K.K., Bigler, M., Clausen, H.B., Dahl-Jensen, D., Fischer, H., Goto-Azuma, K., Hansson, M., Johnsen, S.J., Jouzel, J., Masson-Delmotte, V., Popp, T., Rasmussen, S.O., Rothlisberger, R., Ruth, U., Stauffer, B., SiggaardAndersen, M.L., Sveinbjornsdottir, A.E., Svensson, A., White, J.W.C., 2008. Highresolution Greenland Ice Core data show abrupt climate change happens in few years. Science 321 (5889), 680e684. Vinther, B.M., Jones, P.D., Briffa, K.R., Clausen, H.B., Andersen, K.K., Dahl-Jensen, D., Johnsen, S.J., 2010. Climatic signals in multiple highly resolved stable isotope records from Greenland. Quaternary Science Reviews 29 (3e4), 522e538. Wagner-Cremer, F., Finsinger, W., Moberg, A., 2010. Tracing growingdegree-day changes in the cuticle morphology of Betula nana leaves: a new microphenological palaeo-proxy. Journal of Quaternary Science 25 (6), 1008e1017. Wu, H.B., Guiot, J.L., Brewer, S., Guo, Z.T., 2007. Climatic changes in Eurasia and Africa at the last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modeling. Climate Dynamics 29 (2e3), 211e229.
Contents lists available at ScienceDirect
Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev
Rapid Communication
Spring-season changes during the Late Pleniglacial and Bølling/Allerød interstadial Friederike Wagner-Cremer*, André F. Lotter Palaeoecology, Institute of Environmental Biology, Utrecht University, Budapestlaan 4, NL-3584 CD Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2010 Received in revised form 4 April 2011 Accepted 6 May 2011 Available online 2 June 2011
Earlier spring onset and the associated extension of the growing season in high latitudes belong to the most obvious consequences of global warming. The natural dynamics of growing-season properties during past climate shifts however, are extremely difficult to reconstruct since temperature reconstructions are hardly ever seasonally resolved and the applied proxies such as chirinomid or pollen analysis are mainly sensitive to summer temperatures. Here we apply a newly developed leaf cuticlebased proxy to reconstruct growing degree-days (GDD) in a quantitative way and to estimate changes in the timing of spring onset over the last deglaciation. Cuticle analyses of fossil birch leaves preserved in lake sediments from southern Germany reveal extremely low GDD values during the Late Pleniglacial, which are rapidly increasing at the onset of the Bølling/Allerød interstadial. While temperature and GDDs show a simultaneous warming during deglaciation, a GDD decline precedes lowering of summer temperatures during the Older Dryas cooling. Later bud-burst dates support the hypothesis of a shortening the growing season during this cool pulse. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Last termination Growing season Cuticle analysis Palaeoclimatology
1. Context and aim Seasonality has been identified to be a major player in climate change, not only at present, where the significant lengthening of the growing season is a likely consequence of ongoing global warming (IPCC, 2007), but also during natural climate swings of the past. At the end of the last glacial, about 14,700 years ago, temperature shifts of 10 C or more pushed the climate system within a few years to decades into the warm Bølling/Allerød interstadial (Steffensen et al., 2008). It is well accepted that these estimates, based on Greenland ice-core records, have to be separated into high amplitude changes during wintertime, and only moderate alterations of the summer temperatures (Isarin and Renssen, 1999; Denton et al., 2005, 2010; Wu et al., 2007). Greenlandic ice-core stable isotope records with seasonal resolution clearly demonstrate the importance of resolving subannual signals to understand mechanisms of climate change (Denton et al., 2005; Steffensen et al., 2008; Vinther et al., 2010). Outside Greenland, however, well-constrained quantification of seasonality remains a major challenge for palaeoclimatologists (Caseldine and Turney, 2010) because most climate proxies are usually responding to maximum summer or, to a lesser extent, minimum winter temperatures. Just recently, a new palaeobotanical approach
* Corresponding author. Tel.: þ31 30 2532636. E-mail address: [email protected] (F. Wagner-Cremer). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.05.003
has been introduced, using epidermal cell characteristics of dwarf birch leaves (Betula nana) to determine the amount of growing degree-days (GDD) (Wagner-Cremer et al., 2010). GDD, being the cumulative sum of degrees Celsius above a certain threshold value, are a combined measure for the total duration of the growing season as well as the mean daily temperature during the growing season. In today’s subarctic biomes, the thermal properties of the growing season determine the duration of leaf growth, which produces detectable imprints on the leaf morphology. The high correlation between GDD5 (cumulative temperature above 5 C, the standard threshold temperature for subarctic biomes; growing season defined as May through September) and the shape of the epidermal cells allows to systematically infer GDD5 values from fossil B. nana leaf remains often abundantly preserved in Quaternary sediments. Moreover, a significant correlation between cell shape and the date of bud-burst also allows for the very first time concomitant estimates of the timing of the phenological spring onset (Wagner-Cremer et al., 2010). 2. Evidence from modern and fossil leaves A distinct correlation between epidermal cell morphology of modern B. nana and GDD5 as well as bud-burst date has been described from annually-collected B. nana leaves from Kevo, Northern Finland (69.74 N, 27.14 E). Under subarctic conditions, spring onset in combination with GDD5 determines the period available for leaf growth. Early spring onset, thus prolonged growth
1826
F. Wagner-Cremer, A.F. Lotter / Quaternary Science Reviews 30 (2011) 1825e1828
periods, stimulates epidermal cell wall undulation during leaf maturation, which can be expressed as the undulation index (UI) (Kürschner, 1997). The distinct imprint of GDD5 on the cuticle morphology has been validated by studying B. nana leaf fragments from young peat deposits at the same site. Based on meteorological data from Kevo the modern and sub-fossil leaf material covers a GDD5 range from 450 to 850 C . This modern training set was used to develop inference models for GDD5 and bud-burst date from UI values (Table 1). The response of the B. nana UI to GDD5 from additional peat sections in Sweden and Finland has been validated by comparison with historical instrumental data from Scandinavia for the past 150 years (Wagner-Cremer et al., 2010). In the present study we measured the UI of B. nana leaves preserved in 15.5e13.5 kyr BP old sediments from Schleinsee (Fig. 1), spanning the termination of the last glacial and the onset of the interstadial (i.e. the Bølling/Allerød warm phases). The ageassessment (Table 2) for the 70 cm Schleinsee sediment section presented here is based on the correlation of the bulk carbonate stable oxygen isotope (d18O) record with that of the dated Greenland NGRIP d18O record (Andersen et al., 2004; Rasmussen et al., 2006). Moreover, an Accelerator Mass Spectronomy (AMS) 14 C date on Dryas otopetala leaves, calibrated with CALIB04 (Reimer et al., 2009) provided the basal age of the section. The chronology used for correlation of terrestrial and Greenlandic ice-core correlation is based on the INTIMATE event stratigraphy (Lowe et al., 2008). The high abundance and generally good preservation of B. nana macro remains in these calcareous lacustrine sediments have initially been described by Lang (1952), who tentatively ascribed the larger size of leaves from the interstadial to improved growing conditions. Systematic UI analysis performed on leaf fragments from new Schleinsee sediment cores (Fig. 2a) allow the quantification of both GDD5 (Fig. 2b) and the date of bud-burst (Fig. 2c) on a (multi-)decadal scale. 3. Growing season changes during the Pleniglacial/Bølling-Allerød Lowest UI values (Fig. 2a) are recorded during the late Pleniglacial (GS-2) that translate into GDD5 of w500 C (Fig. 2b). At the transition to the lateglacial interstadial GDD5 start to increase, reaching maximum values of 750 C in the early interstadial (GI-1e, Bølling). GDD5 are constantly high for w300 years before a decrease after 14.3 kyr BP indicates a prolonged phase of low GDD5, lasting until 13.8 kyr BP. The youngest part of the record, from 13.8 to 13.6 kyr BP (GI-1c, Allerød) is again characterized by GDD5 values of w700 C, but not reaching early interstadial (GI-1e) maximum levels any more. Inferred bud-burst dates (Fig. 2c) indicate an advance in spring onset by w18 days, from day-of-year 167 in the late Pleniglacial (GS-2) to 149 in the early interstadial (GI-1e, Bølling), subsequently becoming successively later again with a second peak at 13.9 kyr BP, with inferred bud-burst at dayof-year 161. Table 1 Correlation points and radiocarbon date for the Schleinsee chronology. Core SCH05 II sample depth
NGRIP d18O age tie-points (b2k ¼ years before A.D. 2000)
Age year BP
AMS14C age
Calibrated age year BP (1 sigma)
491e490 cm POZ32706 470e469 cm
e
e
GS-2/GI-1: 14,700 Onset GI-1d: 14,000
14,650
13,020 100 BP e
15,215 e15,959 BP e
13,950
e
e
435e434 cm
Fig. 1. Site map. Location of Betula nana leaf fragments-bearing Schleinsee (SCH) site in southern Germany (47.61 N, 9.63 E, 475 m a.s.l.) and Greenland NGRIP ice-core drill site (75.10 N, 42.43 W).
By correlating the d18O record of the mid-latitude Schleinsee site in Germany to the Greenland NGRIP d18O record, the GDD5 and date of bud-burst data are directly comparable to the air temperature changes detected in the Greenland ice. The extremely low GDD5 values recorded in the Schleinsee leaf material during the Late Pleniglacial parallel the most negative d18O values, thus the coldest temperatures. The transition towards peak GDD5 starts at 14.7 kyr BP, mirroring the transition from GS-2 to GI1e. Due to the lack of B. nana leaves between 14.7 and 14.6 kyr BP the dynamics of the warming, however, are difficult to determine. Vegetation model-based estimates for the region north of the Alps predict 1500e2500 C lower GDD5 during the Last Glacial Maximum compared to present-day (Wu et al., 2007). In our reconstruction, the difference in GDD5 between 15.4 kyr BP and present-day is approximately 1600 C (2100 C MJJAS GDD5 for 2008, reference site Friedrichshafen, Germany), which agrees well with the model results. A striking divergence between the GDD5 and the NGRIP d18O record occurs during the second half of the Bølling (GI-1e), where a rapid decrease in GDD5 values in combination with delayed budTable 2 GDD5 and bud-burst inference models. Inference models for GDD5 and bud-burst based on modern training sets of B. nana UI values from Kevo, climate station data and phenological records from the same site (Wagner-Cremer et al., 2010). GDD5 inference model
Bud-burst inference model
GDD5 ¼ 10(2.4355þ5.5996(logUIfossil)), r2 ¼ 0.63, p¼<0.001, RMSE ¼ 63 GDD5 day-of-year ¼ 10(2.309(1.745(logUIfossil)), r2 ¼ 0.64, p ¼ 0.002, RMSE ¼ 4.3 days
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Fig. 2. Reconstructed GDD5 and bud-burst dates correlated to NGRIP d18O. a. Undulation Index (UI) data (crosses) and mean values per analyzed horizon (black dots) versus d18O measured on Schleinsee sediment bulk carbonates. b. UI inferred GDD5 values (black line, dashed indicates gap in dataset). Grey envelope is the RMSE (63 C) for GDD5. d18O NGRIP record plotted in red. c. UI inferred ordinal date (day-of-year and date) of bustburst (solid black line, dashed indicates gap in dataset). Grey envelope is the RMSE (4.2 days) for budburst date. The chronology is based on a correlation between the d18O Schleinsee record and the Greenland NGRIP record. Ages are given in calibrated years BP. Vertical dashed lines show the d18O tie-points. Arrow indicates the position of the AMS 14C date. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
burst dates indicates a shortening of the thermal growing season, with a later spring onset from 14.3 kyr BP onwards. It culminates around 13.95 kyr BP, which corresponds to the Older Dryas/ Aegelsee Oscillation (Lotter et al., 1992) (GI-1d). The rapid warming marking the onset of the Allerød (GI-1c) is synchronous again in both records. The divergence of the two records points to an asynchronous development of seasonal temperature regimes during the cool pulse of the Older Dyras/Aegelsee Oscillation (GI-1d). From our data, a scenario arises where a significant shortening of the growing
season, with delayed spring onset precedes the slow mean annual temperature decline as reflected in both, the NGRIP and Schleinsee d18O record. The synchronicity of GDD5, bud-burst date, and d18O during the termination of GS-2 (Late Pleniglacial) and the onset of GI-1e (Bølling) on the other hand, indicates contemporaneous rapid warming and increasing length of the thermal growing season. An ultra-high resolution multi-proxy study at Gerzensee (Switzerland) combining bulk sediment d18O analysis with pollen and chironomid analysis (Lotter et al., 2010) shows a rapid July temperature increase at c. 14.7 kyr BP reaching maximum
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temperatures during the early interstadial (Bølling). In the continuation of the record, pollen and chironomid summer temperature records diverge, with chironomids predicting a continuous temperature increase, even over the Aegelsee Oscillation (GI-1d) into the Allerød (GI-1c). At Gerzensee, pollen predict constantly high temperatures until the Aegelsee Oscillation (GI-1d), where a significant lowering in July temperatures is evident. It has been suggested that aquatic biota such as chironomids are following mainly summer temperatures, whereas the terrestrial vegetation combines summer temperature with a seasonality signal (Lotter et al., 2010). Our GDD5 data during this period show a slightly earlier shortening of the thermal growing season than the polleninferred decrease in July temperature. Whether the pollen record indeed holds a seasonality signal is difficult to determine from the comparison with our data, but the overall agreement supports the assumption of a seasonality component in the pollen record that is not reflected in the chironomid data. Stressing the issue of potential biases in established temperature reconstructions caused by unresolved seasonality changes, we provide for the first time evidence from a biological proxy for changes in the crucial spring onset dates and temperatures over the last termination. Recent re-analysis of Greenlandic d18O records for the past 1500 years has demonstrated that large parts of the signal are reflecting changes in November through April temperatures that are dominated by the dynamics of the North Atlantic Oscillation (NAO) (Denton et al., 2005; Vinther et al., 2010). Although rapid atmospheric circulation reorganization is a likely cause for climate change over the last deglaciation (Steffensen et al., 2008), the role of NAO especially during the cold stages is not clear (Pausata et al., 2009). Nonetheless, NAO-like sea level pressure dynamics exert complex spatial variability over the continental North Atlantic realm, which impedes uncertainties in the proxy-based reconstructions of past temperature where the seasonality of surface climate variability is not taken into account (Lotter et al., 2010). Pinning down seasonality through time by a geographicallygridded application of the new UI proxy may enable a sound mapping of seasonal variability that places apparent discrepancies between continental temperature records into a logical context. The determination of spatial patterns in seasonality changes during natural climate shifts may contribute to a better assessment of the consequences resulting from ongoing, human induced warming. Acknowledgements A. van Rosmalen produced parts of data. This research was funded by the Utrecht University. References Andersen, K.K., Azuma, N., Barnola, J.M., Bigler, M., Biscaye, P., Caillon, N., Chappellaz, J., Clausen, H.B., DahlJensen, D., Fischer, H., Fluckiger, J., Fritzsche, D., Fujii, Y., Goto-Azuma, K., Gronvold, K., Gundestrup, N.S., Hansson, M., Huber, C.,
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