Réf. Mangini & al. 2005 - A

Référence bibliographique complète

MANGINI, A., SPOTL, C., VERDES, P. 2005. Reconstruction of temperature in the Central Alps during the past 2000 yr from a δ18O stalagmite record. Earth and Planetary Science Letters, 235, 741-751. DOI: 10.1016/j.epsl.2005.05.010.

Abstract: The precisely dated isotopic composition of a stalagmite from Spannagel Cave in the Central Alps is translated into a highly resolved record of temperature at high elevation during the past 2000 yr. Temperature maxima during the Medieval Warm Period between 800 and 1300 AD are in average about 1.7 °C higher than the minima in the Little Ice Age and similar to present-day values. The high correlation of this record to ∆14C suggests that solar variability was a major driver of climate in Central Europe during the past 2 millennia.

Mots-clés
Medieval Warm Period; temperature; Central Alps; stalagmite record; Holocene; stable isotopes

Organismes / Contact

• Forschungsstelle Radiometrie, Heidelberger Akademie der Wissenschaften, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
• Institut für Geologie und Paläontologie, Leopold-Franzens-Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria


(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature      

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Austria Central Alps Spannagel Cave   2347 m a.s.l. the last 2000 yr

(1) - Modifications des paramètres atmosphériques
Reconstitutions

Derivation of the temperature record for the past 2000 yr

The conversion of the isotopic profile into temperature is performed by applying the transfer function (curve) obtained from 5 points with known temperature from the average yearly temperature reconstruction for the Alps by Luterbacher et al. [17] and the isotopic composition of the stalagmite in the corresponding sections. The uppermost point marks the present-day conditions in Spannagel at +1.8 °C (and δ18O = – 7.8‰).

The average temperature in the Alps during the coldest period between 1688 and 1698 coincided with the Maunder Minimum. As derived from Luterbacher’s data, the average temperature in this period was – 1.0 (± 0.5) °C, about 1.8 °C lower than in the period between 1995 and 1998. The section of the stalagmite that grew during this period has the heaviest δ18O values in the profile. This cold period delivers the second temperature point on the calibration curve. The authors choose as third and lowest point on the calibration curve the lowest isotopic values of calcite, which we assume to be deposited close or slightly below 0 °C, as stalagmite growth will cease when temperature falls below the freezing point. An uncertainty of 0.2 °C were ascribed to this point.

The reconstruction of temperature is corroborated by two further points on the transfer function. The average temperature in the Alps in the period between AD 1800 and 1890 was about 1.15 ± 0.50 °C lower than in the period between 1995 and 1998. Thus, Luterbacher’s reconstruction suggests that then the temperature in the Alps was slightly lower than the 0.7 °C lower global temperature as reported in the IPCC curve [2]. This temperature, together with the average of the δ18O value in the section corresponding to the interval 1800–1890, yields the fourth point on the curve. The fifth point at δ18O at the top of the stalagmite corresponds to the calcite deposited around AD 1950 when temperature was 0.3 ± 0.3 °C lower than today, a period when there was positive glacier growth in the Alps.

Based on this transfer function, temperature reconstructed from the isotopic record ranges between 0 °C and 2.7 °C. Considering the confidence limits of the calibration curve, the uncertainty of the temperature reconstruction for Spannagel is about ± 0.3 °C. The lowest values is observed in the section formed during the Little Ice Age (AD 1400–1850), and maximum values in the section corresponding to the MWP (approx. AD 800–1300). These latter values are even slightly higher than those of the top section of the stalagmite (1950 AD) and higher than the present-day temperature of 1.8 °C.

We cannot exclude that there have been some periods when stalagmite growth stopped in the past, for example if the temperature in the cave went significantly below 0 °C. However, the record of Luterbacher et al. indicates that the Maunder Minimum was one of the coldest periods during the last 500 yr and that colder events were short lasting and infrequent. Therefore we claim that if any hiatus occurred it did not distort significantly our age reconstruction of SPA 12. Furthermore, the high number of precise Th/Uages reduces this uncertainty anyway.

The temperature reconstruction from SPA 12 shows a pattern similar to other climatic archives from the Northern Hemisphere including Greenland ice cores, sea-surface temperatures (SST) from the Bermuda Rise, as well as to the reconstruction of glacier tongue advances and retreats in the Alps [9,8,7,18]. There is a good correspondence to the reconstruction of SST at Bermuda Rise showing a temperature difference of about 2 °C between the MWP and LIA [7,19]. However, the range of relative temperature variations in SPA 12 of about 2.7 °C exceeds that of the ice cores, where ΔT between the LIA minima and MWP maximum is approximately 1.5 °C.

Comparison with the multi-proxy reconstruction by Moberg et al. is shown. Both curves depict the maximum of temperature in the N.H. during the MWP. As expected the multiproxy stack has smaller amplitude of about 0.9 °C than our curve from Spannagel between the minimum in the LIA and the MWP events. The smaller amplitude is obvious, since Moberg’s reconstruction, resulting from a stack of several different archives with independent age control, looses amplitude as a consequence of the uncertainty in the ages of the single curves. In contrast, the temperature record from SPA 12 with an extremely good age control and with a better than decadal resolution of 18O, gives insight into temperature variations that were not recorded in other archives.

Overall, the good correlation of the present reconstruction of temperature from SPA 12 with other archives supports the conclusions. Together, these non-faunal archives indicate that the MWP was a climatically distinct period in the Northern Hemisphere. This conclusion is in strong contradiction to the temperature reconstruction by the IPCC, which only sees the last 100 yr as a period of increased temperature during the last 2000 yr.

The high resolution record from SPA 12 has a number of interesting features:

a) During the MWP we observe periods lasting between 20–50 yr with temperatures higher than the average over the last 2000 yr. One may speculate that these warmer periods are related to the strength of ocean circulation [20] or to changes in the extension of the ice cover of the N. Atlantic Ocean [21], but more data are needed to confirm this conclusion.

b) The temperature derived from SPA 12 compares better to Luterbacher’s reconstruction of the winter temperature than to their yearly average temperature in the Alps.

c) We observe a high correlation between δ18O (and temperature) and Δ14C, that reflects the amount of radiocarbon in the upper atmosphere. The profile of δ18O was tuned to the tree-ring calibrated profile of Δ14C, similarly as done earlier for several other stalagmites [13,22,23]. Tuning delivers a high correlation between δ18O in SPA 12 and Δ14C (r =0.61). The maxima of δ18O coincide with solar minima (Dalton, Maunder, Sporer, Wolf, as well as with minima at around AD 700, 500 and 300). This correlation indicates that the variability of δ18O is driven by solar changes, in agreement with previous results on Holocene stalagmites from Oman, and from Central Germany [13,22,23]. In stalagmites from N. W. Germany we have found that periodic drier sequences correlate with periods of increased Δ14C throughout the last part of the Holocene [22], corroborating the findings of Bond et al. in the North Atlantic [20].

Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

The stalagmite SPA 12 was retrieved from Spannagel Cave, which is part of a high-elevation cave system in the Central Alps of Austria [11]. The cave’s entrance is located at 2524 m a.s.l. and the site at which SPA 12 formed is located at 2347 m a.s.l.. The stalagmite formed in a remote part of the 10 km long cave system where today’s air temperature is constant at +1.8 °C and relative humidity is near condensation throughout the year. When SPA 12 was removed in 1998, its top was wet, but the dripping rate from the stalactite above was extremely slow. The sample is composed of white, densely crystalline low-Mg calcite, showing columnar-shape crystals in thin section. 13 ages were derived from the 20 cm long stalagmite applying the Th/U method (TIMS) as described earlier [12,13]. Dating is possible at high precision due to high uranium content.

The age–depth relationship reveals a fairly constant growth rate of 75 μm/yr during the last 2000 yr and a slower rate of 17 μm/yr in the section between 2000 and 5000 yr at the base of the sample. The youngest age at 0.2 cm is 64 ± 4 yr, and a linear fit through the top 5 points suggests that the age at the top of stalagmite has an approximate date of 1950 AD. Applying a growth rate model the depth profile were converted into an age profile. In this study the authors only discuss the sections with better time resolution deposited over the last 2000 yr. The δ18O age profile consists of 700 samples obtained at 100 μm increments (using micromilling, for analytical details see [14]) thus resulting in an average resolution of slightly over one year per isotope sample.

As the authors exclude kinetic effects, the changes in the isotopic composition of the calcite only depend on the variation in precipitation and temperature during calcite formation [15,16].

The conversion of the isotopic profile into temperature is performed by applying the transfer function (curve) obtained from 5 points with known temperature from the average yearly temperature reconstruction for the Alps by Luterbacher et al. [17] and the isotopic composition of the stalagmite in the corresponding sections.

The relationship between temperature and the isotopic composition of carbonate at Spannagel:
The fact that we are able to determine a relationship between temperature and the isotopic composition of carbonate at Spannagel is certainly unusual, as for most stalagmites the kinetic effect and the source effects overprint the temperature signal. The main reason is that SPA 12 displays a negligible kinetic effect, probably due to the very low temperature in the cave which reduces the outgassing of excess CO2, accompanied by high drip rates during the summer. Consequently, the relationship between temperature and isotopic composition results from a mixing of different types of precipitation that varies with temperature. The isotopic composition of precipitation in Europe strongly depends on the amount of rainout of the clouds; on its path across the continent and as temperature decreases, precipitation becomes lighter. Winter precipitation is significantly lighter than that in summer and precipitation that falls upon Central Europe is more depleted than that from the southern trajectories which partly pass over the Mediterranean Sea (yearly average Stuttgart: –8.1‰, Genoa: –5.6‰) [24].

A first explanation for the observed relationship between isotopic composition and temperature is that the trend at Spannagel relies on a response of hydrology to climate, for example if the supply of winter precipitation to the ground water reservoir feeding the drip water is reduced during periods of colder climate. The drip water isotope data suggest that presently winter precipitation contributes about 40% to the drip water that forms stalagmites. A reduction of the winter contribution to 32% would generate 1‰ heavier drip water. Removal of snow on the ridge above the karst system by wind erosion or a shorter lasting melting period in summer reducing the supply of winter precipitation to the aquifer are possible explanations that are being presently tested.

Another possible explanation is that the isotopic composition of stalagmites in Spannagel reflects changes of the temperature and the strength of the NAO index. In Northern Europe, periods of low NAO index are characterized by colder and drier winters with reduced contribution of precipitation originating from the North Atlantic [25,26]. The opposite mode, a high NAO index, causes wet and warmer winters in Northern Europe. Temperature correlates with the NAO both in Central Europe and in the Alps, whereas it anticorrelates with the NAO in southern Europe [27,25]. Wanner et al. have shown this relationship to have existed since 1700 when meteorological records were first available, although the correlation of temperature to the NAO index at times breaks down.

For a composite of seven stations in Austria, Kaiser et al. [28] have shown that the isotopic composition of precipitation correlates with the NAO index and that a positive NAO index is accompanied by y18O values 1–2‰ heavier than average. This trend may be ascribed to a reduction of the share of lighter winter precipitation during NAO+, resulting in a heavier yearly average precipitation. They also conclude that two localities in Austria, Villacher Alpe and Graz, receive a significant contribution of heavier Mediterranean winter precipitation [28]. A similar situation may arise at Spannagel, where data from regular monitoring of drip water at different locations conducted since 1998 show that drip water is heavier by about 2‰ in comparison to the regional δ18Oppt vs. the altitude trend in the Austrian Alps [29]. This second hypothesis, that could explain the relationship between temperature and isotopic composition in Spannagel, assumes that the cave receives variable amounts of precipitation from northern and southern sources. If so, it becomes sensitive to variations of the NAO index: during high NAO index annual precipitation at Spannagel becomes lighter due to a larger contribution of precipitation from northern trajectories, and vice versa. This behavior is opposite to the trend of isotopic composition of precipitation in Austria, where heavier isotopic composition is related to a positive NAO, as described by Kaiser et al. [28]. Therefore, if this second hypothesis is correct, it requires that the source-effect at Spannagel Cave overcompensates the winter/summer-effect, leading to observed heavier precipitation with colder temperatures.

Both these hypotheses will be tested in the future. Recapitulating, despite that the process is not yet well understood, the good correlation of the temperature in Spannagel both with the winter temperature in the Alps as well as with 14C, clearly suggests that the intensity of the sun plays a mayor role on the isotopic composition of the drip water in the cave. In the first hypothesis, it would explain the variability of hydrology, in the second, it would rather suggest that the sources of precipitation vary in accordance with the solar intensity.


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

 


(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

 


(4) - Remarques générales

The knowledge of the evolution of temperature during the last 2000 yr (directly measured data are available only for the last 200 yr) is important as climate models are often verified to this curve. The temperature reconstruction by the IPCC is utilized in the majority of cases for verification [1,2]. This reconstruction is mainly based on data from tree rings, as well as from high latitude ice cores, corals and seasurface temperature from highly resolved near-shore sediments. This temperature curve only shows small variations during the last 1800 yr, but demonstrates an abrupt temperature increase after 1860, which is generally ascribed to the increase of the greenhouse gases CO2 and CH4 in the atmosphere. However, it is presently being discussed what type of climate existed during Medieval times [3] and if the Medieval Warm Period (MWP) was a global or regional phenomena [4]. The statistical uncertainty in the IPCC curve in the section around 1000 AD is about ± 0.5 °C [1]. However, another major source of uncertainty is that biological samples, which are the foundation of climate reconstructions, tend to accommodate with climate change, and the accuracy of the temperature estimates based on biological proxies is not likely to be better than 1.3 °C [5]. Several other climate archives, such as ice cores [6], sediments from high accumulation sites [7] or mountain glaciers in the Alps [8], suggest that the Northern Hemisphere has undergone major climatic changes during the last 2000 yr. The Roman Period about 2000 yr before present as well as the MWP, lasting from about 800 to 1300 AD were recognized as phases of glacier retreat, whereas the Little Ice Age (LIA), from about 1400 to 1850 AD, marked a period of generally positive glacier mass balances [9]. Recently, Moberg et al. [10] have reconstructed temperatures for the Northern Hemisphere from low and high resolution proxy data from eleven archives. Their multi-proxy reconstruction reveals a relatively large multicentennial variability which clearly contrasts the reconstructions which have mainly used tree-ring data sets of annual and decadal resolution [1]. The amplitude of temperature variations between the LIA and the MWP in their stack is about 0.9 ° C.

Because of this ongoing discussion, more climate records are required to explain the likely causes of climate variations over the last two millennia. Here we reconstruct the evolution of temperature in the Central Alps during the last 2000 yr from the δ18O record of a well-dated stalagmite, SPA 12, retrieved from a high-alpine cave site. Our reconstruction relies on the temperature dependence of processes in the atmosphere that are recorded during the carbonate formation.


(5) - Syntèses et préconisations

Conclusions

Stalagmite SPA 12 from the Alps yields a highly resolved record of changes in climate at high elevation in Central Europe during the past 2000 yr at a resolution comparable to that of the Northern Hemisphere ice core records. The similarity between records in Europe, Greenland and from the Bermuda Rise suggests that the MWP had a major impact on the Northern Hemisphere climate. The temperature difference between the LIA and the MWP is about 1.7 °C on average. This difference is in good agreement with those derived from sediment cores from the Bermuda Rise but is larger than the reconstruction of temperature for the Northern Hemisphere from low frequency stacks and significantly larger than that in the IPCC report. The highly significant correlation with ∆14C underlines the important role of solar forcing as a driver of Northern Hemisphere climate during the past 2 millennia.

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