OXYGEN ISOTOPE COMPOSITION OF HUMAN TEETH AND THE RECORD OF CLIMATE CHANGES IN FRANCE (LORRAINE) DURING THE LAST 1700 YEARS 2,5 ´ ´ ´ ERIC ´ VALERIE DAUX1,4 , CHRISTOPHE LECUYER , FRED ADAM3 , FRANC ¸ OIS MARTINEAU2 and FRANC¸OISE VIMEUX1,6 1
Laboratoire des Sciences du Climat et de l’Environnement, UMR CEA/CNRS 1572, L’Orme des Merisiers, Bˆat. 701, CEA Saclay, 91191 Gif/Yvette Cedex E-mail:
[email protected] 2 Laboratoire CNRS UMR 5125 “Pal´eoenvironnements & Pal´eobiosph`ere”, Universit´e Claude Bernard Lyon 1, Campus de la Doua, F-69622 Villeurbanne, France E-mail:
[email protected] 3 Institut National de la Recherche Arch´eologique Pr´eventive, Direction R´egionale des Affaires Culturelles de Lorraine, Metz, France 4 Universit´e Paris 6, 4 place Jussieu, 75252 Paris Cedex 05, France 5 Institut Universitaire de France, 103 Boulevard Saint-Michel, 75005 Paris, France 6 Institut de Recherche pour le D´eveloppement, France
Abstract. In order to study climate variations during the last 1700 years in eastern France, fifty-eight oxygen isotope compositions of phosphate were measured in human tooth enamel. The individuals, who lived in Lorraine, are assumed to have drunk local water derived directly from rainfall. According to previous work, drinking water is the main source of oxygen that sets the isotopic composition of phosphatic tissues in humans. The empirical fractionation equation determined from our data combined with those of Longinelli’s one [Geochim. Cosmochim. Acta 48 (1984) 385] was used to calculate the oxygen isotope composition of meteoric waters. The mean air temperature was inferred from these isotope ratios and the Von Grafenstein et al.’s [Geochim. Cosmochim. Acta 60 (1996) 4025] relationship between δ 18 O and air temperature. Oxygen isotope composition of present-day individuals yields a mean air temperature of 9.9 ± 1.7 ◦ C which is consistent with meteorological data. Application of this method to historical individuals results in mean air temperatures estimates 0 to 3 ◦ C higher than present-day air temperature. These warm air temperatures are not realistic during the so-called Little Ice Age for which an air-cooling of about 0.5 to 2 ◦ C has been documented. We propose that these relatively high δ 18 O values of human tooth enamel reflect higher mean δ 18 O values of meteoric water which can be attributed to an increased proportion of summer rainfall during the “Little Ice Age” time frame in Lorraine.
1. Introduction Although there is considerable uncertainty with respect to the rate and magnitude of any future warming which may occur as a result of human activities, one aspect of this question is not in dispute: any human-induced change in climate will be superimposed on a background of natural climatic variations. Hence, in order to be able to predict future climatic changes, it is necessary to have an understanding of how and why climates have varied in the past. Of particular relevance are historic climate variations over the last millennium. Climatic Change (2005) 70: 445–464 DOI: 10.1007/s10584-005-5385-6
c Springer 2005
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It has been suggested that a “Medieval Warm period”, a period relatively mild, occurred somewhere around the 10th to the 14th centuries. Climate reconstructions of this time interval are based primarily on dendrochronological data (e.g. Naurzbaev and Vaganov, 2000), glacier moraines (e.g. Grove and Switsur, 1994), and documentary historical data (e.g. Lamb, 1984; Pfister et al., 1998). The climatic signature of this period has been detected throughout Europe. In Western Europe, between A.D. 1090 and 1179, the mean air temperatures for 30-year periods were similar to those of the twentieth century (Pfister et al., 1998); the warm and stable winter climate in the 13th century (warmest time) supported subtropical plants as far north as Cologne, Germany (Pfister et al., 1998). However, regional and temporal climatic variability within the “Medieval Warm Period” time interval has been suggested (e.g. Broecker, 2001) and the validity of this term has been much debated (see e.g. Hughes and Diaz, 1994; Ogilvie and Farmer, 1997; Briffa, 2000; Ogilvie and J´onsson, 2001). The “Little Ice Age” has been defined in many ways (Ogilvie and J´onsson, 2001). Here it is defined as a cold climatic interval from ca A.D. 1350 to the late nineteenth century. Reconstructions of air temperature during the “Little Ice Age” come from datasets that include ice cores, tree rings, mountain glacier movements, lake levels, palynological data, isotope composition of speleothems and coral, and historical documents (e.g. Domack and Mayewski, 1999; Stevenson et al., 1999; Alcoforado et al., 2000; Verschuren et al., 2000; Kirchhefer, 2001; Kuhnert et al., 2002; Cronin et al., 2003). One of the longest instrumental records of atmospheric temperature is from central England. This record suggests that the air temperature during the 18th century was 2 ◦ C cooler than present (Manley, 1974). Most authors propose smaller air temperature differences between the Little Ice Age and the 20th. In some sites from Central and Western Europe, these differences are estimated to be closer to 1–1.5 ◦ C (Trepinska et al., 1997; Filippi et al., 1999; Brooks and Birks, 2001; Bodri and Dˆov´enyi, 2004). Mean values at the European or Northern land area scale are even lower. For Europe, a difference of ∼0.5 ◦ C of winter average temperatures between the period 1500–1900 and the 20th century was indeed calculated by Luterbacher et al. (2004). For the northern land area, the mean 17th summer temperature is also 0.5 ◦ C lower than between 1961–1990 (Jones et al., 2001). The research described here involved the analysis of oxygen isotope composition of phosphate from human tooth enamel in the context of the study of climate variations during the last 1700 years in Lorraine, France (Figure 1). The oxygen isotopic composition of phosphate from human skeleton (bone, tooth) mainly reflects the composition of drinking water (Longinelli, 1984; Luz et al., 1984; Kohn, 1996). In the case of tooth enamel, the oxygen isotope ratio (δ 18 O) record is restricted to the composition of water ingested during childhood until dental growth is complete, which occurs around the age of 12. The δ 18 O composition of tooth enamel may be preserved for a long time after death because the dense and well-crystallized hydroxyl-apatite is resistant to diagenetic alteration (Kolodny et al., 1983, 1996).
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Figure 1. (A) Lorraine and Alsace areas (Fr). S: Stuttgart (D); (B) site locations in Lorraine mentioned in this study. Black circles: tooth sampling, grey circle: tap and mineral water sampling, squares: precipitation sampling.
Longinelli (1984) and Luz et al. (1984) have shown that the δ 18 O composition of human phosphate is linearly related to the composition of drinking water. Except for the most recent periods (post 1960), characterized by a rapid increase of trade exchange that modified dietary behavior, the main source of drinking water was local water reservoirs (wells, springs, creeks, rivers) derived from meteoric waters (e.g. Rozanski, 1984, 1985; Fritz et al., 1987; Ingraham and Taylor, 1991; Kortelainen and Karhu, 2004). Correlations have also been established between the weighted oxygen isotope composition of precipitation and mean annual surface temperature at mid- to highlatitude regions, whereby higher temperatures correspond to higher δ 18 O in precipitation. For temperature below 15 ◦ C, the δ 18 O/Ts gradient is close to 0.6‰/◦ C (e.g. Dansgaard, 1964; Rozanski et al., 1992, 1993; Von Grafenstein et al., 1996; Fricke and O’Neil, 1999). The covariance of oxygen isotopic composition of precipitation and temperature has its origin in the isotope fractionation effects accompanying evaporation from the ocean and subsequent condensation during the atmospheric transport of the vapor. The 18 O/16 O of vapor in the air masses lowers as they move along surface temperature gradients and lose water (i.e. along increasing latitude, continentality and altitude). Stations situated in a mid-continental setting typically portray seasonal change in the isotopic composition of the precipitation, correlated with temperature in most cases (e.g. Rozanski et al., 1993). The lower δ 18 O in winter precipitation is due to lower condensation temperature in winter than in summer. The seasonal variation is smaller than the latitudinal variations of yearly averages.
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By combining the relationship between δ 18 O of tooth enamel and drinking water with the relationship between δ 18 O of meteoric water and temperature, Fricke et al. (1995) proposed that the air cooled during the “Little Ice Age” in Greenland from isotopic measurements of Inuit and Norse tooth enamel. However, Bryant and Froelich (1996) pointed out that part of the isotopic signal recorded could be attributed to sampling strategy. The main comment concerned possible culturally based differences in diet or food processing between the Norse and Inuit. Indeed, the low δ 18 O values that were attributed to the “Little Ice Age” event were derived from Inuit whereas the characterization of the “Medieval Warm Period” was based on Norse populations. Most intriguing is the lack of such a low δ 18 O event in the Dye 3 ice cores from Greenland contemporaneous to the “Little Ice Age”. We propose to investigate the evolution of the oxygen isotope composition of human tooth enamel from a rural, sedentary and geographically confined human population from the northeastern part of France (Lorraine; Figure 1) from the third century until the present. Dating of the human skeletal remains was performed by three methods: (1) 14 C dating or (2) typology of the grave goods associated with the skeletons, and/or (3) monastery chronicles. Present-day oxygen isotope compositions of both human teeth and local drinking water have also been measured in order to verify the relationship between δ 18 O of water and tooth enamel for the studied area. 2. Sample Collection Fifteen millilitres of tap water were collected on a monthly basis at Rehaupal in 2002 (Figure 1). Two samples of mineral water from Vittel and Contrexeville (Figure 1) were also taken from 1.5 L bottles produced in 2002. Atmospheric precipitations were collected from June 2002 to June 2003 at Dogneville, Bulligny and Bure (Figure 1) as part of the French GdR FORPRO program (CNRS). Samples were taken on an irregular basis (between one and three months) but the whole precipitation of the year was collected. To validate the relationship between the oxygen isotopic compositions of rain and tooth enamel, we compared the isotopic composition of present day individuals from two geographically distinct populations to their respective local meteoric water δ 18 0 values. Present-day human teeth were provided by dental surgeons practicing at “Le Tholy”, a little town located in Lorraine at an altitude of 600 m (Figure 1), and Qu´ebec City (Canada). The Canadian tooth samples are isotopically distinct from those of Lorraine (Section 5.1). Historical human teeth were obtained from the archaeological collections of the “Direction R´egionale des Affaires Culturelles de Lorraine” in Metz, France. The individuals studied lived in Lorraine in a rural habitat. Locations and living dates are given in Table I. This rural population used to be very sedentary (Dupˆaquier et al., 1988). However, except for 10 individuals (nuns, priest and castle inhabitants) whose birth and living places are known from chronicles, we cannot rule out the possibility of having foreign people buried in Lorraine (with non-regional dental
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TABLE I Location, type, altitude (m) and dating (year AD) of archeological and contemporaneous sites and oxygen isotope compositions of human tooth enamels (pre-molar or molar of teenagers and young adults) in ‰ relative to SMOW. All the sites, except Qu´ebec (Canada) are situated in Lorraine (France; Figure 1) Altitude
Age
δ 18 O
Necropolis
180
Necropolis
290
Necropolis
420
Necropolis
180
Castle
340
Abbey
325
Priest’s grave Chapel
340
Paupers’grave
240
340–360 340–360 340–360 340–360 700–800 700–800 700–800 900–1100 900–1100 900–1100 900–1100 1100–1200 1100–1200 1100–1200 1100–1200 1100–1200 1100–1200 1100–1400 1100–1400 1100–1400 1100–1400 1500–1600 1500–1600 1500–1600 1500–1600 1500–1600 1500–1550 1500–1700 1500–1700 1500–1700 1500–1700 1500–1700 1600–1650 1600–1650 1600–1650
17.4 17.6 16.7 18.0 16.6 17.3 17.6 17.1 17.2 17.4 15.9 17.4 17.4 18.4 17.2 17.2 17.6 17.5 17.5 17.4 17.1 17.2 15.2 15.8 15.6 17.0 18.5 18.0 17.3 17.3 17.3 17.7 17.7 16.7 17.2
Sample Metz–Colline Ste Croix Metz–Colline Ste Croix Metz–Colline Ste Croix Metz–Colline Ste Croix Nomexy Nomexy Nomexy Vitry/Orne Vitry/Orne Vitry/Orne Vitry/Orne Metz–Intramuros Metz–Intramuros Metz–Intramuros Metz–Intramuros Metz–Intramuros Metz–Intramuros Dommartin/Vraine Dommartin/Vraine Dommartin/Vraine Dommartin/Vraine Ban St Martin Ban St Martin Ban St Martin Ban St Martin Ban St Martin Metz–Intramuros Sepvigny Sepvigny Sepvigny Sepvigny Sepvigny Verdun Verdun Verdun
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TABLE I (Continued) Sample Verdun Verdun Verdun Verdun Verdun Verdun Verdun Epinal Epinal Epinal Epinal Epinal Le Tholy Le Tholy Le Tholy Le Tholy Le Tholy Le Tholy Qu´ebec Qu´ebec Qu´ebec Qu´ebec Qu´ebec
Altitude
Convent
340
Dentistry
600
Dentistry
Age
δ 18 O
1600–1650 1600–1650 1600–1650 1600–1650 1600–1650 1600–1650 1600–1650 1600–1700 1600–1700 1600–1700 1600–1700 1600–1700 1950–2000 1950–2000 1950–2000 1950–2000 1950–2000 1950–2000 1950–2000 1950–2000 1950–2000 1950–2000 1950–2000
16.9 17.6 19.2 18.3 18.3 17.6 18.6 16.3 17.1 18.1 17.4 17.3 17.0 17.3 17.3 16.3 16.2 16.4 14.1 15.0 15.3 14.4 15.3
δ 18 O) included in the dataset. To decrease the probability of sampling emigrants we selected teenagers or young adults only. Dental caries were systematically excluded from the sampling for oxygen isotope analysis. The least damaged molar and pre-molar teeth available were used. Teeth were crushed and enamel was sorted by handpicking under a microscope and cleaned with hydrogen peroxide before being crushed to powder in an agate mortar. 3. Analytical Techniques 3.1.
OXYGEN ISOTOPE ANALYSIS OF PHOSPHATE FROM TOOTH ENAMEL
Most studies of δ 18 O in apatite start by isolating PO3− 4 using acid dissolution and anion-exchange resin. The protocol used is directly derived from the original method published by Crowson et al. (1991) and slightly modified by L´ecuyer et al. (1993).
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After dissolution of 15–30 mg of powdered tooth enamel in 2 M HF at 25 ◦ C for 24 h, the CaF2 that precipitates is separated from the solution, which includes the phosphate, by centrifugation. The CaF2 precipitate is rinsed three times using double deionised water (DDW) and the rinse water is added to the solution which is neutralized with a 2 M KOH solution. Cleaned AmberjetTM resin (2 mL) is added to the neutralized solution in polypropylene tubes. The tubes are placed on a shaker table for 12 h to promote the ion exchange process. Excess solution is discarded and the resin is washed five times with DDW to remove the traces of ionic contaminants. To elute the phosphate ions quantitatively from the resin, 25–30 mL of 0.5 M NH4 NO3 is added to bring the pH of the solution to 7.5–8.5, and the tubes are gently shaken for about 5 h. Silver phosphate is precipitated from the eluted solution following the method of Firsching (1961). The solution is placed in a 250 mL Erlenmeyer flask and about 1 mL of concentrated NH4 OH is added to raise the pH to 9–10. Fifteen millilitres of ammoniacal AgNO3 solution is added to the flask. Upon heating this solution to 70 ◦ C in a thermostatic bath, millimetre-size yellowish crystals of Ag3 PO4 are quantitatively precipitated. The crystals of silver phosphate are collected on a Millipore filter, washed three times with DDW and air dried at 50 ◦ C. The measurement of 18 O/16 O ratios is performed by the reaction of silver phosphates with graphite to form CO2 (O’Neil et al., 1994; L´ecuyer et al., 1998). Samples of Ag3 PO4 are mixed with pure graphite powder in proportions of about 0.8 mg of C to 15 mg of Ag3 PO4 . They are weighed into tin reaction capsules and loaded into quartz tubes and degassed for 30 min at 80 ◦ C in vacuum. The samples are then heated at 1100 ◦ C for 1 min to promote the redox reaction. The CO2 produced during this reaction is directly trapped in liquid nitrogen to minimize isotopic exchange with quartz at high temperature. The CO2 gas is then analysed on a Finnigan DeltaETM mass spectrometer at the University of Lyon. Isotopic compositions are quoted in the standard δ notation relative to V-SMOW. The reproducibility of measurements carried out on tooth enamel samples is better than 0.2‰ (1σ ). Silver phosphate precipitated from standard NBS120c (natural Miocene phosphorite from Florida) was repeatedly analysed (δ 18 O = 21.75 ± 0.20; n = 17) along with the silver phosphate samples derived from the human tooth collection.
3.2.
OXYGEN ISOTOPE ANALYSIS OF WATERS
Aliquots of three millilitres of tap or mineral water (14 samples) are equilibrated with twenty micromoles of CO2 at 25 ◦ C for 48 h. CO2 is analysed with a Finnigan DeltaETM mass spectrometer at the University of Lyon. Reproducibility of oxygen isotope measurements is better than 0.1‰. The SMOW standard was reproducible within 0.17‰. Atmospheric precipitations were analysed in LSCE (same procedure) with a Finnigan MAT 252. Reproducibility of oxygen isotope measurements is ±0.05‰.
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4. Results 4.1.
HUMAN TOOTH ENAMEL
Oxygen isotopic compositions of human tooth enamel are presented in Table I. Six present-day human tooth enamel samples from Le Tholy (Lorraine, Fr) and five samples from Qu´ebec city (Qu´ebec, Ca) have δ 18 O values that range from 16.2 to 17.3‰ (meanvalue = 16.7‰), and 14.1 to 15.3‰ (meanvalue = 14.8‰), respectively. Forty-seven tooth enamel samples of individuals that lived from the third to the seventeenth centuries have δ 18 O values that range from 15.2 to 19.2‰ (Figure 2). The highest values are documented for individuals that lived during the seventeenth century in Verdun and the lowest ones correspond to individuals that lived during the sixteenth century in Ban St Martin (Figure 1). During the “Medieval Warm Period”, we calculate a mean δ 18 O value of 17.3 ± 0.5‰ (n = 14). This average includes samples from three different sites in the region. This value is indistinguishable from the mean value obtained for the teeth of the “Little Ice Age” (17.3 ± 0.9; n = 26; 4 sites). The variability observed during the “Little Ice Age” is large (Figure 2). 4.2.
ENVIRONMENTAL WATERS
The mean oxygen isotopic composition of monthly-sampled tap waters (Table II) from Rehaupal is −8.5‰ with a standard deviation of ±0.16‰, indicating the absence of seasonal variation in the composition of water drunk by the inhabitants. The analysis of two aliquots of mineral waters from the same area (Vittel and Contr´exeville) yields comparable results (Table II). The mean weighted annual oxygen isotopic compositions of the precipitations at Dogneville, Bulligny and Bure are −8.1, −7.9 and −8.1‰ respectively (Table III).
Figure 2. Oxygen isotope compositions (‰ SMOW) of phosphate of tooth enamel from individuals having lived in Lorraine (France) over the last 1700 years. Small open circles: individual data. Black circles: mean values by site except the one at 1550 AD, which is the mean of five data from one site plus one from another site of the same age (see Table I). “X” error bars: uncertainty on the age (as in Table I); “Y” error bars: standard deviation of the mean.
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TABLE II Oxygen isotope compositions of tap waters from Rehaupal (Lorraine, Fr; Figure 1) collected during the year 2002. Values are reported in ‰ relative to SMOW Location Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal Rehaupal
Time Tap water January February March April May June July August September October November December Mineral water
δ 18 O (H2 O) −8.6 −8.4 −8.4 −8.3 −8.4 −8.7 −8.6 −8.5 −8.4 −8.5 −8.2 −8.6 −8.3 −8.4
Vittel Contrexeville
The range of variation of the oxygen isotopic composition through the year is ca. 7‰. The δ 18 O value of local water in Quebec City is calculated to be close to −11.5‰ from a global dataset according to an algorithm developed by Bowen and Wilkinson (2002) and refined by Bowen and Revenaugh (2003). The dataset used is derived from the International Atomic Energy Association/World Meteorological Organization Global Network for Isotopes in Precipitation (IAEA/WMO, 2001). 5. Discussion 5.1.
THE PHOSPHATE-WATER δ 18 O RELATIONSHIP
When our present-day human tooth enamel samples are plotted against the respective local meteoric water δ 18 O values, the data plot close to the fractionation line (Figure 3) determined by Longinelli (1984), which does not significantly differ from Luz et al.’s (1984) regression. Therefore, we suggest that it is appropriate to apply a fractionation equation that relates the δ 18 O of local waters to the δ 18 O of human tooth enamel (δ 18 Owater = 1.49 ± 0.09 × δ 18 Ophosphate − 33.6 ± 1.5) to the analysis of our historic samples.
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TABLE III Amount and oxygen isotopic compositions of precipitation collected at Bure, Dogneville and Bulligny (Lorraine, Fr; Figure 1) in 2002–2003. The date of sampling and the time of collection in number of days after 1st January 2002 are given. The values of δ 18 O are reported in ‰ relative to SMOW Date
Day after 01/01/02
Dogneville, altitude = 320 m 4/7/02 155–185 4/9/02 186–247 4/10/02 248–277 6/11/02 278–310 17/2/03 311–413 12/3/03 414–436 16/4/03 437–471 20/5/03 472–505 26/6/03 506–542 Total Mean weighted annual composition Summer–Winter amplitude Bulligny, altitude = 260 m 4/7/02 155–185 4/9/02 186–247 2/10/02 248–275 6/11/02 276–310 26/2/03 311–422 12/3/03 423–436 16/4/03 437–471 20/5/03 472–505 23/6/03 506–539 Total Mean weighted annual composition Summer–Winter amplitude Bure, altitude = 335 m 4/9/02 155–247 6/10/02 248–275 6/11/02 276–310 6/1/03 311–371 26/2/03 372–422 11/3/03 423–435 16/4/03 436–471 20/5/03 472–505 23/6/03 506–539 Total Mean weighted annual composition Summer–Winter amplitude
Amount (cm)
6.8 15.1 6.2 17.8 34.3 2.6 1.6 5.8 6.1 96.4
δ 18 O (‰) −5.1 −6.3 −8.6 −7.7 −10.6 −11.0 −7.9 −5.9 −3.5 −8.1 7.5
3.1 17.3 4.6 14.2 32.8 3.2 1.6 3.1 10.8 90.7
−6.7 −6.2 −7.7 −7.9 −10.0 −11.7 −8.4 −6.5 −3.8 −7.9 7.9
14.6 2.9 20.5 34.4 10.7 3.9 1.8 7.1 9.5 105.5
−6.7 −6.3 −8.3 −8.9 −10.5 −10.9 −8.2 −6.9 −4.5 −8.1 6.4
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Figure 3. Fractionation equation relating the δ 18 O of human phosphate to the δ 18 O value of local waters. Open circles: data from Longinelli (1984). Filled circles: this study. The δ 18 O values of human phosphate correspond to present-day samples collected at Qu´ebec City (Qu´ebec, Ca) and Le Tholy (Lorraine, Fr). For Qu´ebec, the δ 18 O value of local water is inferred from the IAEA–WMO dataset. For Le Tholy, it corresponds to the mean value of tap waters collected in 2002 at the 10 km far village of Rehaupal.
5.2.
THE WATER δ 18 O-AIR TEMPERATURE RELATIONSHIP IN THE PRESENT-DAY STUDIED AREA
The oxygen isotopic composition of rainfall is not dependent only on temperature but also on the rate of precipitation and on the trajectory of air masses. For mid latitudes, rough linear correlations are observed between the δ 18 O of meteoric waters and air temperature. Several relationships (e.g. Dansgaard, 1964; Yurtsever and Gat, 1981; Rozanski et al., 1992, 1993; Fricke and O’Neil, 1999) have been established. Von Grafenstein et al. (1996) proposed a relationship for mid latitudes (δ 18 OH2 O = 0.58 ± 0.11 × T − 14.48), from the study of the water oxygen ratios of lake Ammersee (southern Germany) that is also conservative through time, at least throughout the quaternary. Note that the use of the different equations leads to sizable differences in the estimates of air temperature. For example, when applied to the waters sampled at Rehaupal and Dogneville in 2002 (mean δ 18 O values of −8.5 and −8.1‰ respectively; Table II), the calculated air temperatures are 7 and 8 ◦ C when using the Dansgaard’s (1964) relation, 10 and 12 ◦ C when using the Yurtsever and Gat’s (1981) relation, and 10.4 ± 1.2 ◦ C and 11.1 ± 1.2 ◦ C using the Von Grafenstein et al.’s (1996) relation. The last estimates match the true 9 and 10 ◦ C mean temperatures measured at or near the sites (data provided by M´et´eo-France). Moreover Lorraine is not far from the Ammersee area where Von Grafenstein et al.’s relationship was defined (ca 400 km). Therefore, for interpreting our δ 18 O dataset of human tooth enamel in terms of mean air temperatures,
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Figure 4. Variations through the last 1700 years of the apparent “atmospheric temperatures” in Lorraine deduced from the oxygen isotope data of human tooth enamel (Figure 2) by using the equations given in Figure 3 and in Von Grafenstein et al. (1996). See text for more explanations. Symbols as in Figure 2.
we have combined the phosphate–water fractionation equation given in Figure 3 with the Von Grafenstein et al.’s (1996) relationship established for mid latitudes to determine past air temperatures. Using this method with δ 18 O of teeth from individuals living today in Lorraine (at Le Tholy) and in Qu´ebec City, mean values of 9.9 ± 1.7 ◦ C and 4.7 ± 1.3 ◦ C are obtained, which is in agreement with recorded annual temperatures (4 ± 0.8 ◦ C at Qu´ebec City; data from the Meteorological Service of Canada). This result suggests that this method can be applied to historical individuals if indeed the relation between δ 18 O in precipitation and air temperature holds for the past 2000 years.
5.3.
THE δ 18 O RECORD OF HUMAN TOOTH ENAMEL OVER 1700 YEARS IN LORRAINE
The differences between historical and present-day air temperatures range from 0 to +3 ◦ C, except at one site (−0.5 ◦ C) during the sixteenth century (Figure 4). Although the error bars are big, the calculated temperatures are not consistent with the air cooling prevailing during the “Little Ice Age” (−0.5 to −2 ◦ C; see references in introduction). An air cooling of −0.5 to −2 ◦ C should correspond to a ≈0.2 to 0.8‰ decrease in the δ 18 O of human tooth enamel, which is not observed (except at one site). Accordingly, another process has to be invoked for explaining the unexpected 18 O-enrichment of the phosphate during the “Little Ice Age.” Fricke et al. (1995) described a 3‰ decrease in the δ 18 O values of Norse and Inuit tooth enamel during the “Little Ice Age”. They interpreted these results as reflecting an air-cooling of about 6 ◦ C over 300 years. The examination of Figure 2 from Fricke et al. (1995) reveals that the distribution of data mainly reflects the latitudinal variation (15◦ ) of the sampling sites across Greenland. As mentioned in Bryant and Froelich’s (1996) comment, the cooling event was defined on the basis of
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Figure 5. δ 18 O (in ‰) in precipitation (circle) and surface waters (squares) in Lorraine versus altitude. Re: Rehaupal (510 m); Vi: Vittel (365 m); Co: Contrex´eville (350 m); Bur: Bure (335 m); Do: Dogneville (320 m); Bul: Bulligny (260 m). The filled symbols correspond to mean annual values, the open ones to a single sample. The line is the European lapse rate proposed by Poage and Chamberlain (2001).
samples coming from only one Inuit site in Julianehaab Bay. Therefore, a spatially biased sampling cannot be excluded as sources of low δ 18 O values in the Little Ice Age. In our study, the geographic area is restricted enough to avoid any latitudinal effect in the isotopic composition of rainfall (Figure 1). However, the comparison of temperatures deduced from the oxygen isotopic composition of enamel may be biased by an altitude effect. The isotopic composition of Lorraine water (precipitation and tap water) decreases linearly with increasing elevation (Figure 5). Our data are in agreement with the lapse rate of −0.21‰/100 m proposed by Poage and Chamberlain (2001) for Europe from a larger dataset including precipitation, surface waters and ground waters. Note that, as a corollary to this relation, the mean weighted annual oxygen isotopic composition of the precipitations and the mean annual isotopic composition of tap water are equal if corrected for the altitude effect. The altitudes of the archeological sites range between 183 and 340 m. The altitude of Le Tholy, where the present-day samples of Lorraine come from, is 600 m (Table I). The altitude effect of −0.21‰/100 m may therefore produce differences in the oxygen isotopic composition among our dataset up to 0.9‰. Coupling this with the T-δ 18 O relationship would yield an altitude related variation of temperature up to 1.6 ◦ C. The data normalised to 300 m, the mean altitude of Lorraine, using the −0.21‰/100 m lapse rate, show no clear variation through time (Figure 6a and 6b). The”Medieval Warm Period” appears as hot as the 20th century (up to 1950–1970, time of growth of the present-day teeth included in our sampling). This observation is in agreement with most estimates of the temperatures prevailing during the “Medieval Warm Period” (e.g. Jones et al., 2001; Esper et al., 2002). The relatively high temperatures calculated for the “Little Ice Age” are more unexpected. One
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Figure 6. Variations through the last 1700 years of the apparent “oxygen isotopic composition” (A) and “atmospheric temperatures” (B) in Lorraine deduced from the oxygen isotope data (Figure 2) in tooth enamel and normalised to an altitude of 300 m using a −0.21‰/100 m lapse rate. Symbols as in Figure 2.
possible interpretation is that the cooling during the “Little Ice Age” has actually been moderate or non-existent in Lorraine. Taking into account the variability of the isotopic signal in human tooth enamel (see contemporaneous samples) and the error on the coefficients of the human phosphate-meteoric water fractionation equation (Section 5.1) and of the meteoric water-mean atmospheric temperature equation (Section 5.2), the effect of a small decrease of the atmospheric mean temperature may not be detectable. Alternatively, if a cooler mean air temperature is postulated, the isotopic composition of tooth enamel should be lower than at present unless the relationship between the mean annual oxygen isotopic composition of precipitation and the mean atmospheric temperature has varied through time. The relatively high δ 18 O values of phosphate during the “Little Ice Age” may then result from an increase in the proportion of summer precipitation relative to winter rainfall. Indeed, the difference of δ 18 O values between summer and winter precipitation may reach several permil in Lorraine (Table III). A shift towards an increasing rate of summer precipitations may result in an increase of the mean δ 18 O value of the local waters (springs, wells, etc.), which could balance the effect of diminishing temperatures. A simple calculation can be performed to test the validity of this hypothesis. We infer that the “Little Ice Age” was 2 ◦ C colder than the present (the most chilly recorded conditions at this time, see ref. in Section 1) and to have experienced a higher proportion of summer precipitation. This type of rain distribution can be
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TABLE IV Amount of precipitation (cm) at Dogneville (Lorraine, Fr) and Munich (D); data provided by M´et´eo-France Precipitation Month
Dogneville
Munich
January February March April May June July August September October November December Total Winter–Summer
7.1 7.3 7.4 6.6 8.1 8.2 7.9 5.8 8.8 8.9 8.8 9.4 94.3 50–50%
7.2 5.2 5.2 6.0 11.2 12.4 13.6 9.6 7.6 4.4 6.0 4.8 93.2 35–65%
found now in eastern Germany (D) for instance. The station of Munich (D) records approximately the same annual amount of rain as Dogneville (F) (93.0 versus 94.3 cm/an) but wetter summers: the summer precipitation (April–September) accounts for 67% of the yearly total, whereas it is 52% at Dogneville. Therefore to simulate a mean annual weighted isotopic composition of precipitation at 300 m in Lorraine during the “Little Ice Age, we use the mean δ 18 O monthly values of the precipitation in Lorraine (calculated from the data available at Dogneville, Bure and Bulligny; Table III) normalised to an altitude of 300 m (with the −0.21‰/100 m lapse rate) and corrected for a uniform 2 ◦ C cooling (−1.2‰) and the same distribution of precipitations as the one prevailing in Munich (Table IV). It yields a mean weighted annual δ 18 O in precipitation of −8.5‰, corresponding to a mean temperature of 10.4 ± 1.2 ◦ C and to a δ 18 O of tooth enamel of 16.9 ± 0.3‰. This last value is close to those deduced from the tooth enamel of individuals of the “Little Ice Age” time period (Figure 2). This simple calculation does not demonstrate unequivocally that the temperatures were lower and the summers were wetter during the Little Ice Age than at present. In particular, we are well aware that the monthly values of δ 18 O in precipitations may have been different from those measured in Lorraine at present if the rain distribution was different during the Little Ice Age. However, the calculation shows that if the “Little Ice Age” was colder (even 2 ◦ C colder) and had wetter summers, the δ 18 O values of the precipitation and therefore of the human phosphates could have been higher.
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Figure 7. Variation of grape harvest dates from the 17th to the 20th centuries in Lorraine, Alsace and western Germany. The y coordinate is the number of days between the 31st of August and the grape harvest date (note that the axis is reversed). The heavy line represents the 10 years running mean curve of the average value of the three sites.
To our knowledge, there is no atmospheric temperature reconstruction in Lorraine during the Little Ice Age available in the literature. Vine development annual cycle strongly depends on climatic conditions – especially temperatures (from March–April to September–October; Pfister, 1979; Le Roy Ladurie and Baulant, 1981). The earlier the grape harvest date the hotter the spring and summer time. We have compiled grape harvest dates for the vineyards of Lorraine (1840–1879; Muller, 1991), Alsace (1700–1947; Figure 1; Muller, 1991, 1993, 1995) and two near sites in western Germany (Stuttgart area in Figure 1; 1680–1830; Angot, 1883). The three series are mutually consistent and show an appreciable warming of this part of the Western Europe since the beginning of the 18th century (Figure 7). This observation is consistent with a “Little Ice Age” significantly colder than the present time in Lorraine. Our hypothesis is also supported by the precipitation measurements undertaken at the Observatory of Paris since 1680 (Slonosky, 2002). The author concluded from the analysis of time series that “the most striking change in the precipitation patterns over the past 300 years is the change in seasonal distribution”. Winter precipitation (October to March) increased on average by 24 mm per century and some intervals (1700–1750 for example) were characterized by winter/summer precipitation ratios as low as 30% (Slonosky, 2002). 6. Concluding Remarks The oxygen isotopic composition of the phosphate of human tooth enamel from individuals who lived in Lorraine in the last 1700 years has not changed significantly. In particular, the signal appears to be almost flat during the “Little Ice Age” period. The δ 18 O of spring and well water is controlled by the isotopic composition of rain, which varies through the year; summer waters being isotopically heavier than winter ones. The δ 18 O of tooth enamel, which integrates the isotopic signal of rain over
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the growth time of the tooth via drinking water, reflects the seasonal distribution of precipitation. Our data are consistent with a change of the seasonality of the rains in the northeastern part of France during the “Little Ice Age” (winter precipitation less abundant than summer precipitation). This study illustrates that variations in air temperatures cannot be deduced from the oxygen isotopic composition of human tooth enamel alone. The relatively high atmospheric temperatures calculated for the “Little Ice Age” are not in agreement with the general literature and the regional pattern of grape harvest dates. This result questions the applicability of spatially derived δ 18 O of precipitation/temperature relationships to study temporal patterns even when the time slice considered is short. Acknowledgments We thank Drs C. Gillet and P. Gaudreault who provided present-day teeth from Vosges and Qu´ebec respectively; A. Bergeron for his assistance in Qu´ebec; Mr. Legendre for the access to the collections of the Direction R´egionale des Affaires Culturelles de Lorraine; B. Daux who sampled tap water monthly at Rehaupal (Vosges, France); J.M. Moisselin from M´et´eo-France for providing meteorological data; M. Fordant-Delvi for assistance in handling samples; and G. Hofman, U. Von Grafenstein, V. Masson-Delmotte, A. Person, V. Balter and L. Simon for fruitful discussions. The paper benefitted by the constructive reviews of K. O’Keefe, P. Koch and two anonymous reviewers. This work is financially supported by the CNRS and ANDRA through the ECLIPSE program and the GdR FORPRO (Research action number 2002.I; GDR FORPRO contribution number 2004/01 A). References Alcoforado, M. J., De Fatima Nunes, M., Garcia, J. C., and Taborda, J. P.: 2000, ‘Temperature and precipitation reconstruction in southern Portugal during the Late Maunder Minimum (AD 1675– 1715)’, Holocene 10(3), 333–340. Angot, M. A.: 1883, ‘Etude sur les vendanges en France’, in Annales du Bureau Central M´et´eorologique de France, M´emoires Divers, Paris. Bodri, L. and D¨ov´enyi, P.: 2004, ‘Climate changes of the last 2000 years from borehole temperatures: Data from Hungary’, Global Planet. Change 41, 121–133. Bowen, G. J. and Wilkinson, B.: 2002, ‘Spatial distribution of δ 18 O in meteoric precipitation’, Geology 30(4), 315–318. Bowen, G. J. and Revenaugh, J.: 2003, ‘Interpolating the isotopic composition of modern meteoric precipitation’, Water Resour. Res. 39(10), 1299, doi:10.129/2003WR002086. Briffa, K. R.: 2000, ‘Annual climate cariability in the Holocene: Interpreting the message of ancient trees’, Quat. Res. Rev. 19, 87–105. Broecker, W. S.: 2001, ‘Was the Medieval period global?’, Science 291, 1497. Brooks, S. J. and Birks, H. J. B.: 2001, ‘Chironomid-inferred air temperaures from lateglacial and Holocene sites in north-west Europe: Progress and problems’, Quat. Sci. Rev. 20, 1723–1741.
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