Introduction

Radiocarbon, a radioactive isotope of carbon, is mainly produced in the earth’s atmosphere due to the interaction between neutrons associated with cosmic rays and nitrogen in the atmosphere1. Thermal neutrons were also produced anthropogenically during atmospheric nuclear weapon tests in 1950s and 1960s, which interacted with nitrogen to form radiocarbon2,3. Immediately after its production, radiocarbon (14C) oxidizes to14CO and 14CO2 subsequently4,5. The 14CO2 in the atmosphere enters different reservoirs like the ocean and biosphere through carbon cycle pathways. Initially, it was assumed that 14C in the atmosphere had been homogeneously distributed, but later, several studies reported spatial heterogeneity or offsets in atmospheric 14C values6,7,8,9,10.

The spatial heterogeneity in atmospheric 14C levels amplified when 14C concentration spiked during the 1950s and 1960s11,12,13,14,15,16. During this period, multiple atmospheric nuclear bomb tests were carried out, most of which were in the northern hemisphere17,18,19,20. It added large excess of 14C into the northern stratosphere, which was then transported to the northern troposphere and subsequently across the equator to the southern hemisphere 21,22,23,24. This addition almost doubled the 14C levels in the atmosphere14,15,25, which was observed in different atmospheric 14C measurements from both hemispheres during this period24,25,26,27,28,29. Due to stratospheric input of bomb-produced excess 14C in the northern hemisphere and its distribution by atmospheric circulations, a latitudinal gradient in atmospheric 14C level was created between the north and south hemispheres,13,16,21. 100–500 ‰ higher 14C levels were observed in the northern hemisphere compared to the southern hemisphere, and even within the northern hemisphere, decreasing trend was observed in 14C levels from high to low latitudes12,16. Large fluxes of excess 14C from the atmosphere to the ocean and biosphere during the bomb peak probably also contributed to regional atmospheric 14C variations13. Latitudinal gradients of 14C levels for a few years following the nuclear tests were dominantly influenced by stratospheric 14C input, and after the late 1960s, ocean and biosphere related 14C fluxes also contributed significantly to the latitudinal 14C gradient16,30,31. In recent years, fossil fuel emissions influenced the spatial distribution of atmospheric 14C level31,32.

In the past few decades, bomb 14C has been applied as a useful tracer to study atmospheric, terrestrial, and oceanic processes13,20,33,34,35,36,37,38,39. Bomb 14C also facilitated the dating of recent organic materials40,41. For the use of bomb 14C in the carbon cycle modelling and age calibration, Hua et al.14 defined different zones in the atmosphere based on the atmospheric 14C concentration between different latitudes. The boundary between these zones has been refined after the introduction of more radiocarbon records, and it led to a better understanding of radiocarbon distribution in the atmosphere15. Currently, there are three zones defined in the northern hemisphere, namely northern hemisphere (NH) zones 1, 2 and 3. The boundary between NH zone 1 and 2 is defined around 40°N by the Ferrel cell-Hedley cell boundary14, and the boundary between NH zone 2 and 3 is defined by TLPB (Tropical Low Pressure Belt) over continents15.

In spite of several measurements, there exist regions on the global map which are poorly represented by atmospheric 14C records during the bomb 14C period. To improve bomb 14C application as a tracer or dating tool, atmospheric 14C records from such underrepresented regions need to be studied. One such region is the eastern Mediterranean region in NH zone 2, which is represented by only about one year of atmospheric 14C measurement during the bomb 14C period15. The other nearest record in NH zone 2 is from the eastern coast of the Atlantic Ocean15. Therefore, more terrestrial bomb 14C records from this region are required to better understand the atmospheric bomb 14C distribution in the region. Tree ring cellulose-based 14C records can act as a reliable proxy for atmospheric 14C levels if the timing of ring growth (cellulose formation) is known and if stored photosynthate carbon is not used by tree for wood production. It becomes more relevant in case of high-resolution radiocarbon records. In present study, pine tree wood from northern Israel has been studied for its sub-annual 14C concentration around the bomb 14C peak period. These values have been compared with other atmospheric and terrestrial 14C records from the northern hemisphere to better understand the distribution of 14C in the atmosphere. Further, the analyzed pine 14C record is used to understand the growing season of olive wood from the same region.

Results

Growth period of earlywood and latewood

In this study, the reported pine tree (Pinus halepensis, HAN 5B) record from Havat Hanania (northern Israel) spans between the years 1964 and 1968. The annual rings in the pine wood were clearly visible (Figure S1). The wood sample showed distinct colour differences between earlywood and latewood, which helped in counting the growth year for each annual ring. Samples were sliced using a microtome, and about 4 sub-samples were obtained from each annual ring. A total of 20 contiguous samples were obtained and processed for alpha-cellulose extraction. These 20 alpha-cellulose samples were analyzed for radiocarbon (Δ14C) and stable carbon isotope (δ13C) measurements. Within each annual ring of Pinus halepensis, the growth season for earlywood and latewood samples was defined as per growth data of unwatered Pinus halepensis by Liphschitz et al.42, which shows two inactive or rest periods each year. Liphschitz et al.42 studied Pinus halepensis from Ilanoth, Israel and observed that in unwatered Pinus halepensis the earlywood grows during autumn and late winter, while its latewood grows during spring and early summer. They suggested that in winter the cambium activity of Pinus halepensis stops due to a drop in temperature and then again it stops in summer. As our sample location is a few kilometers away from Ilanoth and is not irrigated, we assume a similar growing season for the Pinus halepensis samples analyzed in this study. As local rainfall and temperature records do not show anomalously wet or dry years were observed between 1964 and 1968, we assumed the same wood growth season for each year. The earlywood samples were assigned to autumn and late winter to early spring period, and latewood samples were assigned to late spring and early summer period (Figure S2).

Stable carbon isotope ratio in Havat Hanania pine

Results of stable carbon isotope measurements are reported in terms of δ13C, denoting deviations of stable carbon isotope ratio of sample from VPDB standard in per mil unit (‰). The δ13C values of analyzed samples vary between −25.2 ‰ and −20.2 ‰ (Table S1), with a mean value of −22.5 ± 1.4 ‰. A clear sub-annual cycle is observed in the pine δ13C values. Relatively higher δ13C values are observed in the latewood representing late spring to early summer, and relatively lower δ13C values are observed in the later part of earlywood, representing late winter to early spring (Figure S3). The timing of seasonal maxima and minima is similar to that of the seasonal variation in δ13C of atmospheric CO2 in the northern hemisphere43. However, the observed amplitude of the variation is much larger in the tree δ13C values, which is a result of fractionation during CO2 fixation by plants and is influenced by various environmental parameters44,45. It is observed that the amplitude and timing of Havat Hanania pine δ13C are comparable to the δ13C variations observed in Pinus halepensis from the Yatir forest in Israel45, wherein the lowest δ13C values occur in cooler periods and highest during warmer periods. The similarity in δ13C variation of analyzed pine and Yatir forest pines also supports the growth season assignment of pine wood based on Liphschitz et al.42.

Radiocarbon in Havat Hanania Pine

Radiocarbon results are reported as Δ14C (‰), which are corrected for their age and isotopic fractionation based on δ13C values, following conventions of Stuiver and Polach46. The Δ14C values of Havat Hanania pine (HAN 5B) samples vary between 915 ‰ and 584 ‰ with evident sub-annual variations (Figure S2). These sub-annual variations are superimposed on a typical long-term post-bomb period Δ14C declining trend, which starts after the Δ14C peak in the spring and early summer of 1964. The peak value reaches 915 ± 3 ‰ during 1964, and after that, the Δ14C values show a decline rate of about 69 ‰yr-1.

Based on the bomb Δ14C levels, Hua et al.15 grouped all atmospheric Δ14C records spatially into five different zones. The location of Havat Hanania (Israel) falls within NH zone 215. Therefore, Δ14C records of HAN 5B are compared with monthly Δ14C data for NH zone 2 (Fig. 1). The comparison shows that between 1964 and 1968, HAN 5B Δ14C values mostly remained higher than monthly NH zone 2 Δ14C values. However, the HAN 5B Δ14C values match better with the monthly NH zone 1 Δ14C values. The good match between HAN 5B and NH zone 1 Δ14C values also implies that the growth season assignment of pine wood samples based on information from Liphschitz et al.42 is reasonable. The pine latewood Δ14C values are higher than adjacent earlywood Δ14C values for the analyzed period. These differences cannot arise due to the deposition of stored (older) carbon. The match between atmospheric and wood Δ14C values indicates that the difference between pine earlywood and latewood Δ14C values is related to atmospheric Δ14C changes. It is in conformity with the observations of Grootes et al.47 and Svarva et al.48 that coniferous trees can be good archives of sub-annual atmospheric 14C changes. Bomb 14C, majorly produced in the stratosphere, was transported to the troposphere mainly during the spring and summer15. This increased the atmospheric Δ14C values during spring and summer compared to the autumn and winter seasons. These sub-annual fluctuations are well recorded by the analyzed pine tree from the northern Israel.

Figure 1
figure 1

Δ14C values of Havat Hanania pine (HAN 5B) earlywood and latewood samples along with monthly Δ14C values from NH zone 1 and 215 between the years 1964 and 1968. The time period of earlywood and latewood growth of HAN 5B (Pinus halepensis) is based on Liphschitz et al.42.

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Discussion

The NH zone 1 consists of Δ14C records from mid to high latitude regions, and the NH zone 2 consists of Δ14C records between 45°N and TLPB (Tropical Low Pressure Belt) mean position during boreal summer15. In NH zone 1, atmospheric Δ14C records that span the years 1964 to 1968 are from Fruholmen, Norway26, Vermunt, Austria27 and China Lake, USA49,50,51,52,53. In NH zone 2, atmospheric Δ14C records from Izana, Mas Palomas and Santiago de Compostela in Spain and Dakar in Senegal15,26 span more than one year between 1964 and 1968. When these individual atmospheric records from NH zone 1 are compared with the Δ14C records of Havat Hanania (HAN 5B), it is observed that the HAN 5B record is in good agreement with them (Figure S4). Atmospheric records from NH zone 2 show more spread compared to NH zone 1. It is also observed that the HAN 5B values are higher than most of the individual atmospheric records from NH zone 2 (Figure S4). The difference of HAN 5B Δ14C values from contemporaneous monthly Δ14C values of each zone was calculated. The difference between HAN 5B and NH zone 1 Δ14C values ranges from −24 ‰ to 14 ‰ with a mean value of −4 ± 11 ‰. But the difference between HAN 5B and NH zone 2 Δ14C values was much higher, ranging between 14 ‰ to 82 ‰ with a mean value of 43 ± 20 ‰. This again suggests that the HAN 5B record is closer to NH zone 1 values.

To check for the goodness of fit between Δ14C values of HAN 5B and atmospheric curves (NH zones1 and 2) χ2 value was estimated. As per Ramsey et al.54, the χ2 value can be calculated using the following formula,

$$\upchi 2=\sum \frac{{({H}_{i}-{A}_{i})}^{2}}{({\sigma }_{{H}_{i}}^{2}+{\sigma }_{{A}_{i}}^{2})}$$

Here, Hi is the Δ14C values of HAN 5B, and Ai is the Δ14C values of the atmosphere curve (NH zone 1 or 2) for the contemporaneous period. σ represents the uncertainty associated with Hi and Ai values. The uncertainty of HAN 5B Δ14C record is between 2 and 3 ‰ and the uncertainty in the compiled monthly Δ14C values of NH zone 1 and 2 vary between 2 and 7 ‰15. Considering the uncertainty of 1σ in the atmospheric curves, χ2 values of 161.28 and 2389.24 were obtained for the atmospheric curve of NH zone 1 and 2, respectively. The reduced χ2 value in both cases is greater than 1. This suggests that within 1σ HAN 5B do not fit either of the NH 1 or 2 values significantly but Δ14C values of HAN 5B are closer to NH zone 1 Δ14C values compared to NH zone 2 Δ14C values. When 3σ uncertainty in the atmospheric curves was used, χ2 values of 17.92 and 265.47 were obtained for the atmospheric curve of NH zone 1 and 2, respectively. Now, the reduced χ2 value for the NH zone 1 curve is 0.90 (n = 20), and for the NH zone 2 curve, the reduced χ2 value is 13.7. These observations indicate that HAN 5B Δ14C values better fit NH zone 1 values26,27,49,50,51,52,53.

Between 1955 and 1967, due to the injection of bomb 14C into the troposphere, a gradient is observed from high to low latitudes13,15. The highest Δ14C values are observed in the mid to high-latitude region of the northern hemisphere and decrease southwards15. The bomb 14C distribution is influenced by atmospheric circulations, based on which Hua et al.14 defined different zones and boundaries between them. The NH zone 1 and 2 was separated by Ferrel cell–Hadley cell boundaries roughly around 40° N based on available Δ14C records14. With the inclusion of tree-ring Δ14C records from western Oregon, USA55 and Washington state, USA47, this boundary was modified. The current boundary between NH zone 1 and 2 over northwestern USA is considered to be between these two tree sites and south of China Lake (35° N) because China Lake Δ14C record is closer to NH zone 1 and higher than NH zone 2 Δ14C record mostly during winter-spring14,15. The present study location, Havat Hanania (32° N), is at about similar latitude as that of China Lake, and its atmospheric bomb Δ14C record is also closer to NH zone 1 values and relatively higher than NH zone 2 values. The monthly wind climatology map shows that the study location receives wind from higher latitudes in the north during summer (Figure S5). While during winter, the winds are from the west, mainly from the Mediterranean Sea. The winds during summer can bring 14C enriched CO2 from higher latitudes, explaining the good agreement between Havat Hanania and NH zone 1 atmospheric Δ14C record. These observations suggest that the present study area lies within NH zone 1 and the boundary between NH zone 1 and 2 over northern Israel should possibly be placed south of Havat Hanania, Israel, at least for the period which is represented by HAN 5B wood growth. This underlines the fact that the boundary between NH zone 1 and 2 needs to be modified with more such Δ14C records.

This Havat Hanania Δ14C record can be used as a reference for atmospheric Δ14C levels in the northern Israel between 1964 and 1968. Earlier, Ehrlich et al.56 reported bomb Δ14C records from olive and pine latewood collected from Havat Hanania, and they identified the annual nature of olive wood. They further tried to determine the growth season of olive wood by fitting the olive wood’s Δ14C record on the available atmospheric and tree ring Δ14C records from the northern hemisphere. Use of non-local Δ14C records to fit olive wood Δ14C records can possibly add some ambiguity to the fitting. However, it is noted that before the present study, no other local continuous atmospheric sub-annual Δ14C record existed apart from the Rehovot (Israel) atmospheric record between 1967 and 1968. To clearly understand the growing season of the olive wood, a longer sub-annually resolved atmospheric Δ14C record from the local region is required, and the HAN 5B Δ14C record provides us with this opportunity.

Ehrlich et al.56 identified sets of olive Δ14C values belonging to each year using the olive δ13C record. We use the same sets of sub-annual olive Δ14C values of each year between 1964 and 1968 and match them with Δ14C values of HAN 5B, Rehovot and NH zone 1 (Fig. 2). It will provide a better estimate of the growth period of the olive wood in the region. Figure 2 shows a good match between HAN 5B Δ14C record and Rehovot atmospheric Δ14C record which demonstrates that HAN 5B record represents well the atmospheric Δ14C level of the region. Due to the prominent seasonality in atmospheric Δ14C values between 1964 and 1968, the olive Δ14C record can be clearly matched to other Δ14C records. As olive Δ14C values show a clear increasing and decreasing trend in 1964 and 1965, respectively, olive Δ14C values can be easily matched with the corresponding trend in the atmospheric Δ14C values. The Δ14C values matching show that olive wood grew mainly during spring and early summer in the year 1964. However, the next year, in 1965, olive wood grew later, from late summer to winter. Unlike 1964 and 1965, the next three years olive Δ14C values show relatively smaller sub-annual variation, and these values are lower than the corresponding year’s HAN 5B Δ14C values. The lower Δ14C values of olive compared to HAN 5B in 1966, 1967, and 1968 suggest that olive did not grow in the spring or summer period of these years. For the year 1967, when a nearby (Rehovot) atmospheric Δ14C record is available, olive Δ14C values are matched to that (Rehovot) atmospheric Δ14C record. Due to relatively smaller sub-annual Δ14C variation in olive Δ14C values of 1966, 1967 and 1968, these values can be matched to different atmospheric Δ14C values ranging from autumn to winter, and it suggests that olive wood grew sometime between autumn and winter periods in these years. By just looking at records between 1964 and 1966, it is evident that the olive wood growth season shifted from one year (ring) to the other. Understanding the reason for such a shift in olive growth season is beyond the scope of this study, and so it is not discussed here. However, the observation suggests that olive wood can record in some years high 14C levels of early summer and also can record relatively low 14C levels of winter in other years. When olive wood records low Δ14C values of winter, it can possibly reflect offset compared to tree Δ14C records from central and northern Europe and North America (growing during the spring–summer period), which constitute major parts of the calibration curve (IntCal). To check for 14C offset, annual mean Δ14C values of HAN 5B and olive wood were compared with annual mean Δ14C values of trees from NH zone 1 (Figure S6), and it is observed that olive Δ14C values were clearly lower than NH zone 1 tree values apart from the year 1964. As olive appears to have grown around spring and early summer in 1964, its growth period overlapped with that of central and northern Europe and North American trees, and thus in 1964, its Δ14C values were within the observed range of Δ14C values of NH zone 1 trees. For the next three years, olive appears to have grown around autumn and winter, and its Δ14C values are well below the observed range of Δ14C values of central and northern Europe and North American trees. However, pine (HAN 5B) which was growing close to the olive location, always shows Δ14C values comparable to central and northern Europe and North American trees. This highlights the possibility of offset in olive wood's annual average Δ14C value from the region.

Figure 2
figure 2

Δ14C values from Havat Hanania pine (HAN 5B), Rehovot atmosphere26, Havat Hanania pine latewood56 and olive56 along with Fruholmen26, Vermunt27, China Lake49,50,51,52,53 atmospheric Δ14C records, Norway pine Δ14C records48 and monthly NH zone 1 Δ14C values15. Olive data points have been adjusted to match other Δ14C records.

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The large seasonal variation in atmospheric bomb Δ14C values between 1964 and 1968 exaggerates the offset arising due to the difference in the growing season, and it is more than 25 ‰. For the pre-bomb period, seasonal variation in atmospheric Δ14C is inferred to be around 4 ‰8, and so the offset arising from the difference in growing season would be much less in the pre-bomb period. An offset of about 2.5 ‰ for plant materials growing in the northern hemisphere but in season opposite to that of central and northern Europe and North American trees has been observed57,58. A similar offset can be expected in olive wood 14C records from pre-bomb periods, and it can significantly influence calibrated age of events dated using olive wood radiocarbon dates, such as the Minoan eruption event of Santorini59,60. The Minoan volcanic event of Santorini is an important time marker in the archaeological records of the eastern Mediterranean region, however, the date of this event is still under scientific discussion. Disagreement between this event's archaeological and radiometric age estimates did not allow the scientific community to reach a consensus about the event date. Various investigation has been conducted to constrain this Minoan volcanic event date, and among them, the one based on radiocarbon dating of an olive branch from Santorini is widely discussed56,59. The radiocarbon dating of this olive branch put this event in the seventeenth century BCE, but the archaeological evidence places it in the sixteenth century BCE59. Ehrlich et al.56 had shown that 14C dates of this olive branch can be reconciled with archaeological evidence under some possible scenarios. As seen in this study, the presence of offset in the olive wood 14C value can be expected. Such offsets can also influence the calibrated calendar age of the olive branch from Santorini. Applying an offset of 2.5 ‰ calibration model of Santorini olive wood 14C dates can clearly increase the probability for younger event dates, mostly in the sixteenth century BCE (Figure S7). Pearson et al.60 had also shown that model results with an offset of 13.7 ± 2 14C years increased the probabilities of the sixteenth century BCE date for the Minoan eruption event. However, with current understanding, it is difficult to ascertain if these olive wood 14C dates had the offset arising from a shift in the growing season. Nevertheless, this study highlights that offset in olive wood 14C dates is possible, and caution while interpreting such dates is warranted.

Summary

A pine (Pinus halepensis) wood sample from Havat Hanania in northern Israel between 1964 and 1968 was analyzed for its 14C concentrations. It is observed that the Δ14C values of the analyzed pine wood are closer to the atmospheric bomb radiocarbon value of NH zone 1 than NH zone 2 bomb 14C values. This observation underlines the requirement to refine the boundary between NH zone 1 and 2 over northern Israel. The pine Δ14C record also allowed us to assess the growth period of olive wood from the same region. Δ14C values show that the olive wood growth season changed from one year to another. Such growth season shifts in olive wood can result in 14C offsets compared to IntCal curve records. So, 14C ages estimated from olive woods should be carefully interpreted considering the possibility of 14C offset.

Methods

Study area and sampling

Pine tree (Pinus halepensis) was sampled from Havat Hanania (N32°56.153′, E35°25.296′, 415 m) in northern Israel. Figure 3 shows the sampling location of the pine tree from northern Israel analyzed in the present study. The sampling area was within the property of the Olive Division of the Plants Production and Marketing Board. The sampling was coordinated and approved by the head of the Olive Division and was conducted during the years 2013–2014. The plant collection and use were in accordance with all the relevant guidelines. A cylindrical core was obtained using a 5.15 mm increment borer. The cores were polished using an orbit sander (Makita #BO5041) with five grades of grit (240, 320, 600, 800, 1000). Pine tree rings were visually identified using a binocular microscope (M80, Leica). Pine ring width was measured to the nearest 0.001 mm with the help of a sliding micrometer stage (“TA” measurement system, Velmex Inc.). The Tellervo dendrochronological analysis package61 was then used to cross-date the pine ring width data with the master pine ring width record of the region. After cross-dating, rings between 1964 and 1968 were selected for sub-sampling (Figure S1). The sub-sampling was carried out using a WSL-Lab microtome62. About 10 mg of wood samples were collected by pooling thin sections cut from a sampling region. Samples were pooled for every 1 mm step, yielding the desired sample size of about 10 mg of pine wood. These pooled samples were processed further for alpha-cellulose extraction.

Figure 3
figure 3

(a) Sampling location represented by red map pin in the northern Israel region map (https://www.google.com/maps); (b) Satellite image of the sampling location (https://www.google.com/maps).

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Alpha-cellulose extraction

Before processing samples for alpha-cellulose extraction, all glassware required for the process was baked at 450 °C for 1 h to remove organic contaminants. Pooled thin sections of wood (about 10 mg) were placed in borosilicate test tubes (16 × 125 mm).

From these samples, alpha-cellulose was extracted using steps given by Ehrlich et al.56 with a modification. We skipped the last acid step of the ABA process done by Ehrlich et al.56. This step is for removal any absorbed atmospheric CO2 during the previous base step. The last step of holocelluse extraction also includes treatment with acid for absorbed atmospheric CO2 removal56. Therefore, the last acid step of the ABA process by Ehrlich et al.56 was excluded from the present pretreatment process. In present pretreatment, samples were treated with 5 ml of 1N HCl for 1 h, followed by washing with DDW (double distilled water). Then samples were treated with 5 ml of 0.1N NaOH for 1 h, followed by washing with DDW. Then a mix of 2.5 ml of HCl and 2.5 ml of NaClO2 was added to each sample and was kept at 70 °C until samples were bleached. After the bleaching step, samples were washed again with DDW. The washed samples were treated for 1 h with 6 ml of 5N NaOH and again washed with DDW. Then samples were treated with 5 ml of 1N HCl at 70 °C for 1 h, followed by a wash with DDW until reaching a neutral pH. The extracted alpha-cellulose was then placed in an oven at 100°C for drying.

Stable carbon isotope and radiocarbon analysis

About 2–4 mg of alpha-cellulose were weighed and packed in tin foil capsules (Elemental Microanalysis Ltd. 5 × 3.5 mm #D1015). Packed samples were combusted in an Elemental Analyser (Elementar vario ISOTOPE select) linked to an Isotope Ratio Mass Spectrometer (Elementar AMS precision IRMS) and an Automated Graphitization Equipment (Ionplus AGE 3). A fraction of CO2 resulting from sample combustion in Elemental Analyser is analyzed in the IRMS and the rest is graphitized over iron powder by AGE 3. For δ13C measurements, samples were analyzed along with the NBS SRM-4990C oxalic acid II and IAEA-C3 cellulose63 standards, and the precision for these measurements was 0.2 ‰.

For radiocarbon analysis, graphitized samples were pressed into aluminium targets. The targets containing samples and standards are then analyzed in the 500 kV NEC Accelerator Mass Spectrometer at DANGOOR Research Accelerator Mass Spectrometry (D-REAMS) Laboratory in the Weizmann Institute of Science64. To check the accuracy of the radiocarbon measurements, VIRI D65 and VIRI M66 standards were analyzed along with the samples. VIRI D and VIRI M yielded an average value of 2856 ± 18 BP and 73.884 ± 0.072 pMC respectively, which are in very good agreement with the consensus value (VIRI D: 2836 ± 3.3 BP; VIRI M: 73.900 ± 0.0322 pMC)65,66 of these standards.