isotope composition of rain under the assumption that the calcite has been formed under isotope equilibrium conditions using the relation derived by Kim and O’Neil (1997) for the oxygen isotope fractionation factor between water and calcite. The oxygen isotope composition is of drip water equates the oxygen isotopic composition of the rain, since no isotope fractionation processes affect the infiltrated water (Sec. 11.1.3). The calculated calcite δ18O values for each box are illustrated in Fig. 11.7. The blue circles correspond to the winter months and the red circles to the summer months. Note, that the fraction of summer infiltration that contributes to the annual infiltration is negligible in comparison to the winter infiltration. Only for the boxes, which are close to the Northern Sea, does the summer infiltration affect the isotopic composition annual infiltration (Fig. B.2). Hence, the focus is on the δ18O values in the following. The δ18O gradient depends on the gradient of the drip water, i.e., the gradient of the oxygen isotopic composition of the precipitation and the gradient of the mean annual air temperature. Since the temperature gradient is negative along T2 (Fig. 11.7) the equilibrium oxygen isotope fractionation counteracts with the depletion of the drip water in 18O along the transect. This results in the change of the slope for calcite δ18O values (Fig. 11.7) compared to the slope for the oxygen isotope composition of the precipitation (Fig. 11.6). The computed gradient for calciteKim and O’Neil (1997) fits to values from cave-calcite samples of Rana (Norway) and Korrallgrotten cave (Sweden) (Fig. 11.7).
11.3 Sensitivity of the model on model parameters for Central Europe (T1)
In the previous sections two precipitation δ18O gradients and the related gradients in calcite δ18O values were discussed based on the climate conditions in the period between 1901 and 2009. However, within the Holocene the slope of the Central European gradient of speleothem δ18O values has changed (McDermott et al., 2011). To address this question, the reference winter period between 1901 and 2009 is used to study the sensitivity of the slope in precipitation δ18O values and the corresponding calcite δ18O values by changing the temperature and the amount of precipitation. In the following the discussion is focused on T1 with the objective to investigate the changed trends in speleothem δ18O gradients in Central Europe during the Holocene. In the first sensitivity scenario, the monthly temperature is changed by ± 3 °C, resulting in an annual air temperature that is warmer and colder by 3 °C, respectively (Fig. B.3). The results for the stable isotopic composition of the precipitation are shown in Fig. 11.8. The discussion is limited on the δ18O values and is also valid for the δD values. For the temperature scenario the results for the δ18O values are illustrated by the closed circles; red indicates the +3 °C scenario, whereas blue indicates the -3 °C scenario. The blue and red squares are the GNIP station values for winter and summer months (Tab. 11.1). The change in the isotopic composition of the precipitation between the two temperature scenarios (+3 °C and -3 °C) is negligible in
11. Stable isotopes in Precipitation
-10 -5 0 5 10 15 20
-14 -12 -10 -8 -6 -4 -2
longitude (°E)
18 calcite (‰)
Fig. 11.7: Illustrated are the computed calcite δ18O values for T2 for the winter months (October-March) (blue stars) and the summer months (April-September) (red stars). The values are calculated from the corresponding drip water δ18O values and the oxygen isotope fractionation factor ofKim and O’Neil(1997). For the temperature, the mean annual temperature of the respective box is used. The closed black squares are present-day calcite δ18O values from several stalagmites locates close to T2. The speleothems are CC-3, Bilbo, SG-95, SU, K1 and FM-3. The speleothem δ18O values are reported by McDermott et al.(2011).
comparison to the evolution of the precipitation δ18O gradient. This is an effect of the Rayleigh approach, at which the isotopic composition of the formed rain is more sensitive on the amount of formed precipitation than on the changes of the temperature dependent isotope fractionation investigated here. However, colder temperatures leads to a steeper gradient, because more H182 O molecules are condensed, causing a stronger depletion in 18O of the remaining moisture in the atmosphere.
The δ18O gradient of precipitation for the precipitation scenario, where the amount of precipitation changes in every box by ± 30 %, is illustrated by the closed circles in Fig. 11.8.
The +30 % precipitation (wetter) scenario is visualized in red and the -30 % precipitation (drier) scenario in blue (Fig. B.3). Note that the initial amount of precipitation in the atmosphere does not change and is equivalent to the reference winter period. For the +30
% precipitation scenario the slope of the δ18O gradient is much steeper compared to both slopes of the temperature scenarios, whereas the slope for the -30 % precipitation scenario is much flatter. This is caused by the changed amounts of precipitation and the fact that
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11.3. Sensitivity of the model on model parameters for Central Europe (T1)
the initial amount of precipitation is constant. Hence, for a wetter climate, the atmospheric moisture becomes more depleted in 18O along the transect, which is simply a result of the Rayleigh approach. For a drier climate it acts opposed, because less moisture condenses and consequently more18O remains in the atmospheric moisture. A similar result would be observed if not the amount of precipitation would have been changed but the initial amount of moisture, because the two variables are equivalent. The reason for this is the nature of the Rayleigh approach. Hereby the degree of isotopic fractionation is determined by the ratio of N/N0 (Eq. 11.10). Hence, if more moisture is in the atmosphere, the slope is less steep, whereas less moisture leads to a steeper gradient.
The effect of the temperature and the precipitation scenarios is illustrated in Fig. 11.9.
Thereby the reference scenario is the winter session of Fig. 11.3. It is indicated by the grey closed circles. For the temperature scenario (closed circles) the absolute values are shifted towards more negative values for +3 °C scenario (red). In contrast they are shifted towards more positive values for the -3 °C scenario (blue). The reason for this effect is that the counteracting of the isotope fractionation effect of the by the oxygen equilibrium isotope fractionation effects during the calcite formation. Since the sensitivity of oxygen isotope fractionation factor between water and calcite is negative (Fig. 7.5, dotted line), a warming forces a calcite oxygen isotopic composition, that is more depleted in 18O. A cooling in contrast, results in an enrichment of 18O. The slope of the δ18O gradient does not change for the temperature scenarios in comparison to the reference scenario. This is mainly determined by the fact that the slope of the δ18O gradient of the precipitation is almost unchanged for the two temperature scenarios and the the temperature gradient of T1 was changed parallel in all boxes. Only, if there would was an additional temperature gradient added to the reference temperature gradient, the slope would have changed. As an example: if the temperature in West Europe (Ireland) would be colder by 3 °C and warmer by 3 °C in East Europe (Romania), the observed calcite δ18O gradient would be a super-position of the +3 °C and -3 °C scenarios. Consequently, the slope of the calcite δ18O gradient becomes steeper and shallower if there was a warming in West Europe and a cooling in East Europe.
For the precipitation scenarios (open circles) the slope of the calcite δ18O gradient changes, whereas the absolute value in the first box is almost unchanged. For the temperature scenarios in contrast the absolute δ18O value of the precipitated calcite shifts by c. ±1
% caused by the temperature dependent isotope fractionation effects. If the amount of precipitation changes, under the assumption that the initial amount of moisture in the atmosphere is unchanged, the slope of the precipitation δ18O gradient would change (Fig. 11.8). This change is transferred into the cave by the percolation water. Since the temperature is unchanged in the precipitation scenarios the oxygen isotope fractionation effects during the calcite precipitation are the same as for the reference scenario. This results in a steeper slope of the calcite δ18O gradient for the +30 % precipitation scenario (red) whereas the slope for the -30 % scenario (blue) is flatter (Fig. 11.9).
11. Stable isotopes in Precipitation
Fig. 11.8: Illustrated are the computed δ18O and δD for T1 for different sensitivity studies (closed circles). The closed circles (line) indicate the temperature sensitivity study for an increase of the temperature of +3 °C (red) and -3 °C (blue). The open circles (dotted line) depicts the precipitation sensitivity study whereas the amount of precipitation is changed in each box by +30 % (red) and -30 % (blue). The open squares picturing the GNIP station data for the winter months (blue) and the summer months (red).
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11.3. Sensitivity of the model on model parameters for Central Europe (T1)
-10 -5 0 5 10 15 20 25
-13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3
longitude (°E)
18 O calcite (‰)
Fig. 11.9: Illustrated are the computed calcite δ18O values for the sensitivity studies (coloured circles). The closed circles indicate the results of temperature sensitivity study: the red closed circles are the results of the +3 °C scenario and the blue closed circles for the -3 °C scenario. The open circles picturing the results for the precipitation sensitivity study: the red open circles are the results for the +30 % scenario and the blue open circles for the -30 % scenario. The black closed circles illustrate the computed δ18O values for the reference scenario.
11. Stable isotopes in Precipitation