Of the 19 measured samples, I could only use 13 for a detailed analysis of the stable water isotopes in the fluid inclusions, as only these samples have released more than 0.2 µl water. This water volume is considered to be the threshold value for a reliable evaluation (see section3.6). These 13 samples are in an age range of 7.7 to 5.0 ka (185 to 140 mm dft). For the oldest sample (7.7 ka) a correction due to a different sea level, which corresponds to a different isotopic composition of the global ocean would result only in a correction of −0.05h for δ18O and −0.40h for δ2H in VSMOW [Wael-broeck et al., 2002]. This correction is so minor that it is not considered in the further discussion. Nevertheless, the corrected data is listed in table A.26. This correction is an important aspect for samples formed during the Last Glacial Period (115 – 11.7 ka BP [Dansgaard et al., 1993]), since the changing ocean as a reservoir for the resulting precipitation and therefore drip water has a oxygen glacial - interglacial amplitude of
∼ 1h VSMOW [Waelbroeck et al., 2002].
The fluid inclusion data indicate more positive values in both, δ18O and δ2H, with respect to the current drip water composition (see figure 5.5). This suggests that fractionation occurs. I performed a linear regression (red line) considering the x - and y - uncertainties and obtained a slope of (+ 2.27 ± 1.12) δ2H/δ18O. Hu et al. [2009] per-formed evaporation experiments in which the water vapour was not recycled. They found deuterium to be fractionated more strongly than oxygen by a factor of 3.432 with the correlation coefficient (R2) of 0.845 [Hu et al., 2009]. Accordingly, I performed a linear regression (blue line) with a fixed slope of 3.4 δ2H/δ18O. The actual mean drip water composition over the intersection point of the evaporation line (linear regression with a fixed slope) and the GMWL could be calculated. The resulting values close to the modern - day drip water isotope ratios suggest that the deviation from the original stable isotopic composition of the fluid inclusions was caused by evaporation. This is consistent with the consideration of drier climatic conditions during the period be-tween 7.7 - 5.0 ka and therefore a lower drip rate or even a complete stop of dripping. It would allow long residence times on top of the stalagmite surface, which would favour evaporation [Niggemann et al., 2003b]. As already mentioned, the two coralloid layers could only form under extremely dry conditions. Accordingly, it could be possible that the drip water on the stalagmite top has evaporated and thus experienced kinetic fractionation. The hypothesis that the isotopic composition of the fluid inclusions was effected by evaporation seems to be realistic, considering the related evaporation line of y = (3.7 ± 0.2) · x + (5.1 ± 1.8)h which I found for a stalagmite from a different climatic region (see section 6).
- 8 - 7 - 6 - 5 - 4
Figure 5.5: Water stable isotopes of Bu4 (blue circles) together with mean annual drip water (green triangles) and rain water data (light grey dots). The linear regression (red line) for Bu4 fluid inclusion data taking x - and y - uncertainties into account results to a slope of +2.27 ± 1.12 δ2H/δ18O. Considering the findings of Hu et al.
[2009] for fractionation under evaporation with a slope of 3.4 δ2H/δ18O, the point of intersection between the GMWL and the resulting linear regression (blue line) predicts the actual drip water composition.
Where occurs evaporation - soil or stalagmite surface ?
I detect a clear evaporation signal in stable isotopic composition of the fluid inclusions of the stalagmite Bu4. The question arises where this evaporation takes place. Does evaporation already occur in the infiltration process or does it only take place in the cave. Evapotranspiration in the soil above the Bunker Cave was detected by Münsterer et al. [2012] through the analysis of cosmogenic36Cl in drip water. Furthermore, the occurrence of PCP at the drip site TS8 indicates a previous change in the chemical composition of the drip water [Riechelmann et al., 2013]. This mechanism is possi-ble when the water percolates through the air - filled soil/epikarst and an initial CO2 outgassing is enabled. However, previous outgassing does not affect the oxygen and hydrogen isotopic composition of the drip water, because it is representing the mean annual rainwater composition and does not deviate from the GMWL (see figure A.31).
5.4 Discussion
Watch glass experiments today show kinetically fractionated δ18Ocalcite values below TS8, which are not completely understood [Riechelmann et al., 2013]. Riechelmann et al. [2013] exclude evaporation today, because the average humidity in the cave varies around 93 ± 2 %. Still, increased residence times on the stalactite tip or the stalagmite surface could lead to evaporation and has not been investigated so far. Assuming dry climatic conditions in the time period from 7.1 to 5.5 ka, it cannot be excluded that the humidity in the cave was lower than today. In a homogeneously ventilated cave, like Bunker Cave (see figure A.28), the evaporation increases proportional to (1 - h) for a humidity below 95 % and significant effects altering the isotope ratio are possible [Dreybrodt and Deininger , 2014]. If the mean deviation of the current drip water to the measured fluid inclusions is considered, an enrichment of approximately 2h VSMOW for δ18Of luidcan be identified. This enrichment due to evaporation would theoretically be possible in a moderately ventilated cave with a mean cave temperature of 10 ° C, a wind speed of 0.1 m/s and a mean humidity of 80 % [Deininger et al., 2012]. The possibility that evaporation in the cave effects the isotopic composition of the fluid inclusions is also supported by speleothem studies of Holocene stalagmites from Sauer-land. Niggemann et al. [2003b, a] suspect that areas with white porous lamina (the region of Bu4, which contains water - filled inclusions) were precipitated during periods of low humidity in the cave, which enhances evaporation from the thin water film. This assumption is confirmed by significantly increased δ18Ocalcite values (> + 1h) and the correlation of δ18Ocalcite and δ13Ccalcite, which gives evidence of periodic kinetic frac-tionation due to evaporation and fast degassing CO2 [Niggemann et al., 2003a].
In summary, I exclude the possibility of evaporation in the soil or epikarst, as the current drip water data does not provide any indication of this effect. The measured isotopic signal in the fluid inclusions shows a clear evaporation signal, which was prob-ably imprinted during the residence time on the stalagmite surface. Evaporation was favoured by a very thin water film and partial drying of the stalagmite surface. With regard to the interpretation of δ18Ocalcite signals in stalagmites, the effect of kinetic fractionation induced by evaporation should be discussed more intensively.
Findings for the Holocene stalagmite from Central Europe - Bunker Cave
• A strongly changing water content is an indirect indicator for varying climatic conditions.
• I observe a high water content in the facies zone B (white porous lam-inae), which is preferred under drier climatic conditions and slow drip rates and probably reflect periods of stronger evaporation and/or higher supersaturation.
• For facies zone A (dark compact laminae) no water - filled inclusions could be found. The compact crystals develop under constant drip water supply and therefore resulting from a very regular crystal growth under more humid climatic conditions.
• I detect a clear evaporation signal in the stable isotopic composition of the fluid inclusions of the stalagmite Bu4 with a linear regression slope of +2.27 ± 1.12 (δ2H/δ18O), which is comparable to other evaporation exper-iments [Hu et al., 2009].
• I assume that the δ18Of luid- δ2Hf luid evaporation signal was imprinted during the residence time on the stalagmite surface.