## 4 General Discussion
### Comparison with Observations from the Literature
On the other hand, thin section analyses of the limestone sample discussed in this manuscript (Section 3.1) show that its matrix texture contains vugs that are partly filled with secondary crystallised calcite. Assuming that sample TS2-SZL4 contains less, and smaller, voids compared to the limestone reported in [46], most of its open pore space, e.g., voids, may already be closed at \(\sigma_{1}=\sigma_{2}=20\) MPa. The latter would subsequently lead to a roughly constant P-wave velocity at \(\sigma_{1}=\sigma_{2}>20\) MPa, as observed in Figure 13.  
The limestone sample studied in [47] reveals significantly lower P-wave velocities, as a function of increasing \(\sigma_{1}=\sigma_{2}\), compared to sample TS2-SZL4 assessed in this study. Reason for this is most probably the difference in mineral composition. Sample TS2-SZL4 is mainly dominated by calcite and dolomite minerals (Table 7), whereas the limestone investigated in [47] contains calcite minerals only. Calcite minerals reveal P-wave velocities of around 6.26-6.64 km/s [48; 49; 50; 51; 52], where dolomite shows velocities of 7.05-7.34 km/s [55; 56]. The latter most likely (partly) controls the difference in absolute velocities. The velocity trend of [47] corresponds rather well with the trend observed in [46]. It is related to the open pore space present. As the limestone samples studied in [46; 47] both show porosity values of 13%, compaction could continue even after \(\sigma_{1}=\sigma_{2}=20\) MPa, leading to enhanced mineral-to-mineral contact areas, hence increased P-wave velocities. As mentioned before, for sample TS2-SZL4, bulk of its open pore space might already be closed at \(\sigma_{1}=\sigma_{2}=20\) MPa, resulting in rather constant velocities with increasing stress. Figure 14 presents the P-wave velocity-\(\sigma_{1}\) relationship for a limestone sample (TS2-SZL4) that was used in this work as part of experimental series A, i.e., the UCS tests (Figure 5). Furthermore, P-wave velocity data for an Indiana limestone, obtained during an UCS experiment, taken from [57] are shown as well (Figure 14). The Indiana limestone reported in [57] is characterised by its homogeneous nature and low strength. Figure 14 shows that the velocities of both limestones are of the same order of magnitude (4500-4900 m/s) and have a very similar trend. The trend consists firstly of a rather rapidly increasing velocity with stress due to compaction. After that, the velocities increase at a slower rate as a function of stress since bulk of the open pore space is already closed, and hence the mineral-to-mineral contact area can only increase slightly. Just before rock failure occurred (at 44.2 MPa for the Indiana limestone and at 72.5 MPa for TS2-SZL4), the P-wave velocity displays a small decrease. This decrease is a result of rock damage, i.e., microcracking [58; 59; 60; 61]. The absolute differences in velocity between TS2-SZL4 and the Indiana limestone are related to heterogeneity in terms of rock texture and mineralogy. Where the Indiana limestone is well-known for its relatively homogeneous nature, the limestone samples collected at the Kizildeere site have shown a high level of heterogeneity due to diagenesis processes and the potential presence of fossils (Section 3.2). Although absolute velocity values differ between specimens from different regions (Figures 13 and 14), most probably due to variations in mineralogy and microstructure (e.g., pore type and size), the trends in velocities related to the Kizildere samples seem to broadly correspond to velocity trends observed at other sites. This might imply that there is a potential to use limestone and calcschist velocities, recorded at low stress levels, and extrapolate them to larger stress levels using the observed trends of this study. One crucial parameter to consider in this process is the porosity, i.e., open pore space, as it seems to control the slope of the velocity trend (e.g., limestone samples in Figure 13).