Cave sediments are often preserved from erosion when surface sediments are completely removed. Therefore, caves can work similarly to traps in terms of sediment retention. Secondary carbonates (
The quality of paleoenvironmental reconstructions depends on the accuracy of the proxies used for the reconstruction and the accuracy of the studied chronology. High resolution and precise chronologies are relatively easy to complete for young speleothems. In speleothem science, the 230Th/U method is usually the best way to obtain precise chronologies. For older periods (
In this paper, we present the results of studies on speleothem material from Głęboka Cave (Kraków-Częstochowa Upland, Poland). The results of the 230Th/U analysis indicated that this material falls outside of the method age limit (
In summary, the main goal for this work is the improved estimation of studied flowstone chronologies. Oxygen isotopic stratigraphy is tested as an alternative dating method. The second goal is a regional paleoenvironmental interpretation of the obtained records (including δ18O and δ13C records) and the results of the petrographic observations.
Głęboka Cave is located in southern Poland in the Kraków-Częstochowa Upland and is the largest known cave formed of karst from a natural reserve (Mt. Zborów;
Location of the study site. A – location of the Głęboka Cave (black circle) in the southwestern region of Poland (Krajewski and Matyszkiewicz (2009), modified); B – plan view of the Głęboka Cave, which shows the location (asterisk) where the flowstones were collected, cave map and profile (B) after Sznober and Tyc (2010), simplified.Fig. 1
The sampled flowstone is located 16–16.5 m below the cave entrance, however over the profile of calcite flowstones there is more than 20 m of limestone. Currently there are no crystallizing flowstones in the cave; only small stalactites are forming. The cave is currently more than 50 m above the groundwater level. A photo of the entire flowstone and its setting is presented on
The sampled flowstone and its setting.Fig. 2
The studied flowstone consists of four parts, which are separated by surfaces with discontinuities that are underlined with layers of detrital material (
Microscopic description of the studied flowstone. 1 – Discontinuities in the surfaces; 2 – Layers selected for the detailed microscopic studies.Fig. 3
The upper part of the flowstone (depth of 0–56 mm) consists of two layers, which are separated by discontinuities in the surfaces. The lower discontinuities (H2) have two breaks (H2A and H2B), which are separated by a 2–4 mm thick layer of mixtures, including detrital and calcite materials. These two layers are built from different calcite fabrics: almost pure calcite and more detritally-contaminated calcite. The middle, thickest part of the flowstone, which is located between the H2 and H3 discontinuities (56–344 mm depth), has been divided into three slightly different zones, which are mainly based on the colour, transparency and occurrence of lamination. In the lowest part of the flowstone (below the H3 discontinuity), two different zones are distinguished based on the calcite crystal purity and the flowstone porosity.
Six thin sections were cut parallel to the growth direction from locations with visible discontinuities during deposition, and those that differed in the macroscopic characteristic of calcite were used to determine the microscopic description. The main goal of the microscopic observation was to study growth mechanisms and microscopic features such as banding, fabric type, microdiscontinuities and inclusions. The microscopic observations were obtained using the Nikon Eclipse LV100POL microscope from the Institute of Geological Sciences at the Polish Academy of Sciences (Warsaw, Poland). The analysis and characterization of the speleothem fabrics were based on the methodology proposed by Turgeon and Lundberg (2001)1 and Frisia (2015). Based on the microscopic observations, a microstratigraphic log was constructed for selected parts of the flowstone; its construction used a series of codes to identify fabrics similar to those proposed by Muñoz-García
Series of 0.5–1.7 g calcite samples were drilled along the growing layers. The average thickness of the drilled sample was 2 mm. After the thermal decomposition of organic matter, the samples were spiked with a 233U-236U-229Th spike before further chemical treatment. The calcite sample was dissolved in nitric acid. Uranium and thorium were separated from the carbonate matrix using a chromatography method with TRU Resin. Standard samples and blank samples were prepared simultaneously for every series in the studied samples. Mass abundances of 236U, 233U, 234U, 238U, 229Th, 230Th and 232Th were measured by a double-focusing sector field ICP mass spectrometer (Element 2, Thermo Finnigan MAT) in an ICP-MS laboratory at the Institute of Geology (Czech Academy of Sciences, Prague, Czech Republic). The measurement results were corrected to account for background and chemical blanks. The final results were reported as isotopic ratios. U-series ages were iteratively calculated from the 230Th/234U and 234U/238U activity ratios using the following decay constants (yr–1): λ238 = (1.55125 ± 0.0017) ·10–10 (Jaffey
Samples for the δ18O and δ13C analyses were acquired at a 4.5 mm resolution. The analyses were carried out at the Laboratory for Isotope Dating and Environmental Research at the Institute of Geological Sciences, Polish Academy of Sciences (Warsaw, Poland). Oxygen and carbon stable isotope compositions were determined using a Thermo Kiel IV carbonate device connected to a Finnigan Delta Plus IRMS spectrometer in a dual inlet system. CO2 from calcite was extracted using orthophosphoric acid (density: 1.94 g/dm3) at 70°C. International standard NBS 19 was analysed in every ten samples. The isotope ratios were reported as delta (δ) values and expressed relative to the Vienna Pee Dee Belemnite standard. Measurement precisions of 0.07%o and 0.03%o were identified for oxygen and carbon, respectively (at 1 standard deviation).
For age estimation, we used the OIS method. OIS is based on the correlation between the local δ18O record and the global reference δ18O record (Imbrie
Using a genetic algorithm as a tool for correlation is one typical example of an AI technique that solves optimization problems. The final results depend on the initial populations and parameters of the genetic algorithm. The goal of the correlation process is to identify the most similar position in the studied records (
A microscopic analysis of the calcite crystal appearances and the identification of the original texture features of the studied material shows that approximately 90% of the observed flowstone is composed of elongated calcite crystals, which range from 0.2 mm to 3 mm wide and 2 mm to 30 mm long. Overall, the elongated crystals are perpendicular to the growth surface in each layer. Typically, the crystals are flush with each other; they are subparallel with one another and extinguish light at the same angle for the entire length of the crystals during microscopic observation (
Photomicrographs for different types of calcite fabrics observed in thin sections. A – Columnar fabric (compact and open); B – Columnar with hiatuses, columnar fabrics and protruding rhombohedral terminations (Crt) underlined with a higher content of detrital material; C – Columnar fabric with a faint lamination, which has a visible characteristic elongation of the inclusions parallel to the direction of the calcite crystal growth; D – Columnar compact fabric with faint and flat laminae; E – A large discontinuous surface with a blocky crystal fabric that gradually transforms into a columnar fabric; F – A degradation surface that is also visualized by the change in the direction of crystal growth; G – Initial crystalline stage, which precipitates after discontinuity surface with transitional basal crystals; H – Columnar fabric of the microcrystalline type; I – Dark layers (i.e., condensed dark laminae) with possible dissolutions and/or hiatuses.Fig. 4
During petrographic observations, the occurrence of several types of columnar fabrics was observed: elongated – competitive growth with the incomplete coalescence of crystals and a length to width ratio greater than 6:1, which sometimes shows lateral overgrowth; compact – when the crystals form a compact aggregate, and the intercrystalline porosity is not discernible; open – characterized by the presence of linear inclusions or pores; microcrystalline – composed of highly irregular intercrystalline boundaries, has a uniform extinction, and is punctuated by inter- and intracrystalline microporosity (Frisia, 2015).
Other characteristic zones are related to discontinuities in the surfaces. Those zones contain darker layers that are enriched with detrital material. They are often accompanied by visible signs of corrosion in older layers. These layers are essential for the formation of small calcite crystals, which are shaped by small blocks surrounded by quartz, micas and clay materials. Initially, many small crystals grow in different directions gradually with distance from the nucleation surface, then the size of the crystal increases significantly, and their orientation becomes optically uniform (
Additionally, large degradation surfaces appeared in external regions that had several thin sections; this is where the destruction and subsequent overgrowth of younger crystals occurred, which formed in different directions than the original crystals (
As mentioned previously, a microstratigraphic log was constructed for several parts of the flowstone using series of codes to identify fabrics similar to those proposed by Muñoz-García
As a result of the calcite fabric analysis, it was possible to prepare brief descriptions of the conditions that could lead to the formation of these layers (Frisia, 2015; Turgeon and Lundberg, 2001) and the observed depths of their occurrences in the flowstone (
Results of microstratigraphic logging for calcite fabrics in flowstone from the Głęboka Cave.Code Formation environment Depths (mm) 1 Stable growth regime and warm climate with a persistent water film 71–84.5, 129.5–138.5, 143.5–147.5, 155.5–165.5, 426–429, 433–439 2 Correlated with enhanced degassing, which is typical of low water supply rates combined with intense cave ventilation 138.5–143.5, 147.5–150.5, 165.5–173.5, 188.8–198.5, 429–433, 439–450 3 Thicker film of fluid with faster water delivery rate and less efficient degassing than that in columnar compacts 34.5–45.5, 57.5–71, 150.5–155.5, 185.5–188.5, 215.5–228.5, 328.5–332.5, 350.5–356.5, 450–454.5 4 Constant water supply rate, higher precipitation rate, Mg/Ca ratio in water that is higher than those in previous fabrics 0.5–3.5, 8.5–14.5, 15.5–21, 22–33.5, 198.5–215.5, 311.5–328.5, 332.5–346.5 5 Seasonal contrast in temperature, vegetation activity, water supply and changes in cave ventilation and an increase in the flushing of colloidal particles 45.5–51.5, 55.5–57.5 6 Hiatuses, destructive fabric, relatively dry periods, or condensation-corrosion of a primary fabric; these conditions also enable initial crystalline stages of calcites that contain abundant impurities, which often form after discontinuity/periods of low growth rates or precipitation cessations 3.5–8.5, 14.5–15.5, 21–22, 33.5–34.5, 51.5–55.5, 346.5–350.5
For two samples (277 and 278,
Results of U-series dating from the Głęboka Cave flowstone. The reported errors are 2σ. * D.C. – detrital contaminationSample lab no. Depth (mm) U cont. (ppm) 234U/238U 230Th/234U 230Th/232Th Age (ka) Remarks 265 478 ± 2 0.0643 ± 0.0001 1.000 ± 0.004 1.05 ± 0.01 450 ± 5 > 500 267 377 ± 2 0.0715 ± 0.0001 1.007 ± 0.004 1.04 ± 0.01 588 ± 7 > 500 268 350 ± 2 0.0459 ± 0.0001 1.014 ± 0.005 1.08 ± 0.02 60 ± 1 > 500 333 107 ± 2 0.0396 ± 0.0001 1.019 ± 0.007 1.00 ± 0.03 487 ± 14 334 74 ± 2 0.0239 ± 0.0001 1.000 ± 0.007 0.97 ± 0.03 320 ± 10 277 50 ± 2 0.1839 ± 0.0004 1.212 ± 0.004 1.14 ± 0.01 12.5 ± 0.2 *D.C. 278 26 ± 2 0.1046 ± 0.0002 1.316 ± 0.005 1.05 ± 0.01 12.1 ± 0.1 *D.C. 279 14 ± 2 0.0466 ± 0.0001 1.172 ± 0.006 1.04 ± 0.02 290 ± 5 > 500
The U content in all samples, except for the two contaminated samples, was relatively low (below 0.1 ppm), which was typical for speleothems from the Kraków-Częstochowa Upland. Only two samples (333 and 334,
A total of 116 samples have been analyzed for the oxygen and carbon isotopic compositions, and as a result, we obtained the δ18O and δ13C records (
Oxygen (A) and carbon (B) stable isotope compositions in the flowstone from the Głęboka Cave. The vertical lines indicate visible breaks in the flowstone deposition.Fig. 5
The age of the studied flowstone is outside of the U-series limit. Speleothems from the Kraków-Częstochowa Upland are characterized by a low uranium content, which is why there are problems with the application of the uranium-lead method regarding dating.
Therefore, the isotopic stratigraphy method was used to establish a more detailed chronology. Six dated points from the profile (
Oxygen stratigraphy results for the analysed flowstone. (A) 1 – δ18O record from the Głęboka Cave flowstone; 2 – discontinuity surfaces; 3 – the foraminifera δ18O record from the LR04 profile (Lisiecki and Raymo, 2005); (B) Age-depth model.Fig. 6
One of the major objective of the present work was to obtain a chronology of the cave flowstone profile. A first attempt was made using U-series dating. However, the results showed that the studied flowstone age was out of the range of the 230Th/U method. On the other hand, the 234U/238U ratios dated the profile between 1,200 ka and 500 ka. Finally, isotopic stratigraphy was used as a tool to develop a more detailed chronology. The results of the isotopic stratigraphy correlation placed this flowstone profile in the time interval between 935 ± 5 ka and 470 ± 5 ka. The flowstone from Głęboka Cave was divided by three hiatuses. Two of them, H1 and H3, were also detected by numerical correlation. The detection of two hiatuses by numerical correlation confirmed the reliability of the calculated age-depth model. The deposition break time for H2 may be too short for detection by correlation at this record’s resolution.
The speleothem’s growth rate varies from approximately 0.4 mm up to approximately 5 mm/ka and is in range of the typical values reported for flowstones (Ford and Williams, 1989). The growth rate is positively correlated with temperature, thickness of water film and calcium ion concentration in seepage water (Fairchild and Baker, 2012). The fragment located between depths of 110 mm and 340 mm has the faster sedimentation rate and comprises
Results of the isotopic measurements show high variability in the environmental conditions during the formation of the studied flowstone. The time range of the isotopic record spans from MIS 24 to MIS 12 (
The variability of δ18O (B) and δ13C (C) isotopes, with marked discontinuities in the surfaces, along with oxygen isotope stages (A), insolation at 50°N (D) and a fabric log (E) with marked as ellipsoids locations of thin sections on the timescale.. 1 – discontinuities in the surfaces; 2 – boundaries of oxygen-isotope stages (Lisiecki and Raymo, 2005).Fig. 7
In summary, the analyzed record contains a clearly visible succession of marine isotopic stages from MIS 24 to MIS 12 (Lisiecki and Raymo, 2005;
Because the flowstone was older than the method limit, the use of the U-Th method only allowed for the estimation of the flowstone age at 0.5–1.2 Ma, and applying OIS as a tool for speleothem dating enabled the establishment of a precise chronology for the flowstone (from 935 ± 5 ka to 470 ± 5 ka). Consistency with North Atlantic records proves that the climate was also controlled by the NAO. The calcite fabric analysis was a useful tool for the reconstruction of growth stability regime, temperature, the presence and thickness of water films, and the identification of the pediods when the crystallization was interrupted and/or the flowstone was partially eroded. The analysis of stable isotope variability combined with the chronology obtained by oxygen stratigraphy allowed for the estimation of climate changes in the vicinity of the Gleboka Cave during the crystallization of the flowstone. The isotopic record time range spans from MIS 24 to MIS 12, and crystallization during MIS 18 and MIS 16 proves that speleothems can also grow also during cold oxygen isotope stages. All recorded hiatuses in the flowstone from the Głęboka Cave occurred during interglacial periods, which suggests that the major factor controlling the flowstone growth rate is the precipitation amount. Cold periods in the Middle Pleistocene did not always cause the cessation of speleothem growth in Central Europe, but they favoured deposition by increasing the humidity of the climate.