LETTER: IUGS RECOGNITION OF THE RAČIŠKA PEČINA CAVE SECTION AS A GLOBAL GEOLOGICAL HERITAGE SITE SEDIMENTNI PROFIL V RAČIŠKI PEČINI VPISAN V SEZNAM SVETOVNE GEOLOŠKE DEDIŠČINE MEDNARODNE ZVEZE GEOLOŠKIH ZNANOSTI (IUGS) Nadja ZUPAN HAJNA1 ACTA CARSOLOGICA 53/2-3, 211-214s, POSTOJNA 2024 1 ZRC SAZU, Karst research Institute, Titov trg 2, 6230 Postojna, Slovenia; e-mail: nadja.zupan-hajna@zrc- sazu.si Prejeto/Received: 20. 11. 2024 DOI: https://doi.org/10.3986/ac.v53i2-3.14020 1. INTRODUCTION Račiška Pečina, a limestone cave in the Classical karst in southwestern Slovenia, which belongs to the External Di- narides (Slovenian Cave Register No. 935; 45°30’12.10’’N; 14°09’00.83’’E; 609 m a.s.l.), has received international recognition for its exceptional sedimentary section. The cave’s sediments, which span 3.4 Ma, provide a con- tinuous and detailed record of climate, environmental and paleomagnetic changes. This unique archive of the Earth’s history has earned Račiška Pečina a place among the “Second 100 IUGS Geological Heritage Sites”, an award announced by the International Union of Geologi- cal Sciences (IUGS) at the 37th International Geological Congress in Busan, South Korea. To explain, an IUGS Geological Heritage Site is a key place with extraordinary geological elements or processes of the highest scientific relevance, used as a global reference, and/or with a sub- stantial contribution to the development of geological sciences through history (Lozano et al., 2024). The scientific significance of the Račiška Pečina sedimentary section (i.e. RP section) is the result of more than two decades of interdisciplinary research led by the Karst Research Institute ZRC SAZU in Slovenia, mainly in cooperation with the Czech Academy of Sciences and the Polish Academy of Sciences. Through a combination of magnetostratigraphy, isotope studies and paleonto- logical analyses, the researchers have created a compre- hensive framework linking regional geology with global stratigraphic and paleoclimatic events (e.g. Horáček et al. 2007; Mihevc, 2003, 2007; Moldovan et al., 2011; Pawlak et al., 2024; Pruner et al., 2010; Sierpień et al., 2021; Zupan Hajna et al., 2008, 2010, 2020, 2021). My thanks go to the researchers, in- stitutions and international collaborators who made this achievement possible. This milestone once again under- lines the importance of preserving and studying cave sites, which represent an exceptional geological heritage. 2. SIGNIFICANT RESULTS The Račiška Pečina sedimentary section is today recog- nized as a globally significant archive of alternating spele- othem layers (calcite flowstone) and clay or silt deposits, which were formed during periods of interrupted calcite deposition (hiatus). These hiatuses, related to changes in surface hydrology and climate, provide crucial insights into past geological and environmental conditions. Using advanced dating methods, including U-series, radiocar- bon dating, paleomagnetic analyses and paleontologi- cally based magnetostratigraphy, a robust chronology for the section has been established (Horáček et al., 2007; Zupan Hajna et al., 2021). NADJA ZUPAN HAJNA One of the most remarkable discoveries is the Ma- tuyama/Brunhes magnetic reversal, which was precisely dated to 777.7 ± 6 ka and recorded in a very detailed stratigraphic interval of 6 mm (Pawlak et al., 2024). The age model based on oxygen isotope stratigraphy locates the reversal in the middle of MIS 19 (between 777.9 and 777.2 ka), characterized by abrupt changes in stable isotopes (δ¹³C and δ¹⁸O), trace element concentrations (e.g. Mg, Sr, Ba, Na, P, Si, Mn, Fe, Al, Cu, U, Pb, Ti) and calcite microstructure. These changes indicate signifi- cant temperature fluctuations and varying precipitation quantities. In particular, two maxima in the trace element concentrations at the beginning and end of the reversal indicate two periods of intense precipitation, which led to increased leaching of clay minerals into the cave. In addition, the shifts in stable isotopes reflect significant cooling and increased precipitation during the M/B tran- sition, which occurred in the generally warm MIS 19. The section also records other important paleomagnetic events, including the Gauss/Matuyama reversal and the Olduvai subchron. The precise dating of the Matuyama/ Brunhes reversal is consistent with estimates from other records, e.g. from the Mediterranean (Marino et al., 2015; Capraro et al., 2017; Toti & Bertini, 2018) and Osaka Bay (Kitaba et al., 2017). The reversal is estimated to have lasted around 1,000 years and represents an unstable, cooler phase with increased precipitation. These results raise interesting questions about the influence of the Earth’s magnetic field on paleoenvironmental conditions and support the hypothesis that magnetic reversals can affect broader climate patterns. The accurate record and precise dating of the Matuyama/Brunhes boundary, cor- related with stable isotope and faunal data, provide a de- tailed chronostratigraphic framework for the transition and establish Račiška Pečina as a key global terrestrial reference site. The stable isotope analyses of the calcite layers (δ¹³C and δ¹⁸O) provide isotope compositions that reveal long- term climate trends and provide a valuable record of past climate changes that correlate well with global oxygen isotope curves (Sierpień et al., 2021; Zupan Hajna et al., 2021). These data reveal transitions from the Pliocene to the Pleistocene as well as glacial and interglacial cycles during the Quaternary. The RP section is also important for its paleonto- logical discoveries. Various faunal remains were found in the cave sediments, including Ursus ex gr. spelaeus, dated to >72 ka, and fossil gastropods (Zospeum sp.), possi- bly the first troglobiontic snails, stratigraphical dated to around 2 Ma. These finds illustrate the ecological history and the dynamics of biodiversity in the region (Zupan Hajna et al., 2021). Notable discoveries include fragments of molars and incisors attributed to species such as Cle- thrionomys cf. glareolus, indicating a Late Early or Middle Pleistocene age, as well as Apodemus cf. atavus and Plio- mys sp. characteristic of biozones MN17–Q1 (Horáček et al. 2007; Zupan Hajna et al. 2021). These remains extracted from clay layers indicate different ecological conditions, including arboreal and steppe habitats. The combina- tion of paleontological evidence and magnetostrati- graphic data has signifcantly contributed to the chrono- logical framework of the site and enriched our under- standing of Pleistocene ecosystems and their evolution. In addition, radiocarbon-dated soot layers indicate the temporary presence of humans in the cave during the Holocene, adding an archaeological dimension to its significance (Zupan Hajna et al. 2021). Furthermore, Figure 1: Diploma for the inclu- sion of Račiška Pečina in the IUGS Second 100 Geological Heritage Sites, awarded for its exceptional importance for the stratigraphic- sedimentary heritage. 212 ACTA CARSOLOGICA 53/2-3 – 2024 LETTER: IUGS RECOGNITION OF THE RAČIŠKA PEČINA CAVE SECTION AS A GLOBAL GEOLOGICAL HERITAGE SITE the cave sediments provide insights about tectonic uplift and regional climate variability, show how local geological processes interact with global environmental changes. 3. CONCLUSIONS The recognition of the Račiška Pečina by the IUGS high- lights its exceptional global scientific importance. Its sedi- mentary profile, which has been carefully studied over decades, offers a rare opportunity to study important geo- logical events such as the Matuyama/Brunhes magnetic reversal. Beyond its regional significance, Račiška Pečina serves as a crucial archive for understanding the intricate interplay between tectonic processes, climate dynamics and ecological changes over millions of years. Its inclu- sion on the IUGS Geological Heritage List underlines both its extraordinary scientific value and the need for its preservation. The cave is securely locked and protected, ensuring its availability for ongoing and future research. Račiška Pečina is an example of the importance of interdisciplinary research and international cooperation to improve our understanding of the Earth’s geological and environmental history. The well-preserved sedimen- tary record in the caves provides a rare opportunity to integrate regional geological findings into broader, global contexts, cementing their role as an important contribu- tion to the study of Earth’s dynamic systems and a crucial resource for ongoing and future research. ACKNOWLEDGEMENTS The results are mainly due to the cooperation with the Karst Research Institute of the Research Centre of the Slovenian Academy of Sciences and Arts in Postojna, the Institute of Geology of the Czech Academy of Sci- ence in Prague (MOBILITY PLUS project SAZU-24-04) with contributions from the Institute of Geological Sci- Figure 2: The RP section provides information on speleothem dome growth (in relation to the inflow source: 1-5), stalagmites (6), paleo- magnetic reversals (7), dating results (8– 14C, 9– U/Th), paleontology (10), and climate (a schematic log of stable isotopes– 11). Location of the M/B magnetic polarity reversal is marked with a black line. 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