Natural Speleothem Damage in Postojnska Jama, Slovenia, Caused by Glacial Cave Ice? A First Assessment

Povzetek

Naravne poškodbe kapnikov v Postojnski jami so že dolgo znane. Odlomljene in nagnjene stalagmite je na svojih slikah prvi upodobil Schaffenrath. Nekaj teh poškodb je preučil Hohenwart in kot vzrok odločno izključil skalni podor ali potres. V tem prispevku avtor razpravlja o vzrokih za naravno lomljenje kapnikov. Kot najustreznejši vzrok se kaže jamski led. Ko so bila v višku glaciala stalno zamrznjena tla razširjena po vsej severni, vzhodni in srednji Evropi, je v jamah moral nastajati led. Zaradi tega je najustreznejši dejavnik, s katerim bi lahko razložili starejše poškodbe kapnikov. Nadalje je v študiji predstavljenih nekaj zgodovinsko izpričanih lomov kapnikov iz Postojnske jame. O tem teče razprava z vidika jamskega ledu in predstavljen je ad hoc model nastanka ledu. V stranskih Pisanem in Brezimenem rovu so velike količine polomljenih stalagmitov in stalaktitov v nenavadnih položajih. Jamski led je lahko tisti dejavnik, ki razloži ta pojav. Avtor domneva, da je bila Postojnska jama med zadnjim in prejšnjim glacialom zapolnjena z ledom. V kolikor bi se strinjali, da je vzrok poškodovanim kapnikom jamski led, potem bi nam lahko taki kapniki služili kot pokazatelji, do kod v podzemlje je v raznih glacialih segala izoterma 0o C.

Natural speleothem damage has been known from Postojnska jama for a long time. Schaffenrath was the fi rst to depict broken and leaning stalagmites on his pictures from the interior of the cave. Hohenwart analysed some of these damages, clearly excluding rock fall or earthquakes as a cause. Here the author discusses the possibilities, which could cause natural speleothem breakage in general. The most promising cause is cave ice. It must have formed in caves during glacial maxima when permafrost spread throughout northern, eastern and central Europe. Consequently cave ice could be the most prominent factor in explaining non-recent speleothem damage. Next some of the historically known fl owstone breakages from Postojnska jama are presented. These are discussed in view of cave ice and an ad hoc model for the genesis is given. In the side passages Pisani rov and Brezimenski rov there are masses of broken stalagmites and stalactites and speleothem fragments in precarious positions. Cave ice offers an overall process to explain these observations. Thus it is suggested that all or parts of the Postojnska jama were fi lled with ice during the Last and earlier Glacials. If accepting speleothem damage as a consequence of glacial cave ice, then it should be possible to use it as a marker facies for the extent of the zero temperature line during the various glacials.