COBISS: 1.01 HOw TO DATE NOTHING wITH COSMOGENIC NUCLIDES KAKO DATIRATI PRAZNINE S KOZMOGENIMI NUKLIDI Philipp HäUSELMANN1 Abstract UDC 539.16:552.5(494) Philipp Häuselmann: How to date nothing with cosmogenic nuclides A cave is a natural void in the rock. Terefore, a cave in itself cannot be dated, and one has to resort to datable sediments to get ideas about the age of the void itself. Te problem then is that it is never very certain that the obtained age really is coin-cident with the true age of the cave. Here, we present the use of a method which couples sedimentary and morphologic information to get a relative chronology of events. Datings within this relative chronology can be used for assessing ages of forms, processes, and sediments, and the obtained dates also fx some milestones within the chronology, which then can be used to retrace, among other things, paleoclimatic variations. For many cave systems, the dating limits of the most widely used U/T method on speleothems are too low (350 to max. 700 ka) to get ages that inform us about the age of the cave. Te recent use of cosmogenic nuclides on quartz-containing sediment permits to push the datable range back to 5 Ma. while the theoretical background is explained elsewhere (Granger, this volume), we concentrate on the Siebenhengste example (Switzerland). Key words: relative chronology, cosmogenic nuclides, cave dat-ing methodology, Siebenhengste. Izvleèek UDK 539.16:552.5(494) Philipp Häuselmann: Kako datirati praznine s kozmogenimi nuklidi Jame predstavljajo praznino v kamninski masi in jim kot takim ne moremo doloèiti starost. Zato z datiranjem jamskih sedimen-tov sklepamo tudi o starosti jame, pri èemer seveda ne moremo trditi, da je dobljena starost tudi prava starost jame. V èlanku predstavimo metodo pri kateri z združitvijo sedimentarnih in morfoloških izsledkov sklepamo o relativni kronologiji dogodkov. Datiranje v oviru relativne kronologije lahko uporabimo za doloèevanje starosti razliènih oblik, procesov in sedimentov. Dobljene rezultate pa lahko uporabimo kot pomembne mejnike v kronologiji, npr. pri intepretaciji klimatskih sprememb. Veliko jam je starejšiih od zgornje meje starosti (350 do 700 ka), ki jo lahko doloèimo z uran-torijevo metodo, ki je zelo razširjena. V zadnjem èasu se zato uveljavlja metoda datacije s kozmogenimi nuklidi, ki omogoèa datiranje dogodkov do starosti 5 Ma. Ker je teoretièno ozadje te metode predstavljeno drugje (npr. Granger v tej številki), se tu omejimo le na uporabo metode v jamskem sistemu Siebenhengste (Švica). Kljuène besede: relativna kronologija, kozmogeni nuklidi, metodika datiranja jam, Siebenhengste. INTRODUCTION For many cave scientists, it might not be evident that a while the sediments found within the cave give variable cave does not exist - only the surrounding rock gives ex- ages from today (in the case of still active speleothems) istence to the void called cave. Terefore, a cave cannot up to the last stages of speleogenesis (in the case of spe-be dated by conventional methods (Sasowsky 1998), but cifc sand deposits dated by cosmogenic nuclides) and one has to use datable sediments. In karstic caves, the age therefore to the age of nothing itself. of the surrounding rock gives a maximal age of the cave, 1 Swiss Institute of Speleology and Karst studies SISKA, c.p. 818, 2301 La Chaux-de-Fonds, Switzerland, Fax 0041 32 913 3555, e-mail: praezis@speleo.ch Received/Prejeto: 11.12.2006 TIME in KARST, POSTOJNA 2007, 93–100 PHILIPP HäUSELMANN Tis paper contains two parts. In the frst part, the concept of relative chronology is explained. Te link be-tween morphology and sediment succession leads to a relative chronology of erosional and depositional events. Any dating of sediment with the purpose of studying the age of nothing basically requires such a relative chronol-ogy, which places the obtained data into a timeframe. In the second part, the dating of sandy cave sediments with cosmogenic nuclides is briefy presented. Es- THE CONCEPT OF RE INTRODUCTION Geologists and other scientists are usually aware of the laws of stratigraphy, which say that a younger sediment overlies an older one. Tese laws are the base of a relative chronology. Tis chronology is normally used to assess the correctness of an obtained age - the numerical value has to be concordant with stratigraphy, or the dated age may not be correct. Most of the time, this principle is used with stalagmites, where the obtained ages must be older at the base and younger at the top (e.g. Spötl et al. 2002). Morphological indications, on the other hand, also give chronological information. A keyhole passage in-forms us that a phreatic phase was followed by a vadose one. Successions of speleogenetic phases are found in many cave systems. while some of them indicate base level rises (Audra et al., 2004), most of them indicate a downcutting of the regional base level (uplif, valley deepening, e.g. Ford & williams 1989; Rossi, Cortel & Arcenegui 1997). Tis in itself is also a chronological information: the oldest cave passages are on top, the young-est ones near the present baselevel. Te difculty now is to connect the sediments of several, basically independent, sedimentary profles and to link them with the morphological succession of the cave passages. Tus, the sedimentary profles are not in-dependent from each other, and a relative chronology of erosional and depositional events over the whole cave can be made. ExAMPLE Figure 1 shows a real situation encountered in St. Beatus Cave (Switzerland): To the right side is a typical keyhole passage which proves that a phreatic initiation of the ellipse on top was followed by a canyon incision. In the middle part of the fgure, the meander gradually disappears and is replaced by a more or less elliptic passage that continues towards pecially when dealing with sands, a relative chronology is very important to date only meaningful sediments. Te theoretical background is only very briefy presented, and the reader is referred to Granger (this volume) for more thorough information. Te Siebenhengste example, the use of the relative chronology, and the obtained results are presented in more detail. TIVE CHRONOLOGy the lef side of the fgure. we see therefore a transition of a vadose feature into a phreatic one, and thus an old water level. In the profle to the right, we observe fowstone deposition that was truncated by the river incising the meander. Terefore, the fowstone predates the canyon, but postdates the initial genesis of the elliptic passage to the right. Te meander changes into an elliptic passage, thus the two forms are contemporaneous. Consequently, the older fowstone disappears in the area of this transition. within all the passages, silts were deposited. Tey are younger than the meander incision, and younger than the passage to the lef, and prove of an inundation of the whole cave. Stalagmites grow on the silts and are partially still active. Tis example can be written as a table (Tab. 1). -------------------------------------- Phreatic genesis of top ellipse -------------------------------------- Water level lowering Deposition of fowstone Erosion of fowstone Erosion of meander -------------------------------------- Water level lowering Silt deposition Stalagmite growth -------------------------------------- tab. 1: Chronology of erosional and depositional events (Fig. 1) Tis table is a frst relative chronology that links the sediments and the morphology of the cave.For practical reasons, the table presenting the chronology of events in a large cave system is not rewritten with each sedimen-tary succession found. Instead, the single sedimentary sequence is coupled with morphology, and is written as a column in the table. Te next sedimentary sequence, again coupled with morphology, is written as another column. Tus, the above example would then look like Table 2. 94 TIME in KARST – 2007 HOw TO DATE NOTHING wITH COSMOGENIC NUCLIDES Sequence at left Sequence at right Phreatic genesis of top ellipse Water level lowering Phreatic genesis Deposition of fowstone Phreatic genesis Erosion of fowstone Phreatic genesis Erosion of meander Water level lowering Silt deposition Silt deposition Stalagmite growth Stalagmite growth tab. 2: Chronological table with columnar writing of Fig. 1 ExPANSION If we continue up- and downstream of that profle, we fnd several other morphological indications and sedi-mentary successions, each of them having a link with our initial profle - until we encounter the next paleo-water-level and thus the next morphological change. Tere, the links have to be established again. Te table thus slowly grows and gets more complete. Of course, the example presented above is an ideal case. Ofen, the passages lack some information, thus making it difcult to establish an unambiguous chron-Table 3: A more complicated example from St. Beatus Cave Lower passage ? ? ? Upper passage Phreatic genesis Water level lowering Speleothem Silt deposition Speleothem Silt deposition Speleothem Silt deposition Erosion Speleothem Silt deposition Phreatic genesis Water level lowering Pebble deposition Speleothem Erosion Sand deposition Speleothem Sand deposition Silt deposition Pebble deposition Sand deposition Silt deposition Erosion Speleothem Erosion Silt deposition Speleothem Erosion Silt deposition ' tab. 3: A more complicated example from St. beatus Cave ological table. Table 3 give an example: here, the upper passage lacks incision of a canyon. Terefore, it is not clear whether the sediments found in the upper passage were all deposited while the lower passage was still in its initial genesis, or whether the sediments can be partly correlated. In this case, a relative correlation of the sediments by observation only is not possible: some absolute dates have to be obtained. Of course, these ages have to be in stratigraphic order of both the sediment succession and the morphologic indications. Te above example had been dated by U/T on speleothems. Te resulting table is presented in Table 4. Here, the speleothems with roughly the same age have been grouped together. Ten, laminated silt deposits that are thought to be a product of glacial damming (Bini, Tognini & Zuccoli 1998; Audra et al., this volume), are parallelized, inferring that the whole cave was fooded in such conditions. Of course, some un-certainties still persist. Table 4: The more complicated example, dated and expanded Lower passage Phreatic genesis Water level lowering Pebble deposition Speleothem Erosion Sand deposition Speleothem (235 ka) Sand deposition Silt deposition Pebble deposition Sand deposition Silt deposition Erosion Speleothem (180 ka) Erosion Silt deposition Speleothem (91 ka) Erosion Silt deposition tab. 4: Te more complicated Upper passage Phreatic genesis Water level lowering Speleothem (>350 ka) Silt deposition Speleothem (337 ka) Silt deposition Speleothem (114 ka) Silt deposition Erosion Speleothem (99 ka) Silt deposition example, dated and expanded TIME in KARST – 2007 95 PHILIPP HäUSELMANN Fig. 1: Schematic section through a part of St. beatus Cave (Switzerland), showing the relationship between sediments and morphology. wHy A RELATIVE CHRONOLOGy? parallelized all the sedimentary sequences, it is possible Te huge advantage of such a table of relative chronology to make a synthetic and dated sediment profle of the is that it ofers more control on the correct stratigraphic whole cave, which can then be used to get information order than single sections, in ideal cases also the cave on climatic variations and the presence or absence of gla-genesis can be dated, and last but not least, when having ciers damming the cave’s exit (Häuselmann 2002). DATING wITH COSMOGENIC NUCLIDES INTRODUCTION Cosmogenic nuclides are generated by the interaction of cosmic rays (mainly protons, neutrons, and muons) with atoms in the Earth’s atmosphere and lithosphere. Te production rate of cosmogenic isotopes depends on the intensity of the cosmic rays, which is subject to change. Te atmosphere then absorbs most of the primary rays and thus causes production rates to depend on elevation. Finally, the geometry of the sample location (and eventu-al snow or soil cover) also has its efects. Te radioactive nuclides most widely used for dating purposes are 10Be and 26Al produced in quartz. THE PRINCIPLE AND POSSIBILITIES OF BURIAL DATING Burial dating of cave sediments is a relatively new tech-nique that indicates the time sediment has been underground (Granger, Fabel & Palmer 2001). It relays on the radioactive decay of the nuclides that were previ-ously accumulated when the sediment was exposed at the surface. whereas the intensity of the cosmic rays may vary with time, the ratio of produced 10Be to 26Al remains always approximately 1:7. Te 10Be/26Al ratio can thus be calculated from the production rates and radioactive decay. If a sample that contains 10Be and 26Al is washed underground to sufcient depth to be shielded from further radiation, the nuclide concen-trations diminish. Since 26Al has a half-life of 720 ka, 96 TIME in KARST – 2007 opposed to the one of 10Be of 1.34 Ma, the ratio of 1:7 is gradually lowered. Measurement of that ratio there-fore gives a direct indication of the time the sample re-mained underground. Of course, several prerequisites have to be fulflled in order to get a burial age: - First of all, the sediment must contain quartz that was irradiated sufciently prior to burial. Te grain size should be minimally fne sand (otherwise the cleaning process also eliminates the quartz), but may reach pebble size without problem. - Ten, burial should ideally be 20-30 m below the surface to be sufciently shielded from radiation. - In order to make a measurement meaningful, the stratigraphic relationship of the sampled sand with the passage and other sediments should be clearly estab-lished - the relative chronology is needed. Burial dating has a range from about 100’000 years up to 5 Ma. Afer that time, the amount of remaining isotopes is usually too small to be measured accurately (Granger & Muzikar 2001). It is one of only a few radio-metric methods that date lower quarternary and Plio-cene deposits. It is of great interest for cave dating, frst because many old caves were created in the Pliocene or even earlier, and second because caves are very efective at shielding the sediment from further cosmic ray bom-bardment. As with other cave-dating methods, burial dating may also be used to date the age of the passage, HOw TO DATE NOTHING wITH COSMOGENIC NUCLIDES Caves of the region Siebenhengste - Hohgant Perspective view from 370g, as of January, 2002 Only the most important passages shown Spring for 1440 - 558 Aare valley Fitzlischacht ^^jT FItZ 0.5 1 1.5 km mi* m a.s.l. 1950— Siebenhengste JA201HSHP Spring for 1950-1505 Eriz Hohgant m St. Beatus Caves 760 Zone profonde 558 F1 Bätterich Bärenschacht Faustloch K2 Haglätsch by HRH & toporobot Fig. 2: Projection (370 degrees) of the Siebenhengste caves with the speleogenetic phases. Stars indicate sampling places for cosmogenic dates. From häuselmann & Granger (2005), modifed. thus indicating valley deepening rates and evolution of the surface outside the cave. THE SIEBENHENGSTE ExAMPLE we used burial dating to date the old passages of the Siebenhengste cave system in Switzerland. Te Siebenhengste region is situated in the north-western part of the Alps, adjacent to the molasse basin. From Lake Tun, the mountain range extends to the Schrattenfuh, 20 km away. Te cave region is one of the longest and deep-est worldwide, with the Réseau Siebenhengste-Hohgant Fig. 3: Plot of ages (vertical) versus altitude (horizontal). having 154 km length and -1340 m depth. Te caves comprise 14 diferent speleogenetic phases, which can be related to paleo-valley bottoms (Jeannin, Bitterli & Häuselmann 2000). Te highest and oldest fve phases (at presumed spring elevations of >1900, 1800, 1720, 1585, and 1505 m a.s.l.) had their springs in the Eriz valley (Fig. 2). Te next phase, at 1440 m, shows a change in fow direction of 180°. Te spring was then located in the area of Lake Tun. Te infuence of today’s Aare valley (the site of Lake Tun today) therefore became predominant. All subsequent springs (at 1145, 1050, 890, 805, 760, 700, 660, and 558 m a.s.l.) drained to-wards the Aare valley. In the area between Lake Tun and Hohgant, a total of 23 sites were selected for sampling (see Fig. 2: stars indicate sites). Selection was made on the basis of a relative chronology, and care has been given to ensure that ei-ther the oldest possible sediment, or a series in stratigraphic order, was sampled. Due to the limited amount of time in which sam-pling could be done, the relative chronology is incomplete (Tab. 5), although the main events were re-traced. 21 samples were analysed (Häuselmann & Granger 2005). Te results show a great diversity of ages, ranging from 118 ka up to TIME in KARST – 2007 97 0 PHILIPP HäUSELMANN bold = morphologic event (a O denotes phreatic genesis, a v vadose enlargement), italic = dated event A201 ShP low SHP up Haglätsch A2TR A2CHU A2NS RBL L18 Faustloch Beatus Age Interpretation ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- 1800O 1800O 1800O 1800O SHP 7 Sediment 4.39 Erosion Erosion SHP2 2.35 SHP 3 Silt ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- 1720O 1720O 1720O Flowst. Flowst. Silt lake Silt Erosion Erosion Erosion Flowst. Sand Erosion A201 SHP5 1.9-1.84 Sand Sand Silt Silt ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- 1585O 1585O 1585O 1585O 1585O Erosion Parag.? Flowst. Flowst. Silt Flowst. Erosion lake SHP1 SHP6 HGLP Flowst. Flowst. Flowst. Erosion Erosion Canyon HGLS Paragen. epiphreatic L18 Silt 1.54-1.60 1.04-1.09 (.93?) ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- 1505/1440O 1505/1440O SHP4 HGLT A2TR A2CHU A2NS RBL2 Flowst. Erosion 0.78-0.80 (.93?) RBL1 0.63 ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- 1050O 1050O ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- 1050v 890O 890O Flowst. flooding F STL 0.47 ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- 805O 805O 760O 760O BG23 0.23 BG1 0.18 BG20 0.16 ------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------- tab. 5: Relative chronology of events around the Siebenhengste 2000 1500 1000 500 0 1 2 3 4 6 Burial age (10 years) 4.4 Ma (Tab. 6). Te surface sample (MwA) has a burial age of 106 ± 176 ka. Tus, the value is indistinguishable from zero, and we may assume that the sample was never buried. Te sample from St. Beatus Cave (BG1) has an age of 182 ± 122 ka. Its true value, bracketed by U/T Fig. 4: Rate of valley lowering in the Siebenhengste. Only maximum and minimum ages are displayed; however the valley deepening rates as well as the knickpoint at ~800 ka are easily visible. 98 TIME in KARST – 2007 HOw TO DATE NOTHING wITH COSMOGENIC NUCLIDES A relative chronology of events, albeit incomplete, coupled with burial age dating by cosmogenic nuclides, per-mitted to obtain a continuous history of valley incision in the Alps. Such data cannot be obtained in the same precision with other methods or at the surface. Te re-sults presented here are the frst cosmogenic dates for an Alpine cave system in a glacially infuenced area. Te re-sults indicate an onset of karstifcation in the Siebenhengste before 4.4 Ma, that is in the Pliocene or even earlier. Together with U/T dates obtained earlier (Häuselmann 2002), the history of the Siebenhengste cave system and ages, should be between 160 and 235 ka, which is again the case. Tese values indicate that the method yields young ages where expected. A difculty for dating with cosmogenic nuclides is mobility of the sediment. For instance, recent sand can be transported into a fossilized cave by a food and then be deposited. Our results show that this process happens: for any speleogenetic phase, there is a range of ages ob-served (Fig. 3). However, Fig. 3 also indicates that the re-mobilization and re-deposition of old sediments is rarely observable: if this would be the case, we would expect a random distribution of ages throughout the phases. However, the maximum age decreases with the next lower phase. we can thus construct a gradual valley lowering with time which is represented in Fig. 4. we see a knickpoint in the line connecting the ages: this knickpoint occurs at around 800 ka and 1500 m. Tis point refects a dramatic increase in valley deepening rate and coincides with the change in fow direction from Eriz to the Aare valley. tab. 6: Results of dating. its surrounding environment can be traced back over a huge time span. Te construction of a complete relative chronology is very time-consuming, but can be extremely reward-ing given the information one can extract from the cave. If speleogenetic phases, which are related to the overall geomorphic evolution of an area, can be expanded by such relative chronologies as well as absolute dates, the rate, duration, and extent of valley deepenings can be assessed, and a paleoclimatic history can be drawn as well. TIME in KARST – 2007 99 PHILIPP HäUSELMANN REFERENCES Audra, Ph., L. Mocochain, H. Camus, E. Gilli, G. Clauzon & J.-y. Bigot, 2004: Te efent of the Messinian Deep Stage on karst development around the Mediterra-nean Sea. Examples from Southern France. - Geo-dinamica Acta, 17, 6, 27-38. Bini, A., P. Tognini, & L. Zuccoli, 1998: Rapport entre karst et glaciers durant les glaciations dans les val-lées préalpines du Sud des Alpes. - Karstologia, 32, 2, 7-26. Ford, D.C. & P. williams, 1989: Karst geomorphology and hydrology. Chapman & Hall, London, 601 p. Granger, D.E, D. Fabel & A.N. Palmer, 2001: Pliocene-Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic 26Al and 10Be in Mammoth Cave sediments. - GSA Bulletin, 113, 7, 825-836. Granger, D.E. & P.F. Muzikar, 2001: Dating sediment burial with in-situ produced cosmogenic nuclides: theory, techniques, and limitations. - Earth and Planetary Science Letters, 188, 269-281. Häuselmann, Ph., 2002: Cave genesis and its relationship to surface processes: Investigations in the Siebenhengste region (bE, Switzerland). - PhD thesis, Université de Fribourg, 168 p. Häuselmann, Ph. & D.E. Granger, 2005: Dating of caves by cosmogenic nuclides: Method, possibilities, and the Siebenhengste example (Switzerland). - Acta Carsologica, 34, 1, 43-50. Jeannin, P.-y., T. Bitterli, T. & Ph. Häuselmann, 2000: Genesis of a large cave system: the case study of the North of Lake Tun system (Canton Bern, Switzer-land). In: A. Klimchouk, D. C. Ford, A. N. Palmer, & w. Dreybrodt (Eds.), Speleogenesis: Evolution of Karst Aquifers, pp. 338-347. Rossi, C., A. Cortel & R. Arcenegui, 1997: Multiple pa-leo-water tables in Agujas Cave System (Sierra de Penalabra, Cantabrian Mountains, N Spain): Crite-ria for recognition and model for vertical evolution. - Proceedings 12th Int. Congress of Speleology, La Chaux-de-Fonds, Switzerland, 1, 183-187. Sasowsky, I.D., 1998: Determining the age of what is not there. - Science, 279, 1874. Spötl, C., M. Unterwurzacher, A. Mangini & F.J. Long-stafe, 2002: Carbonate speleothems in the dry, inneralpine Vinschgau valley, northernmost Italy: witnesses of changes in climate and hydrology since the last glacial maximum. - Journal of Sedimentary Research, 72, 6, 793-808 100 TIME in KARST – 2007