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SPELEOTHEM
LUMINESCENCE PROXY (Paper Presented at Bergen Meeting, August,1996) Y.Y.Shopov, L.Tsankov, L.N.Georgiev, A.Damyanova, Y. Damyanov, E. Marinova-Section Speleology & Faculty of Physics, University of Sofia, James Baucher 5, Sofia 1126,Bulgaria. E-mail: YYShopov@Phys.Uni-Soria.BG D.C. Ford-Geography Dept., McMaster University, Hamilton, Ontario, L8S lK4, Canada C.J.Yonge, W. MacDonald, H.P.R.Krouse- Dept. of Physics, University of Calgary, Calgary, Alberta, Canada Speleothem growth rate variations represent mainly rainfall variations. Speleothem luminescence visualizes annual microbanding we used to derive proxy records of annual precipitations for the cave site. In case of Rats Nest cave, Alberta, Canada we obtained good correlation (correlation coefficient of 0.57) between annual precipitations (from Banff station 50 km north of the cave) and annual growth rate of the speleothem (Fig. 1). We used obtained regression coefficients to reconstruct annual precipitations for last 280 years at the cave site with estimated error of 80 mm/ year. Annual speleothem growth rate was not dependent on intensly of luminescence, on annual temperature and solar luminosity for the same time span (zero correlation). Fig.1 (Up) Annual growth rate of a stalagmite from Rats Nest cave, Alberta, Canada(Down) Annual precipitations (from August to August) for Banff station, 50 km north of the cave By "tuning" of the time scale of a luminescent record with a geomagnetic dipole intensity record (Mazaud et al., 1991) we obtained a reconstruction of growth rates and precipitations in Bosnek karst region near Duhlata cave, Bulgaria for the last 50000 years (Fig.2). This record show a very prominent peak at 7000- 8000 years B.P., when annual growth rate was over 53 times higher than today. Considering that cave site is located in the place of the oldest civilizations (Mediterranean basin) this event can be related to the Bibles "Deluge". The age of the recorded event is about the age of "The Creation of the world". The duration of the recorded event is of several hundreds years, because of the low resolution of the record. In case that the real flood happened for a short time span it suggests enormous rainfall. Present day precipitation at the cave site is 685mm/yr. This speleothem was dated with 8 TAMS 14C dates. By "tuning" of the time scale of a luminescent record with the calibration 14C record (Stuiver and Kra, 1986) we obtained a reconstruction of growth rates and Fig.2. (Up) Reconstruction of growth rates (proxy of precipitations) of a flowstone from Duhlata cave, Bulgaria for the last 50000 years; (Down) Reconstruction of growth rates (proxy of precipitations) for the last 6400 years with averaged time step of 41 years for Cold Water cave, Iowa, US. precipitations for the last 6400 years with averaged time step of 41 years for Iowa, near Cold Water cave, US (fig.2). It suggests higher speleothem growth rate and higher precipitations between 6400- 2500 years B.P. at the cave site. This speleothem was dated with 7 AMS U/Th dates.
U-Pb dating of
Quaternary age speleothems Abstract U/Pb dating techniques are based on the ingrowth of stable isotope 206Pb from deacy of the parent isotope 238U and have the potential of dating speleothems beyond the range of the more usual 230Th/ 238 U methods. We obtained a 206 Pb */238U age of 255± 12 ka for a stalactite from Winnats Head Cave, Peak District, concordant with a 230Th / 238 U age of 255± 30 ka, assuming an initial state of uranium-series disequilibrium with initial 234U / 238 U ratio of 1.32± 0.05 and negligible initial 230Th and daughter isotopes.Introduction Important information about past climates and landscape evolution can be obtained from speleothems (secondary calcites precipitates in caves, such as stalactites and stalagmites). However, the limited range of 230Th / 238 U dating techniques (less than 500 ka using high-precision thermal ionisation mass-spectrometry or TIMS) means that many potentially important geological samples of Quaternary and Tertiary age remain undated. U-Pb dating techniques are commonly used for materials with a range in age of a few million years to the age of the earth. Successful application of U-Pb techniques to limestones and calcite spars of ancient carbonates and their late stage diagenesis have been made (Smith el al, 1991; Jones et al., 1995 ). Here we assess the application of U-Pb methodology to much younger deposits where the initial state of uranium-series disequilibria must be taken into account.
Methods and Results We demonstrate that a stalactite from Winnats Head Cave in the Peak District, England, analysed by 230Th / 238U methods (Rowe, pers. comm), can be dated using U-Pb methodology. Sub-millimetre wafers, almost entirely composed of clear calcite with a small detrital component, were analysed by TIMS for U and Pb. The calcite phase has high U content (16-30 ug g-1), and low Pb content (<10ng g-1). Using the Pb isotopic results plotted below (Figure 1), we obtain an initial 206Pb / 238U ratio of 17.11 ± 0.04, identical to the measured Figure1. 206Pb / 238U isochron for a stalactite from Winnats Head Cave, Peak District, England.detrital, low -m (i.e 204Pb / 238U ) phase 204Pb / 206Pb, 17.17 ± 0.09. The extremely large range of u values obtained (800 to 1.2´ 106) enables 206Pb*/ 238U and 207 Pb */235U to be estimated with reasonable precisions. Using the independently estimated initial activity ratio for 234U/218U of 1.32± 0.05, and assuming negligible initial 231Pa , 227Ac , 230Th, 226Ra and 210Pb , near-concordant ages of 250 ± 12 ka and 318 ± 54 ka (minimum 2s errors) have been derived. The age calculation uses the Bateman equations for closed-system decay (Ludwig, 1977). These ages compare very well with the alpha-spectrometric 230Th age of 255 ± 30 ka. The feasibility of dating further speleothem deposits is discussed. Only samples with high U/Pb ratios are suitable for U/Pb techniques. Results of a preliminary survey of U and Pb concentrations of many samples from various locations across the globe indicate that U/Pb ratios vary by eight orders of magnitude and it is clear that pre-screening must be adopted before attempts are made to date samples. Conclusions We demonstrate that U-Pb dating of speleothems is feasible by obtaining ages concordant with 230Th/ 238U ages. This has been achieved on an ideal sample with an extremely large range in m values. It is clear that further samples must be attempted before wider applicability can be demonstrated. References Jones, C.E., Halliday, A.N. and Lohmann, K.C. (1995). The impact of diagenesis on high-precision U-Pb dating of ancient carbonates: An example from the Late Permian of New Mexico. Earth and Planetary Science Letters, 134, 409-423. Ludwig, K. (1977). Effect of initial radioactive-daughter disequilibrium on U-Pb isotope apparent ages of young minerals. Journal of Research of the United States Geological Survey, 5, 663 -667. Smith, P.E., Farquhar, R.M. and Hancock, R.G. (1991). Direct radiometric age determination of carbonate diagenesis using U-Pb in secondary calcite. Earth and Planetary Science Letters, 105, 474-491.
ISOTOPIC COMPOSITION OF
PALAEOPRECIPITATION (Paper presented at Bergen Meeting, 1996) School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK The stable isotope composition of ancient speleothems contains an important palaeoclimatic signal. Fluid inclusions, trapped in the speleothem calcite as it is precipitated, are a geologic archive of palaeoprecipitation and palaeorecharge. There are, however, few reliable measurements of the isotopic composition of fluid inclusion water from speleothems. This is largely due to the analytical problems associated with the extraction and separation of less than 1 m l volumes of water from the host calcite and its subsequent analysis for both d 18O and d D. The main source of error is due to adsorption of the inclusion water onto fresh calcite fracture surfaces generated during crushing or decrepitation. Fractionation at this stage leads to depleted isotopic compositions in the recovered water. To overcome these problems we have developed an extraction system based on cold crushing of samples and thermal desorption of the released water. Tests with synthetic inclusions of known isotopic composition in iceland spar show that water can be recovered quantitatively and without isotopic fractionation. Using the technique we have made several new analyses of the hydrogen and oxygen stable isotope composition of fluid inclusions from two Holocene speleothem from GB cave in the Mendip hills. A brief synopsis of the data is given here.Synthetic inclusions were prepared by sealingul m l quantities of water in glass capillary tubes and crushing these at room temperature under static vacuum with dry iceland spar. Water released from the capillary was allowed to adsorb onto the freshly fractured calcite. Typical water calcite weight ratios were in the range 0. 1 to 0.2% and similar to that expected of many speleothems. Following adsorption, the water was cryogenically recovered into a trap whilst heating the crushing cell to 150° C. Measured yields showed a near 100% recovery. Isotopic analysis using micro-CO2 equilibration for d 18O and Zn reduction for d D gave mean compositions of (d 18O = - 6.49± 0.29‰vSMOW and d D = -47.7 ± 0.9‰VSMOW). These compositions, plotted on a d D versus d 18O plot in Figure 1, are very close to starting water composition of d 18O = -6.84‰VSMOW and d D = -44.9‰ VSMOW.Two Holocene stalagmites from GB cave in the Mendip Hills were selected for analysis. GB40 is a milky white specimen displaying fine growth laminae. U-series dates indicate that the basal deposit of the speleothem has an age of 8,700± 500 years. There then followed a hiatus in deposition with a subsequent growth history resuming at 3,700 years and continuing until the immediate past. Microscopic analysis of this stalagmite shows that the fluid inclusion population is dominated by numerous small, sub-micron inclusions concentrated in bands parallel to the growth laminae. Specimen UEA780527 is composed of optically clear, silvery calcite, similar in appearance to iceland spar. It is a good microstructural contrast to GB40 containing few large inclusions as opposed to many finely dispersed sub-micron inclusions. U-series and 14C age determinations give conflicting results, but it is probable that the stalagmite growth spans the entire Holocene. Results for d 18O and d D analysis of both stalagmites are plotted in Figure 1. For specimen GB40 the data are averaged over the growth interval 3,700 to 800 years b.p, and for UEA780527 the results correspond to the early Holocene. The deuterium compositions of GB40 fie in the range -37 to -47 ‰VSMOW and are wholly consistent with modern drip waters from this cave (Figure 1). In contrast the oxygen isotope compositions lie in the range -2.4 to -6.4‰VSMOW and are clearly inconsistent with the modern drip waters. The displacement in oxygen isotope composition with respect to the drip waters and the World Meteoric Line suggests that the post depositional oxygen isotope exchange between the speleothem calcite and fluid inclusion water has occurred. There is no apparent relationship between either oxygen or hydrogen isotope composition and water content.In contrast to specimen GB40, the inclusion waters from near the base of UEA780527 have compositions that are consistent and lie close to the WML. Moreover, using the inclusion and calcite oxygen isotope compositions, calculated cave palaeotemperatures are 9.7 and 12. These are close to present day cave temperatures and within the range expected for the Holocene. A more complete study has been made of the hydrogen isotope composition of inclusions from this stalagmite. The d D composition is plotted as a function of distance (decreasing age) above the base in Figure 2. The data are in excellent agreement with the expected d D compositions of Holocene precipitation. It is worth noting that the isotope composition of recovered water is independent of the water content of this particular sample. Water contents range from less than 0.01 wt.% to 0.1 wt.%. This suggests that recovery is quantitative and that there is little or no isotopic fractionation. The results obtained to date are very encouraging and suggest that we can reliably recover the d D, and in favourable circumstances the d 18O composition of palaeoprecipitation from speleothem fluid inclusions. The results presented in this paper represent the first step in a major programme to characterise palaeoprecipitation isotopic compositions and cave palaeotemperatures in western Europe during the past 250,000 years.
Isotopic composition of present-day
alpine speleothems (Paper Presented at Bergen Meeting, Aug., 1996) 1. Museo Tridentino di
Scienze Naturaii, Trento, Italy The stable isotope composition of present-day carbonate speleothems have been analysed from 3 different caves in Trentino, NW Italy (Grotta Cesare Battisti: entrance elevation 1880 in a.s.l., mean annual temperature 3.5° C; Grotta di Ernesto: entrance elevation 1167 in a.s.l., mean annual temperature 6.4° C; Grotta del Calgeron: entrance elevation 467 in a.s.l., mean annual temperature 7.5° C). All the speleothems come from poorly ventilated areas in the caves where humidity is almost at saturation, i.e. no evaporation effect is to be expected. Consequently, the precipitation mechanism involved is only by CO2 degassing. Speleothem morphology and isotopic composition We analysed active speleothems of different morphology in order to evaluate possible differences in isotopic composition. For the same morphology, and in the same place, differences in isotopic values of up to +/- 0.5‰ d13C and +/- 0.25‰ in d18O can be observed. These could be related to both kinetic effect and drip water provenance. Carbon and oxygen isotopic values display a trend towards increasing positive covariance from high drip-rate related morphology (microcrystalline cylindrical stalactites) to low drip-rate related morphology (soda straws). The following hierarchy in d13C and d18O enrichment has been observed: spray deposits º soda straws > tip stalagmites º cone stalactites > cylindrical microcrystalline stalactites. For Grotta del Calgeron d13C and d18O covariance in different morphologies shows a good correlation (fig. 1), which is related to two different kinetic mechanisms acting together: 1) In low drip-rate related morphology the residence time of percolating water within the rock is longer, between one and several month (Borsato, 1995), consequentely the host rock contribution is enhanced, and the isotopic composition of both drip water and
Figure 1. Isotopic composition of different speleothem morphologies in Grotta del Calgeron speleothems is enriched in 13C. This interpretation is confirmed by the Mg/Ca ratio of the drip water percolating throught dolomitic limestone rocks, which is higher (i.e. higher host rock contribution) during periods of low drip rate. 2) At lower drip rate the longer "residence time" of the drop at the stalactite tip enhances CO2 degassing that causes an enrichment in 13C of up to 2 (Dever et al., 1983). This consideration can also explain the more positive isotopic composition in spray deposits, where each drop splashed twice and causes a higher CO2 degassing. Although degassing may affect only d13C composition (Dever et al., 1983), the lack of evaporation suggests that the slight d18O enrichment (d18O enrichment is about 0.25 times that of d13C) is also related to the degassing process. Consequently, speleothem precipitation can occur in isotopic disequilibrium not only as a result of the evaporation effect (Gonzales & Lohmann, 1987), but also for different rates and velocities of CO2 degassing: This has been documented for other calcite precipitation processes that involve rapid CO2 degassing (Clark & Lauriol, 1992). For this reason, the interpretation of isotopic paleoclimatic series in speleothems are valid only if quantification and modelling of these kinetic effects are possible. Carbon isotopic composition vs. altitudeThe d13C composition of different speleothems with the same morphology (i.e. cone stalactite tips), displays a positive covariance with the altitude of the recharge area above the caves. This covariance is directly related to the pCO2 in the soil above the cave. At high altitude the thickness of the soil, as well as the density and activity of the vegetation, are reduced and, consequentely, there is a low CO2 production. In these conditions the pH at which calcite saturation takes place in percolating water is slightly alkaline (between 7.5 and 8.5) producing a strong positive shift between dissolved CO2 and CaCO3 precipitates (Wigley et al., 1978). At low altitude, high soil pCO2 enhances carbonate dissolution: water calcite saturation occurs at subacid pH (between 6.0 and 7.0), and the C-isotope fractionaction between dissolved CO2 and CaCO3 precipitates is smaller. These considerations offer an alternative interpretation of d13C fluctuation vs. time for interglacial speleothems from alpine and temperate regions, where entirely C3 vegetational assemblages are to be expected, and C3-C4 fluctuaction are unrealistic. Bibliography: BORSATO A., 1995 - Analisi chimico-fisiche e monitoraggio di acque di percolazione nella Grotta del Calgeron (Valsugana): primi risultati e implicazioni idrogeologiche. Studi Trent. Sc. Nat. Acta Geologica, 70 p.79-94. CLARK I.D. & LAURIOL B., 1992 - Kinetic enrichment of stable isotopes in cryogenic calcites. Chemical Geology, 102, p.217-228. DEVER L., DURAN R-, FONTES J. C. & VACIIIER P., 1983 - Etude pedogenetique et isotopique des neoformations de calcite dans un sol sur craie; caracteristiques et origines. Geochim.. Cosmochim. Acta, 47, p.2079-2090. GONZALES L.A. & LOHMANN K.C., 1987 - Controls on mineralogy and composition of spelean carbonates: Carlsbad Cavem, New Mexico. In: James N.P. & Choquette P.W. (ed.): Paleokarst, Springer-Veriag, p.81-101. WIGLEY T.M.L., PLUMMER L.N. &, PEARSON F.J.JR., 1978 - Mass transfer and carbon isotope evolution in natural water systems. Geochimic. Cosmochim .Acto, 42, p. 1117-1139.
sotopic Study of Cave Carbonates from Moravian Karst, Czech Republic (Paper Presented at Bergen Meeting, Aug.,1996) Cave carbonates termed speleothems are mineral formations precipitated in limestone caves from groundwater which has percolated through the adjacent carbonate host rock. Speleothems may provide paleoclimatic record for terrestrial environments (e.g., Bradley 1985, Gascoyne 1992). The Moravian Karst is a famous karst area in Central Europe. Exploration and scientific research started here more than one century ago. In 1938 Professor Absolon conducted a demanding project with the aim to discover a large cave system with an active subsurface stream. A tunnel was dug in cave clastic sediments of the Zazdina jeskyni Cave, which is located near the Punkevni jeskyni show-cave and the famous Macocha Chasm. Unfortunately, the project was interrupted in 1940 and never finished. The largest profile in cave sediments of the Moravian Karst more than 330 m long was exposed. The Zazdina jeskyni Cave consists of two parts. The vertical part is filled by rhythmically bedded sands and silts. This sedimentary accumulation is 33 m thick and in its upper part sediments with a reverse paleomagnetic polarity were found (Sroubek and Diehl, 1995). Consequently, the age of deposition is older than the paleomagnetic boundary Brunhes/Matuyama (780,000 years). In horizontal, part of the Zazdina jeskyni Cave, fluvial sandy gravels, sands and silts were deposited by a subsurface stream. The break in clastic sedimentation followed and a flowstone layer originated on the surface of fluvial sediments in some parts of the cave. During the last sedimentation period the laminated and thin-bedded silts filled most of the cave corridor up to the ceiling. These fine sediments were transported by meteoric waters vertically from the surface through cracks and karst chimneys. The d 13C and d 18O values of layers and separate relics of flowstone document that in the horizontal corridor of the Zazdina jeskyni Cave cave carbonates originated in two periods with different climatic conditions. The redeposited blocks of flowstonepreserved inside fluvial sediments in the frontal part of the cave probably originated in caverns above the main corridor and fell from the chimneys into the sediments deposited in the horizontal part of the cave. The age of the flowstone layer deposited on the surface of fluvial sediments (200 m far from the cave entrance) was determined by U/Th method in the U-Series Dating Laboratory of Bergen University, Norway. The basal part of the flowstone layer was deposited 114.37 (+5.05/-4.85) kyr ago. Carbonate precipitation continued approximately 2,000 years and was interrupted for the following 10,000 years long hiatus as documented also by a 6 mm thick layer of silt. The upper part and the top of the flowstone layer is 99.85 (+3.30/-3.21) kyr old. The carbonate layer was formed at the beginning of the last glacial period, i.e., in isotope substages 5c-d. The same flowstone layer was used for carbon and oxygen isotope studies. A detailed profile (42 samples in total) from base to top of this 10 cm thick layer was sampled and analysed. Small variability in d 13C and d 18O values which don't correlate with each other and the location of the sampling point deep in a narrow cave with limited ventilation suggest that the calcite was deposited in isotopic equilibrium with the seepage water. The oxygen isotopic composition of cave carbonates is controlled by numerous factors. The most important factors are (i) temperature-dependent fractionation between calcite and water at the site of precipitation and (ii) oxygen isotopic composition of the meteoric water penetrating into the cave. The changes in oxygen isotopic composition of meteoric water of the past are influenced by numerous temperature and climate-related factorors, which are difficult to evaluate. Empirical evidence shows that in most caverns the cooler climate periods are accompanied by higher d 18O values of the cave carbonates and vice versa (Gascoyne, 1992). The studied flowstone layer from the Zazdina jeskyni Cave records climatic changes during a ca. 15 kyr long period at the beginning of the last Glacial. A comparison of the d 18O obtained record with the published oscillations of d 18O values of marine benthic Foraminifera from V19-30 core (East Pacific, Shackleton et al., 1983) and V27-60 core (Norwegian Sea, Duplessy and Labeyrie, 1992) shows good correlation (see Fig. 1). The laminated and thin-bedded silts deposited above the flowstone layer also provide some climatic information. These sediments were transported by meteoric water during a very rainy period. Poor vegetation cover on karst surface probably caused instantaneous penetration of meteoric waters to the limestones. The low amount of vegetation may have resulted from deterioration of climatic conditions. The laminated silts were deposited in the Zazdina jeskyni Cave probably during the Early Weichselian/Early Pleniglacial or Middle/Late Pleniglacial. Near the northern rim of the Moravian Karst a large profile in cave sediments is exposed in the Holstejnska jeskyni Cave. Three fluvial accumulations of different age are preserved in this cave. The oldest strongly weathered sandy gravels are covered by relics of a flowstone layer with inverse paleomagnetic orientation (Sroubek and Diehl, 1995). Consequently, the ages of the flowstone and the underlying fluvial sandy gravels are older than paleomagnetic boundary Brunhes/Matuyama (780,000 years). The middle fluvial accumulation is younger than 250,000 years (the age of the stalagmite below these sediments - Glazek et al., 1995). On the surface of the middle accumulation, relics of flowstone are preserved. The youngest sandy and silty fluvial accumulation was deposited probably during the last glacial period. On its surface relics of flowstone layer are also preserved. Approximately 60 analyses of d 13C and d 18O were made from flowstones exposed in the Holstejnska jeskyni Cave. The results show that cave carbonates precipitated under different climatic conditions. The flowstone layer covering the middle accumulation of fluvial sediments consists of two parts - the basal part with a stalagmite (isotopic samples 1-5 in Fig.2) and the upper part of the carbonate layer (samples 6-10). Both parts show different d 13C and d 18O values (see Fig.2) and were deposited under different climatic conditions. Because of the uncertainty in d 18O of meteoric waters at the time of deposition a more precise calculation of the deposition temperature is impossible. The determination of d D values of the water trapped in fluid inclusions within the Fig.2 Isotopic composition of the flowstone layer covering the middle fluvial accumulationin the Holstejnska Jaskyne cave Moravian Karst:a) sampling points in the profile through 25 cm thick layer, b) d 13C and d 18O values from the lower part of flowstone layer with stalagmite and the upper part of the carbonate layer. flowstone will allow the calculation of oxygen isotopic composition of meteoric waters and thus an estimation of mean annual surface temperature in the study area. References Bradley R.S. (1985): Quaternary paleoclimatology.- Allen Unwin Inc., 1-472.
Gascoyne M. (1992): Palaeoclimate determination from cave calcite deposits.- Quaternary Science Research 11, 609-632.
Labeyrie J. (1984): La cadre paleoclimatique depuis 140 000 ans.- L'Anthropologie, 88, 1,19-48. Shackleton N.J., Imbrie J. & Hall M.A. (1983): Oxygen and carbon isotope record of East Pacific core V19-30: implications for the formation of deep water in the late Pleistocene North Atlantic.- Earth Planet.Sci. Lett., 65, 233-244. Sroubek P. & Diehl J.F. (1995): Paleomagnetic/enviromnental magnetic study of cave sediments in Moravian Karst (in Czech).- in Cilek, V.ed.: The Underground World. Library of the Czech Speleological Society, Vol. 25, 29-30.
Acknowledgements This study is supported by Czech Grant Agency in Prague (No.205/93/0726) and by U.S.Czechoslovak Science and Technology Program (No.9505 1).
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