Sea levels, palaeoclimate and uranium-series disequilibria
(Paper presented at Bergen Meeting, Aug. 1996)
David A. Richards1,2) Charles J. Borton3)
Peter L. Smart2) R. Lawrence Edwards3)
1)Department of Earth
Sciences, University of Leeds, Leeds, LS2 9JT, U.K.
2)Department of Geography, University of Bristol, Bristol, BS8 ISS, U.K.
3)Minnesota Isotope Laboratory, Department of Geology and Geophysics,
Minneapolis,University of Minnesota, MN 55455, U.S.A.
Summary - The pattern of growth in a submerged flowstone and stalagmite sequence from Grand Bahama serves as an important source of sea-level and palaeoclimate information for the middle and late Pleistocene and is compared with other sea-level data; a flowstone record from the same island (Lundberg and Ford, 1994), 230Th ages of coral reef terraces and oxygen-isotope records from deepsea cores. Elevations and mass-spectrometriC 238U-234U-230Th ages of multiple phases of calcite growth in submerged speleothems provide maximum constraints on sea-level elevation because calcite deposition could only have occurred when the cave passages were air-filled. Elevation constraints are good because the Bahamas have remained tectonically stable for the period of growth. Non-depositional hiatuses can be attributed to submergence during high sea stands or ceasation of drip during periods of aridity, lack of soil cover or fissure blockage. Ages for initiation of growth after major high sea-stand events in the Sagittarius sequence are 384± 2017ka, 3 15± 13 ka, 190± 5 ka and 80± 2 ka, and constrain the timing of the oxygen-isotope stage boundaries 11/10, 9/8, 7/6 and 5/4, respectively. Numerous hiatuses occur in isotope stage 8, indicating that deposition was sensitive to climate change at this time. Few studies have investigated the U-Th systematics of dissolution and precipitation in carbonate platforms. The longest sequence of flowstone growth from the Bahamas (Sagittarius Cave, Grand Bahama) exhibits a first-order trend of decreasing initial 234U/238U along the axis of growth. This is ascribed to closed-system decay of the overlying carbonate unit since deposition of material with of modern sea water at ~300 and ~400 ka. Second-order variation is probably caused by subsequent periods of more limited deposition of marine carbonates with elevated 234U/238U during high seastand events and development of the network of flowpaths to the drip source.
Samples and methods - Six speleothem samples from -13 to -18 m in Sagittarius Cave, eastern Grand Bahama, show multiple phases of calcite growth separated by growth hiatuses. U and Th isotope measurements were made on 45 sub-millimetre wafers of calcite from the Sagittarius samples, typically 0.2-0.5 g (238U = 100-500 ng g-1), using mass-spectrometric techniques at the University of Minnesota (Edwards el al., 1987; Richards et al., 1994). Duplicate and replicate analyses agreed within analytical precision. Twenty-six analyses were made on sub-samples older than the last interglacial and only 2 of these, from the top of the same growth layer, showed age inversions when compared with stratigraphically related sub-samples.
Hiatuses and sea level (or palaeoclimate?) events - The composite flowstone sequence from Sagittarius Cave exhibits nine or more depositional hiatuses (Fig. 1). Some of these can be attributed, in part, to submergence during high sea-stands because at least three middle to late Pleistocene platform- flooding events are found in shallow marine carbonates from the Bahamas (Aure et al., 1995). The elevations of high sea-stands below present sea level are poorly known and the Sagittarius sequence (-13 to -18 m) can provide important sea level data, but definitive evidence for submergence during hiatuses is needed. Palaeoclimatic control inhibited speleothem growth in the Bahamas during the last de-glaciation (Pichards el al., 1994) and may be important during earlier growth phases, particularly during isotope stage 8, for which three distinctive hiatuses are evident, and late glacial stage 6, when growth ceased prior to the penultimate deglaciation. Discrepancies in timing, and elevation of sea-level events > 200 ka can be seen in two often cited oxygen-isotope records (Fig. 1). Our data provide useful estimates for the maximum or minimum ages of stage boundaries for the Pleistocene. Deposition of calcite in the Sagittarius sequence commences immediately after sea level regression to below - 1 3 to - 1 8 m based on ages of high sea levels from coral reef terraces (Fig. 1). The minimum age estimates for stage boundaries 11/ 10, 9/8, 8.5/8, 7/6 and 5/4 can, therefore, be considered as chronological control points. This is in contrast to cessation ages, where other controls on growth, such as aridity and drip cessation may occur.
Uranium isotopic variation - Speleothems preserve a record
of the 234U/238U of meteoric waters. Higher resolution sampling and
increased precisions of mass-spectrometric analyses in comparison with alpha-spectrometric
methods have revealed significant secular variation of initial 234U/238U
in the Sagittarius sequence. The 234U/238U ratio in the secondary
calcite is inherited from the marine carbonates overlying the cave which were deposited
with 234U/238U similar to present sea water (~1.144). The
first-order trend towards lower values along the axis of growth can be explained by
radioactive decay to secular equilibrium of the overlying unit. Superimposed on this is a
saw-tooth, or step-like, pattern. This sccond-order trend is likely to be caused by 1)
deposition of additional marine carbonates with elevated 234U/238U
during high sea-stands and, 2) increasingly lower and older material dissolved during
glacials because of fissure development and input of organic material.
Aurell, M., McNeill, D.F., Guyomard, T. and
Kindler, P. (I 995). J of Sed. Res., B65,
170-182.
Edwards, R.L., Chen, J.H. and Wasserburg, G.J. (I 986). Earth platiet. Sci Leit., 18, 175-
192
Lundberg, J. and Ford, D.C. (I 994). Qziat. Sci. Rev., 13, 1-14.
Richards, D.A., Smart, P.L. and Edwards, R.L. (I 994). Nature, 367, 357-360,
Shackleton, N.J. and Pisias, N.G. (1985). in The Carbon Cycle and
Atmospheric C02: Natural Varialiotis, Archean to Present (eds Sundquist,
E.T. and Broecker, W.S.), 303-317.
American Geophysical Union, Washin,,ton D.C.
Chemical Kinetics,
Speleothem Growth and Climate
(Paper presented at Bergen Meeting, Aug., 1996)
Wolfgang Dreybrodt
Institute of Experimental Physics, University of Bremen,
D-28334 Bremen, Germany
To interpret climatic records contained in speleothems, such as stalagmites or flowstone knowledge on the basic mechanisms of their growth is required. It is of utmost importance to understand which parameters determine the growth rates of speleothems and how these rates can be estimated. On the basis of a transport model by Buhmann and Dreybrodt (1985) (1) and our recent experimental work on calcite precipitation kinetics (2) it is shown that rates of calcite deposition to speleothem surfaces covered by thin sheets of water do not only depend on. the chemical composition of the supersaturated solution but also to a significant extent on the type of the flow of the water sheet. If the water sheet is stagnant or in laminar flow, the rates are much lower than for turbulent flow. Furthermore, owing to the slow reaction H+ + HCO-3+ CO2 + H2O by which CO2 is released during calcite precipitation, the thickness of the water sheet is also of major importance in determining growth rates. Finally the growth rate of stalagmites fed by dripping water depends critically on the time interval between two drops, whereas stalagmites fed from a continuous source with the same chemical composition will exhibit quite different behaviour not only in growth rate but also in morphology.
Fig. 1 (right panel) illustrates maximal growth rates in mm/year for deposition from water sheets stagnant or in laminar flow with various thickness d as a function of the Ca-concentration in the water. Temperature is 10 and the partial pressure of carbon dioxide in the cave air is 310-4 atm. These curves have been obtained from a modified transport model, taking into account that precipitation of calcite is inhibited for solutions with W = IAP / Kc < 2. Therefore precipitation stops at an apparent equilibrium with 0.72 mmole/l instead of the true chemical equilibrium at 0.63 mmole/l (2). The left panel depicts the rates for water sheets in turbulent flow. Note that the scale is changed by more than one order of magnitude, showing the dramatic increase which can occur at turbulent flow. Recent experiments from our laboratory which suppor these data are presented.
d.One further has to consider that the rates are also dependent on temperature, and within the range of cave temperatures from 5 to 2 increase by more than a factor of 2. Variations
of similar magnitude can be caused by changes of the partial pressure of carbon dioxide in the cave air. The rates decrease with increasing pressure, and deposition may even be inhibited completely if the supersaturated solutions have calcium concentrations below that of apparent equilibrium. As an example this apparent equilibrium is at 1.4 mmole/l for 10 and a carbon dioxide pressure of 10-3 atm. in the cave air. Taken this all together, we conclude that growth rates are determined by many parameters in a very conflicting way. Thus an increase of growth rates caused by a climatically induced increase in soil carbon dioxide might well be counter- balanced by decreasing temperatures which reduce the growth rates.
Not only the growth rates of speleothems but also their shapes depend on variables determined by climate. We present computer simulations for the shapes of stalagmites from the beginning of their growth until they have obtained their final stable equilibrium form (3). This shape follows from two simple principles: a) the growth is always perpendicular to the present surface, and b) the growth rates decrease with increasing distance x from the axis of the stalagmite. Fig. 2 shows the growth history of stalagmites which differ in their growth conditions only by differing initial surfaces upon which they started to grow. The lower curves represent the shapes of stalagmite for time intervals D T in arbitrary units. The upper curves show the stable equilibrium shape, which is independent of the initial surface. Note that the diameter scale is magnified by a factor of 5. The real shape is given by the dotted curves. In all these calculations it was assumed that the growth rate perpendicular to the present surface is given by f(x) = f(0) exp(-l(x)/l ), where l(x) is the length on the surface of the stalagmite from the drip point at x = 0 to x. It turns out that in this case the diameter of the stalagmite is D=2× l .
The diameter of a stalagmite growing under constant conditions is furthermore given by
D2=4V/× (d ×
p[1-exp(-a T/d )] ) (1)where V is the volume of the feeding drop, T the time interval (in year) between two drops, d is the depth (in cm) of the water sheet covering the speleothem, and a is the slope of the deposition rate curve, represented by Fig.1. Note that the diameter does not depend on the Ca-concentration of the supersaturated solution. On the basis of the arguments above we will present simulations, which show how climatic variations can lead to various shapes. Decreasing water supply leads to conical shaped stalagmites, which often are encountered in caves. Increasing water supply gives club shaped stalagmites, whereas periodic changes in feed rates lead to periodic changes in the diameter. Thus studies of the stratigraphy of stalagmites might give supplementary information.
The growth rate of a regular stalagmite is shown to be
W=1.17*106*(Ceq-c)(I-exp(-a T/d ))*d /T [cm/year] (2)
where c is the calcium concentration of the feeding water and Ceq the apparent equilibrium concentration in mole/l. This is valid for D>3.5 cm, the minimum diameter of a stalagmite. In combination with eqn. 1 we therefore can correlate growth rate and diameter, Which can be a useful information when selecting samples in a cave.
1.) D. Bulunann and W. Dreybrodt, The kinetics of calcite dissolution and precipitation in geologically relevant situations of karst areas: 1. Open system. Chem. Geol. V. 48, p. 189-21 1, 1995
2.) W. Dreybrodt, L. Eisenlohr, B. Madry, and S. Rnger, Precipitation kinetics of calcite in the system CaCO3 - H2O - CO2: The reaction H+ +HCO-3 ® CO2 +H2O as a rate limiting step and the reduction of surface reaction rates, Submitted to Geochim. Cosmochim. Acta, March 1996
3.) W. Dreybrodt, Processes in Karst Systems - Physics, Chemistry and Geology. Springer Series in Physical Envirorments 4, Springer
Berlin, New York, 1988
COMPLEX STRATIGRAPHIC SERIES
IN BELGIAN CAVES: CORRELATION WITH THE PALEOCLIMATIC VARIATIONS DURING MIDDLE AND UPPER
PLEISTOCENE
( Paper presented at Bergen Meeting, Aug., 1996)
Yves Quinif
C
entre d’Etudes et de Recherches Appliquees au Karst (CERAK), Faculte Polytechnique de Mons,Rue de Houdain, 9; B-7000 Mons-Belgique
The message of endokarstic sediments.
Deposits in karstic cavities are very good indicators of palaeoenvironmental conditions. But the message is complex. We can recognize four influences. Geological environment determines mineralogical and sedimentological nature of the detritic sediments. Tectonic evolution modifies the energetic inputs into the karstic network and determines the nature of the sedimentation (high or low energetic levels). Climatic factors, of course, have a determining role through the biorhexistasy. Finally, human actions modify very much the "natural" mechanisms. Thus, the sedimentary recordings in caves reflect, because of their stratigraphic complexity, many parameters in which climate is only one.
The deposits in Belgian cavities.
Belgian karstic cavities are very good sediments traps. They belong to a low plateau
karst, with long horizontal galleries which contain complex sedimentary series. Detritic
units (roundstones, sands, clays) alternate with stalagmitic complex (flowstones,
stalagmites). The supply of matter and energy is determined by morpho-structural context.
Allochtonous rivers go deep into the limestones from sandstones and shales, because of the
peneplanation of the hercynian fold structures in the paleozoic rocks. Accordingly, there
are three main types of deposits. [i] : The river deposits (roundstones, sands, clays) are
coming principally from allochtonous terranes.
[ii] : Loams are removed from the cover above the karst and go down by fissures into the
galleries. [iii] : Speleothems covers detritic deposits and can be also covered by other
detritic sediments. All these sedimentary units are integrated in complex stratigraphic
series, with erosion forms into sediments.
The climatic message.
In the regional context, we have demonstrated that those litho-stratigraphic characteristics are a major consequence of climatic variations during the Quaternary and, thus, constitute a very good tool to study those palaeoenvironmental variations. Indeed, the Pleistocene modifications of the relief because of the tectonic factors are low and, thus, sediments reflect particulary climate modifications. The detritic sediments settle in cold conditions (rhexitasy period = glacial period), rivers deposits during wet cold periods and loams during dry cold periods. Speleothems are grown in temperated and wet conditions (biostasy periods = interglacial or interstadial periods). But, More, we can have mechanical erosion of detritic sediments with formation of hollowings, and chemical corrosion of speleothems. This combinaison of sedimentation and erosion process are the consequence of the base level fluctuations, like a consequence of detritic sedimentation in the bottom of the valley during glacial periods.
Because of those phenomena, the sedimentary series in belgian caves are complex, with an Stratification of the different types of deposits. The reconstitution of palaeoenvironments must go through the utilisation of different parameters : litho-stratigraphy, mineralogy, palynology, paleontology. The spelcothems only cannot be used for complete paleoclimatic reconstitution.
The Pleistocene-Holocene rupture. The transition takes place around 12 ky with a speleothems generation (Allerod) after an important detritic sedimentation during isotopic stage 2 (Rochefort cave). Younger Dryas is a period of hollowing into the sediments, following by the speleothems growing during Holocene, which seal the erosion forms. This period is very well known by palynology which shows the installation of forest (Louve, Han, Remouchamps caves).
Fig.2: Evolution of a karstic network during a climatic cycle. Left : allochtonous terranes (shales, sandstones) of the alimentation basin ; right : section through a limestone massif with active and inactive levels.A. First interglacial period. The river does not carry sediments because of the pedogenesis. There is much biological CO2 and the speleothems grow. There is no detritic sedimentation in the cave.
B. Beginning of the glacial period. The disparition of the forest permit the erosion of the soils.The running is loaded ; there is detritic sedimentation in the karst. The development of the speleothems slows down.
C. Wet glacial period. A intense freezing erosion provokes a big sedimentary load in the river. The caves are completely filled in. The speleothems development stops.
D. Dry glacial period. Because of the dryness, we have loess sedimentation. These eolian loams can go down in the karst by the fissures.
E. Second interglacial period. Clear river, without sedimentary load, cut the detritic sediments in the caves. A new speleothems generation develops.
The glacial period sedimentation. Several series prove the input of rivers deposits during the stage 4 (cold and wet) and removed loams during stage 2 (cold and dry). Pollinic analysis has demonstrated this fact, for example finding Selaginella Selaginoides (Vilaine Source, Bohon caves). Those cold periods correspond to an complete filling of the active caves. Different sedimentary sequences can be recognized, like drying sequences.
The isotopic stage 3. This interstade (equivalent to "Interstade des Cottes") is characterized either by a hollowing of stage 4 river deposits (Bohon, Vilaine Source caves) or speleothem generation. The double sequence of the Pere Noel cave can date the isotopic stage 4 (56,5 - 72,3 ky). In other cases, we see an incision through the detritic deposits sedimented during the stage 4.
Interstadial periods. Some speleothems are situated into glacial stages (Han, Neptune caves). They indicate that, during short times, a climate improvement has permitted the return of better conditions (forest of Pinus and betula). For example, we find two periods around 17 and 20-21 ky.
The transition between stages 4 - 5. This transition consists in the passage between stalagmitic sedimentation to detritic sedimentation. U/Th datings situate this transition around 70 ky. Pollinic analysis shows the disparition of forest to steppe environment (Sclayn, Pere Noel, Feluy caves).
The stage 5. We find essentially big speleothems in which we have detected the different substages by U/Th datings and palynology (Han cave). Sometimes, when there was no river flow, one finds colluvial loams (Sclayn cave). The sedimentation is not continue, but is cut by river incisions and corrosion of speleothems (Vilaine Source cave). Cold substages 5.2 and 5.4 are recorded in speleothems (Han cave) or in loams (Bohon cave). Another perturbation is the consequence of earthquakes (stop of the growth, cementery of stalactites and stalagmites). All those speleothems permit by the pollen analysis to reconstruct the aspect of the forest.
Middle Pleistocene. The isotopic stage 6 is characterized by river deposits, like sands and clays in the upper passage of the Han cave and in the Bohon cave. The isotopic stage 7 is not well represented. Lower part of the big flowstone in the Han cave is dated around 200 ky. Many speleothems can not be used, because of their age older than 350 ky.
Bibliographie.
BAST1N B., QUINIF Y., DUPUIS C., GASCOYNE M., 1988 - La Squence sedimentaire de la Grotte de Bohon (Belgique). Ann. Soc. geol. Beig., 111, 1 : 51-60.
BASTIN B., DUPUIS C., QUINIF Y., 1982 - Etude microstratigraphique et palynologique d'une croute stalagmitique de la grotte de la Vilaine Source (Arbre, Belgique). Methodes et resultats. Rev. Belg. Geogr., 106, 1 : 109-120.
QUINIF Y., DUPUIS C., BASTIN B., JUVIGNE E., 1979 - Etude d'une coupe dans les sediments quaternaires de la grotte de la Vilaine Source (Arbre, Belgique). Ann. Soc. Giol. Beig., 102 : 229-241.
QUINIF Y., 1990 - La datation des speleothemes (U/Th) appliquee sequences sedimentaires souterraines pour une mise en evidencc des ruptures paleoclimatiques. Actes du colloque "Remplissages karstiques et paleoclimats". Karstologia Memoires, 2 : 23-32.
QUINIF Y., GENTY D., BASTIN B., 1992 - Une serie sdimentaire endokarstique tardi-glaciaire et holocene : le remplissage de la nouvelle galeric de la Grotte de Rochefort. Speleochronos, 4 : 31-40.
QUINIF Y., BASTIN B., 1993 - Une fin d'interglaciaire : le plancher stalagmitique de Feluy - La transition entre les stadcs isotopiques 5 et 4. Speleochronos, 5 : 19-24.
QUINIF Y., BASTIN B., 1994 - Datation uranium/thorium et analyse pollinique d'une sequcnce stalagmitiquc du stade isotopique 5 (Galeric des Vervietois, Grotte de Han-sur-Lesse, Belgique). C.R.A.S.Paris, 318, serie II : 211-217.
QUINIF Y., GENTY D., MAIRE R., 1994 - Les speleothemes : un outil performant pour les etudes paleoclimatiques. Bull.Soc.Geol.Fr., 165, 6 : 603-612.
Calibration of speleothem stable isotopes
against
historical records:
a Holocene temperature curve for north Norway
(Paper Presented at Bergen Meeting, Aug., 1996)
Stein-Erik Lauritzen
Allegaten 41, N-5007 Bergen, Norway
Introduction
The stable isotope signal in speleothems have been considered as promisingfor paleotemperature estimates for more than two decades. However, the remaining problem has been to find a unique functionality between temperature and the d 18O signal. Provided that the calcite is precipitated in isotopic equilibrium, according to the Hendy (1971) criteria, the T (temperature)-dependence of the oxygen-isotope composition of calcite (d 18Oc) is dictated by the thermodynamic constants in the O'Neill equation (O'Neil et al. 1969) and by the isotopic composition of the dripwater [d 18O w= F(T, g, t) ], which in combination (Dorale et al. 1992) yields:
where a and b are constants of the O'Neill equation, and F(T,g,t) is a function of temperature (T), geographic position (g) and time (t). The t- and g- dependence is intended to describe time- and site- dependent changes in stormtrack patterns (rainout) and changes in the source (seawater, D d 18Osw) due to the ice-volume effect (Lauritzen 1995). F(T,g,t) also include any averaging or biasing taking place when rainwater (or snow meltwater) passes down through the vadose zone before it enters the cave. Therefore, the problem of paleotemperature deduction is to find an approximation to F(T,g,t) that is valid for past climates and longer timespans. In its simplest form, the Dansgaard (1964) relationship may serve as an approximation of F(T,g,t):
F(T,g,t) » c(T-273.15)+d+D d 18OSW(t) (2)
c and d are site-dependent constants. The term D d 18OSW (t) is added to model the time-dependent ice-volume effect. Provided that the constants c and d can be calibrated, equations (1) and (2) can be solved uniquely for T. This was attempted on a Holocene stalagmite from Mo i Rana, north Norway(66° 20' N).
Sample analysis and data processing
The 32 cm tall stalagmite (a very large specimen at this latitude) grew directly on stream gravel and displayed some growth disturbance at the tip. TIMS dating revealed a basal of 8 500± 200yr ,and a top age of 180± 15 yr. It was therefore inferred that the stalagmite ceased to grow during the Little Ice Age (LIA), around 1750AC. The whole sequence was first sampled at every 5 mm for stable isotopes, later for every 1 mm, corresponding to 25-30 years’ resolution. Present-day stalactite tips (immediately above the sample) and dripwater matched the present-day cave temperature (+3.5° C,Einevoll and Lauritzen, 1994). Based on historical and botanical records, it is generally accepted that the LIA climatic deterioration in this region was accompanied by a drop of 1.5° C in mean annual temperature. Using T and d 18Oc for the present-day conditions and for LIA, respectively, is suffcient to calibrate the constants c and d in equation (2), [D d 18Osw (t) = 0 ]. This yielded c = -0.096 and d = - 15.09, which contrasts the positive coefficient (c) of the Dansgaard relationship on modern precipitation.The time-dependent variation of «SMOW»(D d 18Osw (t)) was modeled from a standard, global stable isotope curve (i.e. Imbrie et al. 1984). The equation set (1 & 2) was then solved numerically for each stable isotope measurement. This resulted in the temperature curve in Figure 1.
Discussion.
The resulting temperature curve range over 5° C, with a maximum some 3° C higher than present, and with a cold spike at about 1° C. The spiky nature of the curve is enigmatic, as it may suggest that the mean annual temperature varied rapidly by up to 3 or 4° C over periods of 25-50 years, which is well comparable with the recorded anthropogenictemperature rise since the industrial revolution. This variation is induced by variations in the stable isotope measurements, and cannot easily be explained by post-depositional effects. For instance, the sample spacing is at least an order of magnitude smaller than the size of individual crystallites in the specimen, and structures like visible growth bands and luminescent bands are much finer.
A 5 point running mean of the data set compares favorably with totally independent temperature data from south Norway, based on botanical and glaciological evidence (Nesje and Kvamme 1991, Moe 1995). This suggests that the average temperatures derived from the stalagmite may be an adequate estimate of Holocene temperatures, but closer comparison with historical measurements during the last 200 years (which is lacking in the present record) is needed before confidence can be put on the noisynature of the data.
It is also interesting to note that the climatic optimumin the speleothem temperature series is basically introduced through the D d 18Osw (t)-term.
References.
Dansgaard, W. (1964): Stable isotopes in precipitation.Tellus 16, 436-468.
Dorale, J. A.; Gonzaics, L.A.; Reagan, M. K.: Pickett, D. A., Murrell, M. T. & Baker, R.G. (1992): A High-Resolution Record of Holocene Climate Change in Speleothem Calcite from Cold water cave, Norteast Iowa. Science 258, 1626-1630.
Einevoll, S.L. & Lauritzen, S.E. (1994): Calibration of stable isotope and temperature signal in the percolation zone of a sub-arctic cave, northern Norway. Cave and karst Science 21, 9.
Hendy, C. H. (1971): Tle isotopic geochemistry of speleothems. 1. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as paleoclimatic indicators. Geochimica Cosmocimica Acta 35, 801- 824.
Imbrie,J.; Hays, 1. D.; Martinson, D.G.; McIntyre, A.; Mix, A. C.; Morley, J. J.; Pisias, N. G.; Prell, W.L.&Shackleton, N. J. (1984): The orbital theory of Pleistocene climate: Support from a revised chronology of the marine d
18O record. In: Berger, A.;Imbrie, J.; hays, J.D.; Kukla, G. & Saltzmann, B. (Eds.) Milancovich and climate, Partl, Vol 126. 269- 305.Lauritzen, S.E. 1995: High-resolution paleotemperature proxy record for the Last Interglaciation based on Norwegian speleothems Quaternary Research 43, 133-146
Moe, D (1995): Climatic variations in western Norway during the last 13,000 years. A review. Geologija (in Press)..
Nesje, A; Kvamme, M (1991): Holocene glacier and climate variations in western Norway: Evidence for early Holoccne glacier demise and multiple Neoglacial events. Geology 19, 610-612.
O'Neil. J. R.; Clayton, R.N. and Mayeda, T (1969): Oxygen isotope fractionation in divalent menal carbonates. Jozirnal of Cliemical Physics 51, 5547-5558.
(Paper Presented at Bergen Meeting, Aug.,1996)
Dominique Gentya,
Andy Bakerb, William Barnesc and Marc Massaulta
allniversite de Paris-Sud, Laboratoire
d'hydrologie et de geochimie isotopique,
CNRS URA 723, Bat. 504, F-91405 Orsay Cedex, France.
bDepartment of Geography, University of Exeter, Devon, EX4 4RJ, England.
cDepartment of Physics, University of Exeter, Devon, EX4 4QL, England.
Description and Terminology
Two main kinds of calcite growth laminae fabrics have been defined (Genty, 1992; Genty et al., in press): 1) white porous laminae (WPL) which are white and porous when observed under reflective light, and sometimes show a brown layer of organic matter under transmissive light; 2) dark compact laminae(DCL),which are much more dense and absorb incident light which makes them darker. WP and DC calcite fabrics are different from the speleothem color (which is, generally, more or less brown); WPL and DCL can be found in both dark brownish or colourless stalagmites.
Backgound
Many speleothems are composed of millimetre scale growth laminae; these are particularly visible on polished vertical sections of stalagmites. Recently it has been demonstrated that, in several cases, luminescent and visible laminae are annually deposited: an alternation of WPL and DCL is deposited in one year (Shopov and Dermnendjiev, 1990; Genty, 1992, 1993; Baker et al, 1993; ). The physical and chemical study of the growth laminae is just at its beginning and is of great interest for paleoclimatic reconstruction , principally because of its high resolution chronology. A correlation has been observed between growth laminae thickness of a tunnel stalagmite and water excess, which demonstrates that, besides the calcium concentration (see Baker et al., this volume), the drip rate and/or duration of dripping is an important factor in growth rate (Genty and Quinif, 1996). Under UV light some laminae luminesce, due to organic matter trapped in the calcite lattice; this organic matter comes from microbial breakdown of soil organic matter. Comparison of the luminescent and visible laminae in one stalagmite from the Godarville tunnel (Belgium) has shown that the white porous laminae (WPL) are the most luminescent (Genty et al., in press). Since high organic matter concentrations have has been observed in winter drip waters in a British cave (Baker et al, unpub. data) and because a green chalk mark put on a stalagmite of the Godarville tunnel in Autumn 1992 was found two years later under a WPL, it is highly probable that at least in this tunnel WPL develop in Winter. Other comparisons on Holocene and historical stalagmites of the Villars cave (Dordogne, France) have confirmed the link between luminescence and WPL. Thus, it seems that organic matter is the primary cause of both white porous calcite and luminescence. We show here that grey level and luminescence are strongly linked to the vertical growth rates of stalagmites, which provides further insight into the paleoclimatic significance of calcite fabrics and/or growth rate variations.
Methods
The intensity of the visible light reflected on WPL and DCL (the grey level) is measured by digital image, processing. A video camera, linked to a video computer board, digitalizes an image of a polished section (512“ 5l2 pixels); then, several numerical filters enhance the image and, finally, laminae thickness and grey level are measured by a specific software. Luminescence intensity is obtained by a laser UV excitation at 325nm; a photomultiplier selects a 480nm wavelength. Growth rates have been calculated with annual laminae thickness, which gives a high precision value, and by AMS radiocarbon ages. The latter have been made by microdrilling 10 and 30 mg of carbonate across very few growth laminae (between 1 and 7 annual laminae). When possible, comparison between laminae counting and 14C duration have been made and have confirmed the annuality of the growth laminae studied. Grey level and luminescent profiles have been made on vertical polished sections of stalagmites. Areas where visible changes in the calcite laminae fabrics are observed were preferentially analysed; i.e. discontinuities or hiatuses composed of a few millimeters thick band of dark compact calcite (which is in fact composed of several very thin DCL).
Table 1: Growth rates from laminae measurements and 14C
dating and grey level calculated
by digital image processing.
| Stalagmite name and cm/base | Average growth rate mm/yr | Average grey level | Stalagmite name and cm/base | Average growth rate mm/yr | Average grey level |
| PNst4 above D19 | 1.13 | 148 | PNst4 above D23 | 0.77 | 191 |
| PNst4 D19 | 0.58 | 63 | PNst4 D23 | 0.44 | 42 |
| PNst4 below D19 | 1.08 | 187 | PNst4 below D23 | 0.73 | 101 |
| PNst4 above D58 | 0.50 | 227 | PNst4 above D75 | 0.83 | 107 |
| PNst4 D58 | 0.24 | 70 | PNst4 D75 | 0.35 | 43 |
| PNst4 below D58 | 0.77 | 203 | PNst4 below D75 | 0.89 | 187 |
| PN-stm1 above D42.5 | 0.65 | 204 | PN-stm1 above D47 | 0.56 | 146 |
| PN-stm1 D42.5 | 0.45 | 59 | PN-stm1 D47 | 0.31 | 38 |
| PN-stm1 below D42.5 | 0.54 | 136 | PN-stm1 under D47 | 0.53 | 208 |
| Vil-stm1 1230AD/1600AD | 0.76 | 153 | Vil-stm1 30AD/740AD | 0.36 | 124 |
| Vil-stm1 740AD/1230AD | 0.26 | 113 | Vil-stm1 150BC/30AD | 0.75 | 166 |
Results
Seven discontinuities from two tall columnar stalagmites (Pere Noel cave, Belgium) have been studied. At each discontinuity, grey level and luminescence decrease as the WPL disappear progressively. When grey level and luminescence decrease towards the discontinuity, it appears that the vertical growth rate, measured by laminae counting, decreases by a factor of two or three: i.e. below the D19 discontinuity of PNst4, the vertical growth rate is 1.08mm/yr, at the discontinuity, it is 0.58mm/yr and just above, growth rate increases again to 1.13 mm/yr (Fig 1). For all the discontinuities studied, the average
vertical growth rate variation is linearly correlated to the grey level change; this is true for average measured values but also for each growth laminae measurements (r = 0.73, n = 33, for the D75 discontinuity of PNst4) (Fig.2). A similar situation occurs between the grey level and the 14C growth rate of a stalagmite from a SW France cave (Vil-stml, Grotte de Villars). On this sample, no hiatus are observable, which suggests that average vertical growth rates calculated with 14C ages are true and similar to growth rates calculated with laminae (comparisons of corrected - dead carbon - callibrated 14C ages and laminae counting agree well). The base of this stalagmite, between 150BC and 30AD, is composed of white porous calcite (thick WPL) and the growth rate is very high: 0.75mm/yr; then, abruptly, in a few years around 30AD, calcite fabrics becomes dark and compact, with very few visible laminae, and the growth rate decreases : 0.36mm/yr btween 30AD and 740AD, 0.26mm/yr between 740AD and 1230AD. Then, WPL appear progressively, the grey level increases and the growth rate reaches 0.76mm/yr between 1230AD and 1600AD.
Interpretation
The two main calcite fabrics found in stalagmites are linked to vertical growth rate: white and porous calcite, composed of thick more or less clear WPL, is linked to high growth rate; while dark compact calcite, mainly composed of DCL, is linked to low growth rate. Because high grey level corresponds to a high luminescence, and because luminescence is principally due to organic matter, it appears that high growth rates occurs when calcite organic matter content is higher. Several hypotheses can be suggested to explain this relationship: 1) high growth rate is due to a higher calcium concentration because of a more intense plant activity in the soil; the latter produces more organic matter, which explains why high growth rate sections, have a higher luminescence and grey level intensity; 2) high growth rate is due to a higher flow rate, which brings into the cave all the organic matter content of the soil; however, this could explain short variations of calcite fabrics, but unlikely in those whose duration is several centuries, as observed in Vil-stm I stalagmite; 3) high growth rate is due to the organic matter interaction with calcite during precipitation: it may disrupt crystals and lead to higher growth rate(?).
References
Baker, A., 1993. Ph.D. Thesis, University of Bristol, Bristol (unpublished).
Baker, A., Smart, P.L., Edwards, R.L. and Richards, A., 1993. Nature, 364: 518-520.
Genty, D., 1992. Speleochronos, Faculte Polytechnique de Mons, Belgium, 4: 3-29.
GentY, D., 1993. Academie des Sciences (Paris), Comptes Rendus, Serie II 317: 1229- 1236.
Genty, D. and Quinif, Y., 1996. Journal of Sedimentary Research, 66: 275-288.
Genty, D., Baker, A. and Bames, W. L., 1996. Earth Surface and Pocesses Landform, in
press.
Shopov, Y.Y. and Dermendjiev, V., 1990. Academie Bulgare des Sciences,
Comptes Rendus, 43: 9-12.
This research is part of the IGCP 379 Karst processes and carbon cycle and of the PAGES PEP III programs