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Luminescent Inicrobanding in speleothems:High-resolution chronology and paleoclimate

Y. Y. ShopovD. C. Ford

Department of Geography, McMaster University, Hamilton, Ontario L8S 4K1 , CanadaH. P. Schwarcz

Department of Geology, McMaster University, Hamilton, Ontario L8S 4M1, Canada

ABSTRACTWhen illuminated by ultraviolet light, many calcite speleothems (stalagmites, stalactites,

flowstones) display luminescence caused by the presence of organic (humic) substances oc-cluded in the calcite. The amplitude of luminescence varies in a banded pattern parallel togrowth layering. Through laC and thermal ionization mass spectrometry uranium-series dat-ing, we show that cyclical oscillations in the luminescence have periodicities ranging from a fewdays to t10t yr. A well-defined annual cycle is present in many vadose-zone speleothems andcan be used to define the chronolory of short-term events. This cycle is pmbably a response tohydrological events in the recharge to the cave. Longer term oscillations are inferred to becontrolled by clirnaten through its effect on organic activity in the overlying soil.

INTRODUCTIONSpeleothems such as stalagmites, stalactites, and flowstones

are chemical precipitates of calcite or other minerals deposited incaves. Calcite stalagmites and flowstones contain isotopic records ofpaleoclimate (Schwarcz, 1986; Gascol'ne,1992; Dorale et al.,1992)that can be dated by U-series analysis with a precision of -+l7o oveta 105 yr time scale (Li et al., 1989). Even higher precision chronol-ogies would be useful to permit the calibration of high-resolution Cand O isotope records for such deposits and to detect short-termfluctuations in growth rate, which are also climatically determined(Gascolme et al., 1983; Gent5r, 1992;Baker et al., 1993b). The pur-pose of this paper is to demonstrate that very high temporal reso-

lution may be obtainedquickly

andcheaply

byanalysis of the lu-

minescence of speleothems. This is illustrated with North Americanexamples investigated over a wide range of time scales.

Many speleothems exhibit varvelike submillimetre-scale colorbands. Broecker et al. (1960), using toc dates, showed that thesebands were annual in some rapidly growing stalagmites in temperateclimate regions. The color of such speleothems is chiefly due to thepresence of variable amounts of clay or humic substances (Gas-coyne, 1977; White, 1981), which coprecipitated or adsorbed ontocalcite surfaces from drip waters that passed through the soil on theirpath to the cave.

LUMINESCENT BANDING IN SPELEOTHEMS OF THEVADOSE ZONE

When stimulated by ultraviolet (U\) light, many speleothemsare strongly luminescent. Gilson and Macarthney (1954) were theflrst to suggest that, in vadose-zone deposits, this luminescencemight also be activated by organic substances. Shopov (1987) andWhite and Brennan (1989) showed this to be true; the activators arecalcium salts of humic and fuMc acids, plus some low-molecular-weight organic esters. These excite a broad luminescence spectrum.Luminescence spectroscopy must be applied to distinguish this fromthe luminescence spectra of trace elements such as Mn, P, Pb, U, orrare earth elements that are the predominant activators of lumines-cence in hydrothermal calcites and that also sometimes appear invadose speleothems, rendering them unsuitable for the applicationsdescribed below.

GEOLOGY, v. 22, p. 407-410, May 1994

Luminescence in some speleothems displays banded patternsthat are parallel to the visible color banding but can be resolved toa much finer degree, revealing periodic banding on a micrometrescale. By assuming that the flnest scale, regularly periodic bandswere annual, Shopov (1987) showed that luminescence banding in-tensity displayed ll to 22 yr cycles in parts of some samples. Herewe demonstrate that absolute dating by U series and by 1aC confirmsthat the highest or near highest frequenry cycles are often annualand that other cyclic behavior exists.

Luminescence banding provides us with a record of periodicfluctuations in the abundance of fuMc and humic acids occluded inthe growing speleothem. FuMc acids are produced by photoqm-thesis and are released through plant roots. They are readily soluble,and thus may be expected to enter speleothem feed waters prefer-entially during growing seasons. Humic acids are products of or-ganic decomposition in the soil and any epikarst beneath it, anddissolve more slowly. We suspect that theyprovide more moleculesto the longer ryclic components in the luminescence records dis-cuSsed below. Together, the two groups may be taken as indices ofproductMty in the overlying soil and plant cover and, therefore, asa proxy measure of paleoclimate as it has affected these variables(Lauritzen et al., 1986). In addition, Shopov et al. (1991) proposedthat luminescence banding can provide its own internal high-reso-lution chronology for determining such features as variation in spe-leothem growth rates, i.e., autocalibration dating.

ANALYTICAL PROCEDURESThe luminescence is excited experimentally by UV (200-340

nm) backlighting of a l-5-mm-thick section of speleothem cut nor-mal to its growth layers. The luminescent images of the sections arerecorded on photographic film at magnifications up to 300X at theplane of the film (Fig. 1). The optical density of the film negatives isthen scanned with a densitometer. Density is measured only on arelative scale (log I where 1 = intensity) because the absolute den-sity depends on many variables such as the general transparency ofthe speleothem and the optical characteristics ofthe excitation sys-tem. Scanning is performed at steps (pixels, px) of 10 pm, with anaperture of 25 x 500 pm, on traces normal to the banding. Paralleltraverses a few millimetres apart on a banded specimen give close

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Flgure 1. Luminescencebanding (horizontalstripes) in stalagmltelrom McFail's Cave, NewYork, lrregular stripesnormal to luminescencebands mark boundariesbetween adloining calcitecrystals. True scalelength of lrame (top tobottom) is 1 mm.

agreement, as do reverse scans (when the section is turned over).Luminescence bands are oriented normal to the direction of growth.Their variation in intensify is assumed to represent only variation inthe concentration of exciting substances (humic) in the speleothems,because the speleothems exhibit negligible absorption in the visiblewavelength region.

1aC dates of Holocene stalagmites were determined by accel-erator mass spectrometry (AMS) at the Isotrace Laboratory, Uni-versity of Toronto. U-series dates were obtained by thermal iontza'tion mass spectrometry (TIMS-US; Edwards et al., t987;' Li et al.,1989) on 1-2 g samples chipped from the stalagmites. Oxygen andcarbon isotopic analyses were made of samples drilled from polishedsurfaces of some stalagmites; b18O and D13C are reported in permil

relative to the Peedee belemnite (PDB) standard.

ANNUAL BANDING AND OTHER SHORT-PERIODFEATURES

Our speleothem samples are typically a few centimetres to afew tens of centimetres in height (stalagmites) or thickness (flow-stones). Distinct luminescent banding may not be detectablethroughout a given sample. Where it is detectable, its continuityalong a densitometer trace is often intemrpted by the presence offluid inclusions or other small breaks or distortions in the calcitedeposit. As a consequence, measured unbroken sequences ofbandsare often limited to a few tens or hundreds of individuals, althoughsome samples appearto extend for thousands ofbands. The

laC and

U-series dating show that banding of samples from temperate cli-mate areas occurs on two time scales: (1) 1-100 pm annual bandingand (2) quasiperiodic banding on a spatial scale of tens of microme-tres to millimetres. Absolute dating confirms that the finest scalebanding seen in many specimens has a period of - 1 yr. For example,autocalibration dating of a small section from the middle of sampleMF 1, a Holocene stalagmite from McFail's Cave, New York, sug-gested a growth period (bottom to top) of 178 +30 yli thermal AMS1aC dating of the section gave200 t90 yr (2o; Shopov et al., t992).Baker et al. (1993a) reported a similar result applying TIMS-USdating to an -500 yr section in a Holocene stalagmite from Scotland.

The two styles of banding both appear in sample CW1, a ry-lindrical stalagmite t6.4 cm in height composed of clear, inclusion-free calcite from Coldwater Cave, Iowa. Underlying prairie grass-

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YEARSFigure 2. A: Optical density ol luminescence band pattern (ODL) for com'plete speleolhem, Cwl, trom Coldwater Cave, lowa, with resolution ol1.78 yr per pixel (yrlpx), B: 23-yr-long section at resolution of 2.5 d/px. C:Annual growth; distances (in pm) between successive annual maxima onfi\re-point smoothed spline average of B'

land, this cave has very stable temperature and humidity conditions,making it especially suitable for paleoclimate studies (Harmon et al.,1979). The base of CW1 is dated by the TIMS-US method ro 7075+295 yr. toc dates along its length show that there was fairly regular

growth to the present, at an average rate of 23 p'mly. Figure 2Ashows the complete luminescence banding pattem for CWl, with aresolution of 1.78 yr per pixel (yrlpx). Figure 28 shows a samplesection of 23 yr duration at a resolution of 2.5 dlpx; the annual band-ing is clearly seen in this profile. Figure 2C displays the distancesbetween the luminescence maxima of Figure 28 converted into mi-crometres of calcite deposition per year; there is a 4X variationbetween the greatest and the least of the annual accumulations. Afaster growing Holocene stalagmite from McFail's Cave, New York,showed a similar range of interyear variation during a given decadeand at least a 14X variation in the extremes (fastest to slowest) dur-ing its 10000 yr history.

Similar annual bands are seen in many other speleothems fromtemperate caves that have grown fast enough to allow the bands to

be optically resolved. It is not yet established whether the moreluminescent parts of such bands were deposited in winter orsummer.

Studies at higher resolution are also possible. Figure 3,A. showsa section of the record of CW1 that is 2.4 yr in duration (6 h/px), witha 300 px spline curve fitted to it. The difference between the originaland smoothed curves (Fig. 38) provides an example of the veryshort term variations that can be resolved in luminescence bandingpatterns. A power spectrum of the unsmoothed data set is shown inthe inset: in addition to the strong annual peak and seasonal peaks,there were weaker but well-resolved peaks with periods of 27, 23,and 13.5 d in this particular 2.4yr peiod that may be related to thecharacteristic lifetimes of meteorological sequences (recharge

CpticalDensity

ofLuminescence(relative scale)

ODL(relative scale)

Annual Growthof Calcite

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20

15

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-\:v T*

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0 50 100 t50 200 250Distance (pixels)

0

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ResidualODL

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Figure 3. A: 2.4-yr-long section of annual record of CWl, resolved to 6h/px, with 300 px spllne curve fitted to record, B: Residual after subtract-ing spline curve from original. C: Power spectrum of original data. ODL-optical denslty ol luminescence,

events of frontal origin?) or possibly to tidal pumping of groundwater.

Given that annual calcite accumulation rates are not constant inthese deposits, the variations of optical density of luminescence inthem can be attributed to three different control mechanisms, op-erating singly or in combination: (1) calcite growth rate (i.e., lumi-nescence is weakest where growth was fastest), controlled by thesupersaturation of the feed-water solution with respect to calcite (asin Dreybrodt, 1981); (2) calcite growth rate controlled by feed-waterflow rate (as in Gonzalez et al., 1992); and (3) temporal variations inconcentrations of dissolved humic and fulvic substances in the feedwater overwhelming calcite growth-rate effects. The data of Figure2C would appear to underscore the significance of growth rate

(mechanisms 1 andlor 2); the variationis probably

comparableto the

interannual variation in the amounts of drip-water recharge to thesetypes of speleothems. The strong seasonal ryclicity in Figure 38could represent different combinations of all three mechanisms. It isevident that luminescence banding provides a new method for es-timating ancient rates of deposition on ayearly time scale; however,careful field studies are now needed to evaluate its further potentialfor very high resolution paleoenvironmental reconstructions.

DECADAL AND LONGER TERM BANDING:PALEOCLIMATE RECORDS

Figure 2A also demonstrates that there can be longer time-scalefluctuations in luminescence banding intensity. These are typical ofthe banding records from temperate-zone speleothems in NorthAmerica that we are studying and were emphasized in the earlierBulgarian studies that lacked independent dating control (e.g., Der-mendjiev et al., 1990). Power spectra show peaks in decadal andmultidecadal ranges. We suppose that the long-term rycles repre-sent changes in concentration of humic substances in drip watersdue to climate-driven changes in actMty in the overlying soil. Der-mendjiev et al. (1990) correlated their speleothem findings with sun-spot numbers and longer period solar maxima and minima, thusproposing that these solar processes regulate production of humicsubstances in the soil to a signfficant extent.

Figure 4 compares the part of the CW1 record from -1000 A.D.to the present with a record of residual differences from a spline-function smoothing of the atmospheric A laC production record for

GEOLOGY, May 7994

1000 1200 1400 1600 1800 2000Date (yr A.D.)

Figure 4. A: Optical density ol luminescence (ODL) record of lowasample CWI lrom -1000 A,D. to present. lnset shows originalrecord, in plxels. Peaks and troughs ol dated (maln) graph aretuned to B; graph ol residual difleronces from spline'functlonsmoothlng of A 1aC record (14C calibration cun e; Stuiver andOuay, 1980). lnset shows 20 yr average sunspot record since

1700 A.D,, and the sunspot numbers between 1500 and 1700 A.D.calculated by Shove (1983) from frequency of aurora records.

the same interval (Stuiver and Quay, 1980). The correlation coeffi-cient of the two curves is r = 0.803. These fluctuations may berelated to short-term changes in solar activity, as is also seen inchanges in 20 yr average numbers of sunspots (inset, Fig. 4B).

Sample JC11 from Jewel Cave, South Dakota, provides an ex-ample of luminescence-banding variation at the millenial time scale.It is a large, fallen and shattered flowstone that displays strong colorgrowth bands over a 1.0.6-cm-thick section. From 10 TIMS-US datesit can be determined that this section grew from 432 + 18 to 92 10.8ka. Figure 5A shows the luminescence-banding record of the upper

4.7 cm of growth(24}to92ka) resolved to 100yr/px. Luminescence-banding variations are parallel in their spacing and relative intensityto a much less well resolved record of the variation in 613C (Fig. 5B;

-5000 yrlpoint). Figure 5C shows that there is also a good correla-tion between this midcontinental speleothem and the well-known18O foraminiferal ocean core record, V28-238 (Imbrie et al., 1984).Jewel Cave lies beneath a pinon pine forest, just above a transitionto natural grassland, including C4 grasses that are enriched in 13C

with respect to the pines by up to 13%o (Smith and Epstein, 1971).The 47oa drop in E13C of calcite during isotope stage 5, representinga decrease of -8%o in soil CO, thus could be due to a lowering of thetree line across the top of the cave. This shift was also accompaniedby a decrease in the luminescence.

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JC 11

ODL(Relative units)

JC 11

c13oc

migure 5, A: Optical den-sity of luminescence(ODL) record of upper 4.7cm of flowstone sampleJCl'l trom Jewel Cave,South Dakota. Solid cir-cles-McMaster Univer-sity thermal ionizationmass speclrometry u-se-ries dates with 20 errorbars. B: Record of b13Clrom JCl1, sampled par-allel to trace of ODL tra-verse. c: aecord of 6180of planktonic foraminif-era from ocean core V28-238 (lmbrie et al., 1984). v28-238

c18oo100 150 200 250

ka B.P.

CONCLUSIONSLuminescence banding in speleothems can be used to trace

changes in their growth history on time scales ranging from 10s yr. Annual oscillations are well resolved in many temperate-region stalagmites; higher frequency components may represent as-yet unexplained periodicities in past meteorological rycles. The an-nual and shorter cycles probably represent minor changes in calcitegrowth rate modulated by changes in drip rate and its effect on thekinetics of calcite precipitation.

Stronger, longer term changes in amplitude of the luminescence

reflect shifts in both calcite growth rate and in abundance of humicsubstances in the drip water. These probably reflect climaticallycontrolled changes in soil productivity in many instances, becausethese will influence volume of flow and degree of supersaturation ofthe drip water and thus calcite deposition rate as well as the rate ofproduction of soluble fuMc and humic substances in the soil.Changes in luminescence-banding intensiqz can therefore be used totrack changes in relative states ofcontinental paleoclimate, althoughthey may not be directly translatable into climatic variables such astemperature or relative humidity.

ACKNOWLEDGMENTSSupported by Natural Sciences and Engineering Research Council of

Canada operating grants and a strategic research grant to Ford andSchwarcz. We thanl Martin Knyf for 13C analyses and Karen Goodger andWangxing Li for U-series TIMS dating.

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Manuscript received November 4, 1993Revised manuscript received February 2, 1994Manuscript accepted February 1.1, 1994

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