Controls on trace element (Sr–Mg) compositions of carbonate cave waters: implications for speleothem climatic records

Chemical Geology 166 Ž2000. 255–269 www.elsevier.comrlocaterchemgeo

Controls on trace element žSr–Mg/ compositions of carbonate cave waters: implications for speleothem climatic records Ian J. Fairchild a,) , Andrea Borsato b, Anna F. Tooth a , Silvia Frisia b, Christopher J. Hawkesworth c , Yiming Huang a,c , Frank McDermott d , Baruch Spiro e a

Department of Earth Sciences, Keele UniÕersity, Staffordshire ST5 5BG, UK Museo Tridentino di Scienze Naturali, Via Calepina 14, 38100 Trent, Italy Department of Earth Sciences, Open UniÕersity, Milton Keynes, MK7 6AA, UK d Department of Geology, UniÕersity College, Belfield, Dublin 4, Ireland e NERC Isotope Geology Laboratory, Keyworth, Nottingham NG12 5GG, UK b

c

Received 17 September 1999; accepted 5 November 1999

Abstract At two caves ŽClamouse, S France and Ernesto, NE Italy., cave drip and pool waters were collected and sampled at intervals over a 2–3 year period. MgrCa and SrrCa concentration ratios, corrected for marine aerosols, are compared with those of bedrocks and, in some cases, aqueous leachates of soils and weathered bedrocks. Cave waters do not lie along mixing lines between calcite and dolomite of bedrock carbonate, but typically show enhanced and covarying MgrCa and SrrCa. Four factors are considered as controlling processes. Ž1. The much faster dissolution rate of calcite than dolomite allows for the possibility of increase of MgrCa if water–rock contact times are increased during drier conditions. A theoretical model is shown to be comparable to experimental leachates. Ž2. Prior calcite precipitation along a flow path is a powerful mechanism for generating enhanced and covarying MgrCa and SrrCa ratios. This mechanism requires the solution to lose CO 2 into pores or caverns. Ž3. Incongruent dolomite dissolution has only limited potential and is best regarded as two separate processes of dolomite dissolution and calcite precipitation. Ž4. selective leaching of Mg and Sr with respect to Ca is shown to be important in leachates from Ernesto where it appears to be a phenomenon of calcite dissolution. In general selective leaching can occur whenever Ca is sequestered into precipitates due to freezing or drying of soils, or if there is derivation of excess Sr and Mg from non-carbonate species. The Ernesto cave has abundant water supply which in the main chamber is derived from a reservoir with year-round constant PCO 2 of around 10y2.4 and no evidence of calcite precipitation in the karst above the cave. Two distinct, but overlying trends of enhanced and covarying MgrCa and SrrCa away from the locus of bedrock compositions are due to calcite precipitation within the cave and, at a variable drip site, due to enhanced selective leaching at slow drip rates. Mg-enhancement in the first chamber is due to a more dolomitic bedrock and longer residence times.

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Corresponding author. Tel.: q44-1782584305; fax: q44-1782584305. E-mail address: [email protected] ŽI.J. Fairchild..

0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 2 1 6 - 8

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The Clamouse site has a less abundant water supply and presents geochemical evidence of prior calcite precipitation, both in the cave and in overlying porous dolomiterdedolomitized limestone bedrock. Initial PCO 2 values as high as 10y1 are inferred. Experimental incubations of Clamouse soils which generated enhanced PCO 2 and precipitated CaCO 3 had compositions similar to the karst waters. Calcite precipitation is inferred to be enhanced in drier conditions. Hydrological controls on cave water chemistry imply that the trace element chemistry of speleothems may be interpretable in palaeohydrological terms. Drier conditions tends to promote not only longer mean residence times Ženhancing dolomite dissolution and hence MgrCa., but also enhances degassing and calcite precipitation leading to increased MgrCa and SrrCa. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Karst; Weathering; Aqueous geochemistry; Palaeohydrology

1. Introduction Karstic systems are treasuries of geochemical data related to past environmental conditions, but conversion of these data into measures of palaeoclimate requires a detailed understanding of each system. For example, the oxygen isotopic composition of speleothems can be used to evaluate changes in mean annual palaeotemperatures Že.g., Gascoyne, 1992., but in practice the geochemical processes acting at each site need to be thoroughly understood for such interpretations to be compelling ŽBar-Matthews et al., 1996, 1997.. Variations in trace element chemistry, including regular annual cycles, are now known to exist in speleothems ŽRoberts et al., 1998., but more understanding of cave processes is needed to provide reliable interpretations. Early work by Gascoyne Ž1983. focused on the influence of temperature in equilibrium partitioning of Mg into calcite, but this proved to be insufficient as an explanation ŽGoede, 1994.. Fairchild et al. Ž1996. argued that longer water residence time in the karstic system Ždrier conditions. would lead to enhanced dolomite dissolution relative to calcite dissolution and hence higher MgrCa in cave waters and higher Mg in speleothems. Roberts et al. Ž1998. demonstrated annual antipathetic cycles of Mg and Sr in a Scottish stalagmite, interpreted in this way as reflecting varying residence time. This means that we may be able to derive information about hydrology, and hence the balance of precipitation and evaporation in the past, by study of the trace element compositions of speleothem calcitee. To develop the palaeoclimatological potential, an understanding is needed of each part of the karst system. In this article we discuss the variations in the chemistry of the cave waters which are responsible

for much of the variation in speleothem geochemistry. Whereas there has been some discussion of variations in chemistry within caves as a result of calcite precipitation Že.g. Gonzalez and Lohmann, 1988., we take into consideration also the relationships with host rock and aqueous leachate compositions which have been explored little so far. Weathering behaviour, water routing through different lithologies, and prior calcite precipitation are shown to be key variables. 2. Study methods Cave and rain waters were sampled in 1994–1997 as part of larger studies including modern precipitates and isotopic investigations. Normally, separate acidified and unacidified aliquots were taken for cation and anion analysis respectively by inductively coupled plasma ŽICP. atomic emission spectrometry and ion chromatography. Carbonate saturation was calculated using these analyses together with alkalinity Žcross-checked estimates from charge balance and laboratory titrations. and on-site pH data by using the MIX4 program ŽPlummer et al., 1975; Fairchild et al., 1994a., updated with the calcite solubility product values of Plummer and Busenberg Ž1982.. Carbonate bedrock and soil compositions were determined by analysis of solutes in 0.5 M hydrochloric acid. ICP analytical precision Ž2 s . is 2, 0.1 and 0.002 mgrl respectively for Ca, Mg and Sr at typical concentrations, corresponding to uncertainties of "6 and "0.05 in the respective weight ratios 1000MgrCa and 1000SrrCa at typical ratio values and with 50 mgrl Ca2q. Several leaching experiments to simulate weathering were carried out on materials from both study sites and are summarized in Table 1. Following

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Table 1 Summary of leaching experimental conditions Duration Ernesto (laboratory temperature) E1. 1 h Žair continually bubbled.

Materials

Water:solid ratio

Final PCO 2 as ylog PCO 2

Final saturation index

Weathered bedrocks, broken to cm-sized pieces

5:1 by volume

Atmospheric Ž3.5.

n.d.

5:1 by volume

) atmospheric Ž- 3.5.

n.d.

20:1 by weight

2.8 " 0.15

y0.4 " 0.4

20:1 by weight

2.85 " 0.1

y0.3 " 0.1

100:1 by weight 1:2 by weight

3.25 " 0.25 2.4 " 0.3

y2.2 " 0.3 q1.05 " 0.2

E2. 1 month Žundisturbed closed vessel. Clamouse (308C, undisturbed closed Õessel) C1. 31 days Bedrock broken to 5–8 mm pieces C2. 31 days Weathered bedrocks broken to cm-scale pieces C3. 1 day Stony soils C4. 31 days Stony soils

filtration, samples were analyzed for cations and, in the case of the Clamouse experiments, pH, alkalinity and anions. 3. Study sites and a comparison of cave water and bedrock composition Firstly we summarize the likely history of the waters from the study sites. Our water samples have generally infiltrated sub-vertically into caves at relatively shallow depth. Tracer studies elsewhere suggest that a lateral dispersion of up to 100 m can be expected in such situations ŽBottrell and Atkinson, 1992.. The geological setting is constant in each case over a much bigger area than the likely dispersion of water. Hence the range of soils and bedrocks encountered is well-constrained and the shallow depth minimizes water mixing. High CO 2 pressures in soil zones drive carbonate dissolution reactions into waters percolating through the soil and underlying karstic aquifer. The aquifer at shallow levels Žthe sub-cutaneous zone. is a zone of enhanced solutional porosity ŽGunn, 1983; Williams, 1983., behaving as a dual-porosity system with a baseflow of seepage water held in diffuse matrix pores and more rapid water flow Žquickflow. along fissures, with some conduits only active at times of highest flow. Degassing of CO 2 from dripping water into cave air in turn drives carbonate precipitation, which will be accentuated if any evaporation occurs. Water chemistry thus reflects the initial atmospheric chemistry

variably modified by dissolution and subsequent precipitation reactions. Characteristics of two sites are compared in Table 2. Ernesto ŽAwsiuk et al., 1991. is the cooler and has the smaller volume and occurs in a moist climate. The PCO 2 of waters and cave air average 10y2 .8 whereas maximum soil PCO 2 in the soil profile is around 10y1 .9 in summer to 10y2 .5 in late autumn. The main chamber has dripwater sites displaying either steady or variable dripping, one example of the latter responding to rainfall events within hours by increased drip rate and dilution of composition ŽBorsato, 1997.. Otherwise, calcium concentrations of dripwaters vary little ŽFig. 1a. and hence are likely to reflect the primary cationic composition of fluids derived from weathering reactions. Ca-budget calculations indicate that only 1r20 to 1r80 of aqueous Ca in dripwater becomes precipitated on stalagmite tops, consistent with the normal cone shape of stalagmites indicating continued precipitation on their flanks. A high conductivity drip in the first chamber displayed little change in drip rate and conductivity, consistent with a longer mean residence time of water. During winters 1995r6, 1996r7 and 1997r8 ŽDecember–April., 4–6 weekly monitoring of pool waters in the main chamber showed a drop of up to 25% in electroconductivity and Ca, while dripwaters changed less than 5% ŽFig. 1a.. These conductivity data are interpreted as reflecting the effects of calcite precipitation on stalagmite flanks, flowstones and in pools. These effects coin-

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Table 2 Comparison of some attributes of the studied caves

Location Temperature Rainfall Physical setting and soils Geology Cave geometry

Clamouse

Ernesto

S France 14.58C Low total Ž790 mm., less in summer; strong summer soil water deficit. Poorly vegetated hillslope with thin red, rendzina-like soil with carbonate debris Žin studied part. Variably dipping porous Jurassic dolostones, partly dedolomitized Long system; passages up to 10 m high and 20 m wide; several tens of metres below surface

NE Italy 6.58C 1300 mm distributed throughout the year; spring snowmelt significant Steep forested hillslope with brown earth soil developed on carbonate talus Shallowly dipping Jurassic Žcalcitic. dolostones plus limestones in talus Tube, average 3 m diameter within 20 m of surface

cided with lower PCO 2 in cave air and reduced winter recharge of the karstic aquifer related to a lying snowcover. Samples at Clamouse cave ŽChoppy and Dubois, 1974., which lies in a warmer and drier climatic zone were collected on up to four occasions in different seasons in three chambers ŽSalle de gour sec, Grand CarrefourrBalcon and Couloir blanc.. Water and air PCO 2 values average 10y2 .5. There is little short-term variation in drip rates, pointing to the dominance of storage water in the porous dolostone bedrock. Ca contents correlate with supersaturation at individual sites ŽFig. 2a.. Stalagmites are predominantly candle-shaped, with little precipitation

on their flanks, reflecting relatively efficient CaCO 3 precipitation Žcalculations on one stalagmite showed that 1r7 of aqueous Ca in dripwater was deposited on the stalagmite top.. Precipitation in pools is enhanced by slow evaporation effects. A progressive 10% drop in logged electroconductivity over two months in late summer at one dripwater site Žduring a period of dry weather conditions. is interpreted to reflect prior CaCO 3 precipitation above the cave. Table 3 summarizes compositions of cave waters. In order to compare with bedrock compositions, it is necessary to correct for the estimated contribution from atmospheric aerosols. We are particularly concerned with Sr and Mg since these elements are

Fig. 1. Ernesto data. Ža. Plot of Ca concentration against calcite saturation index. Žb. Comparison of cave water and bedrock compositions expressed as the weight ratios MgrCa and SrrCa. Cave waters are enriched in Sr and Mg relative to Ca compared with bedrocks and show a covarying trend in each of the two chambers.

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Fig. 2. Clamouse data. Ža. Plot of Ca concentration against calcite saturation index; individual locations display covariations. Žb. Comparison of cave water and bedrock compositions at Ernesto expressed as the weight ratios MgrCa and SrrCa. Cave waters are enriched in Sr and Mg relative to Ca compared with bedrocks and show a covarying trend at several individual locations.

associated with the carbonate system. Marine aerosol contribution is calculated assuming that the ratio of each element to Cl is as in sea water ŽSarin et al., 1989., bedrock evaporites being absent. SrrCa and MgrCa ratios are much higher in seawater than in

carbonate rocks, but atmospheric contributions of these elements are negligible at Ernesto, in agreement with rainfall analyses ŽTable 3., despite some evidence ŽCa contents. of limestone dust dissolution in rainfall samples. Aerosol contributions at Clam-

Table 3 Cave water composition and estimated contributions of marine aerosol. Rainfall analyses are successive month-long collection Mean Žand standard deviation. concentration in cave water Žmgrl. Ernesto Cl Na Mg Ca K Sr Si SO4 NO 3 Alkalinity

Ž n s 98, main chamber. 0.8 " 0.4 0.4 " 0.2 5.9 " 1.0 53.1 " 5.2 0.3 " 0.4 0.02 " 0.002 2.2 " 0.2 6.5 " 0.9 6.1 " 2.5 2.6 " 0.3 meqrl

Clamouse Cl Na Mg Ca K Sr Si SO4 NO 3 Alkalinity

Ž n s 40. 10.5 " 4.6 6.1 " 2.3 23 " 9 47 " 11 0.6 " 0.6 0.024 " 0.009 1.9 " 0.5 9.3 " 5.5 1.3 " 1.9 3.9 " 0.7 meqrl

% as marine aerosol

Concentration in rainwater Žmgrl.

110 1 -1 7 2 -1 -1 -1 -1

Ž n s 4. 0.7 0.2 0.2 4.6 - 0.1 to 1.65 - 0.002 - 0.1 1.6 0.7 n.d.

100 3 0.5 27 20 -1 -1 -1 -1

Ž n s 5. 5.6 3.1 0.8 7 - 0.2 to 0.6 0.003 - 0.1 3 0.4 n.d.

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ouse are significant for Sr in relation to the bedrock yields and cave water chemistries are corrected for these inputs where indicated in this paper. In Section 1, we mentioned the concept that karst water compositions were controlled by dissolution of varying proportions of calcite and dolomite. This can be tested by comparison of cave waters and bedrocks ŽFigs. 1 and 2. in terms of SrrCa and MgrCa ratios. In both cases, the variation in bedrock composition is sufficiently limited to allow a dolomite–calcite mixing line to be established. Significantly, the cave waters all lie above the mixing lines and the data display covariation. The same relationship of covarying water data Žcorrected for marine aerosol input. lying above bedrock compositions in SrrCa–MgrCa space has been found in two other caves studied ŽGrotte Pere-Noel, ` ¨ Belgium and Crag Cave, western Ireland., data from which will be presented elsewhere in the light of the discussion in this paper. In principle, Sr could also be derived from dissolution of CaSO4 or by selective leaching of silicates. Sulphate minerals are definitely absent at Ernesto and are undescribed at Clamouse where there is also no correlation Ž R 2 - 0.01. of aqueous SrrCa with sulphate. Instead, the source of karst water sulphate is considered to be pyrite oxidation. The bedrocks contain 0–2% clay minerals, but leaching of any labile Sr from such silicates would already have been included in our dilute HCl leaching procedure for determining bedrock composition. Our purpose is to demonstrate a general approach to understanding how karst water compositions become modified in nature in relation to such bedrock compositions. Determination of the precise proportion of any Sr from silicates Že.g. by using Sr isotope data. is beyond the scope of this paper. 4. Controlling processes We consider the effects on water chemistry of several processes, both in theory and by comparison with experimental data. Then we apply the insights gained to the field sites. 4.1. Differential dissolution of calcite and dolomite The slower weathering of dolomite than calcite is well known from field data ŽCowell and Ford, 1980;

Atkinson, 1983. and is confirmed by experiment ŽChou et al., 1989.. Considerable time Žup to years. is needed for fluids to become close to saturation with dolomite ŽBusenberg and Plummer, 1982., although this is complicated by the derivation of Ca, Mg and CO 32y ions also from calcite dissolution. The transit time of water from the surface to a cave drip can vary from hours to years, with different drip sites exhibiting various hydrological behaviours ŽSmart and Friederich, 1986.. There exists therefore the potential for karst waters to show considerable variation in trace element chemistry depending on the transit time of waters ŽRoberts et al., 1998. as well as the relative proportions of calcite and dolomite encountered by the flows. The kinetics of calcite dissolution are relatively well understood ŽPlummer et al., 1979., but dolomite dissolution is less well known ŽBusenberg and Plummer, 1982; Chou et al., 1989., except far from equilibrium. Busenberg and Plummer Ž1982. measured the dissolution kinetics of a variety of natural dolomites under a range of PCO 2 values and derived a rate expression in terms of three forward and one backward reaction. Chou et al. Ž1989. used only the three forward rate terms to model their data as follows: Rate Ž mol cmy2 sy1 . s 2.6 = 10y7 Ž aq H. U 0.75

= Ž aH 2 CO 3 . U

0.75

q 1.0 = 10y8

q 2.2 = 10y12

where a H 2 CO 3 represents the activity of aqueous CO 2 and H 2 CO 3 combined. Chou et al.’s Ž1989. expressions for dolomite and calcite dissolution were used to calculate the changing solution composition during the progressive dissolution of calcite–dolomite mixtures towards equilibrium in an open system, that is one with a PCO 2 fixed by equilibrium with Žsoil. gas. In the initial stages of dissolution Ž- 5 mgrl Ca2q ., Busenberg and Plummer’s Ž1982. expressions indicate that different specimens of dolomite dissolved between 25 and 250 times slower than calcite compared with 8–9 times slower using Chou et al.’s Ž1989. expression based on one dolomite type. However, calculated dissolution rates of dolomite rapidly

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become negative using the former expressions, indicating their lack of applicability to later stages of dissolution. Fig. 3 represents the results of a computation utilizing Chou et al.’s Ž1989. rate expression in which progressive dissolution of equal surface areas of dolomite and Mg-free calcite is modelled at a fixed PCO 2 of 10y2 .6 , leading Žat line A. to a calcite-saturated water with the Mg and Ca concentration similar to that of the main Ernesto chamber. The MgrCa ratio is not intended to be a precise prediction since the behaviour of different dolomites is so variable and Chou et al.’s rate expression was derived from the initial stages of dissolution only. Also, the final ratio would be greater if significant Mg is derived from calcite dissolution. Nevertheless, Fig. 3 illustrates the tendency for the effects of dolomite dissolution to become relatively more important Žhigher MgrCa. as dissolution proceeds. Once calcite is saturated, only dolomite can dissolve, and the ultimate possible cumulative ratio of calcite to dolomite dissolved is independent of kinetics. Fig. 3 indicates that the MgrCa ratio rises significantly

Fig. 3. Results of a computation to simulate evolution of water chemistry during competitive dolomite and calcite dissolution. Equal surface areas of dolomite and ŽMg-free. calcite are dissolved, in steps of 0.1 pH units, in an open system with PCO 2 s 10y2 .6 . Calculations use the algorithms of Raiswell Ž1984., but with activity coefficients of divalent species reduced to 0.7. The solubility product for dolomite used here is defined as ŽaCa2q . 0.5 ŽaMg 2q . 0.5 ŽCO 32q ., where a refers to activity, by analogy with calcite. The relative rates of calcite and dolomite dissolution is taken from the data of Chou et al. Ž1989.. A s position at calcite saturation; Bs position at dolomite saturation.

261

Fig. 4. Results of computations Ž258C. in which bedrocks with different proportions of calcite and dolomite are dissolved to calcite saturation at three different PCO 2 values. The data points are joined by tie-lines, not reaction path lines. The aqueous leachates have lower MgrCa relative to the bedrocks because of preferential calcite dissolution. This effect is more pronounced at lower PCO 2 values. The simulation of Fig. 3 is represented by the central line emanating from a bedrock composition of 50% dolomite.

between calcite saturation Žline A. and dolomite saturation Žline B.. Fig. 4 summarizes the results of such calculations across a range of calcite–dolomite bedrock compositions illustrating both the drop in MgrCa in calcite-saturated leachates compared with bedrocks due to preferential calcite dissolution and how this is most pronounced under lower PCO 2 conditions. Fig. 5 compares the expected composition of leachates at a PCO 2 of 10y2 .6 with experimental leachates of Clamouse bedrock samples Žexperiment series C1 and C2, Table 1.. A general agreement is found, the variability of results being consistent with the expected heterogeneity of the exposed surfaces of the natural materials. The leachates were still undersaturated Žmean saturation index of y0.35 and y0.45 in the two sets of experiments, Table 1. and so the effect of preferential calcite dissolution should be slightly enhanced Žcf. Fig. 3. compared with the model, as is observed in most cases in Fig. 5. Fig. 5 also shows the expected MgrCa ratios at dolomite saturation for comparison. Fig. 6a and b summarize leachate compositions at respectively calcite and dolomite saturation, by comparison with bedrock compositions, and at PCO 2 of 10y1 .6 . Leachates from bedrocks with less than around 40% dolomite show dramatically higher MgrCa at dolomite saturation ŽFig. 6b. than calcite

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Fig. 5. Comparison of computations of MgrCa ratios of leachates resulting from dissolution of calcite–dolomite bedrock mixtures at PCO 2 of 10y2 .6 Žas in Figs. 3 and 4., with composition of experimental leachates of Clamouse bedrocks Žexperiments C1 and C2, Table 1.. The data points are joined by tie-lines, not reaction path lines.

saturation ŽFig. 6a., although dolomite dissolution times must increase as dolomite abundance decreases. Dolomite-rich Ž90% q . bedrocks are al-

ready close to dolomite saturation even under quickflow conditions and hence show little change in MgrCa with prolonged reaction times.

Fig. 6. Results of computations to illustrate evolution of solution composition in relation to that of parent bedrock in response to various processes. The bottom line with added points in each plot represents the locus of compositions of bedrocks made of calcite–dolomite mixtures from 1 to 99% dolomite Žwith 1000 ) SrrCa arbitrarily defined as 0.2 for both calcite and dolomite end-members.. In diagram Ža. the next line represents the aqueous leachate at the time when calcite saturation is just reached and the top line represents evolution of solution composition following degassing from PCO 2 of 10y1 .6 to 10y2 .1 followed by calcite precipitation to a final saturation index of q0.1. In diagram Žb. the leachate line shows compositions attained at dolomite saturation and the upper two lines show the effect of calcite precipitation Žto a final saturation index of 0.1. after no degassing and after degassing from PCO 2 of 10y1 .6 to 10y2.1. Leachate lines derived as for Figs. 3 and 4; upper lineŽs. derived using modified version of aqueous speciation model MIX ŽPlummer et al., 1975..

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It can be concluded that at relatively short reaction times, a key process is preferential calcite dissolution over dolomite. This leads to leachate compositions being systematically lower in MgrCa with respect to bedrocks. MgrCa ratios may increase given in particularly slow-flowing portions of the shallow karstic aquifers. Other processes are required however, to cause the aqueous trace element to calcium ratios to deviate from those of the bedrocks. 4.2. Calcite precipitation Karstic waters typically become supersaturated for calcite when allowed to equilibrate by degassing with an atmosphere with a lower PCO 2 than that of the soil or sub-cutaneous zone. Since the partition coefficients for Mg and Sr are < 1 ŽMorse and Bender, 1990., the ratios MgrCa and SrrCa rise as calcite precipitation occurs, leading to a vector of constant slope on a log–log crossplot of these parameters ŽFig. 6.. Fig. 6 illustrates the consequences of calcite precipitation, forced by a drop of PCO 2 from 10y1 .6 to 10y2 .1 , on leachates that had reached either calcite or dolomite saturation ŽFig. 6a and b, respectively.. The MgrCa and SrrCa of the final solutions are double those of leachates that were originally at calcite saturation at PCO 2 of 10y1 .6 ŽFig. 6a., and the increase is greater still in Fig. 6b. Since MgrCa and SrrCa increase progressively as Ca falls, this can be used as a test that a given set of waters have evolved by calcite precipitation from an initial constant source. This test is used in Section 5. Fig. 7 illustrates a comparison of two leaching experiments ŽC3 and C4, Table 1. on Clamouse soils. Whereas the short leach experiments, which were undersaturated for calcite, led to the expected decrease in MgrCa due to preferential calcite dissolution, the longer experiments with a lower water– soil ratio ŽC4. yielded MgrCa ratios much higher than in the parent soils. These solutions had high PCO 2 values indicating substantial CO 2 generation during the experiments. Some of this CO 2 must have been lost during sample filtration to generate the extremely high saturation indices calculated. Calcite precipitation will occur rapidly at such supersaturations, which is consistent with the high MgrCa ratios attained.

Fig. 7. Comparison of MgrCa ratios of carbonate fraction of Clamouse soils with leachates in experiments C3 and C4 of Table 1. Preferential calcite dissolution drives the ratios down in C3. In C4 the high ratios are very inferred to have been caused by calcite precipitation.

4.3. Incongruent dolomite dissolution In Section 4.1 we assumed that both calcite and dolomite dissolved congruently. Incongruent dissolution behaviour of dolomite has been studied by Busenberg and Plummer Ž1982. who found incongruent preferential release of Ca from fresh dolomite surfaces during the leaching of the first lattice layer of broken surfaces, but that continued leaching was congruent. The fact that datapoints lie close to the dolomite end-member in Fig. 2b suggests that this phenomenon is not significant. Incongruent dolomite dissolution in a looser sense refers to calcite precipitation forced by supersaturation generated by slow dolomite dissolution Žas at line B on Fig. 3.. Fig. 6b illustrates the possible increases in MgrCa and SrrCa arising from calcite precipitation to equilibrium in a closed system from leachate solutions initially saturated for dolomite. The effects are much less pronounced than those induced by PCO 2 changes. 4.4. SelectiÕe leaching We use the term selective leaching to refer to enhancement of trace element yields with respect to Ca, in waters compared to the Ždilute acid-soluble. bedrock composition, by whatever mechanism. Such preferential release of Mg and Sr with respect to Ca

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compared with parent materials could occur in principle if there were easily leached sources in soluble soil salts or in silicate phases. Such selective release can also occur in weathering of CaCO 3 , which although usually treated as dissolving congruently during weathering Že.g., Palmer and Edmond, 1992., is known to display incongruent effects in young carbonate aquifers Že.g., Reeve and Perry, 1994.. For example, Mg-calcite often releases Mg in transforming to low-Mg calcite with little crystallographic alteration, and thin section to aquifer-scale transformation of aragonite to calcite yields excess Sr. Field evidence ŽFairchild et al., 1994a. of such selective leaching of Mg and Sr in a glacial limestone terrain was confirmed by laboratory leaching experiments ŽFairchild and Killawee, 1995. on a variety of carbonates, including pure calcites, crushed to 10 mm grains. It is not known whether these selective leaching effects reflect only dissolution, or whether they reflect enhanced dissolution at defects Žwhich are enhanced on broken surfaces, Macinnis and Brantley, 1992. and simultaneous reprecipitation of calcite elsewhere on the mineral surface.

Fig. 8. Leaching experiment data from weathered Ernesto bedrocks ŽE1, Table 1. in comparison with data from cave dripwater G2. Strong enrichment in Sr and Mg in leachates from samples near the CaCO 3 end-member indicates that a process of selective leaching from this end-member is occurring. The grid illustrates the compositions of various calcite–dolomite mixtures as defined by the compositions of unweathered Ernesto bedrocks ŽFig. 1., with variable selective leaching of Mg and Sr from the CaCO 3 end-member. Variation in chemistry in G2 drip waters, which is independent of Ca composition ŽFig. 9., may be explained by variable selective leaching of this type.

Experiments on Ernesto materials Žexperiment E1, Table 1. illustrate a selective leaching effect. Fig. 8 illustrates that leachates of weathered bedrocks at atmospheric pressure have higher SrrCa than expected for congruent dissolution. In particular, three samples of dolomite-free limestone showed strong enrichments in both SrrCa and MgrCa representing up to 5-fold selective leaching of Sr and Mg compared with the limestone. A maximum of only an additional 10% of the Mg and Sr can be attributed to a marine aerosol source, based on Na contents of the leachates. The selective leaching effect is thus likely to be due to either a non-congruent calcite dissolution, or the accumulation of soluble Mg and Sr related to cycles of dissolution and calcite precipitation in the weathered materials. In principle, such precipitation could be forced by degassing, evaporation or freezing ŽFairchild et al., 1994b..

5. Application to study sites A general problem in hydrochemistry is that any water could have evolved to its current composition by an infinite number of pathways of gas–water–rock interaction. In our case, we have established some constraints by establishing the range of bedrock compositions and are also dealing with a system where CO 2 tends to decrease from soil to cave. The pattern displayed by families of water samples also suggests likely evolutionary trends. Ideally one would need to know the primary near-surface PCO 2 with which waters equilibrate, but this cannot be precisely determined in practice as soil PCO 2 values vary considerably in time and space. Hydrologically, we can distinguish quickflow and seepage baseflow end-member waters. Given the rapidity of calcite dissolution kinetics, even the rapidly flowing quickflow waters will approach equilibrium with calcite Žline A on Fig. 3., but are not likely to evolve past that line. Seepage baseflow waters could, in principle, reach dolomite saturation Žline B on Fig. 3., but, generally, stored waters in shallow karstic aquifers are likely to lie intermediate between A and B, and where dolomite dissolution occurs below the soil zone, will be more likely to belong to a system closed to CO 2 , rather than the open system depicted in Fig. 3. Closed systems will

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evolve less in solute MgrCa ratios than open systems since dolomite saturation is reached earlier. 5.1. Ernesto The behaviour of drips in the main chamber suggests quickflow behaviour and the waters are undersaturated for dolomite. Measured maximum soil PCO 2 values above Ernesto in summer and autumn are around 10y1 .9 to 10y2 , which places a maximum bound on the initial PCO 2 of waters. Addition of CO 2 to typical dripwater samples using the MIX speciation program, and assuming that no calcite precipitation has already occurred, suggests a parent water with PCO 2 of 10y2 .3 to 10y2 .5. Given an initial 1000 MgrCa ratio of around 80 Žby extrapolation of the data trend to meet the bedrock composition line, Fig. 1b., modelling indicates that the parent carbonate source aquifer has around 50% dolomite. A test for prior calcite precipitation is provided by plots of Ca vs. MgrCa or SrrCa ŽFig. 9.. Dripwaters Žunlike poolwaters. vary little in Ca content at different times in each chamber and in the main chamber show remarkably constant mean value at each site which is consistent with both Ža. minimal prior calcite precipitation and Žb. the rapid equilibra-

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tion of near-surface waters with calcite at a PCO 2 Žaround 10y2 .4 . that is lower than that attained in the soil and which does not vary seasonally. The enhancement in Ca in the first chamber dripwaters ŽFig. 9a. is suggestive of a higher primary PCO 2 of around 10y1 .9 , close to summer soil values. Assuming quickflow conditions allowing dissolution to calcite saturation, a bedrock composition of around 75% dolomite can be inferred. Alternatively, given that the constant drip rate implies longer water-rock contact times, the bedrock may have had as little as 65% dolomite if saturation for dolomite were reached during dissolution in the aquifer. An intermediate bedrock composition and saturation state for dolomite is most likely. The significant winter precipitation that is inferred to occur from drips to pools from conductivity evidence is matched by the data of Fig. 9, which illustrate the spread of points along the model calcite precipitation lines. Given that some stalactitic precipitation occurs at many drip sites, stalagmite waters should also show this effect, but to a more limited extent. The overall trends of data in the first and main chambers, related to calcite precipitation effects, are similar to the trends shown by comparison of leachate

Fig. 9. Ernesto data in relation to influencing factors. Ža. Ca2q vs. MgrCa and Žb. Ca2q vs. SrrCa. Modelled lines represent modelled evolution of fluids precipitating calcite at approximate equilibrium Žsaturation index of q0.05. as PCO 2 falls. Pool waters spread out along the lines, indicating variable prior calcite precipitation, whereas dripwaters do not. Dilution effects should result in vertical data trends which are not prominent. The difference in MgrCa ratios between the two chambers is argued to be due to a higher proportion of dolomite and longer water residence time above the first chamber. The slope of the model lines is not sensitive to the exact partition coefficient; here we use empirical partition coefficients Že.g., for Mg, MgrCa speleothem rMgrCa water . of 0.02 for Mg and 0.1 for Sr derived by comparison of mean water and speleothem composition in chambers from three caves. These values are consistent with our unpublished laboratory growth experiments.

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experiments E1 and E2 ŽTable 1. on Ernesto samples ŽFig. 10.. The Ca contents of the latter require attainment of high PCO 2 conditions during leaching as described earlier in Clamouse experiment C4. Both dripwaters and leaching trends are close to the calcite precipitation vector. Drip G2 provides an exception in that it shows a covariation of MgrCa and SrrCa ŽFig. 8., but the points do not lie along the calcite precipitation lines in Fig. 9. Given that the data trends originate from just above the locus of bedrock compositions, a possible explanation of the covariation at this drip site is that it reflects varying importance of selective leaching. Fig. 11 illustrates that MgrCa decreases with increasing drip rate. This implies that drier conditions, with more sustained water–rock interaction lead to enhanced MgrCa and SrrCa ratios. The seasonal freezing of soil at Ernesto may be a particularly effective way of enhancing such processes both by generation of fresh mineral surfaces and forcing CaCO 3 reprecipitation. Variations in the proportion of dolomite and calcite weathering due to varying residence time should result in antipathetic trends on the MgrCa–SrrCa plot because the dolomite is depleted in Sr compared with the calcite. Such are not observed in the Ernesto data except by comparison of data from the two chambers as a whole.

Fig. 10. SrrCa–MgrCa crossplot of range of composition of Ernesto bedrocks and cave waters compared with slow and fast leachates Žexperiments E1 and E2 respectively, Table 1. from weathered bedrock materials. The overall covarying trends are related to calcite precipitation.

Fig. 11. Variation of MgrCa ratio with drip rate for sites G1 and G2, Ernesto main chamber. An antipathetic variation is observed for the latter, but this is not so clear where variations of drip rate of less than an order of magnitude are present, as in drip G1.

In summary, two types of positively covarying trends are overlaid in the Ernesto data: one apparently related to weathering reactions and one to calcite precipitation. The first chamber is more Mgenriched probably due to a combination of longer water residence time and locally more dolomite-rich bedrock. 5.2. Clamouse At Clamouse, karst waters lie significantly above the bedrock line ŽFig. 2. except for some points close to the position of congruent leaching of dolomite. A selective leaching effect could not be tested for directly in the Clamouse leachates because Sr yields were too low in experiments C1 to C3. Minimal impact of silicate leachates is indicated by experiments in which potassium values typical of the karstic waters yields are achieved by short leaches of soils Ž- 1 day. after which Ca, Mg and Sr yields Ž4.6, 0.7 and 0.003 mgrl. are much lower than those of karst waters. Some soil salts had accumulated in the soils, which were collected during the summer dry season, but the amounts yielded from leaching experiments are equivalent to the evaporation of just one pore volume of soil. Overall, selective leaching effects are not thought to be important at Clamouse. The position of data above the bedrock line can be accounted for by calcite precipitation. Fig. 12

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At Clamouse, both pool water and dripwaters are amongst those that have evolved far from the locus of bedrock compositions. This implies calcite precipitation occurred above the sampling chamber as well as within it. This could be either in unknown higher cave passages or within air pockets in the highly porous, partly dedolomitized aquifer.

6. Implications for palaeoclimate studies

Fig. 12. Ca vs. MgrCa plot for waters from the Grotte de Clamouse compared to calcite precipitation model lines as in Fig. 10. Calcite precipitation can account for most of the data spread, but some variation in the initial fluid composition is also required.

illustrates that there is a spread of data roughly parallel to calcite precipitation lines in several sites Žalthough some variation in initial water composition is also required.. Experiment C4 yielded highly supersaturated solutions which had already evolved to compositions comparable with those of the karst waters ŽFig. 2.. Extrapolation back along the calcite precipitation vector, suggests that the data from Salle de gour sec ŽFig. 2. appear to have been derived by evolution from a bedrock site close to the calcite end-member. The limited availability of dolomite in such a site implies that waters would have evolved little beyond the quickflow condition. A typical water from this chamber has a PCO 2 of 10y2 .6 , but modelling the addition of CO 2 and removing the effects of calcite precipitation indicates that the original water would have had a PCO 2 in excess of 10y1 and 230 mgrl Ca. By contrast, backtracking solutions from the Grand Carrefour yields solutions, approximately saturated for both calcite and dolomite at a PCO 2 of around 10y1 .8. The bedrocks above this passage would have had dolomite contents of 90–100%. Some aragonite precipitates are found in association with the most Mg-enriched waters: aragonite precipitation would increase MgrCa but leave SrrCa virtually unchanged ŽFrisia et al., 1997.. There is no indication of this trend in the data.

We have shown that the predominant variations with time in our cave sites are sympathetic covariations of MgrCa and SrrCa, whereas variations in space tend to be antipathetic, reflecting varying routing through different lithologies with different calcite-dolomite ratios. The sympathetic covariations can arise through enhanced selective leaching effects with prolonged water-rock contact times or prior calcite precipitation along the flowpath, or both. Prior precipitation of calcite along a flowpath degassing can relate to relatively dry conditions when not only will contact times be long, but there also a greater likelihood of degassing into air pockets. Both at Clamouse ŽSection 3. and at Grotte de Pere-Noel, ` ¨ Belgium ŽGenty and Deflandre, 1999., automatic monitoring of conductivity at steady drip sites has shown a progressive fall in the late summer during predominantly dry weather conditions. We interpret this as evidence for such a process of prior calcite precipitation, although this needs to be tested by elemental analyses of waters within such a period. At Ernesto, evidence for increased calcite precipitation in the cave occurs during the winter related to a lower cave PCO 2 despite the constant initial PCO 2 of the waters. This is consistent with reduced karstic recharge whilst the ground is snow-covered. In summary, shifts to higher SrrCa and MgrCa composition are due to enhanced prior calcite precipitation or selective leaching of Sr and Mg during weathering. Both effects should be enhanced during dry weather conditions with lower water availability in the karstic environment. Although not observed in this study, a relative increase in the ratio of dolomite to calcite dissolved during weathering Žleading to higher MgrCa. could also be caused by drier weather leading to longer mean water–rock contact times.

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These trace element trends will be transferred to speleothems, given relatively constant partition coefficients. Partition coefficients are influenced by temperature, growth rate, growth mechanism and structural characteristics of growth surfaces Že.g., Lorens, 1981; Morse and Bender, 1990; Paquette and Reeder, 1995., but indications are that they are quite consistent in the cave environment and experimental analogues Žsee caption to Fig. 9.. We have already shown ŽFairchild et al., 1996. at Pere-Noel ¨ that at more ephemeral drip sites, where seasonal evaporation occurs, strongly covarying trace element signals occur in a recent speleothem, giving a record of the changing hydrological balance of the system. Longer term variations in SrrCa and MgrCa will reflect not only palaeohydrology, but also changing source materials Žespecially soils. and karst plumbing. However, recent studies on Sr isotopes have shown that there can be cyclic palaeoclimatological controls on Sr sourcing ŽGoede et al., 1998; BarMatthews et al., 1999., which suggests that a combination of elemental abundance and isotopic studies would be a powerful approach. Arguably, the interpretation of speleothem trace element chemistry in palaeohydrological terms has the most promise for high resolution studies ŽRoberts et al., 1998; Huang et al., submitted. where annual and sub-annual variations may be detected.

Acknowledgements We are grateful for funding from the European Commission ŽEV5V-CT94-0509. and NERC Žgrant GR3r10801.. We thank the cave owners for their enthusiastic co-operation and Jane Harris, Paul Mugridge, David Emley and Ian Wilshaw for analytical work. Dr. T.C. Atkinson, Dr. M. Bar-Matthews and anonymous reviewers offered valuable criticism of the manuscript. [MB]

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