Holocene calcite-to-aragonite transitions in speleothems from Morocco: Elemental and isotopic evidence

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Geochimica et Cosmochimica Acta 92 (2012) 23–47 www.elsevier.com/locate/gca

Climate and cave control on Pleistocene/Holocene calcite-to-aragonite transitions in speleothems from Morocco: Elemental and isotopic evidence Jasper A. Wassenburg a,⇑, Adrian Immenhauser a, Detlev K. Richter a, Klaus Peter Jochum b, Jan Fietzke c, Michael Deininger d, Manuela Goos a, Denis Scholz e, Abdellah Sabaoui f a Ruhr University Bochum, Universita¨tsstrasse 150, 44801 Bochum, Germany Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany c Helmholtz Centre for Ocean Research Kiel (GEOMAR), Wischhofstrasse 1-3, 24148 Kiel, Germany d Heidelberg Academy of Sciences, c/o Institute of Environmental Physics, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany e Institut for Geosciences, University of Mainz, Johann-Joachim-Becher-Weg 21, 55128 Mainz, Germany f Faculty of Sciences Dhar Mahraz, BP 1796 Atlas, Fe`s, Morocco b

Received 11 January 2012; accepted in revised form 1 June 2012; available online 20 June 2012

Abstract The occurrence of aragonite in speleothems has commonly been related to high dripwater Mg/Ca ratios, because Mg is known to be a growth inhibitor for calcite. Laboratory aragonite precipitation experiments, however, suggested a more complex array of controlling factors. Here, we present data from Pleistocene to Holocene speleothems collected from both a dolostone and a limestone cave in northern Morocco. These stalagmites exhibit both lateral and stratigraphic calcite-to-aragonite transitions. Aragonite fabrics are well-preserved and represent primary features. In order to shed light on the factors that control alternating calcite and aragonite precipitation, elemental (Mg, Sr, Ba, U, P, Y, Pb, Al, Ti and Th) abundances were measured using LA-ICP-MS, and analysed with Principal Component Analysis. Samples were analyzed at 100–200 lm resolution across stratigraphic and lateral transitions. Carbon and oxygen isotope ratios were analysed at 100 lm resolution covering stratigraphic calcite-to-aragonite transitions. Results show that the precipitation of aragonite was driven by a decrease in effective rainfall, which enhanced prior calcite precipitation. Different geochemical patterns are observed between calcite and aragonite when comparing data from the Grotte de Piste and Grotte Prison de Chien. This may be explained by the increased dripwater Mg/Ca ratio and enhanced prior aragonite precipitation in the dolostone cave versus lower dripwater Mg/Ca ratio and prior calcite precipitation in the limestone cave. A full understanding for the presence of lateral calciteto-aragonite transitions is not reached. Trace elemental analysis, however, does suggest that different crystallographic parameters (ionic radius, amount of crystal defect sites, adsorption potential) may have a direct effect on the incorporation of Sr, Mg, Ba, Al, Ti, Th and possibly Y and P. Ó 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

⇑ Corresponding author. Tel.: +49 (0)234 32 27768; fax: +49 (0)234 32 14201. E-mail address: [email protected] (J.A. Wassenburg).

0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.06.002

Speleothems – particularly stalagmites and flowstones – are established archives of continental climate change (Dorale et al., 1992; Neff et al., 2001; Johnson et al., 2006). Processes determining their isotopic and elemental composition, however, are complex (Fairchild and Treble,

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2009; Lachniet, 2009). Whereas most stalagmites currently used for palaeo-climate reconstruction are calcitic, detailed studies using aragonitic stalagmites are less abundant (Holmgren et al., 2003). Aragonitic stalagmites or flowstones are often avoided because aragonite is thermodynamically instable and subject to post-depositional alteration to calcite (Frisia et al., 2002; Martin-Garcia et al., 2009). Climate proxy data from aragonite archives yield promising results, however, when screening for geochemical and petrographical alteration documents well-preserved aragonite fabrics (Finch et al., 2003; Cosford et al., 2008; Li et al., 2011). Nevertheless, poor knowledge of the controls on aragonite precipitation in cave depositional environments remains a significant obstacle in aragonite archive research. The processes involved are complex and the controlling factors may change over time between different caves, within a single cave and even for an individual drip site (Railsback et al., 1994; Frisia et al., 2002; McMillan et al., 2005). From field studies and laboratory experiments, it is suggested that high dripwater Mg/Ca ratios (molar Mg/Ca ratio > 1.1) are an important factor in inducing aragonite precipitation (Frisia et al., 2002; Mcmillan et al., 2005; Fairchild and Treble, 2009) by inhibiting the formation of calcite (Ferna´ndezDı´az et al., 1996; Davis et al., 2000). Other controlling factors include (i) evaporation within the cave combined with elevated cave air temperatures (Railsback et al., 1994); (ii) crystal nucleation parameters combined with air temperature (Kawano et al., 2009); (iii) CO2 degassing rates (Ferna´ndez-Dı´az et al., 1996); (iv) low CaCO3 fluid saturation states combined with high fluid Mg/Ca ratios (De Choudens-Sanchez and Gonzalez, 2009); or (v) CO32 controlled kinetic effects (Zuddas and Mucci, 1998). Processes affecting dripwater Mg/Ca ratio include prior calcite precipitation (PCP), karst water residence times, incongruent dissolution of dolomite and selective leaching of Mg with respect to Ca (Fairchild et al., 2000). Prior calcite precipitation occurs when the water encounters a gas phase with a lower pCO2. Within the karst aquifer, the space occupied by gas increases under more arid conditions, and thus enhances PCP. Increased water residence times potentially affect the amount of dissolution of Mg-rich dolomite, whilst saturation with respect to calcite is reached earlier compared to dolomite. Therefore, increasing water residence times increases the ratio of dolomite to calcite dissolution. Although incongruent dissolution of dolomite and selective leaching may affect dripwater Mg/Ca ratios as well, the potential effects of the former two processes has lead many authors to interpret the presence of aragonite in caves as a consequence of more arid climatic conditions or seasonal aridity (Railsback et al., 1994; McMillan et al., 2005). This may also result in increased evaporation rates of the thin water film on a stalagmite surface. As shown by Frisia et al. (2002), the local karst hydrology plays an important role as well. For example, changes in the aquifer water pathway may affect the encountered amount of gas filled voids, affecting PCP and the amount of encountered dolomite. Specific elemental patterns in speleothems have the potential to shed light on these complex processes and path-

ways (Fairchild et al., 2000; Treble et al., 2003; Borsato et al., 2007). A positive correlation between Mg, Sr and Ba might indicate the existence of PCP in the karst aquifer or at the ceiling of the cave (Tooth and Fairchild, 2003; Mcmillan et al., 2005; Wong et al., 2011), whereas P, Y, Pb and possibly U indicate organic material, which could be flushed into the cave system from the soil (Treble et al., 2003; Borsato et al., 2007; Zhou et al., 2008a). These trace elements have the potential to shed light on microbial activity in the soil zone and or the amount of vegetation decay (Fairchild et al., 2001). It has also been noted that different transport mechanisms may play a role (colloidal, particles, free ions) and that this affects the way these elements are incorporated into the stalagmite (Borsato et al., 2007; Hartland et al., 2012). Furthermore, clay minerals may be detected by high peak concentrations of Al, whereas clay minerals can adsorb a range of trace elements including Th (Dorale et al., 2004) and may therefore obscure the climate signal inferred from other trace elements. Here, we present, discuss and interpret a wide range of elemental data including Mg, Sr, Ba, U, Al, Ti, Th, Pb, P and Y as well as carbon and oxygen isotope ratios from three Pleistocene to Holocene stalagmites. The stalagmites were collected in different caves in the Middle Atlas range of Morocco and are characterized by well-preserved, aragonite-to-calcite (Ar–Cc), and calcite-to-aragonite (Cc–Ar) transitions within individual speleothems. The aim of this study is threefold: first, to present high-resolution elemental and isotope transects across aragonite-to-calcite transitions; second, to provide tentative interpretations of speleothem aragonite data in a process-oriented context; third to assess the significance of Ar–Cc and Cc–Ar transitions as archives of past climate change. 2. CASE SETTING 2.1. Present day climate With respect to its climatic setting, Morocco represents a complex and interesting study area as it is bordered by the North Atlantic to the west, the Mediterranean Sea to the north-east and the Sahara desert to the south-east. Morocco is characterized by the west-south-west to east-north-east trending Atlas mountain range subdivided in the Middle Atlas to the north-east, the High Atlas to the south-west and the Anti-Atlas to the south. The three speleothems discussed here are from caves located in the north-west of the Middle Atlas (Fig. 1A), according to Knippertz et al. (2003) this area falls within the Atlantic domain of Morocco. The present-day climate in the Middle Atlas region is characterized by dry summers and wet winters, related to the strength and position of the Azores subtropical high. Annual rainfall depends on altitude, with mountain ranges being wetter relative to lowlands. For the period 1999– 2008, average annual rainfall in the city of Taza, situated at 450 m above sea level and located in the vicinity of Grotte Prison de Chien (Fig. 1B), was 468 mm. Data were collected by the weather station in Taza and made available by the Institute for Geophysics and Meteorology, University of Cologne, Germany. At Bab Bou Idir, situated at an

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Fig. 1. Regional setting of Northern Morocco bordered by the North Atlantic to the west and the Mediterranean Sea to the north. MA = Middle Atlas; HA = High Atlas; AA = Anti Atlas. (A) Map modified after I. Sadalmelik. Location of study area indicated by square. (B) Geological map modified after Taous et al. (2009), stars indicate location of caves. 1 = Grotte Prison de Chien; 2 = Grotte de Piste. (C) Geological cross section modified after Taous et al. (2009) with indication of relative cave positions. Key to colour scheme in panels B and C: 1 = Palaeozoic schists and sandstones; 2 = Permian, Triassic clays and basalts; 3 = Lower Jurassic dolostones and limestones; 4 = Middle and Upper Jurassic limestones and marls; 5 = Middle Jurassic marls; 6 = Miocene units; 7 = Pliocene/Quaternary units; 8 = Quaternary basalts; 9 = Quaternary and recent units; 10 = fault structures.

altitude of 1500 m and located near the second cave site (Fig. 1B), an annual average 711 mm of rainfall was measured for the period 1999–2008. On decadal timescales, the North Atlantic Oscillation (NAO) plays an important role on the amount of winter rainfall (Ward et al., 1999). Decreasing rainfall amounts in the Atlantic and Mediterranean domains of Morocco after the 1970’s were related to a dominant positive NAO mode (Ward et al., 1999). 2.2. Cave parameters The stalagmites were retrieved from two caves in the Middle Atlas. These caves are referred to as “Grotte Prison de Chien” (Dog’s prison cave), situated at 360 m above sea level (Figs. 1B–C and 2A), and “Grotte de Piste” (Gravel road cave), situated at 1260 m above sea level (Figs. 1B–C and 2B–C). Grotte Prison de Chien lies within a predominantly calcitic (subordinate dolomite content) host rock - as based on XRD analysis – and is formed within Liassic brecciated marine limestones (Fig. 1B and C; Sabaoui et al., 2009; Taous et al., 2009). The cave is overlain by ca. 20 m of host rock and has several open connections with the outside atmosphere (Fig. 2A). The vegetation is restricted to small shrubs, locally small trees and grasses. Approximately 50% of the land surface above the cave is covered by up to 30 cm of lateritic soil; Elsewhere, the limestone host rock is exposed at the land surface. The cave has a steeply downward sloping entrance that is approximately 7 m in diameter.

During winter, a noticeable draft of incoming air is present. Cave air temperature varies on a seasonal scale between 12.2 °C in winter to 15.7 °C in summer, as based on one year of temperature measurements with a resolution of 12 h. These data suggest that the cave is dynamically ventilated. Grotte de Piste lies within a Lower Jurassic south-east dipping, dominantly dolomitic host rock with spatially limited limestone intervals (Fig. 1B and C; Sabaoui et al., 2009; Taous et al., 2009). The vegetation above the cave consists of small (i.e., <2 m tall) oak trees, shrubs and grasses. About 60% of the surface is covered by up to 20 cm of soil, elsewhere the dolomite host rock is exposed at the land surface. The dripwater entering the cave is of local origin due to the surface morphology above the cave, forming a topographic high, and the elevated position of the cave at the slope. The entrance of the cave is about 3 m in diameter and has a steep downward gradient (Fig. 2B). Cave ventilation is evident from the seasonally varying cave air temperature, which ranges between 10.7 (winter) and 12.3 °C (summer) at the bottom of the cave and between 11.8 (winter) and 13.3 °C (summer) at the 2nd cave level (Fig. 2B and C). 3. MATERIALS AND METHODS Mineralogies of cave hostrock and speleothems were determined by X-ray diffraction (XRD) at the Ruhr University Bochum, Germany. Approximately 20 mg of sample powder was drilled for XRD. Ten percent of quartz was

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Fig. 2. Cave maps and cross sections. (A) Map view of Grotte Prison de Chien (GC). (B) Cross section view of Grotte de Piste (GP). (C) Map view of Grotte de Piste. Sampling locations of stalagmites HK3, HK1 and GP2 are indicated.

added to the sample powder as a standard in order to derive offsets in the 104 calcite peaks, and estimate the Mg content within the crystal lattice in mol-% (Fu¨chtbauer and Richter, 1988). Subsequently the samples were homogenised in an agate mortar before being analysed. X-ray diffraction patterns were recorded with a Pananalytical MPD diffractometer, equipped with a copper tube, 0.5° divergent and antiscatter slits, a 0.2-mm high receiving slit, incident and diffracted beam 0.04 rad soller slits, and a secondary graphite monochromator as documented in Miao et al. (2009). In the terminology applied here, speleothem fabrics are referred to as “calcitic” if they contain P99% calcite with the remaining bulk fabric being formed by aragonite and subordinate amounts of clay minerals. Conversely, the term “aragonitic” implies P98% aragonite with the remaining bulk fabric being formed by calcite and subordinate amounts of clay minerals. Similar to this, cave host rocks are referred to as “limestone” if they are formed by at least 70% calcite, whereas the label “dolostone” is used for a host rock containing at least 96% dolomite. The three stalagmites examined in this study were sampled during a field campaign in March 2009, and were not active at the time of collection. Stalagmite HK1 was collected from Grotte Prison de Chien (Fig. 2A). HK1 is 56 cm long and 13 cm wide at its base (Fig. 3A). The stalagmite exhibits a mainly calcitic core with laterally increasing amounts of aragonite towards the flanks of the stalagmite. Therefore, this speleothem allows for the study of lateral changes in calcite and aragonite within single growth layers. Stalagmite HK3 was also collected from Grotte Prison de Chien, but from a different locality (Fig. 2A). HK3 is 26 cm long and 6 cm wide at its base (Fig. 3B). The stalagmite exhibits four aragonite layers alternating stratigraphically with calcite intervals. Two transitions were studied here, one Ar–Cc transition (27.4–17.5 mm) and one Cc–Ar transition (10.1–0.2 mm).

Stalagmite GP2 formed part of a column and was collected from Grotte de Piste, at the 2nd level of the cave (Fig. 2B and C). This speleothem is approximately 100 cm long and 8 cm wide at the base and exhibits one stratigraphic transition from calcite to aragonite. The three stalagmites were cut longitudinally and surfaces were polished. Macroscopically, calcite intervals appear darker whereas aragonite intervals are whitish in appearance (Fig. 3), Cc–Ar and Ar–Cc mineralogical transitions are sharp (i.e. take place over distances of some tens of microns only; Fig. 3). U-series dating of the speleothems was conducted at the IFM GEOMAR, Kiel, Germany, with an AXIOM MICICP-MS (multiple ion counting inductively coupled plasma mass spectrometer). Further methodological details can be found in Fietzke et al. (2005). Polished thin sections were first examined under a polarisation microscope and thereafter sputtered with gold and examined under a cathodoluminescence microscope at the Ruhr University Bochum, Germany. The cathodoluminescence microscope is equipped with a hot cathode (Neuser et al., 1996). Beam current densities were between 5 and 10 lA/mm2, with an acceleration potential of 14 kV. Carbonate detrital material containing traces of Mn range in color from yellow to red under the cathodoluminescence microscope, therefore it can easily be distinguished from the low-Mn speleothem calcite (dark blue) and the green aragonite (Richter et al., 2003). Other types of inclusions like organic material or clay minerals cannot be distinguished with this method. Important to distinguish from each other is the intercrystalline calcite within an aragonite layer which is formed in situ, and carbonate detrital material, which is derived from an allochthonous source. It is difficult to distuinguish the mineralogy of the carbonate detrital material, therefore we will continue to use the term carbonate detrital material.

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Fig. 3. Images of cut and polished speleothems. Positions of geochemical sampling transects for elemental abundances (black lines), isotope ratios (grey shading), XRD analysis (star), and 230Th/U data (arrows) are indicated. (A) Stalagmite HK1. (B) Stalagmite HK3. (C) Stalagmite GP2.

Carbon and oxygen isotope analyses were performed at the Ruhr University Bochum, Germany, with a Gasbench coupled to a Finnigan MAT 253 mass spectrometer. For sampling of the stratigraphical Cc–Ar and Ar–Cc transitions, a micromill (Merchantek, Esi-New Wave) equipped with a flat-tipped, 0.5 mm diameter dentist drill was used. Two 1-mm-deep trenches were drilled with a width of 5 mm (HK3) and 10 mm (GP2). The Ar–Cc and Cc–Ar transitions in stalagmite HK3 (Fig. 3B) were each sampled at a resolution of 100–200 lm over a 4 mm traverse. Sampling of these intervals was extended to both older and younger growth at 500 lm resolution. In stalagmite GP2, the interval 3 mm beneath to 1 mm above the transition was sampled at 100 lm resolution (Fig. 3C). This sampling transect was extended with a hand-held drill at 1 mm resolution in order to cover the same interval analysed for the trace elements. Carbon and oxygen isotope values are expressed in & with respect to the Vienna PDB (VPDB) standard. Sample aliquots weighing between 0.27 and 0.33 mg were dried in an oven at 105 °C for 48 hours. The vials were flushed with He in order to avoid atmospheric contamination. Phosphoric acid (104%) was added to the sample. CO1 and CO8 carbonate standards were used for correction, whereas the NBS19 and the RUB internal carbonate standards were used as a quality control. Four duplicates were analyzed for every sample batch of 48 samples, in order to check for sample homogeneity. Adding the averaged internal standard deviations derived from the analysis of nine peaks per sample, to the averaged difference of each duplicate, suggests a precision of ±0.08& for d13C in both stalagmites and ±0.09& (stalagmite GP2) and ±0.13& (stalagmite HK3) for d18O. Elemental abundances (Mg, Sr, Ba, P, Y, Pb, U, Th, Al, Ti) were analysed with a Thermo Finnigan Element 2 ICPMS at the Max Planck Institute for Chemistry, Mainz, Germany. The analysis is accurate as proved by Jochum

et al. (2012), who focussed on LA-ICP-MS analysis on carbonates including speleothems. Samples were ablated with a New Wave UP213 laser with an energy of 15.7 J/cm2. A round, 100 lm diameter spot was used for all measurements. The relatively large spot size was necessary to average out heterogeneities within a given growth increment (Finch et al., 2003; McMillan et al., 2005). In order to avoid an effect of surface contamination the first two to five scans of every single spot analysis were discarded. Total measurement time per spot analysis was between 100 and 105 s. Intensities or “counts per second” were corrected for background noise, therefore all data shown were significantly elevated above the background value and therefore above the detection limit. The NIST-612 glass reference material and the MACS3 and MACS1 carbonate reference material were measured 9–15 times equally distributed in the sequence in blocks of three individual spot analyses. The MACS1 reference material was not included in the sequence with stalagmite HK3. An averaged relative sensitivity factor (Jochum et al., 2007) from the NIST612 and MACS3 was used to derive absolute concentrations with the newest reference values (Jochum et al., 2011). Detection limits (Jochum et al., 2012) and relative uncertainties are shown in Table 1. The relative uncertainty is derived from the MACS1 elemental abundances, and is hereby defined as the relative standard deviation in percent (Table 1). The detection limits as published in Jochum et al. (2012) are here used as a reference value. If elemental concentrations are close to the referenced detection limit (concentration <10 times the detection limit; Jochum et al., 2012), we assumed a relative uncertainty of 20%, unless the MACS1 values are close to the detection limit and can provide a fundament for a more solid estimate. It has to be stated that detection limits may vary by a factor of three or four per sequence depending on the measurement conditions. In addition, the measured MACS1 elemental concentrations and uncertainties were very similar to earlier

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J.A. Wassenburg et al. / Geochimica et Cosmochimica Acta 92 (2012) 23–47 Table 1 LA-ICP-MS detection limits, and relative uncertainty. Element

Detection limit (ppm)a

Concentration MACS1

Uncertainty (1 standard deviation SD)

Relative uncertainty (1 RSD)b (%)

Al Ba Mg Pb Pb Sr Th Ti U Y

1 0.02 0.2 4 0.003 0.5 0.0002 0.5 0.0002 0.01

27.1 106 10.5 2.5 102 196 0.01 – 0.003 0.06

3.5 6 0.7 0.2 7 13 0.002 – 0.0007 0.01

13 6 7 8 7 7 20 20 23 17

a

Detection limits may vary by a factor of three or four depending on the measurement conditions, the values here were derived from Jochum et al. (2012). b For elements which are close to the referenced detection limits we assumed a relative uncertainty of 20%, unless the MACS1 can provide a fundament for an estimation (for example P).

published MACS1 values (Munksgaard et al., 2004; Mertz-Kraus et al., 2009; Table S1 in supplementary material). A comparison between parallel tracks of ICP-OES and LAICP-MS trace element data across the Cc–Ar transition in stalagmite GP2 shows that potential sample heterogeneity does not play a role (see Fig. S1 in supplementary material). Note that for chalcophile/siderophile elements with low boiling points, a matrix matched calibration is necessary to avoid matrix effects (Jochum et al., 2012). Lead was therefore only corrected with the MACS3 carbonate reference material. For more information on the method, accuracy and precision the reader is referred to Jochum et al. (2007, 2011, 2012) and Mertz-Kraus et al. (2009). In order to compare trace element transects with C and O isotope transects, the exact position of each Cc–Ar and Ar–Cc transition was used as a datum. For stalagmite GP2, the position of the trace elemental transect was parallel to the isotopic transect at a distance within 5 mm (Fig. 3C). For stalagmite HK3, the trace element transects were measured in the same traverse as the isotope data (Fig. 3B). Soil trace element concentrations were analysed from compressed powders consisting of eight g of sample material and one gram of ELVACITE resin. Analysis was performed on a wavelength dispersive X-ray fluorescence spectrometer (WD-XRF; Philips PW 2404) with a Rh-tube. Two soil samples were analysed for Grotte de Piste and one soil sample was analysed for Grotte Prison de Chien. Pearson correlation coefficients (r) have been calculated between all individual elements to identify similar properties of the trace element time series. Correlations are referred to as significant on the 1% significance level (p-value < 0.01) and r > 0.5, unless stated otherwise. For an overview of all correlation coefficients and significance levels, the reader is referred to Tables S2–S6 in the supplementary material. In addition, “Principal Component Analyses1” (PCA) has been performed (von Storch and

1 PCA are also referred in the literature as Empirical Orthogonal Functions (EOF)

Zwiers, 2002; Navarra and Simoncini, 2010). It is acknowledged that PCA represents a non-trivial and abstract statistical method. Therefore a basic explanation of the significance of PCA for the research shown here is given below. A PCA identifies patterns of simultaneous variations between (in this case) different trace element time series and finds a small subspace that contains most of the variability of the complete dataset. This small subspace is formed by the principal components, which explain most of the total variance of the data. Whereas total variance is defined as the cumulative of the variance from the individual trace element time series and can thus be regarded as the total variation of the complete dataset. The principal components can be regarded as new time series derived from a combination of different trace element time series. A single principal component may represent a single forcing mechanism (e.g. climate aridity). Principal components are independent from each other, and are ordered in a way, such that the 1st principal component explains most of the total variance and the 2nd principal component explains the 2nd most of the total variance (von Storch and Zwiers, 2002; Navarra and Simoncini, 2010). Therefore, the higher the explained total variance by the 1st and 2nd principal components the more dominant the forcing mechanisms become. Note, there are as many principal components as dimensions in the multi-dimensional data space (e.g., the number of different trace elements). In this study PCA has been used to visualize correlation patterns between different groups of trace elements by plotting the correlation coefficients of the individual trace element time series with respect to the first and second principal components. In these plots trace elements that are close to each other are in general positively correlated. When trace elements plot on opposite sites, they are generally negatively correlated to each other. In addition PCA has been used to identify the dominant forcing mechanisms affecting the trace element composition of the stalagmites. The advantage of studying PCA in addition to Pearson Correlation Coefficients is that the shape of each principal component versus distance (i.e., time) can be examined.

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Thus, if a principal component is identified as a dominant forcing mechanism (e.g., climate aridity or effective rainfall) higher values of the principal component represent higher climate aridity or lower effective rainfall. In order to be able to compare the trace element time series, and to account for the different amplitudes in their variability, the data was normalized in a way that the average of each trace element time series is zero and the standard deviation of each trace element time series is one (Navarra and Simoncini, 2010). The PCA in this study was performed using MATLAB. 4. RESULTS 4.1.

230

Th/U-dating

Table 2 presents all relevant 230Th/U ages. For dating, most of the sample material was collected from the aragonitic portion of the stalagmites. Since the aragonite contains relatively high amounts of the parent nuclide U (in the order of ppm’s), this results in small analytical errors. For all samples, the 230Th/232Th activity ratio is relatively high, indicating a minimum amount of initial detrital Th in the samples (Table 2). The base of stalagmite HK3 is dated at 27.5 ka BP, the top revealed an age of 4.2 ka BP. This stalagmite has a hiatus between 23.5 and 7.7 ka BP (Table 2; Fig. 3B). The four aragonite layers occur in a time window between 27.5 and 23.5 ka BP. The average precipitation rate in this interval is 17 lm/a. Therefore, a 100 lm spot size corresponds to on average 5.7 years per sample. The base of stalagmite HK1 revealed an age of 36.5 ka, the top was dated 18.9 ka BP. The lateral calcite-to-aragonite transitions studied here were dated 36.5 (base) and 33.4 (top) ka BP (Table 2). Stalagmite GP2 has been dated between 97.7 ka (base) and 2.5 ka (top) BP, a hiatus exists between 44.8 and 11.5 ka BP. At around 2.5 ka BP, the upward growing stalagmite and the downward growing stalactite connected.

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The calcite-to-aragonite transition was dated between 11.5 (base) and 11.1 (top) ka BP (Table 2). During this time period, the average growth rate was 40 lm/year, thus a 100 lm spot size corresponds to 2.5 years. 4.2. Soil mineralogy and trace element composition At present, the soil above Grotte Prison de Chien consists of organic plant debris with mineral components of muscovite, kaolinite in which quartz is volumetrically dominant. Trace element concentrations of the soil are shown in Table 3. Both muscovite (alkaline earths) and kaolinite (alkalis, alkaline earths) may act as a source material for a range of trace elements, suggesting that the soil is an important source in addition to the cave host rock. For Grotte de Piste, the main mineral composition of the soil consists of quartz, muscovite, chlorite, albite, kaolinite and unspecified Fe-oxides/hydroxides. This mineralogical composition again suggests that trace elements in speleothems could be provided by the soil in addition to the cave host rock. Table 3 lists trace element concentrations of the soil above Grotte de Piste. It is acknowledged that mass balance calculations of soil-derived versus cave Table 3 Soil trace element concentrations. Element

Grotte Prison de Chien

Grotte de Piste

SGC 1

SGP1

SGP2

Al2O3 (%) TiO2 (%) MgO (%) P2O5 (%) Pb (ppm) Th (ppm) Y (ppm) U (ppm) Sr (ppm) Ba (ppm)

19.82 1.73 1.12 0.99 63 16 54 3 82 349

19.43 2.16 3.46 0.48 102 19 48 7 75 292

19.15 2.16 3.06 0.28 73 15 47 7 66 256

Table 2 Results from U/Th dating. Sample

Mineralogy

Depth (mm)

238

(230Th/232Th)

(234U/238U)

Initial (234U/238U)

Age (ka)

GP2U3.1 GP2U1.3 GP2U1.2 GP2U1.1 GP2U1 HK3U7 HK3U3 HK3U2 HK3U1.1 HK3U1 HK1U6 HK1U2 HK1U1

Aragonite Aragonite Calcite Calcitea Calcite Calcite Calcite Aragonite Aragonite Aragonite Aragonite Aragonite Calcite

7 582 597 613 987 6 95 156 186 224 16 444 531

1.485 ± t0.002 1.374 ± 0.002 0.013 ± 0.000 0.047 ± 0.000 0.029 ± 0.000 0.234 ± 0.000 0.239 ± 0.000 6.340 ± 0.010 36.996 ± 0.068 10.169 ± 0.020 12.536 ± 0.025 11.041 ± 0.025 0.353 ± 0.000

5345 ± 48 74688 ± 2131 380 ± 6 828 ± 4 9636 ± 1104 82 ± 1 52 ± 0 3710 ± 19 3255 ± 11 13363 ± 107 15662 ± 165 261630 ± 29276 3088 ± 118

6.000 ± 0.012 7.000 ± 0.010 7.050 ± 0.030 5.127 ± 0.011 5.624 ± 0.025 1.313 ± 0.003 1.430 ± 0.002 1.358 ± 0.003 1.020 ± 0.003 1.326 ± 0.003 1.260 ± 0.003 1.200 ± 0.003 1.250 ± 0.003

6.04 7.19 7.25 6.25 6.44 1.32 1.44 1.38 1.02 1.35 1.27 1.22 1.28

2.78 ± 0.02 11.11 ± 0.06 11.49 ± 0.17 44.77 ± 0.57 97.74 ± 0.98 4.24 ± 0.05 7.66 ± 0.07 23.53 ± 0.17 14.36 ± 0.09 27.48 ± 0.20 18.87 ± 0.14 33.41 ± 0.29 36.47 ± 0.26

U (ppm)

Decay constants used: k230 = 9.158  106 y1, k232 = 4.9475  1011 y1, k234 = 2.8263  106 y1, k238 = 1.5513  1010 y1. For the correction of detrital 230Th a 230Th/232Th activity ratio of 0.6 ± 0.2 was used. a Based on macroscopic observation.

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host rock-derived elements might provide useful insights. Given the lack of quantitative data from leaching experiments tailored to the specific geological conditions found in the study areas, however we conclude that this approach is beyond the scope of our work. 4.3. Stalagmites HK1 and HK3 (Grotte Prison de Chien) 4.3.1. Petrography In speleothem HK1, both calcite and aragonite was identified by XRD. Calcite layers contain up to 1.9 mol-%

MgCO3, whereas intercrystalline calcite within the aragonite contains up to 1.3 mol-% MgCO3. Examination from a thin section of a lateral calcite-to-aragonite transition revealed that (i) the mineralogy of both carbonate phases is primary and (ii) no petrographic evidence for secondary dissolution or calcitization of aragonite fabrics was found (Fig. 4A and B). Within the calcite crystals relict aragonite needles cannot be identified, note that the thin section was taken from a position off the growth axis, therefore a dominant growth direction cannot be observed in this thin section. In speleothem HK3, both calcite and aragonite were identified by XRD (Fig. 3B). The calcite layers contain up to 3.25 mol-% MgCO3, whereas the intercrystalline calcite within the aragonite layers contains up to 4 mol-% MgCO3. Thin sections across Ar–Cc and Cc–Ar transitions reveal a fibrous morphology of aragonite. The fibres have a lengthto-width ratio >6. Furthermore, a sweeping extinction across several crystals is observed under the microscope with crossed nichols. This is in agreement with acicular fabrics as described in Frisia and Borsato (2010). Fig. 4C–J gives an overview of the observed fabrics. Inclusions of carbonate detrital material are evenly distributed in the aragonite and increase in abundance close to the flank of the stalagmite (Fig. 4J) as evidenced by the yellowish to red spots under cathodoluminescence microscopy (Fig. 4D, F and J). Solid inclusions forming layers were not observed. Near the Ar–Cc transition, the abundance of intercrystalline calcite between aragonite fibres and the amount of carbonate detrital material both increase (Fig. 4D). Under the cathodoluminescence microscope, the carbonate detrital

3 Fig. 4. Stalagmite HK1 and HK3. Thin section petrography. Red arrows indicate transitions. Calcite is blue, aragonite is green under cathodoluminescence. (A and B) Lateral calcite (Cc) to aragonite (Ar) transition stalagmite HK1. (A) View under plane polarized light. (B) As A under crossed nichols. Thin section was taken from a position off the growth axis of stalagmite HK1, therefore calcite does not show a clear growth direction. (C–J) Petrography stalagmite HK3. (C) Overview of aragonite (dark; Ar) to calcite (light; Cc) transition under plane polarized light. (D) Same as in C, under cathodoluminescence. In the transition interval from aragonite (Ar) towards calcite (Cc) increasing abundance of carbonate detrital material is indicated by increased abundance of yellowish to red spots. (E) Overview of calcite (Cc) to aragonite (Ar) transition under plane polarized light. Porosity (light brown) in calcite is visible. (F) Same section as in E under cathodoluminescence. Porosity (grey; Pr) in calcite is visible. (G) Horizontal layering in calcitic portion under plane polarized light. Red line indicates a distinct transition. (H) As G but under cathodoluminescence. Layering is formed by luminescent detrital carbonate particles and micro- and macro-pores (Pr) and possibly non luminescent material. (I) Columnar to elongated columnar calcite crystals with high undulosity under crossed nichols (Un). Undulosity points to primary nature of the calcite. (J) Cathodoluminescence image taken close to the flank of stalagmite HK3 showing the increased abundance of carbonate detrital material (yellowish to red spots) with slower growth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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material appears as yellowish to red spots, respectively (Richter et al., 2003; Fig. 4D, F, H and J). The calcite crystals forming speleothem HK3 are characterized by columnar to elongated columnar fabric (Frisia and Borsato, 2010). A sweeping extinction across multiple crystals and within every individual crystal (undulosity) is observed under crossed nichols (Fig. 4I). Based on their characteristic extinction pattern, these calcites represent radiaxial fibrous fabrics, which are characterized by crystals with converging C-axes in the direction of growth (Neuser and Richter, 2007; Richter et al., 2011). The radiaxial calcites in sample HK3 contains carbonate detrital material as evidenced by the yellowish to red spots under the cathodoluminescence microscopy (Fig. 4H) and are characterized by layers of macro- and micro-pores (Fig. 4G and H). Neither crystals with round features, nor relicts of aragonite needles within calcite crystals, nor blocky calcite lacking a dominant growth direction has been observed (Fig 4). Therefore, evidence for dissolution or recrystallisation of aragonite is lacking, which is considered solid evidence that the aragonite represents a primary fabric. 4.3.2. Geochemistry Two lateral Cc–Ar transitions within individual growth increments in stalagmite HK1 were analysed in order to assess differences in geochemical concentrations across these intervals. Analytical results are presented in Fig. 5. Trends within the calcite or aragonite intervals may be induced by the fact that the trace element transect was not entirely perpendicular to the growth axis. Across the lateral Cc–Ar transitions, however, Mg concentrations decrease approximately by a factor of 60–90, Ba by a factor of 4–5, Sr by a factor of 7–9 and U by a factor of 40–60. In addition, P and Y increase as well, whereas Pb and Al concentrations remain similar (Fig. 5). The elements Ti, and Th were not significantly elevated above background level during measurement. Therefore, Ti and Th are not shown in Fig. 5. Stalagmite HK3 exhibits four aragonite layers alternating with calcitic intervals that range in age between 27.5 and 23.5 ka BP. Barium, Sr, U (Fig. 6), Al and Ti (Fig. 7) display a distinct change within only 100s of lm across the transitions. With regard to P, Y (Fig. 6) and Pb (Fig. 7) shifts in elemental abundance are only present across Ar–Cc transitions. Magnesium shows a gradually increasing concentration at the Ar–Cc transition, but a very pronounced shift at the Cc–Ar transition. Carbon and oxygen isotope ratios decrease across the Ar–Cc transition, whereas especially carbon isotope ratios increase at the Cc–Ar transition (Fig. 6). In the aragonite section of the Ar–Cc transition, the total variation in the data explained by the first two principal components is 64%. The 1st principal component is made up by Al, Pb, Mg and Sr and U and the 2nd principal component is made up by P and Y (Fig. 8A). Aluminium and Pb are strongly correlated to each other, but only weakly correlated to Mg. Strontium is positively correlated to U and weakly correlated to P (Fig. 8A). Negative correlations exist between U and Al and Pb. Phosphorus and Y are not correlated to each other. Carbon and oxygen isotope values lack a consistent co-variation with elemental patterns

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(Fig. 6). Titanium and Th are mainly below the detection limits and are therefore not further discussed. In the calcitic section, close to the Cc–Ar transition, Mg, Ba and Sr display an increasing trend towards aragonite, similar to carbon and oxygen isotopes, whereas P and Y show a decreasing trend (Fig. 9). The total variation in the data explained by the first two principal components is 77%. Whereas the 1st principal component is made up by Mg, Ba, Sr, P and Y and the 2nd principal component is made up by Al, Ti, Th and Pb (Fig. 8B). Magnesium, Ba, Sr and P and Y show a strong positive correlation to

Fig. 5. Stalagmite HK1, trace elemental concentration in lateral calcite-to-aragonite transitions. From core to flank is to the left, as indicated by the arrow. Horizontal scale indicates distance in mm from aragonite-to-calcite transition. Transect HK1-CA1 is indicated by triangles, HK1-CA2 is indicated by open circles. Aragonite is shown by light grey, calcite by dark grey shading. Calcite is present in the core portions of the stalagmite, aragonite at the flanks. Note, U and Mg are plotted on logarithmic scales in order to reveal the lowest concentrations. Relative uncertainty is not shown in order to be able to compare the two transects.

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each other, whereas P and Y are both negatively correlated to Mg, Ba and Sr (Fig. 8B). Aluminium, Ti, Th and Pb are strongly positively correlated to each other (Fig. 8B). In the aragonitic section from the Cc–Ar transition, d13C, Mg, Sr and Ba co-vary (Fig. 10), Ba is positively correlated to Mg and Sr. The total variation in the data explained by the first two principal components is 60% (Fig. 8C). In general, correlations between elements are low. The 1st principal component is made up by Mg, Sr, Y, Ba, Pb and Al. Phosphorus and U make up the 2nd principal component and show a weak positive correlation to each other. Titanium and Th are mainly below the detection limits and are therefore not discussed (Fig. 7).

Fig. 7. Stalagmite HK3: trace elemental data of Th, Pb, Ti and Al. Stratigraphic top is to the right, as indicated by the arrow. Thorium and Ti values marked by the transparent grey bar were not significantly elevated above the background. Aragonite is shown by light grey, calcite by dark grey shading, white refers to the transition zone (as based on Mg, and U concentrations). Relative uncertainty is indicated by grey shading.

4.4. Stalagmite GP2 (Grotte de Piste)

Fig. 6. Stalagmite HK3: trace elemental data of Mg, Ba, Sr, Y, P, U and carbon and oxygen isotope ratios. Stratigraphic top is to the right, as indicated by the arrow. Aragonite is shown by light grey, calcite by dark grey shading, white refers to transition zone (as based on Mg, and U concentrations). Note, that Mg, Ba, Sr, U and Y are plotted on logarithmic scales, whereas P, carbon and oxygen isotope ratios are plotted on linear scales. Relative uncertainty is indicated by grey shading.

4.4.1. Petrography In speleothem GP2, both calcite and aragonite were identified by XRD for those intervals that were later sampled for trace elemental and isotope analyses. The calcite layer contains up to 4.65 mol-% MgCO3, whereas the intercrystalline calcite within the aragonite contains up to 3 mol% MgCO3. Fig. 11A and B provides an overview on the spatial distribution of the different carbonate mineralogies. After the transition from calcite to aragonite, the mineralogy remains aragonitic up to the tip of the stalagmite. The columnar to elongated columnar calcite crystals (Frisia and Borsato, 2010) show a sweeping extinction over several crystals (Fig. 11C). Undulosity is observed for individual calcite crystals (Fig. 11C). This is evidence for their radiaxial fibrous fabrics with converging C-axes in the direction of growth as previously described from speleothems in Germany (Neuser and Richter, 2007; Richter et al., 2011). The calcite is characterized by low porosity and minor amounts of detrital material, as evidenced by the lack of yellowish to red spots under the CL-microscope as well as the lack of non light transmissive inclusions within the calcite crystals (Fig. 11A–C). The change in mineralogy from calcite to aragonite takes place over a distance of some tens of microns only. The fibrous aragonitic fabric is organised in fans characterized by a sweeping extinction pattern extending over several crystal fibres (Fig. 11A and B). Following Frisia and Borsato (2010) this fabric is classified as acicular. Porosity in the aragonitic fabric is most common between clusters of fans. 4.4.2. Geochemistry The transition from calcite to aragonite in stalagmite GP2 occurred between 11.5 and 11.1 ka BP. At the

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Fig. 8. Correlation plots between trace element time series and the 1st and 2nd principal components from stalagmite HK3 and GP2. The percentage of total data variation explained by the respective principal component is indicated. Trace elements which plot close to each other are generally positively correlated. (A) Aragonite from the aragonite-to-calcite transition stalagmite HK3. (B) Calcite stalagmite HK3. (C) Aragonite from the calcite-to-aragonite transition stalagmite HK3. (D) Calcite stalagmite GP2. (E) Aragonite stalagmite GP2. The specific intervals analysed by the PCA were defined by the Mg and U concentrations in order to avoid data representing an admixture of calcite and aragonite.

transition from calcite to aragonite, Ba, Sr, U, P, Y and Pb show a strong increase in concentration, whereas Mg and Al display a strong decrease (Fig. 12). Especially carbon isotope ratios increase across the Cc–Ar transition, oxygen isotope ratios show a more continuous increase. In the calcite interval, d13C values display a trend towards more negative values (a decrease of 2&) followed by a minor increase near the transition to aragonite (Fig. 13). Oxygen isotope ratios show a small variation. Magnesium, Ba and Sr show an increasing trend towards the Cc–Ar transition, whereas Y, P, Pb, U and Al show a decreasing trend (Fig. 13). The total variation explained by the first two principal components is 83% (Fig. 8D). The 1st principal component is made up by P, Y, Pb, Al and Mg, Ba and Sr. The 2nd principal component is made up by Al and U (Fig. 8D). Phosphorus, Y, Pb, and Mg, Ba and Sr are positively correlated to each other, whereas Mg, Ba, Sr are negatively correlated to P, Y and Pb (Mg versus Pb: r = -0.49; Fig. 8D). In the aragonite interval, Sr, P, Y, Pb and U show a decreasing trend after the transition (Fig. 14). Magnesium, Ba and Al show no consistent co-variation with any other elements described here. Carbon and oxygen isotope ratios co-vary and shift towards more positive values after the calcite-to-aragonite transition. Above a sampling depth of 583 mm both d13C and d18O become more negative. The total variation explained by the first two principal components is 74% (Fig. 8E). The 1st principal component is made up by Sr, P, Y, Pb and U. The 2nd principal component is made up by P and Ba (Fig. 8E). Aluminium is not incorporated in the PCA, because it is not correlated to any of the other trace elements. Strontium is positively correlated to P, Y, Pb and U (Fig. 8e).

5. INTERPRETATION AND DISCUSSION 5.1. Aragonite diagenesis Aragonite is thermodynamically unstable under atmospheric pressure and surface temperature conditions. Therefore, aragonite may be dissolved or re-crystallised to calcite or another, more stable carbonate mineralogy (Frisia et al., 2002; Martin-Garcia et al., 2009). Any study of fossil aragonitic speleothems must, thus, assess the degree of postdepositional diagenetic alteration. The most powerful screening tool for the detection of diagenetic aragonite alteration is careful thin section petrography. Acicular fibres of aragonite can be affected by diagenetic calcitization and micritization. Both of these features are recognizable in thin sections. In the case of the Moroccan speleothems examined here, none of the thin sections show the abovementioned petrographic evidence for diagenetic alteration of aragonite (Figs. 4 and 11). More complex and far more difficult to trace, however, is fabric-preserving diagenetic remobilization of specific elements and isotopes. Detailed investigation of fossil aragonitic corals has shown that the U-series system is more sensitive to post-depositional diagenetic change than any other petrographic or general geochemical parameter (e.g., Chen et al., 1991; Fruijtier et al., 2000; Scholz and Mangini, 2007). A spatially localized example for fabricpreserving geochemical remobilization of U and/or Th isotopes is found in stalagmite HK3. 230Th/U-dating of the aragonite at one of the aragonite-to-calcite transitions resulted in an age that was – in comparison to nearby age data – apparently 10 ka too young (Table 2). Interestingly, the U concentration of this specific interval is 37 ppm,

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Fig. 9. Stalagmite HK3 calcite from calcite-to-aragonite transition: trace elemental data of Mg, Ba, Sr, Y, P, U and carbon and oxygen isotope ratios. Stratigraphic up is to the right as indicated by the arrow. Aragonite is shown by light grey, calcite by dark grey shading, white refers to the transition zone (as based on Mg, and U concentrations). Relative uncertainty is indicated by grey shading. Note, Y is plotted on a logarithmic scale. Note the strong covarying patterns between Mg, Ba, Sr and d13C, which is interpreted as a prior calcite precipitation effect, see text Section 5.2.1 for discussion.

which is four times higher than the average U concentration in HK3 aragonite based on LA-ICP-MS analysis. This may be evidence for post-depositional U-redistribution, which has been observed in fossil reef corals (Scholz et al., 2007). Apart from this localized feature, no further geochemical or petrographic evidence for diagenesis was observed in any of the speleothems studied here. Moreover, the very sharp boundary of the transitions in the trace element concentrations serves as an additional argument against fabric preserving remobilization. In addition, the amount of

Fig. 10. Stalagmite HK3 aragonite from calcite-to-aragonite transition: trace elemental data of Mg, Ba, Sr, Y, P, U and carbon and oxygen isotope ratios. Stratigraphic up is to the right as indicated by the arrow. Aragonite is shown by light grey, calcite by dark grey shading, white refers to the transition zone (as based on Mg and U concentrations). Relative uncertainty is indicated by grey shading.

inter-crystalline calcite within the aragonite intervals is below 2%. Assuming calcite and aragonite end member values for elemental abundances based on the observed difference at the lateral and stratigraphic Cc–Ar and Ar–Cc transitions, simple mass balance calculations suggest that 2% of primary calcite within an aragonitic fabric may affect Mg and U but the effect on other elements is minimal (see supplementary material for calculations and Fig. S2). If the 2% calcite represents calcitized aragonite, then the effect on the

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Fig. 11. Stalagmite GP2, thin section petrography. Stratigraphic up is to the top. (A) Image of calcite-to-aragonite transition under plane polarized light. Note fan-like texture of aragonite (Ar) fibres and clear calcite (Cc) crystal shape at the black arrow oriented in the growth direction, which indicates that this calcite is a primary calcite. Red arrows indicate position of transition. Porosity is indicated (Pr). (B) Same as A under cathodoluminescence. (C) Image of undulous (Un), columnar to elongated columnar calcite crystals under crossed polarized light. Undulosity is evidence for the primary nature of the calcite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

elemental abundances is very small as the composition of this calcite is most likely close to that from the original aragonite. Calcitized aragonite is characterized by low Mg calcite (0.6–1.6 mol-% MgCO3; XRD; Niggemann and Richter, 2006). In the stalagmites studied here, inter-crystalline calcite in aragonite layers probably represent a mixture of both primary and secondary calcite as XRD analysis indicated a range of 0.5–4.6 mol% of MgCO3 for the inter-crystalline calcite. The samples analysed with XRD suggest that the amount of inter-crystalline calcite does not exceed 2%. Therefore, it can be concluded that aragonite in the Moroccan speleothems discussed here is well preserved both with respect to its fabric and geochemistry. Small-scale, spatially localized geochemical remobilization cannot be excluded but is not considered significant for the intervals analysed here. 5.2. Climate forcing of alternating calcite and aragonite precipitation? The key to understand the reason for aragonite precipitation lies in the calcitic section just before the occurrence

of the aragonite. Thus, we focus here on the calcite from the Cc–Ar transitions. Trace elements correlated and grouped by Principal Component Analysis (Fig. 8), are assumed to reflect a common process. Therefore, the following clusters of elements will be discussed and placed in the context of their suggested drivers: (i) Mg, Sr, Ba in the context of PCP; (ii) P and Y in the context of vegetation decay; and (iii) Al, Th, Ti, Pb in the context of clay minerals. 5.2.1. Magnesium Strontium and Barium, relation to prior calcite precipitation Prior calcite precipitation refers to the process of calcite precipitation from aquifer water prior to reaching the stalagmite. This includes precipitation of stalactites at the cave ceiling. Prior calcite precipitation takes place if the aquifer water encounters a gas phase with lower pCO2, which causes CO2 degassing that in turn leads to fluid super-saturation with respect to CaCO3 and precipitation of calcite from the liquid phase (Fairchild and Treble, 2009). As a consequence, PCP affects the CaCO3 saturation state of the dripwater. Furthermore, since the partition coefficients of Mg, Sr and Ba are smaller compared to that of Ca during calcite precipitation, PCP will increase the Mg/Ca, Sr/Ca

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Fig. 12. Stalagmite GP2, trace elemental data of Mg, Ba, Sr, U, P, Y, Pb, Al and carbon and oxygen isotope ratios. Stratigraphic up is to the right as indicated by the arrow. Horizontal scale is distance in mm from top of speleothem. Aragonite is shown by light grey, calcite by dark grey shading, white refers to the transition zone (as based on Mg, and U concentrations). Note, that P and carbon and oxygen isotope ratios are plotted on linear scales, whereas Mg, Ba, Sr, U, Y, Pb, and Al are plotted on logarithmic scales in order to reveal the lowest concentrations. Relative uncertainty is indicated by grey shading surrounding the plots, except for the isotope ratios where relative uncertainties are very small.

and the Ba/Ca ratios of the dripwater. Thus, a pronounced positive correlation between speleothem Mg, Sr and Ba

Fig. 13. Stalagmite GP2 calcite: trace elemental data of Mg, Ba, Sr, Y, P, Pb, U, Al and carbon and oxygen isotope ratios. Aragonite is shown by light grey, calcite by dark grey shading, white refers to the transition zone (as based on Mg, and U concentrations). Note that Y and Pb are plotted on logarithmic scales. Relative uncertainty is indicated by grey shading.

concentrations has been interpreted in terms of variable amounts of PCP (Tooth and Fairchild, 2003; Mcmillan et al., 2005; Wong et al., 2011). Since PCP affects the Mg/ Ca ratio and the CaCO3 saturation state of the dripwater, PCP may be an essential process for inducing aragonite pre-

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Fig. 14. Stalagmite GP2 aragonite, trace elemental data of Mg, Ba, Sr, U, P, Y, Pb, Al and carbon and oxygen isotope ratios. Aragonite is shown by light grey, calcite by dark grey shading, white refers to the transition zone (as based on Mg, and U concentrations). Note that Y and Pb are plotted on logarithmic scales. Relative uncertainty is indicated by grey shading surrounding the plots.

cipitation. Prior calcite precipitation may occur as a consequence of increasingly arid climate (Fairchild et al., 2000) resulting in reduced drip rates. Increasing air temperatures, however, may increase soil CO2 production (Pinol et al., 1995), which could enhance PCP as well. Finally, the effect of re-dissolving carbonate formed during PCP on dripwater chemistry remains difficult to quantify.

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Prior calcite precipitation also affects the d13C value of the dissolved inorganic carbon of the dripwater (Johnson et al., 2006; Scholz et al., 2009; Dreybrodt and Scholz, 2011). Dripwater d13C, however, may also be influenced by several other processes including relative changes between C3 and C4 type vegetation above the cave (Dorale et al., 1992; McDermott, 2004), (micro-) biological activity in the soil zone and the carbonate aquifer (Genty et al., 2006), kinetic effects modulated by changes in drip rate (Mu¨hlinghaus et al., 2007, 2009) and cave ventilation (Spo¨tl et al., 2005; Frisia et al., 2011). Enhanced PCP due to increasing aridity likely coincides with longer water residence time in the aquifer, higher evaporation rates, decreasing drip rates and soil zone activity as well as an increase of C4 type vegetation relative to C3 type vegetation (Fairchild et al., 2000; McMillan et al., 2005; Fairchild and Treble, 2009). All these processes commonly result in increasing d13C values of the dripwater. The calcitic portion of speleothem HK3 is characterized by a strong co-variation between the elements Mg, Sr, Ba and the carbon isotope ratios (Fig. 9). This pattern is considered evidence for PCP. Following the above considerations, the observation that the highest Mg, Sr and Ba concentrations as well as high d13C values occur just before the transition of calcite to aragonite (Fig. 9) suggests that enhanced PCP was of major significance for the onset of aragonite precipitation in stalagmite HK3 (Fig. 15B). A similar relation is observed for the elements Mg, Sr and Ba in the calcitic sections of stalagmite GP2 (Fig. 13). Applying the previous lines of evidence, the presence of aragonite in stalagmite GP2 is probably also related to enhanced PCP. Stalagmites HK3 and GP2, however, differ from each other in one significant aspect. d13C values do not co-vary with Mg, Sr and Ba in stalagmite GP2 (Fig. 13). This may imply that, in the case of stalagmite GP2, PCP is not the dominant process affecting dripwater carbon isotope ratios. Furthermore, the amount of PCP required to induce aragonite precipitation is probably smaller in Grotte de Piste compared to Grotte Prison de Chien. This is because of the different host rocks, which is a Mg-rich dolomite in the case of the Grotte de Piste (speleothem GP2) and low-Mg limestone in the case of the Grotte Prison de Chien (speleothem HK3). In addition, the concentrations of Mg in the calcite just before the Cc–Ar transition are for both stalagmites approximately 8500–9500 ppm (Figs. 9 and 13). This may imply that a certain dripwater Mg/Ca ratio threshold has to be reached in order to precipitate aragonite. This threshold may be calculated assuming a partition coefficient for Mg in calcite of 0.019 at 15 °C, or 0.031 at 25 °C (Huang and Fairchild, 2001). Calcite containing 9000 ppm Mg could therefore be precipitated from dripwater with a molar Mg/Ca ratio of 2.1 or 1.3, respectively. In particular the calculated dripwater Mg/Ca ratio at 25 °C is in good agreement with monitoring data from Frisia et al. (2002). Given the fact that paleo-temperature data for the caves investigated are not well constrained, the correct partition coefficient remains difficult to assess. In summary, Mg, Sr, and Ba elemental data provide strong evidence that enhanced PCP induced aragonite precipitation in both stalagmites. In order to assess whether

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the amount of PCP is related to changes in (i) aridity, (ii) temperature related CO2 production or (iii) changes in local hydrology, other trace elements not affected by PCP (e.g. P and Y), may provide important information. 5.2.2. Phosphorus and Yttrium, relation to vegetation decay Fairchild et al. (2001) and Treble et al. (2003) suggested that speleothem P concentrations and vegetation decay are related. Treble et al. (2003) observed that for regions, where vegetation productivity is stressed by water availability, speleothem P concentrations decreased in years with lower

rainfall. Furthermore, Borsato et al. (2007) concluded that at Grotta di Ernesto (Italy) elevated speleothem P and Y concentrations are related to enhanced transport of (organic) colloids from the soil during the infiltration season, where P most likely originates from microbial breakdown of organic matter. Borsato et al. (2007, pp. 1507) also noted that the “. . .incorporation of P into calcite depends on the relative proportions of free ion versus inorganic and organic colloidal forms. . .” Furthermore, Huang et al. (2001) suggested that P was available as phosphate ions. In the view of the authors, it therefore remains an open question how

Fig. 15. Schematic graphic summary of geochemical patterns in Moroccan speleothems across calcite-to-aragonite transitions and their respective drivers. Stratigraphic up is to the right. Summary is based on the observed first order trends in stalagmite HK3 and GP2. Prior calcite precipitation = PCP; Prior aragonite precipitation = PAP. (A) Carbon and oxygen isotope ratios in stalagmite HK3. (B) Magnesium, Ba and Sr concentrations in stalagmite HK3. (C) Phosphporus, Y and Sr abundances in stalagmite HK3. (D) Carbon and oxygen isotope ratios in stalagmite GP2. (E) Magnesium, Ba, and Sr abundances in stalagmite GP2. (F) Phosphorus, Y and Sr abundances in stalagmite GP2.

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P is actually transported and incorporated into calcite or aragonite (De Kanel and Morse, 1978; Huang et al., 2001; Millero et al., 2001). This may be different between different cave sites. Finally, Treble et al. (2005) identified growth rate related annual patterns of Na, Sr, Ba and U in a stalagmite from Moondyne Cave. Phosphorus did not show annual banding, which may suggest that the incorporation of P into speleothem calcite is not related to changes in speleothem growth rates. It must be noted, however, that P can be transported and incorporated through several mechanisms, and that for example the amount of defect sites under high growth rates could increase, whereas the adsorption potential may remain similar. This still requires more research. The source of Y differs from that of P but has similar chemical properties and atomic structure compared to the rare earth elements (Zhou et al., 2008a), which have the tendency to become adsorbed to organic matter (McCarthy et al., 1998; Tyler, 2004). Therefore, a positive correlation between P and Y in speleothems is indicative of soil-derived organics in cave dripwater (Borsato et al., 2007; Zhou et al., 2008a). Note that the unspecific label “organics” is used here because it remains unclear whether P is transported in form of organic colloids or as organic matter. The source of these organics is, as indicated above, most likely decaying plant remains in the soil zone above the cave. Table 3 provides evidence that the soil above a cave may act as a source for Y. Phosphorus and Y are positively correlated in calcitic intervals of stalagmite HK3 (Fig. 8B). Following the above discussion, we propose that these elements reflect the incorporation of organics in the crystal lattice or inter-crystalline organics. In the calcite interval, near the calcite-to-aragonite transition, both P and Y concentrations are decreasing towards the aragonite interval (Fig. 9). A possible interpretation for this feature may be related to decreasing microbial break down of organic matter in the soil zone. This potentially reflects an effect of decreasing temperature or a decrease in effective rainfall. It must be emphasized here that increasing temperatures could increase both the microbial breakdown of organic matter (thus increasing stalagmite P concentrations) and soil CO2 concentrations (thus enhancing PCP), provided that the available amount of moisture in the soil is sufficient to sustain microbial activity and soil zone CO2 production. Because of the negative correlation between P and Y with Mg, Sr and Ba (Fig. 8B), it is suggested that PCP was induced by increasingly drier climatic conditions rather than changes in local hydrology or increasing temperatures (Fig. 15A–C). The 1st principal component in the calcite section of HK3 made up by Mg, Ba, Sr and P, and Y is therefore representing effective rainfall. Higher values represent lower effective rainfall and vice versa, because the highest values occur just before the Cc– Ar transition, aragonite started to precipitate at the moment effective rainfall was low (Fig. 16). Similar to stalagmite HK3, a negative correlation between P, Y and Mg, Sr and Ba is observed in the calcite layer for stalagmite GP2 (Fig. 8D). This recurrent pattern, combined with low P and Y concentrations in the calcitic speleothem close to the calcite-to-aragonite transition, is

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indicative of enhanced PCP driven by increasing aridity (Figs. 13 and 15D–F). It can therefore be concluded that the 1st principal component in the calcite section of stalagmite GP2 made up by P, Y, Pb, Mg, Sr, and Ba is, similar to stalagmite HK3, representing effective rainfall. Lower values indicate lower effective rainfall and vice versa. The lowest values occur just before the Cc–Ar transition, indicating that aragonite started to precipitate at the moment that effective rainfall was relatively low (Fig. 16). The forcing mechanism behind the 2nd principal component and the negative correlation between Mg and U remains unclear. Due to the differences in the host rock mineralogies in the two caves in Morocco, we suggest that – in the case of stalagmite GP2 – only a moderate decrease in effective rainfall was required to induce aragonite precipitation. This is due to the considerably higher Mg content of the dolomite host rock of Grotte de Piste. Evidence for this comes from the observation that speleothem GP2 d13C values lack a co-variation with Mg, Sr, Ba, P or Y (Figs. 13 and 15D– F), showing that changes in mean drip rates were not sufficient to induce a significant additional amount of CO2 degassing. Instead speleothem GP2 d13C values must have been dominated by other processes. Across the transition from calcite to aragonite in stalagmite GP2, P, Y and Pb concentrations increase with increasing aragonite content (Fig. 12). This was also observed for P and Y in the lateral calcite-to-aragonite transitions in stalagmite HK1 (Fig. 5), but not in stalagmite HK3 (Fig. 6). This observation suggests a crystallographic forcing on P and Y. It can, however, not be explained by simple substitution for Ca, due to the different valence of P (3+ or 5+) and Y (3+) compared to Ca (2+), and for P its smaller ionic radius. Therefore the shift to higher P and Y concentrations at the transition from calcite to aragonite may be related to differences in adsorption potential between calcite and aragonite, the amount of defect sites available for organics or competition effects. The reason why this pattern is not observed in stalagmite HK3 may be related to differences in the relative proportions of P available as free ions versus inorganic and organic colloidal material (Borsato et al., 2007) in the dripwater between the two caves. Furthermore, the correlation between P and Y (indicative for organics) is much weaker in the aragonite compared to calcite. This indicates that P is indeed not only available as organic material but may also be transported and incorporated through other mechanisms. These considerations must be the scope of further research and are not addressed here. 5.2.3. Aluminium, Titanium, Lead and Thorium, relation to clay minerals Higher concentrations of Th are associated with noncarbonate phases (Fairchild et al., 2006). This is because Th is transported as adsorbed phase on detrital materials such as clay minerals (Dorale et al., 2004). Aluminium is also a major constituent of clay minerals, such as kaolinite or montmorillonite and is transported both in colloidal and particulate form (Zhou et al., 2008b; Fairchild and Treble, 2009). Elemental abundances of Al, Ti, Pb and Th in stalagmite HK3 are positively correlated and together make up

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Fig. 16. 1st and 2nd principal components versus distance (time). Aragonite is indicated by light grey shading, calcite (Cc) is indicated by dark grey shading. (A) 1st principal component from GP2 calcite section, high values indicate higher effective rainfall and vice versa. (B) 2nd principal component from GP2 calcite section (unknown forcing mechanism). (C) 1st principal component from the GP2 aragonite section, higher values indicate higher effective rainfall and vice versa. (D) 2nd principal component from GP2 aragonite section (unknown forcing mechanism). (E) 1st principal component from HK3 aragonite section from aragonite-to-calcite transition (unknown forcing mechanism). (F) 2nd principal component from HK3 aragonite section from aragonite-to-calcite transition (unknown forcing mechanism). (G) 1st principal component from the HK3 calcite section, high values indicate lower effective rainfall. (H) 2nd principal component from HK3 calcite section, possibly reflecting a transport mechanism for clay minerals. (I) 1st principal component from HK3 aragonite section from calcite-to-aragonite transition (unknown forcing mechanism). (J) 2nd principal component from HK3 aragonite section from calcite-to-aragonite transition (unknown forcing mechanism).

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the 2nd principal component in the calcitic section of stalagmite HK3 (Fig. 8B). The most likely interpretation of this pattern is a similar source and similar transport mechanism for these elements. Clay minerals are rich in Al and can easily adsorb a range of trace elements including Th (Dorale et al., 2004). They can be transported as colloidal material and as particles (Fairchild and Treble, 2009). Therefore, the Al, Ti, Pb and Th elemental maxima may be related to an increased abundance of colloidal clay material or larger clay particles. At the stratigraphic mineralogical transitions of stalagmite HK3 and stalagmite GP2 a clear shift from higher concentrations in calcite to lower concentrations in aragonite is observed for Al, Ti, Th and Pb (Figs. 7 and 12). Considering the abrupt shift in elemental abundances at the transitions, it is here suggested that this pattern is crystallographically forced. This may be related to different adsorption potentials between calcite and aragonite or competition effects for crystal defect sites. Interestingly the suggested crystallographic forcing on Y and P showed an opposite effect. This indicates that organics may be differently incorporated or are directly competing for crystal defect sites with clay minerals in these stalagmites. Once more, this topic requires much more attention, but is beyond the scope of this paper. An interesting feature of the 2nd principal component is the trend towards more negative values approaching the Cc–Ar transition (Fig. 16). This is not directly visible in Fig. 7, showing the additional value of PCA. Whereas the 1st principal component was interpreted as representing effective rainfall, the 2nd principal component could represent transport mechanisms that may be related to effective rainfall as well, but with a different type of response compared to the 1st principal component. Other potential interpretations could be a gradual increase in aragonite abundance approaching the Cc–Ar transition, because the aragonite is characterized by very low Al, Ti, Pb and Th concentrations, or a temperature induced weathering signal (cooling). Neither of the latter two interpretations is, however, supported by petrographic evidence or other proxies. For this reason, the authors consider effective rainfall as the most likely forcing mechanism. 5.2.4. Detrital layers and hiatus surfaces Under increasingly arid conditions, drip sites may dry out and hiatus surfaces cap speleothems. Hiatus surfaces in speleothems are often marked by detrital material that is delivered as aerosols in cave air or in the dripwater itself. Alternatively, detrital material may accumulate on crystal surfaces due to overall decreasing precipitation rates, which eventually results in a hiatus surface characterized by detrital material (Immenhauser et al., 2007). A third option is that the thin and only temporarily present water film on a speleothem beneath an increasingly dry drip site may be insufficient to remove detrital material from the speleothem surface. These mechanisms might occur in combination or separately. In the aragonite layer from the Ar–Cc transition in stalagmite HK3, it was observed that silt-sized carbonate detrital material increased in abundance at the upper

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boundary of the aragonite layer (Fig. 4D). This material might represent insoluble residue from the carbonate host rock or the soil above the cave. At the flank of the stalagmite where growth is slow the abundance of carbonate detrital material increased as well (Fig. 4J). Therefore, the fine silt probably accumulated in the inter-fibre pore space on top of the aragonite surface from the Ar–Cc transition during time intervals when speleothem growth ceased. Additional evidence for the presence of a hiatus at the top of the aragonite layer comes from the strong shift to higher concentrations in P and Y at the Ar–Cc transition and the absence of such a shift at the Cc–Ar transition (Fig. 6). Therefore, it can be concluded that this shift is not crystallographically forced, but rather indicates the onset of increased soil activity after a relatively dry period. The increasing amounts of calcite observed towards the upper limit of aragonite layers can be explained by the reinitiation of speleothem growth and, thus, more humid climate. Calcite precipitation on top of the aragonite hiatal surfaces occluded pore space between aragonite fibres and encased detrital material on top of the aragonite speleothem surface. This is supported by geochemical evidence based on the Mg concentrations across the Ar–Cc and Cc–Ar transitions. The Ar–Cc transition is much more gradual compared to the Cc–Ar transition (Fig. 6). XRD data from this aragonite layer indicates that the calcite contains 4 mol-% MgCO3 an observation that suggests that the calcite at the transition qualifies as a moderately high Mg calcite rather than calcitized aragonite that would form low-Mg calcite. Although the effect of the carbonate detrital material on the Mg concentrations cannot be excluded, we consider it very likely that the increase of calcite with an elevated Mg content contributed to the more gradual Ar–Cc transition. 5.2.5. Climatic context of stratigraphic calcite-to-aragonite transitions Only a few climate reconstructions from Morocco cover the Holocene and the late glacial (Lamb et al., 1995; Cheddadi et al., 1998, 2009). Lake Tigalmamine is the best studied lake record and is located in the Middle Atlas at an altitude of 1628 meters above sea level. Cheddadi et al. (1998) suggested that during the early Holocene, the region of Lake Tigalmamine was characterized by increasing January temperatures and a decrease in annual precipitation from 900 mm at approximately 12 ka BP to 700 mm around 11 ka BP (14C ages were calibrated using the online calibration http://www.calpal-online.de/, accessed 2011–11–29; Weninger and Jo¨ris, 2008). It must be noted, that these annual precipitation rates were probably lower than 700 mm at the site of Grotte de Piste due to its lower altitude (1260 m) compared to Lake Tigalmamine (1628 m) and the general altitude-to-rainfall amount relation in Morocco. Acknowledging some degree of error in the age model of these climate reconstructions, the warmest and driest period of the Holocene seems to coincide in time with the stratigraphic Cc–Ar transition in stalagmite GP2. This might imply that the occurrence of aragonite in stalagmite GP2 is probably induced by decreasing rainfall amounts. Increasing average air temperatures might have played a role too.

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The climate context of calcite-to-aragonite transitions in stalagmite HK3 is more difficult to assess, whilst climate reconstructions from Morocco for marine isotope stage 2 and 3 are limited to a record from Lake Ifrah (Cheddadi et al., 2009). The age model of the Lake Ifrah record does not allow for a comparison of the age of aragonite layers with the marine isotope stage 2 and 3 climate reconstruction in Morocco. Heinrich event 2 (H2) was recorded by several marine cores surrounding the Iberian Peninsula and Morocco (Turon et al., 2003; Bout-Roumazeilles et al., 2007; Fletcher and Goni, 2008; Nebout et al., 2009; Penaud et al., 2010). Fletcher and Goni (2008) and Nebout et al. (2009) suggested that H2 was characterized by overall dry conditions in the Western Mediterranean. Bout-Roumazeilles et al. (2007) used Artemisia and Ephedra pollen data to reconstruct the % of semi-dessert vegetation combined with the palygorskite content from a marine core in the Alboran Sea (ODP 976), which reflects vegetation cover in Western Morocco. This reconstruction suggests that Western Morocco was characterized by several dry/humid phases between 26.5 and 21.5 ka BP (Bout-Roumazeilles et al., 2007). Stalagmite HK3 commences to grow (aragonite) around 27.5 ka BP. This time interval coincides with Greenland Interstadial 3 as recognized in the Greenland ice cores (Rasmussen et al., 2008). At present, our attempt to date the transition from aragonite to calcite was unsuccessful. Linear interpolation between the two datings suggests that the timing of the more humid calcite phase is centred around 24.5 ka BP. In contrast, the subsequent dry aragonite phase is centred around 23.7 ka BP, although it has to be noted that the presence of the hiatus at the top of the aragonite layer induces uncertainties in the age model. The age of the aragonite phase centred around 23.7 ka BP is, however, robust, as the sample for dating was drilled from this layer. This may confirm Fletcher and Goni (2008) and Nebout et al. (2009) who suggested that dry conditions occurred during H2. The calcite phase, however, indicates a more humid period too that may coincide with one of the more humid phases reconstructed by Bout-Roumazeilles et al. (2007) for the period between 26.5 and 21.5 ka BP. 5.2.6. Lateral calcite-to-aragonite transitions in stalagmite HK1 (Grotte Prison de Chien) In the above discussion, evidence has been presented that stratigraphic transitions from calcite to aragonite may be driven by, amongst other factors, effective rainfall. In the case of lateral calcite-to-aragonite transitions, similar processes inducing aragonite precipitation might have been active. Obviously, the main difference between lateral relative to stratigraphic transitions is that both, calcite and aragonite are precipitated pene-contemporaneously within one growth increment. Precipitation of calcite and aragonite must not, however, be from the same fluid. Alternatively, the physico-chemical properties of the water film on the stalagmite surface may have changed over short distances. This implies that boundary conditions were initially in a calcite mode but very close to the threshold of the aragonite mode. In the case of lateral transitions of calcite

(stalagmite core) to aragonite (stalagmite flanks), relevant parameters of dripwater chemistry include CaCO3 saturation state, dripwater Mg/Ca ratio and CO32- controlled kinetics. Relevant environmental conditions include temperature and evaporation potential (Railsback et al., 1994; Ferna´ndez-Dı´az et al., 1996; Zuddas and Mucci, 1998; Frisia et al., 2002; Kawano et al., 2009). Possibly, the occurrence of both calcite and aragonite within a single growth layer has some implication as climate proxy. This hypothesis may be verified or falsified by analyzing stratigraphic changes in geochemistry. 5.3. Interpretation of trace element abundances in speleothem aragonite Prior calcite precipitation was the main process to induce aragonite precipitation in Grotte Prison de Chien and Grotte de Piste speleothems. In the calcitic portions of stalagmites HK3 and GP2, no difference in the behaviour of Mg, Sr and Ba was observed. Conversely, Mg, Sr and Ba do show a different behaviour in the aragonitic portions of the two stalagmites. In addition the total amount of variance explained by the first two principal components in the aragonite sections are lower compared to the calcitic sections. These differences merit discussion. The number of previous studies focussing on trace elements (particularly Ba and Sr) in speleothem aragonite is indeed limited (Finch et al., 2001, 2003; Mcmillan et al., 2005). Finch and co-workers proposed a possible relation of Ba and Sr concentrations to rainfall amount, with higher concentrations of Ba and Sr corresponding to higher rainfall. A mechanistic model for Ba and Sr incorporation into speleothem aragonite, however, has not yet been brought forward. We consider PCP an unlikely mechanism, because higher rainfall amounts should result in a decrease of PCP and thus, lower speleothem Ba and Sr concentrations. Aragonitic stalagmites as discussed in previous studies were mainly retrieved from caves with a dolomitic host rock. Examples include the Cold Air Cave in South Africa (Holmgren et al., 1999; Finch et al., 2003), the Lianhua cave in China (Cosford et al., 2008), the Joa˜o Arruda Cave in Brazil (Bertaux et al., 2002) and the B7 cave in Germany (Niggemann and Richter, 2006). This fundamental pattern emphasizes the role of Mg in aragonite precipitation. An aspect that has not been given sufficient consideration is “prior aragonite precipitation” (PAP) rather than prior calcite precipitation. This process may be important where Mg concentrations in the dripwater of dolomitic host rock caves are high. Differences between PAP and PCP exist because the partition coefficients of Ba and Sr for aragonite precipitation are closer to unity compared to partition coefficients for calcite precipitation (Fairchild and Treble, 2009). The relation between the dripwater Ba/Ca and Sr/ Ca ratios, however, depends on whether the partition coefficients are smaller or larger than one. Trace element partition coefficients depend on several environmental factors such as temperature and precipitation rate (Huang and Fairchild, 2001; Treble et al., 2005). Cave analogue experiments focussing on partition coefficients in aragonite do not exist to our knowledge. It is therefore only emphasized that

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PAP will have a different signature on the speleothem Ba and Sr concentrations compared to PCP. This feature may explain the absence of a negative correlation between Ba, Sr and annual rainfall at Cold Air Cave in South Africa (Finch et al., 2003). In addition, Sr concentrations in dolomite are relatively low compared to limestone due to the lower amount of Ca-sites available for substitution by Sr (Jacobson and Usdowski, 1976). This implies that in caves with a dolomite host rock, soil-derived Sr is likely to be more significant relative to limestone host rock caves. The interpretation of Mg in aragonite speleothems also differs from that of calcite speleothems. This is because Mg is not likely incorporated into the crystal lattice of aragonite due to differences in ionic radius between Ca and Mg 0 0 ˚ versus 0.72A ˚ ). Magnesium/Ca ratios of the dripwater (1A will, therefore, increase faster in the case of PAP compared to PCP. Magnesium concentrations in aragonite speleothems potentially indicate differences in the amounts of PAP. However, Mg could also be an indicator for co-precipitating magnesian calcite, shown by the Mg concentrations across the more gradual Ar–Cc transition in stalagmite HK3 (Fig. 6) discussed in Section 5.2.4. Alternatively, Mg is associated with colloidal material, organic matter or particles. Organic matter may be incorporated in the aragonite crystal lattice or be present as inter-crystalline phase. Thus, variations in Mg, Sr and Ba in aragonitic speleothems must not be compared with those in calcitic speleothems and may reflect very different processes. 5.3.1. Aragonitic portion of speleothem GP2 Stalagmite GP2 was collected in Grotte de Piste characterized by a dolomitic host rock mineralogy similar to that of the Cold Air Cave (Holmgren et al., 1999). In contrast to the calcitic portions of speleothem GP2, the aragonitic portion lacks a clear positive correlation between Mg, Sr and Ba elemental values. Instead, Sr is positively correlated with P, Y, Pb and U, which together make up the 1st principal component (Fig. 8E). Magnesium does not significantly correlate with any element and Ba is only negatively correlated with Pb. Following the discussion above, the decoupling of Sr, Mg and Ba in aragonite is probably due to the absence of PCP, whereas PAP may have taken over in the karst aquifer (Fig. 15). Strontium, Y, P, U and Pb all show decreasing trends across the aragonite portion of speleothem GP2 (Fig. 14). Acknowledging that the correlation between P and Y is weaker compared to the calcite section, this positive correlation still indicates that organics play a role in determining the P concentrations in the aragonite from speleothem GP2. This is confirmed by the similar trends of U, Y and Pb, elements that are strongly linked to organics (Treble et al., 2003; Borsato et al., 2007). Therefore, it can be concluded that Sr variations observed in aragonite may be related to the presence of organics (Fig. 15) and that the 1st principal component represents effective rainfall, with lower values representing lower effective rainfall and higher values representing higher effective rainfall (Fig. 16). Following this line of evidence, it is suggested that rainfall amounts continued to decrease after the initiation of aragonite precipitation (Fig. 16). It remains, however, unclear why Mg and U show

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a weak positive correlation. Similarly, the main driver of the 2nd principal component remains poorly constrained too. 5.3.2. Aragonitic portions of speleothem HK3 When comparing the outcome of the PCA from both individual aragonite layers in speleothem HK3, similarities in elemental grouping are not obvious (Fig. 8A and C). In the aragonite section beneath the Ar–Cc transition, a correlation between Mg, Sr and Ba is lacking. This implies that PCP is not a dominant factor. A correlation between Sr, P, U, Y and Pb, as observed for speleothem GP2, is not visible in speleothem HK3. Nevertheless, a positive correlation between Sr and P as well as Sr and U exists. This might indicate the influence of organics. Furthermore, compared to speleothem GP2, higher Al concentrations in speleothem HK3 suggest a stronger influence of clay minerals, because Al is positively correlated with Mg, Pb and Y (p < 0.03, r = 0.47; Fig. 8). However, the low total data variation explained by the first two principal components in the aragonite from the Ar–Cc transition suggests, that it is difficult to identify a dominant forcing mechanism for the trace elements in this aragonite interval in contrast to the calcite interval. A positive correlation between Mg and Ba as well as Ba and Sr was observed in the aragonitic section that directly follows the Cc–Ar transition (Fig. 8C). These elements co-vary with d13C, suggesting an influence of PCP (Fig. 10). If enhanced PCP is interpreted in terms of lower effective rainfall, however, a negative correlation of Mg, Sr and Ba with P and Y is expected. This is not the case. Therefore, the interpretation of this set of elements is not straightforward, a conclusion that is also reflected in the Principal Component Analysis. This is because the first two principal components only account for 60% of the total data variation (Fig. 8C), an outcome that contrasts with the calcite interval. This suggests that various environmental and kinetic factors are relevant, which may include different incorporation, transport mechanisms and possibly competition effects for crystal defect sites (Borsato et al., 2007). 6. CONCLUSIONS Three Holocene speleothems from two caves in Morocco were investigated for their petrography as well as their elemental and isotope geochemistry across lateral and stratigraphic calcite-to-aragonite transitions. By using Principle Component Analysis the overall complexity of the dataset was reduced and dominant forcing mechanisms were identified. The 1st principle component in the calcitic sections of stalagmites HK3 and GP2 is coupled to the vegetation-related input of organics and prior calcite precipitation and is reflecting effective rainfall. The 2nd principle component in stalagmite HK3 reflects transport mechanisms of clay minerals (colloidal or particle) that shows a different response to effective rainfall compared to the 1st principal component. The 1st principal component in the aragonite section of stalagmite GP2 reflects the vegetation-related input of organics and is coupled to effective rainfall. In stalagmite HK3, the total data variation

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explained by the first two principal components was very low in the aragonite. This indicates that trace elemental abundances were not dominated by one or two forcing mechanisms. Aragonite precipitation in stalagmites HK3 and GP2 is predominantly controlled by climate forcing. Particularly, the change from more humid to more arid climate is of importance. Arguments for this relation include: (i) The vegetation-related input of organics as documented by speleothem P and Y concentrations reaches a minimum before and during the precipitation of a given aragonite interval. (ii) Prior calcite precipitation was at a maximum just before the presence of aragonite as evidenced from the relatively high Mg, Sr and Ba concentrations in the calcite directly before the aragonite interval. (iii) On top of the aragonite layer from the aragonite-to-calcite transition in stalagmite HK3 the increased abundance of carbonate detrital material is possibly linked to a hiatal surface. Evidence comes from the strong increase in the P and Y concentrations across the aragonite-to-calcite transition and the absence of such a shift across the calcite-to-aragonite transition. The transition from calcite to aragonite in stalagmite GP2 is less significant in terms of climate change compared to that of speleothem HK3. This is mainly due to the dolomitic host rock mineralogy of Grotte de Piste (GP2) resulting in elevated background Mg/Ca ratio in Grotte de Piste dripwater, which in turn decreases the rate of calcite precipitation and enhances aragonite precipitation. The interpretation of trace element abundances in aragonitic speleothems differs substantially from that of calcitic speleothems. Major differences include the absence of prior calcite precipitation and the possible presence of prior aragonite precipitation within caves with mainly dolomitic host rock versus prior calcite precipitation within caves with mainly limestone host rock mineralogies. This is obvious from the decoupling of the positive correlation between Mg, Sr and Ba in the aragonite of stalagmite GP2. Difficulties in the interpretation of trace elements in aragonite may occur as a consequence of small scale dissolution and recrystallization features. In well-preserved aragonitic speleothems, Mg is likely an indicator for the co-precipitation of calcite or detrital material. Conversely, Sr most likely reflects the flux of soil-derived organics evidenced by the positive correlation between Sr, P, U, Y and Pb in the aragonite from stalagmite GP2. The data shown here, clearly document the value of well-preserved speleothem aragonite fabrics. Specifically, speleothems characterized by stratigraphic and/or lateral changes from aragonite to calcite represent sensitive, albeit highly complex, archives of past climate change. ACKNOWLEDGEMENTS This work was financed by the Deutsche Forschungsgemeinschaft (DFG; project IM 44/1). We would like to thank the following people for fruitful discussions: A. Borsato, S. Frisia, C. Spo¨tl, D. Fleitmann and A. Niedermayr. In addition the staff in the isotope laboratories at Bochum and Mainz (U. Weis, B. Stoll, D. Buhl and B. Gehnen) is acknowledged for their help with sample preparations and measurements. C. Kirchmann is thanked for his help with the drilling of isotope samples. Furthermore, we would like

to thank R. Neuser for the help with the cathodoluminescence microscope, T. Reinecke for the X-Ray diffraction measurements, the thin section lab at Bochum, our local speleoguides El Houcine El Mansouri and Tarik Echchibi for their help in the field and A. Fink (Institute for Geophysics and Meteorology, University of Cologne) and Mileud (weather station Bab Bou Idir) for providing rainfall data. I. Sadalmelik is thanked for making the Morocco map (Fig. 1A) available. We greatly acknowledge the very detailed and constructive comments by three GCA reviewers: R. Martı´n-Garcı´a and two annonymous colleagues. We also acknowledge the comments and the editorial handling of this paper by Miryam Bar-Matthews.

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