Relationships between colour and diagenesis in the aragonite-calcite speleothems in Basajaún Etxea cave, Spain

    Relationships between colour and diagenesis in the aragonite-calcite speleothems in Basaja´un Etxea cave, Spain Rebeca Mart´ın-Garc´ıa, Ana M. Alonso-Zarza, Andrea Mart´ın-P´erez, Andrea Schr¨oder-Ritzrau, Thomas Ludwig PII: DOI: Reference:

S0037-0738(14)00135-3 doi: 10.1016/j.sedgeo.2014.08.001 SEDGEO 4769

To appear in:

Sedimentary Geology

Received date: Revised date: Accepted date:

12 June 2014 25 July 2014 3 August 2014

Please cite this article as: Mart´ın-Garc´ıa, Rebeca, Alonso-Zarza, Ana M., Mart´ın-P´erez, Andrea, Schr¨ oder-Ritzrau, Andrea, Ludwig, Thomas, Relationships between colour and diagenesis in the aragonite-calcite speleothems in Basaja´ un Etxea cave, Spain, Sedimentary Geology (2014), doi: 10.1016/j.sedgeo.2014.08.001

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ACCEPTED MANUSCRIPT RELATIONSHIPS BETWEEN COLOUR AND DIAGENESIS IN THE ARAGONITE-

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CALCITE SPELEOTHEMS IN BASAJAÚN ETXEA CAVE, SPAIN.

Rebeca Martín-García1, Ana M. Alonso-Zarza1, Andrea Martín-Pérez2, Andrea Schröder-

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Ritzrau3, Thomas Ludwig4

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1. Department of Petrology and Geochemistry. Faculty of Geology, Complutense

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University of Madrid. IGEO (CSIC-UCM). Madrid, Spain 2. Institute of Palaeontology ZRC SAZU. Ljubljana, Slovenia

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3. Heidelberg Academy of Sciences, Heidelberg, Germany 4. Institute of Earth Sciences, Heidelberg University, Germany

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[email protected]

Abstract

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Isotope geochemistry.

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Keywords: Speleothem, Aragonite, Diagenesis, Secondary ion mass spectrometry (SIMS),

Basajaún Etxea Cave, North Spain, contains a wide morphological and colour variety of speleothems including crusts, stalactites, helictites, stalagmites and flowstones. Most of the speleothems are composed of aragonite, but calcite speleothems are also found. Their most common colour is white, but there are also pink, green and turquoise speleothems, in different areas of the cave. Mg-rich dissolution waters from the cave’s crystalline dolostone and magnesite host rock favour aragonite precipitation and drive important diagenetic changes.

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ACCEPTED MANUSCRIPT In this paper we will discuss how diagenesis modifies speleothem texture, mineralogy and geochemistry, causing significant changes in the colour and geochemical signatures

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commonly used as paleoenvironmental proxies. We also discuss how speleothems that have undergone diagenesis may also be useful indictors of paleoclimatic conditions.

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While micritization did not affect trace element composition, it did increase values of δ13C

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in micritized crystals. In addition, green speleothems become turquoise or white when

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micritized. Changes in mineralogical, textural, and geochemical composition due to aragonite-to-calcite transformation did not include changes in the green colour. The change

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to pink is due to the entrance of Mn in neomorphic calcite, also enriched, with respect to primary aragonite, in Mg and Cu and depleted in Sr and δ13C.

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Our work shows that diagenesis changes the colour of the speleothems because of the

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mobility of the chromophore elements during this processes. Along with this elemental

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alteration comes the loss of the primary isotopic signals of speleothems. Detailed petrological studies of speleothems should precede their analysis for palaeoclimatic reconstruction so that influence of areas affected by diagenesis can be eliminated. In addition, diagenetic signals also contain valuable information on the changes of the waters within the cave, and the overall evolution of speleothems and the cave itself.

1. Introduction The study of petrology and diagenesis of speleothems, recently an interesting area of research, continues that of only a few papers dealing with it before the 1980’s (Folk and Assereto, 1976; Kendall and Broughton, 1977, 1978; Cabrol, 1978). More recent studies dealing with speleothem diagenesis (Frisia et al., 1996; Frisia et al., 2002; Woo and Choi, 2006; Woo et al., 2008; Hopley et al., 2009; Aramburu Artano et al., 2010; Pagliara et al., 2

ACCEPTED MANUSCRIPT 2010, Perrin et al., 2014; Zhang et al., 2014) have shown that diagenetic processes: 1) change the geochemical signals used for dating and/or paleoclimatic studies (Martín-García

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et al., 2009; Frisia and Borsato, 2010); 2) driven by environmental changes, are also

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archives of paleoenvironmental conditions (Cabrol and Coudray, 1982; Frisia et al. 2002); 3) especially in aragonite speleothems (where the driving mechanisms are less well-known

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than those of their marine counterparts), can produce similar features to those observed in

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marine aragonite deposits like micrite, cements and aragonite to calcite transformation (Woo and Choi, 2006; Martín-García et al., 2009; Martín-Pérez et al., 2012; Perrin et al.,

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2014; Zhang et al., 2014); 4) can change the aspect, shine and colour of speleothems, modifying their impression on visitors (in case of tourist-attraction caves). Diagenetic

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processes affect mostly aragonite speleothems, due to its unstable nature under surface

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conditions, transforming them to calcite, the stable polymorph (Hill and Forti, 1997;

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Rowling, 2004). Aragonite is an unstable polymorph of CaCO3 due to its structure, in which the space for the Ca ion is larger than in the calcite (Klein and Hulburt, 1993). However other carbonate cave minerals, such as calcite or huntite, also undergo such diagenetic processes as dissolution, recrystallization, micritization or even dolomitization (Alonso-Zarza and Martín-Pérez, 2008; Jones, 2010; Melim and Spilde, 2011). Diagenetic processes in speleothems have been unequivocally recognized (Railsback et al., 2002; Ortega et al., 2005; Melim and Spilde, 2011; Devès et al., 2012; Lachniet et al., 2012; Martín-Pérez et al., 2012), driven by the differences in chemistry through time of the waters within the cave and by kinetics. However detailed petrological and geochemical studies are critical to distinguish between the primary signals and those imprinted by diagenesis (Cabrol, 1978; Bar-Matthews et al., 1991; Frisia et al., 2002; Woo and Choi, 2006). Usually carbonate speleothems lack very 3

ACCEPTED MANUSCRIPT distinctive colours, this is not the case with those found some caves, such as for example Cliefden, Australia (Turner, 2002), the Blue Cave, France (Cabrol, 1997), Crovassa

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Azzurra Cave, Sardinia (Caddeo et al., 2011) or the Basajaún Etxea Cave studied in this

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paper, all of them showing blue to turquoise speleothems. The colour of speleothems has formerly been related to the presence of Mn, Co, Cu (White, 1997) or humic compounds

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(James, 2003).

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This paper analyses mostly aragonitic speleothems from Basajaún Etxea Cave, Navarra (Spain), which have undergone different diagenetic processes. Some of the speleothems

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show very distinctive green, turquoise or pink colour. The aims of our paper are to: 1) characterize the diagenetic processes that the speleothems have undergone, describing the

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mineralogical, textural and geochemical changes occurring during diagenesis; 2) discuss

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the causes of the diagenetic processes and their possible environmental significance and, 3)

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elucidate what element likely correlates with the colour of speleothems and the possible variation of colour and trace element content during diagenesis. In doing so we intend to offer clear criteria for the recognition of diagenetic processes in speleothems and contribute to a better understanding of the study of early diagenesis in caves.

2. Case setting 2.1. Regional geology and climate Basajaún Etxea cave (also called Ayerdi III) is part of a group of caves located on the northern slope of Mount Ayerdi (815 m a.s.l.), 2.5 km to the north of Lantz, Navarra (Spain) (Fig. 1), within the South Pyrenees. These caves are located within and area used by the Romans for Cu-mining in the 1st and 2nd centuries (Mezquíriz, 1973).

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ACCEPTED MANUSCRIPT The cave is located in the Alduides-Quinto Real Massif, one of the three Basque Massifs, of Palaeozoic age, between the Pamplona fault and the South Pyrenean Zone, in the

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Western Pyrenees (Fig. 1.a).

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The deformation is characterized by the presence of N-S and NNW-SSE oriented folds and E-W and ENE-WSW oriented faults. The most important structure is the thrust fault to the

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south of the Massif, which places the Palaeozoic materials over the Upper Cretaceous ones.

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In the East sector of the Massif, Devonian to Carboniferous deposits are 2500-3000 m thick. The base is characterized by the presence of schists and sandstones of Devonian age,

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followed by an alternation of carbonates and schists of the Devonian-Carboniferous transition. Finally a thick Late-Carboniferous turbiditic succession crops out, pinching out

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onto shallow carbonate platforms (Olmedo et al. 1992)

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These caves are hosted in crystalline dolostones and magnesites interbeded between

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sandstones and shales, all of Carboniferous age (Fig. 1.b). Chalcopyrite mineralizations present in the dolostones have a hydrothermal origin and appear as veins with a N-S orientation (Trapote-Redondo et al. 2006). Sandstones and shales occur in well-laminated decimetre-thick beds. These siliciclastic rocks are in places weathered, mainly by hydrolysis.

The climate of the area is temperate oceanic climate (Cfb according to the Köppen–Geiger climate classification (Peel et al., 2007). The nearest weather station is in Esteribar (8,93 km far, altitude 615 m a.s.l.) and it gives a high average precipitation (1200-2500 mm/year) distributed regularly throughout the year. The mean external annual temperature is between 6 and 8ºC.

2.2. Cave setting 5

ACCEPTED MANUSCRIPT Basajún Etxea Cave is a small sub-horizontal cave, is 900 m in length with interior temperature of about 8°C and relative humidity of 99%, constant throughout the year

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(Lopez-Acevedo, 1976). The waters of the cave are slightly basic, with pH fluctuating from

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7.7 to 8.2, and Mg/Ca molar ratio of 2.3 (Lopez-Acevedo, 1976).

Most of the speleothems in Basajaún Etxea cave are on the ceiling and walls, while scarce

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on the floors (stalagmites). Following the classification of Hill and Forti (1997), the

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morphologies of these speleothems are crusts, frostworks, helictites, stalactites, columns and draperies.

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One of the most striking features of this cave is its green, turquoise and pink speleothems (mostly) crusts, helictites, soda-straws and distributed irregularly throughout the cave (Fig.

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2).

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The cave morphology is characterized by a network of galleries with a NW-SE orientation

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controlled by lithology and bedrock. Of its two levels, the lower is up to 30 m below the surface while the upper level is only 5 meters below surface. Both levels are connected at two points; by a steep ramp at the end of the first upper-level room, and through a hole about 25 m deep at the end of the large upper-level circular room, the largest of the cave, with a height of about 15 m (Fig. 2). The hydrology of the cave has varied over time, as reflected by the presence of fluvial cave sediments that contrast with the present-day almost total absence of water. Locally, fluvial cave deposits are thick and consist of rounded gravels in the upper part of the cave and fine sand in the lower parts. Hydrolysis of silicates has formed red clays that in some places stain the speleothems. Near the entrance of the cave, the dolomitic host rock and the clays that cover it are visible. These clays give a reddish colour to the walls and ceilings throughout the cave. 6

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3. Methodology

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The studied samples correspond to samples taken from 7 speleothems, all of them crusts.

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The samples are pink (1), green (4) and turquoise (2).

Conventional optical petrography was performed on thin sections. First the samples were

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submerged in a resin containing Epofer EX 401 and Epofer E 432 in a vacuum system to

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strengthen them and then cut and polished to 35µm thick.

The speleothems were mineralogically characterised by X-ray diffraction (XRD) using a

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Philips PW-1710 XRD system operating at 40 kV and 30 mA, and employing monochromatic CuKα radiation. XRD spectra were obtained from 2-66° 2θ. The samples

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for XRD were obtained using a micro drill and selecting areas with different colour and/or

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texture.

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Scanning electron microscopy was performed using a JEOL 6400 scanning electron microscope (SEM) working at 20 kV and with a resolution of 35 Å. SEM was performed on 4 gold-coated fresh surfaces. Observations were made in order to get a better comprehension of the different textures. Secondary electron and backscattering detectors were used together with an X-ray detector system (EDS) to obtain semi-quantitative compositions. The Ca, Mg, Sr, Fe, Mn and Cu concentration of the different minerals in selected polished samples were firstly determined using an electron microprobe (JEOL Superprobe JXA8900 M) operating at 15 kV and 20 nA and employing an electron beam diameter of 5 μm. The limit of detection for the elements analysed were as follows: Ca 145 ppm, Mg 125 ppm, Sr 175 ppm, Mn 240 ppm, Fe 295 ppm and Cu 330 ppm. The standards used are

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ACCEPTED MANUSCRIPT described by Jarosewich et al. (1980) and were provided by the Smithsonian Institute, Washington, USA.

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Additional analyses of the trace elements Mg, Mn, Cu and Sr were done with a secondary

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ion mass spectrometer Cameca ims 3f at the Institute of Earth Sciences, Heidelberg University (a preliminary analysis had shown that the concentration of Fe and Co in the

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samples was too low to be detected). The analyses were performed using a 14.5 keV, 20 nA O– primary ion beam with a diameter of ~ 15 μm. Positive secondary ions of the isotopes

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Mg,

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Ca,

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Mn,

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Cu and

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Sr were accelerated to 4.5 keV and energy filtered with an

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offset of 75 eV and an energy window of ±20 eV. The imaged field was limited to a diameter of ~ 12 μm by using a 700 μm field aperture (imaged field mode 25 μm, FA #2)

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and the instrument was set to its lowest mass resolution of ~ 400 (m/m at 10%). The ion yields of Mg, Mn, Cu and Sr relative to Ca were determined using the NIST SRM610 glass

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as reference material and the concentrations (preferred averages) published by Pearce et al. (1997). The relative accuracy of this method is ≤ 20 % (typ. ≤ 10%). The Ca concentration of the unknown samples was assumed to be 40 wt. % (100 % CaCO3). The δ13C and δ18O values of 28 powdered samples were analysed at the Serveis Cientificotècnics of the Universitat de Barcelona. Approximately 1 mg of sample was drilled with a 0,5 mm micro drill, of which 60 micrograms were subsequently dissolved in 100% phosphoric acid at 70ºC for 3 minutes. To assure that individual samples do not represent any mixture of different layers or mineralogies, a previous examination of the thin section and the XRD plots was necessary, and only the areas with no mixture were selected. The CO2 was extracted using a Thermo Finnigan Carbonate Kiel Device III isotopic analyser with a Thermo Finnigan MAT-252 spectrometer, according to the McCrea (1950)

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ACCEPTED MANUSCRIPT method. Values obtained were corrected using the NBS-19 and NBS-18 standard for δ13C and the standard NBS-19 for δ18O.

Results are expressed in parts-per-thousand (‰)

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referred to VPDB standard. Reproducibilities obtained were better than 0.03‰.

4. Terminology

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The meaning and uses of the term micrite are controversial. Folk (1959) originally

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described micrite as a clay-sized matrix, which characterizes low-energy carbonate deposits, referring to mechanically deposited material, giving the term genetic connotations

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that were followed by Friedman (1985), whereas Milliman et al. (1985) used it as a descriptive term (carbonate crystals less than 4 µm) with no genetic implications. The

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application of terms that carry genetic connotations invariably creates problems.

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In this paper micrite is used as a descriptive term that is independent of interpretation or

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mineralogy. Thus, it follows the criteria of Friedman (1964); Alexanderson (1972); Milliman et al. (1985), Jones and Kahle (1995), Pedone and Dickson (2000) or Dix et al. (2005) who used the term micrite for carbonate crystals less than 4 μm long. Micritization is the process of destruction of the internal textures of the carbonate crystals to form cryptocrystalline textures (Reid and Macintyre, 1998). Aragonite-to-calcite transformation is a well-known process that has been described broadly in the literature. Aragonite is metastable under surface temperature and pressure conditions and thus tends to transform into the stable CaCO3 phase, low Mg calcite. Neomorphism is an inclusive term to define isocompositional and replacement processes such as recrystallization and inversion (transformation of one polymorph into another) (Folk, 1965). However, in some cases the process may also involve cementation on a microscale, so, in this paper, it will be referred to as aragonite-to-calcite transformation. 9

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5. Results

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5.1. Mineralogy

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Although some amounts of langite – Cu4SO4(OH)6 · 2H2O – have been found in the speleothems in Basajaún Etxea cave (López-Acevedo, 1976), the samples analysed in this

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work are formed by calcite and aragonite. These carbonates can be found in different

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colours (white, turquoise, green or pink). The coloured speleothems are restricted to certain parts of the cave (Fig. 2) and are mainly arranged in a N-S direction. Each colour has a

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determined fabric.

Green crusts show, in general, acicular aragonite alternating with white micritic bands. The

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turquoise samples show a micritic to microsparitic texture and the pink samples show

5.2. Fabrics

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equant calcite fabrics with areas of white micrite.

Acicular crystals. Aragonite appears as acicular crystals from 1 - 2 mm long with a length/width ratio ≥ 6:1. Both macro- and microscopically, these crystals are shiny and transparent; under the microscope they are arranged as fans growing out from a common point (Fig. 3.b). Micrite and microsparite. Micritization occurs on calcite and aragonite speleothems and destroy their initial coarse texture as only part of the original speleothem is preserved. Thus micrite is a secondary product formed by micritization. In the case of the coloured speleothems this process changes not only the texture, but the colour as well (Fig. 3). Under the SEM micrite is seen as a mass of small crystals from 1 - 4 µm with no preferred

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ACCEPTED MANUSCRIPT orientation. There are other areas where the crystals are bigger varying from 10 - 63 µm, with a very heterogeneous morphology and size, this fabric is called microsparite (Fig. 3.c).

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Sparite: sparry calcite appear when the aragonite-to-calcite transformation occurs. Under

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the microscope, calcite speleothems consist of equant mosaics of small calcite crystals 0.1 1 mm wide that include textural and/or mineralogical aragonite relics (Fig. 4). The

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recognition of an aragonite precursor is generally based on textural evidence since the

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aragonite morphology is often preserved even in the case of complete transformation.

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5.3. Carbon and oxygen isotopes

The isotopic signatures of the speleothems (Fig. 5) show some variations depending on the

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original mineralogy, colour and diagenetic processes. The values of δ18O vary between -2.3

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varied (from -9 to 6 ‰).

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and -5.2 ‰ in all the textures, colours and mineralogies. However the δ13C values are more

The micritized aragonite in the green speleothems shows heavier δ13C (≈ 1‰) values than the non-micritized original crystals and is also slightly (≈ 0.2‰) heavier in the δ18O values. The pink sample shows positive δ13C values in the micritic aragonite and in the secondary calcite.

In the case of the turquoise sample, as there are not different areas only one sample was taken. This sample has a microsparitic to micritic texture and the value in δ 13C is heavier (1.74‰) than in the micrite of the green sample (-4.70‰). The δ18O values are in the same range (-4.60‰).

5.4. Colour and trace elements in speleothems

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ACCEPTED MANUSCRIPT A speleothem of each colour (pink, green and turquoise) was selected for the study (Fig. 6). Within each speleothem analyses of each sample’s textures and mineralogies were carried

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out with special attention paid to the transition zones.

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The pink sample (Fig. 6.a) is composed of two textures, equant calcite and micritic aragonite. The analyses corresponding to the micritic aragonite produced low values of Mg

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(14.9 – 21.7 ppm) and higher values of Sr (455.4 – 608.5 ppm), unlike those of calcite,

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which has a Mg content of around 4300 ppm and a Sr content of around 45 ppm. The Mn content in aragonite has values below detection limits; the content in calcite was between

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75 and 441.6 ppm. The concentration of Cu was similar to Mn, with a maximum of 7.9 ppm in the aragonite and between 231.4 and 289.0 in the calcite crystals (Fig. 7.a).

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In the turquoise sample (Fig. 6.b) the mineralogy is 100% aragonite of two textures, micrite

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and microsparite distributed in patches. Analyses revealed that the Mn content is very low

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in both textures. There is a slightly higher Mg and Cu content in the textures formed by smaller crystals, but the variations are not significant: Mg (24.0 - 56.8 ppm), Cu (234.6 386.9 ppm) and Sr (274.0 - 326.5 ppm) (Fig. 7.b). The green sample consists of micritic and fibrous aragonite and equant calcite (Fig. 6.c). Geochemical analyses were performed in selected points (Fig. 8.a) and along a profile with a resolution of 200 micrometers through the aragonite (micritic and fibrous) and calcite. This profile (Fig. 8.b) shows that the Mg content in aragonite is below limit of detection. The first trace of the profile corresponds to the micritic aragonite (Am), and is fairly homogeneous, with Sr values of 315 - 350 ppm and Cu values of 150 - 177 ppm. In the fibrous aragonite (Af) area, Sr values remain in the same range, and variations at some points are observed in the amount of Cu, reaching 363 ppm. In the transition area from aragonite to calcite, the Mg and Cu concentrations increase while Sr decreases. In the 12

ACCEPTED MANUSCRIPT calcite (Cc), Sr concentrations decrease below limits of detection while both Mg and Cu increase markedly; Cu values are up to 50 times higher than in the aragonite. The final

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section of the profile corresponds to the continuation of the aragonite fibres of the first

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section, and the concentrations of the three elements are similar.

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6. Discussion

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Although calcite is the most common and stable polymorph of CaCO3, aragonite formation is favoured when the Mg/Ca ratio in solutions is high and saturation with respect to CaCO3

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is low (De Choudens-Sánchez and González, 2009). Partially dehydrated Mg adheres to the surface of the nuclei of the incipient calcite crystals inhibiting their growth (Berner, 1975),

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thus favouring the precipitation of the faster growing aragonite (Lippmann, 1973; Gutjahr

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et al, 1996; De Choudens-Sánchez and González, 2009). Mg is released by dissolution of

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the host rock, dolostones in this case, favouring a relatively high Mg/Ca ratio in the parent waters and, thus, if supersaturation is low aragonite precipitation. A similar situation has been described in Ochtiná Cave in Slovakia, where the host rock is siderite and ankerite (Bosák et al. 2002) and in Castañar Cave, Spain (Alonso-Zarza et al. 2011) where the host rock is of dolostone partially magnesitized.

6.1. Micritization Micritization has usually been reported as a shallow marine process related to the action of boring and filling microorganisms over skeletal components (Bathurst, 1966; Calvet, 1982), while, in the absence of microorganisms, diagenetic dissolution has been reported as the driving mechanism for micritization (Neugebauer, 1978). In some aragonitic shells it seems that the presence of porous aragonite zones (chalky) favor the aragonite-to-calcite 13

ACCEPTED MANUSCRIPT transformation (Maliva, 1998; Maliva et al., 2000). Although these studies do not describe this chalky-micritized area, it is supposed to have an inorganic origin. The process of

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micritization in caves is common, although it has usually been reported as condensation

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corrosion (Sarbu and Lascu, 1997; Tarhule-Lips and Ford, 1998; Zupan-Hajna, 2002; Auler and Smart, 2004; Dreybrodt, 2005; Martín-García et al., 2011). This dissolution processes

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in caves occurs normally during a hiatus in the crystalline growth. During these periods, the

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acidic atmospheric water (in equilibrium with the CO2 partial pressure of the cave atmosphere) condenses on and dissolves speleothems (Sánchez-Moral et al. 1999). The

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process of inorganic micritization in speleothems was described in detail by Martín-García et al. (2009) in Castañar Cave. The micrite formed there by etching of crystal surfaces can

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lead to the partial or total micro-disolution of the original large crystals (Perrin et al., 2014).

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The products of this condensation-corrosion process are white opaque porous areas that

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contrast with the bright fresh crystals in which the crystal size varies from 4 to10 μm. This process on speleothem surfaces reproduces their morphology. In the case of successive layering of surfaces, stages of speleothem growth can be reconstructed (Martín-García et al., 2011).

In the case of Basajaún Etxea Cave, it seems that the process of micritization not only occurs on the surfaces, but also in the inner parts (Fig. 9). The presence of micrite on the speleothem surfaces can be due to condensation-corrosion processes, but the internal micritic bands probably formed due to undersaturated waters flowing along the feeding channels of the aragonite fans and discontinuities between aragonite bands. This micritization must have occurred after the formation of the bands since micrite is present on both layers (Fig. 9.c), and in the nucleus of the fans (Fig. 9.b). In the cases where only the surface of the lower band is affected, either condensation or water films from the 14

ACCEPTED MANUSCRIPT percolation of drip waters can be the driving mechanism for the process, but before the precipitation of the next layer.

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In this study it is shown that micritization occurs in two stages (Fig. 3); the first

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corresponding to incomplete micritization process forming microsparite (Fig. 3.c), a mass of high-porosity aragonite crystals. Thus some of the original crystal texture and orientation

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conserved can be recognized under the petrographic microscope. In the second stage the

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dissolution advances and the microsparite crystals evolve into a mass of micritic crystals (Fig. 3.d). The changes in crystal size (microsparitization and micritization) in what were

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originally green speleothems change their colour, since the microsparite areas are dull turquoise and the micritized ones are white.

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The morphology of the micritic and microsparitic zones is controlled by the initial texture

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of the minerals and is limited by the section observed. In samples formed by aragonite fan

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layers (Fig. 9) the micritized areas are distributed as bands only in the base of the fans and separate different layers of crystals (Fig. 9.c). At the contact between the nonmicritized/micritized crystals an irregular porous fringe is observed. The pores are 0.2 - 1 mm in size. Sections perpendicular to the aragonite fans (Fig. 9.b) display micritized areas concentrated in the core of the fans and in the vicinity of the feeding channels.

6.2. Aragonite-to-calcite transformation The most common textural characteristic of the aragonite-to-calcite transformation is the presence of aragonite relics inside the calcite crystals. These relics may preserve the original mineralogy or be textural relics. The presence of relics preserving their original texture inside a later phase is a common feature in transformation processes (Mazullo, 1980). In general, calcite precipitation requires a stable surface as well as a saturated fluid. 15

ACCEPTED MANUSCRIPT While calcite crystals have been most commonly observed to grow on calcite nuclei, since the activation energy of aragonite and calcite is nearly identical, calcite can nucleate

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directly on aragonite (Bathurst, 1976).

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The aragonite to calcite transformation takes place in the presence of a fluid undersaturated with respect to aragonite and saturated with respect to calcite (Kinsman, 1969; Katz et al.,

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1972; Maliva et al., 2000; Frisia et al., 2002) and appears to be driven by the

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microstructural characteristics of the aragonite (Frisia-Bruni and Wenk, 1985). In this case, dissolution of the metastable aragonite phase and precipitation of the stable calcite phase

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occurs (Perdikouri et al. 2008). Very thin water films (100 Å - 1 μm) dissolve aragonite and precipitate calcite virtually at the same time. As the process goes on, the fluid gets locally

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saturated in aragonite, losing its dissolving power and leaving relics (Pingitore, 1976; Frisia

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et al., 2002). This incomplete dissolution may preserve aragonite crystals as solid

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inclusions inside calcite crystals.

In Basajaún Etxea Cave, high initial fan porosity favours the presence of intersticial water. Calcite nuclei precipitate from this water, with the aragonite surface acting as a substrate for growth. This process favours transformation when water saturation varies between undersaturated in aragonite and saturated in calcite (Maliva et al., 2000; Frisia et al., 2002). In this study, successive stages of transformation can be observed (Fig. 4): Stage 1. The initial porosity of the aragonite fans promotes growth of calcite crystals (100 μm) along the aragonite fibres, which act as nuclei for calcite crystallization. Aragonite crystals do not contain Mg, but calcite crystals contain up to 19147 ppm of MgCO3 (Table I; Fig. 4.a, b).

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ACCEPTED MANUSCRIPT Stage 2. Aragonite fibres arranged in fans show patches of scattered calcite crystals affecting several fibres. Aragonite fibres are completely recognizable through the patches

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of semi-transparent and cloudy calcite crystals (Fig. 4.d).

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Stage 3. The number of calcite crystals increases and the crystals merge together (Fig. 4.e). Stage 4. A calcite mosaic is formed covering large areas of the speleothem. In some of the

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calcite crystals the aragonite fibres still can be seen; in other cases no relics can be seen in

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thin section.

Stage 5. The whole aragonite fan is transformed into calcite, with a texture composed of

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large columnar calcite crystals showing undulant extinction and no aragonite relics. Between these columnar crystals some smaller pyramidal calcite crystals (≈ 0.5 mm)

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appear (Fig. 4.a, f).

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Another mechanism of transformation starts with the precipitation of calcite cement in the

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pores left by the aragonite (Fig. 10). When the pore is completely closed by calcite cement, the process continues into the contact between the aragonite and the calcite. Calcite engulfs the aragonite during its growth; the result is the formation of a rim over the calcite borders that are in contact with aragonite (Fig. 10.a). This rim of about 40 µm is slightly darker that the rest of the crystal and it is the result of the transformation (Fig. 10.b), where no aragonite has been preserved, and the original dimensions of the pore probably reached the inner limit of the rim.

6.3. Speleothem colour versus diagenesis Calcite and aragonite can be coloured by any chromophore divalent transition metal substituting Ca2+ or Mg2+ in their sites in the mineral structure (White, 1997). As speleothems grow, trace elements enter the crystal lattice; their ratios depend on such 17

ACCEPTED MANUSCRIPT environmental conditions as temperature or water feed during formation (Finch et al, 2001, Zhou et al. 2007).

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The samples analyzed in this work show green, turquoise and pink colours. All of them

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have Cu values over 230 ppm except the micrite of the pink sample. Cabrol (1978) demonstrated that the blue colour of beaded helictites in a French cave was caused by

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incorporation of copper; moreover, he found that the threshold for the appearance of the

441 ppm) as Mn2+ state (White, 1997).

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blue tint is Cu concentration from 50 to 100 ppm. Pink, is due to the presence of Mn (75 –

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The pink sample is formed by white aragonite micrite and secondary pink calcite replacing the aragonite. The aragonite analysed lacked any of the Mn and Cu chromophore elements

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present in the calcite, indicating that aragonite was white in origin, and during the

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transformation process the colour was taken on by calcite that incorporated Mn and Cu.

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The tinting effect of the more abundant Cu is masked by the less abundant Mn present. In the case of the green-turquoise samples, the calcite analysed has 10 times more Cu than the aragonite replaced, because Cu enters in the calcite lattice more easily than in the aragonite one (Morse and Mackenzie, 1990). For aragonite, depending on the crystal size (100% aragonite according to XRD analysis) the colour changes from green in the fibrous crystals, to turquoise in the microsparite and white in the micrite. However, it has been observed that the concentrations of Cu in all the textures are very similar, with no significant variations to explain the differences in colour. Hence, probably crystallography and crystal orientation determine colour. Beck (1978) observed that the yellow and white colour zones in "fried egg" stalagmites, which are deposited from a single water source, are due to crystal texture differences, not chemical or mineralogical factors.

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ACCEPTED MANUSCRIPT 6.4. Paleoenvironmental significance 6.4.1. Presence of aragonite.

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Normally the presence of calcite and aragonite in caves has been associated with alternating

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wet and arid periods, with calcite precipitate during wet periods and aragonite during arid ones (Railsback et al., 1994; Frisia et al., 2002; McMillan et al., 2005; Wassenburg et al.,

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2012). During the arid periods the water intake is lower and/or the residence time in the soil

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and epikarst increases and therefore the Mg/Ca ratio in the drip waters increases and favours the precipitation of aragonite.

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In the case of the caves in which the host rock is dolomite, as in Basajaún Etxea Cave, the mineralogical changes are less subject to climatic influence because the Mg / Ca ratio is

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always high, enabling the precipitation of aragonite regardless of the climate (Wassenburg

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et al. 2012). In Basajaún Etxea Cave, the presence of calcite is poor, and in all of the

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studied samples it is transforming the aragonite. With this in mind and without more data, aragonite may or may not be an indicator of high evaporation/aridity, as high Mg/Ca ratio alone is enough to form aragonite, but the presence of calcite may give a clue to changes in water chemistry related to changes in the rainfall/climate regime.

6.4.2. Isotope and trace element data. Most of the paleoclimatic interpretations from speleothems are based on their isotopic signals. Our study shows that δ18O values (between -5,2 and -2,3‰) corresponds to meteoric waters (between -5 and -3‰ (Siegenthaler, 1979)). The heavy carbon isotope enrichment of the micrite with respect to the original aragonite may be due to the greater mobility of the light isotope. In laboratory experiments conducted with CaCO3, Skidmore et al. (2004) observed that Ca12CO3 dissolves preferably to Ca13CO3. 19

ACCEPTED MANUSCRIPT The more varied δ13C values may provide significant paleoenvironmental information. Two main aspects have to be considered: the first, in the micritized speleothems the δ13C is

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higher than in the non-micritized ones. Heavier values are commonly considered an

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indicator of lower rainfall or higher evaporation rates, decrease of the dripping waters or changes in the vegetation above the cave, all linked to arid conditions (Stowe and Teeri,

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1978; Turner 1982; Mickler et al., 2006); however, in our case, the micritization process is

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probably diagenetically driven related to microdisollution preferably dissolving the CaC12O3 and producing more positive values of δ13C. The second aspect, the striking

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positive values of δ13C of the pink sample are rare, given the sample’s position (along with the position of the green samples) within a reduced area. The limited spatial distribution of

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these speleothems indicates that their development was conditioned by factors that occurred

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exclusively in that limited part of the cave. Although several factors can lead up to 10 ‰

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heavier δ13C signal in speleothems (e.g. low dripping rates, reduction in growth and changes in vegetation associated with aridity - Stowe and Teeri, 1978; Turner 1982; Mickler et al., 2006), these processes rarely lead to a signal so positive as in the pink speleothem of Basajaún Etxea Cave. These values are comparable with the ones obtained from the study of cryogenic cave carbonates (CCC), whose positive values are due to the kinetic effect of freezing (Clark and Lauriol, 1992), which is not the case in Basajaún Etxea cave. A possible explanation is that these speleothems formed and/or transformed influenced by hydrothermal fluids rich in Mn circulating through host rock discontinuities. These fluids possibly of hydrothermal origin could have become enriched in metals as they circulated through the extensive ophite outcrops associated with Keuper deposits located 500 m to the SW and 1 km to the NW of the cave, and then through faults and fractures that surround the cave. Similar processes of incorporation of Mn into karst (although in 20

ACCEPTED MANUSCRIPT different types of speleothems) have been described in the Cave of Lazalday (Zarate-Alava) by Yusta et al. (2009a, 2009b). Rodríguez-Berriguete et al. (2012) showed that thermogene

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travertines from the Canary Islands have meteoric δ18O values (between -11 and -2‰) and

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hydrothermal δ13C values (between 4 and 11‰).

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7. Conclusions

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Our work on the Basajaún Etxea Cave has shown that the diagenetic processes undergone by the speleothems change their texture, mineralogy, colour and geochemical signals. Our

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main conclusions are:

The main diagenetic processes in Basajaún Etxea Cave are micritization and aragonite-to-

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calcite transformation. Both processes, but specially micritization, highlight the high

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microporosity pattern of the fibrous aragonite. The process of micritization on the green

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speleothems leads to the formation of two textures: in an incomplete process the original crystals transform into a microsparitic texture that changes the green colour to turquoise; in a complete process a micron-sized mass of crystals of white colour is formed. Geochemical analyses show that micritization of aragonite has no effect on the trace element content, whereas the δ13C isotopic values are heavier in the micrite than in the original aragonite. The process of aragonite-to-calcite transformation changes the texture from fibrous to equant in all the samples regardless the colour. During this transformation the green samples preserve their colour, but the pink colour is present only in the calcites transformed from white aragonite, so the Cu and Mn that calcite incorporates during diagenesis are responsible for the colour. Calcite becomes 10 times more enriched in Cu than the original aragonite. The amount of Mg in calcite is higher than in aragonite as well, but Sr is lost during the process, being close to 0 in the calcites. The green samples show lighter δ13C in 21

ACCEPTED MANUSCRIPT the calcites than in the aragonite from which they were transformed, with the δ18O unchanged. The isotopic ratios of the pink (diagenetic) calcites have proved to be heavier in

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both the δ18O and δ13C.

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Our work shows that diagenetic processes modify the texture and geochemical signals that are used as paleoenvironmental indicators. While this variation does not invalidate

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speleothems as environmental records, speleothem formation signals should be

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distinguished from their transformation signals, and so the correct interpretations need to be

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well supported by detailed mineralogical and petrological studies.

Acknowledgements:

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This work received financial support from the Gobierno de Navarra and Project CGL2011-

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27826-C01-01 from the MCINN. Esteban Faci, Alfonso Meléndez and Luis Jordá are

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Skidmore, M., Sharp, M., Tranter, M., 2004. Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments: Implications for detection of microbial CO2 signatures using δ13C-DIC. Geochimica et Cosmochimica Acta, 68, 4309-4317. Stowe, L.G., Teeri, J.A., 1978. The geographic distribution of C4 species of the Dicotyledonae in relation to climate. American Naturalist, 112, 609–623. Tarhule-Lips, R.F.A., Ford, D.C., 1998. Condensation corrosion in caves of Cayman Brac and Isla de Mona. Journal of Cave and Karst Studies, 60, 84-95. Trapote Redondo, M.d.M., Marchán, C., Gómez de las Heras, J., López, M.T., Arránz, J.C., Martínez, B., Locutura, J., Rubio, J., Alberruche, E., Avilés, C., 2006. Informe de la minería en Navarra. IGME, Madrid. Turner, J.V., 1982. Kinetic fractionation of carbon-13 during calcium carbonate precipitation. Geochimica et Cosmochimica Acta, 46, 1183-1191. Turner, K., 2002. Chromophores producing blue speleothems at Cliefden, NSW. Helictite, 38, 3-6.

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ACCEPTED MANUSCRIPT Wassenburg, J.A., Immenhauser, A., Richter, D.K., Jochum, K.P., Fietzke, J., Deininger, M., Goos, M., Scholz, D., Sabaoui, A., 2012. Climate and cave control on Pleistocene/Holocene calcite-

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to-aragonite transitions in speleothems from Morocco: elemental and isotopic evidence. Geochimica et Cosmochimica Acta, 92, 23-47.

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National Speleological Society, Alabama, pp. 239-244.

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White, W.B., 1997. Colour of speleothems. In: Hill C., Forti, P. (Eds.), Cave minerals of the world.

Woo, K. S., Choi, D. W., Lee, K. C., 2008. Silicification of cave corals from some lava tube caves

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in the Jeju Island, Korea: Implications for speleogenesis and a proxy for paleoenvironmental

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change during the Late Quaternary. Quaternary International, 176-177, 82-95. Woo, K.S., Choi, D.W., 2006. Calcitization of aragonite speleothems in limestone caves in Korea: Diagenetic process in a semiclosed system In: Harmon, R.S., Wicks, C. (Eds.), Perspectives

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on karst geomorphology, hydrology, and geochemistry. A tribute volume to Derek C. Ford

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Colorado, pp. 297-306.

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and William B. White. Geological Society of America Special Paper, 404, Boulder,

Yusta, I., Castellano, A., Aranburu, A., Velasco, F., 2009a. Los depósitos de Mn-Al-Fe de la cueva de Lazalday (Zarate, Alava): composición química y mineralogía. Geogaceta, 47, 31-34. Yusta, I., Castellano, A., Aranburu, A., Velasco, F., 2009b. Absorción de metales en espeleotemas de Mn-Al-Fe de la Cueva de Lazalday (Zarate-Alava). Macla, 11, 203-204. Zhang, H., Cai, Y., Tan, L., Qin, S., An, Z., 2014. Stable isotope composition alteration produced by the aragonite-to-calcite transformation in speleothems and implications for paleoclimate reconstructions. Sedimentary Geology, 309, 1-14. Zhou, H., Chi, B., Lawrence, M., Zhao, J., Yan, J., Greig, A., Feng, Y., 2007. High-resolution and precisely dated record of weathering and hydrological dynamics recorded by manganese and rare-earth elements in a stalagmite from Central China. Quaternary Research, 69, 438-446.

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ACCEPTED MANUSCRIPT Zupan-Hajna, N., 2002. Chemical weathering of limestones and dolomites in a cave environment. In: Gabrovšek, F. (Ed.), Evolution of karst: from prekarst to cessation, Zalozba ZRC,

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Postojna-Ljubljana, pp. 347-356.

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Figure captions:

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Table I. Mean chemical composition obtained with an EMPA of the samples depending on

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the colour and texture. (n) number of samples. (> lod) below limit of detection.

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Fig. 1. Geological setting of Basajaun Etxea cave. a) Location of the area studied within the

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Iberian Peninsula. The gray zone corresponds to the Pyrenees. b) Simplified geological map

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of the region. Modified from IGME (1975, 1978).

Fig. 2. Ground plan of the Basajaun Etxea cave. The coloured areas symbolize the zones in which coloured speleothems occur. Photographs of the crusts and the sampling sites are also indicated.

Fig. 3. a) View of the ceiling of the cave. a) Green and turquoise speleothem. The green speleothems are formed by unaltered aragonite crystals, whereas the turquoise ones are formed by microcrystalline aragonite. b, c and d) Microphotographs of thin sections, plainpolarised light. b) Green speleothem showing the fresh aragonite crystals. c) Microsparitic textures found in the turquoise speleothems, the aragonite texture and orientation can still be appreciated. d) Microphotograph of the aragonitic micrite. 32

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Fig. 4. Aragonite-to-sparry calcite transformation process. a) Initial stage of the process,

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small pyramidal calcite crystals (green, Cc) grow on surfaces of the aragonite (Ar). SEM

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image. b) Semiquantitative X-ray EDS analysis of the pyramidal crystal showing that the composition corresponds to calcite as it contains Mg. c, d, e and f) Cross-polarised light

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microphotographs. c) Aragonite fan being transformed into a mosaic of sparry calcite that

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grows upwards. d) Detailed view of an area in which the transformation starts and the calcite crystals are isolated, containing a high amount of aragonite relics. e) The

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transformation advances and the calcite crystals begin to merge and form a mosaic. In this case the amount of aragonite relics in the crystals is much lower than in d. f) Example of a

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undulant extinction.

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fan that has been totally replaced by calcite inheriting the original morphology and

Fig. 5. Photographs of the samples, the points of isotope analyses are marked in red. Below, a table shows the δ18O and δ13C values for each point. a) Green sample. b) Pink sample. c) Turquoise sample. d) Cross plot of the isotopic composition of the points analyzed. The colours correspond to the colour of the sample. The standard deviation is between 0.02 and 0.03 in all cases.

Fig. 6. Coloured samples used for the SIMS study. The squares correspond to the areas selected for the analyses. A) Pink sample formed by calcite (Cc) and aragonite (Ar). B) turquoise sample formed by microcrystalline aragonite. C) Green sample formed by calcite and aragonite of different textures.

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ACCEPTED MANUSCRIPT Fig. 7. a) Secondary electron (SE) image of the pink sample. The textures of calcite (smooth) and aragonite (rough) can be distinguished. The flat areas of calcite in hand

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sample show pink colour, while aragonite is white. The points of analysis are marked in

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yellow. On the right, a table shows the results obtained at each point. The shaded grey corresponds to aragonite and the rest to calcite. b) SE image of the turquoise sample. The

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entire sample consists of microcrystalline aragonite. The different textures are due to

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different crystal sizes. The points of analysis are marked in yellow. On the right, a table shows the results obtained at each point. (SE images in this Figure and in Fig. 8 were taken

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with a LEO 440 SEM at the Institute of Earth Sciences, Heidelberg University)

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Fig. 8. a) SE image of the green sample. The different textures are marked by a dashed line

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(see legend). The points of analysis and the profile are marked in yellow. On the right, a

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table shows the results obtained at each point. b) Detail of the profile and the Mg, Sr and Cu values of the section analysed. The colours correspond to different textures and mineralogy (see legend).

Fig. 9. a) Green speleothem showing micrite (white) lines. b) Plain-polarised light photograph of aragonite spherulites. The micrite corresponds to the dark areas and is limited to the core of the spherulite. The central void represents the feed channel. c) Detailed microphotograph of the micrite on the original aragonite crystals. The micritization process affects the tips of the fibres. Thin section, plain-polarised light.

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The transformation takes place in a rim formed on the calcite crystal.

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n

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24

7

3

CaCO3 (wt. %)

99,87

99,78

99,85

99,82

MgCO3 (ppm)

> lod

47,8

> lod

48,3

SrCO3 (ppm)

901,6

940,1

1062,9

MnCO3 (ppm)

92,8

235,6

> lod

FeCO3 (ppm)

85,2

330,9

CuCO3 (ppm)

> lod

231,5

Transformed Ar (Cc) Green Pink 6

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Original Ar White Green

13 99,66

1279,9

19147,1

1011,6

942,9

614,4

170,9

182

317,6

309,2

182,7

203,7

275,3

> lod

> lod

741,3

81,1

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97,91

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