The Red Coloration of Goikoetxe Cave's Speleothems (Busturia

Quaternary International 566-567 (2020) 141–151

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The red coloration of Goikoetxe Cave’s speleothems (Busturia, Spain): An indicator of paleoclimatic changes T

∗ V. Martínez-Pilladoa,b, , I. Yustaa, E. Iriartec, A. Álvarod, N. Ortegad, A. Aranburua, J.L. Arsuagab,e a Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU. Barrio Sarriena s/n, 48940, Leioa, Bizkaia, Spain b Centro Mixto de Evolución y Comportamiento Humanos UCM-ISCIII. Avda. Monforte de Lemos 5, Pabellón 14, 28029, Madrid, Spain c Laboratorio de Evolución Humana, Dpto. de Historia, Geografía y Comunicación, Universidad de Burgos. Plaza Misael Bañuelos s/n, Ediﬁcio de I+D+i, 09001, Burgos, Spain d Centro Nacional de Investigación sobre la Evolución Humana (CENIEH). Paseo Sierra de Atapuerca, 3. 09002, Burgos, Spain e Departamento de Paleontología, Facultad de Ciencias Geológicas, Universidad Complutense de Madrid. C/ José Novais, 12. 28040, Madrid, Spain

ARTICLE INFO ABSTRACT

Keywords: The most commonly used paleoclimatic proxies in speleothem studies are the carbon and oxygen stable isotopes Speleothem color and the trace elements of calcite. However, assessing the incorporation of other components, such as organic Organic matter matter, may also be of interest in interpreting and reconstructing the climate during speleothem growth. In this Spectroscopy work, the incorporation of humic and fulvic acids derived from overlying soils is proposed as the cause of the red Fluorescence coloration of speleothems from the Goikoetxe Cave (Busturia, Bizkaia). Through the application of petrological studies combined with X-ray ﬂuorescence, UV luminescence, Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR) analysis, it has been possible to correlate a variation of organic content in the overlying soils and the red coloration, being this stain a main proxy to study and reconstruct the seasonal paleoclimatic parameters during the speleothem formation.

1. Introduction analyzed, and it is correlated with the petrological characteristics presented by these stalagmites, providing additional information on the Speleothems are one of the main records for establishing paleocli- climatic conditions during their formation. matic and paleoenvironmental sequences during the Quaternary, as their formation may be related to different climatic variables that un- 2. Regional settings dergo changes over time. For its study, the most commonly used proxies are the stable isotopes of carbon and oxygen (eg. Dorale et al., 1998; The Goikoetxe cave, also known as the "Malloku System", is located Genty et al., 2003; Baldini et al., 2008), and the incorporation of trace within the karst of Peña Forua, in the municipality of Busturia elements in the carbonate structure (eg. Treble et al., 2003; Johnson (Bizkaia), within the Urdaibai Biosphere Reserve (Fig. 1A). The Peña et al., 2006; Fairchild et al., 2007; Fairchild and Treble, 2009; Osácar Forua karst is located on the northern flank of the Gernika diapiric et al., 2013). However, studying the incorporation of other components, anticline (Morales-Juberías and Fernández de Valderrama, 2010). It is such as organic matter, may also be of interest in interpreting and re- developed on reef limestones from the Lower Cretaceous (Aptian-Al- constructing the climate (eg. Baker et al., 1996; McGarry and Baker, bian) in the Urgonian facies (Fig. 1B), emerged and deformed during 2000; van Beynen et al., 2001; Blyth et al., 2008). the Alpine orogeny, from the Eocene to the Miocene (García-Mondéjar Despite the fact that iron is generally assigned as the main re- et al., 1985), when the process of karstification began, giving rise to a sponsible of the red color in both geological and archaeological mate- well-defined cone-doline-type landscape (Aranburu et al., 2015). The rials, it has been found that organic matter can be a main source of main entrance to the system is located inside the Goikoetxe barn (from speleothem color (e.g. Caldwell et al., 1982). which the cave takes its name), at a distance of about 6.5 km from the In this work, the cause of the red coloration that characterizes most mouth of the Oka river (Fig. 1B). of the speleothems formed in the Goikoetxe Cave (Busturia, Bizkaia) is The karstic system presents a principal network of galleries along

∗ Corresponding author. Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU. Barrio Sarriena s/n, 48940, Leioa, Bizkaia, Spain. E-mail address: [email protected] (V. Martínez-Pillado). https://doi.org/10.1016/j.quaint.2020.04.006 Received 31 January 2020; Received in revised form 3 April 2020; Accepted 4 April 2020 Available online 11 April 2020 1040-6182/ © 2020 Elsevier Ltd and INQUA. All rights reserved. V. Martínez-Pillado, et al. Quaternary International 566-567 (2020) 141–151

Fig. 1. A) Peña Forua karst location (red star) within, Bizkaia province (Spain) and the Urdaibai Biosphere Reserve. B) Geological map of the Peña Forua karst area (modified from Aranburu et al., 2015). C) Longitudinal cave profile and location of the Sala Roja (modified from Aranzabal and Maeztu, 2011). D) Malloku System with the three karst levels (modified from Aranburu et al., 2015). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3400 m with N–S orientation (Fig. 1C). It comprises three subhorizontal fluvial origin mainly at the base (Edeso et al., 2011), and chemical levels (Fig. 1D) of marked phreatic origin that present abrupt changes calcite precipitates (flowstones, stalagmites and stalactites) in the upper in the direction of the galleries due to intense structural control. In the part. The speleothem formation in this chamber occurs through long upper level (at about 80 m asl), which is practically full of sediments, it cracks in the ceiling, causing the alignment of the stalactites and sta- has been possible to explore only a few meters (Aranzabal and Maeztu, lagmites along the cavity (Fig. 2A). 2011). The intermediate level (about 50 m asl) constitutes 80% of the The most distinctive feature of the speleothems in the Sala Roja is karst system, and includes abundant speleothems and terrigenous de- the coexistence of abundant colored formations, from honey to red hues posits of different granulometry. The active level of the karst (at about (Fig. 2A), with some white stalagmites and thin colorless tubular sta- 30 m asl) receives water from the area of recharge, marked by a large lactites. This coloration in the speleothems could be related to the development of drains, as well as non-native water from the siliciclastic presence of cations incorporated during the crystalline growth, al- massifs draining through small water courses that run laterally to the though it could also be due to inclusions of other minerals or to the karst. presence of crystalline defects. Vadillo and Barberá (2011) carried out a The Sala Roja (“Red Chamber”), located on the intermediate karstic study of the chemistry of the dripping water along the second level of floor, presents a sequence of mixed filling formed by gravel and sands of the Goikoetxe cave, collected under stalactites of different colors to look

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Fig. 2. A) General view of the Sala Roja, where reddish speleothem formation is abundant. Note the alignment of red speleothems with the fractures in the ceiling. Picture from Grupo Espeleológico ADES. B) General stratigraphic outline of the Sala Roja and relationship between the different endokarstic infillings. Unscaled drawing. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

for a correlation between their colour and the concentration of metals (Fe, Mn and Al) by means of ICP-MS. The results did not show any difference that would allow differentiating the composition of the waters over the red and white stalagmites. Neither the study of dissolved total organic carbon in the dripping water (which varies between 0.68 and 1.38 mg/L over red and 0.38–1.38 mg/L over white speleothems, yielding mean values of 0.94 and 0.9 mg/L respectively) gave results that allowed relating the reddish coloring of some of the speleothems with the inclusion of organic matter from the vegetation cover of the massif (Vadillo and Barberá, 2011). Based on allostratigraphic criteria, at least two growth phases of drip speleothems in this cavity have been determined (Aranburu et al., 2015). The first one was developed over a wide flowstone that went broken and tilted, and where the second speleothem generation grew after its fall (Fig. 2B). Both generations contain red stalagmites, indicating that the coloration of these speleothems has been a continuous process over the years and not punctual.

3. Material and methods

In order to identify the cause of the characteristic reddish coloration of the Goikoetxe cave stalagmites, and given the existence of white speleothems from the same precipitation phase coexisting with the red ones in the Sala Roja, four stalagmites of different colors belonging to the two speleothem generations have been selected (Moreno, Antuá, Gorri and Zuri, Fig. 3) as well as two small tubular modern stalactites. The Moreno stalagmite, with the most intense red color, was collected in situ from the last speleothem generation (Generation II, Fig. 2B). The Antuá stalagmite, with an amberine-reddish color, was collected in situ from the Generation I (Fig. 2B). The Gorri and Zuri stalagmites, with reddish and white colorations respectively, were Fig. 3. Studied stalagmites from the Sala Roja of Goikoetxe's cave to determine found broken and displaced from their original positions, and they are the factors influencing the different colorations. Moreno has an intense red not assigned to any speleothem generation yet. coloration. Antuá and Gorri present a reddish tone, almost honey color. Zuri has no color stain, with a white general aspect. (For interpretation of the references 3.1. Petrology to color in this figure legend, the reader is referred to the Web version of this article.) Detailed petrological studies of the crystal fabrics were carried out to determine the growth of the sampled stalagmites. After cutting and The Antuá stalagmite was dated using an 8-channel ORTEC Octete polishing them, thin sections were made along the precipitation axis in Plus alpha spectrometer at the Jaume Almera Institute of Earth Sciences the laboratories of the Mineralogy and Petrology Department of the (ICTJA-CSIC). Chemical separation of the radioisotopes and purifica- UPV/EHU. The petrological study was performed using Olympus BH2 tion were performed following the procedure described by Bishoff et al. transmitted light petrographic microscope with an attached Olympus (1988). Isotope electrodeposition was carried out using the method DP10 digital photographic system, and was based on previous petro- described by Talvitie (1972) and modified by Hallstadius (1984). Age logical works (Frisia et al., 2000, 2002; Fairchild et al., 2007; Frisia, calculations were based on the computer program by Rosenbauer 2015). (1991). The Moreno stalagmite was dated by an Inductively Coupled Plasma 3.2. Chronology Mass Spectrometer (ICP-MS) at the University of Minnesota using the methodology described in Shen et al. (2002) and Cheng et al. (2009). To place chronologically the studied stalagmites and check the The ages for the Zuri stalagmite were obtained by Thermal Ionization ff di erent stages of speleothem formation in the Sala Roja of Goikoetxe Mass Spectrometer, (TIMS) at the Xi'an Jiaotong University following cave, both base and top of three of the stalagmites (Moreno, Antuá and the methodology from Cheng et al. (2013). The Gorri stalagmite has not 234 230 Zuri) were dated using the Uranium-series ( U/ Th) disintegration been dated yet. method.

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3.3. Trace elements by means X-ray ﬂuorescence 4. Results

In order to evaluate differences in the concentration of trace ele- 4.1. Stalagmites growth pattern ments that could influence these chromatic variations, four stalagmites and two additional colorless tubular stalactites collected from the same The four stalagmites studied present an identical growth pattern in chamber were analyzed. Wavelength dispersive X-ray fluorescence spite of their color differences. They present practically cylindrical (WD-XRF) analysis with a PANalytical Axios Advanced PW4400 spec- morphologies with a single drip point. The exterior of the stalagmites is trometer (Rh tube SST-mAX, at 4 kW) was performed at the SGIker characterized by a totally smooth surface with a soft texture, without facilities (UPV/EHU). Fused beads were prepared mixing 0.2 g of finely any signs of corrosion or significant alterations. The growth mor- ground sample with 3.8 g of a lithium borate flux (Spectromelt A12, phology, according to Miorandi et al. (2010), is of the Conical-shape Merck) and LiBr as non-wetting agent. Lower limit of detection for type, with carbonate precipitation both on the main axis of the sta- major elements are in the range of 0.01 wt% and ~5 ppm for trace lagmites and on the flanks, where it is somewhat less. They show an elements. agradational type in the growth laminae stacking pattern (Muñoz- García et al., 2016). Moreno, Antuá and Zuri stalagmites have a continuous growth 3.4. Speleothem luminescence without allochthonous material neither interruptions. In the case of Zuri, a change in the dripping point can be observed at its top with a It is known that organic matter luminescence provides a useful non- darker surface delimiting the two growth phases, but without erosional destructive and rapid method for assessing dissolved organic matter features. quantity and quality (Blyth et al., 2008). Gorri stalagmite presents a clear breakdown after its first 6 cm of Fluorescence analysis with ultraviolet (UV) light was carried out by growth. Despite this feature, and in harmony with the rest of the stu- means of an XRF-Core Scanner Avaatech at the CORELAB laboratory of died speleothems, the color hue is homogeneous along all the growth, the University of Barcelona, irradiating Moreno, Gorri and Zuri sta- and only small differences in saturation can be observed. lagmites with a UV light source of 380 nm. High resolution photographs were obtained to identify the possible content of fluorescent substances 4.2. Petrology (organic matter) and other impurities inside the stalagmites. The resolution of the photographs was 0.4 mm per pixel, with a focal aperture Petrologically, the beginning of the development of the Moreno of 2.8 and exposure times between 13 and 50 ms. Different color stalagmite is characterized by the presence of a first phase of growth of components were also recorded: RGB (Red-Green-Blue) and L*, a*, mosaic crystals, of small size (< 100 μm), on which prismatic, homo- b*(where L* is Lightness, ranging from 0 to 100; a* is the axis between metric and flat extinction crystals are developed that are practically green and red, and b* is the axis between blue and yellow). parallel to each other in identical crystallographic orientations (Fig. 4A). The rest of the speleothems do not retain their base. All the four stalagmites are composed by large crystals that form a 3.5. Spectroscopic and mineralogical analysis (Raman and FTIR) columnar fabric throughout their development. These crystals have regular contacts with straight edges (Fig. 4B) and develop perpendi- In order to identify the organic substances responsible for this stain, cularly to the speleothems' growth lines, increasing in size as they de- analyses were carried out by Raman spectroscopy and Fourier velop from 1 mm to more than 5 mm. In some areas, these crystals Transform Infrared spectroscopy (FTIR) on Moreno, Gorri and Zuri sta- appear transversely sectioned, simulating a large mosaic fabric lagmites. (Fig. 4C). The three stalagmites were sampled using a hand microdrill with a In most crystals, the typical rhombohedral cleavage of calcite is disc head to extract a wafer of about 1 g and 5 mL of 37% HCl was clearly observed (Fig. 4D and E), being more evident in the upper zone added. The samples were left under acid attack for full digestion. The of the stalagmites, where their development ends (Fig. 4F). There is no residue obtained was neutralized 24 h after reaction with NaOH 1M and presence of micrite or detrital particles, neither of diagenesis features in centrifuged repeatedly, washing the sample with deionized water in any of the stalagmites. between. The excess of water was pipetted and the sample was left to dry at room temperature in an extraction hood. 4.3. Chronology Raman analyses were performed at the CENIEH Archeometry laboratories with a confocal DXR Thermo Fisher Raman dispersive The ages obtained for the Antuá, Moreno and Zuri stalagmites cover spectrometer, with a laser emitting at 532 nm. The power radiation two different chronological ranges (Table 1). measured under the Olympus x50 microscope objective was about 0.2 The oldest stalagmite is Antuá, which starts its formation around mW–0.5 mW. Acquisitions of about 10 s and multiple additions were 165 ka BP and stops its growth near 118 ka BP, placing the Generation I used. The spectrometer worked in a spectral range from 55 to of speleothems formation in the Sala Roja during the Marine Isotopic − 3350 cm 1. The verification of the spectrometer was done with a Stage 5 (MIS-5). Zuri has the largest development spanning from 105 ka − polystyrene standard (main band: 1000 cm 1). BP to 2 ka BP. Taking into account the change in the dripping point FTIR spectra were acquired using a FTIR Nicolet 6700 Thermo observed in this stalagmite, and given the wide temporal range, more Fisher spectrometer. These analyses were performed by diamond atte- dating must be carried out to check the presence of hiatuses along its nuated total reflectance (ATR) where samples were placed directly on growth. This stalagmite covers both speleothem generations. Moreno is an ATR and pressed to achieve good contact with the crystal of the the youngest stalagmite, being developed during the mid-Holocene, − accessory. Spectra were acquired over the range 4000-400 cm 1 between 7 and 5 ka BP, and placing the Generation II during the − averaging 128 scans at 4 cm 1 resolution. All spectra were corrected Holocene. for the ATR and also corrected for attenuation by water vapor and CO2. Due to this, some bands appear as artificial artefacts in the region of 4.4. Trace elements by means X-ray fluorescence − 2400-1900 cm 1. The results for both Raman and FTIR analyses were treated with the Omnic software. In petrographic analysis, the absence of detrital fraction in the mi- crostratigraphy of the stalagmites was confirmed, and XRF analyses were performed to test the presence of iron and other possible

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Fig. 4. Stalagmites crystalline fabric under petrographic microscopy. A) Beginning of the carbonate precipitation with mosaic crystals of sparitic calcite and development of the columnar fabric in the Moreno stalagmite. B) Columnar crystals in which straight contacts between them can be observed. C) Columnar crystals in cross section. D) Detail of the typical rhombohedral cleavage of the calcite in the columnar crystals E) Calcite crystal with highly developed cleavage. F) Terminal zone of the crystals of the stalagmites top. Scale bars A, B, C, D and F = 1000 μm; E = 500 μm. chromophores in the carbonate crystalline network. emission of different speleothem compounds excited under UV light has The results indicate that all speleothems are very pure, composed been widely studied and catalogued (eg. White and Brennan, 1989; almost exclusively of CaCO3, and there are no significant differences in Shopov et al., 1994; Baker et al., 1996, 1998; 1999; Baker and Genty, the geochemical composition between white/colorless and red or honey 1999; Baker and Bolton, 2000; McGarry and Baker, 2000; Baker and stalagmites (Table 2). Spencer, 2004; Shopov, 2004). It has been observed that dissolved or- The range of variation in Fe of the colored stalagmites ganic matter (DOM) inside the calcite lattice emit a higher lumines- (357–392 ppm) coincides with that of the colorless ones (357–406 ppm) cence when illuminated under short-wave UV (SWUV, 200–250 nm) indicating that this element is not the cause of the coloration. In other (Baker, 2005), but at long-wave UV (LWUV > 300 nm) some lumi- chromophore elements as Ti, P or Sr, their concentration is sometimes nescence spectra of organic substances have been measured producing even higher in white and colorless stalagmites, discarding them as the emission colors in yellow or orange shades (Shopov, 2003, 2004). cause of the red coloration. The stalagmites studied in this work have been excited under LWUV (380 nm) and yet in the response to luminescence these orange colors 4.5. Speleothem luminescence are not observed, as well as any other shade both for organic and inorganic substances (Shopov, 2004). The emission colors obtained, both After discarding the presence of iron or other inorganic substances in the white and red stalagmites (Fig. 5), have been in different in- as the main cause of the reddish coloration, it was evaluated whether it tensities of blue, the general emission color of speleothems themselves was related to the presence of organic compounds. For this purpose, the when illuminated with UV light (van Beynen et al., 2001). However, response of calcite to UV light was examined. The intensity and color of certain differences in hue can be observed between the white stalagmite

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Table 1 U/Th dating of Antuá, Moreno and Zuri stalagmites. In each sample name, letter T refers to “top” and B to “base”. A) Dates obtained by alpha spectrometry at the Jaume Almera Institute of Earth Sciences (ICTJA-CSIC) for the Antuá stalagmite. B) 230Th dating results obtained by mass spectrometry at the University of Minnesota for the Moreno stalagmite and at the Xi'an Jiaotong University for the Zuri stalagmite. The error is 2σ error.

Sample name 238U (ppm) 232Th (ppm) 234U/238U 230Th/232Th 230Th/234U Age (yr BP)

Antuá-T 0.31 0.25 0.87 ± 0.02 2.172 ± 0.106 0.65 ± 0.03 118055 + 9890/-9020 Antuá-B 0.16 0.19 0.98 ± 0.03 2.052 ± 0.105 0.78 ± 0.04 165328 + 19475/-16495 B

238 232 230 232 234 a 230 238 230 234 b 230 c Sample name U Th Th/ Th d U (measured) Th/ U Th Age (yr) d UInitial Th Age (yr BP) (ppb) (ppt) (atomic x106) (activity x103) (uncorrected) (corrected) (corrected)

Moreno-T 266.0 ± 0.5 305 ± 6 512 ± 11 −205.8 ± 1.5 35.6 ± 0.2 5008 ± 31 −209 ± 2 4902 ± 43 Moreno-B 250.2 ± 0.4 1302 ± 26 159 ± 3 −198.7 ± 1.3 50.1 ± 0.2 7058 ± 32 −203 ± 1 6804 ± 138 Zuri-T 24.8 ± 0.0 520 ± 10 23 ± 1 210.1 ± 3.1 29.3 ± 1.4 2675 ± 128 211 ± 3 2107 ± 379 Zuri-B 59.2 ± 0.1 891 ± 18 934 ± 19 327.3 ± 2.0 853.1 ± 1.7 105940 ± 452 441 ± 3 105601 ± 500

−10 −6 U decay constants: l238 = 1.55125 × 10 (Jaﬀey et al., 1971) and l234 = 2.82206 × 10 (Cheng et al., 2013). −6 Th decay constant: l230 = 9.1705 × 10 (Cheng et al., 2013). − Corrected 230Th ages assume the initial 230Th/232Th atomic ratio of 4.4 ± 2.2 × 10 6. Those are the values for a material at secular equilibrium, with the bulk earth 232Th/238U value of 3.8. The errors are arbitrarily assumed to be 50%. a 234 234 238 d U=([ U/ U]activity – 1)x1000. b 234 230 234 234 l234xT d Uinitial was calculated based on Th age (T), i.e., d Uinitial =d Umeasured xe . c BP stands for “Before Present” where the “Present” is deﬁned as the year 1950 A.D.

Table 2 the luminescence emission form an alternating lamination of dark and XRF elemental analysis of different speleothem samples from the Sala Roja. All light bluish hues both in Moreno and Gorri stalagmites, and being more data are expressed in ppm except CaO, in wt. %. marked and rhythmic in Moreno (Fig. 5). Sample Color CaO Ti P Fe Sr 4.6. Spectroscopic and mineralogical analysis (Raman and FTIR) Zuri white 56.10 46 36 406 32 Tubular 1 colorless 56.40 11 72 357 12 fi Tubular 2 amber 56.05 33 64 385 10 To con rm the presence of organic matter in the colored spe- Moreno red 56.08 26 11 357 36 leothems, Raman and FTIR spectroscopic analytical techniques were Moreno red 56.43 19 12 357 37 applied. In the spectrum obtained by Raman microscopy (Fig. 6) the − − Antuá reddish 56.00 7 20 392 28 characteristic peaks around 1600-1500 cm 1 and 1400-1300 cm 1 of Antuá reddish 56.48 36 17 364 22 – Gorri reddish 55.25 2 15 357 46 the G and D bands of the C C bond respectively (Gázquez et al., 2012) can be sensed. These signals have been previously identified by Yang and Wang (1997) in graphite and carbons and also in humic acids in natural waters, which could support the presence of organic matter in the analyzed stalagmite. The spectra obtained by FTIR also show characteristic peaks related to the presence of organic compounds (Fig. 7). A summary of the main bands observed in the studied stalagmites is presented in Table 3. In-

dicative signs of aliphatic compounds (C–H bonds of –CH2 and –CH3) − − appear around 2920-2850 cm 1 and 1100-1000 cm 1 (Gázquez et al., 2012; Parolo et al., 2017). − Around 1650-1550 cm 1, the band of C]C bond and H bonded to C]O bonds, typical of aromatic compounds (Gázquez et al., 2012), is − observed. It should be noted at 1575 cm 1 the band of the amide N–H and C]N, that could be related to polypeptides (Zaccheo et al., 2002). − In 1400-1300 cm 1, representative peaks of the COO- functional group appear, indicating the presence of humic and fulvic acids (Anderson − et al., 2004; Gázquez et al., 2012). The intensity of the 1720 cm 1 Fig. 5. Photographs of Moreno, Gorri and Zuri stalagmites obtained under UV absorption band is related to peat decomposition and illustrates pro- light irradiation. gressive free acid release with increasing humification (Artz et al., 2008). and the red ones. Perhaps this difference can be attributed to the same cause of the red coloration, but at the moment it is not possible to say 5. Discussion for sure. In the case of the red stalagmites, a well-defined banding that was 5.1. Common characteristics and general climatic interpretation not noticeable under visible light appears under UV light. A correlation between high intensity of luminescence (with lighter bluish colors) and Despite of the color hue differences between the Goikoetxe spe- those areas with less intense reddish tones under visible light can be leothems, and their different ages, some common features to all of them appreciated. The areas with darker red color under visible light do not can be observed as the growth patterns and the petrological char- seem to emit much luminescence under UV light. These differences in acteristics. The homogeneity observed in the growth morphology, columnar

146 V. Martínez-Pillado, et al. Quaternary International 566-567 (2020) 141–151

Fig. 6. Raman spectra for Moreno, Gorri and Zuri stalagmites. G and D bands of the C–C bond are shown. A sample of pure calcite used in the laboratory as a calibration is included for comparison. crystal fabric, continuous growths and carbonate saturation reveal a The results obtained in the Raman and FTIR analyses corroborate relatively constant dripping with high humidity conditions in the Sala the presence of organic compounds, like aliphatic and aromatic amides Roja during the speleothem formation in the two growth generations. and humic and fulvic acids, derived from overlying soils in the reddish The three dated stalagmites (Moreno, Zuri and Antuá) place these two speleothems, that are not present in the white one. Therefore, the red growth phases in the Holocene and the MIS-5. This would imply a re- color of Moreno and Gorri stalagmites (and by extrapolation Antuá and latively similar climate during these two periods, both considered as the rest of the reddish and amber speleothems of the Goikoetxe cave) homologous in temperature, humidity and vegetation patterns (Klotz should be correlated with the incorporation of this organic matter in its et al., 2003). structure, and not with the presence of detrital particles or chromophore trace elements such as iron. This would coincide with what has 5.2. The cause of the red coloration been stated by some authors (Gascoyne, 1977; Shopov et al., 1994; van Beynen et al., 2001; Verheyden, 2004; Blyth et al., 2008), who de- The existence of abundant iron oxide particles in the endokarstic monstrated that, in addition to certain inorganic elements, there are infilling sequence, as well as in the host-rock of the Goikoetxe karst, other substances such as organic acids capable of intensely coloring induces to assign iron as the main agent responsible for the reddish speleothems, as has also been observed in El Soplao cave (Gázquez coloration of the cave stalagmites. After XRF analysis, it has been ruled et al., 2012). fl out that the cause of the reddish coloration of the speleothems in the The UV image of the three stalagmites (Fig. 5) shows a high uor- Sala Roja of the Goikoetxe Cave is linked to variations in the compo- escent response in blue colors, although Moreno and Gorri present a ff ff sition of inorganic trace elements in the calcite. Due to this, organic di erent hue that Zuri. This small di erence could be related to the high compounds are the most probably cause of this stain. concentration of humic and fulvic substances found in the reddish

Fig. 7. FTIR spectra for Moreno, Gorri and Zuri stalagmites. Blue bands represent the intervals associated with absorbance of aliphatic, humic and aromatic compounds. A sample of pure calcite used in the laboratory as a calibration standard is included for comparison. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the Web version of this article.)

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Table 3 Assignment of the principal descriptive IR adsorption bands in the studied stalagmites.

− Peak position (cm 1) Assignment Characterization Speleothems

Current study Literature

a 2921–2917 2920 Aliphatic vas(C–H) Fats, wax, lipids Moreno, Gorri a 2854–2850 2850 Aliphatic vs(C–H) Fats, wax, lipids Moreno, Gorri 1718 1720 v(C]O) Carboxylic acidsa Moreno 1637 1600–1650 Aromatic v(C = C) Lignin and other aromaticsa Moreno, Gorri

1430 1426 vs(C–O) Carboxylate/Carboxylic structures Moreno, Gorri (humis acids)a 1388–1395 1392–1430 Aliphatic δ(C–H), Phenolic (lignin) and aliphatic structures, calcitea,c Gorri, Zuri 2- vibrations of CO3 b 1260 1270 Phenol vas(C–O), Carboxylic acids Moreno carboxylic acid v(C–O) 2- c 869–875 866–887 Vibrations of CO3 Calcite Gorri, Zuri 2- c 711 712–715 Vibrations of CO3 Calcite Gorri, Zuri

Assignment: v stretching vibration; vas asymmetric stretching vibration; vs symmetric stretching vibration; δ bending vibration. a Artz et al. (2008) and references therein. b Margenot et al. (2015) and references therein. c Parolo et al. (2017) and references therein. stalagmites through the Raman and FTIR analyses, but at the moment major development of trees and edaphic horizon, and with no decom- this statement cannot be confirmed. Several works focused on the lu- position of the vegetation. Therefore, in these areas the accumulation of minescence of organic substances within speleothems (e.g. Baker et al., organic matter is much lower. It is known that every stalagmite location 1996, 1998; McGarry and Baker, 2000; Perrette et al., 2005) relate their within a cave is subject to its own specific karstic drainage regime presence with an intense luminescence response emitting visible light (Perrette, 2000). Therefore, the OM content of a stalagmite is depen- between 390 and 500 nm. Shopov (2003, 2004) point out that under dent on its location, and therefore on the soil and vegetation, hydro- LWUV the organic substances within the calcite network emit yellow logic events, and transfer and storage of OM in the karst (Perrette et al., and orange colors that cannot be seen in Goikoetxe's stalagmites despite 2005). As mentioned before, the formation of drip speleothems in the their high concentration in organic substances. Thus, the luminescence Sala Roja of the Goikoetxe Cave is in favor of large fissures through colors obtained, in blue hues, cannot be related itself with the presence which infiltration water penetrates into the cavity. The network of of these substances. Nevertheless, in the red speleothems, a clear la- fissures originated in the doline areas, with a highly developed edaphic mination pattern appears under the UV radiation, which is not observed horizon, gives rise to the formation of red stalagmites, as the infiltration under visible light. It is remarkable the clear correspondence between water incorporates and carries away the organic compounds derived the lighter shades of luminescence and those areas of the speleothems from the decomposition of the vegetal cover. Those stalagmites fed by where the reddish colors are less intense, while the darker and more the water infiltrated by fissures in areas of bare karst will, in principle, intense reds, related to the high concentrations of organic substances, have a white color. hardly emit any luminescence. Some authors have described this phe- nomenon as quenching effect caused by inner filtering produced by self- 5.4. The red coloration as an indicator of paleoclimatic/ absorbance (Mobed et al., 1996). van Beynen et al. (2001) studied this paleoenvironmental changes effect comparing different colored stalagmites, finding that the lighter speleothems fluoresced more intensely and at shorter wavelengths than The UV image of the reddish stalagmites shows different fluores- the dark samples, even when these were twice as concentrated in or- cence intensity of the carbonate caused by the unequal incorporation of ganic compounds. This result, similar as the observed within the red humic substances which deserves a detailed discussion. stalagmites from Goikoetxe, can be explained by the self-absorbance In the case of Moreno, a well-defined banding can also be seen in- effect by which, at higher concentrations of organic substances in the dicating a variable that modifies cyclically the vegetal cover and calcite, the incident UV light is absorbed by the molecules themselves. probably the rainfall amount. Variations in the rainfall rate will mark Extensive luminescence analyses have to be performed varying the the residence time of the water in contact with the soil and, therefore, wavelength of the UV emission to test the response of these speleothems the amount of organic matter dissolved in water and the differences in and the organic acids included, mostly illuminating with SWUV, where the red tone saturation in the different speleothem generations. In the DOM intrinsic fluorescence properties are better understood (Baker, case of Goikoetxe cave, the rainfall rate has been regulated within the 2005). But what it is confirmed by now, is that the cyclic pattern ob- epikarst as shown by the constant crystal fabric on Moreno stalagmite. served in the reddish stalagmites is due to the variation in the in- These periodic phases are not clearly observed under visible light in this corporation of calcium salts from organic compounds inside the crystals stalagmite, and neither observed under visible or ultraviolet lights in (e.g. van Beynen et al., 2001; Perrette et al., 2005) which are also re- the case of Zuri stalagmite. The chronological data for these two sta- sponsible for their red staining. lagmites place the growth of Moreno simultaneously to part of Zuri's formation, during the mid-Holocene. That means that the environ- 5.3. The differential coloration of the Sala Roja speleothems mental variations causing the cycles observed in Moreno must be af- fecting Zuri as well, but only modify the incorporation of the organic The differentiation in the staining of the speleothems developed in substances that stains the red stalagmites. the Sala Roja seems to be related to the heterogeneous distribution of The presence of organic matter that stains the carbonate crystals in the soil and the vegetal cover outside the cavity, which accumulate and the red stalagmites is, thus, a proxy to determine the rhythmic variation concentrate in dolines and/or penetrative lapies. These areas are the in the vegetal cover derived organic matter. This red coloration records most abundant in the Goikoetxe area, where the large development of climatic changes in the surrounding environment with particular em- holm oaks and gall oaks dominates the landscape. However, there are phasis in the edaphic organic matter derived from overlying vegetation. certain areas in which the karst is almost naked (karren fields), with no In Fig. 8, the intensity of the Lightness (L*) and the Red component

148 V. Martínez-Pillado, et al. Quaternary International 566-567 (2020) 141–151

Fig. 8. Lightness (L*, black line) and Red component (RGB, red line) intensity profiles measured over the UV light photograph of Moreno stalagmite though time. The profiles have been measured in the central part of the growth axis. Darkest luminescence bands are associated with high OM content due to self- absorbance effect (fluorescence quenching). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

(from the RGB colors) measured over the UV photography in Moreno stalagmite shows graphically the variation of the luminescence response of the calcite along the growth axis during its formation. A general trend to light (more fluorescent) calcite is detected from bottom (ca. 6.8 ka) to top (ca. 4.9 ka). This trend englobes the centimeter-scale multidecadal fluorescent cycles mentioned above, especially well developed in the central part of the speleothem (from 6.4 ka to 5.4 ka), which are formed by a light color semi cycle and a dark color semi cycle. Due to the scarcity of dates, the invoked dates and time intervals must be taken cautiously. The cycles become asymmetrical towards the bottom (predominance and amalgamation of dark intervals) and the top (predominance and amalgamation of light intervals). Finally, every cycle includes a millimeter to submillimeter scale lamination where light and dark laminae alternate. All these changes point to different time scale processes controlling the amount of organic matter infiltrated to the endokarst, that probably are related with the available precipitation water/organic matter ratio (Blyth et al., 2008, 2016). A general trend to less organic matter availability and infiltration (maybe less humid conditions) is observed through time, with different order multidecadal (cycles) and perhaps even seasonal variations (laminations). Unfortunately, our UV fluorescence proxy it is not enough to discuss properly the environmental evolution. Ongoing exhaustive UV spectra, stable isotopic analysis and trace elements record will complement the current information and will allow a most complete discussion of the paleoenvironmental/paleocli- matological processes that controlled the observed variations over time.

6. Conclusions

The uniformity in the growth of the calcite crystals of the four stalagmites studied, creating a columnar fabric, indicates a high and constant rate of precipitation within the cave, organic activity in the soils (vegetation cover) and infiltration of organic matter (humic substances). This humidity seems to be equivalent during the Holocene and during the MIS-5, given the ages of the three stalagmites dated. The reddish coloration that characterizes the vast majority of the speleothems in the Goikoetxe's Sala Roja seems to be linked to a high presence of aromatic and aliphatic compounds as well as humic and fulvic acids in the crystalline structure of the calcite. These organic compounds are derived from the organic activity in overlying soils and the degradation of a thick, well-developed vegetal cover over the cavity. The coexistence of several white stalagmites with the red ones in the same chamber seems to be related with the network of fissures in the ceiling, through which rainwater penetrates into the cavity. Even though the landscape in the surrounding area is mostly composed by a dense and well developed deciduous forest, some zones of the karst form lapies (karren fields) with no edaphic horizon. Is in these areas where the infiltration water cannot incorporate humic substances, due to the lack of soils, and forms the colorless and white speleothems. Under UV light, the Moreno and Gorri stalagmites show different lamination patterns with different time scale cyclic variations. This differential response points to a paleoenvironmental process that caused a change in the amount of humic substances dissolved in the infiltration water, probably in relation to different vegetation activity rates and organic matter availability in overlying soils, but not surely with the amount of rainfall. Thus, in the case of the Goikoetxe

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