Impact of wildfires on subsurface volcanic environments: New insights into speleothem chemistry

Science of the Total Environment 698 (2020) 134321

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Impact of wildfires on subsurface volcanic environments: New insights into speleothem chemistry Ana Z. Miller a,b,⁎, José M. De la Rosa b, Nicasio T. Jiménez-Morillo a, Manuel F.C. Pereira c, José A. Gonzalez-Perez b, Heike Knicker b, Cesareo Saiz-Jimenez b a b c

Laboratório HERCULES, Universidade de Évora, Largo Marquês de Marialva 8, 7000-676 Évora, Portugal Instituto de Recursos Naturales y Agrobiología de Sevilla, Consejo Superior de Investigaciones Científicas (IRNAS-CSIC), Av. Reina Mercedes 10, 41012 Sevilla, Spain CERENA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Cave speleothems represent one of the most important climate archives. • Jelly-like speleothems are composed of hydrous gels of amorphous aluminum silicates. • Stable isotopes identify plant-derived organic matter from overlying laurel forest. • Biomarkers of Erica arborea are recorded in speleothems by analytical pyrolysis. • Speleothems are archives of pyrogenic OM from the overlying burnt biomass.

a r t i c l e

i n f o

Article history: Received 17 June 2019 Received in revised form 4 September 2019 Accepted 5 September 2019 Available online 06 September 2019 Editor: Paulo Pereira Keywords: Volcanic caves Wildfires Soil organic matter Analytical pyrolysis Stable isotopes

a b s t r a c t Siliceous speleothems frequently reported in volcanic caves have been traditionally interpreted as resulting from basalt weathering combined with the activity of microbial communities. A characteristic feature in lava tubes from Hawaii, Azores and Canary Islands is the occurrence of black jelly-like speleothems. Here we describe the formation process of siliceous black speleothems found in a lava tube from La Palma, Canary Islands, Spain, based on mineralogy, microscopy, light stable isotopes, analytical pyrolysis, NMR spectroscopy and chemometric analyses. The data indicate that the black speleothems are composed of a hydrated gel matrix of amorphous aluminum silicate materials containing charred vegetation and thermally degraded resins from pines or triterpenoids from Erica arborea, characteristic of the overlying laurel forest. This is the first observation of a connection between fire and speleothem chemistry from volcanic caves. We conclude that wildfires and organic matter from the surface area overlying caves may play an important role in the formation of speleothems found in La Palma and demonstrate that siliceous speleothems are potential archives for past fires. © 2019 Elsevier B.V. All rights reserved.

⁎ Corresponding author at: Laboratório HERCULES, Universidade de Évora, Largo Marquês de Marialva 8, 7000-676 Évora, Portugal. E-mail address: [email protected] (A.Z. Miller).

https://doi.org/10.1016/j.scitotenv.2019.134321 0048-9697/© 2019 Elsevier B.V. All rights reserved.

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1. Introduction Cave speleothems represent one of the most important climate archives as their formation depends, aside from the mineralogy and chemical composition of the host rock, on the amount of groundwater that drips into the cave during cold and warm seasons (Pausata et al., 2011; Deméney et al., 2016; Columbu et al., 2018). Long-term environmental changes can thus be recorded in speleothems and recovered by oxygen and carbon stable isotope analysis (Fairchild and Baker, 2012; Miller et al., 2016; Zhu et al., 2017). In volcanic caves or lava tubes the most common speleothems are of siliceous nature owing to the mineralogy and chemical composition of the host rock (basalts) along with the chemical composition of the seeping water (Onac and Forti, 2011). However, more detailed studies on minerogenetic processes occurring in lava tubes clearly demonstrated that microbial activity supports the weathering process and silica deposition (De los Ríos et al., 2011; Northup et al., 2011; Miller et al., 2014; López-Martínez et al., 2016). The highly variable morphology and coloration of siliceous speleothems depends largely on the deposited minerals, the associated microbial communities and the preserved organic matter (OM) (López-Martínez et al., 2016; González-Pimentel et al., 2018). The two main sources of OM in speleothems are assumed to derive from the overlying soil and sediments or from microbial communities thriving in the cave systems (White, 2010; Mudarra et al., 2011; Northup et al., 2011; Hartland et al., 2012; Hathaway et al., 2014; Daza and Bustillo, 2015). Since the OM in those deposits has the potential to provide information about former climatic conditions and land use (Berna et al., 2012; Miller et al., 2016), a better knowledge of its nature and origin can help to improve our understanding of the impact of environmental changes in volcanic regions.

Frequently reported forms of speleothems in lava tubes are the jellylike deposits found on the walls and ceilings of lava tubes from Hawai'i (USA), Azores (Portugal) and Canary (Spain) Islands (White, 2010; Hathaway et al., 2014; Riquelme et al., 2015). However, neither the composition nor the processes involved in their formation are known. A type of gelatinous speleothems reported by Daza and Bustillo (2014) in lava tubes from Terceira Island, Azores, was associated with the fossilization of plant roots that penetrated into caves. Carbone et al. (2016) reported jelly-like speleothems in Santa Barbara level of the Libiola Mine (NW Italy). These authors described these gelatinous flowstones as jellystone, containing large quantities of water and poorly crystallized minerals. Jelly-like speleothems which cannot be explained by plant roots fossilization are the blackish deposits found in Llano de los Caños Cave in the volcanic island of La Palma, Canary Islands, Spain (Fig. 1). Andosols are a type of soil developed in volcanic regions. It is generally composed of poorly crystalline clay minerals, such as colloidal allophanes and imogolite, and other Al and Fe oxyhydroxides, produced by the weathering of volcanic rocks, which usually form jelly-like organomineral structures (Parfitt, 2009; De la Rosa et al., 2013; Takahashi and Dahlgren, 2016). In acid soils, such as Andosols, the release of hydroxides and numerous cations by the ashes of burnt plant biomass contribute to an increase in their pH which can cause a further loss of organic acids after the fire (Ulery et al., 1993; Certini, 2005; Neris et al., 2016). Such events lead to accumulation of black pyrogenic OM and desiccated soils. Therefore, post-fire soil erosion may subsequently promote leaching and transport of OM to the underlying cave environments. To test this hypothesis, we provide the first detailed report on black jelly-like deposits in lava tubes under forested fire-prone areas and the impact of wildfires on speleothem composition. For elucidating

Fig. 1. Satellite image of Llano de los Caños area and topography of the lava tube showing the sampling sites of speleothems BS1 and BS2. Courtesy of O. Fernández (GE Tebexcorade - La Palma).

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the origin of OM in these speleothems, the overlying andic soil (uncultivated land) and the most abundant vegetation (E. arborea) were also studied by elemental analysis, solid-state 13C nuclear magnetic resonance (13C NMR) spectroscopy, analytical pyrolysis (Py-GC/MS) and stable isotope analyses. 2. Material and methods 2.1. Site description and sampling La Palma is the northwestern-most island of the volcanic archipelago of Canary Islands, Spain, located in the Atlantic Ocean (28°40′N, 17°52′ W). Two main volcanos, Taburiente and Cumbre Vieja, separated by a valley confer the characteristic topography of the island (Troll and Carracedo, 2016). The geology of La Palma was extensively described by Carracedo et al. (2001). Cumbre Vieja was the most active volcano of the Canary Islands in historic times. The site studied is Llano de los Caños Cave (Fig. 1), located in Mazo, SE of La Palma Island (28°34′52″ N, 17°48′02″W), at 1050 m a.s.l. near the La Horqueta mountain. This lava tube is located in the Cumbre Vieja volcano area, which has lava compositions ranging from basanite to phonolite with titaniferous clinopyroxene, olivine and kaersutite as the major phenocryst phases in the mafic rocks (Klügel et al., 2005). The climate of La Palma Island is subtropical oceanic, classified as BSh by the system Köppen-Geiger, which is very mild and sunny most of the year, with rainfall concentrated in autumn and winter. It is strongly influenced by the humid northeast trade winds, which combined with the altitude and the northwest dry winds, produce an inversion layer and marked vegetation areas with a great floristic diversity (Santos Guerra, 1983). In inland areas, the weather becomes cooler and wetter with increasing altitude. The vegetation of Cumbre Vieja area is a laurel forest mainly composed of Erica arborea and Myrica faya, with dispersed Pinus canariensis (Fig. S1). These climatic conditions and vegetation favor the appearance of wildfires, which is a recurrent event in the Cumbre Vieja area. In the period 2000–2017, 343 wildfires and extreme rainfall events affected this island (Cabildo de La Palma, 2018), resulting in extensive soil erosion and forest devastation. The overlying cave area was affected by an intense wildfire in August of 2012 that devastated over 2000 ha (Sarriá, 2016; Fig. S1). In Llano de los Caños Cave, brown to black-colored jelly-like speleothems were observed coating the wall and ceiling of the cave, from an area of ca 1 m2. Samples of the black speleothems were collected in 2015 with a sterile scalpel, gathering it into sterile vials and storing at 4 °C until laboratory analyses. The selected sampling area is located at Galería de los Zapadores, one of the newest cave branches discovered in 2007 (Fernández et al., 2015), with a depth of approximately 4 m below ground level. The average temperature inside the cave is 15 °C and 92% relative humidity. The soil over the cave is an ash soil (Andosol) of volcanic origin, with a depth of ≤40 cm. Topsoil (0–5 cm) and the most abundant vegetation over the cave (Erica arborea) were also sampled. Composite soil samples were prepared by combining four subsamples taken within an area of ca 10 m2 above the cave sampling points. Litter layer was removed and the soil material was then transported in glass containers, dried at room temperature, sieved to fine earth (b2 mm) and homogenized for analytical determinations. Samples of Erica arborea overlying the cave sampling points were also collected by cutting small fragments of the plant with a sterile scalpel, which were gathered into sterile containers. 2.2. Morphological and microstructural characterization of the black speleothems Black speleothem samples were examined under an Olympus SZ51 stereomicroscope for describing sample colors and textures, as well as for hand-picking homogeneous materials for further mineralogical analyses.

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For field emission scanning electron microscopy (FESEM), bulk speleothem samples were sputtered with an ultra-thin gold/palladium (Au/Pd) coating for preventing charging of the samples and observed using a Jeol JSM-7001F microscope equipped with an Oxford energy dispersive X-ray spectroscopy (EDS) detector. FESEM examinations were operated using the secondary electron (SE) mode, a working distance of 10 mm and an acceleration voltage of 15 kV. X-ray micro-computed tomography (micro-CT) was performed with a Skyscan 1172 using the following conditions: accelerating potential of 100 kV, current intensity of 100 μA, a pixel spatial resolution of 17.79 μm, and an average of five radiographs was recorded at each position. During acquisition, 360 X-ray digital radiographs were recorded at different angles during step-wise rotation between 0° and 360° around the vertical axis. To reduce artefacts, an Al-filter (0.5 mm) was placed between the X-ray source and the object to attenuate the low photon energies. The resulting sample scanning time was about 70 min. Three-dimensional (3-D) reconstructions were performed using the instrument software packages NRecon 1.6.3, DataView and CTvox. 2.3. Geochemical and mineralogical characterization Mineralogical composition of the black speleothem samples was determined by X-ray diffraction (XRD) using a X'PERT-PRO (PANalytical) diffractometer with CuKα radiation and a X'Celerator detector. Powdered samples were measured at 40 kV and 35 mA, with a step size of 0.002°2θ and 20 s of counting time. Phase determination was achieved using the X'PERT “High Score Plus” analytical software and PDF2 database. The composition of OM was unveiled by elemental analysis (C and N), solid-state cross polarization magic angle spinning (CPMAS) 13C NMR spectroscopy and Py-GC/MS. These techniques have been successfully combined for the characterization of OM from complex matrices, such as soils and sediments (Fabbri et al., 1998; De la Rosa et al., 2012). Carbon (C) and nitrogen (N) contents of the black speleothem samples, topsoil and Erica arborea were determined in triplicate by dry combustion using a flash 2000 HT elemental micro-analyzer equipped with a thermal conductivity detector (Thermo Scientific, Bremen, Germany) at 1020 °C. The Total Organic Carbon (TOC) was determined after the removal of carbonates for the black speleothems and topsoil. Approximately 20 mg of each sample was ground to a fine powder, homogenized in a ball mill and treated with 1 M HCl, then washed and dried (40 °C, 48 h). Bulk carbon and nitrogen isotope composition (δ13C and δ15N, respectively) of the black speleothem samples, topsoil and Erica arborea was determined as described by Miller et al. (2016) for assessing the origin of the OM recorded in speleothems. In short, an elemental microanalyzer (flash 2000 HT, Thermo Scientific, Bremen, Germany) combined with a continuous flow isotope ratio mass spectrometer (IRMS) (Delta V Advantage, Thermo Scientific, Bremen, Germany) via a ConFlo IV interface unit (Thermo Scientific, Bremen, Germany) were used. Isotope analysis was carried out in triplicate. Speleothem and Andosol samples were decarbonated prior to IRMS analysis. Stable isotope results were expressed in the delta notation (δ) as parts per thousand deviation (‰) from corresponding international standards of Pee Dee Belemnite for carbon (13C/12C, PDB) and Vienna-Air (V-Air) for nitrogen, which are recognized by the International Atomic Energy Agency (IAEA). The standard deviation of bulk δ13C and δ15N were ±0.05‰ and ±0.3‰, respectively. The solid-state 13C NMR spectra of the black speleothem BS2, topsoil and Erica arborea were obtained in a Bruker Avance III HD 400 MHz instrument (Bruker, Billerica, MA, USA) operating at a frequency of 100.64 MHz and using ZrO2 rotors of 4 mm OD for 13C with Kel-F caps. The cross polarization (CP) technique was used during magic-angle spinning (MAS) of the rotor at 14 kHz (De la Rosa et al., 2017a). The solid-state 13C NMR spectra were acquired using a pulse delay of

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500 ms, a line broadening of 50 Hz, a contact time of 1 ms and a ramped 1H-pulse to circumvent Hartmann-Hahn mismatches. Between 40,000 and 100,000 scans were accumulated for each sample. The 13C chemical shifts were calibrated relative to tetramethylsilane (0 ppm) with glycine (COOH at 176.08 ppm). The spectra were quantified using the MestreNova 10 software by integrating the following chemical shift regions as described by Knicker (2011): alkyl C (0–45 ppm); N-alkyl/ methoxyl C (45–60 ppm); O-alkyl C (60–110 ppm); aromatic C (110–160 ppm); carbonyl/amide C (160–245 ppm). Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was performed using a 2020i double-shot pyrolyser (Frontier Labs, Fukushima, Japan) attached to a GC/MS system, as described elsewhere (De la Rosa et al., 2017a, 2017b). Briefly, approximately 1–2 mg of sample (speleothems, topsoil and plant biomass) was introduced into crucible capsules and pre-heated for 1 min in a micro-furnace at 300 °C for thermal desorption and subsequently at 500 °C for pyrolysis. The compounds evolved were then directly injected into a Agilent 6890 N gas chromatograph (Agilent Technologies Wilmington, DE, USA), which was equipped with a HP-5 ms-UI capillary column (Agilent Technologies, Basel, Switzerland), and an Agilent 5973 mass selective detector (Agilent Technologies Wilmington, DE, USA). Compound identification was attained via single-ion monitoring (SIM) and by comparison with mass spectra libraries (NIST14 and Wiley7). From the structural information provided by the pyrolysis analysis it was possible to construct contour van Krevelen density diagrams as described by Almendros et al. (2018). In short, pyrolysis data were represented by plotting pyrolytic yields for individual compounds calculated as total abundances (z), as density surfaces in the x,y plane defined by its atomic H/C (y) and O/C (x) ratios.

FESEM-EDS (Fig. S2), this material thus comprises a mixture of organic and hydrated aluminum silicate compounds. 3.2. Isotopic (δ13C and δ15N) signatures To further illuminate the composition of the speleothems, we determined the total organic C (TOC) content of the bulk speleothems, which are 3.1 ± 0.2% and 6.5 ± 0.9% for samples BS1 and BS2, respectively (Table 1). This confirms the organic nature of the black fraction. The δ13C value of BS1 is similar to that measured for the top 5 cm of the overlying soil and the most abundant vegetation (E. arborea), whereas that of BS2 is more negative (Table 1). Cosford et al. (2009) stated that the carbon isotope composition of a non-calcite speleothem is controlled by: (1) the sources of C dissolving in the groundwater (i.e. soil, atmosphere and bedrock), (2) evaporation in the cave, and (3) the extent of degassing of CO2 from emergent drip water due to changes in drip rate or ambient cave air pCO2. The contribution of CO2 from the dissolution of bedrock can be discarded due to the range of δ13C values. The contribution of CO2 from the atmosphere to the groundwater is negligible relative to biological sources in the soil. Thus, the difference existing between δ13C of BS1 and BS2 (−25.9 ± 0.6 and − 30.1 ± 0.1‰ respectively) is probably attributed to differences in the organic composition of drip water transported from the overlying soil. In this sense, the δ 13 C value of BS1 is in the range of diterpenoids, as reported by Simoneit (2005). In contrast, the δ13C value of the speleothem BS2 fairly agrees with values reported for triterpenoids (Simoneit, 2005). Both types of terpenoids are commonly present in laurel forests. The values of δ15N obtained for the black speleothems are typical of plants (from 0 to 7‰) (Maksymowska et al., 2000), as confirmed by the average value of the overlying E. arborea (4.0‰).

3. Results and discussion 3.3. Solid-state 13C NMR spectroscopy 3.1. Mineralogical composition of the jelly-like speleothems Among the black jelly-like speleothems from Llano de los Caños Cave, we selected two representative samples (BS1 and BS2) for a more detailed analysis (Fig. 2A, B). All collected replicates showed a brown to black color with hydrous jelly-like appearance and comparable mineralogy. When air-dried the samples exhibited a heterogeneous texture, from black compact material to a very yellowish fine powder (Fig. 2C, D). When dried at 50 °C overnight the speleothems lost 95% (±2%) of their weight, revealing very high water retention capacity of the jellylike material. This property is characteristic of the colloidal constituents of Andosols (Karube and Abe, 1998). Volcanic rock particles, depicting phenocrystals and vacuolar structure were observed (Fig. 2C, D). The overall volume of the samples showed low opacity or high transparency to X-rays as revealed by micro-CT (Fig. 2E). A diffuse layering was also observed suggesting the existence of different compaction levels during speleothem growth (Fig. 2F). FESEM-EDS of the bulk samples revealed a jelly-like material (Fig. S2A), mainly composed of C, Al and Si (Fig. S2B). In addition, clay-like minerals embedded in the jelly matrix were observed (Fig. S2C). Minor contributions of Ca and Fe suggest the presence of calcium carbonates (calcite or aragonite) and iron oxide (Fig. S2D). The X-ray diffraction of the bulk samples showed the typical pattern of the local basalt rocks (Fig. 3A). As particles with varying color and texture were identified under the stereomicroscope, we separated them into a fraction of light colored particles and a dark brownish slimy material (Fig. 2C,D). The light colored fraction exhibited an XRD pattern similar to the bulk samples, comprising minerals of the basaltic parent rock, such as augite, sodian anorthite, olivine, goethite, maghemite, analcime and nepheline, with minor calcite and illite (Fig. 3B). For the dark product (Fig. 3C), only two broad reflection bands in the range 15–35°and 35–45°2θ were observed, which are characteristic of amorphous aluminum silicates, such as imogolite and allophane, from volcanic ash soils (Arancibia-Miranda et al., 2013). As also revealed by

All CPMAS 13C NMR spectra were dominated by signals in the Oalkyl C region (60–110 ppm) (Fig. 4). The peak at 71 ppm together with the signals at 101–105 ppm are assigned to carbohydrates (Fig. 4A), probably resulting from plant-derived fresh OM and microorganisms. The signal in the alkyl C region (0–45 ppm) is present in all the samples with a maximum at 29 ppm. The high intensity (24 to 26% of the total 13C intensity) in the well-resolved alkyl C region and the low intensity assigned to carboxyl/amide C between 185 and 160 ppm of E. arborea (4%) suggests that long chain lipids, including waxes, resins or terpenoids dominate over peptide C contributions (Knicker et al., 2006). This is supported by the relatively low N-content and the corresponding high C/N ratio of this sample (Table 1). The 13C NMR spectrum of the Andosol topsoil showed a high relative contribution of aromatic C (160–110 ppm) with 25% of the total 13C intensity (Fig. 4B). However, the peaks at 143 and 151 ppm evidence that phenolic C (lignin) is also present. A comparable pattern exhibits the solid-state 13C NMR spectrum of the speleothem BS2 (Fig. 4C). Although the latter shows less aryl C contributions than the Andosol topsoil, it is still higher than that identified for strongly humified plant residues (Knicker and Lüdemann, 1995) or fire-unaffected andic soils (De la Rosa et al., 2013). Possibly, aside from biogenic sources, soluble pyrogenic residues, formed during wildfires, served as feedstock for the OM of the speleothem. The O-alkyl C/alkyl C ratio has been reported as an indicator of OM transformation, since carbohydrates are preferentially used as energy and C source for the build-up of microbial biomass, which is dominated by peptides and lipids (contributing to 45 to 0 ppm). The relative increase of carboxylic C, together with a low O-alkyl C/alkyl C ratio and a C/N ratio of 10–11 for the Andosol topsoil and BS2 (Table 1), reveal high maturation of their OM. The fact that the O-alkyl C/alkyl C ratio is slightly higher for the BS2 than the Andosol (1.4 vs. 1.0) suggests that the proportion of less degraded OM is higher in BS2, probably due to better preservation conditions for labile OM in the cave.

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Fig. 2. Black jelly-like speleothems from Llano de los Caños Cave, La Palma Island, Spain. (A, B) Photography of the black speleothems in the lava tube. (C) Stereomicroscopy image of the airdried speleothem. (D) Stereomicroscopy image of the hand-picked yellowish particles. (E, F) Reconstructed micro-CT image of the internal 3D structure depicting low opacity to X-rays. Grayish regions are organic-rich and brighter zones are attributed to minor carbonate deposition.

3.4. Biomarkers of Erica arborea and Pinus canariensis in speleothems With the goal of identifying potential biomarkers related to the origin of the OM contained in the black jelly-like speleothems, samples BS1 and BS2, E. arborea biomass and Andosol topsoil were subjected to double-shot Py-GC/MS. The first desorption step (at 300 °C) was followed by a second shot at pyrolysis temperature (500 °C), allowing the sequential examinations of the volatile products at mild conditions (thermal desorption) and the subsequent analysis of the more refractory organic compounds (pyrolysis step). The total ion current (TIC) chromatograms of speleothems BS1 and BS2, E. arborea biomass and Andosol topsoil at 300 °C are shown in Fig. 5. A total of forty-five compounds were identified (Fig. 5; Table S1). The speleothem BS1 (Fig. 5A) is dominated by methylinositol (peak 3), dehydroabietic acid (peak 16), abietic acid (peak 18), stigmastan-3,5-diene (peak 30) and β-sitosterol (peak 36). DPinitol (D-3-O-methyl-chiro-inositol) has been isolated from needles and wood of different pine species (Assarsson, 1958; Poongothai and Sripathi, 2013), while the other abundant compounds are commonly present in resin and bark of Pinus (Rowe, 1965; Joye and Lawrence, 1967). In addition, a diterpenoid, podocarpa-8,11,13-trien-15-oic acid

(peak 12), a few unsaturated stigmastanes (stigmastan-3,5-dien-7one: peak 43; stigmastan-4-en-3-one: peak 44), retene (peak 11), a pyrolysis product from proteins (9H-pyrido(3,4b)-indole, peak 9), and some minor peaks of methyl-6-dehydrodehydroabietate (peak 13), αtocopherol (peak 32), β-amyrin (peak 40) and α-amyrin (peak 42) were identified. The TIC chromatogram of BS2 (Fig. 5B) released at 300 °C is dominated by isomers of pentacyclic triterpenoids with formula C30H48 and C30H50O, which include oleanane, ursane, and related triterpenes (peaks 26, 27, 29, 40 and 42). The stable carbon isotope analysis (δ13C) of BS2 already suggested the dominance of triterpenoids, according to Simoneit (2005). The TIC chromatogram of E. arborea biomass at 300 °C (Fig. 5C) is dominated by aliphatic compounds such as phytadienes and squalene (peaks 5–7, 16), n-alkanes (peaks 17, 19, 21, 28), and fatty acids (peaks 8, 10). The presence of pentacyclic triterpenoids, previously identified in the speleothem BS2, was evident. The Andosol topsoil (Fig. 5D) and E. arborea biomass (Fig. 5C) TIC chromatograms were similar regarding the cluster of triterpenoids, similarity also shown by the TIC chromatogram of BS2 (Fig. 5B). The TIC chromatogram of sample BS2 (Fig. 5B) also revealed the presence of miristic acid (peak 4), palmitic acid (peak 8) and a few long chain n-

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Fig. 3. XRD patterns of the black jelly-like speleothems. (A) Bulk speleothem sample. (B) Hand-picked yellowish fraction. (C) Air-dried dark brown jelly-like material. Peaks are labelled from 1 to 7 and comprise: 1-augite; 2-anorthite sodian; 3-olivine; 4-goethite; 5-maghemite; 6-analcime, and 7-nepheline. Some peaks show more than one mineral assignation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

alkanes, such as n-nonacosane (peak 21), n-hentriacontane (peak 28), n-tritriacontane (peak 34) and squalene (peak 20). All of these compounds are present in the chromatograms of E. arborea (Fig. 5C) and Andosol topsoil (Fig. 5D). In general, C25-C33 n-alkanes are related to epicuticular waxes (Simoneit, 2005), which is a further evidence of the influence of the vegetation in speleothem BS2. Pentacyclic triterpenoids are chemical constituents of higher plants, especially in plant leaves where the presence of amyrin-type triterpenes in epicuticular waxes is common. However, their identification by GC/ MS is difficult as they differ only in the position of one methyl group and therefore show very similar mass spectra. For this reason, we relied on mass chromatograms of selected ion monitoring (SIM). The extracted ion chromatogram of m/z 408 suggested that most of the peaks in the black speleothem BS2 correspond to oleanane or ursane triterpenes (Fig. S3A). The same is observed in the chromatograms of E. arborea (Fig. S3B) and Andosol overlying the cave (Fig. S3C). Additional ion chromatograms of m/z 218 indicated that triterpenes are present in both speleothems (Fig. S3D), as well as in E. arborea (Fig. S3E) and the Andosol (Fig. S3F). The triterpene peaks identified in the speleothems were taraxera-2,14-diene (peak 26), oleana-2,12diene (peak 27), ursa-2,12-diene (peak 29), ursa-9(11),12-dien-3-ol (peak 37), taraxerol (peak 39), β-amyrin (peak 40), urs-12-en-3-one

Table 1 Elemental analysis (TN, TOC) and bulk isotopic signatures (δ13C and δ15N) of the studied samples (mean ± SD in % and ‰, respectively; n = 3). Sample

TC (%)

TN (%)

TOCa (%)

δ13Ca (‰)

δ15N (‰)

Speleothem BS1 Speleothem BS2 Topsoil (0-5 cm) Erica arborea

4.3 ± 0.5 7.2 ± 1.1 8.5 ± 0.4 53.7 ± 2.1

0.36 ± 0.06 0.65 ± 0.03 0.74 ± 0.09 1.14 ± 0.12

3.1 ± 0.2 6.5 ± 0.9 8.3 ± 0.2 n.d.

−25.9 ± 0.6 −30.1 ± 0.2 −26.6 ± 0.4 −27.1 ± 0.6

5.5 ± 2.5 7.4 ± 0.6 8.3 ± 0.9 4.0 ± 1.5

a TOC and δ13C analyses were performed after the removal of the inorganic C by treating the samples with HCl (1 M). n.d. means not determined.

(peak 41), α-amyrin (peak 42) and olean-12-en-3-yl acetate (peak 45) (Table S1). Compounds with the lupane, oleanane and ursane skeletons have been identified in geological samples and related to various early diagenetic degradation products (Ten Haven et al., 1992; Nakamura et al., 2010). For Ten Haven et al. (1992), taraxera-2,14-diene, oleana-2,12diene and ursa-2,12-diene, identified in Holocene buried mangrove sediments, were diagenetic products formed by dehydration of the alcohol group of precursor compounds, such as α- and β-amyrin, and taraxerol. The presence of these triterpenes in the speleothems could support this hypothesis. However, their identification among the thermal desorption products of E. arborea suggest that these three compounds were likely formed by dehydration during the wildfire, or alternatively during the analytical thermal desorption protocol which provoked vaporization of the pentacyclic triterpenes, which have melting points around 200 °C. αTocopherol, a compound present on plant chloroplasts, is identified in the desorption chromatogram of both speleothems. It is ubiquitously distributed throughout the plant kingdom (Brendolise et al., 2011; Newton and Pennock, 1971) including in E. arborea, as shown in Fig. 5C. The cumulative values of the main chemical compounds relieved by pyrolysis at 500 °C (Table S2), as well as the contour van Krevelen density plots (Fig. 6), simplified the comparison of the chemical organic compounds present in the different samples. The H/C and O/C values are in the range 0.20–2.50 and −0.20–1.20, respectively (Fig. 6). The organic compounds from the E. arborea sample are positioned in the chemical regions of compounds derived from polysaccharides, lignin and lipids (Fig. 6A; Sutton and Sposito, 2005). Lipids can be separated into hydroaromatic (terpenes and sterols), and alkyl compounds (mainly epicuticular waxes, such as n-alkanes, alkenes and n-alkanoic acids). In the Andosol (Fig. 6B), the main clusters are positioned both in the lipid (alkyl and hydroaromatic compounds) and lignin-derived compound regions. These compounds can have as original source the OM coming from the overlying vegetation (Almendros et al., 2018; Jiménez-Morillo et al., 2016, 2018), particularly Erica arborea.

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Fig. 4. CPMAS 13C NMR spectra of the studied samples. (A) Erica arborea biomass. (B) Andosol topsoil. (C) Black jelly-like speleothem BS2.

Concerning the speleothem samples (Fig. 6C,D), differences are clearly observed at the compositional level. Organic matter from sample BS1 is mainly dominated by polycyclic aromatic compounds (Fig. 6C), whereas hydroaromatic (terpenes and sterols) and polycyclic aromatic compounds are dominant in BS2 (Fig. 6D). The contour van Krevelen plot of speleothem BS2 (Fig. 6D) shows the predominance of hydroaromatic compounds, which are in line with those identified for E. arborea (Table S2, peaks 192–194, 198, 201–203). Therefore, organic matter in the speleothem BS2 may also have a direct contribution from fresh biomass of this plant species. 3.5. Implications of wildfires in the formation of volcanic cave speleothems The particular characteristics of the volcanic island of La Palma, with steep forested slopes, dense fire-prone forests and frequent rainstorms, draw a scenario that can help to elucidate the processes involved in the formation of the studied speleothems. Wildfire can lead to severe alterations of the forest soil structure and physical properties, such as aggregate stability, bulk density, pore size distribution and hydrological behavior (Neris et al., 2016; Jiménez-Morillo et al., 2017). In addition, it has been reported that intense fires increase soil water repellency, reduce the infiltration capacity and increase runoff rates immediately

after the fire (Robichaud et al., 2016; Pereira et al., 2018). However, soil water repellency decreases and water infiltration increases after 1 year of the fire event (Robichaud et al., 2016). This is partially related to the recovery of protective vegetation cover. Another important factor is the possible effect of the ashes formed by the combustion of vegetation and litter during the fire. There is a general consensus that ash affects the immediate post-fire hydrological response. It is commonly found that the ash layer increases water infiltration, mainly by intercepting and storing rainfall (Cerdà and Doerr, 2008; Zavala et al., 2009). However, Onda et al. (2008) reported the opposite trend. Woods and Balfour (2010) attributed this variability to site differences in soil texture and ash. Consequently, the effect of ash on water infiltration seems to be ambivalent, and it depends mostly in ash properties and soil-ash interactions (Prats et al., 2018). Under these circumstances leaching of dissolved organic compounds (either from the decomposition of natural plant remains or products of charred residues) and transport of fine particulate matter and ashes into the subsoil may be favored by the low thickness and the high porosity of the volcanic rock (Coleborn et al., 2018; Bian et al., 2019). This process may be accompanied by the presence of aluminum silicates forming gels, characteristic of Andosols (Parfitt, 2009), in which organic C is entrapped or adsorbed and will lead to the

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Fig. 5. Total ion current chromatograms of two black speleothems, Erica arborea biomass and Andosol topsoil subjected to thermal desorption. (A) Speleothem BS1. (B) Speleothem BS2. (C) Erica arborea biomass. (D) Andosol topsoil. The identified peaks are (Appendix A, Table S1): 1–Allyl-syringol; 2–Quinic acid; 3– Methyl-inositol; 4– Miristic acid; 5–7 Phytadiene; 8– Palmitic acid; 9– 9H-Pyrido(3,4b)-indole; 10– Linolenic acid; 11– Retene; 12– Podo-8,11,13-trien-16-oic acid; 13– Methyl-6-dehydrodehydroabietate; 14– Methyldehydroabietate; 15– Copalic acid; 16– Dehydroabietic acid; 17– n-Pentacosane; 18– Abietic acid; 19– n-Heptacosane; 20– Squalene; 21– n-Nonacosane; 22– A-neo-Olean-3(5)12-diene; 23– Cholesta-3,5diene; 24– A-neo-Ursa-3(5)12-diene; 25– A:D-neo-Oleana-12,14-diene; 26– Taraxera-2,14-diene; 27– Oleana-2,12-diene; 28– n-Hentriacontane; 29– Ursa-2,12-diene; 30– Stigmastan-3,5-diene; 31– A-neo-Oleana-3,12-diene; 32– α-Tocopherol; 33– A-neo-Ursa-3,12-diene; 34– n-Tritriacontane; 35– Taraxer-14-en-3-one; 36 –β-Sitosterol; 37– Ursa-9 (11),12-dien-3-ol; 38– Olean-12-en-3-one; 39– Taraxerol; 40– β-Amyrin; 41– Urs-12-en-3-one; 42– α-Amyrin; 43– Stigmastan-3,5-dien-7-one; 44– Stigmast-4-en-3-one; 45– Olean12-en-3-yl acetate. Ph is bis(2-hexylethyl) phthalate.

formation and growing of the speleothems. It is well known that forest burning produces smoke, particulate matter and charred wood, which are released to the atmosphere and soils (Chrysikou et al., 2008). Generally, burnt particles are made up of polycyclic aromatic compounds, which have been attributed the role of markers subrogated to forest fires (González-Pérez et al., 2014). These condensed compounds are produced during incomplete combustion of organic matter (Ravindra

et al., 2008; De la Rosa et al., 2019), being wildfires one of their main environmental sources (Ramesh et al., 2011). The contour van Krevelen plots of the studied samples show a conspicuous cluster in the region dominated by polycyclic aromatic compounds (low value of H/C and O/C), with the exception of the fresh biomass sample (E. arborea) (Fig. 6). A detailed study of this chemical region shows the existence of aromatic compounds with a high number of condensed rings, such

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Fig. 6. Contour van Krevelen density plots displaying cumulative abundances of pyrolysis compounds identified in Table S2 for: A) Erica arborea biomass, B) Andosol topsoil, C) Black jellylike speleothem 1 (BS1), and D) Black jelly-like speleothem 2 (BS2).

as fluorene (Table S2, peak 115), phenanthrene (Table S2, peak 130) and methyl-pyrene (Table S2, peak 157), confirming wildfire episodes in the studied area. In addition, the existence of these recalcitrant compounds in the speleothems may indicate that these secondary mineral deposits can be seen as a record of pyrogenic organic compounds from the overlying biomass (fresh vegetation and soil organic matter). This finding demonstrates that wildfires and organic matter from the surface area overlying caves may play an important role in the formation of jelly-like speleothems found in La Palma.

molecular analyses of the adsorbed OM recorded in the black jelly-like speleothems from Llano de los Caños Cave revealed the existence of two different organic matter pools: 1) a fresh pool from the overlying vegetation, and 2) a pyrogenic pool, from the fire-affected shallow organic matter (vegetation canopy and soil), which can be mobilized and transported into the underground environment through the soil and volcanic rocks. Considering that the organic composition of the deposits can be seen as a record of the aboveground vegetation and past fire events, they have a potential role as ecological or even climatic archives.

4. Conclusions Declaration of competing interest This study provided a comprehensive set of geochemical indicators in combination with mineralogical and organic assemblages to discern the origin of the black jelly-like speleothems found at Llano de los Caños Cave in La Palma Island. We confirm that amorphous aluminum silicates (e.g. imogolite, allophane) and iron oxyhydroxides (e.g. goethite, ferrihydrite), commonly found in volcanic soils, are able to form hydrous gels that can adsorb OM, which results in black gels. The

The authors declare no competing interests. Acknowledgements A.Z. Miller acknowledges the support from the Marie Curie IntraEuropean Fellowship of the European Commission seventh Framework

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Programme (grant PIEF-GA-2012-328689) and the CEECIND/01147/ 2017 contract funded by Fundação para a Ciência e a Tecnologia (Portugal). The authors acknowledge the Spanish Ministry of Economy and Competitiveness (project CGL2013-41674-P) and ERDF funds for financial support. The authors are grateful to the speleologist Octavio Fernández (GE Tebexcorade – La Palma) for the assistance during the field trip and photographic documentation in the lava tubes from La Palma Island. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.134321. References Almendros, G., Tinoco, P., De la Rosa, J.M., Knicker, H., González-Pérez, J.A., González-Vila, F.J., 2018. Selective effects of forest fires on the structural domains of soil humic acids as shown by dipolar dephasing 13C NMR and graphical-statistical analysis of pyrolysis compounds. J. Soils Sediments 18, 1303–1313. https://doi.org/10.1007/s11368-0161595-y. 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