GEOLOGICA BELGICA (2006) 9/3-4: 323 - 335

COMPARATIVE ANALYSIS OF FROM TWO OF NORTHERN . IMPLICATIONS FOR HOLOCENE PALEOCLIMATE STUDIES

Javier MARTÍN-CHIVELET1, María Belén MUÑOZ-GARCÍA1, David DOMÍNGUEZ-VILLAR1, María Jesús TURRERO2 & Ana Isabel ORTEGA3

1. Departamento de Estratigrafía, Instituto de Geología Económica (CSIC-UCM), Facultad Ciencias Geológicas, Universidad Complutense, 28040 Madrid, Spain 2. Departamento de Medioambiente, CIEMAT, Avda. Complutense 22, 28040 Madrid, Spain 3. Departamento de Ciencias Históricas y Geografía. Facultad de Humanidades. Universidad de Burgos, 09001 Burgos, Spain

(7 fi gures)

ABSTRACT. Two caves from Northern Spain (Cueva del Cobre and Kaite) and their Holocene speleothems are comparatively analyzed in order to discuss their potential for paleoclimate studies. Both caves are almost at the same latitude (43º 01’ N), located in the southern slope of the Cantabrian ranges, and separated by only ~60 km. Despite the proximity, their karstologic, hydrogeochemical and climatic characteristics determine a markedly diff erent speleothem development. Kaite is a small developed in Upper Cretaceous and is located only 12-20 m below the surface, which today shows a poor vegetal soil cover. It is a relatively warm cave (10.4 ± 0.1 ºC) with permanent but variable water drip, richly decorated with speleothems. In contrast, Cueva del Cobre is a larger cave, developed in Car- boniferous limestones, at depths of more than 100 m below surface. It is also colder (5.6 ± 0.1 ºC), with net infl uence of high mountain climate, and has scarcer speleothem development. O e stalagmites of Kaite are cylindrical, thin and elongated, whereas those from Cobre are more conical, shorter and thicker. All the studied samples consist of , and their oxygen and carbon isotope signals are valid for paleoclimate studies. A wide range of crystal fabrics is present in Kaite samples, refl ecting changes in the forming conditions, whereas in Cobre columnar calcite is the dominant one. Annual laminations are easily recognizable in Kaite samples, but absent or diff use in Cobre. Finally, hiatuses are more abundant in Kaite samples, refl ecting a rapid response of the cave to abrupt changes in climate, and scarcer in Cobre, where the system, more isolated from surface, seems to have a larger environmental inertia. Kaite speleothems are thus more sensitive to rapid variations in climate, and an annual resolution can be reached in best cases. Cobre stalagmites in general show more continuous records and are quite sensitive to long-term (decadal to centennial) climate trends. O e paleoclimate information that is being obtained from both caves is complementary and its integration should contribute decisively to the construction of paleoclimate series for Northern Spain.

Keywords: Speleothems, , stable isotopes, drip waters.

1. Introduction speleothem typology and their forming karstic systems arises as an essential task for deciphering any paleoclimate Speleothems are considered as an unique data source for signal from these deposits. obtaining high-resolution continental paleoclimate series Under that philosophy, two caves in Northern Spain, due to two factors mainly. First, they record a wide range selected for a broad paleoclimatic research project, are of proxy climatic data, such as stable isotopes, annual being investigated. O ese caves, named Cueva del Cobre growth laminae, fabrics, etc. And second, these proxy and Kaite (Fig. 1) are located in the southern slope of records can be precisely dated by U-series techniques, the Cantabrian Ranges. Although placed almost at the yielding a time scale in calendar years. O e composition, same latitude (43º 01’ N), and separated away by only 60 growth patterns, and internal microstratigraphy of these km, they contain contemporaneous calcite speleothems carbonates are however quite variable and diffi cult to showing marked diff erences in their isotopic composi- interpret, mainly because multiple parameters control tion, morphology, microfabrics and microstratigraphy. their development. As these parameters depend on both O e aim of this paper is to present a comparative study of the host rock (endokarst) and the surface environment, these speleothems, trying to interpret their diff erences in the understanding of the genetic relationships between the genetic framework of each karstic system. O en, the 324 J. MARTÍN-CHIVELET, M.B. MUÑOZ-GARCÍA, D. DOMÍNGUEZ-VILLAR, M.J. TURRERO & A.I. ORTEGA

2. The two caves

As indicated below, the two studied caves are situated in Northern Spain, a key region for understanding the evo- lution of the European climate because of its geographic position set between the North Atlantic, Mediterranean and North African regimes. O e climate is temperate, with marked seasonality, and a limited maritime infl uence be- cause of the location of the caves in the southern fl ank of the Cantabrian Ranges. O e main geographic and climatic characteristics of each cave are the following:

2.1. Kaite Cave

Kaite is located in the northern part of the Burgos Prov- ince, close to the small village of Cueva (Fig. 1). It is a small (~300 m), isolated, hung cave located over the six known karstic levels that form the main system of the Ojo Guareña Karst (e.g. Martín-Merino, 1986), a large Figure 1. Location map of the two studied caves in Northern endokarstic complex that comprises more than 120 km of Spain. Both are located in the southern fl anck of the Cantabrian mapped galleries. O is complex is a phreatic water-table Ranges. system that has been developed on gently dipping Upper Cretaceous carbonate rocks by the successive incision of potential of these speleothems and their forming caves the phreatic level (e.g. Eraso, 1986). for the obtention of paleoclimate record will be analyzed Kaite is developed 12-20 m below the surface in the car- and discussed. bonates of the Cueva Formation, Turonian - Coniacian O is study is limited to Holocene speleothems because in age (Alonso et al., 1982). O e host rock is essentially their forming conditions should not diff er excessively , partially dolomitized and with minor amounts from those of today, and thus can be interpreted with the of argillaceous material. O e cave is 860 m above sea level aid of the present day physicochemical and hydrologi- and the area has a relatively humid, temperate climate cal conditions inside and outside the caves. Pleistocene with marked Mediterranean infl uence. O e annual mean speleothems are also abundant in both caves and will be precipitation usually exceeds 1000 mm, with maximum described in further papers. pluviosity in October to January but also notable pre-

Figure 2. Average monthly precipitation in Kaite and Cueva del Cobre areas, based on the climatological data from the nearest offi cial stations of the Instituto Nacional de Meteorología (1960-2003). In the case of Kaite, the data correspond to averaged values for the complete years available from Villarcayo, Espinosa de los Monteros and Quisicedo stations. For Cueva del Cobre area, the data come from the town of Santa María de Redondo. The climate over this cave can be notably wetter than in that town, which is topographically located 400 m below the cave entrance. COMPARATIVE ANALYSIS OF STALAGMITES FROM TWO CAVES OF NORTHERN SPAIN 325 cipitation until April-May, and typically dry summers 3. Hydrogeochemistry and drip water (Fig. 2). During winter, part of the precipitations can fall monitoring as snow, but this never lasts on the ground more than several days or, exceptionally, a few weeks. O e present Drip water chemistry can vary over time in response to day vegetal cover above the cave is scarce probably due factors such as temperature, rainfall seasonality, hydrologi- to anthropogenic clearance, although small Quercus and cal features, or time-dependent water-rock interactions. Juniperus are present. For this reason, characterization of forming O e selected site for water and rock sampling is located drip waters is considered a key task for understanding at the end of a gallery (Las Velas Hall), separated 350 m the geochemical behaviour of a specifi c system (Frisia et from the main entrance and with very little anthropogenic al., 2000, Genty et al., 2001, Tooth and Fairchild, 2003, disturbance. O is point is characterized by approximately Musgrove and Banner, 2004, Baldini et al., 2005). 14 m of rock covering and by a stable cave climate (T = A sampling and monitoring program commenced in 10.4 ± 0.1 ºC, relative humidity > 98 %, absence of sig- June 2002 at Kaite Cave and is still active. O e details on nifi cant air currents). O ere, the seepage water is frequent the monitoring program, the sampling and the analyti- and speleothems are abundant, some of which are growing cal techniques employed can be found in Turrero et al. at the present time. (2004). Drip waters have also been collected from Cueva del Cobre in order to get a record of its chemical and 2.2. Cueva del Cobre isotopic composition. O e results of both caves and the

Cueva del Cobre is a vast karst system in Sierra de Peña- labra (Palencia Province, Fig. 1). It is a water-table cave with an active low-gradient stream passage and a series of higher, relict low-gradient levels developed within 200 m of height (Rossi et al., 1997). O is cave is notably deeper than Kaite, located between 100-200 m below the surface in a steeply dipping lime- stone unit, namely Agujas Limestone, which is Carbon- iferous in age (Westphalian-D) (Van der Graph, 1971). O e cave entrance is located at ~1600 m a.s.l., which is roughly the elevation of the local timberline. O e forest in the area is essentially formed by Quercus and Fagus, whereas the vegetal cover above the cave consists of high mountain grasses and bushes. O e climate of the area is notably colder and moderately wetter than in Kaite, mainly because of its higher topographic height and its more western location (most fronts come from the W). As a minimum value for precipitations over Cueva del Cobre we will use the data from the nearest meteoro- logical station of Santa María de Redondo, a little town located at 1200 m a.s.l. (i.e. 5 km away the cave entrance and topographically 400 m below). O ere, the mean an- nual precipitation has been around 1000 mm for the last 15 years (Fig. 2), although it notably varied, from >1200 to <700 mm. As for Kaite, maximum precipitation occurs during autumn to winter (October through Feburary) and, to a lesser extend, from March to May. Cueva del Cobre has a remarkable high-mountain infl uence, and during Figure 3. A. Schoeller diagram comparing chemical composi- most of the winter precipitation consists of snow. O is tion of drip waters from Cueva del Cobre and Kaite. The plot snow can stay over the cave for months, until the spring includes the mean concentrations of various analyses, with is well advanced. error bars indicating the standard deviations of the mean. The The speleothems of Cueva del Cobre considered in diagram illustrates a lower ionic content at Cueva del Cobre than at Kaite, except for the alkalinity and calcium. B. Mean this paper were collected in three diff erent galleries. All three-months values of drip water calcium from the monitor- the sampling sites are located at least 1 km away from ing area at Kaite (total dissolved calcium was determined by the main entrance, far enough to ensure the absence of inductively coupled plasma atomic emission spectrometry anthropogenic disturbance, and in areas of negligible ICP-AES). According to the data there is a trend defi ned by air current. O e average temperature of the cave is 5.6 a signifi cant decrease of calcium concentration from the fi rst ± 0.1 ºC and its relative humidity exceeds 98 %. sampling campaigns to the present. 326 J. MARTÍN-CHIVELET, M.B. MUÑOZ-GARCÍA, D. DOMÍNGUEZ-VILLAR, M.J. TURRERO & A.I. ORTEGA

comparison of their main hydrogeochemical aspects are less than 1 drop/min or even a negligible drip. O e average described below. δ18O measured at this cave is -10.3 ± 0.4 ‰ (SMOW). O e sets of data collected in both sites exhibit a very diluted composition, with a marked chemical variability Discussion: O ere are evident and signifi cant diff erences between drip waters. A diagram with the ionic compo- between Kaite and Cueva del Cobre caves and, after sition of both caves is given in Fig. 3A. For Kaite, the almost three years monitoring Kaite, a strong infl uence ionic content is higher (electrical conductivity = 261-572 of climatic factors in variables such as drip rate, pH and μS/cm) than for Cueva del Cobre (electrical conductivity calcium concentration has been demonstrated. As an = 174-203 μS/cm). Kaite contains a signifi cant concen- example, in Fig. 3B seasonal and inter-annual variations tration of chlorine and sulphate, possibly derived from in calcium concentration can be observed. O is fact has atmospheric input and the lixiviation of remnant Tertiary an eff ect on the growth process of calcite. O e saturation lutites located over the host-rock limestones. indices varied from 0.9 to 0.4 and the mass of calcite O e concentration of calcium is higher in Kaite (60 ± 17 collected decreased with time. Variations in the calcium mg/L) than in Cueva del Cobre (36 ± 3 mg/L) and varies concentration not only aff ect the quantity (mass) of calcite seasonally and annually (Fig. 3B). Although 2002 is not a precipitated but, according to Frisia et al. (2000), can also complete year, there is a seasonal variation refl ected in a aff ect the habit and fabric of calcite crystals. similar trend in the curves of the three years, with a max- As above mentioned, intra-site variation in seepage fl ow imun calcium concentration in summer, a decreasing in have been recognized in both caves. Preliminary studies autumm and winter and a sligth increasing during spring. on calcite crystals collected from sites of high and low O e interannual variability is defi ned by a signifi cant de- drip rates reveal diff erences in growth rates and in the crease of calcium concentration from the fi rst sampling size of the resulting crystals, being smaller in those zones campaigns on 2002 to the present. O ere is a relationship of very high drip rates and larger and more regular in between variations in calcium concentration and the ex- those of moderate but continuous drip. O is fact allows to ternal environment, mainly changes in temperature and recognize the drip rate as one of the variables responsible precipitation (Turrero et al., unpublished data). of this specifi c characteristic in the speleothems. On-site pH measurements provide pH values of 8.1 ± 0.2 All these aspects together are worthy of note to identify in Kaite and 7.5 ± 0.1 in Cueva del Cobre. O erefore, drip factors aff ecting growth of speleothems in the past, as -3.0 waters of Kaite have a CO2 partial pressure of 10 atm., will be discussed later. -2.5 lower than CO2 pressure of Cueva del Cobre (10 atm). So, Kaite is supersaturated with respect to calcite (satura- tion index (S.I.) = 0.4-0.9) while Cobre is undersaturated 4. Speleothems with respect to it (S.I. = -0.5). At Kaite, there is a seasonal variability in the present day precipitation of calcite, with Calcite speleothems can conform multi-proxy archives a higher growing rate associated with the autumn and of paleoenvironmental changes in extra-glacial conti- winter periods (around 0.38 mg of precipitate collected), nental conditions. O ey can record key aspects of climatic although with intra-site and inter-annual variations, than variability in the area over the cave, such as changes in in the spring and summer periods (around 0.20 mg of temperature, rainfall or vegetation. O e proxy-data come precipitate collected). from a series of parameters that include, among others, At Kaite, the seepage fl ow is frequent during all the year the growth patterns, the microstratigraphy, the crystal and drip rate measurements reveal intra-annual (seasonal) fabric, and the isotopic composition, all of them sensitive and intra-site variability depending on the site selected to climate or environmental changes. In this paper we for measuring. O e points with a moderate-high and vari- comparatively describe these parameters in the Kaite and able drip rate (8-45 drops/min) could indicate that the Cobre Holocene speleothems and discuss their potential permeability is high and the storage component of the to be used as paleoclimate proxies in each case. overburden can be considered negligible. Further stud- Four stalagmites from Cobre and two from Kaite have ies should clarify this aspect. O ese dripping points are been chosen as representative of the Holocene spele- largely infl uenced by the precipitations. Other sites are othems of each cave. O e samples from Kaite come from not so sensitive to seasonal changes and maintain a low Las Velas Hall, already described above, whereas those drip rate along the year (< 1 drop/min or negligible drip), from Cueva del Cobre have been collected in diff erent independently of rainfall. Besides, at these points scarce sites of the cave, all of them in relict low-gradient canyons variability in the δ18O is found (-7.3 ± 0.3 ‰ SMOW, or halls, hundreds of meters away from the main entrance, average value and standard deviation of a continuous and isolated from air currents and the main water stream. record of three years), what could be explained by long Absolute dates were determined by U-series dating us- water residence times in the aquifer. ing TIMS and ICP mass spectrometers. 13 TIMS and At Cueva del Cobre, drip rate measurements greatly de- 2 ICP-MS dates were obtained from Cueva del Cobre pend on the site. Of the few selected sites, one has drip speleothems, while Kaite Cave speleothems counts with rates in winter around 25 drops/min, while others have 24 ICP-MS dates. O e chronology of the speleothems COMPARATIVE ANALYSIS OF STALAGMITES FROM TWO CAVES OF NORTHERN SPAIN 327

Figure 4. Kaite Cave. A. Main entrance. B. Partial view of the cave interior, with a well-decorated roof and very elongate sta- lagmites. C. Monitoring equipment installed in Kaite for obtaining physicochemical parameters and drip water rate. D & E. Two longitudinal sections of Holocene stalagmites, showing their dominant cylindrical morphology, their aggradational growth pattern, and the abundance of microstratigraphic discontinuities (D: Basal portion of LV3, Early Holocene; E: Middle and upper portion of LV5, Holocene). F. Annual laminations in stalagmite LV3. Polished section. G to J. Dominant crystal fabrics in Kaite stalagmites. Thin sections under the petrographical microscope. Note that, in the four samples, the growth bands are easily distinguishable. G) Elongate columnar or fi brous fabric. Cross-polarized light. H) Columnar to elongate columnar or fi brous fabric, Partially cross polarized light. I) Dendritic to microcrystalline fabric. Plane polarized light. H) Microcrystalline fabric. Plane polarized light. Nomenclature based in Frisia et al. (2000). 328 J. MARTÍN-CHIVELET, M.B. MUÑOZ-GARCÍA, D. DOMÍNGUEZ-VILLAR, M.J. TURRERO & A.I. ORTEGA

Figure 5. Cueva del Cobre A. Main entrance to the cave. B. Partial view of the cave interior with three Holocene stalagmites. The polished section of one of them is shown in C. C & D. Two longitudinal sections of mid-late Holocene stalagmites showing their dominant conical morphology and their progradational-aggradational growth pattern. C: Sample C-11, D: Sample C-7. Note the different scale of the photographs. E & F. Different types of lamination in C. del Cobre stalagmites under the petrographical micro- scope. The general structure is based on millimetrical growth bands with different size, density and origin of porosity. COMPARATIVE ANALYSIS OF STALAGMITES FROM TWO CAVES OF NORTHERN SPAIN 329

has been established on the basis of these datings, and Vertical growth rates of stalagmites depend on three main provide a quite complete chronostratigraphic record of the factors: temperature, drip rate, and calcium ion concen- Holocene (Muñoz-García et al., 2004, 2006; Domínguez- tration in drip waters (Dreybrodt, 1988), which directly Villar et al., 2004, 2006; and unpublished data). depend on the external climate and environment (pre- cipitation, temperature and soil activity). O e lower tem- 4.1. Stalagmite morphology and growth rates perature could possibly explain the broadly lower growth rates of Cueva del Cobre speleothems together with the Although the morphological variability of the stalag- lower saturation of the dripping waters. O e changes in mites within each cave is considerable, a fi rst qualitative drip rates and in the concentration of calcium in the description to the dominant shapes and dimensions is waters could explain the presence of hiatuses and abrupt presented in order to illustrate the main diff erences be- changes in speleothem growth in each cave during the tween the two caves. In the same way, the growth rates Holocene. In the case of Kaite, located only 12-20 meters of the speleothems herein included, although probably below the surface, the variations in the aquifer recharge representative of each cave, should be considered as a fi rst must have a rapid response in drip rates. Furthermore, its approach, based in a limited number of samples, and will thin soil cover (comparable to other European caves, e.g., be refi ned in the near future. Genty et al., 2001), should determine any abrupt (natural O e stalagmites of Kaite are cylindrical, thin and elon- or anthropogenic) change in vegetation to have drastic gated, and their apices are commonly concave (Fig. 4). consequences in stalagmite growth. A temporary soil

O e diameter of the studied samples averages 4-5 cm, loss would derivate in a reduction of groundwater CO2, and their length usually reaches one meter or even more. which would, in consequence, generate a period of very O e growth patterns are strongly aggradational (i.e., slow growth rates or even corrosion. O ese two processes dominated by vertical accretion), as can be seen in Fig. 4, could explain the discontinuous development of Kaite and the development of the stalagmites is notably discon- speleothems, and the presence of more abundant hiatuses tinuous. All the studied samples show abundant internal than in C. del Cobre. O is latter cave, more isolated from stratigraphic discontinuities that correspond to hiatuses surface and with thicker soil cover, seems to have had of diff erent duration and intervals of non-deposition and slower and smoother responses to external changes in frequent corrosion. Growth rates calculated according to precipitation and vegetation. the U/O dates and annual laminae counting in intervals of continuous growth (between successive hiatuses) dem- 4.2. Crystal fabrics onstrate a strong variability within each sample. Early Holocene growth rates are high, usually between 300-500 O e mineralogical composition of the Holocene stalag- µm/year, whereas Late Holocene rates are substantially mites from both Kaite and Cueva del Cobre is calcite. No lower, normally around 100 µm /year. traces of aragonite or other minerals were found in the O e external shape of Cueva del Cobre stalagmites is studied samples. Calcite crystals fabrics, and their distri- usually conical (Fig. 5), and their apices can be concave bution in the speleothems can be considered as climate or convex. O is conical shape is due to the aggradational proxies (e.g., Kendall & Broughton, 1978; Gonzalez et – progradational pattern of growth (i.e., coexistence of al. 1992; Quinif et al., 1994; Frisia et al., 2000). For this vertical and lateral accretion), controlled by the addition of reason, a short introduction to them in Kaite and Cobre thin layers all along the sides of the stalagmite. O e growth caves is herein included. of the studied samples is more continuous than those of A wide range of crystal fabrics is present in Kaite samples Kaite, and hiatuses, although present, are less abundant. (Fig. 4). Following the classifi cation proposed by Frisia Growth rates for intervals of continuous development et al. (2000), columnar, elongate columnar (or fi brous), ranges from 23 to 140 µm/year. microcrystalline and dendritic fabrics are well represented in the studied stalagmites. All fabrics occur in the axial Discussion: O e dominant shape, the average diameter part of the stalagmites, whereas the fl anks usually consist and the vertical growth of the stalagmites from each cave of dendritic and microcrystalline fabrics (Fig. 4). O e refl ect diff erent forming conditions probably determined crystal fabrics are much less variable in Cueva del Cobre to a great extent by the environment. According to math- stalagmites, in which the columnar and elongate columnar ematical models on stalagmite growth and stratigraphy (or fi brous) ones are clearly dominant (Fig. 5). (Dreybrodt, 1999; Kauff mann & Dreybrodt, 2004) the more conical shape and the larger diameter of Cueva del Discussion: Distribution of fabrics along individual stalag- Cobre stalagmites could be related to higher drip rates and mites can be used as a record of changes in paleoenviron- lower temperatures than those of Kaite. According to the ment and paleoclimate through time. Accordingly, Kaite theoretical approach of these authors, the thin stalagmites stalagmites off er an independent paleoclimate proxy that from Kaite would be then generated under relatively low can be analyzed and compared with other proxies such as drip rates (of about 1 drip each 1000 seconds). the isotopic signal or the growth layer microstratigraphy. 330 J. MARTÍN-CHIVELET, M.B. MUÑOZ-GARCÍA, D. DOMÍNGUEZ-VILLAR, M.J. TURRERO & A.I. ORTEGA

According to works by other authors in diff erent caves, But these patterns of changes in lamina thickness also each fabric is formed under specifi c physicochemical constitute an independent climate proxy. O is is because conditions, mainly concerning drip rates and drip water variations in the thickness of successive growth lamina chemistry. According to Frisia et al. (2000), the columnar depend on environmental factors such as temperature, aggregates could have been formed under low calcium drip rate and soil activity above the cave (e.g., Baker et supersaturation waters, being the crystals more elongated al., 1998; Genty et al., 2001). In a theoretical approach, when higher drip rates occur; and the microcrystalline and Dreybrodt (1999) related thicker laminas of stalagmites dendritic fabrics, related to more chaotic crystal growths, with intervals of higher drip rates, generally associated could be interpreted as typical of conditions of higher su- to humid years or periods. Recent experimental studies, persaturation and disequilibrium (Frisia et al., 2000). High however, demonstrated that, when drip rates are high supersaturation could be attained through prolonged enough, the main control on lamina thickness could be outgassing at the speleothem surface (Dreybrodt, 1988), the mean annual temperature outside the cave, due to the which usually occurs when drip rates are low. correlation that exists between calcium ion concentration

in drip water, soil pCO2, and surface temperature (Genty 4.3. Lamination et al., 2001). Experimental studies on Kaite cave reveal that seasonal Internally, in longitudinal sections, the central or axial changes have a notable infl uence in the composition of part of the cylindrical stalagmites of Kaite (Fig. 4) shows drip waters and in the drip rates (Turrero et al., 2004 remarkable changes in texture and color, which allow and unpublished data). O ese changes would justify the to stratigraphically subdivide the studied samples. O at formation of the well defi ned annual laminations of central part can be characterized by the presence of quite Kaite speleothems. Cueva del Cobre, notably deeper and transparent calcite, amber in color, that commonly shows colder than Kaite, seems to be less sensitive to intra-an- regular laminations defi ned by the alternation of more nual changes, and laminations, if annual, are scarce and and less porous layers. Also, this central part can be more less defi ned. chaotic and consist of white or pale cream, more opaque calcite that can show more delicate and irregular lamina- 4.4. Stable Isotopes tions. Gradual or abrupt vertical transitions between these two main textures can occur throughout the stalagmites, About 330 samples of calcite from Kaite and 305 from probably indicating changes in the drip rate and in the Cueva del Cobre have been analyzed for oxygen and water chemistry. carbon stable isotopes. Although the fi nal objective of Surrounding this central part, an outer or lateral zone, these analysis will be the reconstruction of high-resolu- about 0.5 cm thick, has been observed in the studied tion paleoclimate series, in this paper we only present a samples (Fig. 4). O is part, which shows a rapid but fi rst approach to the analytic data, trying to characterize gradual transition from the axial one, is formed by white, the average isotopic composition of the Holocene stalag- quite opaque, dendritic calcite, that also shows delicate mites in the two caves. A comparative analysis of these laminations tangent with the speleothem surface in their records permits to discuss the potential of each cave for outermost part. paleoclimate reconstruction. Microstratigraphic work and U/O dates have demon- Fig. 6 plots δ18O and δ13C data from two stalagmites of strated these thin laminations of both the inner and each cave which, according to the U/O dates (Muñoz- the outer part of the stalagmite to refl ect annual cycles García et al., 2004, 2006; Domínguez-Villar et al., (Domínguez Villar et al., 2004). 2004, 2006; and unpublished data), include most of the Cueva del Cobre stalagmites are internally more homoge- Holocene. neous, and the laminations are usually not as well defi ned In the samples from Cueva del Cobre, the δ18O yields a as in Kaite. O ere is not a marked textural diff erentiation mean value of –8.01 ‰ (VPDB), with a variance of 0.152 between the axial part of the stalagmites and the outer and a range of 2.12, between –9.36 and –7.24 ‰; and zone. Growth layers occur however in some parts of the the δ13C ranges between –11.05 and –2.10 ‰ (VPDB), speleothem, defi ned by the presence of bands with abun- averaging –5.91 ‰, and showing a variance of 5.83. dant inclusions, which can be linear or rounded in shape. Covariation of δ18O and δ13C is low, with a correlation In some areas, delicate growth layers of possible annual index r2 of 0.292 for stalagmite C-11 and of 0.236 for origin appear. O ese layers are thin (10-100 µm thick) C-7 (Fig. 6). and show only rounded inclusions (Fig. 5). In the speleothems from Kaite, the mean value for the δ18O is –6.33 ‰ (VPDB), with a variance of 0.39 and Discussion: Annual laminations recognized in Kaite spe- a maximum range of 2.34, between –7.64 and –5.30 ‰. leothems can provide a high-resolution record of growth O e mean value of δ13C in the same samples is –8.32 ‰ rate simply by measuring the distance between laminae, (VPDB), the variance is 1.75 and the variation range is as well as a precise chronostratigraphic framework in between –11.28 and –3.87 ‰. Covariation of δ18O and which to plot climate proxies such as the isotopic data. δ13C is even lower than in Cobre, showing a correlation COMPARATIVE ANALYSIS OF STALAGMITES FROM TWO CAVES OF NORTHERN SPAIN 331

cold climate and heavy δ13C values has been reported from caves in SW and conform the basis of the paleoclimate interpretation of the δ13C time series of the Villars Cave (Genty et al., 2003). In the speleothems from that cave, the higher δ13C values are correlated with colder time intervals, and this relationship interpreted as the result of changes in the ratio between organic carbon (soil derived) and inorganic carbon (atmospheric) of the drip waters. During warm periods, a higher soil develop-

ment induces greater biogenic CO2 and the calcite of the speleothems become impoverished in C-13, whereas cold ones determined diminished biologic soil activity and higher δ13C values in the stalagmites. As indicated above, all the studied samples show very low correlation indices between δ18O and δ13C. O e lack of substantial covariation between the two isotopic ra- tios supports that the stalagmites essentially grew under conditions of isotopic equilibrium, i.e., without relevant Figure 6. Diagram showing the isotopic composition of Holo- kinetic isotopic fractionation. O is aspect is further dis- cene speleothems from Kaite and Cueva del Cobre. Two sta- cussed in the following point. lagmites of each cave have been chosen for the diagram (Cueva del Cobre: C7 and C11. Kaite: LV-3 and LV-5). The correlation 4.5. Hendy Tests index r2 is included as an indicator of possible covariation 18 13 between δ O and δ C for each stalagmite. The low values of Stalagmites that grew under conditions of isotopic equi- this index in all the cases reveal a bad correlation between the librium are usually regarded as ideal for the construction two variables and strongly suggest that calcite precipitation took place under conditions of isotopic equilibrium. Note the of stable isotope time series of paleoclimate signifi cance markedly different isotopic values for the stalagmites of each (e.g., Schwarcz, 1986), although the isotopic signal of sta- cave, particularly those of the δ18O. These differences should lagmites that suff ered some kinetic isotopic fractionation be interpreted in the framework of the different environmental during their growth can also give excellent paleoclimate conditions of each cave. See further explanation in the text. information (e.g., Spölt & Magnini, 2002). In order to check for equilibrium versus kinetic isotopic fractionation during stalagmite formation, a series of index r2 of 0.105 for stalagmite L-V5 and of 0.184 for Hendy tests (Hendy, 1971) has been performed in the LV-3 (Fig. 6). studied samples (see various examples in Fig. 7). O e test needs a series of oxygen and carbon isotope analyses Discussion: O e isotopic characteristics from the Holocene along an individual growth layer, these extracted from the stalagmites of both caves (Fig. 6) reveal markedly heavier stalagmite axis (closest zone to the drip point) laterally δ18O values for Kaite than for Cueva del Cobre (–6.33 ‰ down to the sides of the stalagmite, i.e. along the former vs. –8.01 ‰) and lighter δ13C (–8.32 ‰ vs. –5.91 ‰). fl ow path of the water fi lm. According to Hendy (1971), O e diff erences in δ18O between the stalagmites of the the oxygen isotopic composition of the calcite along each two caves could be interpreted as related to the lighter layer should be largely independent from the distance composition of the drip waters of Cueva del Cobre, be- from the apex when no kinetic fractionation occurs and ing at the present time nearly 3 ‰ lower than in Kaite calcite precipitates in isotopic equilibrium. It occurs if 18 (δ OW = -10.0 ± 0.4 in Cobre versus –7.3 ± 0.3 in Kaite). drip rates are not excessively low, the relative humidity 18 Interestingly, the range of variability of δ O data is re- of the cave approaches to 100 % and the pCO2 gradient ally similar in both caves, a point that strongly suggests between the karst aquifer and the cave air is low. On the that the isotopic record obtained from the stalagmites is contrary, when these conditions do not occur, a more representative of the karst water signal. extensive degassing of CO2 takes place along the water With respect to the δ13C record, the diff erence between path from the apex (drip point) and the stalagmite laterals. Cueva del Cobre and Kaite (–5.91 ‰ and –8.2 ± 0.3 ‰ O e result is a progressive increase in both δ18O and δ13C respectively) is quite small if we compare it with the due to the Rayleigh-type distillation (Frisia et al., 2000, high variability that δ13C values show in each cave, with Spölt & Magnini, 2002). ranges that exceed 8 ‰ in both cases. O e slightly higher O e studied samples gave variable results to Hendy tests average δ13C of Cueva del Cobre could be due to a minor (Fig. 7). Most of the performed tests suggest that precipi- biological productivity over this cave, related to its colder tation of calcite during speleothem formation took place climate and with the permanence of a snow cover during under conditions of (or near to) isotopic equilibrium in nearly half of the year. A similar relationship between both caves, but some others could be indicating some 332 J. MARTÍN-CHIVELET, M.B. MUÑOZ-GARCÍA, D. DOMÍNGUEZ-VILLAR, M.J. TURRERO & A.I. ORTEGA

Figure 7. Selected cases of Hendy Test from Cueva del Cobre (C10 and C7) and Kaite Cave (LV5 and LV3). The linear correlation index r2 between carbon and oxygen stable isotopes within the same depositional band is included in each graph. Note the different scale for each lamina depending on the size of the speleothem. See text for explanation. kinetic fractionation. O ese conclusions are based in the in poorly laminated samples (C. del Cobre) or in samples following aspects: that show important lateral changes in lamina thickness, a) In most cases, the δ18O shows only minor changes which rapidly thins from the center to the sides (Kaite). along each single growth layer and does not refl ect any In both cases, precise sampling the outermost part of the well defi ned trend or pattern that could indicate kinetic growth bands involves some diffi culty, and some co-drill- isotopic fractionation along the fl ow path on the stalag- ing of contiguous laminations is diffi cult to avoid. O e mite (e.g., enrichment in 18O towards the sides), aspects most extreme variations (essentially occurring in Kaite that support prevailing equilibrium conditions. Cueva del samples, see Fig. 7D), however, can be hardly attributed to Cobre tests give a δ18O variability always below 0.4 ‰, instrumental limitations, and some kinetic fractionation whereas Kaite ones usually give slightly larger values, in can not be excluded, probably restricted to the edges of the range of 0.3-0.6 ‰, and occasionally they can be the stalagmites. It should be noted that dendritic crystals even higher. It should be noted that the minor changes are common in those lateral areas of Kaite speleothems, recorded along sigle grouth bands can be related at least and that these crystal fabrics usually develop when en- in part to instrumental errors due to the diffi culty of hanced degassing (or even evaporation) occurs (Frisia et precisely tracking and sampling individual growth layers al., 2000). Similar observations have been reported from COMPARATIVE ANALYSIS OF STALAGMITES FROM TWO CAVES OF NORTHERN SPAIN 333

other studied caves by diff erent authors (e.g., Talma and these caves determine a notably diff erent development of Vogel, 1992; Repinski et al., 1999), although these micro- calcite speleothems, a fact that has key implications for the stratigraphic patterns do not seem to aff ect the validity obtention of paleoclimatic records. O e main conclusions and the quality of the isotopic series, as these are extracted of this comparative analysis are the following: from the stalagmite growth axis. Kaite stalagmites are longer and narrower than those b) O e performed tests show a notable variability in the from Cueva del Cobre, and grew more rapidly. In them, δ13C along each single growth layer, often accompanied an annual resolution can usually be reached because an- by a well outlined progressive increment of the values nual laminations are well recorded. O is allows a great from the central part towards the sides (Fig. 7). O is chronostratigraphic resolution in these stalagmites and pattern clearly contrasts with the low variability of the the possibility of analyzing their growth patterns from δ18O and would determine a bad correlation between the changes in lamina thickness. two data sets, which supports that isotopic equilibrium O e stratigraphic record in Kaite stalagmites is, however, existed during calcite precipitation. Correlation indexes more incomplete. O ese stalagmites are clearly more sensi- (r2) between δ13C and δ18O in single growth layers are tive to rapid variations in climate than those from Cueva in fact usually low (ranging from 0.002 to 0.3 in most del Cobre, probably because of the shallowness of that performed tests, see Fig. 7A, 7B and 7E as examples) cave. O is point can be regarded as an advantage for paleo- although occasionally they can reach noticeably higher climate studies, particularly for obtaining high-resolution values (up to 0.86 in Cueva del Cobre and 0.61 in Kaite, records for discrete time intervals. However, for some Fig. 7C and 7D). O ese unusual, relatively high values periods the changes outside the cave can determine the could indicate kinetic fractionation in some layers. In fact, formation of abundant hiatuses. O en, the stratigraphic most anomalous r2 values in Kaite samples were obtained record is too incomplete to obtain representative series. in those growth bands for which non-equilibrium condi- O e isotopic record (δ18O and δ13C) of the stalagmites tions were suspected by the presence of dendritic crystals seems to be strongly sensitive to environmental changes and their δ18O variability higher than average. in the two caves and thus constitutes a powerful proxy of climate. O e δ18O signal of the studied stalagmites refl ect Discussion: Most of the samples studied seem not to be variations in the isotopic composition of the meteoric aff ected by kinetic fractionation and, in general, calcite waters which can be related to paleotemperature changes precipitation under conditions of isotopic equilibrium outside the cave. O e δ13C signal probably records in could be assumed for the Holocene speleothems of the both caves changes in the soil activity, conditioned by two caves. Some Hendy tests however suggest some minor environmental changes. kinetic fractionation, occurring probably in the lateral Crystal microfabrics are more heterogeneous in Kaite parts of some samples. O is kinetic fractionation, although stalagmites, and their variations through the growth probably does not aff ect the isotopic signature of the axial axis of the stalagmite can effi ciently reveal long-term part of the stalagmites (from where the data for the time environmental changes. O ese type of microstratigraphic series are obtained) reveals the necessity of further studies studies are more diffi cult to perform in Cueva del Cobre addressed to assure the quality of the isotopic series before stalagmites, characterized by a higher homogeneity in interpreting them from a paleoclimatic perspective. crystal habits. All these points reveal the great potential of both caves for Holocene paleoenvironmental research, and the comple- 5. Conclusions mentary character of the data sets that their stalagmites can yield. O e integration of these data should contribute Kaite and Cueva del Cobre caves show, despite their decisively to the construction of paleoclimate series of geographical proximity in the Cantabrian mountains, high resolution for Northern Spain, a key climatic area marked diff erences in climate and environment. Kaite, because of its intermediate position between the Northern formed on Upper Cretaceous limestones, is a shallow (12- Atlantic, the Mediterranean and the Subtropical North 20 m below surface), warm cave (10.4 ± 0.1 ºC) which African zones. today shows a poor vegetal soil cover and a permanent but variable water drip. Drip waters are supersaturated in

CO3Ca (S.I.=0.9), have pH values of about 8.1 ± 0.2, and 6. Acknowledgements an average δ18O of -7.3 ± 0.3 ‰ (SMOW). In contrast, Cueva del Cobre, developed on Carboniferous limestones O e authors acknowledge the constructive comments and and covered by a thicker soil, is a deep cave (>100 m below suggestions by Dr. Sophie Verheyden and an anonymous surface) notably colder (5.6 ± 0.1 ºC) than Kaite. Its drip referee. O e work was carried out within the framework of waters show, in the sampling sites, lower CO3Ca satura- the research projects “PAREK” (REN2001-1409, 2002- tion index (-0.5), lower pH = 7.6 ± 0.1 and average δ18O 2004) and “CLISP” (CGL2004-01866), both founded of -10.3 ± 0.4 ‰ (SMOW) than the former cave. O e by the DGI of Spain. We greatly thank the facilities and climatic, karstologic and environmental characteristics of permissions given by the authorities of Ojo Guareña 334 J. MARTÍN-CHIVELET, M.B. MUÑOZ-GARCÍA, D. DOMÍNGUEZ-VILLAR, M.J. TURRERO & A.I. ORTEGA

Natural Monument and Fuentes Carrionas-Fuente Co- FRISIA, S., BORSATO, A., FAIRCHILD, I. J. & MC- bre Natural Park and by the D.G. de Patrimonio de la DERMOTT, F., 2000. Calcite fabrics, growth mecha- Junta de Castilla y León. We are also indebted to many nisms, and environments of formation in speleothems colleagues for their valuable contributions, advice and from the Italian Alps and Southwestern Ireland. Journal help during the research work, in special to Carlos Rossi of Sedimentary Research, 70: 1183-1196 (UCM), who introduced us to the knowledge of C. del GENTY, D., BAKER, A. & VOKAL, B., 2001. Intra- Cobre and collected the samples of that cave, Derek Ford and inter-annual growth rate of modern stalagmites. (McMaster Univ.), Henry Schwarcz (McMaster Univ.), Chemical Geology, 176: 191-212 Paloma Gómez (Ciemat), Antonio Garralón (Ciemat), GENTY, D., BLAMART, D., OUAHDI, R., GIL- Lorenzo Sánchez (Ciemat), Miguel Ángel Martín-Me- MOUR, M., BAKER, A., JOUZEL, J. & VAN-EXTER. rino (Grupo Espeleológico Edelweiss, Burgos), Larry Ed- S., 2003. Precise dating of Dansgaard-Oeschger climate wards (Univ. Minnesota) and Alberto Quejido (Ciemat). oscillations in western Europe from stalagmite data. Isotope analyses where carried out in the University of Nature, 421: 833-837 Minnesota (USA), McMaster University () and GONZÁLEZ, L. A., CARPENTER, S. J. & LOHM- Centro Experimental Zadín (CSIC, Spain). O anks are ANN, K. C., 1992. Inorganic calcite morphology:roles extended to Gilberto Herrero (UCM), Beatriz Moral of fl uid chemistry and fl uid fl ow. In 382-399 (UCM) and María Isabel Sevillano (CSIC) for prepara- HENDY, C. H., 1971. 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