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Beachrocks from the island of La Palma (Canary Islands, Spain)
Marine Geology 197 (2003) 75^93 www.elsevier.com/locate/margeo
Beachrocks from the island of La Palma (Canary Islands, Spain) F. Calvet a;1 , M.C. Cabrera b , J.C. Carracedo c , J. Mangas b , F.J. Pe¤rez-Torrado b , C. Recio d , A. Trave¤ a; a
d
Departament de Geoqu|¤mica, Petrologia i Prospeccio¤ Geolo'gica, Facultat de Geologia, Universitat de Barcelona, 08071 Barcelona, Spain b Departamento de F|¤sica-Geolog|¤a, Facultad de Ciencias del Mar, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain c Estacio¤n Volcanolo¤gica de Canarias, CSIC, P.O. Box 195, La Laguna, Tenerife, Spain Servicio General de Ana¤lisis de Iso¤topos Estables, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain Received 14 June 2002; accepted 10 March 2003
Abstract Beachrocks on La Palma Island developed on platform-forming lavas of the Cumbre Vieja volcano. Some of these lavas are related to the 1585 (Puerto Naos), 1677 and 1971 (Echentive) eruptions. Radiocarbon dating of the Charco Verde beachrock gives a conventional age of 33 330 8 490 BP, while that at Playa Chica beach gives a calibrated age of 14 940 8 525 BP. The beachrocks, up to 1.5 m thick and some tens of metres wide, consist of several decimetre-thick horizons dipping 2^15‡ seaward. Petrographically, they can be classified as rudstones and arenites, with volcanic clasts as their main component. The original porosity of the beachrocks was intergranular (and occasionally intragranular) and was partially occluded by cementation and locally by internal sediments. The main cements are fibrous aragonite and micrite high-magnesium calcite (HMC). Spar aragonite, peloidal HMC and microbotryoidal HMC are scarce. The elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters. The evolution of beach deposits, and especially the beachrocks in La Palma island, follows three stages: (1) beach deposition, (2) beachrock formation, and (3) beach retrogradation and/or erosion. The studied beachrocks prompt us to make some important considerations. (1) The mean tidal range in the Canary Islands has not varied over the last thousand years. (2) The position of the beachrocks at the present-day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years. (3) Volcanic activity supplies the sediment that forms the beaches. (4) A dry warm climate with a very low rainfall (below 250 mm/year) and a high insolation rate (6^11 h/day) favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches. (5) The presence of beachrocks in the La Palma beaches prevents the total disappearance of the beaches. @ 2003 Elsevier Science B.V. All rights reserved.
1
Deceased in January 2002. * Corresponding author. E-mail address: [email protected] (A. Trave¤).
0025-3227 / 03 / $ ^ see front matter @ 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0025-3227(03)00090-2
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Keywords: beachrocks; volcanic conglomeratic beaches; marine cements; trace elements; stable isotopes; Quaternary; La Palma (Canary Islands)
1. Introduction Beaches are an ideal location for the precipitation of marine cements. Intertidal lithi¢cation of beach sand and gravel by carbonate cementation results in a rock termed ‘beachrock’ (Ginsburg, 1953). The high-energy setting of beaches (owing to wave and tidal action) and the presence of sand- and/or gravel-sized sediment with high porosity and permeability allows for the necessary volume of water to move through the sediment, giving rise to its cementation (Moore, 1989). Cementation usually occurs in the intertidal zone (Neumeier, 1998), but it can also occur in the upper part of the subtidal zone (Alexandersson, 1972), the lower part of the supratidal zone (Holail and Rashed, 1992) and in the swash zone (Bernier and Dalongeville, 1996). Generally, beachrocks are arranged parallel to the coastline forming linear bodies ranging from several tens of kilometres to metre-size patches or even decimetre-size nodules (Bricker, 1971). Beachrocks rarely exceed 1 m in thickness, though some can attain a thickness of 5 m (Amieux et al., 1989). They normally develop in the lower part of the foreshore and dip gently (maximum 15‡) seaward following the arrangement of the beach. Beachrocks are often made up of several horizons (‘dalles’, ‘beds’, ‘bands’) whose thicknesses range from several centimetres to several decimetres. Although a number of authors consider beachrock formation a feature of tropical or subtropical zones (Bricker, 1971), beachrocks are also found in temperate climates. The principal zones of beachrock development are: (1) tropical or subtropical zones with a humid climate such as the Caribbean area (e.g., Ginsburg, 1953; Moore, 1973; Hanor, 1978; Beier, 1985), the coasts of Africa (e.g., Amieux et al., 1989), the Paci¢c Ocean (Meyers, 1987; Bernier et al., 1990; Neumeier, 1998), the Indian Ocean (Font and Calvet, 1997), Australia (Neumeier, 1998; Webb et al., 1999) and Brazil; (2) tropical or subtropical zones
with an arid or semiarid climate such as the Persian Gulf (Taylor and Illing, 1969) and the Red Sea (Friedman and Gavish, 1971; Holail and Rashed, 1992; Neumeier, 1998); (3) temperate areas such as the Mediterranean Sea (e.g., Friedman and Gavish, 1971; Alexandersson, 1972; Bernier and Dalongeville, 1988; Holail and Rashed, 1992; Neumeier, 1998). The Canary Islands are located in a special subtropical area and, though rarely documented, beachrock formation is relatively common in Fuerteventura, La Palma and La Graciosa (north of Lanzarote), and less frequent in Gran Canaria, Lanzarote, La Gomera and El Hierro. In these islands beachrocks have been reported by Tietz and Mu«ller (1971) in Fuerteventura, by Pe¤rezTorrado and Mangas (1994) in Gran Canaria and by Calvet et al. (2000) in La Palma. The main purpose of this study is to present a model of beachrock formation and to consider the implications of its formation in the island of La Palma. To accomplish this, we have carried out an in-depth study of the geometry, age, petrography and geochemistry of these marine deposits.
2. Methods and analytical techniques Standard thin sections of approximately 50 rock samples from the ¢ve outcrops were examined for their petrography under an optical microscope. C-metallised polished thin sections of some selected samples were analysed for their elemental geochemistry at the Serveis Cienti¢cote'cnics of the University of Barcelona, using a CAMECA model SX-50 microprobe equipped with four vertically displayed WD X-ray spectrometers. The microprobe was operated using 20 kV excitation potential, 15 nA current intensity and 10 Wm beam diameter. Ca, Mg, Sr, Fe, Mn and Na contents of the di¡erent cements were determined. Detection limits were 395 ppm for Mg, 185 ppm for Sr, 275 ppm for Fe, 180 ppm for Mn and 195 ppm for Na. Fe and Mn contents were always
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below the detection limits. Precision on major element analyses averaged 6.32% standard error at 3c con¢dence level. Isotopic ratios (D/H, 13 C/12 C and 18 O/16 O) were determined at the Stable Isotope Laboratory of the University of Salamanca. Results are reported in the usual delta notation relative to PDB and SMOW. CO2 from carbonates was evolved by reaction with 103% H3 PO4 at 25‡C, employing standard techniques, or at 90‡C, employing an ISOCARB0 device coupled on line to a Micromass SIRA-II0 mass spectrometer (McCrea, 1950; Craig, 1957). Precision was monitored by repeated analysis by both internal and international (NBS-19) standards under identical analytical conditions. Average precision obtained was 8 0.02x (N13 C) and 8 0.12x (N18 O). D/H ratios of waters were measured on hydrogen gas evolved by the U reduction method (Bigeleisen et al., 1952), and measured on the same mass spectrometer employing the split-£ight tube for D/H measurement. ND values were normalised to the SMOW-SLAP scale, and GISP was used as an additional reference standard. Long-term reproducibility for repeated determination of ND on reference samples was better than 8 1x. N18 O in waters was measured on CO2 that had equilibrated with water at 40‡C for 5 h using a Micromass Multi£ow0 device coupled on line to an Isoprime0 CF mass spectrometer. As for D/H, 18 O/16 O ratios were normalised to the SMOWSLAP scale, and GISP was used as an additional reference for calibration. Long-term reproducibility for repeated determination of N18 O on reference samples was better than 8 0.27x. Radiocarbon dating was performed at Beta Analytic Inc. at Miami (FL, USA) on two aragonite samples without pretreatment. Both provided suf¢cient carbon for accurate accelerator mass spectrometer (AMS) standard analysis. AMS results were derived from reduction of sample carbon to graphite (100% C), along with standards and backgrounds. 14 C measurements on the graphite were done on an AMS. The conventional 14 C age resulted from applying 13 C/12 C isotopic fractionation corrections to the measured age. Calendar calibration was only applied to the sample giving a conventional radiocarbon age of 14 589 8 130
77
BP. The other sample was too old for an appropriate calibration.
3. Seawater and climate characteristics in the Canary Islands Although the Canary Islands are located in a subtropical zone, the physical and chemical parameters of their climate and surrounding marine waters di¡er considerably from other regions located along the same zone. These di¡erences are due to the fact that the Canary Islands are affected by upwellings, which occur along the African coast, and mesoscale instabilities like cyclonic and anticyclonic eddies produced by islands (Barton et al., 1998). The temperature of open-sea surface waters varies from a maximum of 25‡C in September and October to a minimum of 17‡C during the winter months. In shallow waters, especially in quiet bays or in beaches with a low slope, heat exchange with the atmosphere results in an increase in water temperature of about 1^2‡C. The salinity of seawater in the Canary Islands decreases with depth. If it were only for the e¡ects of evaporation and rainfall, it would reach a maximum value in summer and a minimum in winter. However, these two meteorological parameters are in general masked by upwelling and eddy effects, which produce a salinity of approximately 36.2x in summer and 37.2x in winter. The tidal regime is semidiurnal, with two high tides and two low tides each day. The mean tidal range is about 1.2^1.3 m. The tidal amplitude varies from a maximum (around 3 m) during the spring and autumn equinoxes to a minimum (around 0.7 m) during the summer and winter solstices. There are two types of swell: groundswell (bottom currents) and wind swell. The wind swell is in£uenced by the northeasterly trade winds, which are very persistent during the summer months, but weakened by the Azores anticyclone during the winter. The samples of seawater (collected during August 1997) from the foreshore zone in the di¡erent beaches of La Palma island show conductivity
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Table 1 pH, conductivity, N18 O and ND data of seawater collected from the island of La Palma Beach
pH
Conductivity (Ws/cm)
N18 O (x SMOW)
ND (x SMOW)
Puerto Naos Charco Verde Las Zamoras Chica Echentive (foreshore) Echentive (lake)
8.2 7.6 8.2 7.5 8.2 7.0^7.5
45 600 64 900 48 800 58 900 59 800 42 600
1.1 1.0 1.1 ^ 1.1 0.4
6.0 4.8 4.9 ^ 5.4 3.8
varying between 45 600 and 64 900 Ws/cm, and pH between 7.5 and 8.2 (Table 1). These values are consistent with those of standard seawater, whose pH ranges from 7.8 to 8.4 (Neumeier, 1998). Water from the innermost part (lakes) of the Echentive beach has pH values that vary from 7.0 to 7.5, and has lower conductivity (42 600 Ws/cm) than the foreshore zone water. In addition, the N18 O SMOW value of the foreshore zone seawater is about +1.1x, while that of the Echentive beach lakes is about +0.4x (Table 1). These values suggest a slight dilution of the lake’s water by inland groundwaters £owing into it. The bicarbonate fraction dissolved in modern seawater from La Palma gave a N13 C of +0.51x PDB. The Canary Islands have a speci¢c ‘dry warm’ climate resulting from several environmental factors (Marzol, 1988): (1) the atmospheric dynamics of subtropical latitudes, (2) the in£uence of the cool Canaries current, (3) the proximity to the African continent, especially the Sahara Desert, and (4) the presence of islands with abrupt topography. The most common winds are the cool humid northeasterly trade winds. As they collide with the islands, they produce clouds at an altitude of between 900 and 1500 m at the northern side of the islands (the cloud sea) but scarce precipitations. Sometimes easterly or southeasterly winds coming from the Sahara Desert make it hot and dry (dry-haze weather). Rarely are there northwesterly climatic perturbations in£uenced by polar fronts, or southwesterly storms producing heavy precipitation and strong winds. In general,
the climate of the Canary region is mild with mean temperatures at coastal zones ranging between 18.5 and 21‡C. There are, however, great contrasts of precipitation within the di¡erent parts of the islands. The windward slopes have mean precipitations around 550 mm/year while the leeward slopes have less than 300 mm/year, reaching maxima of 1100 mm/year at some high northern zones of the islands and minima of less than 100 mm/year at southern coastal zones. The studied zone along the southwestern coast of La Palma is characterised by a semi-desert or dry warm climate. That is, mean temperatures range between 18 and 25‡C, with highs around 40‡C during dry-haze weather and lows around 13‡C during winter. The mean rainfall is below 250 mm/year in this zone, but it is possible to have precipitation exceeding 100 mm/day (recurrent time each 30 year) during the southwesterly storms. The insolation of this area varies between 6 h/day during winter and 11 h/day during summer.
4. Geological setting of the La Palma island The island of La Palma is located at the western edge of the Canarian Archipelago (upper inset in Fig. 1) and above the mantle plume that generated the Canaries in the last 20^30 Myr. La Palma and El Hierro are the youngest islands ( 6 1.8 Myr) and both are in the juvenile shield stage of development (Carracedo et al., 1998).
Fig. 1. Geological map of the La Palma island and location of the beachrocks studied: Puerto Naos beach (PN), Charco Verde beach (CH), Las Zamoras beach (PZ), Playa Chica beach (PC), Echentive beach (EC).
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La Palma is formed by two consecutive main shield volcanoes (Fig. 1): the Taburiente volcano (1.7^0.4 Myr) to the north, with the seamount stage uplifted more than 1000 m asl, and the Cumbre Vieja volcano to the south, which developed during the last 125 kyr. Two main volcanostratigraphic units have been de¢ned in the Cumbre Vieja volcano using the low sea level stand of the last glacial maximum, about 20 kyr BP. This limit separates the cli¡-forming and the platformforming volcanic eruptions. The beachrocks observed in La Palma are located at the present sea level and on the platformforming lavas of the western £ank of the Cumbre Vieja volcano (lower inset in Fig. 1). However, as will be discussed in detail below, some of the ages obtained for these beachrocks are older than the last glacial maximum.
pebbles (see lower inset in Fig. 1). These observations constrain the relative age of the Puerto Naos beachrock between 1585 and 1949. 14 C dating has been impossible due to the di⁄culty of sampling the small-size cements of this beachrock. The beachrock is 20 m wide, 200 m long and 1.5 m thick. It dips seaward between 5 and 15‡ (Fig. 2.1) and presents two horizons. The lower horizon, with 1 m of visible thickness, is made up of an alternance of well-cemented rudites and arenites (Fig. 2.2). It presents an irregular upper surface with decimetre-sized channelled depressions oriented perpendicular to the coastline (Fig. 2.1) similar to the system of ridges and furrows perpendicular to the shoreline (e.g., McLean, 1967; Alexandersson, 1972). The upper horizon, in contrast, consists of poorly cemented rudites and arenites outcropping in metre-sized patches. 5.2. Beachrock of Charco Verde beach
5. Beachrock location, dating and description The beachrocks are located on the southwestern coast of La Palma island, between the village of Puerto Naos and the Fuencaliente lighthouse, namely in the Puerto Naos, Charco Verde, Las Zamoras, Playa Chica and Echentive beaches (lower inset in Fig. 1). 5.1. Beachrock of Puerto Naos beach The Puerto Naos beach developed on volcanic lavas generated by the 1585 eruption (Ben|¤tez, 1952; Santiago, 1960; Machado, 1962^1963; Carracedo et al., 1997). While the recent beach consists mainly of pebbles derived from the Cumbre Vieja basaltic lavas, the beachrock contains a large proportion (about 40%) of pebbles derived from the Pliocene submarine volcano situated in the Taburiente caldera. The Pliocene pebbles could only have come from the interior of the Taburiente caldera, been transported along the Barranco de Las Angustias and, subsequently, by littoral drift to the Puerto Naos beach. The delta formed by the lavas of the 1949 eruption (Carracedo et al., 1997), located to the north of Puerto Naos beach, probably interrupted the littoral drift and thus the transport of the Pliocene
The Charco Verde beach formed on prehistoric platform-forming lavas of the Cumbre Vieja volcano (Carracedo et al., 1997). The beach deposits, and consequently the beachrock, were fossilised by the lavas of the 1585 eruption. This beachrock gave a conventional radiocarbon age of 33 330 8 490 BP (measured age of 32 850 8 490 BP). This age should be considered with caution because, on the one hand it approaches the limit of the 14 C method and, on the other hand, it is older than the underlying platform-forming lavas which have been dated by K^Ar as younger than the last maximum glacial at about 20 kyr (Carracedo et al., 1997; Guillou et al., 1998). Nonetheless, this beachrock can be considered the oldest of La Palma island. The beachrock is approximately 200 m long, 10 m wide and up to 1 m thick. It dips between 2 and 4‡ seaward and presents only one horizon which is made up of rudites grading to arenites and is very well-cemented (Fig. 2.3). 5.3. Beachrock of the Playa Chica beach Playa Chica beach developed on prehistoric platform-forming lavas of the Cumbre Vieja volcano (Carracedo et al., 1997), separated by a nar-
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1
3
81
2
10 cm
4
Fig. 2. (1) Beachrock at Puerto Naos beach presenting an irregular upper surface. (2) Detail of the beachrock at Puerto Naos beach. (3) Cemented volcanic clasts in the beachrock at Charco Verde beach (¢brous aragonite cement). (4) The beachrock pictured in this photograph was destroyed by a storm on 6 January 1999. Las Zamoras beach.
row ( 6 5 m) ‘tongue’ of lava from Las Zamoras beach (Fig. 3). The beachrock gives a conventional radiocarbon age of 14 580 8 130 years BP (measured age of 14 090 8 130 years BP) and forms a metre-sized patch dipping between 10 and 15‡ seaward. It is made up of well-cemented rudites and arenites, very similar to the lower horizon of Las Zamoras beachrock. 5.4. Beachrock of Las Zamoras beach Las Zamoras beach formed on prehistoric platform-forming lavas of the Cumbre Vieja volcano (Carracedo et al., 1997). The beachrock located here was almost completely destroyed by a storm on 6 January 1999. Due to its proximity to Playa Chica beach and the similar characteristics of both beachrocks, it can be considered
that Las Zamoras and Playa Chica beachrocks were formed at the same time (ca. 14 000 years BP). The beachrock was 50 m long, 15 m wide and up to 1 m thick. It dipped 10‡ seaward and presented two horizons (Figs. 2.4 and 3), a lower horizon made up of well-cemented rudites and arenites with an irregular upper morphology and an upper horizon consisting basically of poorly cemented arenites. Today, only a poorly preserved metre-sized patch exists at the inner part of the beach, which probably corresponds to the upper horizon. 5.5. Beachrock of Echentive beach Echentive beach developed on volcanic lavas produced by the 1677 eruption and is partially
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Fig. 3. Beachrock map and schematic cross-section of the beachrock at Las Zamoras, Playa Chica and Echentive beaches. 1: Cli¡-forming lavas (older than 20 kyr); 2,3: prehistoric platform-forming lavas (ca. 18 kyr and 3 kyr, respectively); 4,5: historic platform-forming lavas (1677 and 1971, respectively); 6: beachrocks; 7: volcanic pebbly beaches.
overlapped by the Tenegu|¤a volcanic lavas of the 1971 eruption (Afonso et al., 1974; Carracedo et al., 1997). Consequently, the beach dates from between 1677 and 1971 and the beachrock developed at this beach had to form later than 1677.
Hence, Echentive beachrock is the youngest of all La Palma island beachrocks. Echentive beach shows a complete beach pro¢le : the shoreface zone, the foreshore zone and the backshore zone (Fig. 3). The backshore
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Fig. 4. (1) Fibrous aragonite cement arranged in a pseudospherulitic fashion ¢lling intergranular porosity. Sample PN 1. (2) Fibrous aragonite cement arranged in a pseudospherulitic fashion ¢lling intergranular porosity. Spar aragonite cement ¢lls the inter¢bre porosity. Sample PC 2. (3) Detail of ¢brous aragonite cement arranged in a pseudospherulitic fashion. Sample CH 1, SEM. (4) Detail of ¢bres showing di¡erent sections. Sample CH 1, SEM. (5) a: Micrite HMC cement presenting an irregular morphology with protuberances. b: Peloidal cement. Sample PN 4. (6) a: Micrite HMC cement with meniscus fabric, which is the ¢rst stage of cementation. b: Spar aragonite cement forming a discontinuous rim, which is the second stage of cementation. c: Micrite HMC cement, which is the third stage of cementation. Sample PN 6.
zone, some 400 m wide, presents pebbly backshore ridges up to 4 m high, dipping landward. In the inner part of the backshore zone, and in contact with the volcanic lavas, there are two lakes several metres deep. The beachrock crops out along the seaward
margin of the lakes and disappears below the backshore ridge deposits. It is approximately 1 m thick, which is more or less the current tidal range in this inner zone, and consists of breccias made up of subangular volcanic lava clasts 2^20 cm in size.
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6. Beachrock composition and cements Texturally, the beachrock varies from poorly sorted rudites to arenites. Pebbles are rounded to subrounded (except those from Echentive beachrock) and consist of volcanic rocks of di¡erent nature, mainly alkaline basalts. The grains of the arenites and of the matrix of rudites are made up of volcanic rocks and a small proportion (2%) of crystals (predominantly pyroxenes, olivines and amphiboles). Porosity is mostly intergranular, only locally intragranular (the vesicles of the volcanic lavas). Older beachrocks seems to be more cemented than younger ones. The most common cements are ¢brous aragonite and micritic high-magnesium calcite (HMC) cement. Microbotryoidal HMC, spar aragonite and peloidal HMC cements are locally present. 6.1. Fibrous aragonite cement The ¢brous aragonite cements ¢ll both intergranular and intragranular porosities. Cement ¢bres are arranged either in a pseudospherulitic fashion (Fig. 4.1^3), or form isopachous rims, or are randomly oriented. Pseudospherulite sizes vary from 100 Wm to 3 mm and locally display a meniscus geometry. The aragonite ¢bres range from 10 Wm in length and 3 Wm in width to large crystals up to 3 mm by 200 Wm. The ¢brous crystals exhibit a prismatic habit with blunt terminations (Fig. 4.4) or else an acicular habit with pointed terminations. In general, the larger ¢bres present a pseudoundulating extinction. The ¢brous aragonite cement is non-luminescent. Similar cements have been reported in many beachrocks (e.g., Ginsburg, 1953; Taylor and Illing, 1969; Moore, 1973; Siesser, 1974; Beier, 1985; Strasser et al., 1989; Font and Calvet, 1997; Neumeier, 1998). 6.2. Micrite HMC cement The micrite HMC cement ¢lls intergranular porosity and presents discontinuous submillimetresized laminae or massive texture. It includes variable contents of silt-sized volcanic fragments and,
locally, some scattered bioclastic fragments. The presence of trapped detritus within micrite cements in a beachrock is relatively common (Alexandersson, 1972; Moore, 1973; Meyers, 1987; Bernier and Dalongeville, 1988; Webb et al., 1999). The micritic HMC cement displays an irregular morphology with protuberances (Fig. 4.5) and, locally, can have a meniscus fabric (Fig. 4.6), a feature that has already been reported by Meyers (1987) and Strasser et al. (1989). The micrite HMC cement in the studied beachrocks is related to micro¢laments produced by di¡erent types of micro-organisms. The main type of micro¢lament has a diameter varying from 20 to 40 Wm, is several millimetres in length, and presents dichotomic bifurcations (Fig. 5.1). It is interpreted as fungi. Micro¢lament walls consist of HMC crystals measuring 1^5 Wm (Fig. 5.2). Locally there is a network of ¢laments 1^3 Wm thick interpreted as cyanobacteria. Micro-organisms in beachrock micrite cements have been reported by several authors (Moore, 1973; Strasser et al., 1989; Font and Calvet, 1997). Given this relationship, micritic HMC cement is attributed to microbial activity (e.g., Bernier and Dalongeville, 1988; Neumeier, 1998). The meniscus fabric indicates, in most cases, vadose conditions. Textures similar to the studied micrite HMC cement have been reported in several beachrocks (e.g., Friedman and Gavish, 1971; Bernier et al., 1990; Holail and Rashed, 1992). The micrite HMC cement of the La Palma beachrocks contains 11.2^15.2 mol% of MgCO3 (based on 12 analyses), which is within the same range as those reported by Tietz and Mu«ller (1971), Moore (1973), Magaritz et al. (1979), Meyers (1987) and Font and Calvet (1997). 6.3. Microbotryoidal HMC, spar aragonite and peloidal HMC cements Microbotryoidal HMC cement is present only in the beachrock at Charco Verde beach. This cement is arranged in a uniformly distributed rim (Fig. 5.3) ¢lling intergranular and locally intragranular porosities. Rim thickness varies from 27 to 70 Wm and consists of a single microbotryoid (Fig. 5.4). The microbotryoids are made
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Fig. 5. (1) Micrite HMC cement related to micro¢laments. Sample PN 2. (2) Detail of the micro¢lament. Micrite crystals on the surface of the micro¢lament. Sample PN 2. (3) a: The microbotryoidal HMC cement forms an isopachous rim and is the ¢rst stage of cementation. b: Fibrous aragonite cement is the second stage of cementation. Sample CH 1a. (4) Detail of the microbotryoids. Sample CH 1a, crossed nicols.
up of HMC ¢bres measuring from a few micrometres in width and from 27 to 70 Wm in length. The microbotryoidal HMC cement has 8.2^15.2 mol% of MgCO3 (based on 25 analyses). The spar aragonite cement either forms a continuous or discontinuous rim (Fig. 4.6) or ¢lls in the spaces between the ¢bres of the ¢brous aragonite cement (Fig. 4.2). The continuous rim is 21^35 Wm thick, while the discontinuous rim varies from 0 to 150 Wm. The crystals of the spar aragonite cement have an equant morphology from 7 to 35 Wm, although locally there are crystals with bladed morphology. Although the morphology of this cement is unusual for aragonite, the elemental geochemistry points out to this mineralogy. The peloidal cement consists of round to ovoid peloids measuring from 21 to 50 Wm (Fig. 4.5,6) and is similar to those described in other beach-
rocks (e.g., Moore, 1973; Meyers, 1987; Amieux et al., 1989; Font and Calvet, 1997). 6.4. Cement stratigraphy The cement stratigraphy of the di¡erent beachrocks from La Palma island is variable even within the same beachrock (Fig. 6). Consequently, it is di⁄cult to establish a general rule for the stages of cementation. The general trend cited by some authors (e.g., Taylor and Illing, 1969; Siesser, 1974; Bernier and Dalongeville, 1988; Strasser et al., 1989; Bernier et al., 1990), with the micrite HMC cement as the ¢rst stage of cementation and the ‘crystalline’ cements (aragonite or HMC) as the second, is not common in the studied beachrocks. The main observations of the cementation processes of La Palma beachrocks are : (1) in general,
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Fig. 6. Scheme of the main patterns of cementation in the La Palma beachrocks.
rudites and arenites present ¢brous aragonite cement with a pseudospherulitic arrangement; (2) the micrite HMC cement laminae are never associated with the ¢brous aragonite cement ; (3) when the microbotryoidal HMC cement is present, it always represents the ¢rst stage of cementation; and (4) when the peloidal HMC cement is present, it is always the last stage of cementation.
7. Cement elemental geochemistry The Sr content of the ¢brous aragonite cement
(n = 153 analyses) varies from 4250 ppm to 13 450 ppm (with a mean of 8992 and a standard deviation of 1691); the Na content ranges from 245 ppm to 2530 ppm (with a mean of 1053 and a standard deviation of 630). The Mg content is always below the detection limit (Table 2). These Sr values of the ¢brous aragonite cement of La Palma beachrocks, equivalent to 1^2% of SrCO3 , are similar to those reported by Moore (1973), Strasser et al. (1989) and Font and Calvet (1997) in beachrocks from Grand Cayman Island, southeastern Tunisia and Reunion island respectively. The Sr content of the spar aragonite cement (n = 10 analyses) varies from 6635 ppm to 12 665 ppm (with a mean of 9385 and a standard deviation of 1818). The Na content ranges from 1250 ppm to 2495 ppm (with a mean of 1948 and a standard deviation of 401) and the Mg content is always below the detection limit (Table 2). The Sr content of the microbotryoidal HMC cement (n = 25 analyses) ranges from 780 ppm to 1490 ppm (with a mean of 1161 and a standard deviation of 184), the Na content varies from 315 ppm to 705 ppm (with a mean of 477 and a standard deviation of 108) and the Mg content varies from 8.1 to 15.2% of MgCO3 (with a mean of 11.7 and a standard deviation of 1.7) (Table 2). The Sr content of the micrite HMC cement (n = 12 analyses) varies from 185 ppm to 1195 ppm (with a mean of 886 and a standard deviation of 341), the Na content ranges between 210 ppm and 810 ppm (with a mean of 569 and a
Table 2 Elemental geochemistry of beachrock cements Beachrock
Puerto Naos
Cement
Fibrous aragonite Spar aragonite Micrite HMC Charco Verde Fibrous aragonite Microbotryoidal HMC Playa Chica Fibrous aragonite Spar aragonite Micrite HMC Echentive Fibrous aragonite
No. of analyses
40 4 3 40 25 40 6 9 33
Mg CO3
Sr
Na
(%) min^max (mean, S.D.)
(ppm) min^max (mean, S.D.)
(ppm) min^max (mean, S.D.)
^ ^ 14.3^15.2 (14.9, 0.5) ^ 8.2^15.2 (11.7, 1.7) ^ ^ 11.2^14.7 (12.5, 1.1) ^
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7350^12970 (9351, 1485) 9560^11390 (10142, 851) 185^855 (546, 338) 4250^12310 (8050, 2077) 780^1490 (1161, 184) 5905^13450 (9284, 1628) 6635^12665 (8880, 2180) 315^1195 (999, 272) 7760^10980 (9345, 951)
385^2525 (1362, 621) 1930^2200 (2060, 122) 325^565 (433, 121) 245^770 (461, 170) 315^705 (477, 108) 545^2530 (1599, 467) 1250^2495 (1873, 514) 210^810 (614, 167) 430^1290 (733, 279)
F. Calvet et al. / Marine Geology 197 (2003) 75^93 Table 3 Oxygen and carbon isotopic data from the ¢brous aragonite cements Beachrock
Sample
N18 O (x PDB)
N13 C (x PDB)
Charco Verde
mean CH-1a CH-1b CH-3a CH-3b CH-4a CH-4b CH-5 BLV-3 BLV-4 BLV-3a BLV-4a mean PC-2 PC-3a PC-3b BLC-2 BLC-3 BLC-2a BLZ-4
34.0 34.1 34.2 34.1 33.9 33.7 34.2 34.0 34.0 33.9 34.1 33.9 33.0 33.7 33.1 33.2 32.4 32.4 33.4 33.2
+4.5 +4.4 +4.4 +4.7 +4.4 +4.4 +4.4 +4.6 +4.7 +4.7 +4.6 +4.6 +4.5 +4.3 +4.3 +4.0 +4.9 +4.8 +4.6 +4.1
Playa Chica
Las Zamoras
87
beachrocks are similar to the values of aragonite cements in some beachrocks in French Polynesia (Neumeier, 1998) but, in general, the La Palma values are lower than those of aragonite cements in the beachrocks cited by Magaritz et al. (1979) and Beier (1985), and much lighter than the values reported for aragonite and/or HMC cements in beachrocks surrounding the Mediterranean and the Red Sea (Holail and Rashed, 1992). Possible factors explaining the relatively lighter oxygen isotopic values measured in the studied beachrocks could be: (1) the salinity of the seawater in the Canary Islands ; (2) the temperature of the seawater in the intertidal zone in La Palma beaches; and (3) the in£uence of meteoric water. 8.1. Salinity of the seawater
standard deviation of 172) and the Mg content varies from 11.2 to 15.2% of MgCO3 (with a mean of 13.1 and a standard deviation of 1.5) (Table 2). The Sr values of the micrite HMC cement of La Palma beachrocks are lower than the values reported by Moore (1973), Strasser et al. (1989) and Font and Calvet (1997) in their studies of Grand Cayman Island, southern Tunisia and Reunion island.
8. Oxygen isotopic composition Because of the size of the crystals, only the ¢brous aragonite cement was sampled for N18 O and N13 C analyses. N18 O PDB values measured on the ¢brous aragonite cement of the La Palma beachrocks range between 34.2x and 32.4x (Table 3 and Fig. 7). The aragonite cements of the beachrock at the Charco Verde beach (with a mean of 34.0x PDB) are isotopically lighter than those at Playa Chica beach (with a mean of 33.0x PDB). The oxygen isotopic values of the La Palma
Seawater salinity in the Canary Islands ranges from 36.2 to 37.2x (Barton et al., 1998). In the Mediterranean it varies between 38.8 and 39.5x in Crete (Neumeier, 1998) and between 37.5 and 38.0x in Corsica (Bernier et al., 1997). The salinity in the Red Sea ranges from 39.8 to 40.8x Neumeier, 1998) whereas in French Polynesia, it varies between 35 and 36x (Neumeier, 1998). Consequently, aragonite cements in the beachrocks tend to present lighter oxygen isotopic values when water salinity is low (Canary Island and Polynesia) and heavier values when water salinity is high (Mediterranean and Red Sea). The equation of Railsback et al. (1989) makes it possible to calculate the expected oxygen isotopic composition of the seawater as a function of salinity. Application of this equation to the salinity range in the Canary Islands results in expected N18 O values of the water ranging between +0.7 and +1.1x SMOW. These results are consistent with the mean measured N18 O value of La Palma seawater, which is +1.1x SMOW (Table 1). Thus, the positive correlation between salinity and oxygen isotopic composition of the cements is very likely due to equilibrium conditions (Railsback et al., 1989). 8.2. Temperature of the seawater Seawater temperature in the Canary Islands
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Fig. 7. Cross-plot of stable isotope values of the aragonitic cements of La Palma beachrocks and examples from the literature.
varies between 17 and 25‡C (Barton et al., 1998). In Crete it ranges between 15 and 25‡C (Neumeier, 1998), in Corsica between 13 and 23‡C (Bernier et al., 1997), in the Red Sea between 21 and 29‡C (Neumeier, 1998), and in French Polynesia between 27 and 29.5‡C (Neumeier, 1998). There seems to be no relation between the oxygen isotopic composition of the aragonite cements of the beachrocks and seawater temperature. The equilibrium between H2 O, aragonite cements and temperature is calculated using the equation of O’Neil et al. (1969, modi¢ed): 103 lnK CaCO3 H2 O ¼ 2:78ð106 T 32 Þ32:89
Applying this equation, and using the measured oxygen isotopic composition of La Palma seawater (+1.1x SMOW), equilibrium temperature is between 30.4‡C and 39.9‡C (Table 4).
If the oxygen isotopic composition of the seawater which precipitated the analysed ¢brous aragonite cements was that of the current seawater around La Palma (+1.1x SMOW), much higher temperatures than those currently measured would be inferred. But these estimations are made using open-sea temperatures (17^25‡C), which are lower than the temperatures of the water in the inner part of the beaches. The temperature of the water in such zones reaches 30‡C or more during summer time or dry-haze weather (Marzol, 1988). Thus, the temperature of the seawater could well account for the lighter oxygen isotopic composition of the ¢brous aragonite cements of La Palma beachrocks. The Charco Verde beachrock shows heavier N18 O values towards the top of the pro¢le (Fig. 8) suggesting a decrease in temperature from the
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89
Table 4 Calculated temperatures and N18 O values at 17‡C and 25‡C in equilibrium with the ¢brous aragonite cements (based on the equation by O’Neil et al., 1969, modi¢ed) Beachrock
Sample
Equilibrium temperature
N18 O in equilibrium with carbonate at 17‡C
N18 O in equilibrium with carbonate at 25‡C
33.5 33.6 33.5 33.3 33.1 33.6 33.4 33.4 33.3 33.5 33.3 33.1 32.5 32.6 31.8 31.8 32.8 32.6
31.8 31.9 31.8 31.6 31.4 31.9 31.7 31.7 31.6 31.8 31.6 31.4 30.8 30.9 0.0 0.0 31.1 30.9
(‡C) Charco Verde
Playa Chica
Las Zamoras
CH-1a CH-1b CH-3a CH-3b CH-4a CH-4b CH-5 BLV-3 BLV-4 BLV-3a BLV-4a PC-2 PC-3a PC-3b BLC-2 BLC-3 BLC-2a BLZ-4
39.3 39.9 39.3 38.2 37.1 39.9 38.7 38.7 38.2 39.3 38.2 37.3 34.1 34.6 30.4 30.4 35.7 34.6
lower to the upper part of the intertidal water lens. 8.3. Meteoric in£uence N18 O values for rain waters in La Palma range from 32.51x SMOW (ITGE, 1993) to 33.3x SMOW (Ko«nig, 1997) for samples taken at 1400 and 1200 m asl, respectively. Groundwater N18 O values in the study area vary between 34.54x
SMOW (ITGE, 1993) and 34.0x SMOW (Veerger, 1991), pointing to recharge at higher zones. Applying the equation of O’Neil et al. (1969, modi¢ed), the calculated N18 O values of waters in equilibrium with the ¢brous aragonite cements of La Palma beachrocks at the temperature range measured in seawater (17^25‡C) would be 33.6 to 31.8x SMOW (at 17‡C) and 31.9 to 0.0x SMOW (at 25‡C) (Table 4). These values suggest that these cements precipitated from seawater with meteoric in£uence. Similar values have been cited in several beachrocks (Moore, 1973; Magaritz et al., 1979; Neumeier, 1998) and interpreted in the same way. The meteoric in£uence is higher in the inner part of the beaches. For instance, in Echentive beach, the oxygen isotopic composition of the lake water (+0.4x SMOW) is lighter than the corresponding value in the foreshore zone (+1.1x SMOW).
9. Carbon isotopic composition Fig. 8. Vertical pro¢le of the Charco Verde beachrock and isotopic vertical pro¢le of ¢brous aragonite cement.
The carbon isotopic composition of the ¢brous
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aragonite cement of La Palma beachrocks ranges from +4.0 to +4.9x PDB (Table 3 and Fig. 7), which is very similar to that of aragonite cements of the Bahamas beachrocks (Beier, 1985) and relatively similar to the aragonite cements of the Mediterranean (Magaritz et al., 1979) and French Polynesia (Neumeier, 1998). There are no di¡erences in N13 C between the cements of the beachrocks at Charco Verde and Playa Chica beaches. N13 C values measured are similar to those of recent or subrecent marine cements (Tucker and Wright, 1990). The carbon isotopic composition of present-day seawater HCO3 3 in the Canary Islands (+0.51x PDB) is consistent with that determined on the cements. CO2 degassing from seawater would preferably carry light carbon. Subsequent fractionation between aragonite and HCO3 3 (which is +2.9x according to Robinson and Clayton, 1969) would result in N13 CW+4x PDB, as measured in the beachrock aragonite cements. If correct, degassing would have been the main driving force for cement formation, as suggested by Hanor (1978).
10. Development of the beachrocks of La Palma The evolution of the beach deposits and beachrocks in La Palma involved the following stages (Fig. 9): (1) beach development, (2) beachrock formation, and (3) retrogradation and/or erosion of beaches. (1) Beach development requires an input of sediment (pebbles and sands) and a relatively £at marine bottom. The beach sediment is exclusively of terrigenous origin (clasts and grains derived from volcanic lavas) and there is essentially no material from the marine platform such as skeletal grains. Hence, terrigenous material was supplied by marine erosion of volcanic lavas and from the interior of the island transported along the gullies. Although marine erosion is a continuous process, the period of major sediment input is probably related to volcanic eruptions, when the lavas are brecciated following contact with seawater. Furthermore, on reaching the sea, the lavas can in£uence the littoral morphology, creating capes
Fig. 9. Scheme of the evolution of beachrocks in the island of La Palma.
and bays. In the inner part of the bays, beaches can develop and migrate seaward (progradation stage). Lavas can also form a relatively £at and shallow marine bottom where beaches can originate. Beach pro¢les include a poorly developed shoreface zone and well-developed foreshore and backshore zones. The Echentive beach pro¢le corresponds to this stage of beach development. (2) The beachrocks are located in the intertidal zone and in the inner part of the beach pro¢le. The Echentive beachrock could be the key to understanding beachrock formation in volcanic pebbly beaches. At present, beach deposit cementation occurs in the inner part of the Echentive beach (lakes). Newly formed cements consist of ¢brous aragonite and HMC micrite. Probably,
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cement formation continues in the intertidal zone in a seaward direction, but it is covered by supratidal deposits (the backshore ridges). The oxygen isotopic composition of cements of the beachrocks studied shows that these cements precipitated from seawater under a meteoric in£uence. Moreover, the oxygen isotopic composition, the conductivity and the pH of the present-day Echentive lakes indicate that the original marine water was slightly modi¢ed by meteoric water in£uence. In conclusion, the beachrocks studied are cemented in the intertidal zone and probably in the inner zone of the beaches where seawater presents a meteoric water in£uence, and slightly higher temperatures. (3) The beachrocks at Puerto Naos, Charco Verde, Las Zamoras and Playa Chica beaches, probably originating in the inner part of the backshore zone, crop out today in the foreshore zone. This indicates that a large part of the original beach pro¢le has been eroded due to retrogradation. Beachrocks can be completely eroded if the retrogradation process is very energetic. A good example of this is the Las Zamoras beachrock described above which cropped out until 6 January 1999, when it was almost totally destroyed by a winter storm.
11. Implication of the presence of beachrocks The beachrocks of La Palma prompt us to make some general considerations on the evolution of the relative position of La Palma island with respect to sea level, the relationships between beachrocks and beaches, palaeoceanography, geology and palaeoclimate. (1) Sea level. The location of all the studied beachrocks, with di¡erent ages, at the present sea level would have required a combination of eustatic and isostatic movements to keep the sea level stable at the present level during the past 33 kyr. (2) Beach retrogradation. The outcropping of beachrocks on the beaches implies a retrogradation stage in these littoral deposits (Strasser and Davaud, 1986). Retrogradation is related to a
91
slow sedimentation rate on the beaches and an increase in the erosion rate. The reason for the slow sedimentation rates in La Palma, where the beachrocks are made up exclusively of volcanic clasts and grains, may be a lack of continuous input of these materials to the beaches, resulting in their retrogradation and consequent beachrock exposure. The presence of beachrocks in these beaches, however, prevents the total disappearance of these beaches. (3) Palaeoceanographic implications. The beachrock thickness corresponds in general to the tidal range. The similarity between the thickness of the studied beachrocks (between 1 and 1.5 m) and the actual mean tidal range in the Canary Islands indicates that the mean tidal range in the Canary Islands has been the same for the last thousand years. (4) Geological implications. The beachrocks in La Palma are located along the coastline of the Cumbre Vieja volcano, which is the volcanically active part of the island and, consequently, the only part of the island which has received a signi¢cant amount of sediments during the last 125 kyr. If beaches with beachrocks ever existed in the older part of the island, they have been eroded away. This observation is consistent with the aforementioned model of the development of the beaches: sediment derived from volcanic lavas provides the necessary material to form beaches. (5) Palaeoclimatic implications. The restricted location of the beachrocks along the western coast, which is dominated by a dry warm climate with a very low rainfall (below 250 mm/year) and a high insolation rate (6^11 h/day), indicates that these parameters favour, and favoured, cement precipitation and beachrock formation by increasing water temperature in the intertidal zone and in the inner backshore zone.
12. Conclusions (1) The beachrocks of La Palma are located in the volcanically active part of the island (coastline of the Cumbre Vieja volcano), especially on the southwestern coast, which is characterised by a dry warm climate. Volcanic activity supplied the
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sediment to form beaches and changed the morphology of the coast, whereas the climatic parameters favoured cement precipitation and beachrock formation. The presence of beachrocks prevents the total disappearance of beaches. (2) The beachrocks formed in volcanic pebbly beaches. They are 1^1.5 m thick and up to some tens of metres wide. The coincidence between beachrock thickness and the present mean tidal range in the Canary Islands indicates that the tidal amplitude has been relatively constant over the past thousand years. (3) The age of the beachrocks varies from ca. 33 000 years to recent. The position of the beachrocks at the present-day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level during the past 33 kyr. (4) The beachrocks are cemented by ¢brous aragonite and micrite HMC, and locally by microbotryoidal and peloidal HMC, and spar aragonite. The elemental geochemical values of all cements are consistent with a marine origin. Cementation occurred either in the intertidal zone or in the inner part of the beaches, as occurs today in Echentive beach. (5) The oxygen isotopic composition of the ¢brous aragonite cement, ranging between 34.2 and 32.4x PDB, suggests that the precipitation occurred from marine water slightly modi¢ed by meteoric water at relatively low salinity (36.2^ 37.2x) and at relatively high temperature ( v 30‡C). The carbon isotopic values of this cement (+4.0 to +4.9x PDB) suggest that the principal cause of the precipitation was degasi¢cation.
Acknowledgements This work was carried out within the framework of Project 68/98 of the Fundacio¤n Universitaria de Las Palmas. The analytical part was partially supported by Project CICYT PB97-0883, RpD Project BTE 2002-04453-C02-01 and Grup Consolidat de Recerca ‘Grup de Geologia Sedimenta'ria’ (2001/SGR/75). The authors thank J. Agullo¤, X. Llovet, R. Fontarnau, A. Dom|¤nguez, A. Samper, V. Panella and J. Illa for technical
assistance, and G. Von Knorring and F. Luttikhuizen for the revision of the English version of the manuscript. We are indebted to D. Gimeno for helpful discussions. The manuscript has been considerably improved by constructive criticism from P. Kindler and E. Gischler.
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F. Calvet et al. / Marine Geology 197 (2003) 75^93 tidal Cement. Carbonate Cements. The Johns Hopkins Press, Baltimore, MD, pp. 1^3. Calvet, F., Aguilar, A., Carracedo, F.J., Pe¤rez-Torrado, F.J., Recio, C., Trave¤, A., 2000. ‘Beachrocks’ de La Palma, Islas Canarias. Geotemas 1, 213^217. Carracedo, J.C., Day, S.J., Guillou, H., Gravestock, P.I., 1997. Geological map Cumbre Vieja Volcano (La Palma, Canary Island). International Workshop on Volcanism and Volcanic Hazards in Immature Intraplate Oceanic Islands, La Palma. Carracedo, J.C., Day, S.J., Guillou, H., Rodr|¤guez Badiola, E., Canas, J.A., Pe¤rez-Torrado, F.J., 1998. Hotspot volcanism close to a passive continental margin: the Canary Islands. Geol. Mag. 135, 591^604. Craig, H., 1957. Isotopics standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide. Geochim. Cosmochim. Acta 12, 133^149. Font, Y., Calvet, F., 1997. ‘Beachrocks’ Holocenos de la Isla de La Reunio¤n, Oce¤ano IŁndico. Cuad. Geol. Ibe¤r. 22, 87^ 102. Friedman, G.M., Gavish, E., 1971. Mediterranean and Red Sea (Gulf of Aqaba) beachrocks. In: Bricker, O.P. (Ed.), Carbonate Cements. The Johns Hopkins Press, Baltimore, MD, pp. 13^16. Ginsburg, R.N., 1953. Beachrock in South Florida. J. Sediment. Petrol. 23, 85^92. Guillou, H., Carracedo, J.C., Day, S.J., 1998. Dating of the Upper Pleistocene^Holocene volcanic activity of La Palma using the unspiked K^Ar technique. J. Volcanol. Geotherm. Res. 86, 137^149. Hanor, J.S., 1978. Precipitation of beachrock cements: mixing of marine and meteoric waters vs. CO2 -degassing. J. Sediment. Petrol. 48, 489^501. Holail, H., Rashed, M., 1992. Stable isotopic composition of carbonate-cemented recent beachrock along the Mediterranean and the Red Sea coasts of Egypt. Mar. Geol. 106, 141^ 148. ITGE (Instituto Tecnolo¤gico Geo-Minero de Espan‹a), 1993. Estudio isoto¤pico de las aguas subterra¤neas de la isla de La Palma, 73 pp. Ko«nig, B., 1997. Determination of solute £uxes and balances in the groundwater of the volcanic island of La Palma (Canary Islands). Geol-Pala«ontol. Inst. Univ. Kiel, Report 84, 201 pp. Machado, F., 1962^1963. Erupc\oes do ilha de La Palma (Cana¤rias). Bol. Mus. Lab. Mineral. Geol. Fac. Ci. 9, 143^ 156. Magaritz, M., Gavish, E., Bakler, N., Kafri, U., 1979. Carbon and oxygen isotope composition ^ indicators of cementation environment in Recent, Holocene, and Pleistocene sediments along the coast of Israel. J. Sediment. Petrol. 49, 401^412. Marzol, M.V., 1988. El clima. In: Geograf|¤a de Canarias, Editorial Interinsular Canaria, Santa Cruz de Tenerife, Tomo 1 (IX), pp. 157^202. McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotemperature scale. J. Chem. Phys. 18, 849^857.
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McLean, R.F., 1967. Origin and development of ridge-furrow systems in beachrock in Barbados, West Indies. Mar. Geol. 5, 181^193. Meyers, J.H., 1987. Marine Vadose beachrock cementation by cryptocrystalline magnesian calcite ^ Maui, Hawaii. J. Sediment. Petrol. 57, 558^570. Moore, C.H., 1973. Intertidal carbonate cementation Grand Cayman, West Indies. J. Sediment. Petrol. 43, 591^602. Moore, C.H., 1989. Carbonate Diagenesis and Porosity. Developments in Sedimentology 46. Elsevier, Amsterdam, 338 pp. Neumeier, U., 1998. Le ro“le de l’activite¤ microbienne dans la cimentation pre¤coce des beachrocks (se¤diments intertidaux). Terre Environ. 12, 1^183. O’Neil, J.R., Clayton, R.N., Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys. 51, 5547^5558. Pe¤rez-Torrado, F.J., Mangas, J., 1994. Origen y evolucio¤n geolo¤gica de la barra de Las Canteras (Las Palmas de Gran Canaria). Vector Plus 1, 4^16. Railsback, L.B., Anderson, T.F., Ackerly, S.C., Cisne, J.L., 1989. Paleoceanographic modeling of temperature-salinity pro¢les from stable isotopic data. Paleoceanography 4, 585^591. Robinson, M., Clayton, R.N., 1969. Carbon-13 fractionation between aragonite and calcite. Geochim. Cosmochim. Acta 33, 997^1022. Santiago, M., 1960. Los volcanes de La Palma (Islas Canarias). Datos histo¤rico-descriptivos. Mus. Canario 21, 281^ 346. Siesser, W.G., 1974. Relict and recent beachrock from southern Africa. Geol. Soc. Am. Bull. 85, 1849^1854. Strasser, A., Davaud, E., 1986. Formation of Holocene limestone sequences by progradation, cementation, and erosion: two examples from the Bahamas. J. Sediment. Petrol. 56, 422^428. Strasser, A., Davaud, E., Jedoui, Y., 1989. Carbonate cements in Holocene beachrock: example from Bahiret el Biban, southeastern Tunisia. Sediment. Geol. 62, 89^100. Taylor, J.C.M., Illing, L.V., 1969. Holocene intertidal calcium carbonate cementation, Qatar, Persian Gulf. Sedimentology 12, 69^107. Tietz, G., Mu«ller, G., 1971. High-magnesian calcite and aragonite cementation in recent beachrocks, Fuerteventura, Canary Islands, Spain. In: Bricker, O.P. (Ed.), Carbonate Cements. The Johns Hopkins Press, Baltimore, MD, pp. 4^8. Tucker, M.E., Wright, P.V., 1990. Carbonate Sedimentology. Blackwell Scienti¢c, Oxford, 482 pp. Veerger, A., 1991. Geochemical Methods for Evaluating the Origin and Evolution of Groundwater in Volcanic Rocks. PhD Thesis, Arizona University, Tucson, AZ, 237 p. Webb, G.E., Jell, J.S., Baker, J.C., 1999. Cryptic intertidal microbialites in beachrock, Heron Island, Great Barrier Reef: implications for the origin of microcrystalline beachrock cement. Sediment. Geol. 126, 317^334.
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Beachrocks from the island of La Palma (Canary Islands, Spain) F. Calvet a;1 , M.C. Cabrera b , J.C. Carracedo c , J. Mangas b , F.J. Pe¤rez-Torrado b , C. Recio d , A. Trave¤ a; a
d
Departament de Geoqu|¤mica, Petrologia i Prospeccio¤ Geolo'gica, Facultat de Geologia, Universitat de Barcelona, 08071 Barcelona, Spain b Departamento de F|¤sica-Geolog|¤a, Facultad de Ciencias del Mar, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain c Estacio¤n Volcanolo¤gica de Canarias, CSIC, P.O. Box 195, La Laguna, Tenerife, Spain Servicio General de Ana¤lisis de Iso¤topos Estables, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain Received 14 June 2002; accepted 10 March 2003
Abstract Beachrocks on La Palma Island developed on platform-forming lavas of the Cumbre Vieja volcano. Some of these lavas are related to the 1585 (Puerto Naos), 1677 and 1971 (Echentive) eruptions. Radiocarbon dating of the Charco Verde beachrock gives a conventional age of 33 330 8 490 BP, while that at Playa Chica beach gives a calibrated age of 14 940 8 525 BP. The beachrocks, up to 1.5 m thick and some tens of metres wide, consist of several decimetre-thick horizons dipping 2^15‡ seaward. Petrographically, they can be classified as rudstones and arenites, with volcanic clasts as their main component. The original porosity of the beachrocks was intergranular (and occasionally intragranular) and was partially occluded by cementation and locally by internal sediments. The main cements are fibrous aragonite and micrite high-magnesium calcite (HMC). Spar aragonite, peloidal HMC and microbotryoidal HMC are scarce. The elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters. The evolution of beach deposits, and especially the beachrocks in La Palma island, follows three stages: (1) beach deposition, (2) beachrock formation, and (3) beach retrogradation and/or erosion. The studied beachrocks prompt us to make some important considerations. (1) The mean tidal range in the Canary Islands has not varied over the last thousand years. (2) The position of the beachrocks at the present-day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years. (3) Volcanic activity supplies the sediment that forms the beaches. (4) A dry warm climate with a very low rainfall (below 250 mm/year) and a high insolation rate (6^11 h/day) favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches. (5) The presence of beachrocks in the La Palma beaches prevents the total disappearance of the beaches. @ 2003 Elsevier Science B.V. All rights reserved.
1
Deceased in January 2002. * Corresponding author. E-mail address: [email protected] (A. Trave¤).
0025-3227 / 03 / $ ^ see front matter @ 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0025-3227(03)00090-2
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Keywords: beachrocks; volcanic conglomeratic beaches; marine cements; trace elements; stable isotopes; Quaternary; La Palma (Canary Islands)
1. Introduction Beaches are an ideal location for the precipitation of marine cements. Intertidal lithi¢cation of beach sand and gravel by carbonate cementation results in a rock termed ‘beachrock’ (Ginsburg, 1953). The high-energy setting of beaches (owing to wave and tidal action) and the presence of sand- and/or gravel-sized sediment with high porosity and permeability allows for the necessary volume of water to move through the sediment, giving rise to its cementation (Moore, 1989). Cementation usually occurs in the intertidal zone (Neumeier, 1998), but it can also occur in the upper part of the subtidal zone (Alexandersson, 1972), the lower part of the supratidal zone (Holail and Rashed, 1992) and in the swash zone (Bernier and Dalongeville, 1996). Generally, beachrocks are arranged parallel to the coastline forming linear bodies ranging from several tens of kilometres to metre-size patches or even decimetre-size nodules (Bricker, 1971). Beachrocks rarely exceed 1 m in thickness, though some can attain a thickness of 5 m (Amieux et al., 1989). They normally develop in the lower part of the foreshore and dip gently (maximum 15‡) seaward following the arrangement of the beach. Beachrocks are often made up of several horizons (‘dalles’, ‘beds’, ‘bands’) whose thicknesses range from several centimetres to several decimetres. Although a number of authors consider beachrock formation a feature of tropical or subtropical zones (Bricker, 1971), beachrocks are also found in temperate climates. The principal zones of beachrock development are: (1) tropical or subtropical zones with a humid climate such as the Caribbean area (e.g., Ginsburg, 1953; Moore, 1973; Hanor, 1978; Beier, 1985), the coasts of Africa (e.g., Amieux et al., 1989), the Paci¢c Ocean (Meyers, 1987; Bernier et al., 1990; Neumeier, 1998), the Indian Ocean (Font and Calvet, 1997), Australia (Neumeier, 1998; Webb et al., 1999) and Brazil; (2) tropical or subtropical zones
with an arid or semiarid climate such as the Persian Gulf (Taylor and Illing, 1969) and the Red Sea (Friedman and Gavish, 1971; Holail and Rashed, 1992; Neumeier, 1998); (3) temperate areas such as the Mediterranean Sea (e.g., Friedman and Gavish, 1971; Alexandersson, 1972; Bernier and Dalongeville, 1988; Holail and Rashed, 1992; Neumeier, 1998). The Canary Islands are located in a special subtropical area and, though rarely documented, beachrock formation is relatively common in Fuerteventura, La Palma and La Graciosa (north of Lanzarote), and less frequent in Gran Canaria, Lanzarote, La Gomera and El Hierro. In these islands beachrocks have been reported by Tietz and Mu«ller (1971) in Fuerteventura, by Pe¤rezTorrado and Mangas (1994) in Gran Canaria and by Calvet et al. (2000) in La Palma. The main purpose of this study is to present a model of beachrock formation and to consider the implications of its formation in the island of La Palma. To accomplish this, we have carried out an in-depth study of the geometry, age, petrography and geochemistry of these marine deposits.
2. Methods and analytical techniques Standard thin sections of approximately 50 rock samples from the ¢ve outcrops were examined for their petrography under an optical microscope. C-metallised polished thin sections of some selected samples were analysed for their elemental geochemistry at the Serveis Cienti¢cote'cnics of the University of Barcelona, using a CAMECA model SX-50 microprobe equipped with four vertically displayed WD X-ray spectrometers. The microprobe was operated using 20 kV excitation potential, 15 nA current intensity and 10 Wm beam diameter. Ca, Mg, Sr, Fe, Mn and Na contents of the di¡erent cements were determined. Detection limits were 395 ppm for Mg, 185 ppm for Sr, 275 ppm for Fe, 180 ppm for Mn and 195 ppm for Na. Fe and Mn contents were always
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below the detection limits. Precision on major element analyses averaged 6.32% standard error at 3c con¢dence level. Isotopic ratios (D/H, 13 C/12 C and 18 O/16 O) were determined at the Stable Isotope Laboratory of the University of Salamanca. Results are reported in the usual delta notation relative to PDB and SMOW. CO2 from carbonates was evolved by reaction with 103% H3 PO4 at 25‡C, employing standard techniques, or at 90‡C, employing an ISOCARB0 device coupled on line to a Micromass SIRA-II0 mass spectrometer (McCrea, 1950; Craig, 1957). Precision was monitored by repeated analysis by both internal and international (NBS-19) standards under identical analytical conditions. Average precision obtained was 8 0.02x (N13 C) and 8 0.12x (N18 O). D/H ratios of waters were measured on hydrogen gas evolved by the U reduction method (Bigeleisen et al., 1952), and measured on the same mass spectrometer employing the split-£ight tube for D/H measurement. ND values were normalised to the SMOW-SLAP scale, and GISP was used as an additional reference standard. Long-term reproducibility for repeated determination of ND on reference samples was better than 8 1x. N18 O in waters was measured on CO2 that had equilibrated with water at 40‡C for 5 h using a Micromass Multi£ow0 device coupled on line to an Isoprime0 CF mass spectrometer. As for D/H, 18 O/16 O ratios were normalised to the SMOWSLAP scale, and GISP was used as an additional reference for calibration. Long-term reproducibility for repeated determination of N18 O on reference samples was better than 8 0.27x. Radiocarbon dating was performed at Beta Analytic Inc. at Miami (FL, USA) on two aragonite samples without pretreatment. Both provided suf¢cient carbon for accurate accelerator mass spectrometer (AMS) standard analysis. AMS results were derived from reduction of sample carbon to graphite (100% C), along with standards and backgrounds. 14 C measurements on the graphite were done on an AMS. The conventional 14 C age resulted from applying 13 C/12 C isotopic fractionation corrections to the measured age. Calendar calibration was only applied to the sample giving a conventional radiocarbon age of 14 589 8 130
77
BP. The other sample was too old for an appropriate calibration.
3. Seawater and climate characteristics in the Canary Islands Although the Canary Islands are located in a subtropical zone, the physical and chemical parameters of their climate and surrounding marine waters di¡er considerably from other regions located along the same zone. These di¡erences are due to the fact that the Canary Islands are affected by upwellings, which occur along the African coast, and mesoscale instabilities like cyclonic and anticyclonic eddies produced by islands (Barton et al., 1998). The temperature of open-sea surface waters varies from a maximum of 25‡C in September and October to a minimum of 17‡C during the winter months. In shallow waters, especially in quiet bays or in beaches with a low slope, heat exchange with the atmosphere results in an increase in water temperature of about 1^2‡C. The salinity of seawater in the Canary Islands decreases with depth. If it were only for the e¡ects of evaporation and rainfall, it would reach a maximum value in summer and a minimum in winter. However, these two meteorological parameters are in general masked by upwelling and eddy effects, which produce a salinity of approximately 36.2x in summer and 37.2x in winter. The tidal regime is semidiurnal, with two high tides and two low tides each day. The mean tidal range is about 1.2^1.3 m. The tidal amplitude varies from a maximum (around 3 m) during the spring and autumn equinoxes to a minimum (around 0.7 m) during the summer and winter solstices. There are two types of swell: groundswell (bottom currents) and wind swell. The wind swell is in£uenced by the northeasterly trade winds, which are very persistent during the summer months, but weakened by the Azores anticyclone during the winter. The samples of seawater (collected during August 1997) from the foreshore zone in the di¡erent beaches of La Palma island show conductivity
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Table 1 pH, conductivity, N18 O and ND data of seawater collected from the island of La Palma Beach
pH
Conductivity (Ws/cm)
N18 O (x SMOW)
ND (x SMOW)
Puerto Naos Charco Verde Las Zamoras Chica Echentive (foreshore) Echentive (lake)
8.2 7.6 8.2 7.5 8.2 7.0^7.5
45 600 64 900 48 800 58 900 59 800 42 600
1.1 1.0 1.1 ^ 1.1 0.4
6.0 4.8 4.9 ^ 5.4 3.8
varying between 45 600 and 64 900 Ws/cm, and pH between 7.5 and 8.2 (Table 1). These values are consistent with those of standard seawater, whose pH ranges from 7.8 to 8.4 (Neumeier, 1998). Water from the innermost part (lakes) of the Echentive beach has pH values that vary from 7.0 to 7.5, and has lower conductivity (42 600 Ws/cm) than the foreshore zone water. In addition, the N18 O SMOW value of the foreshore zone seawater is about +1.1x, while that of the Echentive beach lakes is about +0.4x (Table 1). These values suggest a slight dilution of the lake’s water by inland groundwaters £owing into it. The bicarbonate fraction dissolved in modern seawater from La Palma gave a N13 C of +0.51x PDB. The Canary Islands have a speci¢c ‘dry warm’ climate resulting from several environmental factors (Marzol, 1988): (1) the atmospheric dynamics of subtropical latitudes, (2) the in£uence of the cool Canaries current, (3) the proximity to the African continent, especially the Sahara Desert, and (4) the presence of islands with abrupt topography. The most common winds are the cool humid northeasterly trade winds. As they collide with the islands, they produce clouds at an altitude of between 900 and 1500 m at the northern side of the islands (the cloud sea) but scarce precipitations. Sometimes easterly or southeasterly winds coming from the Sahara Desert make it hot and dry (dry-haze weather). Rarely are there northwesterly climatic perturbations in£uenced by polar fronts, or southwesterly storms producing heavy precipitation and strong winds. In general,
the climate of the Canary region is mild with mean temperatures at coastal zones ranging between 18.5 and 21‡C. There are, however, great contrasts of precipitation within the di¡erent parts of the islands. The windward slopes have mean precipitations around 550 mm/year while the leeward slopes have less than 300 mm/year, reaching maxima of 1100 mm/year at some high northern zones of the islands and minima of less than 100 mm/year at southern coastal zones. The studied zone along the southwestern coast of La Palma is characterised by a semi-desert or dry warm climate. That is, mean temperatures range between 18 and 25‡C, with highs around 40‡C during dry-haze weather and lows around 13‡C during winter. The mean rainfall is below 250 mm/year in this zone, but it is possible to have precipitation exceeding 100 mm/day (recurrent time each 30 year) during the southwesterly storms. The insolation of this area varies between 6 h/day during winter and 11 h/day during summer.
4. Geological setting of the La Palma island The island of La Palma is located at the western edge of the Canarian Archipelago (upper inset in Fig. 1) and above the mantle plume that generated the Canaries in the last 20^30 Myr. La Palma and El Hierro are the youngest islands ( 6 1.8 Myr) and both are in the juvenile shield stage of development (Carracedo et al., 1998).
Fig. 1. Geological map of the La Palma island and location of the beachrocks studied: Puerto Naos beach (PN), Charco Verde beach (CH), Las Zamoras beach (PZ), Playa Chica beach (PC), Echentive beach (EC).
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La Palma is formed by two consecutive main shield volcanoes (Fig. 1): the Taburiente volcano (1.7^0.4 Myr) to the north, with the seamount stage uplifted more than 1000 m asl, and the Cumbre Vieja volcano to the south, which developed during the last 125 kyr. Two main volcanostratigraphic units have been de¢ned in the Cumbre Vieja volcano using the low sea level stand of the last glacial maximum, about 20 kyr BP. This limit separates the cli¡-forming and the platformforming volcanic eruptions. The beachrocks observed in La Palma are located at the present sea level and on the platformforming lavas of the western £ank of the Cumbre Vieja volcano (lower inset in Fig. 1). However, as will be discussed in detail below, some of the ages obtained for these beachrocks are older than the last glacial maximum.
pebbles (see lower inset in Fig. 1). These observations constrain the relative age of the Puerto Naos beachrock between 1585 and 1949. 14 C dating has been impossible due to the di⁄culty of sampling the small-size cements of this beachrock. The beachrock is 20 m wide, 200 m long and 1.5 m thick. It dips seaward between 5 and 15‡ (Fig. 2.1) and presents two horizons. The lower horizon, with 1 m of visible thickness, is made up of an alternance of well-cemented rudites and arenites (Fig. 2.2). It presents an irregular upper surface with decimetre-sized channelled depressions oriented perpendicular to the coastline (Fig. 2.1) similar to the system of ridges and furrows perpendicular to the shoreline (e.g., McLean, 1967; Alexandersson, 1972). The upper horizon, in contrast, consists of poorly cemented rudites and arenites outcropping in metre-sized patches. 5.2. Beachrock of Charco Verde beach
5. Beachrock location, dating and description The beachrocks are located on the southwestern coast of La Palma island, between the village of Puerto Naos and the Fuencaliente lighthouse, namely in the Puerto Naos, Charco Verde, Las Zamoras, Playa Chica and Echentive beaches (lower inset in Fig. 1). 5.1. Beachrock of Puerto Naos beach The Puerto Naos beach developed on volcanic lavas generated by the 1585 eruption (Ben|¤tez, 1952; Santiago, 1960; Machado, 1962^1963; Carracedo et al., 1997). While the recent beach consists mainly of pebbles derived from the Cumbre Vieja basaltic lavas, the beachrock contains a large proportion (about 40%) of pebbles derived from the Pliocene submarine volcano situated in the Taburiente caldera. The Pliocene pebbles could only have come from the interior of the Taburiente caldera, been transported along the Barranco de Las Angustias and, subsequently, by littoral drift to the Puerto Naos beach. The delta formed by the lavas of the 1949 eruption (Carracedo et al., 1997), located to the north of Puerto Naos beach, probably interrupted the littoral drift and thus the transport of the Pliocene
The Charco Verde beach formed on prehistoric platform-forming lavas of the Cumbre Vieja volcano (Carracedo et al., 1997). The beach deposits, and consequently the beachrock, were fossilised by the lavas of the 1585 eruption. This beachrock gave a conventional radiocarbon age of 33 330 8 490 BP (measured age of 32 850 8 490 BP). This age should be considered with caution because, on the one hand it approaches the limit of the 14 C method and, on the other hand, it is older than the underlying platform-forming lavas which have been dated by K^Ar as younger than the last maximum glacial at about 20 kyr (Carracedo et al., 1997; Guillou et al., 1998). Nonetheless, this beachrock can be considered the oldest of La Palma island. The beachrock is approximately 200 m long, 10 m wide and up to 1 m thick. It dips between 2 and 4‡ seaward and presents only one horizon which is made up of rudites grading to arenites and is very well-cemented (Fig. 2.3). 5.3. Beachrock of the Playa Chica beach Playa Chica beach developed on prehistoric platform-forming lavas of the Cumbre Vieja volcano (Carracedo et al., 1997), separated by a nar-
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1
3
81
2
10 cm
4
Fig. 2. (1) Beachrock at Puerto Naos beach presenting an irregular upper surface. (2) Detail of the beachrock at Puerto Naos beach. (3) Cemented volcanic clasts in the beachrock at Charco Verde beach (¢brous aragonite cement). (4) The beachrock pictured in this photograph was destroyed by a storm on 6 January 1999. Las Zamoras beach.
row ( 6 5 m) ‘tongue’ of lava from Las Zamoras beach (Fig. 3). The beachrock gives a conventional radiocarbon age of 14 580 8 130 years BP (measured age of 14 090 8 130 years BP) and forms a metre-sized patch dipping between 10 and 15‡ seaward. It is made up of well-cemented rudites and arenites, very similar to the lower horizon of Las Zamoras beachrock. 5.4. Beachrock of Las Zamoras beach Las Zamoras beach formed on prehistoric platform-forming lavas of the Cumbre Vieja volcano (Carracedo et al., 1997). The beachrock located here was almost completely destroyed by a storm on 6 January 1999. Due to its proximity to Playa Chica beach and the similar characteristics of both beachrocks, it can be considered
that Las Zamoras and Playa Chica beachrocks were formed at the same time (ca. 14 000 years BP). The beachrock was 50 m long, 15 m wide and up to 1 m thick. It dipped 10‡ seaward and presented two horizons (Figs. 2.4 and 3), a lower horizon made up of well-cemented rudites and arenites with an irregular upper morphology and an upper horizon consisting basically of poorly cemented arenites. Today, only a poorly preserved metre-sized patch exists at the inner part of the beach, which probably corresponds to the upper horizon. 5.5. Beachrock of Echentive beach Echentive beach developed on volcanic lavas produced by the 1677 eruption and is partially
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Fig. 3. Beachrock map and schematic cross-section of the beachrock at Las Zamoras, Playa Chica and Echentive beaches. 1: Cli¡-forming lavas (older than 20 kyr); 2,3: prehistoric platform-forming lavas (ca. 18 kyr and 3 kyr, respectively); 4,5: historic platform-forming lavas (1677 and 1971, respectively); 6: beachrocks; 7: volcanic pebbly beaches.
overlapped by the Tenegu|¤a volcanic lavas of the 1971 eruption (Afonso et al., 1974; Carracedo et al., 1997). Consequently, the beach dates from between 1677 and 1971 and the beachrock developed at this beach had to form later than 1677.
Hence, Echentive beachrock is the youngest of all La Palma island beachrocks. Echentive beach shows a complete beach pro¢le : the shoreface zone, the foreshore zone and the backshore zone (Fig. 3). The backshore
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Fig. 4. (1) Fibrous aragonite cement arranged in a pseudospherulitic fashion ¢lling intergranular porosity. Sample PN 1. (2) Fibrous aragonite cement arranged in a pseudospherulitic fashion ¢lling intergranular porosity. Spar aragonite cement ¢lls the inter¢bre porosity. Sample PC 2. (3) Detail of ¢brous aragonite cement arranged in a pseudospherulitic fashion. Sample CH 1, SEM. (4) Detail of ¢bres showing di¡erent sections. Sample CH 1, SEM. (5) a: Micrite HMC cement presenting an irregular morphology with protuberances. b: Peloidal cement. Sample PN 4. (6) a: Micrite HMC cement with meniscus fabric, which is the ¢rst stage of cementation. b: Spar aragonite cement forming a discontinuous rim, which is the second stage of cementation. c: Micrite HMC cement, which is the third stage of cementation. Sample PN 6.
zone, some 400 m wide, presents pebbly backshore ridges up to 4 m high, dipping landward. In the inner part of the backshore zone, and in contact with the volcanic lavas, there are two lakes several metres deep. The beachrock crops out along the seaward
margin of the lakes and disappears below the backshore ridge deposits. It is approximately 1 m thick, which is more or less the current tidal range in this inner zone, and consists of breccias made up of subangular volcanic lava clasts 2^20 cm in size.
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6. Beachrock composition and cements Texturally, the beachrock varies from poorly sorted rudites to arenites. Pebbles are rounded to subrounded (except those from Echentive beachrock) and consist of volcanic rocks of di¡erent nature, mainly alkaline basalts. The grains of the arenites and of the matrix of rudites are made up of volcanic rocks and a small proportion (2%) of crystals (predominantly pyroxenes, olivines and amphiboles). Porosity is mostly intergranular, only locally intragranular (the vesicles of the volcanic lavas). Older beachrocks seems to be more cemented than younger ones. The most common cements are ¢brous aragonite and micritic high-magnesium calcite (HMC) cement. Microbotryoidal HMC, spar aragonite and peloidal HMC cements are locally present. 6.1. Fibrous aragonite cement The ¢brous aragonite cements ¢ll both intergranular and intragranular porosities. Cement ¢bres are arranged either in a pseudospherulitic fashion (Fig. 4.1^3), or form isopachous rims, or are randomly oriented. Pseudospherulite sizes vary from 100 Wm to 3 mm and locally display a meniscus geometry. The aragonite ¢bres range from 10 Wm in length and 3 Wm in width to large crystals up to 3 mm by 200 Wm. The ¢brous crystals exhibit a prismatic habit with blunt terminations (Fig. 4.4) or else an acicular habit with pointed terminations. In general, the larger ¢bres present a pseudoundulating extinction. The ¢brous aragonite cement is non-luminescent. Similar cements have been reported in many beachrocks (e.g., Ginsburg, 1953; Taylor and Illing, 1969; Moore, 1973; Siesser, 1974; Beier, 1985; Strasser et al., 1989; Font and Calvet, 1997; Neumeier, 1998). 6.2. Micrite HMC cement The micrite HMC cement ¢lls intergranular porosity and presents discontinuous submillimetresized laminae or massive texture. It includes variable contents of silt-sized volcanic fragments and,
locally, some scattered bioclastic fragments. The presence of trapped detritus within micrite cements in a beachrock is relatively common (Alexandersson, 1972; Moore, 1973; Meyers, 1987; Bernier and Dalongeville, 1988; Webb et al., 1999). The micritic HMC cement displays an irregular morphology with protuberances (Fig. 4.5) and, locally, can have a meniscus fabric (Fig. 4.6), a feature that has already been reported by Meyers (1987) and Strasser et al. (1989). The micrite HMC cement in the studied beachrocks is related to micro¢laments produced by di¡erent types of micro-organisms. The main type of micro¢lament has a diameter varying from 20 to 40 Wm, is several millimetres in length, and presents dichotomic bifurcations (Fig. 5.1). It is interpreted as fungi. Micro¢lament walls consist of HMC crystals measuring 1^5 Wm (Fig. 5.2). Locally there is a network of ¢laments 1^3 Wm thick interpreted as cyanobacteria. Micro-organisms in beachrock micrite cements have been reported by several authors (Moore, 1973; Strasser et al., 1989; Font and Calvet, 1997). Given this relationship, micritic HMC cement is attributed to microbial activity (e.g., Bernier and Dalongeville, 1988; Neumeier, 1998). The meniscus fabric indicates, in most cases, vadose conditions. Textures similar to the studied micrite HMC cement have been reported in several beachrocks (e.g., Friedman and Gavish, 1971; Bernier et al., 1990; Holail and Rashed, 1992). The micrite HMC cement of the La Palma beachrocks contains 11.2^15.2 mol% of MgCO3 (based on 12 analyses), which is within the same range as those reported by Tietz and Mu«ller (1971), Moore (1973), Magaritz et al. (1979), Meyers (1987) and Font and Calvet (1997). 6.3. Microbotryoidal HMC, spar aragonite and peloidal HMC cements Microbotryoidal HMC cement is present only in the beachrock at Charco Verde beach. This cement is arranged in a uniformly distributed rim (Fig. 5.3) ¢lling intergranular and locally intragranular porosities. Rim thickness varies from 27 to 70 Wm and consists of a single microbotryoid (Fig. 5.4). The microbotryoids are made
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Fig. 5. (1) Micrite HMC cement related to micro¢laments. Sample PN 2. (2) Detail of the micro¢lament. Micrite crystals on the surface of the micro¢lament. Sample PN 2. (3) a: The microbotryoidal HMC cement forms an isopachous rim and is the ¢rst stage of cementation. b: Fibrous aragonite cement is the second stage of cementation. Sample CH 1a. (4) Detail of the microbotryoids. Sample CH 1a, crossed nicols.
up of HMC ¢bres measuring from a few micrometres in width and from 27 to 70 Wm in length. The microbotryoidal HMC cement has 8.2^15.2 mol% of MgCO3 (based on 25 analyses). The spar aragonite cement either forms a continuous or discontinuous rim (Fig. 4.6) or ¢lls in the spaces between the ¢bres of the ¢brous aragonite cement (Fig. 4.2). The continuous rim is 21^35 Wm thick, while the discontinuous rim varies from 0 to 150 Wm. The crystals of the spar aragonite cement have an equant morphology from 7 to 35 Wm, although locally there are crystals with bladed morphology. Although the morphology of this cement is unusual for aragonite, the elemental geochemistry points out to this mineralogy. The peloidal cement consists of round to ovoid peloids measuring from 21 to 50 Wm (Fig. 4.5,6) and is similar to those described in other beach-
rocks (e.g., Moore, 1973; Meyers, 1987; Amieux et al., 1989; Font and Calvet, 1997). 6.4. Cement stratigraphy The cement stratigraphy of the di¡erent beachrocks from La Palma island is variable even within the same beachrock (Fig. 6). Consequently, it is di⁄cult to establish a general rule for the stages of cementation. The general trend cited by some authors (e.g., Taylor and Illing, 1969; Siesser, 1974; Bernier and Dalongeville, 1988; Strasser et al., 1989; Bernier et al., 1990), with the micrite HMC cement as the ¢rst stage of cementation and the ‘crystalline’ cements (aragonite or HMC) as the second, is not common in the studied beachrocks. The main observations of the cementation processes of La Palma beachrocks are : (1) in general,
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Fig. 6. Scheme of the main patterns of cementation in the La Palma beachrocks.
rudites and arenites present ¢brous aragonite cement with a pseudospherulitic arrangement; (2) the micrite HMC cement laminae are never associated with the ¢brous aragonite cement ; (3) when the microbotryoidal HMC cement is present, it always represents the ¢rst stage of cementation; and (4) when the peloidal HMC cement is present, it is always the last stage of cementation.
7. Cement elemental geochemistry The Sr content of the ¢brous aragonite cement
(n = 153 analyses) varies from 4250 ppm to 13 450 ppm (with a mean of 8992 and a standard deviation of 1691); the Na content ranges from 245 ppm to 2530 ppm (with a mean of 1053 and a standard deviation of 630). The Mg content is always below the detection limit (Table 2). These Sr values of the ¢brous aragonite cement of La Palma beachrocks, equivalent to 1^2% of SrCO3 , are similar to those reported by Moore (1973), Strasser et al. (1989) and Font and Calvet (1997) in beachrocks from Grand Cayman Island, southeastern Tunisia and Reunion island respectively. The Sr content of the spar aragonite cement (n = 10 analyses) varies from 6635 ppm to 12 665 ppm (with a mean of 9385 and a standard deviation of 1818). The Na content ranges from 1250 ppm to 2495 ppm (with a mean of 1948 and a standard deviation of 401) and the Mg content is always below the detection limit (Table 2). The Sr content of the microbotryoidal HMC cement (n = 25 analyses) ranges from 780 ppm to 1490 ppm (with a mean of 1161 and a standard deviation of 184), the Na content varies from 315 ppm to 705 ppm (with a mean of 477 and a standard deviation of 108) and the Mg content varies from 8.1 to 15.2% of MgCO3 (with a mean of 11.7 and a standard deviation of 1.7) (Table 2). The Sr content of the micrite HMC cement (n = 12 analyses) varies from 185 ppm to 1195 ppm (with a mean of 886 and a standard deviation of 341), the Na content ranges between 210 ppm and 810 ppm (with a mean of 569 and a
Table 2 Elemental geochemistry of beachrock cements Beachrock
Puerto Naos
Cement
Fibrous aragonite Spar aragonite Micrite HMC Charco Verde Fibrous aragonite Microbotryoidal HMC Playa Chica Fibrous aragonite Spar aragonite Micrite HMC Echentive Fibrous aragonite
No. of analyses
40 4 3 40 25 40 6 9 33
Mg CO3
Sr
Na
(%) min^max (mean, S.D.)
(ppm) min^max (mean, S.D.)
(ppm) min^max (mean, S.D.)
^ ^ 14.3^15.2 (14.9, 0.5) ^ 8.2^15.2 (11.7, 1.7) ^ ^ 11.2^14.7 (12.5, 1.1) ^
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7350^12970 (9351, 1485) 9560^11390 (10142, 851) 185^855 (546, 338) 4250^12310 (8050, 2077) 780^1490 (1161, 184) 5905^13450 (9284, 1628) 6635^12665 (8880, 2180) 315^1195 (999, 272) 7760^10980 (9345, 951)
385^2525 (1362, 621) 1930^2200 (2060, 122) 325^565 (433, 121) 245^770 (461, 170) 315^705 (477, 108) 545^2530 (1599, 467) 1250^2495 (1873, 514) 210^810 (614, 167) 430^1290 (733, 279)
F. Calvet et al. / Marine Geology 197 (2003) 75^93 Table 3 Oxygen and carbon isotopic data from the ¢brous aragonite cements Beachrock
Sample
N18 O (x PDB)
N13 C (x PDB)
Charco Verde
mean CH-1a CH-1b CH-3a CH-3b CH-4a CH-4b CH-5 BLV-3 BLV-4 BLV-3a BLV-4a mean PC-2 PC-3a PC-3b BLC-2 BLC-3 BLC-2a BLZ-4
34.0 34.1 34.2 34.1 33.9 33.7 34.2 34.0 34.0 33.9 34.1 33.9 33.0 33.7 33.1 33.2 32.4 32.4 33.4 33.2
+4.5 +4.4 +4.4 +4.7 +4.4 +4.4 +4.4 +4.6 +4.7 +4.7 +4.6 +4.6 +4.5 +4.3 +4.3 +4.0 +4.9 +4.8 +4.6 +4.1
Playa Chica
Las Zamoras
87
beachrocks are similar to the values of aragonite cements in some beachrocks in French Polynesia (Neumeier, 1998) but, in general, the La Palma values are lower than those of aragonite cements in the beachrocks cited by Magaritz et al. (1979) and Beier (1985), and much lighter than the values reported for aragonite and/or HMC cements in beachrocks surrounding the Mediterranean and the Red Sea (Holail and Rashed, 1992). Possible factors explaining the relatively lighter oxygen isotopic values measured in the studied beachrocks could be: (1) the salinity of the seawater in the Canary Islands ; (2) the temperature of the seawater in the intertidal zone in La Palma beaches; and (3) the in£uence of meteoric water. 8.1. Salinity of the seawater
standard deviation of 172) and the Mg content varies from 11.2 to 15.2% of MgCO3 (with a mean of 13.1 and a standard deviation of 1.5) (Table 2). The Sr values of the micrite HMC cement of La Palma beachrocks are lower than the values reported by Moore (1973), Strasser et al. (1989) and Font and Calvet (1997) in their studies of Grand Cayman Island, southern Tunisia and Reunion island.
8. Oxygen isotopic composition Because of the size of the crystals, only the ¢brous aragonite cement was sampled for N18 O and N13 C analyses. N18 O PDB values measured on the ¢brous aragonite cement of the La Palma beachrocks range between 34.2x and 32.4x (Table 3 and Fig. 7). The aragonite cements of the beachrock at the Charco Verde beach (with a mean of 34.0x PDB) are isotopically lighter than those at Playa Chica beach (with a mean of 33.0x PDB). The oxygen isotopic values of the La Palma
Seawater salinity in the Canary Islands ranges from 36.2 to 37.2x (Barton et al., 1998). In the Mediterranean it varies between 38.8 and 39.5x in Crete (Neumeier, 1998) and between 37.5 and 38.0x in Corsica (Bernier et al., 1997). The salinity in the Red Sea ranges from 39.8 to 40.8x Neumeier, 1998) whereas in French Polynesia, it varies between 35 and 36x (Neumeier, 1998). Consequently, aragonite cements in the beachrocks tend to present lighter oxygen isotopic values when water salinity is low (Canary Island and Polynesia) and heavier values when water salinity is high (Mediterranean and Red Sea). The equation of Railsback et al. (1989) makes it possible to calculate the expected oxygen isotopic composition of the seawater as a function of salinity. Application of this equation to the salinity range in the Canary Islands results in expected N18 O values of the water ranging between +0.7 and +1.1x SMOW. These results are consistent with the mean measured N18 O value of La Palma seawater, which is +1.1x SMOW (Table 1). Thus, the positive correlation between salinity and oxygen isotopic composition of the cements is very likely due to equilibrium conditions (Railsback et al., 1989). 8.2. Temperature of the seawater Seawater temperature in the Canary Islands
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Fig. 7. Cross-plot of stable isotope values of the aragonitic cements of La Palma beachrocks and examples from the literature.
varies between 17 and 25‡C (Barton et al., 1998). In Crete it ranges between 15 and 25‡C (Neumeier, 1998), in Corsica between 13 and 23‡C (Bernier et al., 1997), in the Red Sea between 21 and 29‡C (Neumeier, 1998), and in French Polynesia between 27 and 29.5‡C (Neumeier, 1998). There seems to be no relation between the oxygen isotopic composition of the aragonite cements of the beachrocks and seawater temperature. The equilibrium between H2 O, aragonite cements and temperature is calculated using the equation of O’Neil et al. (1969, modi¢ed): 103 lnK CaCO3 H2 O ¼ 2:78ð106 T 32 Þ32:89
Applying this equation, and using the measured oxygen isotopic composition of La Palma seawater (+1.1x SMOW), equilibrium temperature is between 30.4‡C and 39.9‡C (Table 4).
If the oxygen isotopic composition of the seawater which precipitated the analysed ¢brous aragonite cements was that of the current seawater around La Palma (+1.1x SMOW), much higher temperatures than those currently measured would be inferred. But these estimations are made using open-sea temperatures (17^25‡C), which are lower than the temperatures of the water in the inner part of the beaches. The temperature of the water in such zones reaches 30‡C or more during summer time or dry-haze weather (Marzol, 1988). Thus, the temperature of the seawater could well account for the lighter oxygen isotopic composition of the ¢brous aragonite cements of La Palma beachrocks. The Charco Verde beachrock shows heavier N18 O values towards the top of the pro¢le (Fig. 8) suggesting a decrease in temperature from the
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Table 4 Calculated temperatures and N18 O values at 17‡C and 25‡C in equilibrium with the ¢brous aragonite cements (based on the equation by O’Neil et al., 1969, modi¢ed) Beachrock
Sample
Equilibrium temperature
N18 O in equilibrium with carbonate at 17‡C
N18 O in equilibrium with carbonate at 25‡C
33.5 33.6 33.5 33.3 33.1 33.6 33.4 33.4 33.3 33.5 33.3 33.1 32.5 32.6 31.8 31.8 32.8 32.6
31.8 31.9 31.8 31.6 31.4 31.9 31.7 31.7 31.6 31.8 31.6 31.4 30.8 30.9 0.0 0.0 31.1 30.9
(‡C) Charco Verde
Playa Chica
Las Zamoras
CH-1a CH-1b CH-3a CH-3b CH-4a CH-4b CH-5 BLV-3 BLV-4 BLV-3a BLV-4a PC-2 PC-3a PC-3b BLC-2 BLC-3 BLC-2a BLZ-4
39.3 39.9 39.3 38.2 37.1 39.9 38.7 38.7 38.2 39.3 38.2 37.3 34.1 34.6 30.4 30.4 35.7 34.6
lower to the upper part of the intertidal water lens. 8.3. Meteoric in£uence N18 O values for rain waters in La Palma range from 32.51x SMOW (ITGE, 1993) to 33.3x SMOW (Ko«nig, 1997) for samples taken at 1400 and 1200 m asl, respectively. Groundwater N18 O values in the study area vary between 34.54x
SMOW (ITGE, 1993) and 34.0x SMOW (Veerger, 1991), pointing to recharge at higher zones. Applying the equation of O’Neil et al. (1969, modi¢ed), the calculated N18 O values of waters in equilibrium with the ¢brous aragonite cements of La Palma beachrocks at the temperature range measured in seawater (17^25‡C) would be 33.6 to 31.8x SMOW (at 17‡C) and 31.9 to 0.0x SMOW (at 25‡C) (Table 4). These values suggest that these cements precipitated from seawater with meteoric in£uence. Similar values have been cited in several beachrocks (Moore, 1973; Magaritz et al., 1979; Neumeier, 1998) and interpreted in the same way. The meteoric in£uence is higher in the inner part of the beaches. For instance, in Echentive beach, the oxygen isotopic composition of the lake water (+0.4x SMOW) is lighter than the corresponding value in the foreshore zone (+1.1x SMOW).
9. Carbon isotopic composition Fig. 8. Vertical pro¢le of the Charco Verde beachrock and isotopic vertical pro¢le of ¢brous aragonite cement.
The carbon isotopic composition of the ¢brous
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aragonite cement of La Palma beachrocks ranges from +4.0 to +4.9x PDB (Table 3 and Fig. 7), which is very similar to that of aragonite cements of the Bahamas beachrocks (Beier, 1985) and relatively similar to the aragonite cements of the Mediterranean (Magaritz et al., 1979) and French Polynesia (Neumeier, 1998). There are no di¡erences in N13 C between the cements of the beachrocks at Charco Verde and Playa Chica beaches. N13 C values measured are similar to those of recent or subrecent marine cements (Tucker and Wright, 1990). The carbon isotopic composition of present-day seawater HCO3 3 in the Canary Islands (+0.51x PDB) is consistent with that determined on the cements. CO2 degassing from seawater would preferably carry light carbon. Subsequent fractionation between aragonite and HCO3 3 (which is +2.9x according to Robinson and Clayton, 1969) would result in N13 CW+4x PDB, as measured in the beachrock aragonite cements. If correct, degassing would have been the main driving force for cement formation, as suggested by Hanor (1978).
10. Development of the beachrocks of La Palma The evolution of the beach deposits and beachrocks in La Palma involved the following stages (Fig. 9): (1) beach development, (2) beachrock formation, and (3) retrogradation and/or erosion of beaches. (1) Beach development requires an input of sediment (pebbles and sands) and a relatively £at marine bottom. The beach sediment is exclusively of terrigenous origin (clasts and grains derived from volcanic lavas) and there is essentially no material from the marine platform such as skeletal grains. Hence, terrigenous material was supplied by marine erosion of volcanic lavas and from the interior of the island transported along the gullies. Although marine erosion is a continuous process, the period of major sediment input is probably related to volcanic eruptions, when the lavas are brecciated following contact with seawater. Furthermore, on reaching the sea, the lavas can in£uence the littoral morphology, creating capes
Fig. 9. Scheme of the evolution of beachrocks in the island of La Palma.
and bays. In the inner part of the bays, beaches can develop and migrate seaward (progradation stage). Lavas can also form a relatively £at and shallow marine bottom where beaches can originate. Beach pro¢les include a poorly developed shoreface zone and well-developed foreshore and backshore zones. The Echentive beach pro¢le corresponds to this stage of beach development. (2) The beachrocks are located in the intertidal zone and in the inner part of the beach pro¢le. The Echentive beachrock could be the key to understanding beachrock formation in volcanic pebbly beaches. At present, beach deposit cementation occurs in the inner part of the Echentive beach (lakes). Newly formed cements consist of ¢brous aragonite and HMC micrite. Probably,
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cement formation continues in the intertidal zone in a seaward direction, but it is covered by supratidal deposits (the backshore ridges). The oxygen isotopic composition of cements of the beachrocks studied shows that these cements precipitated from seawater under a meteoric in£uence. Moreover, the oxygen isotopic composition, the conductivity and the pH of the present-day Echentive lakes indicate that the original marine water was slightly modi¢ed by meteoric water in£uence. In conclusion, the beachrocks studied are cemented in the intertidal zone and probably in the inner zone of the beaches where seawater presents a meteoric water in£uence, and slightly higher temperatures. (3) The beachrocks at Puerto Naos, Charco Verde, Las Zamoras and Playa Chica beaches, probably originating in the inner part of the backshore zone, crop out today in the foreshore zone. This indicates that a large part of the original beach pro¢le has been eroded due to retrogradation. Beachrocks can be completely eroded if the retrogradation process is very energetic. A good example of this is the Las Zamoras beachrock described above which cropped out until 6 January 1999, when it was almost totally destroyed by a winter storm.
11. Implication of the presence of beachrocks The beachrocks of La Palma prompt us to make some general considerations on the evolution of the relative position of La Palma island with respect to sea level, the relationships between beachrocks and beaches, palaeoceanography, geology and palaeoclimate. (1) Sea level. The location of all the studied beachrocks, with di¡erent ages, at the present sea level would have required a combination of eustatic and isostatic movements to keep the sea level stable at the present level during the past 33 kyr. (2) Beach retrogradation. The outcropping of beachrocks on the beaches implies a retrogradation stage in these littoral deposits (Strasser and Davaud, 1986). Retrogradation is related to a
91
slow sedimentation rate on the beaches and an increase in the erosion rate. The reason for the slow sedimentation rates in La Palma, where the beachrocks are made up exclusively of volcanic clasts and grains, may be a lack of continuous input of these materials to the beaches, resulting in their retrogradation and consequent beachrock exposure. The presence of beachrocks in these beaches, however, prevents the total disappearance of these beaches. (3) Palaeoceanographic implications. The beachrock thickness corresponds in general to the tidal range. The similarity between the thickness of the studied beachrocks (between 1 and 1.5 m) and the actual mean tidal range in the Canary Islands indicates that the mean tidal range in the Canary Islands has been the same for the last thousand years. (4) Geological implications. The beachrocks in La Palma are located along the coastline of the Cumbre Vieja volcano, which is the volcanically active part of the island and, consequently, the only part of the island which has received a signi¢cant amount of sediments during the last 125 kyr. If beaches with beachrocks ever existed in the older part of the island, they have been eroded away. This observation is consistent with the aforementioned model of the development of the beaches: sediment derived from volcanic lavas provides the necessary material to form beaches. (5) Palaeoclimatic implications. The restricted location of the beachrocks along the western coast, which is dominated by a dry warm climate with a very low rainfall (below 250 mm/year) and a high insolation rate (6^11 h/day), indicates that these parameters favour, and favoured, cement precipitation and beachrock formation by increasing water temperature in the intertidal zone and in the inner backshore zone.
12. Conclusions (1) The beachrocks of La Palma are located in the volcanically active part of the island (coastline of the Cumbre Vieja volcano), especially on the southwestern coast, which is characterised by a dry warm climate. Volcanic activity supplied the
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sediment to form beaches and changed the morphology of the coast, whereas the climatic parameters favoured cement precipitation and beachrock formation. The presence of beachrocks prevents the total disappearance of beaches. (2) The beachrocks formed in volcanic pebbly beaches. They are 1^1.5 m thick and up to some tens of metres wide. The coincidence between beachrock thickness and the present mean tidal range in the Canary Islands indicates that the tidal amplitude has been relatively constant over the past thousand years. (3) The age of the beachrocks varies from ca. 33 000 years to recent. The position of the beachrocks at the present-day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level during the past 33 kyr. (4) The beachrocks are cemented by ¢brous aragonite and micrite HMC, and locally by microbotryoidal and peloidal HMC, and spar aragonite. The elemental geochemical values of all cements are consistent with a marine origin. Cementation occurred either in the intertidal zone or in the inner part of the beaches, as occurs today in Echentive beach. (5) The oxygen isotopic composition of the ¢brous aragonite cement, ranging between 34.2 and 32.4x PDB, suggests that the precipitation occurred from marine water slightly modi¢ed by meteoric water at relatively low salinity (36.2^ 37.2x) and at relatively high temperature ( v 30‡C). The carbon isotopic values of this cement (+4.0 to +4.9x PDB) suggest that the principal cause of the precipitation was degasi¢cation.
Acknowledgements This work was carried out within the framework of Project 68/98 of the Fundacio¤n Universitaria de Las Palmas. The analytical part was partially supported by Project CICYT PB97-0883, RpD Project BTE 2002-04453-C02-01 and Grup Consolidat de Recerca ‘Grup de Geologia Sedimenta'ria’ (2001/SGR/75). The authors thank J. Agullo¤, X. Llovet, R. Fontarnau, A. Dom|¤nguez, A. Samper, V. Panella and J. Illa for technical
assistance, and G. Von Knorring and F. Luttikhuizen for the revision of the English version of the manuscript. We are indebted to D. Gimeno for helpful discussions. The manuscript has been considerably improved by constructive criticism from P. Kindler and E. Gischler.
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