Quaternary palaeoclimates in the eastern Canary islands

Quaternary International, VoL 31, pp. 37-46, 1996. CopyrightO 1995INQUA/ElsevierScienceLtd Printed in Great Britain.All rights reserved. 1040--6182/96$29.00

Pergamon '1040-6182(95)00019--4

QUATERNARY

PALAEOCLIMATES

IN THE EASTERN CANARY ISLANDS

B. Damnati,* N. Petit-Maire,* M. Fontugne,t J. Mccoy: and D. Williamson* *Laboratoire de Gdologie du Quaternaire, CNRS, Luminy - - Case 907 - - Marseille 13288, cedex 09, France tCFR, CNRS-CEA, Gif sur Yvette, France ~tULPGC, Universidad de la Laguna, Las Palmas, Spain

In the eastern Canary islands, aeolian formations interbedded with palaeosois have been studies. The sequences cover the last 40 ka, as evidenced by one ~'C accelerator analysis and several classical radiocarbon dates. As in the Sahara, at least two wet episodes characterize isotopic stage 3. A short pedogenesis corresponds to the first deglaciation step at 15 ka. After a new aeolian episode, the Holocene wet phase begins at around 10 ka. The aeolian sedimentation is of Saharian dust, in particular during the stage 3 dry episode(s). U/Th and OSL dating are current.

INTRODUCTION

LITHOLOGIC DESCRIPTION OF THE SECTIONS

The Fuerteventura and Lanzarote Canary islands, 125 km off the coast of Morocco at Cape Juby, are characterized by an arid climate, due to their setting, at latitude 28"N, in the full blast of the cold Canary Current. Mean annual rainfall is only about 100 mm and those islands are considered as the western outpost of the Saharan Zone. Important geological records of the past quaternary climatic variations have been observed in both islands: marine terraces with a warm fauna dated of the Last Interglacial (Meco and Petit-Maire, 1986; Meco et al., 1992) and thick aeolian formations interbedded with several fossiliferous paleosols (Petit-Maire et al., 1986, 1987). Those latter are of great interest for palaeoclimatology, since they imply an alternation of arid and humid phases directly related with diachronic changes in the thermic barrier presently induced by the cold Canary Current, i.e. with variations of the local sea surface temperatures (SSTs) and of the North-South oceanic circulation. In 1992 and 1993, a multidisciplinary study of those formations was performed in the two islands by a CLIP scientific team covering geology, sedimentology, isotope studies, palaeomagnetism, OSL, ~4C and U/Th dating, palaeontology. Continuous sections were observed and sampled in three quarries: La Rosa Negra and La Cascada in Fuerteventura, La Mala in Lanzarote. One outcropping palaeosol was also sampled in southern Fuerteventura, Jandia isthmus, on account of its original palaeontological context (Fig. 1). In this paper, we shall give the preliminary results of this collaborative research, in particular for La Rosa Negra section, in which area some radiocarbon dates have already been obtained.

La Mala quarry profile, in Lanzarote island, is 20 m thick. It may be divided into three distinct parts (Fig. 2): at the bottom, 2 m of reddish silty sand, with fossil Helicidae shells; in the middle, crossbedded aeolian sand evolving into horizontal layers and ending in unstratified sand; at the top, 3 m of silty sand which divide into three reddish palaeosols interbedded with greyish palaeosols. La Caseada quarry profile, in Fuerteventura island (Fig. 2), is 9 m thick. It also divides into three facies: a lower zone of yellowish sand, interbedded with several fossiliferous layers of abundant Helicidae shells and incredible quantities of Hymenoptera nests, well consolidated by silt and calcium carbonate. The middle zone is of greyish, very well stratified silty sand with occasional gypsum crystals. The upper zone is of four red palaeosols with Hymenoptem nests alternating with layers of yellowish sand. La Rosa Negra section (Fig. 2) was obtained by caving through an ancient quarry site. It rests upon a basaltic substratum. Four red palaeosols are interbedded with yellowish or whitish sand layers, sometimes stratified. At the top of the section, a brown palaeosol is characterized by the great number of Helicidae shells and Hymenoptera nests. The very top is made of two gravel layers. A sedimentologieal study of La Rosa Negra section has been made and a relative chronology can already be presented, based upon lithological correlations established with dated palaeosols in nearby areas (Petit-Marie et al., 1986, 1987): at Corralejo (Figs 2 and 3), the Molluscs layer separated from the lower one by 1.30 m of aeolian sand has been dated 15,000 ± 200 BP. The same basaltic substratum is observed at Rosa Negra, Corralejo and El Quemado, the lava flow being given at 35 ka or less (Pomel et al., 1985). 37

38

B. Damnati et al.

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METHODS The granulometry was measured by sifting, separating the fractions superior and inferior to 63 ktm. Helicidae shells fragments were eliminated to avoid errors. A Bernard caleimeter was used, the average error being 5%. All samples were subjected to X-ray diffraction analysis of the < 2 ktm decalcified particles, as follows. The samples were dissociated under water, decarbonized in 5N hydrochloric acid, and then dispersed in distilled water. The defloculation was carded out by repeated centrifugation and mierohomogenization after mixing. The particles inferior to 2 ktm selected by decantation, using Stokes law in 50 p.m, and then prepared in oriented pastes on calibrated glass slides. The X-ray investigation dealed with passages between 1" and 16" tote: natural condition, ethylenic glycol saturation, heating for 2 hr at 500"C, hydrazin hydrate saturation. The quantative estimations took into account the area of basal peaks, which is diminished for the lowcrystallized minerals (smectite) and enlarged or the wellorganized minerals (kaolinite; Chamley, 1971, 1979).

Some samples of special interest were washed in distilled water, separated into the < 63 ~tm and > 63 ~tm fractions and observed in a scanning electron microscope (SEM) coupled with a microprobe (EDS). RESULTS AND DISCUSSION The following results were obtained: - - Carbonate: All sediments are rich in calcium carbonate and/or magnesium. At 9-8 m, the CaCO3 ratio is 70% to 50% and rises again to 70-90% between 7.5 and 6.5 m. From 6 to 4 m, the average percent is 80%. The lowest one is 40% between 4 and 3 m. The upper crust is 90 to 100% CaCO 3 (Fig. 4). Magnesium calcite and aragonite are also represented. - - Granulometry: the gross fraction > 63 ~tm is the most important one, although some thin layers (< 63 gm) exist: the base of the section (9-7.5 m) is composed of 60% fine grain sediments. In contrast, between 5.5 and 4 m, the gross fraction is nearly 90% of the total sediment. A new fine grain level is observed between 4 and 3 m. The top of the section

39

Palaeoclimates in the Eastern Canary Islands

Rosa Negro (L|Jare:, Fuerteventura)

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La Ca$¢sda Fuerteventura

La Mala (Lan~rote)

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FIG. 2. La Mala, La Cascada and La Rosa Negra profiles.

(3-0 m) is homogenously composed of gross sand and 90% smectite, which indicates a local secondary origin of the gravel. clay, possibly a pedogenesis in an alcalinolytic environment - - Clay minerals: The clayey fraction is not very or an alteration of recent basalt. In brief, the Rosa Negra section thus shows well important, but smectite, illite and kaolinite are present. Smectite is the most frequent (40-85%), followed by illite correlated granulometry and lithology. In the palaeosols, the percent of the fine fraction is very high relative to the gross (10-50%) and kaolinite (5-20%) (Fig. 5). Two main mineral sedimentary types may be fraction, while the upper part or in dunes is composed by distinguished. One (A: 9 to 3 m) is characterized by a highly 80% sand. The sediments are enriched in calcium carbonate variable percent of the clay minerals. In a second one (B: 3 (calcite). The silty layers present a lower carbonate content decreasing from bottom to top, this probably resulting from to 0 m), smectite is constantly dominant at 80%. Five intervals within A have the following characteristics: carbonate dissolution. Various origins and formation may explain the (a) Constant percents for smectite (60-70%), illite mineralogical diversity of clays: illite forms in cold and/or (10-20%) and kaolinite (5-20%). (b) High percent of smectite (80%), low percent of illite dry climates (Millot, 1964; Paquet, 1969; Chamley, 1971) through alteration of the silicate rocks. Kaolinite is related and kaolinite (10%). (c) Low percent of smectite (40-50%), higher percent of with a warm-wet climate, when runoff results into drainage of ions. Smectite is also related with an alternation of wet and illite and kaolinite (35%-15%). dry episodes in warm climatic conditions (Millot, 1964; (d) Rise of smectite percentage. Paquet, 1969; Chamley, 1971). (e) The same as c. The dune sands in a, c and e are richer in illite and Within B, the clay fraction in the calcified dune has up to

40

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kaolinite than in smectite, while the sands encrusted with calcite are enriched in pedogenic smectite. As observed by Coud6-Gaussen, et al. (1982) for the Matmata loess (Tunisia), aeolian clays from the Sahara are characterized by a near-constant mineralogy. Therefore we used the kaolinite+illite/smectite ratio as an indicator of the relative in situ clay genesis. The illite+kaolinite group is a good marker of remote aeolian transport, in contrast with smectite which is also linked to local processes. A clear Saharan influence thus appears in layers located at 3--4 m, 5-6 m and 7-8 m, though the high value of the illite+kaolinite/smectite ratio (Fig. 5), eonforted by the abundance of wind-eroded quartz. The SEM and microprobe analyses have shown that all samples contain quartz grains (20 to 300 m, Fig. 6 PL, a) marked with aeolian impacts (Fig. 6 PL, b) and sometimes

covered by a fine layer of palygorskite (Fig. 6 PL, c and d). The carbonates are generally polycrystalline, enriched in calcium, automorphic or xenomorphic (Fig. 6 PL, e, f and g), either small (60 ~tm) or large (300--400 lxm); some of them have geometrical characters or pores, resulting from postdeposit aeolian processes. These observations indicate the allochtonous origin of part of the sediments in Rosa Negra profile. The present day Saharian dust has the same micromorphological (Coud6Gaussen and Rognon, 1988) and mineralogical (Lange, 1982) characteristics. Western Sahara is the main origin of those sediments, which fits the known processes of aeolian transport in those areas (Chamley et al., 1977; Coud6Gaussen, 1981; Whalley and Smith, 1981; McTainsh and Walker, 1982; Chamley et al., 1987).

Palaeoclimates in the Eastern Canary Islands

41

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42

B. Damnati et al.

FIG. 6. SEM observations of shapes and surficial features of Rosa Negra sediment. (PL, a) Quartz grain with surficial carbonate layer. (PL, b) Quartz aeolian choc marks.

Palaeoclimates in thz Eastern Canary Islands

FIG. 6. (PL, c) Ronded quartz with probably palygorskite fibres. (PL, d) Close up of (c), feited palygorskite fibres.

43

44

B. Damnati et al.

FIG. 6. (PL, e and f) Grain gross of calcium carbonate with pores resulted from dissolution and aeolian action.

Palaeoclimatesin the Ba~earnCanaryIslands

45

FIG.6. (PL,g) Graingrossof calciumcarbonatewithporesresultedfromdissolutionand aeolianaction. Two major sedimentation phases may be distinguished after the volcanic crisis at _<35 ka: (1) A first phase with alternate sort wet/dry episodes. During the wet episodes, alteration of the basaltic substratum produces smectite and favours pedogenesis. During the dry (colder?) episodes, aeolian action is important (rise of the kaolinite+illite/smectite ratio and of the quartz percent). Dust is brought over from the Sahara, testifying to a dominant wind direction. Thirteen classical radiocarbon ages have been obtained in Fuerteventura island for the palaeosols in this phase, 4 in the Rosa Negra-Lajares area:33.8 ka, 32.5 ka and 28.4 ka. At Corralejo, 7 km from Lajares (Fig. 1), two ages also correspond to stage 3:37.7 ka and 23.2 ka. In southern Fuerteventura, the same sedimentological series were obtained from diverse palacobiological material: 32.1 ka (bird's egg shell), 31.8 ka (Rumina decollata), 31.7 ka (opercules from marine molluscs shells), 30.0 ka (Helicidae shells), 29.6 ka (Helicidae shells), 28.9 ka (bird's eggshell), 23.6 ka (Helicidae shells). All the ages superior to 25 ka are suspicious, being close to the lowest limit of 14C dating. However, one marine shell from the same palaeosol layer, dated between 31 ka and 23 ka from its numerous palaeontological remains (mostly eggshells and birds bones) by conventional radiocarbon, was submitted to ~4C accelerator analysis: it also resulted into an age of 30 ka, which may be taken as a confirmation of the other results. In both western and eastern Sahara, a wet episode is dated 32 ka to 22 ka (Petit-Maire, 1991). It is not surprising that the

eastern Canaries have followed the same climatic pattern during isotopic stage 3, also characterized by two or three alternations of water/drier episodes. (2) Deglaciation is marked by a pedogenesis dated 15 ka at Corralejo and 12.5-13.5 ka Tarfaya, across the Cape Juby 'strait' (Petit-Maire et al., 1987), followed by a short aeolian episode. The beginning of a new wetter phase, with abundant smectite and pedological development of the carbonated cementation, is dated 9.8 ka in the southern area of the island, and 8.8 ka in the northern one. Detailed sampling of the three new profiles observed in 1993 will provide more information upon the first continental palaeoclimatic record from the sub-tropical Atlantic. In particular, OSL dating of the aeolian sediments is currently endeavoured (Bouab and Lamothe), as well as rock-magnetic measurement performed on 8 cc sediments sub-samples. Preliminary results demonstrate some major differences between the palaeosols and the loessic layers. Palaeosols horizons present relatively high low-field magnetic susceptibility (LFMS) values (Z > 10-6 m3"kg~m) and high values of the frequency-del~ndence of LFMS (FD < 8%). In contrast, aeolian silts have low FMS value (Z< 10-7 m3.ka-1) and near-zero FD values. This suggests that the palaeosols from the islands are relatively enriched in ultrafine ferrimagnetic minerals. Further studies (remanence measurements, hysteresis loop parameters, magnetic minerals extraction and SEM observations) are current and will allow comparison of rock magnetic behaviour with other loess records at higher latitudes.

B. Damnati et al.

46

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McTainsh, G.H. and Walker, P.H. (1982). Nature and distribution of l-lm,mattan dust. Zeitschriflflir Geolmorpholo&ieNF. 26(4), 417-435. Meco, J. and Petit-Make, N. (nds) (1986). Le Quatezmfire r6cent des lles Canaries: Pal~o-climatolosie. Niveanx matins. Mu~o Canario, Las Palmas (ES)/Ma~Ue (FR), Labu. G~Ol. Quatem. CNRS, 98 pp. Meco, J., Petit-Make, N. and Reyss, J.-L. (1992). Le courant des Canaries pendant le stade isotopique 5, d'apr~s la composition fannistique d'un haut niveau matin it Fuerteventura (28"N) n Comptes Rendus de I'Aced/nie des Science Paris, sSrie II, 314, 203--208. Millot, G. (1964). G6ologie des arsiles. Masson, Paris, 499 pp. Paqnet, H. (1969). Evolution g(~chimique des ~ x ergileux clans les alt~xations et les sols des climats m 6 d i ~ s et tmpicaux it saisons c ~ . Mqhn. Serv. Carte 8~ol. Alsace-Lorraine, Stresbourg, 30. Petit-Make, N,, Delibrias, G., Meco, J., Pomel, R. and Rosso, J.C. (1986). ~ l i m a t o l o g i e des Canaries orientales (Fuerteventure). Comptes Rendus de l'Acad6min des Sciences Paris, s&'ie II, 303, 1241-1246. Petit-Maim, N., Rosso, LC., Delilxias, G., Meco, J. and Pomel, S. (1987). P~limats de l'tle de Fuerteventura (Archipel Canarien). Palaeoecolosy of Africa and the S ~ n d i n g Islands, 18, 351-356. Petit-Make, N. (ed.). (1991). Paleoenvironments du Sahara. Lacs holocSnes it Taondenni (Mall). Centre r68ional de Publication - - CNRS, Mm'seille/Paris (PR), 239 pp. Pomel, R.S., MiaUier, D., Fain, J. and Sanzelle, S. (1985). Datation d'un sol brun rouge calcifere par une coul6e d'ige WOrm ancien (51,000 arts) tt Fuerteventura (lies Canaries). Mdditerrande, 4, 59--68. Wbulley, W.B. and Smith, B.J. (1981). Mineral content of Harmatan dust from no~tm'n Nigeria examined by scanning electron microscopy. J. Arid Environments, 4, 21-29.