Vicente Soler

Pleistocene raised marine terraces of the Spanish Mediterranean and Atlantic coasts: records of coastal uplift, sea-level highstands and climate changes

Detailed geological mapping, morphostratigraphic, palaeontological and geochronological (uranium-series) analyses were undertaken on the raised marine terraces and interbedded terrestrial deposits along the Spanish peninsular and insular Atlantic and Mediterranean coasts. Several sets of Pleistocene shallow-marine to coastal deposits exposed in a staircase arrangement are interpreted as being emplaced during sea-level highstands coeval with interglacials or interstadials correlating with marine Oxygen Isotopic Stages (OIS) 5a/5c, 5e, 7, 9/11 and older. Up to three highstands have been identified in deposits formed during OIS 5e. Close to the end of OIS 5e there is a record of sudden changes in sea-surface conditions and climate marked by the disappearance of a major proportion of the warm ‘Senegalese’ fauna, switches from oolitic to non-oolitic facies, and accumulation of boulder beaches. Dating of the coral Cladocora caespitosa, found in a layer that also contains Strombus bubonius, confirms the occurrence of warm fauna in the Mediterranean basin during OIS 7, as previously suggested by Hillaire-Marcel et al. (1986), Goy et al. (1986a,b), Zazo and Goy (1989). Also the occurrence of warm faunas in deposits corresponding to an older interglacial, probably OIS 9 or 11, in the Balearic Islands suggests similar oceanographic conditions (sea-surface temperature, assuming constant salinity) during the last interglacial and at least two interglacials of the Middle Pleistocene in the western Mediterranean. A 2002 Elsevier Science B.V. All rights reserved.

Paleobiology and comparative morphology of a late Neandertal sample from El Sidrón, Asturias, Spain

Fossil evidence from the Iberian Peninsula is essential for understanding Neandertal evolution and history. Since 2000, a new sample ≈43,000 years old has been systematically recovered at the El Sidrón cave site (Asturias, Spain). Human remains almost exclusively compose the bone assemblage. All of the skeletal parts are preserved, and there is a moderate occurrence of Middle Paleolithic stone tools. A minimum number of eight individuals are represented, and ancient mtDNA has been extracted from dental and osteological remains. Paleobiology of the El Sidrón archaic humans fits the pattern found in other Neandertal samples: a high incidence of dental hypoplasia and interproximal grooves, yet no traumatic lesions are present. Moreover, unambiguous evidence of human-induced modifications has been found on the human remains. Morphologically, the El Sidrón humans show a large number of Neandertal lineage-derived features even though certain traits place the sample at the limits of Neandertal variation. Integrating the El Sidrón human mandibles into the larger Neandertal sample reveals a north–south geographic patterning, with southern Neandertals showing broader faces with increased lower facial heights. The large El Sidrón sample therefore augments the European evolutionary lineage fossil record and supports ecogeographical variability across Neandertal populations.

The 1730–1736 eruption of Lanzarote, Canary Islands: a long, high-magnitude basaltic fissure eruption

Abstract The eruption that took place in Lanzarote between 1 September 1730 and 16 April 1736 differs from the normal style of the historic (last 500 years) volcanism of the Canary Islands. The duration (2,056 days), extent (200 km2), volume of materials emitted (3–5 km3) and the evolution of magmas from extremely SiO2-undersaturated lavas (melanephelinites) towards olivine tholeiite compositions are quite unique in the historic trend of volcanism in the Archipelago, and, apparently, even in the Earths historical record of basaltic fissure eruptions. However, no specific work has been undertaken until now to attempt the reconstruction of this eruption. We present here a detailed reconstruction of the eruption, based on field observations and data provided by eye-witness accounts, one of which is a hitherto unpublished manuscript. The “anomalous” nature of this eruption in relation to the historic volcanism of the Canaries—especially the continuation of the eruption after the initial phase was completed (some 3–4 months, the maximum duration of any historic eruption in the Canarian Archipelago)—might be the result of upward movement of the magma generation front to an intermediate depth along a large fracture.

Absolute paleointensity and magnetomineralogical changes

Determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings. Such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization (PTRM) are reflected by negative pTRM checks, a deviation between the original pTRM and a repeated measurement after heating at a higher temperature. Because the deviations indicated by the PTRM checks can provide a direct estimate of the effects to the pTRMs, we suggest that they can be used to correct pTRMs. The correction method we present requires additional measurements, in particular pTRM checks performed after each pTRM and specific diagrams to detect acquisition of chemical remanence. Performing multiple successive pTRM checks can also provide information on the blocking temperatures of the alteration product. These experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step. The corrections have been tested on recent and historic lava flows from several localities. We have also compared results on samples from the same lava flows that were heated in air and in vacuum. The mean paleointensity values were found to lie within less than 10% from the expected field intensity at each site. The corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65% additional data for specimens heated in air. Successive heatings at the same temperature in air are often accompanied by a time-dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field, a problem that is not related to the corrections. We conclude that corrections can be used with fairly high confidence to the pTRMs performed in vacuum and in air.

Paleointensity variations across the last geomagnetic reversal at La Palma, Canary Islands, Spain

A detailed volcanic record of absolute paleointensity has been obtained for the period encompassing the last geomagnetic reversal, from a succession of 69 distinct lava flows in two adjacent sections on the island of La Palma. The correlation between the two sections relies on the positions of the Brunhes-Matuyama boundary and on the flow chronology obtained by a refined potassium/argon (K/Ar) dating technique. Several pretransitional flows exhibit very shallow paleomagetic inclinations, but they cannot be considered as really transitional. Paleointensity experiments conducted on 288 specimens provided 168 successful determinations (14 were obtained after correction) for 50 lava flows that were considered to represent 45 distinct units. The evolution of the paleofield intensity can be constructed by assuming uniform extrusion rates between three dated lava flows. The period preceding the transition is characterized by large fluctuations with typical peak to peak amplitudes between 20 and 40 μT which decrease down to 7 μT prior to the transition. The 100 kyr-long interval following the reversal is associated with field intensities as high as 60 μT and depict a strong and rapid restoration of the dipole field. The virtual dipole moments (VDMs) have been compared with sedimentary VDMs generated from the record of relative paleointensity from Ocean Drilling Program site 851, to which have been added non dipole components similar to the present geomagnetic field. A reasonable fit can be made between the two curves which implies variations in the eruption rates but not incompatible with the uncertainties in the K-Ar age determinations. Thus these results can support but do not fully demonstrate the existence of asymmerical saw-toothed variations in field intensity.

Magnetic anomalies of lava fields in the Canary islands. Possible consequences for paleomagnetic records

Abstract Measurements of the total magnetic field have been performed above the surface of 12 lava flows in the Canary islands of La Palma and Tenerife. Directions can differ by up to 15° and intensity changes reach 20%. The mean-flow directions do not coincide and the mean-flow virtual dipole moments (VDMs) differ by more than 10%. The deviations are linked to topographic features at the surface of the flows and should persist during emplacement of the next overlying flow. Such effects can thus limit the resolution of paleomagnetic records of detailed geomagnetic features and the field intensity variations above the flows (and between the mean-flow values) could explain a significant part of the scatter inherent to many paleointensity studies. Historical field variations have been recorded from the same flows and are in agreement with the European curve for the same period. However, there is also some correlation with the present field which suggest that the underlying magnetized bodies could have affected the paleomagnetic records. Biases induced by the crustal field would increase considerably during periods of reduced field intensity like during reversals and induce erroneous records. Samplings performed over large distances within each lava flow would provide the best way to average out the contribution of the anomalies. In any case, such effects should not be neglected in paleomagnetic studies.

Intrinsic and scattering seismic wave attenuation in the Canary Islands

The Canary Islands volcanic complex is studied in terms of coda wave attenuation. A multiple lapse time window method, based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers, is applied to the available seismic data in order to obtain the intrinsic absorption (Qi−1) and the scattering attenuation (Qs−1) in the Canarian lithosphere. The analysis is performed for two hypocentral distance ranges: from 0 to 80 km and from 0 to 140 km. Results show that in both cases and for all the studied frequency bands (1–2, 2–4,6–8 and 8–10 Hz ) intrinsic absorption dominates. The low albedos found in the region indicate the low degree of heterogeneity in the Canarian lithosphere at the scale length of the studied frequencies. On the other hand the coda attenuation (Qc−1) calculated on the basis of the single-scattering theory gives values near (Qi−1) for low frequencies and near the total attenuation (Qt−1) for high frequencies. The degree of frequency dependence of the attenuation parameters is strong in all cases. A correlation of the observed attenuation parameters with the geological evidence for a hotspot-type archipelago and with other geophysical studies is suggested.

The age and duration of the Matuyama–Brunhes transition from new K–Ar data from La Palma (Canary Islands) and revisited 40Ar/39Ar ages

Abstract Paleomagnetic investigations conducted on lava sequences from La Palma (Canary Islands) yielded several independent records of the Matuyama–Brunhes transition (MBT). Seven K–Ar ages of flows sampled across the MBT suggest a duration between 0 and 11 kyr, and provide a weighted mean age of 786±3 ka (analytical uncertainty only) for the transition. This value is significantly older than the previously admitted age of 779±2 ka, derived from volcanic or sedimentary sequences. When the 1% uncertainty on the calibration standard used in the present study is considered, our age for the transition recorded at La Palma becomes 786±8 ka. We have recalculated the 40Ar/39Ar determinations from the previous studies using recent revisited ages for 40Ar/39Ar standards. This yields a global dataset of 23 K–Ar and 40Ar/39Ar determinations. Our best estimate for the MBT age derived from this compilation is 789±2 ka (analytical error), or 789±8 ka (total error). Within this 8 kyr uncertainty, which could be considered as a minimum estimate, this age remains compatible with recent determinations made by the astronomical time scale. However, the present study suggests that better control of the lock-in depth, as well as better quantification of uncertainties involved in the tuning of δ18O records to orbital forcing models, are needed to improve further the astronomical polarity time scale derived from sedimentary records. In addition, absolute dating by K–Ar and 40Ar/39Ar will gain in accuracy when uncertainty determinations on standards will be significantly improved.

Evidence for a persistent uplifting of La Palma (Canary Islands), inferred from morphological and radiometric data

Abstract The morpho-structural evolution of La Palma (Canary Islands) is characterized by vertical movements responsible for/or favoring large landslides. In its northern part, conglomerate terraces, nowadays topographically hanged and re-incised by the present-day ‘Barranco de Las Angustias’, reveal a recent ongoing uplift process. Lava flows covered by this geological unit have been sampled and dated by the K/Ar Cassignol technique at a mean age of 507±11 ka. The minimum rate of uplift inferred, of about 0.4 mm/yr, is similar to the one deduced from the abnormal elevation of the early submarine series [Staudigel and Schmincke, J. Geophys. Res. 89 (1984) 11195–11215]. This indicates that the northern shield may have experienced a global rise fairly constant throughout most of its geological history. It probably results from the combination of regional Atlas tectonic processes and the existence of a swell at the island scale. This global rise, together with the existence of a north–south fault in the basement, may have controlled triggering of the SW flank collapse of the northern shield and the subsequent concentration of magma along a southern, still active topographic ridge, the Cumbre Vieja (CV). Morphological asymmetry of this structure may result from the destabilization of its distal western part, favored by strong injections at the axis. Tilting processes along lateral listric faults can then explain the existence of slope break-ups, especially on the western side of the ridge. Such tectonic processes are likely responsible for local superimposed uplifting contributing to the genesis and development of huge western coastal cliffs, even in sub-historic and historic times. We show that, unlike many oceanic islands, La Palma Island has experienced important positive vertical movements rather than subsidence.

Remagnetization in lava flows recording pretransitional directions

Basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of La Palma are characterized by overlapping components with very high unblocking temperatures. A medium high-temperature (MHT) component with normal polarity is resistant up to 500–550°C, while in most cases the high-temperature (HT) primary reverse component cannot be isolated below 520°C. The same situation (with opposite polarities) is observed also in a lava flow associated with the onset of the upper Reunion reversal in the Gamarri sequence in Ethiopia. The presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling. The direction of MHT is either close or similar to the direction of the HT component of the overlying flows, which indicates that remagnetization occurred shortly after emplacement. Rock magnetic studies, microscopic observations, microprobe analyses and remagnetization experiments have been carried out to investigate the origin of MHT. The rock magnetic parameters do not exhibit specific anomalies. The unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling. The most plausible scenario is that baking by the overlying flows was accompanied by low-temperature oxidation of titanomagnetite to cation-deficient titanomagnetite. This description also includes the presence of cracks, fractures and other heterogeneities of the lava which induce variability between parallel profiles. Similarities with characteristics observed at Steens Mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution.