Joaquín Meco

Cenozoic volcanism II: the Canary Islands

The Canarian archipelago comprises seven main volcanic islands and several islets that form a chain extending for c. 500 km across the eastern Atlantic, with its eastern edge only 100 km from the NW African coast. The islands have had a very long volcanic history, with formations over 20 million years old cropping out in the eastern Canaries. Thus all stages of the volcanic evolution of oceanic islands, including the submarine stage as well as the deep structure of the volcanoes, can be readily observed. Rainfall and vegetation cover are relatively low, with the exception of the island of La Palma, favouring both geological observation and rock preservation. Furthermore, the absence of surface water has promoted groundwater mining by means of up to 3000 km of subhorizontal tunnels (locally known as ‘galerias’). These galerias are especially numerous in Tenerife, La Palma and El Hierro, and allow the direct observation and sampling of the deep structure of the island volcanoes without requiring expensive and indirect geophysical methods.

The maximum warmings of the Pleistocene world climate recorded in the Canary Islands

Abstract Evidence of the warmest Pleistocene climatic changes is preserved in the eastern Canary Islands. Although the existence of raised fossiliferous marine deposits in northern Gran Canaria has been known since the 19th century, their chronology and significance with regard to past sea levels and temperatures have remained uncertain. Here, we show three marine transgressions recorded by raised marine deposits. The highest (85 m above sea level (masl)) and oldest marine deposit is interbedded in basaltic lava flows and has been dated using both palaeomagnetic and radiometric methods. The age of these deposits, located on the northwest coast of Gran Canaria, near the town of Agaete, places the marine transgression in a first Pleistocene interglacial into the Plio–Pleistocene limit. The marine deposit at an intermediate altitude (35 masl) has been dated using the unspiked K–Ar method on a pillowed basaltic lava flow. The age of these deposits in Arucas places the transgression in the penultimate (European Mindel/Riss sense) interglacial. The deposits at 12 masl in Las Palmas are attributed to the last Pleistocene interglacial on the basis of their fossil content, which is similar to that of the Matagorda deposits in Lanzarote and Matas Blancas in Fuerteventura. Radiometric dating of a lava flow overlying similar deposits at El Cotillo in Fuerteventura has been carried out. The molluscan fauna associated with the deposits permits an estimation of the sea temperatures by reference to their present biogeographical distributions. The Plio–Pleistocene boundary coincides at Agaete (end of Olduvai event) with a high sea level between 1.8 and 1.75 Ma (oxygen isotopic stage (OIS) 63), the dated marine deposits at Arucas (0.42 Ma) place an interglacial in OIS 11.3 and finally we confirm that, in the Canaries, with only one high sea level with a Senegalese fauna (OIS 5.5), they are not an outpost for the Mediterranean Tyrrhenian (deposits with Strombus) oscillations.

Paleotemperature of the last interglacial period based on δ18O of Strombus bubonius from the western Mediterranean Sea

Abstract The objective of the present study is to quantify sea surface temperature (SST) by measuring δ18O in shells of Strombus bubonius. First we present data obtained from modern shells collected at different locations in the Gulf of Guinea where Strombus bubonius is now geographically restricted (islands of Corisso, Bioko and Pagalu). Next we give the results of our investigations of fossils found in two classical Mediterranean sites: Monastir in Tunisia and Palma Nova, a Pleistocene deposit on the island of Majorca. Both sites have been assigned an age corresponding to the last interglacial period (LIP) by stratigraphic correlation with terraces dated by the UTh method. The sampling scheme followed precisely the growth spiral of the mollusk shell in order to obtain time series of 3–4 years, depending on the size of the specimen. As expected, the δ18O records generally exhibit a cyclic pattern which can attributed of the seasonality of SST. δ18O-SSTs reconstructed for the modern shells are in rough agreement with SST values obtained from climatological maps, which suggests that Strombus shells may be useful in paleotemperature reconstructions. The difference between δ18O maxima and minima (isotopic seasonality) is generally in aggreement with the modern SST seasonality range inferred from climatological data. This δ18O-SST seasonal temperature difference ranges from 2°C to 6°C for the modern shells from the Gulf of Guinea and from 7°C to 9°C for the Mediterranean fossils, which is not significantly different from modern seasonality at these locations. The δ18O records obtained from the fossils suggest that the SSTs were higher by several °C during the LIP. However, the magnitude of this temperature difference is difficult to quantify precisely due to our lack of knowledge of the sea surface salinity (SSS) distribution during the LIP. We chose two different working hypotheses: (1) that the Mediterranean SSSs were the same as today, and (2) that the SSSs were typical for periods of sapropel deposition. Mean temperature differences of about 7°C and 3°C can be deduced under the first and the second working hypotheses, respectively. The second value is in agreement with other temperature proxies from marine and terrestrial environments which lends support to the second hypothesis.

The Barranco de Tirajana basin, Gran Canaria (Spain). A major erosive landform caused by large landslides

The Barranco de Tirajana (BdT), located on the island of Gran Canaria (Spain), has some specific features that differentiate it from the ravines of other volcanic islands in the Atlantic Ocean. The origin of this unusually wide upper basin (35 km 2 ) has been under discussion over the last century although its erosional origin is nowadays widely accepted. The purpose of this paper is to describe the landslide deposits that appear at the bottom of the basin and to assess their role in the geomorphological evolution of the basin. We suggest that the BdT basin is a major erosional landform initiated by important ravine incision and widened by a large number of landslides. There are 28 large landslides within the BdT basin. The main movements were of rock-slide and debris-slide types, affecting 70% and 25% of the total area, respectively. In addition, modes of displacement were predominantly translational (rock-, debris-, and earth-slides) consisting of 89% of the total, compared to rotations and flows that constitute only 11%. Three main periods of landslide activity have been established in the development of the BdT basin, using geomorphological criteria. Period I includes ancient movements that could have started at about 0.6 Ma or even 2.7 Ma ago and are considered as abandoned landslides. Period II corresponds to old landslides considered as dormant, which occurred within the Middle‐Upper Pleistocene. Finally, period III includes recent Upper Pleistocene landslides and Holocene landslides that are still active. We suggest that interglacials in the Canary Islands and NW Africa included humid and wet episodes that could account for the occurrence of periods of landslide activity in the BdT basin. D 2002 Elsevier Science B.V. All rights reserved.

Güímar and La Orotava Mega-Landslides (Tenerife) and Tsunamis Deposits in Canary Islands

More than 20 mega-landslides have been described in the Canary Islands affecting the flanks of the volcanic edifices. Guimar and La Orotava landslides, in Tenerife, are two exceptional cases due to their huge dimensions and outstanding geomorphological features. The estimated volume of these landslides exceed tens of cubic km. Tsunami deposits have been also identified in some of the islands of the archipelago probably associated to the large landslides of the islands flanks.