ALBERT

Description: Despite the many advances recently made in palaeointensity (PI) studies, more high quality PI data are required for a better understanding of the full-vector evolution of the ancient geomagnetic field. In this study, we applied the recently-proposed multispecimen parallel differential pTRM (MS) method for absolute PI determination to rocks older than those studied hitherto with this method. Fifty-one lava flows from within the Trans-Mexican volcanic belt, which are generally younger than 1 Ma, returned 32 new PI estimates. Rock magnetic investigations revealed that in most cases, the remanence was carried by Ti-poor titanomagnetites of pseudo-single domain magnetic structure although titanomagnetites with higher Ti-contents and additional contributions of (titano-) haematite were also occasionally present. Comparisons with reconstructions of the global dipole moment reveal that the PI estimates from this study were, on average, around 30 per cent higher than expected. Other, mainly Thellier-type, PI data from Mexico were also observed to be high relative to global records, which could indicate that persistent non-dipole features might be responsible for the higher than expected results. However, the paucity of available data obscures the significance of this observation and the balance of evidence rather suggests an artificial biasing of most measurements towards high values. Our results seem to corroborate results from previous studies on historical lava flows and synthetic samples in which domain state effects were found to cause overestimates of the PI by up to 30 per cent in the MS method. We expect that the degree of the overestimate in the majority of these new MS results is no larger than what might be expected from Thellier experiments performed on samples with a similar given degree of multidomain behaviour. However, seven of the lava flows which were studied here using the MS method were subjected to a preceding demagnetisation step of 200 °C in order to erase viscous remanences and these may have produced results which were further biased towards higher intensities. This additional bias is expected from theoretical considerations and may represent a challenge to future application of the MS method to older rock units. It could, however, potentially be avoided in future studies by performing measurements of the sample moment at temperatures higher than the blocking temperature of any overprint.

Description: This study presents paleomagnetic data from 59 independent lava flows from the trans-Mexican volcanic belt (TMVB) with ages from 6.4 Ma to recent, 52 being younger than 1 Ma, and 11 new 40Ar/39Ar age determinations. Most remanence carriers are Ti-poor titanomagnetite of pseudosingle-domain magnetic structure, nine lavas contain small amounts of titanomaghemite, and four lavas additional (titano-) hematite. Paleosecular variation of lava flows younger than 1.7 Ma is consistent with latitude-dependent Model G and also in agreement with other Pleistocene paleomagnetic data from the TMVB. The directional record of Brunhes and Matuyama Chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions. One lava flow dated at 592 ± 20 ka has a fully reversed paleodirection and most likely erupted during the Big Lost excursion. Another fully reversed flow, dated at 671 ± 12 ka, gives new volcanic evidence for the Delta/Stage 17 excursion. This excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 ± 10 ka. From the Matuyama age lavas, one flow with normal polarity magnetization, dated at 949 ± 37 ka, could either be related to the Kamikatsura or the Santa Rosa excursion and a normal polarity flow, dated at 1628 ± 56 ka, could have been emplaced during the Gilsa excursion. The results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions.

Description: A combination of marine (Alboran Sea cores, ODP 976 and TTR 300 G) and terrestrial (Zo˜nar Lake, Andalucia, Spain) geochemical proxies provides a highresolution reconstruction of climate variability and human influence in the southwestern Mediterranean region for the last 4000 years at inter-centennial resolution. Proxies respond to changes in precipitation rather than temperature alone. Our combined terrestrial and marine archive documents a succession of dry and wet periods coherent with the North Atlantic climate signal. A dry period occurred prior to 2.7 cal ka BP – synchronously to the global aridity crisis of the third-millennium BC – and during the Medieval Climate Anomaly (1.4–0.7 cal ka BP).Wetter conditions prevailed from 2.7 to 1.4 cal ka BP. Hydrological signatures during the Little Ice Age are highly variable but consistent with more humidity than the Medieval Climate Anomaly. Additionally, Pb anomalies in sediments at the end of the Bronze Age suggest anthropogenic pollution earlier than the Roman Empire development in the Iberian Peninsula. The Late Holocene climate evolution of the in the study area confirms the see-saw pattern between the eastern and western Mediterranean regions and the higher influence of the North Atlantic dynamics in the western Mediterranean.

Description: The present project is a joint effort between different institutions to deploy a dense seismic network at Gran Canaria island (Canary Islands, Spain). The interstation distance is around 20 km. The broadband seismic network is composed of one permanent (Guralp CMG-3T 120 s) and five temporary stations (Guralp CMG-3ESP 60 s). The permanent station is a 120 s Guralp CMG-3T and belongs to the Canary Island Seismic Network, run by the Instituto Geográfico Nacional (IGN) of Spain. The temporary stations are 60 s Guralp CMG-3ESP, provided by the GFZ seismic pool. The deployment was carried out in December 2009. The stations will be recording during two years. The improvement of the seismic network allow us to tackle the following issues: the detection and analysis of any local seismicity of tectonic and/or volcanic origin at Gran Canaria island; to contribute to the understanding of the regional seismicity with special interest in the oceanic channel between Tenerife and Gran Canaria Island in collaboration with a project running a dense temporary seismic network in Tenerife; to study the crustal and upper mantle structure, under Gran Canaria to constrain the crustal structure, the source of the volcanism, and better sample the mantle discontinuities and anisotropy. To study the Earth structure, we use receiver function analysis, ambient seismic noise and SKS anisotropy techniques, This project is part of a long-term research of the crustal and the mantle structure of the Canary Islands, which has started with Gran Canaria and Tenerife Islands and will eventually continue with the rest of the archipelago. The origin of the Canary Islands is generally attributed to a broad mantle upwelling under a slow moving plate, resulting in spatially and temporally distributed volcanic activity and a large number of seamounts and islands. A controversial discussion has been going on about the factors that control the evolution of the volcanic edifices, the type of the melting anomaly (a single, well defined mantle plume of a larger area of diffuse mantle upwelling), and the tectonic control of this evolution. This study is foreseen to provide important clues to understand the volcanic structure and tectonic evolution of the Gran Canaria Island.

Description: High-resolution geochemical analysis of a 6-m-long sediment core from Zoñar Lake, southern Spain, provides a detailed characterization of major changes in lake and watershed processes during the last 4,000 years. Geochemical variables were used as paleolimnological indicators and complement Zoñar Lakes’s paleoenvironmental reconstruction based on sedimentological and biological proxies, which define periods of increasing allochthonous input to the lake and periods of dominant autochthonous sedimentation. Chemical ratios identify periods of endogenic carbonate formation (higher Ca/Al, Sr/Al and Ba/Al ratios), evaporite precipitation (higher S/Al, Sr/Al ratios), and anoxic conditions (higher Mo/Al, U/Th ratios and Eu anomaly). Higher productivity is marked by elevated organic carbon content and carbonate precipitation (Mg/Ca). Hydrological reconstruction for Zoñar Lake was based on sedimentological, mineralogical and biological proxies, and shows that lower lake levels are characterized by Sr-rich sediments (a brackish lake with aragonite) and S-rich sediments (a saline lake with gypsum), while higher lake levels are characterized by sediments enriched in elements associated with alumino-silicates (Al, K, Ti, Fe, trace and rare earth elements), reflecting fresher conditions. Geochemical indicators also mark periods of higher detrital input to the lake related to human activity in the watershed: (1) during the Iberian Roman Humid Period (650 BC–AD 300), around the onset of the Little Ice Age (AD 1400), during the relatively drier Post-Roman and Middle Ages (AD 800–1400), and over the last 50 years, due to mechanized farming practices. Heavy metal enrichment in the sediments (Cu and Ni) suggests intensification of human activities during the Iberian Roman Period, and the use of fertilizers during the last 50 years.