Foraminiferal assemblages in active volcanic area of Azores Islands

The present study shows the preliminary micropaleontological results carried out on grab and box corer samples collected during a marine geological survey in the Terceira offshore (Azores Archipelago, Portugal) occurred in September 2011, in the framework of the Eurofleet program. This multidisciplinary project aims to reconstruct high-resolution morpho-bathymetry offshore Terceira Island and to define the evolution history of the Terceira volcanic edifice. A main WNW-ESE oriented fissural zone, characterized by the alignment of scoria cones and eruptive fractures, crosses the island extending down to the sea, along the Serreta Ridge. An intermediate-depth submarine eruption occurred in 1998-2001 producing a consistent amount of volcanic materials (lava balloons, highly vesciculated basaltic scoria, lapilli, etc.) scattered around the vent. In this study benthic and planktic foraminiferal assemblages were studied in order to obtain an ecological characterization of sea-floor and water masses in natural stressed environments like volcanic area of the Azores Islands, after the last volcanic eruption. The samples collected in a bathymetrical interval ranging between 260 and 410 m water depth, are constituted of blackish, sand-sized glass shards, likely originating by volcanic explosive events, in which highly vesicular to scoriaceous clasts prevail on fluidal striated fragments. All samples were stained with Rose Bengal to distinguish living and dead assemblages. Diversity index (α-Fisher index), Faunal density and Foraminiferal Abnormality Index (FAI) were calculated to define the structure of the assemblage and the degree of environmental stress. The preliminary results show that in the most part of the samples, the living taxa number ranges from 18 to 43 while the α-Fisher index shows constant values around 15-16. The living benthic assemblage (Rose Bengal stained specimens) is dominated by deep infaunal species like Angulogerina angulosa, and Bolivina spp. Angulogerina angulosa is considered in some cases epifaunal and in other ones infaunal species. In this study, the record of living specimens of A. angulosa along the whole box-corer, almost until 10 cm depth, suggests a certain capability of this taxon to migrate into the bottom sediments. Probably this species is rather mobile and able to shift its microhabitat from infaunal to epifaunal, somehow reflecting stress forcing. Subordinately, epifaunal (Lenticulina spp., Eherenbergina bradyi, Stomatorbina concentrica, Quinqueloculina seminulum) and agglutinated (Aplophragmoides canariensis, Ammoglobigerina globigeriniformis, Spiroplectinella wrightii, Textularia spp., Eggerelloides scabrus) species are recorded too. Moreover, small sized specimens of Anomalinoides spp. and Miliolinella subrotunda seem to prefer a way of life attached on volcanic shards. Also some agglutinated foraminifers (mainly Aplophragmoides canariensis) were found clinged to glass fragments. Their test are constituted by exclusively glass grains reflecting the compositions of the bottom. Significant frequencies of specimens, showing signs of decalcification of the tests, were found too. Similar test alterations, due to hydrothermal fluids, were recorded in foraminiferal specimens coming from other volcanic bottoms (Aeolian Arc, Tyrrhenian Sea).The dominance of living and dead A. angulosa specimens allows to consider this species as opportunistic pioneer taxon able to colonize stressed environments. In fact, the highest values of FAI (>2), recorded in the most samples, indicate a persisting environmental stress due to the occurrence of repeated volcanic events in the past years. Moreover A. angulosa and Bolivina spp. are the species showing more abundant deformed specimens. The test abnormalities consist mainly of irregular development of chambers and aberrant tests (e.g., double apertures). The total planktic foraminiferal association reflects a clear seasonality. In fact, the living planktic assemblage is characterized by temperate-warm taxa like Globorotalia inflata, Globorotalia truncatulinoides (left), Globorotalia hirsuta, Globigerinoides spp. while cold affinity taxa as Globigerina bulloides and Turborotalita quinqueloba are frequent mainly in the dead assemblage. Plankton seems to be not affected by the Azores Front-Current System, that is centred at 34° N, south of Azores Islands.