Comparison of living and dead benthic foraminifera on the Portuguese margin: Understanding the taphonomical processes

Abstract

Dead benthic foraminifera (> 150 μm) were studied in 23 sediment cores from the Portuguese Margin at water depths between 20 and 2000 m and located on 4 transects off the Douro, Mondego, Tagus and Sado river mouths and 1 transect in the Estremadura. For 10 stations, the dead faunal vertical distribution (0？8 cm) was first investigated in 4 different sediment horizons per core to evaluate the sampling effort necessary to have a representation of the dead fauna deposited under different environmental areas. As a result, it appears that the faunal vertical distribution is constant, except for the deepest environments where fragile taxa were identified in the top layers only. Dead foraminiferal assemblages in the 4？5 cm layer for all stations were then compared to previously published living foraminiferal assemblages (of March 2011) from the same cores to evaluate the taphonomical processes affecting major species. This improves the knowledge of the faunal distribution for a better benthic foraminiferal proxy for paleostudies. There was a considerable loss of some species in the dead fauna. Firstly, this concerns the fragile organic-cemented agglutinated taxa such as Reophax spp., Glomospira charoides, or Bathysiphon spp. Secondly, some calcitic species such as Nonion scaphum, Cancris auriculus, Ammonia beccarii or Bulimina aculeata that were particularly abundant in the living fauna on the inner shelf under the late winter high river discharge conditions, were also far less dominant in the dead fauna. Lastly, other species like Cassidulina carinata, Valvulineria bradyana, and Bulimina marginata systematically showed higher abundance in the dead fauna at the mid shelf. These species, related to eutrophic conditions occurring in summer during the upwelling activity, were therefore not well represented in the living fauna, collected in March. Transport of allochthonous specimens may also account for higher contribution in the dead community of some species like Cibicides lobatulus,Asterigerinata mamilla or Haynesina depressula, especially in coastal environments where hydrodynamic processes (river flood, winter storm, coastal drift) are more vigorous. Several species (U. mediterranea, U. bifurcata, T. agglutinans, H. balthica or B. costata), however, show little or no difference in both abundance and spatial occurrence between the living and dead faunas and provide a stable signal for paleoclimatic investigations.