Silvia Hess

BENTHIC FORAMINIFERAL RECOVERY AFTER RECENT TURBIDITE DEPOSITION IN CAP BRETON CANYON, BAY OF BISCAY

The vertical distribution of living (rose Bengal stained) benthic foraminifera from three multiple cores from Cap Breton Canyon, Bay of Biscay, France, has been investigated with the objective of monitoring the recolonization and subsequent evolution of the foraminiferal fauna following the deposition of a turbidite layer. The first samples, taken in May 2000 in the axis of Cap Breton Canyon, contained a young turbiditic sequence, most likely deposited during the heavy storm of December 1999. Four months after this sedimentological event, the composition of the living benthic foraminiferal fauna was almost monospecific in the >150 μm fraction, which contained mainly Technitella melo, a species which is otherwise very rare or absent in the Cap Breton Canyon and open slope assemblages. This species was accompanied in the 63–150 μm fraction by adult specimens of Cassidulina carinata and Fursenkoina bradyi, and exclusively juvenile specimens of Bolivina subaenariensis and Bulimina marginata. This fauna represents the first stage of foraminiferal colonization after the turbidite deposition. The samples taken one year later, in June and September 2001, at approximately the same location, contained a more variable foraminiferal assemblage strongly dominated by Bolivina subaenariensis. Foraminiferal assemblages in samples taken just below the successive turbidite sequences contained nearly the same faunal elements as the surface assemblages sampled in 2001. The benthic foraminiferal assemblages from the canyon axis sampled in 2001 show the same composition as other canyon axis faunas dominated by B. subaenariensis. We suggest that the recovery of the foraminiferal faunas in this extremely unstable environment takes about 6–9 months, and that the community structure more or less permanently stays in an early stage of ecosystem recolonization.

Benthic foraminifera from the Lower Jurassic transgressive mudstones of the south-western Barents Sea—a possible high-latitude expression of the global Pliensbachian–Toarcian turnover?

The Lower Jurassic section of two commercial wells (7119/12-1 and 7119/12-2), drilled in the Hammerfest Basin, was analysed for foraminiferal assemblages in order to reconstruct palaeoenvironmental changes. Foraminifera occurred singularly in transgressive mudstone intervals of the Stø Formation in well 7119/12-1. High-energy marginal marine and offshore sandstones and freshwater sand- and mudstones were barren of foraminifera. Three distinct foraminiferal assemblages could be distinguished. They contain exclusively agglutinated taxa and have extremely low species diversities, indicating restricted environmental conditions. The main restricting factor was low salinity in strongly delta-influenced waters. Foraminiferal assemblage 1 is developed at the base of the Stø Formation and suggests transgressive, extremely hyposaline lagoonal conditions. Assemblages 2 and 3 occur in hyposaline prodelta shelf mudstones of the Stø Formation and reveal increased diversities in major local transgressive phases. The foraminiferal assemblages indicate Pliensbachian–Toarcian ages. The mudstones of the Stø Formation are regarded as a local expression of the global Pliensbachian–Toarcian turnover recording regional transgressive developments in the Toarcian.

Spatial and temporal organic carbon burial along a fjord to coast transect: A case study from Western Norway

We investigated spatial and temporal changes in accumulation rate and source of organic carbon on a gradient along the Lysefjord and the more coastal Høgsfjord, Western Norway. This was achieved through analysis of total organic carbon and nitrogen content of sediment cores, which were radiometrically dated to the early 19th and 20th centuries for the Høgsfjord and Lysefjord, respectively. Benthic foraminifera (protists) were utilized to determine changes in organic carbon supply and Ecological Quality Status (EcoQS) by their accumulation rate (benthic foraminiferal accumulation rate (BFAR)), assemblage composition, species diversity, individual species responses and the composition of stable carbon isotopes of the tests (shells) of Cassidulina laevigata, Hyalinea balthica and Melonis barleeanus. Organic carbon accumulation rates were greatest closest to the river Lyse at the head of the Lysefjord (83–171 g C m−2 yr−1). The organic carbon at the head of the fjord is mainly terrestrial in origin, and this terrestrial influence becomes progressively less seaward. The δ13C in H. balthica tests as well as the benthic foraminiferal assemblage composition also showed a clear fjord to coast gradient. Organic carbon accumulation rates were lower and less variable at the seaward study sites (13–61 g C m−2 yr−1). We observe no temporal trend in organic carbon, carbon isotopes, EcoQS or foraminiferal assemblage composition in the Lysefjord. In contrast, in the Høgsfjord, there seems to have been an increase in organic carbon accumulation rates during the 1940s. Subsequent accumulation rates are stable. The foraminiferal assemblages in the surface sediments reflect a recent transition from good/moderate to moderate/bad EcoQS.