Stable isotope and biotic evolution in the North Sea during the early Eocene: the Albæk Hoved section, Denmark

Abstract Stable isotope (δ13C, δ18O) and biostratigraphic data are presented for a 20 m thick section spanning the Røsnæs Clay Formation at Albæk Hoved in Denmark. This early Eocene formation is the first calcareous deposit in the North Sea after a several million year period of non-calcareous sedimentation. Nannofossil and dinoflagellate data indicate that the section is unusually complete, spanning lower zone NP11 to lower NP13. Throughout the section, Subbotina spp. dominate the planktonic foraminiferal assemblages. Benthonic foraminiferal assemblages indicate middle bathyal water depths (600–1000 m). Water exchange between the semi-enclosed North Sea and the open ocean has been estimated by comparing the North Sea isotopic records with coeval records for DSDP Hole 550 in the northeastern Atlantic Ocean. Anomalously low δ18O values (−4 to −5‰) for bulk samples and planktonic deep-dwelling Subbotina from the Røsnæs Clay indicate a significant freshwater component in the North Sea. Average salinities in the euphotic zone ranged between 26 and 30 ppt throughout the early Eocene. The benthonic foraminiferal δ18O values indicate generally somewhat more saline and stable conditions in the water mass near the seafloor. During the early Eocene, three principal conditions alternated in the North Sea, depending on regional sea level, position of critical sills and the extent of water exchange with the open ocean. (1) At times of strongly restricted water exchange, calcite dissolution was complete. Grey clays formed and sediment oxygen content was low. Non-calcareous agglutinated foraminifera dominated. This condition prevailed in the earliest Eocene (NP10), during deposition of the lowermost Røsnæs Clay Formation, and at the end of the early Eocene. (2) At times of moderately restricted water exchange, calcite dissolution was important. Calcite content and planktonic/benthonic foraminiferal ratios in the sediment were low. Different grey or reddish brown clays formed. Oxygen content at the seafloor was low to intermediate. Subbotina δ18O values (−2 to −4‰) were generally a few per mil lower than in the coeval open ocean and fluctuated dramatically, due to freshwater admixture. Subbotina-benthonic Δδ13C gradients were high, because of low biological productivity and slow renewal of bottom water in connection with temporarily strongly density-stratified water masses. This condition prevailed during the later half of Biochron NP11, and possibly in latest NP12. (3) At times of more open water exchange, calcite-rich sediments dominated. Deep-dwelling planktonic foraminifera invaded the North Sea and planktonic/benthonic foraminifera ratios were high. Surface-thriving morozovellids, however, were absent, probably because of reduced surface salinities. Subbotina δ18O values (−2 to −3‰) indicate that a freshwater component was present at mid-depth, but salinities were higher and more stable than during more restricted water exchange. Subbotina-benthonic Δδ13C gradients were low, reflecting higher productivity and invigoration of bottom-water circulation. Water mass density stratification was less profound. Reddish brown marls dominated, and oxygen content in the sediment was moderately high. This condition prevailed during most of NP12 and probably during early NP11. High-resolution isotopic profiles over a 5 m thick interval in the upper NP12 part of the Røsnæs Clay Formation reveal that three distinct lithological ‘event beds’ are associated with profound short-term negative shifts (1–2‰) in δ13C and δ18O. The isotopic shifts are of the same magnitude in bottom waters as at mid-depth, implying that they reflect rapid changes in the chemistry of the entire water mass of the North Sea. The events may reflect short-term sea-level falls and/or rapid water mass exchange with other semi-enclosed basins to the north.