Description: Arctic Ocean circulation as we know it today is an exceptional situation compared with the geological past. This was shown from geochemical analyses of a unique marine sediment core recovered in the central Arctic Ocean. A major transition from oxygen-poor sediments to well oxygenated sediments 17.3 million years ago indicates that the Fram Strait, which is the only deep water connection of the Arctic Ocean with the Atlantic Ocean, already opened at this time and allowed the establishment of a well-ventilated ocean basin. Isotope geochemical results suggest that the Arctic deep circulation was strongly influenced by sea ice formation during most of the past 15 million years and was not predominantly controlled by inflowing Atlantic waters, as is the case today.

Description: Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, δ13C, δ18O and 87Sr/86Sr values. The results are compared with the chemistry, and δ13C and 87Sr/86Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre-dates the formation of framboidal pyrite. Most dolomite layers show 87Sr/86Sr-ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the δ13C values of the dolomite are not in equilibrium with the δ13C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87Sr/86Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (〈30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co-occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate–methane interface. It has been hypothesized that microbial ‘hot-spots’, such as the sulphate–methane interface, may act as focused sites of dolomite precipitation. Varying δ13C values from −15‰ to +15‰ for the dolomite are consistent with precipitation at a dynamic sulphate–methane interface, where δ13C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate–methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon-rich sedimentary sequences can provide a useful window into the palaeo-dynamics of the deep biosphere.