Depositional environments within Middle Jurassic oyster-dominated lagoons: an integrated litho-, bio- and palynofacies study of the Duntulm Formation (Great Estuarine Group, Inner Hebrides)

ABSTRACTThe Duntulm Formation of the Bathonian (Middle Jurassic) Great Estuarine Group represents a marine-brackish lagoonal complex which was transgressively established on the drowned delta of the underlying Valtos Sandstone Formation. Duntulm Formation sediments accumulated in adjacent basins, the Sea of the Hebrides-and Inner Hebrides-basins. Litho-, bio-, and palynofacies analysis of these rocks show that the most distinctive lithologies, monotypic shell banks of the oyster Praeexogyra hebridica (Lithofacies 1), accumulated under variable, but distinctly marine conditions. Argillaceous carbonate muds (Lithofacies 2), probably accumulated in the lee of the oyster banks under variably marine-freshwater conditions, while the supralittoral lagoon shores were fringed by algal marshes (Lithofacies 3). The palynology of Lithofacies 3 shows that some marshes accumulated close to the lagoon (dominated by marine dinocysts), while others formed further inland (dominated by terrestrial pollen and spores). In the N of the Sea of the Hebrides Basin, small deltas continued to prograde into the lagoons (Lithofacies 4), and toward the end of Duntulm Formation times, muds and sands colonised by freshwater molluscs (Unio and Neomiodon), the planktonic alga Botryococcus, and dominated by terrestrial pollen and spores (Lithofacies 5), accumulated in this area, representing a basin-wide change from marine to freshwater conditions. The structural high which divided the basins exerted a strong control on lithofacies evolution, effectively preventing any material coarser than silt grade entering the Inner Hebrides Basin. Thickness variation in the Formation is similarly structurally/facies linked, with thick sequences accumulating where sandstones (Lithofacies 4) predominate, and thin sequences occurring close to the structural high.

Selenium Requirement of a Bloom-Forming Planktonic Alga from Softwater and Acidified Lakes

Lakewide odor episodes have been associated with blooms of the planktonic alga Chrysochromulina breviturrita Nicholls (Prymnesiophyceae) in lakes influenced by acidic precipitation. We demonstrate an absolute requirement in this flagellate for selenium (Se) in axenic culture. The alga is capable of utilizing several forms of Se: selenite, selenate, dimethylselenide (DMSe), and selenomethionine. This is believed to be the first demonstration of the utilization of DMSe as a Se source by any organism. As the microflora of aquatic sediments recycle Se as volatile DMSe, this may have significance in its biogeochemistry in acidified waters. Bioassays of lake waters from the Muskoka–Haliburton region of Ontario show that in one lake (Dickie) a simulated Se spike resulted in a significant increase (70%, p < 0.05) in algal growth. Growth in Cinder Lake water, which had supported a population of C. breviturrita, was marginally stimulated (24%, p < 0.05). Distant coal-fired power plants, which have been implicated in the episodic deposition of Se, may account in part for the unpredictable blooms of the alga. Chrysochromulina breviturrita cannot be maintained in culture without this micronutrient, and several Se sources may be important triggering factors for the initiation of blooms.