Article: Ostracoda and palaeosalinity fluctuations in the Middle Jurassic Lealt Shale Formation, Inner Hebrides, Scotland

Molluscs, conchostracans, algae, palynomorphs and, in particular, ostracods are used to define fluctuating salinities in the Lealt Shale Formation, Great Estuarine Group (Upper Bajocian-Bathonian, Middle Jurassic). Salinities ranged from oligohaline (0-5-5%o) to mesohaline (5-18%o). Fluctuations were often rapid, occurring over only 10-20 mm of sediment. The Kildonnan Member of the Lealt Shale Formation is shown to have experienced a more unstable salinity history than the Lonfearn Member of the same formation. The ostracods are shown to provide a more accurate interpretation of the palaeosalinity fluctuations in the sections studied than do the molluscs. Occasionally only integrated studies allow a reasonably viable determination of palaeosalinity. Salinity tolerance ranges are estimated for 26 ostracod species in the sequence studied. Published oxygen isotope analyses of the mollusc Praemytilus strathairdensis are shown to correlate well with the salinity fluctuations determined using the ostracod fauna. High abundances of the fresh to brackish water tolerant alga Botryococcus correlate with occurrences of freshwater ostracods (Limnocythere, Theriosynoecum and Darwinula), as well as with freshwater molluscs (Unio and Viviparus), and with conchostracans.