Jurassic paleosalinities and brackish-water communities; a case study

The Great Estuarine Group (late Bajocian-Bathonian, Middle Jurassic, Scotland) yields benthic molluscan faunas whose distribution was believed to be controlled largely by salinity; different genera characterize a spectrum of mainly brackish-water environments. Benthic molluscan assemblages are also affected by substrate. A test of the salinity-control hypothesis is to compare the distribution of other groups, with differing environmental constraints, by means of bed-by-bed collecting. We have studied the 26m thick type section of the Kildonnan Member of the Lealt Shale Formation, and present new data on bivalves, gastropods, ostracods, conchostracans, dinoflagellate cysts, acritarchs and Botryococcus. Assemblages change consistently over centimetres or millimetres as regards inferred salinity tolerance; the few examples of "mixed" faunas can be explained by special circumstances. Dinoflagellate cysts occur exclusively in beds also yielding marine-brackish bivalves. Botryococcus percentages correlate with low-salinity molluscs and ostracods, and with conchostracans. The bivalve Praemytilus and some gastropods were euryhaline. Large salinity changes in this shallow-lagoonal setting could take place in years or less, but at intervening times the environment may have been stable, apart from seasonal variations recorded in bivalve shell structures, for decades or centuries. Stable isotope data support these conclusions, particularly for Praemytilus. Our results increase confidence in the use of the groups investigated as paleosalinity indicators, and provide data for studies on the changing nature of brackish-water communities over time.