Calcareous sediments on the nearshore continental shelf of western Scotland

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Abstract
The highest carbonate values are associated with exposed coasts of Lewisian gneiss, reaching 95% CaCO3 in SW Tiree: around Iona average values are 80 to 85% CaCO3. Maximum values of 60 to 70% on Islay indicate a regional gradient which reflects a reduced fetch southwards, and a greater input from reworked glacial drift and fluvial material.High energy carbonate deposits are distinguished by the low diversity of their components, nearly all coming from fragmented molluscs and barnacles. Low energy carbonates have diverse components, with foram and echinoid values each reaching 10%. The ratio most easily separates the two facies (high energy mean = 18-5: low energy mean = 2-1).In the immediate offshore tidal-swept zones such as the Sound of Iona barnacles are the most important contributors to the sediment.Most of the carbonate is fresh and by its relationship to contemporary shore ecology can be shown to be of present-day origin. Offshore, however, relict grains are widespread. Their pitted surfaces are inhabited by very abundant diatoms, and it is tentatively suggested that local carbonate dissolution may be caused by them. Additionally they represent a much more abundant source of silica for eventual silicification of the carbonate than do sponge spicules, which are rare.