Marine Studies Group Meeting 24 November 1970 Biogenesis and chemogenesis of recent marine sediments. Chairman: Professor B. M. Funnell.

1. Early biogenic history of lime sand, Bimini Lagoon, Bahamas. By R. G. C. Bathurst

On the floor of Bimini Lagoon the results of biological and related activities are so severe that many of the grains of calcium carbonate are made unrecognisable in terms of their origins as skeletal debris, faecal pellets and ooids—even while they lie at the sediment surface. The Lagoon floor communities are dominated in some places by Thalassia (turtle grass) with an epifauna of miliolids and coralline algae on its leaves. In other places the benthos consists largely of calcareous algae and non-calcareous green and brown algae. In yet other parts there are concentrations of corals and sponges. Foraminiferids live attached to any bush-like or blade-like organism. Bivalves abound in the sediment and echinoids, holothurians, gastropods and fish graze on its surface. The sediment is unconsolidated and is made of the aragonite debris of algae, molluscs and corals and the high-magnesian calcite debris of foram-iniferids and echinoderms.

As the skeletons of the calcareous organisms accumulate on the Lagoon floor they are broken as a result of the feeding of starfish, echinoids, burrowing shrimps and parrot fish, aided by boring sponges, algae and fungi. Most of the floor is stabilised by the roots and rhyzomes of Thalassia, by holdfasts of algae and by a ubiquitous algal mat. The mat-bound sediment is highly resistant to erosion. Areas cleared

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