Mineral coatings on gastropods at hydrothermal vent sites: implications for fossilization processes

Supervisor: Dr Crispin Little

Department: School of Earth and Environment, University of Leeds


Hydrothermal vents are extreme environments where hot, acidic fluid is ejected onto seafloor, usually at great depths. These conditions would seem to be inimical to the preservation of organisms, and yet there is fossil record of hydrothermal vent animals that stretches back hundreds of millions of years, and includes tube worms, brachiopods, gastropods, and bivalves. In part this record can be explained because vent sites are also places where very rapid mineralization occurs, which can lead to exceptional preservation. Indeed, seafloor experiments at hydrothermal vents have shown that mineralization of mollusc shells and worm tubes can take place in under a year. This mineralization is by sulphide minerals, particularly pyrite. However, the details of the early stages of this mineralization process are largely unknown, and yet crucial to understanding the preservation of animals at vents, and thus the biases present in the fossil record of vent communities. Over the past few decades sampling of live gastropods from active hydrothermal vent sites worldwide has revealed that many specimens have shells that are coated by variable thicknesses of reddish or black minerals. These coatings have not been studied, but rather are removed by taxonomists as they obscure shell morphological details. Nevertheless, the mineral coatings are of scientific interest because they likely represent the first stages of a taphonomic (or preservational) pathway to eventual fossilization, with replacement of the original carbonate shell by more recalcitrant oxide or sulphide minerals.

Experimental approach
A collection of 37 gastropod specimens with mineral coatings belonging to 12 species from four Pacific hydrothermal vent areas will be investigated by the student using a binocular microscope to select those with representative coating morphologies (i.e., thin to thick) and mineralogy (i.e., reddish oxides and black sulphides). Any soft tissues remaining in the shells will then be carefully removed and stored for potential later investigations. The specimens will then be critically point dried to avoid as much as possible the coatings becoming detached from the remaining carbonate shells. The shells with their coatings will then be divided, with the major portion being set into resin blocks and subsequently polished, and a minor piece set onto Scanning Electron Microscope (SEM) holders (called stubs). The samples in the polished blocks and stubs will then then be imaged in detail using an SEM at the University of Leeds. This will allow the mineralogy of the coatings to be determined and show, down to very fine scale, the nature of interaction between the mineral coatings and the carbonate shells. The information from the modern material will be compared to data from fossilized vent gastropods, both experimental and natural, where the mineral replacement process been complete. The expected outcome from the project is data that can be written up into a peer-reviewed paper.