Corroding cannons measure dynamic power in reef ecosystems

Historic shipwrecks provide a unique insight into how the flux of dissolved oxygen, caused by waves breaking on coral and reefs, varies with depth and with seabed topography.

As cast iron corrodes it loses the iron-rich phases in sequential order of its iron content—leaving behind the corroded matrix which retains the inert graphite phase, but retaining the original shape.

The depths of corrosion on the cast iron cannon on the Batavia wreck site varies from 19mm to 61mm, while the long-term average for marine iron is 0.1mm/year or 36.5mm of graphitisation for the period between sinking in 1629 and being recovered in 1994.

The reason for the range of corrosion rates is due to the precise topography on the wreck site—some guns are located on top of coral platforms, in shallower parts of the wrecksite, so they corrode at a much faster rate than those which remain partly buried in the deeper sections of the wrecksite.

We have attempted to provide a cumulative index of the flux of dissolved oxygen to sedentary epifauna by depositing lumps of cast iron (heavy enough to withstand being relocated by the surge of wave action in reef-strewn waters) in strategically chosen sections of a reef in Port Phillip Bay in Victoria.

By periodically returning and determining the corrosion rate we have been able to measure the impact of wave action on the supply of nutrients to the marine organisms.

These experiments have confirmed that the corrosion rates fall linearly with the square root of time before reaching a plateau after six to seven years.

The log of the corrosion rate falls linearly with increasing water depth.

We have also learned interesting details about weather patterns by examining the corrosion pattern on a copper wire inside an oil lubrication device on the wreck of the pearling transport Xantho (1872) at Port Gregory in the Mid West.

The wire showed periodic precipitation patterns which demonstrate that the site conditions have changed every seven years since the ship sank 112 years ago.

Corrosion is stopped when the wrecksite is exposed to flowing oxygenated seawater, through galvanic protection from the iron engine.

When the site is buried under a couple of metres of sand, the anaerobic conditions which corrode copper produce the bands of the copper sulphide chalcocite (Cu2S).