The Origin of Biogenic Growths on Gold Recovered from the SS Central America Shipwreck: Implications for Geoarchaeology

SMSU Event Center BC

Black crusts which have formed on US $20 gold coins recovered from the 1857 wreck of the SS Central America display three distinct layers that reveal a complex history of geochemical and biological corrosion and re-precipitation of metals within the shipwreck. Initial corrosion of the shipwreck produced laminar black iron crusts on the gold coins by geochemical processes. These crusts subsequently protected the coins from pitting and corrosion, and as gold it toxic to most life, armored the coin surface and provided an environment for later bacteria to thrive. Specifically, two types of mineralized bacteria were found within the middle layer of the black crusts, and are associated with formation of secondary nano-particulate gold. The source of this gold is likely depletion gilding of placer gold from the wreck, while the nano-particulate gold in the black crusts results from local detoxification by gold-tolerant bacteria. The two types of nano-particulate gold (silver-rich and copper-rich) are presumed to be waste from the distinct metabolic pathways of the two types of mineralized bacteria preserved in the black crusts.