Impact of diagenesis and maturation on the survival of eumelanin in the fossil record

Melanins are polymeric phenolic pigments classified into two groups based on their chemical structures and molecular precursors: eumelanin (brown–black) and pheomelanin (yellow–red). Eumelanin is highly resilient and has a proven fossil record, extending back at least ∼200 Ma. It is widespread in the biological world, occurring in fungi, the ink sacs of cephalopods, the feathers of birds, and the hair, skin, eyes, brain and inner ears of mammals. Although the presence and chemical attributes of fossil eumelanin have been documented, there are few data constraining its long term survival. Here we use a diversity of analytical techniques to compare the chemistry and morphology of fossilized cephalopod ink from three deposits of similar age and lithology, but different maturation histories. We demonstrate that the chemistry of eumelanin begins to alter at the onset of the oil window and is largely independent of age. The decrease in surviving melanin is accompanied by an increase in the relative abundance of organic macromolecular material (kerogen) but, critically for the correct interpretation of fossils, is not accompanied by a consistent change in granule morphology.