TurtleChron – Developing a Scute Growth Increment Pattern Chronology for Radiocarbon Calibration and Paleoclimatology

TurtleChron aims to provide the proof of concept for a growth increment pattern chronology in tortoise carapaces, which could provide an unexplored climate and isotope archive independent of dendrochronology.

The shell of a turtle consists predominantly of keratin, which contains (as all proteins) carbon. This carbon derives from the atmosphere, and is bound by plants, which then get eaten by the turtle, and eventually deposited in the keratin in its shell. Thus, the growth rings in a turtle shell thus preserved the isotopic environment in the year that it formed. This isotopic signal of each individual ring can be measured using radiocarbon dating, resulting in a curve representing the changes over time.

This project aims to examine this isotopic archive in the turtle shell for the first time. TurtleChron could allow us to identify regional variations in radiocarbon and thus provide regional, which is not only important for reconstructing past climates, but would also allow to build a regional and thus more accurate calibration curves for radiocarbon dating.

As nearly all reptiles and amphibians, turtles and tortoises are ectothermic (“cold-blooded”), which means that they cannot regulate their body temperature, and are thus depended on the environmental conditions. As a result, their growth speeds up in the warmer months of spring and summer, and slows down in winter. Much like tree rings in the cross-section of the trunk of a tree, this seasonally varied growth shows up a distinct line on the carapace (shell) of a turtle, which can also be used to assess the age of the turtle. Depending on the climatic fluctuations over time, these growth lines form a pattern which is shared by individuals which lived in the same conditions.

As with tree-rings, this shell-growth pattern can be matched between different individuals, if their lifespan overlapped sufficiently. This allows to build a master sequence, starting with a living turtle, and going back in time with turtle shells found in herpetological collections and archaeological contexts.