ShellChron 0.2.8: A new tool for constructing chronologies in accretionary carbonate archives from stable oxygen isotope profiles

This work presents ShellChron, a new model for generating accurate age-depth models for high-resolution paleoclimate archives, such as corals, mollusk shells and speleothems. Reliable sub-annual age models form the backbone of high-resolution paleoclimate studies. In absence of independent sub-annual growth markers in many of these archives, the most reliable method for determining the age of samples is through age modelling based on stable oxygen isotope or other seasonally controlled proxy records. ShellChron expands on previous solutions to the age model problem by modelling seasonal variability in the proxy record using a combination of growth rate and temperature sinusoids in sliding window approach. This new approach creates smoother, more precise age-depth relationships for multi-annual proxy records with the added benefit of allowing assessment of the uncertainty on the modelled age. The modular script of ShellChron allows the model to be tailored to specific archives, without being limited to δ18Oc proxy records or carbonate archives, with high flexibility in assigning the relationship between the input proxy and the seasonal cycle. The performance of ShellChron in terms of accuracy and computation time is tested on a set of virtual seasonality records and real coral, bivalve and speleothem archives. The result shows that several key improvements in comparison to previous age model routines enhance the accuracy of ShellChron on multi-annual records while limiting its processing time. The current full working version of ShellChron enables the user to model the age of a 10-year long high-resolution (16 samples/yr) carbonate records with monthly accuracy within one hour of computation time on a modern personal computer. The model is freely accessible on the CRAN database and GitHub. Members of the community are invited to contribute by adapting the model code to suit their research topics.