ShellChron 0.4.0: 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 internal age 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 the 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 modeling based on stable oxygen isotope or other seasonally controlled proxy records. ShellChron expands on previous solutions to the age model problem by fitting a combination of a growth rate and temperature sinusoid to model seasonal variability in the proxy record in a sliding window approach. This new approach creates smoother, more precise age-distance relationships for multi-annual proxy records with the added benefit of allowing assessment of the uncertainty in the modeled age. The modular script of ShellChron allows the model to be tailored to specific archives, without being limited to oxygen isotope 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, mollusk, 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 10-year-long high-resolution (16 samplesĝyr-1) carbonate records with monthly accuracy within 1ĝh of computation time on a 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 and encouraged to cite the original work of Judd et al. (2018) alongside this work when using ShellChron in future studies.