Synchronicity testing for Event Records (SyncER) using log-ratios and tied age-depth models

Determining whether events preserved in different geological sequences formed simultaneously is a key challenge in paleoenvironmental research and the reconstruction of regional event chronologies (e.g., earthquakes, floods, tsunamis, volcanic eruptions, pollution, climate change). Variations in dating techniques, sampled material, age uncertainties, and site-specific effects often introduce substantial ambiguity when integrating multiple depositional records. Here, we present a new framework to robustly test the synchronicity of event horizons using an age log-ratio approach that quantifies timing differences between records. By tying age-depth models through independently constrained and correlated stratigraphic marker horizons (isochrons), such as geochemically identified tephra layers or biomarkers, we improve chronological precision and therefore the ability to assess potential (non-)synchronicity. This provides a statistically robust and reproducible relative dating framework to quantify the model-conditioned probability that candidate event horizons are compatible with synchronous deposition within a user-defined temporal tolerance. The approach is implemented in the R package SyncER (Synchronicity testing for Event Records) and is applicable across disciplines that rely on continuous geological archives, including subaquatic paleoseismology, paleoecology, and paleoclimatology.