Defining Chronostratigraphic Time-Marker Horizons: A Framework for Correlating the Onset of Earth-System Perturbations

The term “event” is widely used in geosciences but encompasses phenomena with highly variable durations, mechanisms, and stratigraphic expressions, limiting its precision for chronostratigraphic correlation. In response, a Working Group of the Subcommission on Stratigraphic Classification (SSC) has developed the concept of Time Markers and their stratigraphic expression, Time-Marker Horizons, defined as the discrete, correlatable onset of Earth-system perturbations recorded in the rock record. This contribution at the STRATI2026 conference is conceived as a discussion forum to evaluate whether formalizing this concept within the International Commission on Stratigraphy (ICS) would be beneficial for improving chronostratigraphic resolution. The proposed framework distinguishes explicitly between the Earth-system perturbation itself, often involving cascading impacts and feedbacks over extended timescales, and its earliest stratigraphic expression, which constitutes the Time-Marker Horizon. Only the latter is relevant for chronostratigraphic purposes. The Working Group has outlined criteria for identifying Time Markers, including a clear physical expression in the stratigraphic record, an effectively isochronous and correlatable onset, a transition that is brief relative to surrounding strata, a distinctive signal, and sufficient geographic extent. These criteria aim to ensure that Time-Marker Horizons represent robust and reproducible correlation levels across multiple sections. The framework is flexible enough to accommodate both globally expressed markers and regionally significant horizons that enhance stratigraphic resolution. However, the introduction of formally recognized Time Markers raises several open questions. Would their formalization within ICS stratigraphy meaningfully improve correlation beyond existing tools such as biozones, magnetostratigraphy, and isotope stratigraphy? How should redundancy with established chronostratigraphic frameworks be avoided? At what scale does the addition of Time Markers enhance rather than complicate stratigraphic practice? And should recurring phenomena (e.g., Quaternary deglaciations) be formalized as multiple Time Markers, or remain within existing stratigraphic schemes? By framing these questions, this contribution invites the stratigraphic community to critically assess the role of Time-Marker Horizons as either a formal stratigraphic tool or a conceptual framework. The goal is not to prescribe a specific outcome, but to stimulate discussion on whether and how such markers could contribute to refining the resolution and utility of the chronostratigraphic record.