The new IntCal20 radiocarbon record continues decades of successful practice by employing one calibration curve as an approximation for different regions across the hemisphere. Here we investigate three radiocarbon time-series of archaeological and historical importance from the Mediterranean-Anatolian region, which indicate, or may include, offsets from IntCal20 (~0-22 14C years). While modest, these differences are critical for our precise understanding of historical and environmental events across the Mediterranean Basin and Near East. Offsets towards older radiocarbon ages in Mediterranean-Anatolian wood can be explained by a divergence between high-resolution radiocarbon dates from the recent generation of accelerator mass spectrometry (AMS) versus dates from previous technologies, such as low-level gas proportional counting (LLGPC) and liquid scintillation spectrometry (LSS). However, another reason is likely differing growing season lengths and timings, which would affect the seasonal cycle of atmospheric radiocarbon concentrations recorded in different geographic zones. Understanding and correcting these offsets is key to the well-defined calendar placement of a Middle Bronze Age tree-ring chronology. This in turn resolves long-standing debate over Mesopotamian chronology in the earlier second millennium BCE. Last but not least, accurate dating is needed for any further assessment of the societal and environmental impact of the Thera/Santorini volcanic eruption.

Archaeological Service Kanton Thurgau 8510 Frauenfeld Switzerland

Centre for Isotope Research Faculty of Science and Engineering University of Groningen Nijenborgh 6 9747 AG Groningen The Netherlands

Chair of Forest Growth and Dendroecology Institute of Forest Sciences University of Freiburg Freiburg Germany

Cornell Tree Ring Laboratory Department of Classics Cornell University Ithaca NY 14853 USA

Department of Geography Faculty of Science Masaryk University 611 37 Brno Czech Republic

Department of Geography University of Cambridge Cambridge CB2 3EN UK

Global Change Research Institute CAS 603 00 Brno Czech Republic

Institute of Environmental Physics University of Heidelberg 69120 Heidelberg Germany

Laboratory for Ion Beam Physics Swiss Federal Institute of Technology in Zurich 8093 Zurich Switzerland

Research Laboratory for Archaeology School of Archaeology University of Oxford Oxford OX1 3TG UK

Swiss Federal Research Institute WSL 8903 Birmensdorf Switzerland

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The importance of "year zero" in interdisciplinary studies of climate and history