234U/230Th coral growth dating yields reliable ages in restricted basins despite anomalous δ234Ui values

Late Quaternary coral growth ages from uplifted coastal regions, such as marine terraces and associated palaeoshorelines, are an essential tool used to derive tectonic and fault controlled uplift and deformation rates, and thus contribute to seismic hazard analysis and constrain past global sea levels. Fossil coral growth ages are assessed for reliability based upon whether the δ234Ui (or initial activity ratio 234U/238U) matches the present-day oceanic value of ~145 ±1.5‰. However, δ234Ui values outside of this range have been measured from corals at palaeoshoreline locations throughout the world (e.g. Greece, Italy, western USA; Iran) where the coral ages obtained are typically consistent with glacio-eustatic sea-level highstand timing, their stratigraphic/tectonic settings and other age constraints. We explore this controversy with a detailed analysis of coral growth ages from within the semi-restricted Gulf of Corinth, where only 4% of 154 dated corals display δ234Ui within ±10‰ of the open ocean value but appear to have growth ages that agree with highstand timing, stratigraphy and ages from 36Cl exposure dating of wave-cut platforms. We undertook multiple 234U/230Th analyses on individual corallites which are used in combination with analysis of existing coral growth age data and 87Sr/86Sr values to suggest that δ234Ui within the Gulf of Corinth was elevated throughout Marine Isotope Stages 5e-7a (125-200 ka highstands), as a consequence of growth within a basin subject to limited seawater entry during marine highstands and freshwater input from rivers and groundwater combined with episodic fault uplift. Within the Gulf of Corinth, our findings are used to explore the late Quaternary pattern of gulf-wide fault-related uplift, and suggest that where corals from marginal coastal/restricted basins are dated, deviations in δ234Ui from the open ocean value should be carefully considered.