40Ar/39Ar geochronology of Roman volcanic province tephra in the Tiber River valley: Age calibration of middle Pleistocene sea-level changes
The close proximity of the Roman volcanic province to the Tyrrhenian Sea coastline provides a unique opportunity to combine clastic stratigraphy with 40 Ar/ 39 Ar geochronology to constrain the timing of Pleistocene sea-level oscillations. The main eruptions from the Monti Sabatini volcanic district occurred during the interval 560–280 ka, and the Alban Hills volcanic district main eruptions span 560–350 ka. The interfingering of volcanics from these two centers with fluvial and shallow-marine sediments of the Tiber River and delta provides a datable relative sea-level record for this portion of middle Pleistocene time. We calculate the timing of glacial terminations using analytical errors only, then assess age uncertainties that include analytical plus systematic errors; the latter is required to compare 40 Ar/ 39 Ar ages with those from other dating methods. Terminations III, V, and VI occur at 278 (261, 285) ka (95% confidence interval), 430 (422, 442) ka, and 534 (520, 541) ka, respectively, when only analytical uncertainties are used to calculate the ages of bracketing volcanic horizons. The confidence interval expands significantly when full external errors are considered, with predicted ages of 276 (258, 289) ka, 430 (416, 448) ka, and 533 (512, 548) ka for the terminations. The resultant 40 Ar/ 39 Ar chronology is generally consistent with the deep-sea δ 18 O record of sea-level change tuned to Earth9s obliquity cycle for glacial terminations VI, V, and III. In addition, the 40 Ar/ 39 Ar constrained Tiber River delta sea-level record has the added benefit of identifying when coastal sections respond to complex (multistep) terminations.