The calibrated ages are calculated using known-age tree ring radiocarbon measurements (INTCAL13) utilizing the OxCal program Version 4.2 (http://c14.arch.ox.ac.uk). Samples were prepared at the Tuross Laboratory at Harvard University, and the δ¹³C values were measured in continuous flow using a Costech ECS 4010 elemental analyzer coupled to a Thermo Delta Plus XP mass spectrometer and reported in ‰ relative to PDB. Accelerator mass spectrometry was performed at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) at the Woods Hole Oceanographic Institute in Woods Hole, Massachusetts.