Text S4. Chronology
AMS 14C dating was conducted on fragmentary plant of seventeen samples from the key strata of KhqS profile, KHQ-14 and XS-1603 cores, including two seeds and six charcoal fragments from KhqS profile, nine plant fragments from KHQ-14, one charred seed from XS-1603. The plant materials were pretreated by washing in NaOH and HCl and reduced to neutral PH and then converted to graphite. Radiocarbon ages were calculated after measurement in the STAR accelerator at Beta Analytic Inc., Miami, USA. The calibration to calendar dates used theOxCal online software (OxCal v4.4.2) and the INTCAL20 dataset (Reimer et al., 2020). Results of the AMS 14C dating of samples of plant fragments and charred material are summarized in Table S1.
Six dates from U2 (-20.85 to -2.65 m OD) were used as the basis for age-depth modelling for the section of KHQ-14 (Fig. S4). This age-depth model was subsequently used to estimate the age of sedimentary evidence in U2 of KHQ-14. The oldest date of those obtained, 9227 ± 105 cal BP, is for a sample just above the boundary between U1 and U2 (Fig. 3), indicating that U1 corresponds with the First Hard Clay Layer, which is of Last Glacial age. The age-depth relationship for KHQ-14 dates the boundary between U2 and U4 at -2.65 m OD, or the switch from an estuary mud to a tidal-flat deposit, at 7,687±70 cal BP. This is almost the same date as that for the onset of tidal-flat conditions at KhqS (the transition is dated by two bracketing AMS 14C dates, both of which yielded a calibrated age of 7617±50 cal BP (Fig. 4). The same transition in the far less precisely dated XS-1603 occurred sometime after the calibrated 14C AMS date of 7811±68 cal BP (Fig.S5).