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).