Text S5. Grain size analysis
Evidence of variations in coastal environmental conditions can be inferred from the particle sizes of inorganic fractions of deposits (e.g. Machado, 2016). The particle size composition of a total of 601 sediment samples from the three sites was determined using a Malvern Mastersizer 2000 laser particle size analyzer (data expressed as % of the total), having first been pretreated with H2O2 to remove organic materials and HCl to remove carbonates, and with the addition of sodium hexametaphosphate as a dispersing agent.
The lowermost stratigraphical unit (U1), present in both KHQ-14 (base of core to -20.5 m OD) and XS-1603 (base of core to -8.5 m OD), is medium to fine silt-sized particles. U2 (KHQ-14, -20.85 to -2.65m OD; KhqS, base of the profile sampled to -3.30m OD; XS-1603, -8.54 to -2.50 m OD), overlying U1 and generally comprising a fine, silty deposit. Overall, the deposit has a fine texture, although U2 has a higher sand content at XS-1603 relative to KHQ-14 and KhqS. Median particle size increases in the upper parts of U2 at KHQ-14 and KhqS. U3, presenting at KhqS (-3.30 to -2.35 m OD) and XS-1603 (-2.50 to -2.25 m OD), is a distinctive fine-grained, relatively organic-rich layer containing abundant fragments of wood and pottery and is representative of the “human-affected”, or occupation, phase (Zong et al., 2007) at Kuahuqiao and Xiasun Site. The uppermost stratigraphical unit U4 at all three sites (KHQ-14, -2.65m OD to the top of the core; KhqS, from above -2.35m OD; XS-1603, from above -2.25 m OD), is sandy silt, with relatively high (but varying) sand and a comparatively high median grain size. U4 of KHQ-14 contains intercalations of relatively dark, organic-rich horizons, which are not evident at the other two sample sites. The most distinctive feature of U4 across the three sample sites, particularly at KHQ-14 and XS-1603, is its rhythmitic structure represented by vertically stacked, parallel-bedded, alternating couplets of relatively fine-grained and relatively coarse-grained laminae. Such heterolithic beds are associated with deposition on inter-tidal flats in fluvial deltaic environments with a strong tidal influence (Fan et al., 2015). They likely reflect the strong variation in depositional conditions as the tide ebbs and flows, while an absence of massive sandy layers suggests deposition on the upper part of the tidal-flat (Fan et al., 2015).