Quantitative reconstruction of late Holocene coastal paleoenvironment is essential for understanding the response of land-sea transitional zones to relative sea-level change and climate variability. Diatoms, as environmentally sensitive bioindicators, provide robust proxies for high-resolution paleoenvironmental reconstruction. In this study, we investigated the river-estuary-nearshore continuum along the southeastern coast of Fujian Province, China, and established two independent diatom-based transfer functions, including a diatom-salinity transfer function (DSTF) and a diatom-elevation transfer function (DETF), based on modern surface sediment samples. These transfer functions were subsequently applied to sediment core K5 from the Zhangjiang River estuary to quantitatively reconstruct paleosalinity and paleoelevation variations since the late Holocene. The DSTF was developed using 44 modern surface samples and calibrated with a weighted averaging partial least squares (WA-PLS) model, yielding an optimal cross-validated performance of r²_jack = 0.83 and a root mean square error of prediction (RMSEP) of 3.79. The DETF was constructed using 68 intertidal surface samples, with the optimal model showing r²_jack = 0.40 and an RMSEP of 38.94.
Both transfer functions demonstrate significant statistical relationships between diatom assemblages and environmental variables, indicating reliable predictive capability for paleoenvironmental reconstruction. The age-depth model of core K5 spans approximately 4364-279 cal yr BP, with an average sedimentation rate of 0.57 mm yr⁻¹. The reconstructed results reveal a long-term decreasing trend in paleosalinity, and the paleoelevation fluctuated around the modern sea-level. Furthermore, multiple coarse-grained sand layers and age reversals observed in core K5 suggest that the sedimentary record does not represent a purely continuous depositional sequence, but rather reflects an alternating sedimentation-erosion system controlled by long-term relative sea-level change and superimposed short-term high-energy hydrological events, such as floods, typhoon-induced storm surges, and intensified tidal currents. These event-driven processes significantly modulated the stratigraphic record and produced short-term deviations from the background environmental trends. This study provides new quantitative insights into coastal sediment dynamics along the southeastern coast of China.