Holocene sedimentary environment and sea level significance of sedimentogenic Sr/Ba in Fujian tidal bays

Tidal bays along the Fujian coast lack microorganisms (e.g., foraminifera) and poor fossil preservation, resulting from wide tidal flats and shallow waters. This makes it difficult to rely on the fossils to understand the Holocene sea-level variation course and the sedimentary environment responses in this region. Therefore, this study aims to use the sedimentogenic Sr/Ba ratio index to identify the sedimentary facies in the Sansha Bay, Ningde, Fujian. In addition, AMS ¹⁴C dating was done to explore the sedimentary environment evolution and regional sea-level change since the Holocene. The findings revealed that: 1) The mean value of sedimentogenic Sr/Ba ratio in the tidal flats sediments (9.06) is obviously lower than the surface sediments of Sansha Bay (20.43). However, the sedimentogenic Sr/Ba ratio in tidal flat sediments is significantly higher than those of large estuaries such as the Yangtze River estuary (1~3) where lower freshwater input and higher salinity in the tidal flat, suggesting that the regional hydrological characteristics shall be considered when applying the ratio index for examining the sedimentary facies in sea-land transition zones. 2) The sedimentogenic Sr/Ba ratio of Core NDGK2 sediment of the Late Quaternary could be divided into three layers (Ⅰ-Ⅲ). From the bottom, Layer Ⅰ is variegated hard clay, has a Sr/Ba mean value of 5.29, which is still greater than typical freshwater environments, indicating exposure and alteration of older sediments. Layers Ⅱ and Ⅲ consist of dark gray clay, of which Layer Ⅱ has a mean value of 10.77, similar to the tidal flats facies, while Layer Ⅲ has the greatest mean value of the entire core for 13.44, indicating the bay facies. The AMS ¹⁴C dates indicate the evolution of the fluvial-tidal flat-bay sedimentary facies in Core NDGK2 was controlled by the Holocene sea-level rise. The tidal flat facies was formed at ~8.9 cal.kaBP when the sea level was at 21.0±2.5 m. The maximum flooding surface appeared at ~8.2 cal.kaBP, followed by a long period of low sedimentation rate for 6 ka, and the sedimentation rate in the bay was accelerated again at ~2.2 cal.kaBP. 3) The sedimentogenic Sr/Ba ratios in all 15 cores at Pleistocene-Holocene boundary in the region show obvious tidal flat facies characteristics, indicating the applicability of sedimentogenic Sr/Ba ratio for reconstructing the Holocene relative sea-level change in the region.