Abstract Coralline algae are excellent archives for recording paleo-water depth, but to date few studies have used them for long-term paleo-water depth reconstruction. This study focuses on the Plio-Pleistocene section of Well CK2, which was drilled in the northern South China Sea. The section consists of 55.2 m of volcanic basement overlain by 873.55 m of carbonates. The chronological framework of Well CK2 has been well established using magnetostratigraphy and bulk sediment strontium isotope ratios. The Plio-Pleistocene section includes the uppermost 341 m (5.18 Ma). Based on observations of 201 petrologic thin sections, we identified nine coralline algae genera (nongeniculate: Mesophyllum, Spongites, Lithothamnion, Lithoporella, Lithophyllum, and Hydrolithon; articulated: Amphiroa, Jania, and Corallina), and five coralline algal assemblages (Mesophyllum assemblage, Spongites assemblage, Lithothamnion assemblage, Mesophyllum-Lithophyllum assemblage, and Lithoporella-Lithophyllum assemblage). The Mesophyllum assemblage dominates in the interval between 341 and 312 m (5.18–4.36 Ma; early Zanclean) and developed in a water depth between 15 and 25 m. The Spongites assemblage dominates in the interval between 312 and 309 m (4.36–4.28 Ma; middle Zanclean) and is characterized by subspherical rhodoliths (3 to 9 cm in diameter). This assemblage is probably related to the very shallow (0 to 5 m) water of the distal reef flat/reef crest. The Lithothamnion assemblage dominates in the interval between 309 and 200 m (4.28–2.20 Ma) and developed in a reef platform zone at a water depth of more than 25 m. The Mesophyllum- Lithophyllum assemblage dominates in the interval between 200 and 80 m (2.20–1.16 Ma) and developed in a water depth between 15 and 20 m. The Lithoporella-Lithophyllum assemblage dominates in the interval between 80 and 0 m (1.16–0 Ma) and develops in a water depth of 15 m or less. Our paleo-water depths are basically in agreement with global sea-level variations over time, indicating that paleo-water depths in the South China Sea were controlled mainly by eustatic variations. Based on the relationship between coral reef development and water depth, we further divided the Quaternary intervals into four long-term developmental stages. They consist of two catch-up stages: 1) from 5.18 to 4.28 Ma and 2) from 2.2 to 1.16 Ma; a give-up stage from 4.28 to 2.2 Ma; and a keep-up stage from 1.16 Ma to present. Our results confirm that the coralline algae in the coral reefs of the South China Sea can accurately record past water depth variations.
