Fig. 1

Location of the NK-1 well drilled on Meiji Reef in the Nansha Islands and its regional geological setting. (a) Current coral reef distribution (red dots) (UNEP-WCMC et al, 2008), as well as the locations of the polar ice caps (white), the Tibetan Plateau (brown), and the warm pool (pink). The dashed box indicates the South China Sea and adjacent waters. The black star indicates the NK-1 well, and the numbers denote annual mean sea surface temperature (℃); (b) schematic diagram of the locations of the NK-1 (black star) and XK-1 (yellow star) wells on Yongxing Island in the South China Sea (Yi et al, 2018), and the CK 2 well (16°27'N, 111°43'E) on Chenhang Island in the northern South China Sea (Fan et al, 2020); (c) specific locations of coral reefs (yellow) around the Nansha Islands. The dashed box marks Meiji Reef; (d) detailed location of the NK-1 well on Meiji Reef"

Fig. 2

Display of typical exposure surfaces of coral reefs at various depths in the Meiji Reef NK-1 well. Typical exposure surfaces observed at different depths in the Meiji Reef well in the tropical region during the late Cenozoic are shown in hand specimens (left) and thin sections (right). Sample images at depths of 126.5, 210.7, 359.1, 442.3, 538.0, 558.0, 587.9, 646.2, 699.8, 810.0, and 854.4 m are presented. Red arrows indicate the dissolution boundaries of the coral reefs"

Fig. 3

Comparison of the 87Sr/86Sr ratio versus depth profile from Meiji Reef with the global seawater strontium isotope reference (McArthur et al, 2001; pink line)"

Fig. 4

Lithologic profile and stage division of coral reef development on Meiji Reef in the South China Sea. The lithologic profile of coral reef development on Meiji Reef is displayed, along with the eight stages defined according to changes in minerals, isotopes, and elements. All measured parameters correspond to bulk powder samples, except for the concentrations of major elements (Al2O3 and MgO), which correspond to powder samples washed with deionized water. The figure also shows the relative sea-level data of the Xisha area (redrawn from Shao et al, 2017), the formation times of the Antarctic and Greenland ice caps (formed 23 and 2.6 Ma ago, respectively; Miller et al, 2005), the latest expansion time of the South China Sea (approximately 20 to 16 Ma ago; Sun et al, 2019), the accelerated uplift times of the Tibetan Plateau (21 and 3.6 Ma ago; An et al, 2001), and the formation and development times of the western Pacific warm pool (initially formed around 11 Ma ago and fully developed around 3.2 Ma ago; Jian et al, 2006)"

Fig. 5

Characteristic XRD patterns of minerals in the eight different stages of the Meiji Reef NK-1 well. The lithologic profile on the left side of Fig. 2 provides the geological context for the distribution of these minerals. The legend for the vertical figure is shown in Fig. 2. To more clearly display the mineral composition of each stage, sample curves from different depths were selected from each divided stage of the coral reef. Above these curves, colored numbers are used to label the corresponding minerals: 1 for aragonite, 2 for high-Mg calcite, 3 for low-Mg calcite, 4 for siderite, 5 for dolomite, 6 for chlorite, 7 for illite, 8 for amphibole, 9 for quartz, 10 for microcline, and 11 for albite"

Fig. 6

Normalized depth profile of Meiji Reef, which is adjusted based on the strontium isotope curve (McArthur et al, 2001). (a) normalized depth of Meiji Reef. Blue lines indicate different depths corresponding to the same measured age, whereas brown lines indicate different ages corresponding to the same depth; (b) estimated depths of coral reefs of various ages. In particular, due to the exposure of the coral reef to the atmospheric environment, long-term sedimentary hiatuses occurred, which are marked by black dashed lines and gray shading for intervals exceeding 50 meters"

Fig. 7

Estimated accumulation and loss lengths of coral reefs on Meiji Reef from the CK-2 well on Chenhang Island, Xisha Islands (a) (Fan et al 2020) and the NK-1 well on Meiji Reef, Nansha Islands (b)"

Tab. 1

Interpretation of sedimentary hiatuses and the presence of isolated ages in some layers of Meiji Reef using a staged erosion-deposition model, along with the actual and possible depths of calcareous microfossil and paleomagnetic evidence"

Fig. 8

Conceptual model illustrating the staged erosion-deposition process of Meiji Reef during the Middle-Late Cenozoic, presenting its typical developmental patterns. (a, b) horizontal and vertical profile development of the coral reef are shown respectively. The dashed and solid contour lines represent the erosion and deposition of the coral reefs affected by long-term exposure to atmospheric/aqueous environments and erosion, respectively, which led to sedimentary hiatuses and deposition against the backdrop of gradual tectonic subsidence. The gray line on the right indicates the location of the NK-1 well. In (b), purple arrows indicate tectonic subsidence. The coral reef of Meiji Reef developed on a volcanic basement, and during its formation, the South China Sea region experienced five major erosion periods."