Fig. 1

(a) Map showing the Nansha Islands in deep southern South China Sea; (b) stratigraphic information of Well NK-1, which includes chronology, lithology, and mineralogy. The red box represents the stratigraphy of the Lower Nanwan Formation"

Fig. 2

Composite plots. (a) Stratum, mineral composition, and porosity; (b) Sr isotopic composition. Sr isotope data (solid blue circle) is from our previously published work (Li et al, 2023) and new Sr isotope data are shown using red open squares; (c) δ13C and δ18O; (d) MgO and CaO contents; (e) Sr content"

Fig. 3

Core photographs and photomicrographs from the Nanwan Formation in Well NK-1. (a) Core photos showing numerous moldic voids around the exposed surface; (b) grainstone with calcite (white) and dolomite (brown). Red solid circle represents the micro-drilling position for Sr isotope analysis; (c) replacive dolomite with coralline algae. The precursor bioclast structure is well preserved, and there is no dolomite cement at the edge of the pore; (d) dolomitic limestone stained with Alizarin red S, showing dolomite cements floating in the calcite; (e) lime dolostone stained with Alizarin red S, showing undolomitized coral skeletons and fine and dirty replacive dolomite with cloudy cores and clear rims; (f) dolomitic limestone stained with Alizarin red S, containing undolomitized foraminifera; (g, h) lime dolostone showing replacive dolomite, dolomite cement at the edge of the void, and undolomitized calcite. The depths specified in the top right of each image are below the present-day sea level"

Tab. 1

Mineral compositions and dolomite stoichiometry of the partially dolomitized carbonates from the lower Nanwan Formation in Well NK-1"

Fig. 4

Backscatter images and EDS spot analysis of dolostones within the Nanwan Formation. (a, b) Backscatter images of dolostones at depths of 455.5 m and 492.0 m. Light gray = calcite; dark gray = dolomite; black = porosity. Dolomite is homogeneous, and foraminifera (F) is undolomitized; (c, d) EDS data of the spot from subplot (a); (e, f) EDS data of the spot from subplot (b)"

Fig. 5

Thin-section photomicrographs and the corresponding cathodoluminescence images. (a, b) Fabric-destructive dolostone showing replacive dolomite with reddish orange luminescence and limpid dolomite cements with zoned luminescence. DC-Ⅰ = dolomite cement Ⅰ with reddish orange luminescence located in the core of the rhombs, DC-Ⅱ = dolomite cement Ⅱ with dark luminescence. Note that DC-Ⅰ (the core of dolomite cement) has similar luminescence to that of the replacive dolomite. Depth = 443.1 m; (c, d) fine and dirty replacive dolomites. Microcrystalline dolomite shows reddish orange luminescence, whereas dolomite cement shows darker luminescence. RD = replacive dolomite, DC = dolomite cement"

Tab. 2

Sr isotope ages and in situ U-Pb ages of calcite (coral skeleton) and replacive dolomite from two samples"

Fig. 6

Cross-plot of Sr isotope ages and in situ U-Pb ages at depths of 493.0 m and 494.3 m in Well NK-1. Two U-Pb ages of dolomite have a good fit with 87Sr/86Sr isotope ages; (b, c) harmonic diagram of U-Pb dating in the dolomite zone from two samples. The inset in the bottom left corner of each image shows ablation points of U-Pb dating"

Fig. 7

Composite plots. (a) PAAS-normalized REY distribution patterns from calcite, replacive dolomite, and dolomite cement in partially dolomitized carbonates in Unit II using laser ablation; (b) carbon and oxygen isotopic compositions of limestone and dolostone"

Tab. 3

In situ elemental composition (average Mg/Ca ratio and Mn, Fe, and Sr contents, Mn/Sr) using the laser ablation technique"

Fig. 8

In situ element composition using laser ablation. (a, b) Mg/Ca ratios (mol/mol); (c, d) Mn contents; (e, f) Sr contents. The numbers marked in the figure represent the corresponding element contents or element ratios"

Fig. 9

The distribution curves of 87Sr/86Sr values and ages of dolostones in the Nanwan Formation using class intervals of 0.00001 (a) and 0.00002 (b)"

Fig. 10

Dolomitization stages of the Nanwan Formation and sea level history. (a) The mineralogy and Sr isotope of the lower Nanwan Formation in Well NK-1. Sr isotope data (solid blue circle) is from Li et al (2023) and new Sr isotope data are shown using red square. (b) Dolomitization age of the Nanwan Formation. There are two age intervals of 11.5-8.5 Ma (blue dot) and 8.5-6.0 Ma (red plus), corresponding to two stages of dolomitization. (c) Global sea level curve (Data from Miller et al, 2020). (d) The sea level variation in the SCS inferred from the organic proxy (BIT) from Well XK-1 drilled in the Xisha Islands (Data from Shao et al, 2017)"

Fig. 11

Schematic diagrams showing the dolomitization process in the lower Nanwan Formation on the Meiji Island. (a-c) Large-scale diagenetic evolution; (d-f) microscopic evolution pathway"