Fig. 1

Geographical location of Well NK-1"

Fig. 2

Depth-dependent characteristics of reefal carbonate rocks from the NK-1 well. (a) Mineralogical composition and depth of the exposure surface; (b) Mg/Ca ratios; (c) Fe and Mn contents; (d) δ18O and δ13C isotopes (VPDB); (e) Mn/Sr and Fe/Sr ratios; (f) 87Sr/86Sr ratios (showing a monotonic increase); (g) numerical ages based on Sr isotopes; (h) water-soluble sulfate (WSS) and carbonate-associated sulfate (CAS) contents; (i) δ34SCAS values; (j) δ18OCAS values. Blue and red wavy lines in (f) and (g) indicate two depositional hiatuses. The gray vertical line in (e) represents Mn/Sr = 0.2, the red vertical line in (i) represents S isotope (21.2‰), and the black vertical line in (j) represents O isotope (8.7‰). Mineralogical, Mn, Fe, Sr data, and δ¹³Ccarb-δ¹⁸Ocarb values are from Li et al (2023) and Liu et al (2022)"

Fig. 3

Diagenetic assessment of CAS. (a) Cross-plot of δ13C vs δ18O in reefal carbonates from the NK-1 well; (b) δ34SCAS vs δ18Ocarb; (c) δ34SCAS vs 1/CAS (in mg·kg-1), showing no significant correlation between sulfur isotopic composition and 1/CAS; (d) CAS content vs δ18Ocarb. The dashed lines in different colors in Fig. a represent the carbon-oxygen correlations at different depths"

Fig. 4

(a) Cross-plot of CAS content versus WSS content in reef carbonate samples from the NK-1 well; (b) cross-plot of δ34SCAS versus δ34SWSS for selected samples"

Fig. 5

Effects of meteoric diagenesis and dolomitization on CAS. (a) In meteorically altered samples, CAS content shows no clear correlation with Mg/Ca ratios, whereas dolomitized samples (Mg/Ca > 4) exhibit a weak negative correlation (R2 = 0.29); (b) no significant correlation is observed between δ34SCAS and Mg/Ca, indicating minimal impact of these processes on sulfur isotopic composition"

Appendix Tab. S1

Content of residual water-soluble sulfate and carbonate-bound sulfate, sulfur and oxygen isotope ratios, and Mg/Ca and Mn/Sr ratios of reef carbonate rocks"

Fig. 6

Stratigraphic plots of $ \delta^{34} \mathrm{~S}_{\mathrm{SO}_{4}} $ and $ \delta^{18} \mathrm{~S}_{\mathrm{SO}_{4}} $ values of CAS versus geologic age, based on the preferred age model. Coeval barite $ \delta^{34} \mathrm{~S}_{\mathrm{SO}_{4}} $ and $ \delta^{18} \mathrm{~S}_{\mathrm{SO}_{4}} $ records (black lines) are shown for comparison (Paytan et al, 1998; Kurtz et al, 2003; Turchyn et al, 2006)"

Fig. 7

(a) Cross-plots of δ18OCAS versus Mn/Sr ratios; (b) cross-plots of δ18OCAS versus δ34SCAS"

Fig. 8

Differences between bulk CAS and coeval barite in $ \delta^{34} \mathrm{~S}_{\mathrm{SO}_{4}} $ and $ \delta^{18} \mathrm{~S}_{\mathrm{SO}_{4}} $ for reefal carbonates. The gray area in the upper right quadrant represents the expected range for carbonate recrystallization in contact with isotopically evolved pore fluids, and the gray area on the left indicates the expected range for sulfide oxidation (Rennie and Turchyn, 2014)"