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doi:10.2204/iodp.proc.307.201.2009

Results

The results are compiled in Table T1 and presented in vertical profiles (Fig. F1). Instead of element concentrations, Mg/Ca and Sr/Ca are shown in Figure F1. Mg/Ca ratios range from 0.3 to 1.5, Sr/Ca ratios range from 0.6 to 1.9, and the two ratios are inversely correlated (R² = 0.58) (Fig. F2). Incorporation of Sr and Mg to CaCO₃ largely depends on carbonate mineralogy (Morse and Mackenzie, 1990). Sr/Ca ratio is high in the aragonite fraction that likely originated from corals in the case of the mound sediments. On the other hand, Mg/Ca ratio is high in calcite, such as foraminiferal tests. However, the exception is calcareous nannoplankton, which that has low-Mg calcite skeletons (e.g., Stoll et al., 2002).

Carbonate contents fluctuate from 45 to 73 wt%. At the levels where carbonate content is high, the Sr/Ca ratio is also high but the Mg/Ca ratio is generally low (Fig. F2).

The δ¹³C values range from –6.08‰ to –1.91‰, and the δ¹⁸O values range from 0.42‰ to 2.88‰. Where both values are low, the Sr/Ca ratio and carbonate contents are high. These levels correspond to mottled lithified horizons (Fig. F1).

Cross-plots of geochemical properties are shown in Fig. F2. Carbon and oxygen isotopes show a positive correlation (R² = 0.45). Sr/Ca and Mg/Ca ratios represent a negative correlation. The most distinct correlation was obtained between oxygen isotopes and Sr/Ca ratios (R² = 0.72) (Fig. F2), and the correlation coefficient between carbon isotopes and Sr/Ca ratios is also significant (R² = 0.56). These negative correlations are likely reflected by an abundance in the aragonitic coral fraction. It is widely known that skeletons of L. pertusa bring disequilibrium values of both δ¹³C and δ¹⁸O, which are far lower than the equilibrium values (Adkins et al., 2003).

The thin section images and sketch are shown in Figure F3A–F3D. The sediments are mainly composed of clastic corals, mostly L. pertusa, and matrix. In thin section, it was observed that the mottled parts are mainly positioned around coral skeletons (Fig. F3A–F3D). This is consistent with the result that high Sr/Ca ratios correspond to the mottled parts.

We observed that the residues from the mottled parts were black and clearly differ from the white to gray colored residues from the other subsamples. SEM observation identified the black residues as framboidal pyrites. They are spherical aggregates ~10 mm in diameter, which are composed of subcrystals 1~2 mm wide (Fig. F3E). The origin of framboidal pyrite has been debated since Love (1957). However, in our case, activities of sulfate-reducing bacteria (SRB) might be the process inducing precipitation of framboidal pyrites. SRB decompose organic matter by the following reaction,

Therefore, the framboidal pyrites were formed around coral containing organic matter. The bacterial sulfate reduction also increases alkalinity and induces carbonate precipitation. A similar process by SRB has been also observed in lithification of modern stromatolites (Reid et al., 2000; Andres et al., 2006).

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