Pyritization of iron in tropical coastal sediments: Implications for the development of iron, sulfur, and carbon diagenetic properties, Saint Lucia, Lesser Antilles

Pore water chemistries and rates of pyritization in tropical coastal sediments were investigated in two semi-protected bays, Praslin Bay and Savannes Bay, along the eastern side of the Caribbean island of Saint Lucia. Sediments were collected from deltaic, nearshore non-deltaic, and offshore environments and 210Pb-derived sedimentation rates ranged between 0.1 and 1.7 g/cm2/yr. Sediments were mixed carbonate-siliciclastic (2–44% CaCO3) and had higher average FeHR/FeT ratios (0.4–0.6) than either global averaged continental margin sediments (0.28) or local bedrock and soils (0.11–0.15). Organic carbon concentrations, C/N ratios, and δ13COC measurements demonstrated that the organic carbon in offshore Savannes Bay sediments was dominantly marine in origin while the nearshore Savannes Bay and offshore Praslin Bay sediments contained organic carbon from both marine and terrestrial sources. Organic matter in Praslin Bay deltaic sediments and in organic-rich storm deposits was mainly (61–100%) terrestrial in origin.In most sediments, Spy concentrations and DOP values increased from zero to 0.4–1.5 wt.% and 0.18–0.56, respectively, as FeD/FeT and C/S ratios decreased with depth despite limited net sulfate reduction, as marked by lower pore water SO4/Cl ratios and higher alkalinity values relative to overlying seawater. In the upper 15 cm of sediment, average DGT (diffusive gradients in thin-films)-sulfide fluxes were similar for all environments (5–8 nmol/cm2/d), except for Savannes Bay nearshore sediments, which had a much higher average flux (76 nmol/cm2/d). Average DGT-iron fluxes were 5–13 times average DGT-sulfide fluxes except in Savannes Bay nearshore sediments where the rates were comparable. Savannes Bay nearshore sediments were also notable for possessing the greatest organic carbon (22 mg/cm2/yr) and pyrite sulfur (9.7 mg/cm2/yr) burial rates and represented the only environment where FeDGT- and SDGT-based pore water compositions reached saturation with amorphous FeS or mackinawite. In all other environments, sulfide production and likely iron-sulfide formation preferentially occurred in microniche environments. Most δ34Spy values (− 22 and − 34‰ VCDT) indicated that the supply of sulfate was not limited during sulfate reduction, though much higher values (up to − 7.8‰) identified pyrite formed in relatively closed system conditions. Sediment burial time vs. ln (FeD/FeT) and C/S values showed that half-lives of FeD sulfidation were 71–159 years and that a normal marine C/S ratio of 2.8 was attained after 22–79 years. A comparison of net rates of pyritization with DGT-iron fluxes demonstrated that iron was recycled less than 1.9 times before final burial. Overall, pyrite sulfur burial rates correlated linearly with organic carbon burial rates, providing further evidence that pyrite formation was primarily controlled by the deposition of organic carbon.