Authigenic pyrite formation and re-oxidation as an indicator of an unsteady-state redox sedimentary environment: Evidence from the intertidal mangrove sediments of Hainan Island, China

Abstract

Two cores of intertidal mangrove sediments from the Tanmen and Qinglan Harbors on Hainan Island, China, were investigated for their geochemical characteristics of carbon, nitrogen, iron and sulfur and the pyrite morphology and framboidal pyrite size distribution.A modified sequential iron extraction procedure revealed extremely high FeHR/FeT ratios (0.81±0.07, n=28). The pyrite results determined by the nitric acid digestion and chromium reduction method show a strong correlation (r=0.91, n=28), indicating that most of the chromium-reducible sulfur is pyrite, whereas the proportion of elemental sulfur is minor. The organic carbon concentrations and the atomic C/N ratios demonstrate that the organic carbon in the mangrove sediments is derived predominantly from higher plants. The chromium-reducible sulfur (CRS) values show a good linear logarithmic correlation with the total organic carbon (TOC), indicating that the process of sulfate reduction increases rapidly with the concentration of TOC at Qinglan Harbor (QL), which has low TOC contents (<5wt%). In contrast, sulfate reduction increases slowly with high TOC (>5wt%) at Tanmen Harbor (TM). These data suggest that pyrite formation at the QL site is controlled by the TOC contents, whereas at the TM site, the primary factor controlling the pyritization process is the supply rate of sulfate. Both sites have significantly high sulfate contents (average 1.67±0.45wt% and 0.80±0.32wt% at Tanmen and Qinglan, respectively), which are isotopically depleted in 34S (average −6.15±7.17‰ and −6.72±7.33‰ at Tanmen and Qinglan, respectively) suggesting that the sulfate is mainly from the reoxidation of reduced sulfides (mainly pyrite) instead of seawater sulfate during burial.The distributions of pyrite textures suggest that the pyrite in the mangrove swamps is formed mainly as framboids and only a few pyrite crystals are formed directly as euhedral crystals. The high mean diameters and standard deviations (7.0±4.6μm and 6.1±4.1μm at Tanmen and Qinglan, respectively) are compatible with previous observations in oxic and dysoxic environments. At the TM site, we observed a notable double-hump phenomenon in the framboid size distribution and a greater number of secondary framboids between the 50 and 100cm layers, suggesting different pyrite growth events than are present at QL site. There are remarkable increases in the sulfur isotopic fractionations between the CRS and the acid-dissolvable sulfate in several layers of the QL site (e.g.,>10‰ at 10, 20, 80 and 110–120cm depths), demonstrating that the reoxidized sulfate could then be re-reduced to create greater 34S depletions in sulfides relative to the acid-dissolvable sulfate. In addition, the small mean diameters and standard deviations of framboid size in these layers are much closer to those of dysoxic sediments (e.g., the Peru margin) where the redox conditions may be suitable for the re-reduction of partial sulfate derived from the reoxidation of reduced sulfur. Generally, these data provide evidence that the sudden redox turnover caused by the oxygen input during ebb tides or bioturbation occurs in both mangrove swamps, leading to a significant reoxidation of reduced sulfurs.