부산대학교 도서관

In the tropics, continental weathering and erosion are major sources of trace metals towards estuaries and lagoons, where early diagenesis of sediments may influence their mobility and bioavailability. Determining trace metals speciation in tropical sedimentary settings is thus needed to assess their long-term fate and potential threat to fragile coastal ecosystems. In this study, we determined Fe, Ni and S speciation across a shore-to-reef gradient in sediments from the New Caledonia lagoon that receive mixed contribution from lateritic (iron-oxyhydroxides and clay minerals), volcano-sedimentary (silicates) and marine (carbonate) sources. Sulfur K-edge XANES data indicated a major contribution of pyrite (FeS 2) to S speciation close to the shore. However, this contribution was found to dramatically decrease across the shore-to-reef gradient, S mainly occurring as sulfate close to the coral reef. In contrast, Fe and Ni K-edge XANES and EXAFS data indicated a minor contribution of pyrite to Fe and Ni speciation, and this contribution could be evidenced only close to the shore. The major fractions of Fe and Ni across the shore-to-reef gradient were found to occur as Ni- and Fe-bearing clay minerals consisting of smectite (~nontronite), glauconite and two types of serpentines (chrysotile and greenalite/berthierine). Among these clay minerals, greenalite/berthierine, glauconite and possibly smectite, were considered as authigenic. The low contribution of pyrite to trace metals speciation compared to clay minerals is interpreted as a result of (1) a reduced formation rate due to the low amounts of organic carbon compared to the Fe pool and (2) repeated re-oxidation events upon re-suspension of the sediments top layers due to the specific context of shallow lagoon waters. This study thus suggests that green clay authigenesis could represent a key process in the biogeochemical cycling of trace metals that are delivered to lagoon ecosystems upon continental erosion and weathering. Unlabelled Image • Pyrite contribution to Ni and Fe speciation is low and restricted to the sediments close to the shore • Clay minerals are the major host for Fe and Ni across the shore-to-reef gradient • Fe-rich smectite, glauconite, chrysotile and greenalite/berthierine are the major Fe and Ni-bearing clay minerals • Greenalite/berthierine is the most Ni-rich clay mineral and it is considered to have formed in-situ upon early diagenesis • Green clay authigenesis might represent a major process for trace metals cycling in shallow lagoon sedimentary settings [ABSTRACT FROM AUTHOR]