Syndepositional formation of Fe-rich clays in tropical shelf sediments, San Blas Archipelago, Panama

Solid-phase chemistry of modern carbonate–siliciclastic marine sediments from the San Blas Archipelago, Panama, reveals the occurrence of an Fe–Mg authigenic clay facies that is dominated by a ∼7.1-Å ferric clay mineral. These Fe-rich clays are highly susceptible to HCl-attack and can comprise a large fraction (up to 5.4 wt.% Fe) of the sediment. Some of the clays are clearly forming in the marine environment as they coat grains, infill carbonate micropores, and replace faecal pellets and carbonate shells. The mode of occurrence, structure, and ferric nature of the clay assemblage identifies the facies as verdine, which has odinite, a dioctohedral–trioctohedral Fe3+-rich 1:1 clay mineral as its key member. rdine facies in San Blas occurs in suboxic sediments where iron reduction can be a prevalent organic carbon oxidation pathway. Sediment radiocarbon ages show that the San Blas verdine clays formed within the last few thousand years, and in some cases, the clays have been extensively converted into pyrite. The verdine clays most likely grew by incorporating terrestrially derived Fe and Al, seawater Mg, and pore water Si derived from biogenic opal dissolution. Similar clay formation processes may be common in other tropical nearshore environments where sediments are derived from highly weathered lateritic soils and may prove to be an important oceanic elemental sink.