Iodine diagenesis in pelagic deep-sea sediments

Abstract

Nineteen sediment cores from the Madeira, Seine, Tagus and Nares Abyssal Plains and the Alboran Sea have been used to evaluate the speciation, fluxes and diagenesis of iodine in the deep sea. The sediments have surficial molar I/C ratios of 10–30 × 10 −4 in excess of previous reported values for planktonic material (~1 × 10 −4). Solid phase I contents decrease exponentially with depth corresponding to decomposition rate constants of 5–260 × 10 −6 yr −1 which vary with the carbon accumulation rate. Iodine species in the pore waters follow a vertical sequence of four zones: 1. a zone of I − production where total dissolved iodine (∑I) concentrations initially increase at the seawater-sediment interface; 2. a zone of I − oxidation where interconversion of I − to IO − 3 occurs; 3. a zone of IO − 3 reduction where interconversion of IO − 3 back to I − occurs which corresponds to the suboxic part of the sediment column; and 4. a further zone of I − production which is confined to the lower anoxic part of the sediment column. Benthic ∑I fluxes in the Madeira Abyssal Plain measured from shipboard incubation experiments and calculated from porewater gradients are similar, averaging 0.55 and 0.36 × 10 −8 μmol cm −2 sec −, respectively. In the surface sediment the observed I enrichment results from a quasi-closed cycle for iodine initially involving release of I − from decomposing marine organic matter followed by rapid removal onto organic matter at the sediment-seawater interface where I/C regeneration ratios of up to 200 × 10 −4 are found, lodate reduction occurs during suboxic diagenesis, after denitrification and before MnO 2 reduction, consistent with the sequence of reactions predicted from the free energy yields for organic matter oxidation. There is some further I − production in the anoxic section of sediments but at much smaller rates than occur during the interfacial diagenetic cycling.