Distributions Of Iodate Research Articles

Meridional Survey of the Central Pacific Reveals Iodide Accumulation in Equatorial Surface Waters and Benthic Sources in the Abyssal Plain

AbstractThe distributions of iodate and iodide were measured along the GEOTRACES GP15 meridional transect at 152°W from the shelf of Alaska to Papeete, Tahiti. The transect included oxygenated waters near the shelf of Alaska, the full water column in the central basin in the North Pacific Basin, the upper water column spanning across seasonally mixed regimes in the north, oligotrophic regimes in the central gyre, and the equatorial upwelling. Iodide concentrations are highest in the permanently stratified tropical mixed layers, which reflect accumulation due to light‐dependent biological processes, and decline rapidly below the euphotic zone. Vertical mixing coefficients (Kz), derived from complementary 7Be data, enabled iodide oxidation rates to be estimated at two stations. Iodide half‐lives of 3–4 years show the importance of seasonal mixing processes in explaining north‐south differences in the transect, and also contribute to the decrease in iodide concentrations with depth below the mixed layer. These estimated half‐lives are consistent with a recent global iodine model. No evidence was found for significant inputs of iodine from the Alaskan continental margin, but there is a significant enrichment of iodide in bottom waters overlying deep sea sediments from the interior of the basin.

The Distribution and Redox Speciation of Iodine in the Eastern Tropical North Pacific Ocean

AbstractThe distributions of iodate (IO3−), iodide (I−), nitrite (NO2−), and oxygen (O2) were determined on two zonal transects and one meridional transect in the Eastern Tropical North Pacific (ETNP) in 2018. Iodine is a useful tracer of in situ redox transformations and inputs within the water column from continental margins. In oxygenated waters, iodine is predominantly present as oxidized iodate. In the oxygen deficient zone (ODZ) in the ETNP, a substantial fraction is reduced to iodide, with the highest iodide concentrations coincident with the secondary nitrite maxima. These features resemble ODZs in the Arabian Sea and Eastern Tropical South Pacific (ETSP). Maxima in iodide and nitrite were associated with a specific water mass, referred to as the 13 °C Water, the same water mass that contains the highest concentrations of iodide within the ETSP. Physical processes leading to patchiness in the 13 °C Water relative to other water masses could account for the patchiness frequently observed in iodide and nitrite, probably reflecting subsurface mesoscale features such as eddies. Throughout much of the ETNP ODZ, iodine concentrations were higher than the mean oceanic value. This “excess iodine” is attributed to lateral inputs from sedimentary margins. Excess iodine maxima are centered within a potential density of 26.2–26.6 kg/m3, a density range that intersects with reducing shelf sediments and is almost identical to the ETSP. Evidently, margin input processes are significant throughout the basin and can influence the nitrogen and iron cycles as well, as in the ETSP.

Preliminary Evidence for Iodate Reduction in Bottom Waters of the Gulf of Mexico During an Hypoxic Event

The distributions of iodate and total inorganic iodine concentrations in the waters on the Texas–Louisiana shelf in April, June, and August 2004 are described. Iodine–salinity graphs show three-end-member mixing involving onshore and offshore surface waters and deep offshore water. The April survey showed simple mixing on the surface, but in the later surveys, iodate concentrations were often much lower than predicted by the mixing curve while those for total inorganic iodine were higher. This demonstrated both iodate reduction in the water and iodide addition, although individual samples did not show equivalent speciation changes. Hydrographically, the system consists of the estuaries of the Mississippi and Atchafalaya rivers as they spill onto the shelf. The waters are stratified seasonally by a robust halocline, leading to hypoxia in the bottom waters from the combined effect of restricted downward diffusion of oxygen and the sinking of the luxuriant growth of phytoplankton induced by riverine nutrient supply. The distributions of iodate and total inorganic iodine are, therefore, interpreted in terms of water–sediment interaction as the shelf shoals to the north.

Dissolved inorganic and organic iodine in the Chesapeake Bay and adjacent Atlantic waters: Speciation changes through an estuarine system

The distributions of iodate, iodide and dissolved organic iodine (DOI) were determined in two deep sub-basins in the Chesapeake Bay, the shallow waters at the mouth of the Bay and the adjacent North Atlantic between the late spring and the early fall along the net flow-path of the water entering and exiting the Chesapeake Bay by using an improved analytical scheme designed for the quantitative recovery of DOI. The concentration of R-DOI found in the surface mixed layer in the upper Bay was about twice of those found at the same location in previous studies. (R-X was the concentration of a dissolved iodine species X that had been normalized to a constant salinity of 35.) Thus, DOI in estuarine waters might have been underestimated significantly in the earlier studies. Following the water along its net flow-path, iodate initially constituted more than 60% of total iodine (TI) in the source water in the Middle Atlantic Bight off the Delmarva Peninsula. As this water entered the Chesapeake Bay through the northern part of its mouth, the concentration of R-iodate decreased while that of R-iodide increased progressively until the former became undetectable in the surface mixed layer while the latter reached a maximum of 0.42 μM in the deep water in the upper Bay. Then, the concentration of R-iodate rebounded while that of R-iodide decreased in the outflowing water that exited through the southern part of the mouth of the Bay and was later entrained by the Gulf Stream. The concentration of R-DOI in the surface waters followed the same pattern as R-iodide and reached a maximum of 0.20 μM in the upper Bay. However, R-DOI was depleted in the deep water in the sub-basins. Its concentration dropped to around the detection limit in the suboxic waters in the upper Bay. R-TI in the Bay far exceeded that in the incoming Middle Atlantic Bight water and reached 0.55 μM in the upper Bay. These distributions of the iodine species suggest that, as water from the Middle Atlantic Bight intruded into the Chesapeake Bay, in the well oxygenated surface mixed layer, iodate was reduced to iodide, and the inorganic iodine species could also be converted to DOI. In the deep water, iodate and DOI were converted to iodide. Superimposed on these inter-conversions among the iodine species, dissolved iodine, possibly in the form of iodide, was also added to the water column from the underlying sediments and the process was especially significant in the suboxic deep water in the upper Bay. Mixing between the surface mixed layer and the deep water could also have increased the concentrations of iodide and total iodine in the former.

The distributions of iodate and iodide in Rogoznica Lake (East Adriatic Coast)

This study was performed in order to obtain information on the influence of an acute anoxic event (September, 1997) on distribution and speciation of inorganic iodine in the water column of a small, intensely eutrophicated salt lake. The variations in iodate and iodide depth distributions during the investigated period (1998–2000) were in accord with seasonal changes in redox conditions. During the stratification period (spring and summer), the concentration ratio between iodate and iodide in the upper layers was high, whereas during late summer and autumn, as a result of water column de-stratification and mixing of highly reducing deep water with the oxic layer, lower ratios and more uniform depth distributions were observed. The massive mortality of lake organisms induced by anoxic conditions and sulphide presence throughout the water column was registered by the end of September 1997, when overturn of the lake occurred. The concentrations of iodate in the oxic upper layers were elevated for more than a year after the mass-mortality event (up to 0.55 μmol L −1), whereas iodide concentrations remained high for more than 2 years in deep anoxic water (up to 2.27 μmol L −1). These data suggest that biogeochemical renewal processes affecting the concentrations of inorganic iodine in the lake water are slow compared to those that govern the speciation of iodine. The role of sediment–water interactions and iodine-rich organic species in the production of iodide are discussed.

Geochemical dynamics of iodine in marginal seas: the southern East China Sea

The distributions of iodate and iodide were determined in a transect across the southern East China Sea Shelf, the upwelling center at the outer shelf-upper slope, the adjoining Kuroshio to the Okinawa Trough. In the surface waters, the variations in the concentrations of iodate and iodide among the surface waters masses, namely, the Coastal Water, the Taiwan Current Warm Water and the Kuroshio Surface Water, were relatively small. The composition of the upwelling Kuroshio Subsurface Water was distinctly different and it could be readily distinguished from the surface water masses by its elevated concentration of iodate, depressed concentration of iodide, and the resulting elevated concentration ratio of iodate to iodide. In contrast, since the variations in salinity and temperature in the surface water masses and the upwelling water were less systematic, neither salinity nor temperature could be used effectively for distinguishing these two types of water from each other. Thus, the iodine system was a complementary, or even a superior, tracer for the upwelling water. In the subsurface, the upwelling water was depicted as a dome of cold, nitrate-rich, iodate-rich and iodide-poor water, with molar ratios of iodate to iodide exceeding 10 that extended from the outer shelf-upper slope to the middle shelf. The influence of the Coastal Water was manifested as a tongue of fresher, iodide-rich and iodate-poor surface water, with molar ratios of iodate to iodide of <2.5 that extended from the inner shelf into the middle shelf. The oligotrophic Kuroshio Surface Water was also iodide-rich and iodate-poor. A box model calculation revealed that iodate was imported into the East China Sea Shelf system by frontal exchanges with the adjoining Kuroshio and upwelling at the shelf edge. Within the Shelf system, iodate was consumed and iodide was produced. As a result, during frontal exchanges between the Shelf system and the Kuroshio, there was also a net export of iodide from the Shelf system to the Kuroshio. These behaviors of the iodine system in the East China Sea Shelf system are similar to those found in the South Atlantic Bight and suggest that the marginal seas may be a preferred site for the reduction of iodate to iodide so that exchanges between the marginal seas and the open oceans may constitute a significant net source of iodide to the ocean interior.

Dissolved iodine in waters overlying and in the Orca Basin, Gulf of Mexico

The distribution and speciation of iodine, a biophilic redox-sensitive trace element, in waters overlying and in the Orca Basin, Gulf of Mexico, which contains hypersaline, anoxic and yet non-sulfide-bearing brine have been determined. The distribution of iodate and iodide in the oxic waters overlying the anoxic brine are similar to those reported in other oceans. However, in the oxic-anoxic mixing zone, iodate disappears while the concentration of iodide reaches a maximum of 8.1 μM, the highest concentration ever reported in open oceans. There is also a maximum in specific iodine of 30.7 nM‰ −1 at this depth. Specific iodine in oxic seawater is only about 10–14 nM ‰ −1. These features may be explained by the preferential dissolution of biogenic particles that have accumulated in a strong pycnocline. In the anoxic brine proper, the concentration of iodide is 3.8 μM and can be explained almost entirely by the simultaneous mobilization of chloride and iodide during the dissolution of evaporite beds as the specific iodine of 14.5 nM‰ −1 is only slightly higher than those observed in the oxic waters.

The marine chemistry of iodine in anoxic basins

The distribution of dissolved iodate and iodide has been determined in two anoxic basins, the Black Sea and the Cariaco Trench; and the oxic Venezuela Basin which serves as a comparison for normal oceanic conditions. In normal oceanic waters, iodate is the predominant species; its concentration is lowest at the surface ( ca. 0.3 μM) and increases with depth to ca. 0.5 μM. In contrast, the iodide concentration shows maximum values in surface waters and rapidly decreases to <0.01 μM below the euphotic zone. In anoxic basins, the reduced pE reverses this trend. The concentration of iodide increases rapidly in the oxygen-sulfide mixing zone from 0.02 to 0.46 μM, and 0.01 to 0.43 μM, in the Cariaco Trench and the Black Sea, respectively. The iodate concentration, meanwhile, decreases to zero. The total iodine to salinity ratio is lower in the surface waters with a range of 7.3–12.1 nmoles/g suggesting a possible depletion by organisms. In the anoxic basins, a maximum in this ratio is observed just above the oxygen-sulfide boundary (15–17 nmoles/g) and is indicative of particle dissolution in a strong pycnocline. In the anoxic zone of the Cariaco Trench, the ratio is constant at 12.3 nmoles/g, whereas in the Black Sea, it increases with depth from 10.0 to 19.3 nmoles/g, suggesting a possible flux of iodide from the bottom sediments. By considering the distribution of iodate and iodide in oxic and anoxic basins, the lower limit of the pE of the oxic ocean is estimated to be 10.7, given our present analytical capability. Thermodynamic considerations further suggest that the iodide-iodate couple is a poor indicator for the pE of the oceans with a limited usable range of 10.0–10.7.

The sorption of iodate and iodide by riverine sediments: its implications to dilution gauging and hydrochemistry of iodine

A study of the sorption of iodate and iodide from solution (40 μg 1 −1 - I) onto fourteen natural river sediments, peat and twelve sediment components showed that in rivers where the suspended sediment load is less than 0.1 gl −1 significant sorption (> 0.5 μg 1 −1) of iodide is unlikely to occur. With iodate there is a greater chance of sorption, particularly on iron hydroxide. The results suggest therefore that iodide should be a reliable tracer during measurement of river flow rates by the dilution technique. Nevertheless, two simple tests which establish whether sorption occurs, are described. Experiments conducted at different ionic strengths (NaCl solution and seawater) suggest that sorptive processes will exert some control over the distribution of iodate and iodide in some estuaries. A mechanism by which the freshwater could be enriched with iodate and iodide from seawater, is proposed. The sorptive behaviour of iodate and iodide on peat conflicts with Kharkar, Turekian and Bertine's contention that where a trace element is adsorbed from freshwater it is always released to a greater or lesser extent on contact with seawater because of displacement by other ions. This paradox is resolved.