Marine Iodine Research Articles

Feasibility of 129I groundwater dating calibrated by both 81Kr and 4He for the assessment of deep geological repositories in Japan

Iodine-129, which is a promising tracer for dating old groundwater, has been used as a tracer for deep upwelling groundwater. The nuclide is expected to be one of the key factors for site selection for high-level radioactive waste disposal, which is a global societal issue. The pre-anthropogenic 129I/127I ratio for marine iodine is (1.50 ± 0.15) × 10−12, which could be considered the initial value for 129I dating. This study identifies the challenges in groundwater age dating using 129I/127I. We measured the ratios of 129I/127I and 81Kr/Kr and concentration of 4He in groundwater from boreholes on the northern coast of Japan. The 129I dating results were not coincident with the other groundwater dating results. The iodine in the groundwater was inferred to be released in situ from marine organisms in sediments of various ages. We estimated that the primordial iodine ratio originating from seawater was ~ 1 × 10–13 (8 × 10–14 ~ 2 × 10–13). The groundwater age deduced from the 129I/127I ratio using this value agrees with other groundwater dating results.

Editorial: The marine iodine cycle, past, present and future

Editorial: The marine iodine cycle, past, present and future

Speciation analysis of both inorganic and organic 129I in seawater and its application in the study of the marine iodine cycle.

A complete protocol is presented for the speciation analysis of 129I for both inorganic and organic iodine in seawater using coprecipitation and solid-phase extraction (SPE) combined with accelerator mass spectrometry (AMS). By modifying the iodide separation process and adding a crossover removal step, the improved coprecipitation method significantly reduces the cross-contamination of iodide and iodate to less than 0.05% in the speciation analysis of inorganic 129I, with the separation efficiencies of about 95% and 93% for iodide and iodate, respectively. The SPE-DOI method for the dissolved organic 129I (DO129I) analysis was developed, whereby we report the first direct observation of DO129I/DO127I atom ratios in seawater in this paper. 129I species in seawater from Tokyo Bay were analysed. The 129I results demonstrated that our protocol for speciation analysis of 129I is reliable and provided new insights into understanding the iodine cycle.

The Re/Mo redox proxy reconsidered

Compared to continental crust, marine sediments accumulating beneath O2 depleted waters are enriched in Re and Mo, with Re/Mo ratios that vary by more than two orders of magnitude, a remarkably large range. Deposition of both elements begins simultaneously, and both come to finite aqueous concentration asymptotes together, neither being drawn down to analytical detection limits. To explain these aqueous-phase observations and to reconcile them with laboratory evidence, a previous coprecipitation hypothesis is further developed here. The proposed coprecipitate is a nanoscale sulfide, Fe(Mo,Re)S4 that transforms irreversibly by internal electron transfer to a product represented as Fe(Mo,Re)(S2)S2. Irreversibility preserves information about redox conditions at the time of sedimentation unless sedimentary rocks later become subjected to oxidation or thermal maturation. Compelling support for this mechanism comes from the continuously euxinic Black Sea. A Rayleigh fractionation process characterized by a partition coefficient of 0.38 explains Re/Mo ratios in both its water column and its sediments. On the other hand, in sediments where overlying waters range from oxic to intermittently euxinic, and where Re and Mo deposition occurs only beneath the sediment–water interface, the observed >100-fold range of Re/Mo ratios is too large to be explained solely by Rayleigh fractionation. Instead, it is attributed to modification of pore waters by decomposition of unstable precursory phases. Low solid phase Re/Mo ratios are produced by pore waters modified by dissolution of Mo-rich, Mn,Fe oxyhydroxides; high ratios reflect dissolution of the so-far uncharacterized Re-rich particles that are known to create dissolved Re anomalies in shallow pore waters. Rhenium often follows iodine in biology, suggesting that the Re-rich particles could be biogenic and might participate in the marine iodine cycle. A significant correlation between particulate Re/Mo ratios and biogenic CaCO3 in the Cariaco Basin water column suggests that some of these Re-rich particles could be unstable biocalcification precursors like vaterite and amorphous CaCO3. Compared to δ98Mo, another redox proxy, Re/Mo ratios provide more nuanced redox information, especially under conditions that fall between oxic and sulfidic extremes. Thus Re/Mo ratios are potentially advantageous redox proxies that so far have been underutilized.

Oxidation of iodide to iodate by cultures of marine ammonia-oxidising bacteria

Reaction with iodide (I−) at the sea surface is an important sink for atmospheric ozone, and causes sea-air emission of reactive iodine which in turn drives further ozone destruction. To incorporate this process into chemical transport models, improved understanding of the factors controlling marine iodine speciation, and especially sea-surface iodide concentrations, is needed. The oxidation of I− to iodate (IO3−) is the main sink for oceanic I−, but the mechanism for this remains unknown. We demonstrate for the first time that marine nitrifying bacteria mediate I− oxidation to IO3−. A significant increase in IO3− concentrations compared to media-only controls was observed in cultures of the ammonia-oxidising bacteria Nitrosomonas sp. (Nm51) and Nitrosoccocus oceani (Nc10) supplied with 9–10 mM I−, indicating I− oxidation to IO3−. Cell-normalised production rates were 15.69 (±4.71) fmol IO3− cell−1 d−1 for Nitrosomonas sp., and 11.96 (±6.96) fmol IO3− cell−1 d−1 for Nitrosococcus oceani, and molar ratios of iodate-to-nitrite production were 9.2 ± 4.1 and 1.88 ± 0.91 respectively. Preliminary experiments on nitrite-oxidising bacteria showed no evidence of I− to IO3− oxidation. If the link between ammonia and I− oxidation observed here is representative, our ocean iodine cycling model predicts that future changes in marine nitrification could alter global sea surface I− fields with potential implications for atmospheric chemistry and air quality.

Marine iodine emissions in a changing world.

Iodine is a critical trace element involved in many diverse and important processes in the Earth system. The importance of iodine for human health has been known for over a century, with low iodine in the diet being linked to goitre, cretinism and neonatal death. Research over the last few decades has shown that iodine has significant impacts on tropospheric photochemistry, ultimately impacting climate by reducing the radiative forcing of ozone (O3) and air quality by reducing extreme O3 concentrations in polluted regions. Iodine is naturally present in the ocean, predominantly as aqueous iodide and iodate. The rapid reaction of sea-surface iodide with O3 is believed to be the largest single source of gaseous iodine to the atmosphere. Due to increased anthropogenic O3, this release of iodine is believed to have increased dramatically over the twentieth century, by as much as a factor of 3. Uncertainties in the marine iodine distribution and global cycle are, however, major constraints in the effective prediction of how the emissions of iodine and its biogeochemical cycle may change in the future or have changed in the past. Here, we present a synthesis of recent results by our team and others which bring a fresh perspective to understanding the global iodine biogeochemical cycle. In particular, we suggest that future climate-induced oceanographic changes could result in a significant change in aqueous iodide concentrations in the surface ocean, with implications for atmospheric air quality and climate.

Archean to Paleoproterozoic seawater halogen ratios recorded by fluid inclusions in chert and hydrothermal quartz

AbstractPast changes in the halogen composition of seawater are anticipated based on the differing behavior of chlorine and bromine that are strongly partitioned into seawater, relative to iodine, which is extremely depleted in modern seawater and enriched in marine sediments due to biological uptake. Here we assess the use of chert, a chemical sediment that precipitated throughout the Precambrian, as a proxy for halide ratios in ancient seawater. We determine a set of criteria that can be used to assess the primary nature of halogens and show that ancient seawater Br/Cl and I/Cl ratios can be resolved in chert samples from the 2.5 Ga Dales Gorge Member of the Brockman Banded Iron Formation, Hamersley Group, Western Australia. The values determined of Br/Cl ~2 × 10-3 M and I/Cl ~30 × 10-6 M are comparable to fluid inclusions in hydrothermal quartz from the 3.5 Ga North Pole area, Pilbara Craton, Western Australia, that were the subject of previous reconstructions of ancient ocean salinity and atmospheric isotopic composition. While the similar Br/Cl and I/Cl values indicate no substantial change in the ocean halide system over the interval 2.5–3.5Ga, compared to modern seawater, the ancient ocean was enriched in Br and I relative to Cl. The I/Cl value is intermediate between bulk Earth (assumed chondritic) and the modern seawater ratio, which can be explained by a smaller organic reservoir because this is the major control on marine iodine at the present day. Br/Cl ratios are about 30% higher than both modern seawater and contemporary seafloor hydrothermal systems, perhaps indicating a stronger mantle buffering of seawater halogens during the Archean.

Surface Inorganic Iodine Speciation in the Indian and Southern Oceans From 12°N to 70°S

Marine iodine speciation has emerged as a potential tracer of primary productivity, sedimentary inputs, and ocean oxygenation. The reaction of iodide with ozone at the sea surface has also been identified as the largest deposition sink for tropospheric ozone and the dominant source of iodine to the atmosphere. Accurate incorporation of these processes into atmospheric models requires improved understanding of iodide concentrations at the air-sea interface. Observations of sea surface iodide are relatively sparse and are particularly lacking in the Indian Ocean basin. Here we examine 127 new sea surface (≤10 m depth) iodide and iodate observations made during three cruises in the Indian Ocean and the Indian sector of the Southern Ocean. The observations span latitudes from ∼12°N to ∼70°S, and include three distinct hydrographic regimes: the South Indian subtropical gyre, the Southern Ocean and the northern Indian Ocean including the southern Bay of Bengal. Concentrations and spatial distribution of sea surface iodide follow the same general trends as in other ocean basins, with iodide concentrations tending to decrease with increasing latitude (and decreasing sea surface temperature). However, the gradient of this relationship was steeper in subtropical waters of the Indian Ocean than in the Atlantic or Pacific, suggesting that it might not be accurately represented by widely used parameterizations based on sea surface temperature. This difference in gradients between basins may arise from differences in phytoplankton community composition and/or iodide production rates. Iodide concentrations in the tropical northern Indian Ocean were higher and more variable than elsewhere. Two extremely high iodide concentrations (1241 and 949 nM) were encountered in the Bay of Bengal and are thought to be associated with sedimentary inputs under low oxygen conditions. Excluding these outliers, sea surface iodide concentrations ranged from 20 to 250 nM, with a median of 61 nM. Controls on sea surface iodide concentrations in the Indian Ocean were investigated using a state-of-the-art iodine cycling model. Multiple interacting factors were found to drive the iodide distribution. Dilution via vertical mixing and mixed layer depth shoaling are key controls, and both also modulate the impact of biogeochemical iodide formation and loss processes.

Global sea-surface iodide observations, 1967\u20132018

The marine iodine cycle has significant impacts on air quality and atmospheric chemistry. Specifically, the reaction of iodide with ozone in the top few micrometres of the surface ocean is an important sink for tropospheric ozone (a pollutant gas) and the dominant source of reactive iodine to the atmosphere. Sea surface iodide parameterisations are now being implemented in air quality models, but these are currently a major source of uncertainty. Relatively little observational data is available to estimate the global surface iodide concentrations, and this data has not hitherto been openly available in a collated, digital form. Here we present all available sea surface (<20 m depth) iodide observations. The dataset includes values digitised from published manuscripts, published and unpublished data supplied directly by the originators, and data obtained from repositories. It contains 1342 data points, and spans latitudes from 70°S to 68°N, representing all major basins. The data may be used to model sea surface iodide concentrations or as a reference for future observations.

Experimental observations of marine iodide oxidation using a novel sparge-interface MC-ICP-MS technique

Iodine speciation is used as a redox indicator and water mass tracer. However, the kinetics and mechanisms of oxidation of the reduced iodine species, iodide (I−), in seawater remain largely unresolved. Here, we provide constraints on I− oxidation rates from seawater incubation experiments using a sensitive radiotracer–incubation approach with high-precision mass spectrometry. Samples are inoculated with iodine-129 as I−, incubated under controlled conditions, then oxidized and reduced iodine species are isolated using liquid chromatographic separation. Samples are subsequently aspirated using a novel sparging interface, and the 129I/127I ratio of both iodine species is determined via multi-collector inductively-coupled plasma mass spectrometry. This highly-sensitive analytical approach is optimized for estimating the predicted slow rates of iodine redox transformations in small volume seawater samples at marine-relevant total iodine concentrations. Application of this method to coastal samples from Martha's Vineyard Sound, USA, evidences the heretofore-elusive oxidation of I− under marine-analogue conditions. Our results demonstrate an increase in the 129I/127I of aqueous oxidized iodine species in unamended seawater, most likely due to formation of hypoiodous acid and then dissolved organic iodine, at rates of 118–189 nM/ yr. These rates are comparable to those inferred from previous open ocean mass balance approaches but have the benefit of directly linking iodine speciation changes to in situ I− oxidation. Though our treatment experiments do not definitively isolate the oxidation mechanism, they provide preliminary insight into important roles for both abiotic and biotic factors as well as iodine transformations between dissolved, particulate, and gaseous forms. Ultimately, the derived rates and novel experimental technique have potential to improve marine iodine cycling models and provide constraints on the importance of in situ processes in setting patterns of iodine speciation.

The distribution of iodide and iodate in anchialine cave waters — Evidence for sustained localised oxidation of iodide to iodate in marine water

A study of inorganic iodine speciation in the waters of seven Croatian coastal caves is described. These are anchialine caves as they are connected hydraulically with Adriatic Sea surface water, with the tide inside the cave rising and falling with that outside, but replenishment of the water is restricted by the karst rock. In effect, the water in the cave probably acts more like a piston, and although moving slightly vertically, has a long residence time compared to a fully-flushing cave. Anchialine environments display a number of unusual features, e.g., a well-developed pycnocline, hypoxia and endemic fauna. Iodate and iodide were determined by differential pulse voltammetry and cathodic stripping square wave voltammetry, respectively. Low iodide concentrations (< 10 nM) have been consistently identified in the bottom water of the caves where concentrations of 90–100 nM would ordinarily be expected from intrusion of Eastern Adriatic surface seawater. Where total inorganic iodine concentrations behave conservatively with salinity the loss of the iodide implies oxidation to iodate. As iodide oxidation remains one of the enduring academic problems of the marine iodine system the study of iodine in anchialine caves may help unravel it. Iodate reduction was observed in mid-water, at the halocline, and mechanisms for the reduction involving either respiration or chemolitho-autotrophic bacteria are considered. The respiration mechanism is favoured because of enhanced alkalinity found in the near surface waters of the caves.

Studies with natural and anthropogenic iodine isotopes: iodine distribution and cycling in the global environment

Iodine is an important trace element in geological and biological processes. We summarize here recent results and new data of experiments and observations carried out to improve the understanding of concentration levels and behavior of natural and anthropogenic iodine nuclides in the global environment. The distribution of stable iodine in the Earth’s crust was estimated using concentration data in a suite of representative samples and the influence of subduction on the marine iodine cycle was investigated using 129I systematics on iodine-rich brines from Japan. The importance of microorganisms for the natural iodine cycle is shown in recent studies of iodine sorption on soil and of iodine volatilization from terrestrial and marine environments. Levels of anthropogenic 129I were measured in samples collected around a spent fuel reprocessing plant in Japan.

On the possibility of iodide oxidation in the near-surface of the Black Sea and its implications to iodine in the general ocean

Iodide oxidation to iodate in near-surface waters of the open oceans is an elusive process, and an unequivocal demonstration of it would simplify modelling of the marine iodine system. In the open ocean, the upward advection of iodate complicates any mathematical treatment of the problem. In this context, the high concentration (0.1 μM) of iodate in the Black Sea surface waters suggested that this Sea might be a place where oxidation might be demonstrated. Hydrologically, the surface waters of the Black Sea appear to be downstream of the deeper waters and, given the latter's anoxicity, the surface waters seemed likely to gain most of their iodine as iodide by upward advection. To test this further, prior to experimentation, an iodine budget for the near-surface waters, based upon the latest hydrological model of the Sea was prepared; this predicts a minimum oxidation flux of 3.89×10 −4 mol I m −2 a −1. The chemistry of this oxidation is discussed in the light of existing knowledge of the sulfide system. It is argued that as the redox potential of the IO 3 −/I − and I 2/I − couples at pHs typical of the Black Sea (7.75) are much higher than that of the sulfate–sulfide couple, iodide is probably oxidized in the near-surface domain. This contrasts with sulfide oxidation in the suboxic zone. The possible role of nitrifying bacteria in the oxidation is discussed.

Variations in [formula omitted] ratios in recent marine sediments: evidence for a fossil organic component

The value of the pre-bomb 129 I/ 127 I ratio in the hydrosphere is important for the use of this isotope system for dating and tracing purposes. In order to determine this value and to examine spatial variability, 129 I/ 127 I ratios were measured in 25 sediment samples from five cores taken at geographically distinct coastal areas. The results indicate a pre-anthropogenic 129 I/ 127 I ratio of 1500×10 −15, in good agreement with values derived from previous sediment and groundwater studies. Results from shallow sediments show that the layer of bioturbation is dominated by the addition of anthropogenic 129 I . The good agreement between results for this layer from different cores supports the understanding that marine iodine is isotopically homogeneous and associated with organic material which is actively remineralized at the sediment–water interface. Lower ratios (down to 354×10 −15, corresponding to an age of 32.5 Ma) and larger variations than expected were found in sections of the cores below the layer of bioturbation. These observations can be explained by the presence of refractory fossil organic material, transported by rivers to the sediments from uplifted, continental shales. The results suggest that marine iodine is present in two forms, a labile component which is isotopically homogeneous, and a refractory component which is associated with kerogen (or other fossil organic compounds) whose 129 I/ 127 I ratio reflects the age of these compounds. 129 I/ 127 I ratios can be used to identify and date fossil organic material in Recent sediments, the presence of which can alter the assumptions underlying models of the global carbon cycle.

Potential impact of iodine on tropospheric levels of ozone and other critical oxidants

A new analysis of tropospheric iodine chemistry suggests that under certain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests that modest shifts could result in the critical radical ratio HO2/OH. This analysis is based on the first ever observations of CH3I in the middle and upper free troposphere as recorded during the NASA Pacific Exploratory Mission in the western Pacific. Improved evaluations of several critical gas kinetic and photochemical rate coefficients have also been used. Three iodine source scenarios were explored in arriving at the above conclusions. These include: (1) the assumption that the release of CH3I from the marine environment was the only iodine source with boundary layer levels reflecting a low‐productivity source region, (2) same as scenario 1 but with an additional marine iodine source in the form of higher molecular weight iodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurring in a region of high biological productivity. Based on one‐dimensional model simulations, these three source scenarios resulted in estimated Ix (Ix = I + IO + HI + HOI + 2I2O2 + INOx) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningful enhancements in O3 destruction estimated. Total column O3 destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv Ix source scenario to be more typical of the tropical marine environment; however, for specific regions of the Pacific (i.e., marine upwelling regions) and for specific seasons of the year, much higher levels might be experienced. Even so, significant uncertainties still remain in the proposed iodine chemistry. In particular, much uncertainty remains in the magnitude of the marine iodine source. In addition, several rate coefficients for gas phase processes need further investigating, as does the efficiency for removal of iodine due to aerosol scavenging processes.

Iodine in inshore and off-shore marine waters

The temporal variation of iodate and total iodine in the Menai Straits and Irish Sea is discussed together with station data for the Atlantic Ocean. Surprisingly little temporal variation was found in either iodate or total iodine even though seasonal nutrient cycling occurred within these water bodies. The iodate concentrations of the temperate Atlantic Ocean, Irish Sea and Menai Straits surface waters were significantly different. These results, together with the known hydrology of the Menai Straits, make it likely that iodate-reducing substances present in terrestrial run-off will be responsible for relatively low iodate concentrations in shallow-seawaters, such as the Irish Sea. However, it is suggested that the low iodate concentrations of tropical surface waters are more likely to have a biological origin. The possibility that the marine iodine system is open to a thermo-dynamical interpretation, as some workers have suggested, is refuted in the case of temperate surface waters as well as deep ocean waters (>200 m). It is shown that existing analyses indicate that below 200 m at oceanic stations, iodate does not vary significantly with depth.