Iodine diagenesis in non-pelagic deep-sea sediments

Measurements of sediment geochemistry and porewater speciation have been made using eight cores containing turbidite sections from the Madeira and Nares Abyssal Plains. The results have been used to evaluate how the diagenetic chemistry of iodine in these sediments compares with that in sediments undergoing steady-state diagenesis. The behaviour of iodine is related to the development of a redox front within the turbidite, between the organic-rich anoxic sediment and its oxic cap, and the downward migration of the front through the turbidite with time. In contrast to the steady-state case, sediment I contents and I/ C ratios increase downwards through the oxidised section reaching a maximum at the redox front (up to ~ 100 μ/g I; molar I/C~ 20 × 10⁻⁴) below which values drop dramatically (I/C ~ 5 × 10⁻⁴). A strong iodate enrichment (up to ~3 μmol kg⁻¹) is observed in the oxidised section of the sediment. At the front interconversion of I⁻ and IO⁻₃ species occur and below the front porewater IO⁻₃ is absent and I~ concentrations increase with depth (as in other cases of anoxic diagenesis) up to ~ 10 μmol kg⁻. In the oxidised section of the sediment the I enrichment has been supplied by upward transport of iodide with the increasing I content, with depth being accounted for by progressive diagenetic enrichment with time.     

Dating of Dissolved Iodine in Pore Waters from the Gas Hydrate Occurrence Offshore Shimokita Peninsula,Japan: 129I Results from the D/V Chikyu Shakedown Cruise

Iodine concentration and radioisotopic composition (¹²⁹I/I) were measured in the pore waters from the gas hydrate occurrence in the forearc basin offshore Shimokita Peninsula, north-eastern Japan, to determine the source formation of I and accompanying hydrocarbons. Iodine concentrations correlate well with the alkalinity and SO₄ patterns, reflecting degradation stages of I-rich buried organic matter, rapidly increasing in the sulfate reduction interval, and becoming constant below 250 meters below the seafloor with an upwelling flux of 1.5 × 10⁻¹¹ µmol cm⁻² year⁻¹. The ¹²⁹I/I ratios of 300 × 10⁻¹⁵–400 × 10⁻¹⁵ in deep pore waters suggest ages for iodine and hydrocarbon sources as old as 40 Ma. These ages correlate well with the coaly source formations of the Eocene age thought to be responsible for the conventional natural gas deposits underlying the gas hydrate stability zone. Similar profiles are observed in ¹²⁹I/I ratios of pore waters in the gas hydrate stability zone from the forearc basin in the eastern Nankai Trough, offshore central Japan, where pore waters are enriched in I and reach ages as old as ∼50 Ma through the sediment column. At the outer ridge site along the trough, on the other hand, relatively younger I are more frequently delivered probably through thrusts/faults associated with subduction. The nature of source formations of I and hydrocarbons in the offshore Shimokita Peninsula has a more terrestrial contribution compared with those in the Nankai Trough, but these formations are also considerably older than the host sediments, suggesting long-term transport of I and hydrocarbons for the accumulation of gas hydrates in both locations.     

Rock-magnetic characterization of early, redoxomorphic diagenesis in turbiditic sediments from the Madeira Abyssal Plain

Rock‐magnetic measurements of two sediment cores from the Madeira Abyssal Plain (MAP), north Atlantic, are used to investigate post‐depositional changes in the concentration, grain size and composition of magnetic minerals in the sediments that have occurred within organic‐rich turbidite horizons. The changes are associated with an initial stage of suboxic (reductive) diagenesis, following depletion of porewater O₂, and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment, as a result of diffusion of O₂ from the overlying sea water. The turbidites are of late Quaternary age (δ¹⁸O stages 1–3) and derive both from different sites on the NW African continental margin, and from the flanks of the Canary Islands. Thus, the turbidites are variable compositionally, especially in terms of carbonate, detrital magnetic mineral and organic carbon content. Diagenetic changes in these sediments have been identified using solid‐phase geochemical data (U, Mn, C_org and CaCO₃) reported previously in more than one study. Rock‐magnetic parameters of the sediments, when expressed on a carbonate‐free basis, reveal that significant depletion of detrital ferrimagnetic iron (Fe²⁺/Fe³⁺) oxide grains has occurred within organic‐rich turbidites during redoxomorphic diagenesis. Normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size‐selective process, but it has a minimal effect on the canted‐antiferromagnetic Fe³⁺ oxides in the sediment. Such components, if present, therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively, and bulk magnetic concentration is thus depleted. There is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic‐rich turbidites, beneath both active and fossil oxidation fronts. These grains are most probably associated with populations of live magnetotactic bacteria, which commonly inhabit such organic‐rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis. These results show that simple and rapid rock‐magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep‐sea sediments, at least as effectively as more laborious, time‐consuming and sample‐destructive geochemical measurements.     

The geochemistry of iodine in near-shore carbonate sediments

The total concentration of I is commonly higher in surface terrigenous sediments relative to more deeply buried material. Diagenetic release, loss of dissolved I during burial, and back-reaction of I with the solid phase under oxidizing conditions contribute to I enrichment near the sediment/water interface. In order to differentiate between scavenging of dissolved I by organic matter or metal oxides, the diagenetic behavior of I was examined in the Fe-poor carbonate sediments of Florida Bay, Florida. In this environment I is released by organic decomposition at I/C ratios similar to terrigenous environments (~0.5 mmole/mole), transported to the oxygenated sediment/water interface, and lost to the overlying water. The dissolved I flux from these deposits is roughly equivalent to the production rate within the deposit (~10 μmole/m²/day at 28°C). No significant enrichment is observed in the solid phase.Dissolved iodine transport within the sediment column may also be controlled by non-steady-state lateral diffusion into burrows. These observations, together with laboratory experiments which demonstrate IO^?₃ scavenging by Fe-oxyhydroxides at pH ≤ 8, imply that enrichment of I in terrigenous surface sediments results predominantly from the initial oxidation of I^? to IO^?₃ by microorganisms, followed by sorption on Fe oxides. Upon burial and reduction during anaerobic decomposition, this metal-associated I is released to solution, in a manner similar to phosphate.     

Soil column experiments for iodate and iodide using K-edge XANES and HPLC–ICP-MS

Radioactive iodine is one of the most problematic radionuclides because of its long half life and high mobility. Mobility of iodine depends on the chemical form to a great extent. This paper reports the results of soil column experiments we conducted to evaluate the mobilities of IO₃⁻ and I⁻. In order to determine the mechanisms of adsorption of IO₃⁻ and I⁻ on soil, adsorption isotherms were obtained by batch experiments. Both adsorption isotherms of IO₃⁻ and I⁻ are well explained by Langmuir model. The adsorption maximum of IO₃⁻ is about five times larger than that of I⁻. In the column experiments, iodine distributions between soil and pore water in the soil column were determined at various depths. Chemical forms of iodine in soil and pore water were determined by X-ray absorption near-edge structure (XANES), and high performance liquid chromatography connected to inductively coupled plasma mass spectrometry (HPLC–ICP-MS), respectively. Vertical profiles of iodine in pore water were simulated using Visual MODFLOW. Our results showed, upon I⁻ infiltration through the column, that a small amount of I⁻ adsorbed on soil, and its mobility is mainly controlled by advection and dispersion. The profile of iodine concentration in pore water was well simulated by assuming equilibrium-controlled Langmuir type adsorption without considering any chemical transformations. For the IO₃⁻ addition system into the column, however, IO₃⁻ adsorbed to soil to a larger degree, which causes a much larger retardation effect than I⁻. In addition, reduction of IO₃⁻ to I⁻ was also confirmed in both soil and pore water by XANES and HPLC–ICP-MS, respectively. The fraction of I⁻ increased toward the deeper end in both phases because of its lower affinity for soil than IO₃⁻, where the reduced I⁻ was released to the pore water and transported by the water flow. In this study, such reduction effect was clearly demonstrated by the speciation analyses of iodine in both soil and water phases, which confirmed that the mobility of I⁻ is a dominant factor that controls the fate of iodine in the surface environment.     

The contrasting geochemical behaviours of iodine and bromine in recent sediments from the Namibian shelf

The vertical distribution of iodine, bromine and organic carbon has been examined in sediment cores from a range of environments on the Namibian shelf. The relationship between Br and C org. is linear, and that between I and C org. is variable, for all surface sediments; I/C org. ratios show a decrease of about one order of magnitude between the outer shelf oxidising sediments ($\text{250 × 10}{}^{\mathrm{?4}}$) and the organic-rich inner shelf sediments ($\text{20 × 10}{}^{\mathrm{?4}}$). The contrasting behaviour of the halogens in surface sediments is explained by differences in the amount of halogen absorbed by living organisms within the euphotic zone and on seston on the seabed. It is suggested that sorption by seston occurs only in oxidising sediment where free O₂ is available. Hence, iodine is sorbed by seston in the outer shelf environment, but is not sorbed by the reducing sediments of the inner shelf. Here the iodine in the sediment represents only that taken up by plankton. On the outer shelf, 50–80% of the total iodine in the organic matter is sorbed by seston. The principal site of Br uptake is not known.The distribution of C org. in subsurface sediments broadly reflects that found at the surface, although there is a slight decrease with depth in the outer shelf cores due to dilution by terrigenous materials. In the inner shelf cores, there is no change in the relationship of iodine and bromine to organic carbon at depth. Those from the mid shelf, and especially the outer shelf, on the other hand, show decreases in both I/C org. and Br/C org. ratios, reaching values at about 70 cm depth that are similar to those in surface reduced sediments from the inner shelf. Over this depth interval I/C org. ratios decrease by a factor of five while Br/C org. ratios show a two fold decrease. These changes in the ratios at depth imply that diagenesis within the reducing cores is negligible compared with that of oxidised sediments. The implications of diagenesis with regard to halogen recycling in sediments are briefly discussed.     

Effect of confining pressure on the diffusion of HTO, 36Cl− and 125I− in a layered argillaceous rock (Opalinus Clay): diffusion perpendicular to the fabric

The through- and out-diffusion of HTO, ³⁶Cl⁻ and ¹²⁵I⁻ in Opalinus Clay, an argillaceous rock from the northern part of Switzerland, was studied under different confining pressures between 4 and 15 MPa. The direction of diffusion and the confining pressure were perpendicular to the bedding. Confining pressure had only a small effect on diffusion. An increase in pressure from 4 to 15 MPa resulted in a decrease of the effective diffusion coefficient of ∼20%. Diffusion accessible porosities were not measurably affected. The values of the effective diffusion coefficients, D_e, ranged between (5.6±0.4)×10⁻¹² and (6.7±0.4)×10⁻¹² m² s⁻¹ for HTO, (7.1±0.5)×10⁻¹³ and (9.1±0.6)×10⁻¹³ m² s⁻¹ for ³⁶Cl⁻ and (4.5±0.3)×10⁻¹³ and (6.6±0.4)×10⁻¹³ m² s⁻¹ for ¹²⁵I⁻. The rock capacity factors, α, measured were circa 0.14 for HTO, 0.040 for ³⁶Cl⁻ and 0.080 for ¹²⁵I⁻. Because of anion exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than the uncharged HTO. Unlike ³⁶Cl⁻, ¹²⁵I⁻ sorbs weakly on Opalinus Clay resulting in a larger rock capacity factor. The sorption coefficient, K_d, for ¹²⁵I⁻ is of the order of 1–2×10⁻⁵ m³ kg⁻¹. The effective diffusion coefficient for HTO is in good agreement with values measured in other sedimentary rocks and can be related to the porosity using Archie's Law with exponent m=2.5.     

Diel biogeochemical cycling in a hyperventilating shallow estuarine environment

A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemical cycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhom Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (>560 μM O₂: >250% saturation) during sunny days to suboxic conditions (<5 μM) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O₂ concentrations ranged from 346 μM to sustained non-detectable levels in the night hours. In concert with the DO fluctuations, diel phosphate cycling was on the order of 4 μM in response to tidal flushing events and biological assimilation and regeneration. The IO₃ ⁻/I⁻ redox couple quickly responded to suboxic conditions in the water column by a marked increase in I⁻ concentrations and corresponding depletion of IO₃ ⁻. The extreme fluctuations of the p∈ in the water column resulted in diel dissolved Mn²⁺ variations of nearly 5 μM, with observed dissolved Mn removal rates on the order of 1 μM h⁻¹. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments.     

Biomarkers for aerobic methanotrophy in the water column of the stratified Gotland Deep (Baltic Sea)

Filter samples from the oxic zone and suboxic zone of the physically stratified water column and sediment samples of the Gotland Deep, Baltic Sea, were analyzed for bacteriohopanepolyol (BHP) and phospholipid fatty acid (PLFA) concentrations. In total, eight BHPs were identified, with the greatest diversity in the suboxic zone. There, 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol) and 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), whose concentrations decreased concurrently from the lower to the upper suboxic zone, indicated type I methanotrophic bacteria and thus aerobic oxidation of methane. The presence and activity of type I aerobic methanotrophic bacteria was further supported by the presence of ¹³C-depleted PLFAs, specifically 16:1ω8c and 16:1ω5c (δ¹³C as low as −41.2‰). However, the relative amount of methanotroph-specific compounds was low (aminopentol, <0.2% of total BHPs; 16:1ω8c, ca. 0.5% of total PLFAs), suggesting a minor contribution of aerobic methanotrophic bacteria to the particulate organic matter. The distinctive BHP pattern in the suboxic zone, including aerobic methanotroph biomarkers and a tentative marker for a pelagic redoxcline [putative 22S isomer of the ubiquitous 22R-bacteriohopanetetrol (BHT)], was mirrored in the sediment samples. Our data indicate that a major portion of the sedimentary hopanoids of the Gotland Deep is sourced from the suboxic part of the water column via an effective but unknown transport mechanism.     

Utilisation de la méthode rubidium-strontium pour la datation de niveaux sédimentaires du Précambrien supérieur de l'Adrar mauritanien (Sahara occidental) et la mise en évidence de transformations précoces des minéraux argileux

Amineralogical-geochronological study has been made of sedimentary levels I₂ and I₅ in the upper Precambrian of the Adrar, Mauritania, showing that clay minerals can be used for geochronology. This is, however, only possible where early diagenesis has homogenised the ⁸⁷Sr/⁸⁶Sr ratio of clay minerals and interstitial waters. Rocks whose clay minerals consist of unaltered mica or illite are not suitable for this approach, as is the case for sediments rich in detrital feldspars: carbonates may however be used.Ages of 964 ± 32 M.Y. (I₂) and860± 35 M.Y. (I₅) were obtained using a⁸⁷Rb decay constant of 1.47 × 10?11 yr^?1, confirming that the beginning of the late Precambrian sedimentation in the Adrar may be placed at 1000 M .Y.     

The geochemistry of iodine in oxidised and reduced recent marine sediments

Surface sediments from the South West Africa shelf and the Gulf of California have been analysed for iodine and organic carbon. The iodine contents range from 96 to 1990 ppm. While iodine and organic carbon show certain anomalies on the South West Africa shelf, the trend of the $\text{I}\text{C}$ ratios is smooth and decreases from the shelf edge, an area of oxidising surface sediment, shorewards to reducing sediments, formed as a result of intense water upwelling. In the Gulf of California, a similar distribution of $\text{I}\text{C}$ ratios with surface sediment type occurs; lowest values occur in the reduced sediments and highest in oxidised sediments. Values of the $\text{I}\text{C}\text{(× 10}{}^4\text{)}$ ratio of the oxidised sediments (~250) are more than an order of magnitude higher than in reduced sediments, and are similar to some other surface oxidised sediments.The high I content of oxidised sediments is mostly due to uptake of I on to plankton seston on the seabed. In reduced sediments, I contained in planktonic matter originating in surface waters forms the bulk of iodine in the sediment.With sediment burial, oxidised sediments lose most of the iodine through degradation of unspecified organic constituents. This does not happen to the same extent in reduced sediments. The geological implications of these reactions are briefly discussed.     

Recurrent uranium relocations in distal turbidites emplaced in pelagic conditions

The sediments of the Madeira Abyssal Plain, east of Great Meteor Seamount, are dominated by distal turbidite deposition. While the turbidites exhibit a wide compositional range (25–80% CaCO₃), individual examples can be correlated over a wide area and are relatively homogenous. Organic C oxidation, by bottom water oxygen, proceeds from the turbidite tops downwards after emplacement in pelagic conditions, and the progress of this oxidation front is marked by a sharp colour contrast in the sediments (Wilsonet al., 1985). In turbidites with C_org ? 0.5%, redistribution of authigenic U occurs to form a concentration peak (4–9 ppm U), just below the oxidation front or colour change. Several tens μg U/cm² may be mobilised, and in all examples studied ?60% of the remobilised U is relocated into the peak. Following burial by subsequent turbidites, such U concentration peaks are persistent as relict indicators of their extinct oxidation fronts for at least 2 × 10⁵ years. In the case of thin turbidites where labile C_org is almost exhausted, the U peaks may be located in underlying sedimentary units because of their relationship to the oxidation front. A redox mechanism for U peak formation is suggested from these data rather than a complexation with organic matter.     

Iodine-129 chronological study of brines from an Ordovician paleokarst reservoir in the Lunnan oilfield,Tarim Basin

Previous studies have shown that brines in an Ordovician paleokarst reservoir of the Lunnan oilfield in the Tarim Basin, China, are the product of mixing of paleo-evaporated seawater in the east with paleometeoric waters in the west. In order to put time constraints on the brine and related hydrocarbons in this field, 10 brine samples were collected, for which the iodine concentrations and ¹²⁹I/I ratios were measured and discussed. The iodine concentration (3.70–31.2 mg/L) and the ¹²⁹I/I ratio (189–897 × 10⁻¹⁵) show that the iodine in the paleoseawater and meteoric water (MW) had different origins and ¹²⁹I characteristics. The paleoseawater has a high iodine content (∼31 mg/L), indicating that iodine was introduced into the reservoir along with thermally generated hydrocarbons, possibly in the Cretaceous, from the Caohu Sag in the eastern area. Based on consideration of all possible origins of iodine and ¹²⁹I in the brines, it is suggested that the meteoric water maintained its initial iodine content (0.01 mg/L) and ¹²⁹I/I ratio (1500 × 10⁻¹⁵), whereas the iodine-enriched paloseawater (IPSW) exhibited a secular ¹²⁹I equilibrium (N_sq = 39 atom/μL) as a result of fissiogenic ¹²⁹I input in the reservoir over a long period of time. The model of brine evolution developed on that basis confirmed that meteoric water entered the reservoir in the Miocene at about 10 Ma, and partially mixed with the iodine-enriched paleoseawater. The movement of meteoric water was facilitated by faults created during the Himalayan orogeny, then became more dense after dissolving Paleogene halite and infiltrated into the reservoir at high pressure. The iodine and ¹²⁹I concentration in the brine contains information about the path and history of the fluid in the reservoir. This may be useful in oil exploration, since the movement of water was, to some extent, related to hydrocarbon migration.     

Redox zonation and oscillation in the hyporheic zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the fate of groundwater arsenic during discharge

Riverbank sediment cores and pore waters, shallow well waters, seepage waters and river waters were collected along the Meghna Riverbank in Gazaria Upazila, Bangladesh in Jan. 2006 and Oct.–Nov. 2007 to investigate hydrogeochemical processes controlling the fate of groundwater As during discharge. Redox transition zones from suboxic (0–2 m depth) to reducing (2–5 m depth) then suboxic conditions (5–7 m depth) exist at sites with sandy surficial deposits, as evidenced by depth profiles of pore water (n = 7) and sediment (n = 11; diffuse reflectance, Fe(III)/Fe ratios and Fe(III) concentrations). The sediment As enrichment zone (up to ∼700 mg kg⁻¹) is associated with the suboxic zones mostly between 0 and 2 m depth and less frequently between 5 and 7 m depth. The As enriched zones consist of several 5–10 cm-thick dispersed layers and span a length of ∼5–15 m horizontally from the river shore. Depth profiles of riverbank pore water deployed along a 32 m transect perpendicular to the river shore show elevated levels of dissolved Fe (11.6 ± 11.7 mg L⁻¹) and As (118 ± 91 μg L⁻¹, mostly as arsenite) between 2 and 5 m depth, but lower concentrations between 0 and 2 m depth (0.13 ± 0.19 mg L⁻¹ Fe, 1 ± 1 μg L⁻¹ As) and between 5 and 6 m depth (1.14 ± 0.45 mg L⁻¹ Fe, 28 ± 17 μg L⁻¹ As). Because it would take more than a few hundred years of steady groundwater discharge (∼10 m yr⁻¹) to accumulate hundreds of mg kg⁻¹ of As in the riverbank sediment, it is concluded that groundwater As must have been naturally elevated prior to anthropogenic pumping of the aquifer since the 1970s. Not only does this lend unequivocal support to the argument that As occurrence in the Ganges-Brahmaputra-Meghna Delta groundwater is of geogenic origin, it also calls attention to the fate of this As enriched sediment as it may recycle As into the aquifer.     

Distribution of glycerol dialkyl glycerol tetraether lipids in the water column of Lake Tanganyika

We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in suspended particulate matter from the water column of Lake Tanganyika (East Africa), where sediment studies had shown the applicability of the TEX₈₆ proxy for reconstructing surface lake water temperature. GDGTs, in particular crenarchaeol, showed maximum abundance within the suboxic zone (100–180 m), suggesting that this is the preferred niche of ammonia-oxidizing Thaumarchaeota. Despite evidence for anaerobic methane oxidation in deep anoxic water (300–1200 m) no unambiguous evidence for an imprint of methanotrophic archaea on GDGT distribution was found. Comparison of TEX₈₆ and BIT indices with those of surface sediments suggests that the sedimentary GDGTs are derived predominantly from the oxic zone and suboxic zone of the lake.     

Nonconservative behavior of Br−/Cl− ratios during alteration of volcaniclastic sediments

Other than halite diagenesis and organic matter degradation, Cl⁻ and Br⁻ are considered to be conservative in marine pore fluids. Consequently, Br⁻/Cl⁻ ratios should remain constant during most diagenetic reactions. Nonetheless, Br⁻/Cl⁻ molar ratios decrease to 1.27 × 10⁻³ (≈18% less than seawater value) in pore fluids from Site 833 in the Aoba Basin of the New Hebrides convergent margin despite the lack of halite diagenesis and little organic matter. Sediment at this site is largely volcanic ash, which becomes hydrated with depth as it converts to clay and zeolite minerals. These hydration reactions remove sufficient water to increase the concentrations of most solutes including Cl⁻ and Br⁻. The resulting concentration gradients drive diffusion, but calculations indicate that diffusion does not decrease the Br⁻/Cl⁻ ratio. Some Cl⁻ may be leached from the ash, but insufficient amounts are available to cause the observed decrease in Br⁻/Cl⁻ ratio. The limited source of Cl⁻ suggests that proportionately more Br⁻ than Cl⁻ is lost from the fluids to the diagenetic solids. Similar nonconservative behavior of Cl⁻ and Br⁻ may occur during fluid circulation at ridge crests and flanks, thereby influencing the halide distribution in the crust.     

Bioaccumulation of arsenic,chromium and lead in fish: constraints imposed by sediment geochemistry

Determining metal bioavailability is critical in assessing the necessity to remediate contaminated sediments. In the Halls Brook Holding Area Pond (HBHAP) sapropel, As (3000 mg/kg), and Cr (1400 mg/ kg), are sequestered by amorphous Fe(OH)₃ (Kd_As=560; Kf_Cr=59,0001/kg), while Pb solubility is limited by PbS_(am). Fillet As concentrations in detritivorous and omnivorous fish were similar in the HBHAP (1.19 mg/kg), and the adjacent unimpacted Phillips Pond (1.18 mg/kg). Cr and Pb in both HBHAP and Phillips Pond fish were below analytical detection limits, except for one (0.73 mg/kj Pb), in the HBHAP. The low sediment bioaccumulation factors for As, Cr, and Pb (6.5 × 10⁻⁴, < 1.1 × 10, and 1.8 × 10⁻⁶, respectively in HBHAP) suggest that the sediment acts to sequester metals, rendering them non-bioavailable due to precipitation of solids, and sorption to iron phases.     

Origin of iodine in volcanic fluids: I results from the Central American Volcanic Arc

The largest reservoir of crustal iodine is found in marine sediments, where it is closely associated with organic material. This presence, together with the existence of a long-lived, cosmogenic radioisotope ¹²⁹I (t_1/2 = 15.7 Ma), make this isotopic system well suited for the study of sediment recycling in subduction zones. Reported here are the results of ¹²⁹I/I ratios in volcanic fluids, collected during a comprehensive study of fluids and gases in the Central American Volcanic Arc. ¹²⁹I/I ratios, together with I, Br, and Cl concentrations, were determined in 79 samples from four geothermal centers and a number of crater lakes, fumaroles, hot springs, and surface waters in Costa Rica, Nicaragua, and El Salvador. Geothermal and volcanic fluids were found to have iodine concentrations substantially higher than values in seawater or meteoric waters. ¹²⁹I/I ratios in most of the geothermal fluids are below the preanthropogenic input ratio of 1500 × 10⁻¹⁵, demonstrating that recent anthropogenic additions are largely absent from the volcanic systems. The majority of the ¹²⁹I/I ratios are between 500 and 800 × 10⁻¹⁵. These ratios indicate minimum iodine ages between 25 and 15 Ma, in good agreement with the age of subducted sediments in this region. In all four geothermal systems, however, a few samples were found with iodine ages older than 40 Ma—that is, considerably below the expected age range for subducted sediments from the Cocos Plate. These samples probably reflect the presence of iodine derived from sediments in older accreted oceanic terraines. The iodine ages indicate that the magmatic end member for the volcanic fluids originates in the deeper parts of the subducted sediment column, with small additions from older iodine mobilized from the overlying crust. The high concentrations of iodine in geothermal fluids, combined with the observed iodine ages, demonstrate that remobilization in the main volcanic zone (and probably also in the forearc area) is an important part in the overall marine cycle of iodine and similar elements.     

Early diagenesis in sediments from Danish coastal waters: Microbial activity and Mn-Fe-S geochemistry

Depth distributions of bacterial respiration of O₂, NO₃⁻ and SO₄²⁻ were compared with geochemical data for Mn, Fe and S in coastal sediments from water depths between 26 and 520 m. As water depth increased, the zone of SO₄²⁻ respiration was found deeper in the sediment and was eventually separated from the surface-located activity of O₂ and NO₂⁻ respiration. At the deepest station additional SO₄²⁻ reduction activity was observed in small, detrital aggregates on the sediment surface. Dissolved Mn²⁺ and Fe²⁺ appeared between the O₂- plus NO₃⁻-containing surface layer and the H₂S-plus FeS-containing sediment below. This was a result of Mn and Fe reductions coupled to either the oxidation of sulfide or the mineralization of organic matter. Tracer experiments showed that both FeS, FeS₂ and S⁰ were important radiolabelled products of sulfate respiration in this intermediate zone. In the same zone, the overall degradation of organic matter seemed to be underestimated by the assay of SO^2-₄ respiration and additional mineralization by Mn and Fe reductions was likely.