Udo Fehn

Variations in 129I/127I ratios in recent marine sediments: evidence for a fossil organic component

Abstract 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.

Recycling of iodine in fore-arc areas: evidence from the iodine brines in Chiba, Japan

The distribution of iodine in the Earth’s crust is dominated by its accumulation in marine sediments. If fluxes between terrestrial and marine compartments are considered, however, a significant imbalance exists between known sources and sinks of iodine. We present here evidence from the fore-arc area near Chiba, Japan, the world’s largest brine-iodine producing area, that iodine is mobilized from marine sediments during the early stages of subduction. Based on detailed chemical analyses of 22 brines and 129I dating of 13 of these samples collected from the Kazusa Formation, we show that iodine in these fluids is derived from organic-rich marine sediments with a minimum age of 50 Myr. Geochemical characteristics of the brines and the age of the iodine indicate that the iodine enrichment is caused by mobilization from subducting marine sediments and not by derivation from the host formation (age 1–2 Myr). The direct return of iodine from marine sediments into the oceans during the subduction of oceanic plates could provide the missing link in the iodine cycle and be an important pathway also in the marine cycle of carbon.

Development of 36Cl standards for AMS

Abstract Large-quantity dilutions of the National Institute of Standards and Technology (NIST) 36Cl standard (SRM 4943) to 36 Cl Cl ratios of 5.000 × 10−13, 1.600 × 10−12, 5.003 × 10−12 and 1.000 × 10−11 have been prepared for AMS standards. Dilutions were made with an estimated uncertainty better than 1% and were then checked in three separate runs using the AMS facility at the University of Rochester. The results show excellent agreement between the gravimetric dilution factors and AMS measurements.

Determination of source ages and migration patterns of brines from the U.S. Gulf Coast basin using 129I

Abstract The long-lived cosmogenic and fissiogenic isotope 129I has been applied to the study of source ages and migration patterns of oil field brines from the U.S. Gulf Coast basin. 129 I I ratios were measured in sixty samples of saline formation water from depths of 660–4503 m in several oil fields in southeast Texas, south-central Louisiana, and offshore Louisiana. Comparisons between measured ratios and the decay curve for surface hydrospheric 129I result in minimum source ages much older than present host formation ages, indicating vertical migration of brine from older, deeper sources. Conservative corrections for a fissiogenic component due to spontaneous fission of 238U in the subsurface result in the following median source ages: Iberia field: 55 Ma, Port Barre field: 53 Ma, and Eugene Island Blocks 316 and 330: 55 Ma, pointing to Wilcox Group shales as source formations. Mesozoic sources cannot be ruled out, due to the uncertainties in estimating the fissiogenic component. Some of the ratios measured in Texas Gulf Coast brines show evidence of a greater fissiogenic component which indicates that the brines have resided in formations with locally high U concentrations.

129I AND 36CL CONCENTRATIONS IN WATERS OF THE EASTERN CLEAR LAKE AREA, CALIFORNIA : RESIDENCE TIMES AND SOURCE AGES OF HYDROTHERMAL FLUIDS

Abstract The Clear Lake area of northern California is the location of hot spring activity, some of which is associated with the formation of Au and Hg deposits. We measured 129 I I and 36 Cl Cl ratios in nine warm springs (formation springs), six cold springs with elevated levels of Cl and I (mineral springs), and five springs of recent meteoric origin. Dating of the I in the formation waters indicated that sources of I in these waters are formations with minimum ages between 60 and 80 Ma. This age range is in good agreement with the ages of the Franciscan Complex and the Great Valley Sequence, the dominant formations in this area. Because the mineral waters are essentially formation waters diluted with meteoric water, I in these waters is of the same origin. Residence times of the waters were calculated based on the build-up of129I and 36Cl as a consequence of the presence of U and Th in the crust. The residence time of the formation waters in the Great Valley Sequence, the location for most of these springs, was probably not longer than 84,000 yr. The concentrations found for 36Cl and 129I in the mineral waters indicate that these waters have residence times of similar magnitude in formations such as the Franciscan Complex or the Clear Lake Volcanics, which have slightly higher levels of U and Th than the Great Valley Sequence.

129I in Gulf of Mexico waters

Abstract Nuclear weaponry and power generation have released far more 129 I onto the Earths surface than was formerly present. The effects of this release are clearly visible in the waters of the Gulf of Mexico, far from the major sources. New 129 I is found down to at least 500 m, and probably to 1000 m. Surface values are approximately 60 times the pre-nuclear era levels. The 129 I penetration depth of 470 m agrees well with that determined from bomb fallout isotopes 3 H and Δ 14 C during the 1970s and 1980s in the Gulf of Mexico and in the western North Atlantic at similar latitudes. The observed levels exceed amounts expected from globally distributed nuclear test fallout. The observed surplus must come primarily from spent fuel reprocessing plants in the eastern North Atlantic, followed by rapid circulation of the North Atlantic gyre, and subsequent delivery into the Caribbean Sea and thence into the Gulf of Mexico. Airborne transport also appears significant.

Sources and reservoirs of anthropogenic iodine-129 in western New York

Large quantities of iodine-129 have been released during nuclear weapons testing, and from nuclear power and fuel reprocessing plants. The distribution of this isotope was investigated in 110 surface water and soil samples from western New York (where several potential point sources are located) and other areas of North America, to evaluate its sources, transport pathways, and reservoirs. Elevated 129I concentrations associated with a former reprocessing facility at West Valley, NY, can be tracked to Lakes Erie and Ontario via site drainage, and for over 200 km via atmospheric transport, while only a negligible signal is associated with active power plants in the area. The results point to local reprocessing as the major source of 129I in western New York, while bomb fallout constitutes less than 0.5% of the signal. Surface soil is the dominant reservoir for anthropogenic 129I in this region. Across North America, 129I concentrations are lower than in western New York, although still significantly higher than expected weapons fallout. Reprocessing releases are currently seen to be the major source for elevated 129I concentrations on a global scale, in contrast to previous suggestions that most anthropogenic 129I was still derived from weapons fallout. Concentrations of 129I and iodine in surface reservoirs are generally found to be uncorrelated, implying that natural iodine and anthropogenic 129I are not yet in equilibrium. The results suggest that anthropogenic 129I is cycled between the atmosphere-soil-vegetation systems more rapidly than natural, pre-anthropogenic iodine.

Origin of iodine in volcanic fluids: 129I results from the Central American Volcanic Arc

Abstract 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 (t1/2 = 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.

Iodine dating of pore waters associated with gas hydrates in the Nankai area, Japan

The Nankai hydrate field, Japan, is an example of gas-hydrate deposits associated with an active subduction zone. In order to determine the origin of gas hydrates in this area, 129I/I ratios together with halogen concentrations were measured in a set of pore-water samples collected from two boreholes in the Nankai hydrate field. Iodine concentrations are between 100 and 230 μM, i.e., strongly enriched compared to seawater, while Cl concentrations were found to be close to that of seawater. Except for one sample, 129I/I ratios are between 180 and 520 × 10−15, giving minimum ages between 24 and 48 Ma. Because these ages are considerably older than present host sediments (<2 Ma) and subducting marine sediments (<21 Ma) in this area, iodine (and, by association, methane in the gas hydrates) must have been derived from source formations located in the continental side of the subduction zone. The results do not support derivation of gas hydrates from present host sediments or currently subducting sediments, but could be related to release and long-time recycling of fluids from marine formations of early Tertiary age.

Origin and history of waters associated with coalbed methane: 129I, 36Cl, and stable isotope results from the Fruitland Formation, CO and NM

Abstract The Fruitland Formation of the San Juan Basin was deposited during the late Cretaceous and is associated with significant reservoirs of coalbed methane (CBM). The purpose of this study is to determine the origin and history of waters associated with the formation, using long-lived cosmogenic and stable isotope systems. Ratios of 129 I/I and stable isotope values (δD and δ 18 O) were determined in waters from close to 100 wells, 36 Cl/Cl ratios for a subset of these samples. A significant group of samples has 129 I/I ratios between 100 × 10 −15 and 200 × 10 −15 , indicating minimum iodine ages close to 60 Ma. If these ages are corrected for the addition of fissiogenic 129 I, they are compatible with the depositional age of the Fruitland Formation (Late Cretaceous). Several sets of waters are clearly present within the data. A group dominated by infiltration of recent surface waters is restricted to the uplifted basin margins, with a lateral extent of less than 5 km from outcrop, and is characterized by 129 I/I ratios in excess of 1500 × 10 −15 and meteoric δD, δ 18 O, and 36 Cl/Cl signatures. The rest of the basin is characterized by several subsets of formation waters which have undergone variable degrees of iodine enrichment through diagenesis as well as variable degrees of dilution. The first subgroup is found in coals of relatively low vitrinite reflectance and moderate enrichment of iodine. This subgroup predominantly consists of entrapped pore fluids, although it may also contain waters which infiltrated the coals at the time of the Laramide uplift, between 25 and 30 Ma. A second subgroup consists of formation waters associated with coals of high vitrinite reflectance. Despite subsequent uplift, the high iodine concentrations and low 129 I/I ratios of this subgroup, as well as a moderate depletion of deuterium relative to 18 O, suggest that these waters were not significantly altered since the time when diagenetic reactions occurred in the deepest portion of the basin. A third subgroup, with higher δD and δ 18 O values as well as higher 129 I/I ratios, extends roughly west to east at the New Mexico–Colorado state line and corresponds to a region of extensive fracturing of the coalbeds. In this case, the higher 129 I/I ratios are probably due to contributions of fissiogenic 129 I through fracture flow, perhaps from deeper formation waters. Our results do not support models of subsequent basin-wide groundwater migration in the Fruitland Formation. The combined use of 129 I and 36 Cl with stable isotope studies provides valuable information as to the hydrologic history of coalbed methane deposits, as well as their potential for commercial exploitation.