A deep sleep in coal beds Deep below the ocean floor, microorganisms from forest soils continue to thrive. Inagaki et al. analyzed the microbial communities in several drill cores off the coast of Japan, some sampling more than 2 km below the seafloor (see the Perspective by Huber). Although cell counts decreased with depth, deep coal beds harbored active communities of methanogenic bacteria. These communities were more similar to those found in forest soils than in other deep marine sediments. Science , this issue p. 420 ; see also p. 376

Pore waters associated with gas hydrates at Blake Ridge in the Atlantic Ocean were dated by measuring their iodine-129/iodine ratios. Samples collected from sediments with ages between 1.8 and 6 million years ago consistently yield ages around 55 million years ago. These ages, together with the strong iodine enrichment observed in the pore waters, suggest that the origin of iodine is related to organic material of early Tertiary age, which probably is also the source of the methane in the gas hydrates at this location.

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[1] Gases and fluids from four geothermal fields of Central America were analyzed for nitrogen, methane, and helium concentrations, isotopic composition, and 129 I/I ratios in order to determine the sources of volatiles in these systems. Results for gas ratios and isotopic compositions for three of the fields are consistent with observations from other subduction zones. Ratios of N 2 / 3 He are only slightly higher than average arc values of 1 Â 10 8 and the volcanic flux of N 2 for the Central American systems is estimated to be between 1.6 Â 10 8 and 3.2 Â 10 8 mol/yr. Analysis of 129 I/I ratios indicates the presence of a subducted organic component (25-30 Ma) as well as of a much older crustal component (40-65 Ma) throughout the study area. The magmatic flux of nitrogen and noble gases in Central America was then extrapolated to determine the degree of nitrogen recycling in island arc systems. Global N 2 flux is estimated at 2.7 Â 10 9 to 5.4 Â 10 9 mol/yr, which is comparable to the global mid-ocean ridge flux, and represents 29-58% of the subducted sediment flux. This flux estimate is consistent across the N 2 -CO 2 -He systems and suggests that nearly all of the nitrogen supplied to the mantle wedge is devolatilized beneath the volcanic front. The Momotombo geothermal field of Nicaragua is characterized by exceptionally high excess nitrogen and methane values, and the close correlation of these two gases indicates a common source. While it is not uncommon for sedimentary basins with high heat flow to have excess nitrogen, the Momotombo geothermal field is unique in that the high N 2 / 3 He gases have essentially magmatic 3 He/ 4 He ratios. The high excess nitrogen component of Nicaragua is related to the older iodine end-member, pointing to a crustal origin. The crustal nitrogen contributions along the Nicaraguan portion of the arc are on the order of 2.2 Â 10 8 mol N 2 /yr or roughly equal to the magmatic contribution along the entire Central American volcanic arc. The results for Momotombo indicate that the release of nitrogen during reorganization of island arc systems may have a significant impact on the global flux of volcanic nitrogen.

[1] Since the advent of the nuclear age in the mid-1940s, the mass of radioactive 129 I (t 1/2 = 15.7 Myr) circulating in the Earth's hydrosphere has increased nearly fortyfold from its natural background level of 140 kg. Nuclear fuel reprocessing has been by far the major contributor, responsible for releasing 5400 kg of 129 I, primarily into the North Atlantic Ocean. Regional and global trends in the distribution of the 129 I inventory are elucidated from an examination of more than 600 determinations of 129 I in environmental samples from around the world. Because the major point sources are located in Europe and the United States, more than 99% of the present 129 I reservoir is distributed in the Northern Hemisphere, where both 129 I concentrations and 129 I/I ratios in rivers, lakes, and shallow seawater are several orders of magnitude above the preanthropogenic background. Downwelling in the North Atlantic presently provides a major sink for marine 129 I; however, marine upwelling along the margins of the Pacific will eventually return part of this anthropogenic input to the ocean surface, where it will find its way back into surface waters and the atmosphere. Iodine-129 has a long half-life (15.7 Myr), and consequently, there is also the possibility that climate change will influence the dynamics of iodine transfer in surface reservoirs. We model the effect of a collapse in thermohaline circulation and project a concentration increase of more than 3 orders of magnitude in shallow oceans over the 10,000 years that follow if nuclear reprocessing is to continue at the present rate.

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[1] Both the concentration and the carbon isotope composition of dissolved inorganic carbon (DIC) vary considerably across the sulfate-methane transition (SMT) in shallow marine sediment at locations with gas hydrate. This variability has led to different interpretations for how carbon, including CH 4 , cycles within gas-charged sediment sequences over time. We extend a one-dimensional model for the formation of gas hydrate to account for downhole changes in dissolved CH 4 , SO 4 2− , DIC, and Ca 2+ , and the d 13 C of DIC. The model includes advection, diffusion, and two reactions that consume SO 4 2− : degradation of particulate organic carbon (POC) and anaerobic oxidation of methane (AOM). Using our model and site-specific parameters, steady state pore water profiles are simulated for two sites containing gas hydrate but different carbon chemistry across the SMT: Site 1244 (Hydrate Ridge; DIC = 38 mM, d13 C of DIC = -22.5‰ PDB) and Site Keathley Canyon (KC) 151-3 (Gulf of Mexico; DIC = 16 mM, d13 C of DIC = −49.6‰ PDB). The simulated profiles for CH 4 , SO 4 2− , DIC, Ca 2+ , and d 13 C of DIC resemble those measured at the sites, and the model explains the similarities and differences in pore water chemistry. At both sites, an upward flux of CH 4 consumes most net SO 4 2− at a shallow SMT, and calcium carbonate removes a portion of DIC at this horizon. However, a large flux of 13 C-enriched HCO 3 − enters the SMT from depth at Site 1244 but not at Site KC151-3. This leads to a high concentration of DIC with a d 13 C much greater than that of CH 4 , even though AOM causes the SMT. The addition of HCO 3 − from depth impacts the slope of certain concentration crossplots. Crucially, neither the DIC concentration nor its carbon isotope composition at the SMT can be used to discriminate between sulfate reduction pathways.

[1] Abstract: Gas data were collected from geothermal production fields, fumaroles, and hot springs in Central America in order to investigate the relation between volatile output and spatial distribution of volcanic systems. The 3 He/ 4 He ratios are 6.5 ± 0.7 R a throughout the region, indicating that helium is predominantly of mantle origin and is largely independent of variations in the characteristics of the arc. Lower ratios produced by radiogenic production within the Chortis Block are restricted to the Berlín geothermal field and the region behind the volcanic front. Ratios of CO 2 / 3 He are inversely related to the distance between the volcanic system and the trench. In the southwestern portion of the arc, where the arc-trench gap is short and the subduction angle is shallow (Miravalles, Costa Rica), decarbonation is enhanced relative to the mantle helium flux resulting in higher CO 2 / 3 He. In the northwest, where the gap is greater and the subduction angle steeper (Ahuachapán, El Salvador), decarbonation decreases relative to the helium flux. While variations in the carbon isotopic signature have traditionally been linked to the composition of the subducted sediments, the Central American data provide evidence that other factors within the convergent plate boundary such as arc-trench gap, crustal thickness, and subduction angle play an important role in controlling the flux of CO 2 from the subducting slab. The Central American Volcanic Arc gases show no apparent contribution of carbon dioxide derived from subducted organic sediments. Shallow crustal processes, including partitioning and isotopic fractionation, account for the minor deviations from direct mixing of mantle and carbonate-derived end-members. Given that the Central American arc system is not unique in terms of the composition of the subducted sediments or the volcanic output, previous interpretations of global volcanic flux in terms of carbonate and sediment output should be reconsidered. Carbon-helium relationships in Central America require that only 0.3-3.3% of the subducted carbon is released in volcanic eruptions, while the rest is presumably reintroduced into the deeper mantle. This is generally an order of magnitude lower than global averages and is limited by the availability flux of mineral-bound water and the temperature of release. The d 13 C and CO 2 / 3 He ratios suggest that even though the amount of carbon that is released from the slab and subducted sediments is relatively low in Central America, it still makes up 86-98% of the total carbon released from arc volcanics.