Dissimilatory Fe(III) and Mn(IV) reduction.

Thawing permafrost and the resulting microbial decomposition of previously frozen organic carbon (C) is one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing climate. In this article we present an overview of the global permafrost C pool and of the processes that might transfer this C into the atmosphere, as well as the associated ecosystem changes that occur with thawing. We show that accounting for C stored deep in the permafrost more than doubles previous high-latitude inventory estimates, with this new estimate equivalent to twice the atmospheric C pool. The thawing of permafrost with warming occurs both gradually and catastrophically, exposing organic C to microbial decomposition. Other aspects of ecosystem dynamics can be altered by climate change along with thawing permafrost, such as growing season length, plant growth rates and species composition, and ecosystem energy exchange. However, these processes do not appear to be able to com...

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Vulnerability of Permafrost Carbon to Climate Change: Implications for the Global Carbon Cycle

Clouds and Aerosols

REDUCTION of the soluble, oxidized form of uranium, U(VI), to insoluble U(IV) is an important mechanism for the immobilization of uranium in aquatic sediments and for the formation of some uranium ores1–10. U(VI) reduction has generally been regarded as an abiological reaction in which sulphide, molecular hydrogen or organic compounds function as the reductant1,2,5,11. Microbial involvement in U(VI) reduction has been considered to be limited to indirect effects, such as microbial metabolism providing the reduced compounds for abiological U(VI) reduction and microbial cell walls providing a surface to stimulate abiological U(VI) reduction1,12,13. We report here, however, that dissimilatory Fe(III)-reducing microorganisms can obtain energy for growth by electron transport to U(VI). This novel form of microbial metabolism can be much faster than commonly cited abiological mechanisms for U(VI) reduction. Not only do these findings expand the known potential terminal electron acceptors for microbial energy transduction, they offer a likely explanation for the deposition of uranium in aquatic sediments and aquifers, and suggest a method for biological remediation of environments contaminated with uranium.

Microbial reduction of uranium

This review shows that thermodynamic and kinetic constraints largely prevent large-scale methanogenesis in the open ocean water column. One example of open-ocean methanogenesis involves anoxic digestive tracts and fecal pellet microenvironments; methane released during fecal pellet disaggregation results in the mixed-layer methane maximum. However, the bulk of the methane in the ocean is added by coastal runoff, seeps, hydrothermal vents, decomposing hydrates, and mud volcanoes. Since methane is present in the open ocean at nanomolar concentrations, and since the flux to the atmosphere is small, the ultimate fate of ocean methane additions must be oxidation within the ocean. As indicated in the Introduction and highlighted in Table 3, sources of methane to the ocean water column are poorly quantified. There are only a small number of direct water column methane oxidation rates, so sinks are also poorly quantified. We know that methane oxidation rates are sensitive to ambient methane concentrations, but we have no information on reaction kinetics and only one report of the effect of pressure on methane oxidation. Our perspective on methane sources and the extent of methane oxidation has been changed dramatically by new techniques involving gene probes, determination of isotopically depleted biomarkers, and recent 14C-CH4 measurements showing that methane geochemistry in anoxic basins is dominated by seeps providing fossil methane. The role of anaerobic oxidation of methane has changed from a controversial curiosity to a major sink in anoxic basins and sediments. © 2007 American Chemical Society.

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Oceanic methane biogeochemistry.

We report here that a manganese nodule from the Central Pacific manganese nodule province has been dated by fossil diatoms found in mud scraped from near the nodule's centre. The nodule was taken at DOMES Site B from Core B55–56 at 11°50.3′N and 137°28.2′W in a water depth of 4,892 m. It was resting on the sediment surface, with about 1.5 cm of the nodule bottom (of a total nodule height of 4.2 cm) buried in the mud. The top surface of the nodule was covered with a smooth manganese coating, but the bottom had a very rough, crusty texture. It was found that recent mud had leaked in through cracks in the nodule bottom, but that there were no pre-Pleistocene diatoms in this material. The date obtained was compared with the growth rate determined by the 230Th excess method and found to be in reasonable agreement. This study adds to the work of Harada1,2 on the biostratigraphy (mainly coccoliths) of manganese nodules.

Manganese nodule growth rates determined by fossil diatom dating

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(5), (2020): e2019JC016007, doi:10.1029/2019JC016007.

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Observational and Modeling Evidence of Seasonal Trends in Sediment-Derived Material Inputs to the Chukchi Sea

Evidence of shelf-water transfer from temperature, salinity, and 228Ra/226Ra sampling from the nuclear submarine USS L. Mendel Rivers SCICEX cruise in October, 2000 demonstrates the heterogeneity of the Arctic Ocean with respect to halocline ventilation. This likely reflects both time-dependent events on the shelves and the variety of dispersal mechanisms within the ocean, including boundary currents and eddies, at least one of which was sampled in this work. Halocline waters at the 132 m sampling depth in the interior Eurasian Basin are generally not well connected to the shelves, consonant with their ventilation within the deep basins, rather than on the shelves. In the western Arctic, steep gradients in 228Ra/226Ra ratio and age since shelf contact are consistent with very slow exchange between the Chukchi shelf and the interior Beaufort Gyre. These are the first radium measurements from a nuclear submarine.

Glimpses of Arctic Ocean shelf-basin interaction from submarine-borne radium sampling

Records of trace metals in sediments from the Oregon shelf and slope: Investigating the occurrence of hypoxia over the past several thousand years

Publisher Summary This chapter summarizes the systematics of U- and Th-series nuclides in “marine groundwaters (MGW).” In this chapter, the discussion is confined to hydrothermal systems and to the pore waters of deep-sea and nearshore/estuarine sediments. Radionuclide distributions have been extensively modeled in terrestrial groundwater systems and the same approach can be applied to MGW. The supply rate of U- and Th-series radionuclides to MGW comprises several processes: recoil associated with production from radioactive decay of a parent in the solid phase, in situ decay of a dissolved parent, and other reactions between the fluid and solid phases. The ratios of 228 Ra/ 226 Ra and 210 Pb/Pb in vent fluids have been used to constrain the residence time of the circulating hydrothermal fluid in the crust from the onset of high temperature seawater–basalt interaction to venting at the seafloor. Both low- and high-temperature hydrothermal fluids are greatly enriched in 222 Rn over ambient seawater. Temporal variability of the radon content has been used to gain insight into sub-sea floor processes. The marine sediments serve as both important sources and sinks for U- and Th-series radionuclides. The flux of 226 Ra and 228 Ra out of sediments constitutes the dominant source of these radionuclides to the oceans, and sediments are locally important as sources for 222 Rn and the short-lived Ra isotopes 223 Ra and 224 Ra. Marine sediments also constitute an important sink for U supplied to the oceans. There have been relatively few measurements of the activities of thorium isotopes in sediment pore waters. The presence of elevated 234 Th only in the upper few centimeters of the sediments suggests that it is instead related to the presence of excess 234 Th scavenged from the overlying water column.

David Kadko

Papers

Manganese nodule growth rates determined by fossil diatom dating

Observational and Modeling Evidence of Seasonal Trends in Sediment-Derived Material Inputs to the Chukchi Sea

Glimpses of Arctic Ocean shelf-basin interaction from submarine-borne radium sampling

Records of trace metals in sediments from the Oregon shelf and slope: Investigating the occurrence of hypoxia over the past several thousand years

Chapter 10 Uranium- and Thorium-Series Radionuclides in Marine Groundwaters