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.

Ocean turbulent mixing is a key process affecting the uptake and redistribution of heat, carbon, nutrients, oxygen and other dissolved gasses. Vertical turbulent diffusivity sets the rates of water mass transformations and ocean mixing, and is intrinsically an average quantity over process time scales. Estimates based on microstructure profiling, however, are typically obtained as averages over individual profiles. How representative such averaged diffusivities are, remains unexplored in the quiescent Arctic Ocean. Here, we compare upper ocean vertical diffusivities in winter, derived from the 7Be tracer-based approach to those estimated from direct turbulence measurements during the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, 2019–2020. We found that diffusivity estimates from both methods agree within their respective measurement uncertainties. Diffusivity estimates obtained from dissipation rate profiles are sensitive to the averaging method applied, and the processing and analysis of similar data sets must take this sensitivity into account. Our findings indicate low characteristic diffusivities around 10−6 m2 s−1 and correspondingly low vertical heat fluxes. 

Winter vertical diffusion rates in the Arctic Ocean, estimated from
 7
 Be measurements and dissipation rate profiles

Upwelling Rates and Vertical Diffusivities Determined During the 2013 US GEOTRACES Eastern Tropical Pacific Zonal Transect: Results from 7Be Analysis

Deposition of aerosols to the surface ocean is an important factor affecting primary production in the surface ocean. However, the sources and fluxes of aerosols and associated trace elements remain poorly defined. Aerosol 210Pb, 210Po, and 7Be data were collected on US GEOTRACES cruise GP15 (Pacific Meridional Transect, 152°W; 2018). 210Pb fluxes are low close to the Alaskan margin, increase to a maximum at ∼43°N, then decrease to lower values. There is good agreement between 210Pb fluxes and long‐term land‐based fluxes during the SEAREX program (1970–1980s), as well as between GP15 and GP16 (East Pacific Zonal Transect, 12°S; 2013) at adjacent stations. A normalized fraction f(7Be, 210Pb) is used to discern aerosols with upper (high f) versus lower (low f) troposphere sources. Alaskan/North Pacific aerosols show significant continental influence while equatorial/South Pacific aerosols are supplied to the marine boundary layer from the upper troposphere. Lithogenic trace elements Al and Ti show inverse correlations with f(7Be, 210Pb), supporting a continental boundary layer provenance while anthropogenic Pb shows no clear relationship with f(7Be, 210Pb). All but four samples have 210Po/210Pb activity ratios <0.2 suggesting short aerosol residence time. Among the four samples (210Po/210Pb = 0.42–0.88), two suggest an upper troposphere source and longer aerosol residence time while the remaining two cannot be explained by long aerosol residence time nor a significant component of dust. We hypothesize that enrichments of 210Po in them are linked to Po enrichments in the sea surface microlayer, possibly through Po speciation as a dissolved organic or dimethyl polonide species.

210Pb and 7Be as Coupled Flux and Source Tracers for Aerosols in the Pacific Ocean

Numerous studies address changes in wetland deposition in response to saltwater encroachment driven by the accelerating rate of sea-level rise, by quantifying temporal changes recovered from a vertical sediment sequence. This is the first landscape scale study, based upon 10 core transects representing the heterogeneity of the Southeast Saline Everglades, Florida. By utilizing the known salinity preferences of molluscan assemblages, a Salinity Index was calculated for each core sequence and the recorded salinity changes identified and dated. Radiometric dating utilizing the 210Pb method provides the rate of sediment accumulation and the date of changes identified in the core. The core transects provide the basis for calculation of the rate of saltwater encroachment by comparing the date of saltwater encroachment and the distance between two cores. Thereby, temporal and spatial changes in other sediment parameters in a landscape can also be quantified, such as organic carbon. This paleo-ecological approach to rapidly changing coastal conditions can be utilized to provide scientists and land managers with a record of the past, rate of changing conditions and provide the basis for predicting the future trajectory of their site. Application of this paleo-ecological approach documented increasing rates of saltwater encroachment associated with accelerating rate of sea-level rise: an average rate of 49.1 between 1895 and 1940, 69.2 between 1940 and 1968, 73 between 1968 and 1995 and 131.1 m/yr between 1995 and 2015. Approximately 1.79 km of saltwater encroachment has occurred since 1995, with three partial reversals because of increased freshwater delivery. Associated with saltwater encroachment are changes in sediment organic carbon, decreasing area of marl production and increasing distribution of mangrove. Although the distance of saltwater encroachment is greater in Florida Bay, both changes in sediment organic carbon and mangrove distribution are much less than in Biscayne Bay coastal basins. This heterogeneity is likely the result of differences in tidal ingress efficiency. At the present rate of saltwater encroachment, the freshwater wetlands are predictably lost within a century.

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The paleo-ecological application of mollusks in the calculation of saltwater encroachment and resultant changes in depositional patterns driven by the Anthropocene Marine Transgression

David Kadko

Papers

Winter vertical diffusion rates in the Arctic Ocean, estimated from 7 Be measurements and dissipation rate profiles

Upwelling Rates and Vertical Diffusivities Determined During the 2013 US GEOTRACES Eastern Tropical Pacific Zonal Transect: Results from 7Be Analysis

210Pb and 7Be as Coupled Flux and Source Tracers for Aerosols in the Pacific Ocean

The paleo-ecological application of mollusks in the calculation of saltwater encroachment and resultant changes in depositional patterns driven by the Anthropocene Marine Transgression

Atmospheric deposition of bio-essential trace elements in the southwestern indian ocean: results from the 2021 swings research cruise