Volume 1 Issue 7, July 2008

Volume 1 Issue 7

Eclogites have been suggested as reservoirs with high niobium/tantalum ratios that complement the low niobium/tantalum ratios of the silicate Earth. However, the hafnium isotopic composition of eclogite fragments suggests that they did not possess high niobium/tantalum ratios to begin with. Instead, the high ratios probably reflect chemical modification during residence in the subcontinental lithospheric mantle. The image shows a photomicrograph of a rutile-bearing eclogite fragment entrained in the Lac de Gras kimberlites from the central Slave Craton, Canada.

Letter p468

Cover design by Karen Moore

Editorial

  • Editorial |

    The discovery that biogenic methane production may not be limited to oxygen-free environments throws conventional thinking into turmoil, and calls into question basic assumptions regarding the global methane budget.

Correspondence

Commentary

  • Commentary |

    Modern farms produce particulate matter and gases that affect the environment and human health and add to rising atmospheric greenhouse-gas levels. European policymakers have made progress in controlling these emissions, but US regulations remain inadequate.

    • Viney P. Aneja
    • , William H. Schlesinger
    •  & Jan Willem Erisman

Feature

  • Feature |

    The northern and southern hemispheres of Mars are topographically distinct. Crustal thickness analyses and numerical simulations suggest a giant impact just after the crust differentiated 4.4 billion years ago as a plausible cause for this dichotomy.

    • H. J. Melosh

Books and Arts

Research Highlights

News and Views

  • News & Views |

    Most of the world's surface oceans are oversaturated with respect to atmospheric methane and emit large quantities of this greenhouse gas. Aerobic decomposition of phosphorus-containing organic compounds may be responsible.

    • Ellery D. Ingall
  • News & Views |

    The uneven distribution of biological nitrogen fixation in terrestrial ecosystems has yet to be explained. Latitudinal gradients in temperature and phosphorus may hold the answer.

    • Eric A. Davidson
  • News & Views |

    Ninety-five million years ago, ocean bottom waters were much warmer than at present. Some of this warmth could have come from the proto-North Atlantic's continental shelves after the balmy surface waters became increasingly salty through evaporation.

    • Silke Voigt

Progress Article

  • Progress Article |

    European forests are intensively exploited for wood products, yet they are also a potential sink for carbon. European forest inventories combined with timber harvest statistics from sixteen European countries show that between 1950 and 2000 forest biomass increased faster than the amount of timber harvests. Silviculture, which has developed over the past 50 years, can efficiently sequester carbon on timescales of decades, while maintaining forests that meet the demand for wood.

    • P. Ciais
    • , M. J. Schelhaas
    • , S. Zaehle
    • , S. L. Piao
    • , A. Cescatti
    • , J. Liski
    • , S. Luyssaert
    • , G. Le-Maire
    • , E.-D. Schulze
    • , O. Bouriaud
    • , A. Freibauer
    • , R. Valentini
    •  & G. J. Nabuurs

Review

  • Review Article |

    Land and ocean carbon sinks play a critical role in determining atmospheric carbon dioxide levels. Nitrogen-induced increases in land and ocean sink strength are unlikely to keep pace with future increases in carbon dioxide.

    • Dave S. Reay
    • , Frank Dentener
    • , Pete Smith
    • , John Grace
    •  & Richard A. Feely

Letters

  • Letter |

    The surface waters of the North Atlantic subtropical gyre are depleted in phosphate, relative to the South Atlantic gyre. Despite this nutrient limitation, the two gyres have comparable rates of carbon fixation. Measurements of enzyme activity suggest that dissolved organic phosphorus may be fuelling northern productivity.

    • Rhiannon L. Mather
    • , Sarah E. Reynolds
    • , George A. Wolff
    • , Richard G. Williams
    • , Sinhue Torres-Valdes
    • , E. Malcolm S. Woodward
    • , Angela Landolfi
    • , Xi Pan
    • , Richard Sanders
    •  & Eric P. Achterberg
  • Letter |

    Abrupt changes in the African Monsoon, which have been recorded throughout the late Pleistocene and early Holocene epochs, tend to coincide with changes in North Atlantic thermohaline circulation. A numerical simulation shows that the interaction between thermohaline circulation in the North Atlantic Ocean and wind-driven currents in the topical Atlantic Ocean contributes to the rapidity of African Monsoon transitions during abrupt climate change events.

    • Ping Chang
    • , Rong Zhang
    • , Wilco Hazeleger
    • , Caihong Wen
    • , Xiuquan Wan
    • , Link Ji
    • , Reindert J. Haarsma
    • , Wim-Paul Breugem
    •  & Howard Seidel
  • Letter |

    Although North Atlantic deep-water formation was greatly reduced during the last glacial maximum, bottom-water currents were as vigorous as at present. However, they were weakened during periods of North Atlantic surface freshening. A strong correlation can be seen between bottom-water-current strength and Greenland air-temperature records, thus confirming a close connection between ocean circulation and abrupt climate change.

    • Summer K. Praetorius
    • , Jerry F. McManus
    • , Delia W. Oppo
    •  & William B. Curry
  • Letter |

    During the Cretaceous period, warm deep and intermediate waters filled the oceans. Evidence from benthic foraminferal δ18O and Mg/Ca ratios suggests that the intermediate water masses in the proto-Atlantic Ocean formed from high salinity waters sinking from shallow shelf seas.

    • Oliver Friedrich
    • , Jochen Erbacher
    • , Kazuyoshi Moriya
    • , Paul A. Wilson
    •  & Henning Kuhnert
  • Letter |

    The ancient Farallon plate subducted under North America in two distinct stages. High-resolution tomographic images show large pieces of the plate, including the currently active piece, which descends from the Pacific Northwest coast to 1,500 km depth, and its stalled predecessor, which now occupies the transition zone and lower mantle beneath the eastern half of the continent.

    • Karin Sigloch
    • , Nadine McQuarrie
    •  & Guust Nolet
  • Letter |

    The rifting of the Seychelles microcontinent from India involved two phases of extensional activity. The initial separation of the Laxmi Ridge from India was accompanied by extensive magmatism but the later separation of the Seychelles from the Laxmi Ridge was only weakly magmatic.

    • Timothy A. Minshull
    • , Christine I. Lane
    • , Jenny S. Collier
    •  & Robert B. Whitmarsh
  • Letter |

    Eclogites have been suggested as high niobium/tantalum reservoirs that complement the low niobium/tantalum ratios of the silicate Earth. However, the hafnium isotopic composition of eclogite fragments suggest that the high niobium/tantalum signature of eclogites is unlikely to be primary. Instead, it probably reflects chemical modification during residence in the subcontinental lithospheric mantle.

    • Sonja Aulbach
    • , Suzanne Y. O’Reilly
    • , William. L. Griffin
    •  & Norman J. Pearson

Article

  • Article |

    Surface waters of most of the world’s oceans are supersaturated with respect to atmospheric methane. Measurements in seawater samples suggest that an aerobic methane production pathway, which involves the decomposition of phosphorus-containing organic compounds, may be responsible.

    • David M. Karl
    • , Lucas Beversdorf
    • , Karin M. Björkman
    • , Matthew J. Church
    • , Asuncion Martinez
    •  & Edward F. Delong

Erratum

Backstory

  • Backstory |

    Rhiannon Mather, Sarah Reynolds and colleagues criss-crossed the Atlantic Ocean armed with pumps and plastic bottles in search of the nutrients that feed open-ocean productivity.

  • Backstory |

    Having managed to get themselves and all their instruments on board a ship not too far away from an imminent war zone, Jenny Collier and colleagues enjoyed the serenity of life at sea as they investigated the rifted continental margin of India.