In the modern ocean, oxygen minimum zones (OMZs) are
potential traces of a primitive ocean in which Archean bacteria lived and reduced chemical
anomalies occurred. But OMZs are also keys to understanding the present unbalanced nitrogen cycle
and the oceans' role on atmospheric greenhouse control. OMZs are the main areas of nitrogen loss
(as N(2), N(2)O) to the atmosphere through denitrification and anammox, and could even indirectly
mitigate the oceanic biological sequestration of CO(2). It was recently hypothesized that OMZs
are going to spread in the coming decades as a consequence of global climate change. Despite an
important OMZ role for the origin of marine life and for the biogeochemical cycles of carbon and
nitrogen, there are some key questions on the structure of OMZs at a global scale. There is no
agreement concerning the threshold in oxygen that defines an OMZ, and the extent of an OMZ is
often evaluated by denitrification criteria which, at the same time, are O(2)-dependent. Our work
deals with the identification of each OMZ, the evaluation of its extent, volume and vertical
structure, the determination of its seasonality or permanence and the comparison between OMZs and
denitrification zones at a global scale. The co-existence in the OMZ of oxic (in its boundaries)
and suboxic (even anoxic, in its core) conditions involves rather complex biogeochemical
processes such as strong remineralization of the organic matter, removal of nitrate and release
of nitrite. The quantitative OMZ analysis is focused on taking into account the whole water
volume under the influence of an OMZ and adapted to the study of the specific low oxygen
biogeochemical processes. A characterization of the entire structure for the main and most
intense OMZS (O(2) < 20 mu M reaching 1 mu M in the core) is proposed based on a previously
published CRIO criterion from the eastern South Pacific OMZ and including a large range of O(2)
concentrations. Using the updated global WOA2005 O(2) climatology, the four known tropical OMZs
in the open ocean have been described: the Eastern South Pacific and Eastern Tropical North
Pacific, in the Pacific Ocean; the Arabian Sea and Bay of Bengal, in the Indian Ocean. Moreover,
the Eastern Sub-Tropical North Pacific (25-52 degrees N) has been identified as a lesser known
permanent deep OMZ. Two additional seasonal OMZs at high latitude have also been identified: the
West Bering Sea and the Gulf of Alaska. The total surface of the permanent OMZs is 30.4 millions
of km(2) (similar to 8% of the total oceanic area), and the volume of the OMZ cores (10.3
millions of km(3)) corresponds to a value similar to 7 times higher than previous evaluations.
The volume of the OMZ cores is about three times larger than that of the associated
denitrification zone, here defined as NMZ ('nitrate deficit or NDEF > 10 mu M' maximum zone).
The larger OMZ, relative to the extent of denitrification zone, suggests that the unbalanced
nitrogen cycle on the global scale could be more intense than previously recognized and that
evaluation of the OMZ from denitrification could underestimate their extent. (C) 2008 Elsevier
Ltd. All rights reserved.