A re-evaluation of the magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the ocean

We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition, and compare this to fluvial inputs and di-nitrogen fixation. We evaluate the scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological di-nitrogen fixation is the main external source of nitrogen to the open ocean, i.e. beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of ~0.4% (equivalent to an uptake of 0.15 Pg C yr-1 and less than the Duce et al., 2008 estimate). The resulting reduction in climate change forcing from this ocean CO2 uptake is offset to a small extent by an increase in ocean N2O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs

JGOFS-Indik III. Quantifizierung und Modellierung des Umsatzes von Kohlenstoff und assoziierten Elementen im Arabischen Meer Abschlussbericht der Koordination fuer die dritte Projektphase, 01.01.1999-31.12.2000

The program investigates the processes controlling the transfer of climate-relevant trace gases between the ocean and the atmosphere. It contributes to the international Joint Global Flux Study (JGOFS) process study in the Indian Ocean and is part of the International Geosphere-Biosphere Program (IGBP). The general aims are: - Characterization of the monsoon-controlled biogeochemical processes and a quantification of the role of the Arabian Sea fold global biogeochemical cycles, esp. of the importance of this region as a source or a sink of CO_2 and other green house gases - Examination of the seasonality of biogeochemical processes with high temporal resolution -Quantification of primary productivity and the transformation as well as the whereabouts of biologically fixed carbon - Reconstruction of monsoon-variability during the Quaternary. The main specific goals of the 3"r"d phase of the program were: - Evaluation and interpretation of gathered data and their compilation in a national data bank - Development and validation of coupled physical-biogeochemical models with variable ecosystem complexity - Integration of results into the international JGOFS synthesis. Scientists from 10 German research institutions participated within 15 projects of the program. (orig.)Das Vorhaben untersucht Prozesse im Arabischen Meer, die den Austausch von klimarelevanten Spurengasen zwischen Atmosphaere und Ozean steuern und stellt ein Beitrag zur 'Joint Global Ocean Flux Study' (JGOFS) Prozessstudie im Indischen Ozean dar, die Bestandteil des 'International Geosphere-Biosphere Program' (IGBP) ist. Die allgemeinen Ziele sind: - Charakterisierung der vom Monsun gesteuerten biogeochemischen Prozesse und Quantifizierung der relativen Bedeutung des Arabischen Meeres fuer die globalen biogeochemischen Kreislaeufe, insbesondere der Rolle dieser Region als Quelle oder Senke fuer CO_2 und andere klimarelevante Gase, - Untersuchung der Saisonalitaet biogeochemischer Prozesse in hoher zeitlicher Aufloesung, - Quantifizierung der Produktion, der Umsetzung und des Verbleibs biologisch fixierten Kohlenstoffs, - Rekonstruktion der Monsunvariabilitaet im Quartaer. Die wichtigsten spezifischen Ziele im Foerderzeitraum (3. Projektphase) waren: - Auswertung und Interpretation der Daten sowie ihre Zusammenfassung in einer Datenbank - Entwicklung und Validierung von gekoppelten, physikalisch-biogeochemischen Modellen mit unterschiedlicher Oekosystem-Komplexizitaet, - Einbindung der Ergebnisse in die internationale JGOFS-Synthese. Im Foerderzeitraum waren Wissenschaftler in 15 Teilprojekten aus 10 nationalen Forschungseinrichtungen beteiligt. (orig.)SIGLEAvailable from TIB Hannover: F02B448 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung und Forschung, Berlin (Germany)DEGerman

Changes in ocean denitrification during Late Carboniferous glacial–interglacial cycles

Denitrification (the process by which nitrate and nitrite are reduced to nitrogen gas) in the oxygen-minimum zones of modern oceans is an important part of the global nitrogen cycle. Variations in rates of denitrification over Quaternary glacial-interglacial timescales may have affected global climate. Evidence of denitrification has been reported from some older marine systems, but it is unclear whether denitrification rates varied during pre-Quaternary glacial cycles. Here we present ratios of organic carbon to nitrogen and nitrogen isotope data from the Upper Carboniferous black shales of the North American mid-continent. In these cyclic deposits, we find evidence of variations in the intensity of denitrification in the eastern tropical Panthalassic Ocean associated with glacially driven sea-level changes. Sedimentary 15N increases during the interval of rapid sea-level rise in each cycle, indicative of intensified denitrification, before returning to background levels as sea level stabilized during the interglacial phase. Nearly identical patterns of denitrification have been observed in the eastern tropical Pacific during the Quaternary period. We therefore conclude that ice ages have produced similar oceanographic conditions and nitrogen cycle dynamics in these regions over the past 300 million years. © 2008 Macmillan Publishers Limited