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Methane Output from Cold Seep Structures Along the Central American Continental Margin

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Methane Output from Cold Seep Structures Along the Central American Continental Margin METHANE OUTPUT FROM COLD SEEP STRUCTURES ALONG THE CENTRAL AMERICAN CONTINENTAL MARGIN Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Susan Mau Kiel 2004 Referent/in: ...Prof. Erwin Suess............................................................. Korreferent/in: ...PD. Dr. Klaus Wallmann ............................................ Tag der mündlichen Prüfung: ...7. Dezember 2004................................ Zum Druck genehmigt: ...Kiel, den 15. Dezember 2004........................ Der Dekan Hiermit erkläre ich, daß ich die vorliegende Doktorarbeit selbständig und ohne unerlaubte Hilfen erstellt habe. Ferner habe ich weder diese noch eine ähnliche Arbeit an einer anderen Abteilung oder Hochschule im Rahmen eines Prüfungsverfahrens vorgelegt, veröffentlicht oder zur Veröffentlichung vorgelegt. Susan Mau Preface This study comprises a general introduction presented in Chapter I, three stand-alone publishable papers – Chapter II to IV and an overall summary and conclusion – Chapter V. Each of the Chapters II, III and IV contains a separate introduction, description of methods, presentation of data and discussion as well as a separate reference list. Chapter II has already been submitted for publication in Marine Geology; Chapters III and IV are to be submitted to Global Biochemical Cycles and Geo-Marine Letters, respectively and might be subject to revisions. The titles and authors of the papers (Chapters II, III and IV) are briefly listed below: Chapter II Estimates of methane output from mud extrusions at the erosive convergent margin off Costa Rica Authors S. Mau, H. Sahling, G. Rehder, E. Suess, P. Linke and E. Soeding Status submitted to Marine Geology Chapter III Sources, fate and output of methane from cold seeps in Jaco Scarp, an embayment caused by seamount subduction offshore Costa Rica Authors S. Mau, G. Rehder, E. Soeding, H. Sahling, E. Suess and K. Stange Status to be submitted to Global Biogeochemical Cycles Chapter IV Variations in CH4 seepage from mud extrusions and landslides offshore Costa Rica affected by seismo-tectonics Authors S. Mau, G. Rehder, I. G. Arroyo, J. Gossler and E. Suess Status to be submitted to Geo-Marine Letters I contributed to these papers by: work at sea, acquisition and processing of all methane data and current velocity data, interpretation of the data and calculation of methane output (Chapter II and III), correlation of methane data to oceanographic data and earthquake data (Chapter IV), graphical presentation of methane data, writing and preparation of manuscript. Abstract Methane seepage from different geological structures was investigated along the convergent margin off the Pacific coast of Costa Rica. This segment of subduction zone is dominated by tectonic erosion rather than sediment accretion. Thus, in contrast to accretionary margins, where sediments accumulate and fluids derive mainly from compaction and diagenesis of these sediments, the sediment load at an erosive margin is much less and fluids derive predominantly from the subducted sediments from greater depths. These fluids ascend through high-permeability conduits to the seafloor and discharge into the ocean at various vent sites. Mud extrusions were found to be the most abundant fluid expelling structures along the Costa Rican margin. Therefore, four mud extrusions were investigated in detail to identify the fate and the amount of CH4 discharging into the ocean. Similarly, one of the several embayments caused by the subduction of seamounts (scarps) was investigated, because hardly anything is known of their seeping potential. In addition, several of these structures were repeatedly sampled over the course of four years to study long-term variations in CH4 seepage. All investigated sites show active fluid seepage indicated by chemosynthetic communities, authigenic carbonates and CH4 plumes in the water column. The vent derived CH4 is mainly dispersed and diluted by mixing with background water. Only a part of the CH4 is oxidized in the center of the plume in Jaco Scarp where concentrations of CH4 are high, and close to the scarp’s slopes. This results probably from diminished current flow and/or other components influencing the pathway of oxidation. However, the fate of the emitted CH4 is rather dilution than oxidation. The CH4 outputs were estimated based on measurements of near-bottom CH4 concentrations and current velocities. This approach comprises transient, fast and slow CH4 seepage. The estimates per mud extrusion range between 1 – 10 Mg yr-1 (1 Mg = 106 g) and are lower than those reported from other mud extrusions at accretionary and passive margins which may be due to the erosive nature of the subduction. Assuming a similar vent activity at all 48 mounds -1 discovered offshore Costa Rica and constant seepage over time, 307 Mg yr of CH4 are emitted from mud extrusions along this segment of the subduction zone. The CH4 output from -1 Jaco Scarp yields a value of 61 Mg yr . This amount equals the CH4 emission of ~10 mud extrusions, but other scarps in the area seem to be less active. Assuming a vent activity of 5%, I which is deduced from data at Parrita Scarp, at each of the other three scarps compared to -1 -1 Jaco Scarp, 70 Mg yr would be emitted from all four scarps. In total, 377 Mg yr of CH4 is discharged into the ocean. However, repeated measurements at these geological structures indicate long-term variations in methane seepage. These changes appear to be related to earthquake activity. Higher CH4 concentrations in 1999 and 2002 correlate with major earthquakes occurring in these years whereas CH4 content was less in 2003 – a year of lowered seismic activity. The temporal changes of CH4 emission emphasize that estimates of CH4 output cannot be simply extrapolated. The amount of CH4 released from mud extrusions and scarps offshore Costa Rica adds up to ~0.2‰ of the global CH4 output from the seafloor, which was estimated to be in the order of 20 Tg yr-1 (1 Tg = 1012 g). The total amount emitted from the margin is most likely higher, because the estimated value does not include CH4 seepage from other vent sites e.g. small landslides and enhanced CH4 output from sites, that have not been investigated in detail. However, the estimated CH4 outputs present the first ones from an erosive margin and are, thus, of great significance for assessments of the global methane output in general and of specific geological structures, e.g. mud extrusions, in particular. II Zusammenfassung Das Ausströmen methanreicher Fluide von unterschiedlichen geologischen Strukturen wurde entlang des aktiven Kontinentalrandes der pazifischen Küste Costa Ricas untersucht. Dieser Abschnitt der Subduktionszone ist von tektonischer Erosion anstatt von Sedimentakkretion geprägt. Im Unterschied zu einem akkretionären Kontinentalrand, an dem Sedimente akkumuliert werden und Fluide vor allem aus diesen durch Kompaktion und Diagenese beeinflußten Sedimenten stammen, sind an erosiven Kontinentalrändern bedeutend weniger Sedimente zu finden. Fluide, die an diesen Kontinentalrändern austreten, stammen größtenteils aus den subduzierten Sedimenten und somit aus größerer Tiefe. Sie steigen entlang hoch permeabler Zonen bis zum Meeresboden auf, wo sie an verschiedenartigen Austrittsstellen in den Ozean strömen. Schlammextrusionen sind die am häufigsten auftretenden Fluidquellen am Kontinentalrand von Costa Rica. Aufgrund dessen wurden vier Schlammextrusionen im Detail untersucht. Insbesondere galten die Untersuchungen den Reaktionen von CH4 in der Wassersäule und dienten dazu, die Menge von austretendem CH4 abzuschätzen. Eine Hangabrutschung, die durch die Subduktion von Seamounts verursacht wurde, wurde ebenfalls untersucht. Über Methanaustritte an diesen Strukturen ist bis heute kaum etwas bekannt. Zusätzlich wurden einige Schlammextrusionen und Hangabrutschungen über einen Zeitraum von vier Jahren mehrfach beprobt, um zeitliche Veränderungen der Methanausflüsse festzustellen. Chemoautotrophe Organismen, authigene Karbonate und Methananreicherungen in der Wassersäule wurden an all diesen geologischen Strukturen gefunden. Diese Merkmale deuten auf ein aktives Austreten von Fluiden hin. Das in den Ozean strömende CH4 wird hauptsächlich durch Vermischung mit Meerwasser verteilt und verdünnt. Dagegen wurde Oxidation von CH4 nur im Zentrum der „Methanwolke“ im Jaco Scarp – wahrscheinlich aufgrund stark erhöhter Methankonzentrationen – und entlang der Ränder dieser Hangabrutschung festgestellt. Letzteres ist vermutlich zurückzuführen auf geringere Strömungsgeschwindigkeiten und/oder zusätzlicher Komponenten, die den Ablauf der Oxidation beeinflussen. Die Methanemissionen wurden basierend auf gemessenen bodennahen CH4-Konzentrationen und Strömungsgeschwindigkeiten abgeschätzt. Diese Abschätzung beinhaltet kurzzeitiges, schnelles und langsames Ausströmen von CH4. Die abgeschätzten Methanflüsse pro III Schlammextrusion von 1 – 10 Mg a-1 (1 Mg = 106 g) sind gering im Vergleich zu anderen Extrusionen, die an akkretionären und passiven Kontinentalrändern vorkommen. Das hängt wahrscheinlich mit dem erosiven Charakter der Subduktion vor Costa Rica zusammen. Unter der Annahme, daß alle 48 vor Costa Rica entdeckten Schlammextrusionen gleich aktiv sind -1 und der Ausfluß über die Zeit konstant ist, würden 307 Mg a CH4 entlang dieses Teiles des Kontinentalrandes aus Schlammextrusionen ausströmen. Der Methanfluß von Jaco Scarp beträgt 61 Mg a-1. Diese
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