Fluid discharge at different seep environments : distribution, discharge rate, influence on particle resuspension.

Karpen, Volker A. (2002) Fluid discharge at different seep environments : distribution, discharge rate, influence on particle resuspension. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Mathematisch-Naturwissenschaftliche Fakultät, Kiel, Germany, 108 pp.

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Abstract

In this study submarine seepages were quantified and their implication within the marine environment was investigated. This work was accomplisheJ wiLh Lhe m;e of a new optical schlieren technique application. In different benthic environments - from the littoral to the abyssal - groundwater discharges, cold seeps, and hot vents have been known since deep sea exploration became possible. Nevertheless, many of the seeps are not obviously visible and a high technical and logistical effort is needed to explore them. A new instrument was developed, which enables an optical investigation of different seep types. This instrument was successfully deployed in two different marine environments influenced by different seep systems. First deployments were performed in Eckernforde Bay, western Baltic Sea where the impact of seep activities was observed at numerous stations. A general distinction between active seep sites and seep-influenced stations was carried out. The simultaneous deployment of a CTD-sen sor was used to confirm the video-recordings. Heterogeneous water masses with smallest density anomalies of Δσt = 0.049 were detected optically and by the CTD. In this study it was discovered that even a fluid with a density less than that of the ambient seawater does not ascend directly but is captured within the benthic boundary layer. Eckernförde Bay is characterized by the influence of an aquifer system, which has been the focus of several studies. The determination of the outer border of this seepage-influenced area is still vague. A general survey around known fluid-influenced pockmarks was carried out to detect new active seeps and the distribution of discharged groundwater into the ambient bottom-water. For this study a seep-influenced area of 6.3 km² was determined. Within this area it was possible to observe the distribution of discharged fluid at the sediment water interface. Several active seep sites were detected and a determination of the fluid discharge rates was performed using image processing software. Flow rates varied between 0.05 and 0.7 lm-2min-1 and the flux of a single focussed plume was determined to be as high as 59.6 ml cm-2 min-1. The results present new insights into the variability of discharge patterns as well as into the dispersion of fluids into adjacent regions. A first deployment at a water depth of about 800 m was performed at Hydrate Ridge, Cascadia subduction zone. Active cold seeps were investigated using the new optical application with a TV-guided frame. At two of the stations, discharged fluid was recorded, thus the optical investigation of fluid discharge even at this depth was successful. Further experimental investigations were carried out to study the interaction of fluid and gas bubble discharges forcing the resuspension of aggregates from the sediment-water interface. Sediment cores from both environments described above were obtained and integrated into seawater flumes. A fluid discharge through the sediment cores was simulated and recorded. The resuspended aggregates were counted and measured using image processing software. The median diameters of aggregates from Hydrate Ridge sediments varied between 0.121 and 0.183 mm. The aggregates from the Eckernförde Bay were larger with medians of 0.163-0.612 mm. The escape of bubbles leads to the resuspension of particles in the same size range. A simulated focussed plume which was comparable to a naturally observed one, had a discharge rate of 0.13 ml s-1. Thirteen aggregates per second were resuspended in this water mass. Also, the fluid entrainment due to the bubble discharge was made visible and the transport mass of pore fluid was determined with image processing software. A single ascending bubble (5 x 3.5 mm) with a vertical velocity of 20 cm s-1 entrained a fluid body of 30 μl in its wake up to 4 cm into the water column. These new experiments revealed that active seeps have to be taken into account for the calculation of resuspension fluxes. The size of the resuspended fraction is comparable to known size ranges of aggregates within the benthic boundary layer.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Suess, Erwin
Additional Information: Buch vorhanden in der GEOMAR-Bibliothek / Book in stock in the GEOMAR Library: GEOCH-MAR 45
Keywords: Eckernförde Bay; Cold Seeps; Cascade Range; Subduction; Fluid flow
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Projects: Dynamik globaler Kreisläufe im System Erde
Date Deposited: 26 Jul 2012 09:06
Last Modified: 31 Jan 2023 09:42
URI: https://oceanrep.geomar.de/id/eprint/14921

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