To better understand the present and past carbon cycling and transformation processes in methane-influenced fjord and shelf areas of northern Norway, we compared two sediment cores from the Hola trough and from Ullsfjorden. We investigated (1) the organic matter composition and sedimentological characteristics to study the sources of organic carbon (C org ) and the factors influencing C org burial, (2) pore water geochemistry to determine the contribution of organoclastic sulfate reduction and methanogenesis to total organic carbon turnover, and (3) the carbon isotopic signature of hydrocarbons to identify the carbon transformation processes and gas sources. High sedimentation and C org accumulation rates in Ullsfjorden support the notion that fjords are important C org sinks. The depth of the sulfate-methanetransition (SMT) in the fjord is controlled by the supply of predominantly marine organic matter to the sediment. Organoclastic sulfate reduction accounts for 60% of the total depth-integrated sulfate reduction in the fjord. In spite of the presence of ethane, propane, and butane, we suggest a purely microbial origin of light hydrocarbons in the sediments based on their low d 13 C values. In the Hola trough, sedimentation and C org accumulation rates changed during the deglacial-to-post-glacial transition from approximately 80 cm ka 21 to erosion at present. Thus, C org burial in this part of the shelf is presently absent. Low organic matter content in the sediment and low rates of organoclastic sulfate reduction (only 3% of total depth-integrated sulfate reduction) entail that the shallow depth of the SMT is controlled mostly by ascending thermogenic methane from deeper sources.

No abstract

No abstract

Zusammenfassung81 Kr (T1/2 229.000 a) ist ein idealer Datierungstracer für alte Tiefengrundwässer. Die Oberjura-Formation im tiefen Teil des Molassebeckens stellt ein herausragendes Georeservoir für thermale Tiefenwässer (bis 140°C) dar. Über die genutzten Thermalwässer mit zumeist kaltzeitlicher Bildungscharakteristik (Na-HCO3-Cl-Typ) ist jedoch im Hinblick auf die Neubildungsprozesse, Herkunftsgebiete und Fließdynamik wenig bekannt. Für die Interpretation der Genese und Entwicklung (Ionen-und Isotopenaustausch, Gasflüsse, etc.) fehlen bislang verlässliche Altersinformationen. Erstmals wurden nun neun thermale Tiefenwässer erfolgreich durch 81 Kr/ 85 Kr-ATTA-Untersuchungen datiert. Die abgeleiteten Altersinformationen zeigen im westlichen und zentralen Molassebecken vorherrschend eine Bildung während der letzten Kaltzeit (Würm-Glazial), die sehr gut zur subglazialen Bildungshypothese über alpennahe, sehr mächtige Deckschichten hinweg passt. Im Ostteil des Molassebeckens weisen die Tiefenwässer hingegen einheitlich deutlich höhere Alterscharakteristiken (Günz/Mindel Interglazial) bzw. ein langsameres Strömungssystem auf, das allenfalls durch geringe Neubildungsanteile aus den jüngeren alpinen Vergletscherungen beeinflusst ist. 81 Kr/ 85 Kr-Dating of thermal groundwaters in the Upper Jurassic (Molasse Basin)Abstract 81 Kr (half-life 229,000 years) is an ideal tracer for old groundwater. The Upper Jurassic rock in the deep Molasse Basin is an outstanding geothermal groundwater reservoir (with temperatures up to 140°C). However, due to the complex groundwater evolution (ion and isotope exchange, gas flux, etc.), comprehensive hydrogeological studies completed to date, including 14 C-DIC and He isotopes, could not resolve the recharge dynamics and residence times. Nine geothermal wells were therefore sampled for 81 Kr/ 85 Kr employing the laser-based atom trap tracer analysis technique (ATTA). In the western and central basin, the results reveal predominant groundwater recharge during the last glacial period with one sample influenced by infiltration during the earlier glacial period. Recharge signatures and 81 Kr-model-ages fit very well to subglacial recharge with cross-formational flow through the sedimentary cover (600 to >3000 m deep). In the eastern basin, the results point to the Cromerian complex, indicating a slower flow system with less influence from recharge during glacial periods.

No abstract