Die Regulation von Eisen- und Sulfat-Reduktion in den Sedimenten arktischer Fjorde in Zeiten des Klimawandels
Mikrobielle Ökologie und Angewandte Mikrobiologie
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
The goal of my research was to elucidate how glacial retreat and reduced sea ice cover, caused by the ongoing climate change, may influence the balance between iron and sulfate reduction in Arctic sediments and how this will change the general sediment biogeochemistry. To achieve this goal, I investigated sedimentary Fe, S and C biogeochemistry in longitudinal transects in three contrasting fjords along the west coast of Svalbard, as fjord sediments are known to exhibit strong latitudinal gradients in biogeochemical conditions. The fjords differed in their bedrock lithology and/or the type of glacier. Kongsfjorden (KF) and Lilliehöökfjorden (LF) both have large tidewater glaciers at their head but the glaciers override bedrocks with different lighologies, which enabled to assess the effect of differing bedrock lithology on the Fe, S and C biogeochemistry of fjord sediments. The glaciers of KF and Dicksonfjorden (DF) override similar bedrocks, but DF has, in contrast to KF, only land-terminating glaciers. The comparison of these two fjords enabled to assess the effects of retreating glaciers on Fe, S and C biogeochemistry of fjord sediments. The results show that, as expected, there are strong gradients of organic carbon content and quality in all three investigated fjord sediments, with increasing organic carbon content and decreasing C:N ratios with increasing distance from the glacial source. Investigation of the sedimentary content of Fe(III) and its reactivity showed that in all three fjords there was relatively little reactive Fe(III) (FeR) in sediments close to the glacier, but that the amount and reactivity of FeR increased with increasing distance from the glacial source. At the same time, the sedimentary content of total Fe did not change significantly along the fjord axis. This increase in the amount and the reactivity of FeR with increasing distance from the glacial source is likely caused by an “activation” of relatively unreactive glacially delivered Fe(III) through benthic Fe-cycling by an interplay of biotic and abiotic processes. During this cycling, the, under conditions as they are found in the fjord sediments, unsoluble Fe(III) gets reduced to soluble Fe(II), which diffuses into the oxic zone of the sediments, where it oxidizes and forms highly reactive, authigenic, poorly crystalline Fe(III) (oxyhydr)oxides. The most important factor leading to the observed gradient in the content and reactivity of FeR with distance from the glacial source is the drastic change in sedimentation rate along the longitudinal fjord axis, giving sedimentary processes more time for Fe “activation” before burial with increasing distance from the glacier. Moreover, the gradient in organic carbon and abundance of bioturbating benthic fauna along the fjord axis, increase the potential for Fe-cycling and thus “activation” of unreactive Fe further away from the glacier. In KF and DF the trend in reactive Fe(III) over distance from the glacial source looked very similar despite the difference in glacier type. However, while in KF the amount and reactivity of FeR was highest at the sediment surface and decreased downcore, in DF this trend looked different and the highest amount of FeR was found several cm below the sediment surface and Fe2+ was absent from the top few centimeters of the sediment. It is well established that the hydrology of a fjord changes when glaciers retreat on land, leading to lower primary productivity, as upwelling of nutrients is attenuated. If the primary productivity is lower in fjords that only have land-terminating glaciers, such as DF, sediments in these fjords also receive less input of fresh organic material. Therefore, microbial activity in the sediments is low and oxidants penetrate deeper into the sediment, preventing reduced substances, such as Fe2+, from reaching the sediment surface and thereby lowering the potential flux to the water column. This likely is an additional negative feedback mechanism for primary productivity. The results of the project highlight the importance of fjord sediments as active biogeochemical interfaces, in which the relatively unreactive glacially derived Fe(III) gets “activated” and becomes more reactive i.e. less crystalline. Moreover, I could show that the sediment’s function as a source of Fe to the water column could change with retreating glaciers, what could be an additional negative feedback for primary productivity.
Projektbezogene Publikationen (Auswahl)
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04/2018: 5th International Symposium on Microbial Sulfur Metabolism, Vienna, Austria. “Glacial influence on cryptic sulfur cycling in Arctic fjord sediments”
Laufer K, Michaud AB, Antler G, Pellerin A, Findlay A, Røy H, Jørgensen BB
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(2019). Complex microbial communities drive iron and sulfur cycling in Arctic fjord sediments. Applied and Environmental Microbiology, 85(14): e00949-19
Buongiorno J, Herbert L, Wehrmann L, Michaud AB, Laufer K, Røy H, Jørgensen BB, Szynkiewicz A, Faiia A, Yeager K, Schindler K, Lloyd K
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01/2019: IASC workshop, Geilo, Norway. “Impact of glacially-derived Fe(III) on carbon mineralization pathways in arctic fjord sediments”
Laufer K, Michaud AB, Røy H, Jørgensen BB
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08/2019: Goldschmidt Conference, Barcelona, Spain. “Spatial distribution of reactive Fe(III) in Arctic fjord sediments – impact of different glacial sources and benthic Fe-cycling”
Laufer K, Michaud AB, Røy H, Jørgensen BB
