ALBERT

Beschreibung: This work was integrated into the Russian-German joint venture project System Laptev Sea 2000. The focus of this project was to extent our knowledge about the Laptev Sea system in north-east Siberia obtained through previous Russian-German projects, such as the Laptev Sea System (1994-1997), and Taymyr (1994-1997). Studies on Quaternary environmental changes, Arctic coastal dynamics and recent periglacial processes including ecological studies on permafrost soils and ecosystems of the central Lena Delta were in the focus of the recent project (Schirrmeister et al., 2004). The present work contributes to the last aspect by investigating the field of methane fluxes in polygonal tundra environments of the Lena Delta. Field work and sampling of this study was conducted during the expeditions LENA 2002 and LENA 2005 (Samoylov Island, Lena Delta, Siberia) with a personal participation in the last expedition. The work was performed in the frame of the International Max Planck Research School for Marine Microbiology (MarMic) mainly at the Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam. Some analyses were conducted at the Max Planck Institute for Marine Microbiology in Bremen. This study is presented in English as a cumulative Ph.D. thesis at the University of Bremen (Fachbereich 02). The thesis consists of a general introduction to the particular research field including the scientific background as well as aims and objectives of this study. The study area is described in an extra chapter. The main part of this thesis consists of three manuscripts with first authorship and a final synthesis representing the conclusions as well as critical remarks and future prospects of this work.

Beschreibung: Permafrost region subsurface organic carbon (OC) pools are a major component of the terrestrial carbon cycle and vulnerable to a warming climate. Thermokarst lagoons are an important transition stage with complex depositional histories during which permafrost and lacustrine carbon pools are transformed along eroding Arctic coasts. The effects of temperature and salinity changes during thermokarst lake to lagoon transitions on thaw history and lagoon deposits are understudied. We analyzed two 30-m-long sediment cores from two thermokarst lagoons on the Bykovsky Peninsula, Northeast Siberia, using sedimentological, geochronological, hydrochemical, and biogeochemical techniques. Using remote sensing we distinguished between a semi-closed and a nearly closed lagoon. We (1) characterized the depositional history, (2) studied the impact of marine inundation on ice-bearing permafrost and taliks, and (3) quantified the OC pools for different stages of thermokarst lagoons. Fluvial and former Yedoma deposits were found at depth between 30 and 8.5 m, while lake and lagoon deposits formed the upper layers. The electrical conductivity of the pore water indicated hypersaline conditions for the semi-closed lagoon (max: 108 mS/cm), while fresh to brackish conditions were observed beneath a 5 m-thick surface saline layer at the nearly closed lagoon. The deposits had a mean OC content of 15 ± 2 kg/m3, with higher values in the semi-closed lagoon. Based on the cores we estimated a total OC pool of 5.7 Mt-C for the first 30 m of sediment below five mapped lagoons on the Bykovsky Peninsula. Our results suggest that paleo river branches shaped the middle Pleistocene landscape followed by late Pleistocene Yedoma permafrost accumulation and early Holocene lake development. Afterward, lake drainage, marine flooding, and bedfast ice formation caused the saline enrichment of pore water, which led to cryotic talik development. We find that the OC-pool of Arctic lagoons may comprise a substantial inventory of partially thawed and partially refrozen OC, which is available for microbial degradation processes at the Arctic terrestrial-marine interface. Climate change in the Arctic leading to sea level rise, permafrost thaw, coastal erosion, and sea ice loss may increase the rate of thermokarst lagoon formation and thus increase the importance of lagoons as biogeochemical processors of former permafrost OC.