Methane measurements during POLARSTERN cruise ARK-XXII/2

A methane surplus relative to the atmospheric equilibrium is a frequently observed feature of ocean surface water. Despite the common fact that biological processes are responsible for its origin, the formation of methane in aerobic surface water is still poorly understood. We report on methane production in the central Arctic Ocean, which was exclusively detected in Pacific derived water but not nearby in Atlantic derived water. The two water masses are distinguished by their different nitrate to phosphate ratios. We show that methane production occurs if nitrate is depleted but phosphate is available as a P source. Apparently the low N:P ratio enhances the ability of bacteria to compete for phosphate while the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) is utilized as a C source. This was verified by experimentally induced methane production in DMSP spiked Arctic sea water. Accordingly we propose that methylated compounds may serve as precursors for methane and thermodynamic calculations show that methylotrophic methanogenesis can provide energy in aerobic environments.

Bulk properties and isotopic data of sediment cores from the Bounty Trough and Tasman Sea off New Zealand

Glacial/interglacial changes in Southern Ocean's air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate- to deep-water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943 - 2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

Station characteristics and copepod fatty alcohol, fatty acid and lipid composition during MSM02/4 in Fram Strait

The biodiversity of pelagic deep-sea ecosystems has received growing scientific interest in the last decade, especially in the framework of international marine biodiversity initiatives, such as Census of Marine Life (CoML). While a growing number of deep-sea zooplankton species has been identified and genetically characterized, little information is available on the mechanisms minimizing inter-specific competition and thus allowing closely related species to co-occur in the deep-sea pelagic realm. Focussing on the two dominant calanoid copepod families Euchaetidae and Aetideidae in Fram Strait, Arctic Ocean, the present study strives to characterize ecological niches of co-occurring species, with regard to vertical distribution, dietary composition as derived from lipid biomarkers, and trophic level on the basis of stable isotope signatures. Closely related species were usually restricted to different depth layers, resulting in a multi-layered vertical distribution pattern. Thus, vertical partitioning was an important mechanism to avoid inter-specific competition. Species occurring in the same depth strata usually belonged to different genera. They differed in fatty acid composition and trophic level, indicating different food preferences. Herbivorous Calanus represent major prey items for many omnivorous and carnivorous species throughout the water column. The seasonal and ontogenetic vertical migration of Calanus acts as a short-cut in food supply for pelagic deep-sea ecosystems in the Arctic.