ALBERT

Description: Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can mod- ify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predic- tions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes con- trolling organic matter cycling in the coastal Peruvian up- welling system. Eight mesocosms, each with a volume of ∼ 55 m3, were deployed for 50 d ∼ 6 km off Callao (12◦ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected sub- surface waters at two different locations in the regional oxy- gen minimum zone (OMZ) and injected these into four meso- cosms (mixing ratio ≈ 1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the indi- vidual mesocosms. The mesocosm phytoplankton commu- nities were initially dominated by diatoms but shifted to- wards a pronounced dominance of the mixotrophic dinoflag- ellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. san- guinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decou- pled from surface production and sustained by the remain- ing plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self- shading by phytoplankton and by inorganic nitrogen limita- tion which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few oppor- tunities for blooms, which represents an event where the sys- tem becomes unbalanced. Overall, our mesocosm study re- vealed some key links between ecological and biogeochem- ical processes for one of the most economically important regions in the oceans.

Description: We report on the development of a three-axis absolute vector magnetometer suited for mobile operation in the Earth’s magnetic field. It is based on low critical temperature dc superconducting quantum interference devices (LTS dc SQUIDs) with sub-micrometer sized cross-type Josephson junctions and exhibits a white noise level of about 10 fT/Hz 1/2 . The width of superconducting strip lines is restricted to less than 6 μ m in order to avoid flux trapping during cool-down in magnetically unshielded environment. The long-term stability of the flux-to-voltage transfer coefficients of the SQUID electronics is investigated in detail and a method is presented to significantly increase their reproducibility. We further demonstrate the long-term operation of the setup in a magnetic field varying by about 200 μ T amplitude without the need for recalibration.

Description: A new ultra-low temperature experiment including a superconducting vector magnet has been developed for soft x-ray absorption spectroscopy experiments at third generation synchrotron light sources. The sample is cooled below 50 mK by a cryogen free 3 He- 4 He dilution refrigerator. At the same time, magnetic fields of up to ±7 T in the horizontal direction and ±0.5 T in the vertical direction can be applied by a superconducting vector magnet. The setup allows to study ex situ and in situ prepared samples, offered by an attached UHV preparation chamber with load lock. The transfer of the prepared samples between the preparation section and the dilution refrigerator is carried out under cryogenic temperatures. First commissioning studies have been carried out at the Variable Polarization XUV Beamline P04 at PETRA III and the influence of the incident photon beam to the sample temperature has been studied.

Description: The isotopic composition of water in ice sheets is extensively used to infer past climate changes. In low-accumulation regions their interpretation is however challenged by poorly constrained effects that may influence the initial isotope signal during and after deposition of the snow. This is reflected in snow-pit isotope data from Kohnen Station, Antarctica, which exhibit a seasonal cycle but also strong inter-annual variations that contradict local temperature observations. These inconsistencies persist even after averaging many profiles and are thus not explained by local stratigraphic noise. Previous studies have suggested that post-depositional processes may significantly influence the isotopic composition of East Antarctic firn. Here, we investigate the importance of post-depositional processes within the open-porous firn (〉 10 cm depth) at Kohnen Station by separating spatial from temporal variability. To this end, we analyse 22 isotope profiles obtained from two snow trenches and examine the temporal isotope modifications by comparing the new with published trench data extracted 2 years earlier. The initial isotope profiles undergo changes over time due to downward-advection, firn diffusion and densification in magnitudes consistent with independent estimates. Beyond that, we find further modifications of the original isotope record to be unlikely, or small in magnitude (〈〈 1 per mil RMSD). These results show that the discrepancy between local temperatures and isotopes most likely originates from spatially coherent processes prior to or during deposition, such as precipitation intermittency or systematic isotope modifications acting on drifting or loose surface snow.

Description: Isotopic measurements of seawater sampled on-board Polarstern research vessel

Description: Isotopic measurements of seawater sampled on-board Polarstern research vessel

Description: Isotopic measurements of seawater sampled on-board Polarstern research vessel

Description: Isotopic measurements of seawater sampled on-board Polarstern research vessel