@misc{HodgkinsRichardsonDommainetal.2018, author = {Hodgkins, Suzanne B. and Richardson, Curtis J. and Dommain, Ren{\´e} and Wang, Hongjun and Glaser, Paul H. and Verbeke, Brittany and Winkler, B. Rose and Cobb, Alexander R. and Rich, Virginia I. and Missilmani, Malak and Flanagan, Neal and Ho, Mengchi and Hoyt, Alison M. and Harvey, Charles F. and Vining, S. Rose and Hough, Moira A. and Moore, Tim R. and Richard, Pierre J. H. and De La Cruz, Florentino B. and Toufaily, Joumana and Hamdan, Rasha and Cooper, William T. and Chanton, Jeffrey P.}, title = {Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1125}, issn = {1866-8372}, doi = {10.25932/publishup-45965}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-459658}, pages = {15}, year = {2018}, abstract = {Peatlands represent large terrestrial carbon banks. Given that most peat accumulates in boreal regions, where low temperatures and water saturation preserve organic matter, the existence of peat in (sub)tropical regions remains enigmatic. Here we examined peat and plant chemistry across a latitudinal transect from the Arctic to the tropics. Near-surface low-latitude peat has lower carbohydrate and greater aromatic content than near-surface high-latitude peat, creating a reduced oxidation state and resulting recalcitrance. This recalcitrance allows peat to persist in the (sub)tropics despite warm temperatures. Because we observed similar declines in carbohydrate content with depth in high-latitude peat, our data explain recent field-scale deep peat warming experiments in which catotelm (deeper) peat remained stable despite temperature increases up to 9 degrees C. We suggest that high-latitude deep peat reservoirs may be stabilized in the face of climate change by their ultimately lower carbohydrate and higher aromatic composition, similar to tropical peats.}, language = {en} } @misc{PudellMaznevHerzogetal.2018, author = {Pudell, Jan-Etienne and Maznev, Alexei and Herzog, Marc and Kronseder, M. and Back, Christian and Malinowski, Gregory and von Reppert, Alexander and Bargheer, Matias}, title = {Layer specific observation of slow thermal equilibration in ultrathin metallic nanostructures by femtosecond X-ray diffraction}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {797}, issn = {1866-8372}, doi = {10.25932/publishup-42623}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-426233}, pages = {7}, year = {2018}, abstract = {Ultrafast heat transport in nanoscale metal multilayers is of great interest in the context of optically induced demagnetization, remagnetization and switching. If the penetration depth of light exceeds the bilayer thickness, layer-specific information is unavailable from optical probes. Femtosecond diffraction experiments provide unique experimental access to heat transport over single digit nanometer distances. Here, we investigate the structural response and the energy flow in the ultrathin double-layer system: gold on ferromagnetic nickel. Even though the excitation pulse is incident from the Au side, we observe a very rapid heating of the Ni lattice, whereas the Au lattice initially remains cold. The subsequent heat transfer from Ni to the Au lattice is found to be two orders of magnitude slower than predicted by the conventional heat equation and much slower than electron-phonon coupling times in Au. We present a simplified model calculation highlighting the relevant thermophysical quantities.}, language = {en} } @misc{FondellEckertJayetal.2017, author = {Fondell, Mattis and Eckert, Sebastian and Jay, Raphael Martin and Weniger, Christian and Quevedo, Wilson and Niskanen, Johannes and Kennedy, Brian and Sorgenfrei, Nomi and Schick, Daniel and Giangrisostomi, Erika and Ovsyannikov, Ruslan and Adamczyk, Katrin and Huse, Nils and Wernet, Philippe and Mitzner, Rolf and F{\"o}hlisch, Alexander}, title = {Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {780}, issn = {1866-8372}, doi = {10.25932/publishup-43752}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-437529}, pages = {12}, year = {2017}, abstract = {We present a setup combining a liquid flatjet sample delivery and a MHz laser system for time-resolved soft X-ray absorption measurements of liquid samples at the high brilliance undulator beamline UE52-SGM at Bessy II yielding unprecedented statistics in this spectral range. We demonstrate that the efficient detection of transient absorption changes in transmission mode enables the identification of photoexcited species in dilute samples. With iron(II)-trisbipyridine in aqueous solution as a benchmark system, we present absorption measurements at various edges in the soft X-ray regime. In combination with the wavelength tunability of the laser system, the set-up opens up opportunities to study the photochemistry of many systems at low concentrations, relevant to materials sciences, chemistry, and biology.}, language = {en} } @misc{SchroenKoehliScheiffeleetal.2017, author = {Schr{\"o}n, Martin and K{\"o}hli, Markus and Scheiffele, Lena and Iwema, Joost and Bogena, Heye R. and Lv, Ling and Martini, Edoardo and Baroni, Gabriele and Rosolem, Rafael and Weimar, Jannis and Mai, Juliane and Cuntz, Matthias and Rebmann, Corinna and Oswald, Sascha and Dietrich, Peter and Schmidt, Ulrich and Zacharias, Steffen}, title = {Improving calibration and validation of cosmic-ray neutron sensors in the light of spatial sensitivity}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe}, number = {636}, doi = {10.25932/publishup-41913}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-419134}, pages = {5009 -- 5030}, year = {2017}, abstract = {In the last few years the method of cosmic-ray neutron sensing (CRNS) has gained popularity among hydrologists, physicists, and land-surface modelers. The sensor provides continuous soil moisture data, averaged over several hectares and tens of decimeters in depth. However, the signal still may contain unidentified features of hydrological processes, and many calibration datasets are often required in order to find reliable relations between neutron intensity and water dynamics. Recent insights into environmental neutrons accurately described the spatial sensitivity of the sensor and thus allowed one to quantify the contribution of individual sample locations to the CRNS signal. Consequently, data points of calibration and validation datasets are suggested to be averaged using a more physically based weighting approach. In this work, a revised sensitivity function is used to calculate weighted averages of point data. The function is different from the simple exponential convention by the extraordinary sensitivity to the first few meters around the probe, and by dependencies on air pressure, air humidity, soil moisture, and vegetation. The approach is extensively tested at six distinct monitoring sites: two sites with multiple calibration datasets and four sites with continuous time series datasets. In all cases, the revised averaging method improved the performance of the CRNS products. The revised approach further helped to reveal hidden hydrological processes which otherwise remained unexplained in the data or were lost in the process of overcalibration. The presented weighting approach increases the overall accuracy of CRNS products and will have an impact on all their applications in agriculture, hydrology, and modeling.}, language = {en} } @misc{LevermannWinkelmann2016, author = {Levermann, Anders and Winkelmann, Ricarda}, title = {A simple equation for the melt elevation feedback of ice sheets}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {529}, issn = {1866-8372}, doi = {10.25932/publishup-40983}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-409834}, pages = {9}, year = {2016}, abstract = {In recent decades, the Greenland Ice Sheet has been losing mass and has thereby contributed to global sea-level rise. The rate of ice loss is highly relevant for coastal protection worldwide. The ice loss is likely to increase under future warming. Beyond a critical temperature threshold, a meltdown of the Greenland Ice Sheet is induced by the self-enforcing feedback between its lowering surface elevation and its increasing surface mass loss: the more ice that is lost, the lower the ice surface and the warmer the surface air temperature, which fosters further melting and ice loss. The computation of this rate so far relies on complex numerical models which are the appropriate tools for capturing the complexity of the problem. By contrast we aim here at gaining a conceptual understanding by deriving a purposefully simple equation for the self-enforcing feedback which is then used to estimate the melt time for different levels of warming using three observable characteristics of the ice sheet itself and its surroundings. The analysis is purely conceptual in nature. It is missing important processes like ice dynamics for it to be useful for applications to sea-level rise on centennial timescales, but if the volume loss is dominated by the feedback, the resulting logarithmic equation unifies existing numerical simulations and shows that the melt time depends strongly on the level of warming with a critical slow-down near the threshold: the median time to lose 10\% of the present-day ice volume varies between about 3500 years for a temperature level of 0.5 degrees C above the threshold and 500 years for 5 degrees C. Unless future observations show a significantly higher melting sensitivity than currently observed, a complete meltdown is unlikely within the next 2000 years without significant ice-dynamical contributions.}, language = {en} }