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The Lena Delta in Siberia is the largest delta in the Arctic and as a snow-dominated ecosystem particularly vulnerable to climate change. Using the two decades of MODerate resolution Imaging Spectroradiometer satellite acquisitions, this study investigates interannual and spatial variability of snow-cover duration and summer vegetation vitality in the Lena Delta. We approximated snow by the application of the normalized difference snow index and vegetation greenness by the normalized difference vegetation index (NDVI). We consolidated the analyses by integrating reanalysis products on air temperature from 2001 to 2021, and air temperature, ground temperature, and the date of snow-melt from time-lapse camera (TLC) observations from the Samoylov observatory located in the central delta. We extracted spring snow-cover duration determined by a latitudinal gradient. The ‘regular year’ snow-melt is transgressing from mid-May to late May within a time window of 10 days across the delta. We calculated yearly deviations per grid cell for two defined regions, one for the delta, and one focusing on the central delta. We identified an ensemble of early snow-melt years from 2012 to 2014, with snow-melt already starting in early May, and two late snow-melt years in 2004 and 2017, with snow-melt starting in June. In the times of TLC recording, the years of early and late snow-melt were confirmed. In the three summers after early snow-melt, summer vegetation greenness showed neither positive nor negative deviations. Whereas, vegetation greenness was reduced in 2004 after late snow-melt together with the lowest June monthly air temperature of the time series record. Since 2005, vegetation greenness is rising, with maxima in 2018 and 2021. The NDVI rise since 2018 is preceded by up to 4 °C warmer than average June air temperature. The ongoing operation of satellite missions allows to monitor a wide range of land surface properties and processes that will provide urgently needed data in times when logistical challenges lead to data gaps in land-based observations in the rapidly changing Arctic. Peer Reviewed REKLIM BMBF GeoX ERC

Trace elements incorporation of certain trace elements like strontium (Sr) and magnesium (Mg) among others and their isotope composition in different CaCO3 polymorphs (e.g. calcite and aragonite) as archives provide valuable for proxy information, that can be used as important tools for reconstructing the paleo- environmental conditions of the oceans throughout time. However, data on Sr incorporation into inorganic precipitated CaCO3 (calcite and aragonite), Mg incorporation into aragonite and Sr isotopic fractionation during minerals formation are still very rare. In addition, literature values available concerning Ca isotopic fractionation between calcite and aqueous solution are discrepant to a certain extent, while on the other hand the data available concerning Ca isotopic fractionation between inorganic precipitated aragonite and aqueous solution are also scarce. In order to overcome this lag of information in this study calcite and aragonite were precipitated at three different temperatures (12.5, 25.0 and 37.5±0.2 °C), different precipitation rates (R*) and solution composition by diffusing NH3 and CO2 gases into aqueous solutions containing trace elements and NH3 ions. For the kinetic study we used the initial rate method to solve the rate equations (rate law) with R* values in the range between 2.5 to 4.5 μmol/m2.h. We find that both calcite and aragonite have exactly the same order of reaction only differing in their activation energy (114 kJ/mol for calcite and 149 kJ/mol for aragonite) and rate constants at 25 °C (80.6*10-4 for calcite and 17.3*10-4 mM-2.h-1 for aragonite). The order of reaction with respect to Ca2+ ions is ≈ 1 and temperature dependent, while the order of reaction with respect to HCO3- ions is temperature dependent decreasing from 3 via 2 to 1 as temperature increases from 12.5 via 25.0 to 37.5°C, respectively. Calcium isotope fractionation for both calcite and aragonite (Δ44/40Ca) was found to be R* and temperature dependent. For 12.5 and 25.0 °C we observe a general increase of the Δ44/40Ca values as a function of R*, whereas at 37.5 °C decreasing Δ44/40Ca values are observed relative to increasing R*. It is suggested that the temperature triggered change from a Ca2+-NH3-aquacomplex covalent controlled bonding to a Ca2+-H2O-aquacomplex van-der-Waals controlled bonding caused the change in sign of the R* - Δ44/40Ca slope due to the switch of an equilibrium type of isotope fractionation related to the covalent bonding during lower temperatures to a kinetic type of isotope fractionation at higher temperatures. This behavior of Ca is in sharp contrast to the Sr isotopes which do not show any change of its fractionation behaviour as a function of complexation in the liquid phase. For both polymorphs of CaCO3 as a function of increasing R* the Δ88/86Sr-values become more negative and as temperature increases the Δ88/86Sr values also increase at constant rate. However effect oft R* on the Δ88/86Sr values is more significant in calcite than in aragonite. Magnesium incorporated into aragonite (expressed as DMg= [Mg/Ca] aragonite/ [Mg/Ca] solution) increases with decreasing temperature and also increases with increasing R* and as temperature increases the R* effect decreases. Later behavior is opposite to Mg in calcite (as temperature increases DMg also increases) as already known from earlier studies. Strontium incorporated into both calcite and aragonite (expressed as DSr= [Sr/Ca] solid/ [Sr/Ca] solution) was found to be R* and temperature dependent. Rate effect is more dominant over temperature effect in calcite, while on the other hand temperature effect is more dominant over rate effect in the case of aragonite. In calcite DSr increases with increasing R* and decreasing temperature. In aragonite also DSr increases with decreasing temperature. However concerning R* it responds differently: at 37.5°C DSr as R* increases DSr values increase, but decrease at 12.5°C. At 25.0°C, both behaviors are detected depending on the molar [Sr]/[Ca] ratio of the reacting solution (0.005 or 0.01). In the frame of a qualitative model to explain our trace element and isotope observations we speculate that increasing Mg2+ -concentrations control the material flux back (R*detach) from the crystal to the solution to a large extend. As a consequence R* values for aragonite tend to be lower than for calcite as observed from our data. Hence, Sr incorporation into aragonite is affected as function of temperature to a higher degree when compared to the R* effect. This is also reflect on the Δ88/86Sr values and decreasing the R* effect when compared to the temperature effect. Moreover concerning Ca isotope fractionation, the switch of direction in Ca isotope fractionation above ~25°C may be either due to the Mg2+ blocking effect or due to the switch of complexation from NH3 at and below 25 °C to H2O complexation at 37.5 °C. Plotting DSr versus Δ88/86Sr may be used as a proxy to reconstruct precipitation rates of calcite and of precipitation temperature of inorganic aragonite. Latter correlation may also have important implications for the verification of CaCO3 diagenesis.

Radiogenic lead (Pb) and neodymium (Nd) isotopes are sensitive paleoceanographic proxies for the reconstruction of ocean circulation changes in the past. The goal of this dissertation is to develop improved approaches to recover past seawater Pb and Nd isotope signals from marine sediments and ferromanganese (Fe-Mn) crust, and apply it for tracing water mass sourcing changes in the Southern Ocean at (sub-)millennial resolution.

keine engl.-spr. Zusammenfassung vh. Die vorliegende Arbeit untersucht den Einfluss der Wolken auf die Fernerkundung des Wasserdampfes. Globale Klimatologien des Wasserdampfes sind auf Messungen im infraroten Spektralbereich aufgebaut. Hier sind Wolken undurchlässig, daher müssen bewölkte Szenen bei der Ableitung der Klimatologie ausgeschlossen werden. Dadurch kommt es zu einer Unterschätzung des Wasserdampfes, da bewölkte Szenen mehr Wasserdampf enthalten als unbewölkte. Diese Unterschätzung wird in dieser Arbeit quantifiziert. Aus Messungen des Mikrowellenradiometers AMSU werden sowohl im bewölkten als auch im unbewölkten Fall der Wasserdampf über den Ozeanen bestimmt.

This thesis focuses on the eco-evolutionary processes that drive novelty, using the Caribbean hamlets as a model system to answer this question.

Unveiling the factors and processes that shape the dynamics of host associated microbial communities (microbiota) under natural conditions is an important part of understanding and predicting an organism's response to a changing environment. The microbiota is shaped by host (i.e., genetic) factors as well as by the biotic and abiotic environment. Studying natural variation of microbial community composition in multiple host genetic backgrounds across spatial as well as temporal scales represents a means to untangle this complex interplay. Here, we combined a spatially-stratified with a longitudinal sampling scheme within differentiated host genetic backgrounds by reciprocally transplanting Pacific oysters between two sites in the Wadden Sea (Sylt and Texel). To further differentiate contingent site from host genetic effects, we repeatedly sampled the same individuals over a summer season to examine structure, diversity and dynamics of individual hemolymph microbiota following experimental removal of resident microbiota by antibiotic treatment. While a large proportion of microbiome variation could be attributed to immediate environmental conditions, we observed persistent effects of antibiotic treatment and translocation suggesting that hemolymph microbial community dynamics is subject to within-microbiome interactions and host population specific factors. In addition, the analysis of spatial variation revealed that the within-site microenvironmental heterogeneity resulted in high small-scale variability, as opposed to large-scale (between-site) stability. Similarly, considerable within-individual temporal variability was in contrast with the overall temporal stability at the site level. Overall, our longitudinal, spatially-stratified sampling design revealed that variation in hemolymph microbiota is strongly influenced by site and immediate environmental conditions, whereas internal microbiome dynamics and oyster-related factors add to their long-term stability. The combination of small and large scale resolution of spatial and temporal observations therefore represents a crucial but underused tool to study host-associated microbiome dynamics. © 2016 Lokmer, Goedknegt, Thieltges, Fiorentino, Kuenzel, Baines and Wegner.

The aim of this thesis is to explore and understand some major climate mechanisms that were responsible for atmospheric and oceanic changes during the LGM (21,000 years ago). A coupled global atmosphere ocean model of intermediate complexity is used to study the influence of glacial boundary conditions on the climate system during the LGM in a systematical manner. A web of atmospheric interactions is disentangled which involves changes of the meridional temperature gradient and an associated modulation of the atmospheric baroclinicity. This in turn drives anomalous transient eddy momentum flux which feedback onto the zonal mean circulation. Moreover, the modified transient activity, weakened (strengthened) in the North Pacific (Atlantic), leads to a meridional re-organization of the atmospheric heat-transport, thereby feeding back to the meridional temperature structure. Furthermore, it is argued that modifications of the large-scale atmospheric circulation during the LGM may have led to a slowdown of the Pacific subtropical gyre as well as to an intensification of the Pacific subtropical cell. These oceanic circulation changes generate an eastern North Pacific warming, an associated cooling in the Kuroshio area, as well as a cooling of the tropical oceans, respectively. The tropical cooling pattern resembles a permanent La Nina state which in turn forces atmospheric teleconnection patterns that lead to an enhancement of the subtropical warming by reduced latent and sensible cooling of the ocean. In addition, the radiative cooling due to atmospheric CO2 and water vapour reductions imposes a cooling tendency in the tropics and subtropics, thereby intensifying the permanent La Nina conditions. Hence, a delicate balance between oceanic circulation changes, remotely induced atmospheric flux anomalies as well as local radiative cooling is established which controls the tropical and the North Pacific temperature anomalies during the LGM. The LGM simulation exhibits an intensified Atlantic overturning cell, associated with an enhanced formation of North Atlantic Deep Water. This enhancement can be attributed to the strong surface cooling in high latitudes and brine release in areas of seasonally varying sea-ice extent. In turn, the intensified meridional overturning circulation leads to an enhanced poleward heat transport that is required to equilibrate the strong tropical-extratropical temperature contrast during the LGM. The modeling results compare well with some recent paleoreconstructions.

Gas hydrates are fascinating ice-like compounds made of water cages that retain various types of guest molecules. Natural gas hydrates on Earth form below the seafloor and permafrost and contain mainly methane (CH4). Methane from hydrate deposits could be considered as an energy resource. One possible production scenario of CH4 from hydrates is the injection of carbon dioxide (CO2) or carbon dioxide-nitrogen(CO2-N2) mixed gas into the reservoir. Depending on the thermodynamic constraints, the composition of the gas hydrate guest molecules changes: the energy source CH4 is released and the greenhouse gas CO2 is trapped. The aim of the present work is to study the mixed gas hydrates that form in gas hydrate reservoirs after injection of CO2 or CO2-N2 gas mixtures, using laboratory experiments and modeling.

The Arctic is rich in aquatic systems and experiences rapid warming due to climate change. The accelerated warming causes permafrost thaw and the mobilization of organic carbon. When dissolved organic carbon is mobilized, this DOC can be transported to aquatic systems and degraded in the water bodies and further downstream. Here, we analyze the influence of different landscape components on DOC concentrations and export in a small (6.45 km2) stream catchment in the Lena River Delta. The catchment includes lakes and ponds, with the flow path from Pleistocene yedoma deposits across Holocene non-yedoma deposits to the river outlet. In addition to DOC concentrations, we use radiocarbon dating of DOC as well as stable oxygen and hydrogen isotopes (δ18O and δD) to assess the origin of DOC. We find significantly higher DOC concentrations in the Pleistocene yedoma area of the catchment compared to the Holocene non-yedoma area with medians of 5 and 4.5 mg L−1 (p < 0.05), respectively. When yedoma thaw streams with high DOC concentration reach a large yedoma thermokarst lake, we observe an abrupt decrease in DOC concentration, which we attribute to dilution and lake processes such as mineralization. The DOC ages in the large thermokarst lake (between 3,428 and 3,637 14C y BP) can be attributed to a mixing of mobilized old yedoma and Holocene carbon. Further downstream after the large thermokarst lake, we find progressively younger DOC ages in the stream water to its mouth, paired with decreasing DOC concentrations. This process could result from dilution with leaching water from Holocene deposits and/or emission of ancient yedoma carbon to the atmosphere. Our study shows that thermokarst lakes and ponds may act as DOC filters, predominantly by diluting incoming waters of higher DOC concentrations or by re-mineralizing DOC to CO2 and CH4. Nevertheless, our results also confirm that the small catchment still contributes DOC on the order of 1.2 kg km−2 per day from a permafrost landscape with ice-rich yedoma deposits to the Lena River. Peer Reviewed