Stable Isotope Depth Research Articles (Page 1)

Abstract. Oxygen-minimum zones (OMZs) play an important role in the global oceanic nitrogen cycle because they account for 20 % to 40 % of the global loss of bioavailable nitrogen despite covering only about 1 % of the global ocean volume. The intermediate waters of the Bay of Bengal (BoB) host one of the most pronounced OMZs with near-anoxic conditions. However, it has not yet been recognized as a site with significant nitrate reduction. In this study, we examined the nitrogen-cycling processes in the East Equatorial Indian Ocean (EEIO) and the BoB by measuring water column properties, including temperature, salinity, oxygen, and nutrient concentrations, as well as nitrate isotope signatures, collected during the SO305 BIOCAT-IIOE2 cruise in April and May 2024. Potential temperature and salinity profiles showed distinct water masses and limited mixing between the BoB and the EEIO at 5° N. Nitrate stable isotope depth profiles varied significantly, driven by water mass distribution below 300 m and in situ fractionation above 300 m. Phytoplankton uptake acts as a nitrate sink in the surface waters, showing a significant isotopic enrichment and nitrogen deficit. In subsurface waters, nitrification was observed, primarily through regenerative production using previously assimilated biomass rather than newly fixed nitrogen from N2 fixation. Within the OMZ of the BoB, we identified a persistent nitrogen deficit and slightly enriched nitrate isotopes between 100 and 300 m, indicating nitrogen loss, which we attributed to anammox as the dominant nitrogen loss pathway in the BoB.

Peatland degradation impairs soil functions such as carbon storage and the existence of biodiversity hotspots. Therefore, and in view of the ongoing climate change, an efficient method of evaluating peatland hydrology and the success of restoration efforts is needed. To understand the role of microbial groups in biogeochemical cycling, gaseous loss and isotopic fractionation that lead to specific isotopic depth patterns (δ13C, δ15N), we integrated previously published stable isotope data with a membrane fatty acid (mFA) analysis related to various microbial groups that are known to be common in peatlands. We performed two sampling campaigns to verify the observed stable isotope depth trends in nutrient-poor peatlands in Northern Europe. Cores were taken from adjacent drained (or rewetted) and undrained sites. Fungal-derived mFA abundance was highest in the uppermost part of the drained layer. We found increasing bacterial-derived mFA concentrations with depth peaking in the middle of the drained layers, which correlates with a δ15N peak of bulk material. The results support our hypothesis that changing peatland hydrology induce a shift in microbial community and metabolism processes and is therefore also imprinted in stable isotope values. Under waterlogged conditions overall levels of microbial-derived mFAs were generally low. Drained layers showed simultaneous changes in microbial abundance and composition and depth trends in stable isotope bulk values. Bacteria, particularly acidobacteria, can be expected to dominate increased denitrification with low oxygen saturation accompanied by increased δ15N bulk values in the remaining substrate. Interestingly, cores from recent rewetted peatlands show no depth trend of δ15N in the layers grown under rewetting conditions; this is congruent with relatively low concentrations of microbial-derived mFAs. Hence, we conclude that stable isotopes, especially δ15N values, reflect changing microbial metabolic processes, which differ between drained and undrained - and especially also for recent rewetted–peatlands. As today stable isotope measurements are routine measurements, these findings enable us to get cost- and time efficient reliable information of drainage and restoration success.

Palsas develop as permafrost aggradation uplifts peat out of the zone influenced by groundwater. Here we relate δ15N values to C/N ratios along depth profiles through palsas in two peatlands near Abisko, northern Sweden, to identify perturbation of the peat. The perturbation by uplift as well as the potential nutrient input from the adjacent hollows can be detected in soil δ15N values when related to the C/N ratio at the same depth. Nine out of ten profiles show a perturbation at the depth where peat was uplifted by permafrost. Palsa uplift could be detected by the δ15N depth pattern, with the highest δ15N values at the so-called turning point. The δ15N values increase above and decrease below the turning point, when permafrost initiated uplift. Onset of permafrost aggradation calculated from peat accumulation rates was between 80 and 545 years ago, with a mean of 242 ( ±66) years for Stordalen and 365 ( ±53) years for Storflaket peatland. The mean ages of permafrost aggradation are within the Little Ice Age. Depth profiles of δ15N, when related to C/N ratio, seem to be a suitable tool to detect perturbation and uplift of palsas. Copyright © 2016 John Wiley & Sons, Ltd.

Diffusion and conductivity properties of cerium niobate

Numerical simulations of transport and isotope fractionation provide a method to quantitatively interpret vadose zone pore water stable isotope depth profiles based on soil properties, climatic conditions, and infiltration. We incorporate the temperature‐dependent equilibration of stable isotopic species between water and water vapor, and their differing diffusive transport properties into the thermodynamic database of the reactive transport code TOUGHREACT. These simulations are used to illustrate the evolution of stable isotope profiles in semiarid regions where recharge during wet seasons disturbs the drying profile traditionally associated with vadose zone pore waters. Alternating wet and dry seasons lead to annual fluctuations in moisture content, capillary pressure, and stable isotope compositions in the vadose zone. Periodic infiltration models capture the effects of seasonal increases in precipitation and predict stable isotope profiles that are distinct from those observed under drying (zero infiltration) conditions. After infiltration, evaporation causes a shift to higher δ18O and δD values, which are preserved in the deeper pore waters. The magnitude of the isotopic composition shift preserved in deep vadose zone pore waters varies inversely with the rate of infiltration.

Numerical simulations of transport and isotope fractionation provide a method to quantitatively interpret vadose zone pore water stable isotope depth profiles based on soil properties, climatic conditions, and infiltration. We incorporate the temperature-dependent equilibration of stable isotopic species between water and water vapor, and their differing diffusive transport properties into the thermodynamic database of the reactive transport code TOUGHREACT. These simulations are used to illustrate the evolution of stable isotope profiles in semiarid regions where recharge during wet seasons disturbs the drying profile traditionally associated with vadose zone pore waters. Alternating wet and dry seasons lead to annual fluctuations in moisture content, capillary pressure, and stable isotope compositions in the vadose zone. Periodic infiltration models capture the effects of seasonal increases in precipitation and predict stable isotope profiles that are distinct from those observed under drying (zero infiltration) conditions. After infiltration, evaporation causes a shift to higher δ18O and δD values, which are preserved in the deeper pore waters. The magnitude of the isotopic composition shift preserved in deep vadose zone pore waters varies inversely with the rate of infiltration.

Development of a novel SIMS technique for oxygen self-diffusion and surface exchange coefficient measurements in oxides of high diffusivity