BG2.1 | Application of Stable Isotopes in Biogeosciences
Application of Stable Isotopes in Biogeosciences
Co-organized by GMPV1, co-sponsored by EAG
Convener: Michael E. Böttcher | Co-conveners: Kirstin Dähnke, Gerd Gleixner, Anne-Désirée Schmitt
Orals
| Fri, 28 Apr, 08:30–10:15 (CEST)
 
Room 2.17
Posters on site
| Attendance Thu, 27 Apr, 08:30–10:15 (CEST)
 
Hall A
Orals |
Fri, 08:30
Thu, 08:30
This session is open to all contributions in biogeochemistry and ecology where stable isotope techniques are used as analytical tools, with foci both on stable isotopes of light elements (C, H, O, N, S, …) and new systems (clumped and metal isotopes). We welcome studies from both terrestrial and aquatic (including marine) environments as well as methodological, experimental and theoretical studies that introduce new approaches or techniques (including natural abundance work, labelling studies, multi-isotope approaches).

Orals: Fri, 28 Apr | Room 2.17

Chairperson: Michael E. Böttcher
08:30–08:35
08:35–08:45
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EGU23-8338
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On-site presentation
Sam Barker, Calum Preece, Rob Berstan, and Mike Seed

Identifying and quantifying sources and cycling of nitrogen is important for understanding not only aquatic ecosystems but also planning water resource management, mitigating urban and agricultural pollution, and optimizing government policy. Stable isotopes of dissolved nitrate and nitrite (δ15N, δ18O and δ17O) have been useful in distinguishing between the diverse nitrogen sources and sinks and help understand large scale global ocean processes as well as revealing major changes in agricultural land use and urbanization. 

Despite the strength of dissolved nitrate and nitrite stable isotope analysis, the strong barrier for uptake using the favored contemporary methods (bacterial denitrifier and Cd-azide reaction) due to the laborious multi-step methods, maintenance of anerobic bacterial cultures and use of highly toxic chemicals has limited the analysis to highly specialized laboratories. We evaluate the performance of the Elementar EnvirovisION using the new Titanium (III) reduction method (Altabet et al., 2019) for one step conversion of nitrate into N2O for IRMS analysis.   

The EnvirovisION has been developed for high performance analysis of CO2, N2O and CH4 and dissolved nitrate. The system has the capacity to be rapidly customized for specific needs with options for dual GC columns supporting the Weigand ‘heart-cut’ N2O method (Weigand et al., 2016) and sequential N2 and N2O analysis from a single atmospheric sample. 

How to cite: Barker, S., Preece, C., Berstan, R., and Seed, M.: Analysis of dissolved nitrate stable isotopes using the one-step Ti (III) reduction method and Elementar EnvirovisION System  , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8338, https://doi.org/10.5194/egusphere-egu23-8338, 2023.

08:45–08:55
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EGU23-12808
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On-site presentation
Orlando Sébastien Olivieri, Alberto Vitale Brovarone, Jens Fiebig, Francesco Ressico, Valentina Marassi, Sonia Casolari, and Olivier Sissmann

For decades, the search for terrestrial abiotic CH4 has been a central quest in geology and astrobiology. Most of this research has focused on crustal fluid samples collected at surface seeps, hydrothermal vents, and wells. Nonetheless, in such open systems processes like mixing with shallow biotic CH4, oxidation, and diffusion can lead to large uncertainties in the initial composition of deeply originated CH41. Extracting natural CH4 from fluid inclusions entrapped in minerals could overcome the effects of shallow contamination occurring in natural open systems and shed light on the origin of deep CH4 in a wide variety of geodynamic settings2,3.  

In this abstract, we present a novel approach for bulk off-line fluid inclusion extraction for the analysis of δ13C-CH4 using a Cavity Ring-Down Spectroscopy (CRDS) analyzer (Picarro G2201-i). Two fluid extraction techniques were compared: ball milling in ZrO2 jars and sample crushing in a stainless-steel sealed tube under a piston. The accuracy and precision of the different protocols was evaluated with blanks, CH4 isotopic labelling and interlaboratory comparisons.   

Blanks and isotopically labelled tests with the ball milling technique suggested that milling speed, and duration, and sample mass and type may strongly affect the CH4 concentrations and isotopic compositions measured by the CRDS analyser. These effects are mainly ruled by the blank production of CH4 –demonstrated by gas chromatography analyses– and potentially other molecules induced by frictional heating. The blank gases may cause interference effects on the absorption bands detected by the CRDS analyser. This effect was marked by a large offset in the δ13C-CH4 measured in the high-range analytical modes of the Picarro G2201-i. The magnitude of the interference was inversely correlated to the CH4 concentration.Other processes such as CH4 diffusion and adsorption may play a role in the observed changes in CH4 concentrations and isotopic compositions. Experimental conditions involving high CH4 concentrations and sample mass could well reproduce CH4 isotopic composition. 

Crushing in a sealed stainless-steel tube produces lower blank levels compared to ball milling, nevertheless the crushing efficiency and CH4 release is lower due to smaller rock sample size.   

The presented ball milling protocol provides a simple and fast way to extract and accurately analyse CH4 hosted in fluid inclusions even if great care should be considered for samples with low CH4 concentrations where contamination and interferences can potentially alter the CH4 isotopic signature of natural samples.   

 

References 

  • 1 - Young et al. (2017). 10.1016/j.gca.2016.12.041 
  • 2 - Klein et al. (2019). 10.1073/pnas.1907871116 
  • 3 - Grozeva et al. (2020). 10.1098/rsta.2018.0431 

 

This work is part of project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 864045).    

How to cite: Olivieri, O. S., Brovarone, A. V., Fiebig, J., Ressico, F., Marassi, V., Casolari, S., and Sissmann, O.: Exploring new protocols for bulk off-line fluid inclusion extraction for the analysis of δ13C-CH4 using a Cavity Ring-Down Spectroscopy (CRDS) analyzer., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12808, https://doi.org/10.5194/egusphere-egu23-12808, 2023.

08:55–09:05
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EGU23-8103
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ECS
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On-site presentation
Irina Yankelzon, Lexie Schilling, Nicole Wrage-Moennig, Arne Tenspolde, Ulrike Ostler, Klaus Butterbach-Bahl, Lorenz Hartl, Rainer Gasche, Amanada Matson, Reinhard Well, Clemens Scheer, and Michael Dannenmann

Measuring soil dinitrogen (N2) emissions is notoriously challenging under field conditions. Hence, N2 emissions represent a significant uncertainty in the nitrogen mass balance of terrestrial ecosystems. The 15N gas flux (15NGF) method is the only method currently available for directly quantifying N2 emissions in situ. However, this method has rarely undergone independent validation under field conditions. In this study, our objectives were to: (1) Quantify N2 emissions and their role in the fertilizer N mass balance of a wheat rotation using the 15NGF method (2) Verify the obtained quantities of N2 emissions using a mass balance approach and (3) Verify the temporal N2 emission dynamics at the soil-atmosphere interface using vertical soil profiles of 15N2 enrichment.

To achieve these objectives, we grew winter wheat in lysimeters and applied 15N enriched mineral fertilizers via fertigation in three doses (sum 170 kg N ha-1). We then analyzed gaseous (NH3, N2O, N2) and hydrological N losses, as well as fertilizer N fates in plant and soil, and 15N2 enrichment in soil air.

Our results showed that N2 emissions directly measured using the 15NGF method amounted to 30 ± 4 kg N ha-1, which was equivalent to 18 ± 3 % of the applied fertilizer N. These measurements agreed with unrecovered fertilizer N obtained from the 15N fertilizer mass balance, although the latter had large inherent uncertainty (21 ± 21 kg N ha-1). N2O emissions, however, were negligible (0.14 ± 0.02 kg N ha-1). The temporal variability of measured N2 emissions after fertilizer additions was generally well explained by 15N2 enrichment in soil gas.

Overall, we provide independent validation of the 15NGF method in measuring N2 emissions in the field and highlight the significant role of these emissions in the nitrogen balance of crop systems. Our data also suggest that soil gas measurements in combination with diffusion modeling could serve as an alternative method for quantifying N2 emissions. These results should encourage a wider application of the 15NGF method in order to improve our understanding of N2 emissions and reduce the current uncertainties in estimates of these emissions.

How to cite: Yankelzon, I., Schilling, L., Wrage-Moennig, N., Tenspolde, A., Ostler, U., Butterbach-Bahl, K., Hartl, L., Gasche, R., Matson, A., Well, R., Scheer, C., and Dannenmann, M.: Lysimeter-based full N balance as a tool to test field N2 flux measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8103, https://doi.org/10.5194/egusphere-egu23-8103, 2023.

09:05–09:15
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EGU23-15191
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On-site presentation
Alberto Andrino, Asunción Morte, Francisco Arenas, Aline Figueiredo, Leopold Sauheitl, Alfonso Navarro, Ángel L. Guarnizo, Georg Guggenberger, and Jens Boy

Plants in semi-arid environments have adapted to scarce nitrogen (N) resources by becoming more efficient at using and taking it up, or by relying on symbiotic organisms. Common mycelial/mycorrhizal networks (CMNs) may help plants access and assist to a spatio-temporal redistribution of resources in soil. While CMNs have been extensively investigated in temperate forests and grasslands, their importance in semi-arid environments is still uncertain. This study evaluates the existence and importance of CMNs in N translocation in semiarid environments, using Helianthemmum almeriense as the host plant mycorrhized with Terfezia claveryi. We hypothesize that the presence of CMNs is a response mechanism to N scarcity due to soil heterogeneity. Through this mechanism, host plants and mycorrhizal fungi provide redistribution of N, playing a determinant role at all spatial scales, from the facilitation of seedling establishment to the persistence and coexistence of different plant communities. To test our hypothesis, we designed a mesocosm that allowed only hyphae to cross into an adjacent compartment. Three different tests were used to assess the existence and directionality of CMN. In the first test (T1), an adult plant was labeled with 15N and on the other side, only unlabeled soil was present. In this way, we could check if the mycorrhizal fungus tends to homogenously distribute the 15N to places where there is no other plant. In the second test (T2) we had an adult plant and in the adjacent compartment, four-week-old seedlings that were already mycorrhized with mycelium coming from the compartment with the adult plant. In this case, the 15N marker was applied where the adult plant was located, and we checked whether there was a transfer of 15N to the seedlings. In T3, we used a set of mesocosms equal to T2, but this time 15N was applied on the side where the seedlings were located. The idea was to determine whether the distribution of the 15N was proportional to the size of the plant that could receive it. The three types of mesocosms were sampled before labeling (day 0), 7 and 14 days after labeling. Our results reveal that 15N translocation to adjacent compartments occurred in all three tests, but in significantly different amounts. The translocation of 15N was significantly higher in those tests where there was a plant in the adjacent compartment (Tests 2 and 3) compared to T1. We also found that the contribution (%) of 15N to the total plant N pool was significantly higher for one-month-old seedlings in both T2 and T3, compared to adult plants. Under controlled greenhouse conditions, we have shown that the mycelium seems to act as an effective hub for N translocation, but we have not found the amounts transferred under our experimental conditions to be nutritionally remarkable. Our results should be further evaluated under natural conditions, to verify whether this N transfer has a greater nutritional significance than that found under controlled conditions, and also whether this CMN may play a more important role in signaling between plants adapted to semi-arid regions.

How to cite: Andrino, A., Morte, A., Arenas, F., Figueiredo, A., Sauheitl, L., Navarro, A., Guarnizo, Á. L., Guggenberger, G., and Boy, J.: Determining the relevance of common mycelial networks in the nitrogen nutrition of plants adapted to semi-arid ecosystems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15191, https://doi.org/10.5194/egusphere-egu23-15191, 2023.

09:15–09:25
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EGU23-4677
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ECS
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On-site presentation
Paul Magyar, Damian Hausherr, Robert Niederdorfer, Kun Huang, Joachim Mohn, Helmut Bürgmann, Adriano Joss, and Moritz Lehmann

Anammox plays a pivotal role in both natural and engineered systems as a process that simultaneously converts fixed nitrogen to N2 and regenerates NO3. In aquatic and terrestrial ecosystems, isotopic measurements, especially of the NO3pool, provide an essential constraint on the processes that regulate the supply and elimination of fixed nitrogen, but the isotope effects of anammox remain poorly constrained.

We present measurements of the δ15N and δ18O of NO3, NO2, and NH4+ as processed by anammox in a mixed microbial community enriched for N removal from wastewater. We find that oxygen isotope effects expressed in NO2include a substantial contribution from equilibration reactions with water superimposed on kinetic isotope effects. Equilibrium between water and NO2during processing by anammoxis greatly accelerated above rates observed under abiotic conditions even during growth phases when NO2 is rapidly being consumed. In turn, δ18O of NO3 nearly completely reflects the incorporation of O atoms derived from water with little additional isotopic fractionation. The δ15N values of NO3 and NO2 also show evidence for an equilibrium isotope exchange reaction between these molecules, which raises the possibility that nitrite oxidation is partially reversible, while introducing a high degree of variability into the δ15N of NO3 generated by anammox. Finally, variation observed in the δ15N of NH4+ consumed by anammox can be connected to physiological limitations within the anammox cell.

Despite this complexity, we were able to use NO2 and NO3isotope measurements to diagnose changes in the activity of anammox and related processes within the wastewater treatment system during a low-temperature perturbation experiment. These results provide new constraints for interpreting the variability in δ15N and δ18O of NO3in natural systems, with implications for estimating relative rates of fixed N turnover processes.

How to cite: Magyar, P., Hausherr, D., Niederdorfer, R., Huang, K., Mohn, J., Bürgmann, H., Joss, A., and Lehmann, M.: Equilibrium and kinetic controls contribute to nitrogen and oxygen isotope effects during anammox in a wastewater treatment system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4677, https://doi.org/10.5194/egusphere-egu23-4677, 2023.

09:25–09:35
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EGU23-11001
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ECS
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On-site presentation
Hadar Cohen-Sadon, Yoav Oved Rosenberg, Shimon Feinstein, and Alon Amrani

The sulfur (S) cycle is directly linked to the global carbon and iron cycles. Sulfur plays an important role in the preservation of organic matter (OM) over geological time scales. Assimilatory and dissimilatory sulfate reductions (ASR and DSR, respectively) are the main processes shaping the sulfur cycle and carry different sulfur isotopic fractionation (-1 to -3 ‰, and -20 to -75 ‰, respectively). The reduced S species produced by DSR react during early diagenesis with OM (sulfurization) to form organic-S, and/or with iron (Fe) to form pyrite. Although in most cases organic- and pyritic-S have a common origin (i.e., DSR), organic-S in marine sediments is typically 34S enriched relative to its co-existing pyrite by up to 40 ‰ (global average is ~10‰). This isotopic difference is assumed to depend on specific paleo-environmental conditions and different sulfurization pathways (open-closed system for sulfate, Fe availability, redox state, OM type, etc.). Different sulfurization pathways may affect the type and distribution of S-bonds in sedimentary OM and thus can strongly affect the structure and character of sedimentary OM.

Recently, new instrumentations and methods were developed for the rapid determination of organic- and pyritic-S concentrations and δ34S values using a Rock-Eval analyzer (RE) coupled to a MC-ICPMS. A new parameter, Tmax-S (the temperature at maximum peak of organic-S generation), was suggested to represent the organic-S thermal stability in pyrolysis conditions. Applying this parameter to thirteen organically rich and thermally immature samples of various geological settings and paleo-environmental origins revealed several interesting empirical correlations. The Tmax-S value was found to differ among the rocks and to linearly correlate (R2=0.98) with the percentage of pyrolyzed organic-S out of the total organic-S in the rock. Moreover, Tmax-S was strongly correlated with the distribution of sulfidic and thiophene compounds in the rock (R2=0.87). This suggests that Tmax-S may be used as a tool to evaluate the distribution of different S-bonds in the organic molecule, and, as a proxy, their sulfurization and paleoenvironmental conditions.

The rock samples' Tmax-S values also correlate with their isotopic difference between organic- and pyritic-S (Δ34Sorganic-pyrite; R2=0.76). The Δ34Sorganic-pyrite values of the rocks extended between 1 to 40‰ where marine samples characterized by low Tmax-S values (~400-450 °C) and large Δ34Sorganic-pyrite values (~20-40‰) and lacustrine samples by high Tmax-S values (~450-480 °C) and small Δ34Sorganic-pyrite values (~1-5‰). This correlation further supports the link between paleo-environmental conditions, specific sulfurization pathways, and the organic-S structures (represented by Tmax-S). It may shed light on some fundamental questions regarding the role of S isotopic distribution between pyrite and OM during deposition and diagenesis.

How to cite: Cohen-Sadon, H., Rosenberg, Y. O., Feinstein, S., and Amrani, A.: Development of new proxies for sulfurization and paleo-environmental conditions using a Rock-Eval coupled to MC-ICPMS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11001, https://doi.org/10.5194/egusphere-egu23-11001, 2023.

09:35–09:45
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EGU23-16235
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ECS
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Highlight
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On-site presentation
Rhodelyn Saban, Michael Ernest Boettcher, Anna Kathrina Jenner, Sara Elizabeth Anthony, Gerald Juransinski, Catia Ehlert von Ahn, Patricia Roeser, and Iris Schmiedinger

Soils from a coastal peatland (Drammendorf,  southern Baltic Sea) were investigated for the biogeochemical impact of flooding with brackish seawater. The peatland was rewetted in late 2019 through the partial removal of a dyke and brackish water with high sulfate concentration from a lagoon (Kubitzer Bodden) allowed to intrude into the peatland. Soil cores were retrieved about 2 and 3 years after the initial rewetting event. Pore waters were extracted from soil cores using rhizons and samples were analyzed besides physical parameters for major ions, nutrients, water stable isotopes, dissolved inorganic carbon (DIC) concentrations, and stable isotopes in C and S species. Solid phase samples were analyzed for contents of CNS and acid-extractable metal and nutrient species, and the stable isotope composition of acid-volatile sulfide (AVS), chromium-reducible sulfide (CRS, pyrite).Results from the post-event campaigns are compared with pre-flooding conditions. Poree water generally showed a trend towards freshening with depth as remains from the pre-flooding conditions. . Different sites are furthermore characterized by different amounts of diagenetically released dissolved inorganic carbon (DIC). A mixing evaluation of ẟ13C-DIC signatures together with major ion concentrations reveals potential DIC sources, like organic matter/methane oxidation, carbonate dissolution and mixing with seawater-derived DIC. DIC from the dissolution of minor soil carbonates may lead to a relative enrichment of 13C in DIC. Brackish water intrusion and cation exchange are reflected by the downward gradients in pore water compositions, with Na and Mg decreasing and Ca increasing with depth. Soil organic carbon is dominating with inorganic carbon being a minor fraction in most parts. Dissolved pore water sulfate and high total sulfur in the top soils with decrease downward trends further reflect the importance of post-event enhanced sulfur cycling leading to characteristic sulfur isotope signatures. AVS is depleted in the top soils with highest contents at about 5 cmbsf. Pyrite sulfur dominates and show different enrichment zones with the sediment columns. Contents in CRS contents covary with TOC, indicating that the benthic diagenetic system is controlled by organic matter availability, as in normal marine sediments.  The study demonstrates the role of electron acceptor availability for benthic carbon cycling, and the kinetics of biogeochemical interactions upon oxidation of reduced carbon, mineral authigenesis/dissolution, and ion exchange processes. The results of implications for coastal processes in the humid climate zone during times of increasing seawater level rise.

 

 

How to cite: Saban, R., Boettcher, M. E., Jenner, A. K., Anthony, S. E., Juransinski, G., Ehlert von Ahn, C., Roeser, P., and Schmiedinger, I.: Isotope biogeochemistry of the carbon-iron-sulfur cycle in a temperate coastal peatland after flooding by brackish seawater, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16235, https://doi.org/10.5194/egusphere-egu23-16235, 2023.

09:45–09:55
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EGU23-6603
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ECS
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On-site presentation
Male Köster, Michael Staubwasser, Anette Meixner, Simone A. Kasemann, Hayley R. Manners, Yuki Morono, Fumio Inagaki, Verena B. Heuer, Sabine Kasten, and Susann Henkel

Microbially mediated iron (Fe) reduction is suggested to be one of the earliest metabolic pathways on Earth and Fe(III)-reducing microorganisms might be key inhabitants of the deep and hot biosphere [1, 2]. Since microbial Fe cycling is typically accompanied by Fe isotope fractionation, stable Fe isotopes (δ56Fe) are used as tracer for microbial processes in modern and ancient marine sediments [3, 4]. Here we present Fe isotope data for dissolved and sequentially extracted sedimentary Fe pools from subseafloor sediments that were recovered during International Ocean Discovery Program Expedition 370 from a 1,180 m deep hole drilled in the Nankai Trough off Japan where temperatures of up to 120°C are reached at the sediment-basement interface. The expedition aimed at exploring the temperature limit of microbial life and identifying geochemical and microbial signatures that differentiate the biotic and abiotic realms [5, 6]. Dissolved Fe (Fe(II)aq) is isotopically light throughout the ferruginous sediment interval but some samples have exceptionally light δ56Fe values. Such light δ56Fe values have never been reported in natural marine environments and cannot be solely attributed to microbially mediated Fe(III) reduction. We show that the light δ56Fe values are best explained by a Rayleigh distillation model where Fe(II)aq is continuously removed from the pore water by diffusion and adsorption onto Fe (oxyhydr)oxide surfaces. While the microbially mediated Fe(II)aq release has ceased due to an increase in temperature beyond the threshold of mesophilic microorganisms, the abiotic diffusional and adsorptive Fe(II)aq removal continued, leading to uniquely light δ56Fe values. These findings have important implications for the interpretation of Fe isotope records especially in deep subseafloor sediments.

 

References:

[1] Vargas, M. et al., 1998. Nature 395: 65-67.

[2] Kashefi, K. and Lovley, D.R., 2003. Science 301: 934-934.

[3] Beard, B.L. et al., 1999. Science 285: 1889-1892.

[4] Anbar, A.D. and Rouxel, O., 2007. Annu. Rev. Earth Planet. Sci. 35: 717-746.

[5] Heuer, V.B. et al., 2017. In Proc. IODP Volume 370.

[6] Heuer, V.B. et al., 2020. Science 370: 1230-1234.

How to cite: Köster, M., Staubwasser, M., Meixner, A., Kasemann, S. A., Manners, H. R., Morono, Y., Inagaki, F., Heuer, V. B., Kasten, S., and Henkel, S.: Uniquely low stable iron isotopic signatures in deep marine sediments caused by Rayleigh distillation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6603, https://doi.org/10.5194/egusphere-egu23-6603, 2023.

09:55–10:05
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EGU23-13891
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ECS
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On-site presentation
Megan Reich, Sana Ghouri, Samantha Zabudsky, Gerard Talavera, and Clement Bataille

Migratory insects serve an important role to ecosystems and economies as they participate in the long-distance transfer of nutrients, pollen, and biomass. However, migratory insects are understudied, especially compared to birds and mammals, partially because traditional tracking techniques (e.g. mark-recapture, biologgers) are often ineffective for insects because insects are small, short-lived, and numerous. Thus, isotope geolocation has become an effective tool for studying dispersing insects. The painted lady butterfly (Vanessa cardui (L.)) is a virtually cosmopolitan species that was recently found to make regular, annual multi-generational migrations across the Sahara Desert. Previous work geolocating painted ladies with hydrogen isotopes has shown early spring migratory movements of painted ladies from sub-Saharan Africa to Mediterranean Europe and autumn movements to the sub-Sahara from Europe. However, these previous works using hydrogen isotopes were unable to offer refined estimates of natal origin due to the inherent limitations of the technique. Here, we update previous hydrogen isotope-based geographic assignment by (1) using an updated model of hydrogen isotope variations across the landscape (i.e., isoscape), (2) combining with strontium isotope-based geographic assignment, and (3) expanding the number of geolocated butterflies to include capture locations on both sides of the Sahara Desert across different years. Using this method, we spatially refine previous estimates of the natal origins of successful trans-Saharan migrants and estimate the distances that successful migrants travelled. Overall, this study demonstrates the advantages of combining hydrogen and strontium isotopes for the geographic assignment of migratory butterflies and further advances our understanding of long-distance insect migration.

How to cite: Reich, M., Ghouri, S., Zabudsky, S., Talavera, G., and Bataille, C.: There and back again: Combining hydrogen and strontium isotopes refines the trans-Saharan migratory patterns of the butterfly Vanessa cardui, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13891, https://doi.org/10.5194/egusphere-egu23-13891, 2023.

10:05–10:15
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EGU23-1093
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On-site presentation
Fanny Thibon, Jean Goedert, Nicolas Séon, Lucas Weppe, Jeremy E. Martin, Romain Amiot, Sylvain Adnet, Olivier Lambert, Paco Bustamante, Philippe Telouk, Christophe Lécuyer, and Nathalie Vigier

Life evolution has been shaped by marine and continental environmental dichotomy. Particularly, the ecological history of vertebrates is divided into several aquatic and terrestrial phases. Even today, some species spend time in both marine and continental environments during their lifetime. Nevertheless, the timing and location of past ecological transitions, as well as the monitoring of current migration, are still challenging to trace.

To reconstruct the aquatic environments of vertebrates (i.e. seawater vs freshwater), stable (δ13C, δ18O, δ34S) and radiogenic (87Sr/86Sr) isotope systems applied to mineralized tissues have been commonly used in the past decades1–6. Nevertheless, these methods hold some limitations as they cannot be applied universally.

Here, we measured the lithium stable isotope composition of mineralized tissues (δ7Li) from extant vertebrates living in various aquatic environments (seawater, freshwater/terrestrial, and "transitional environments”). We highlight the potential of δ7Li to decipher vertebrates that live in these different environments, in contrast to δ34S and δ18O that cannot distinguish – in some cases – species living in intermediate waters from those living in seawater. Furthermore, we measured the δ7Li values of fossil apatites from extinct vertebrates and obtained values that fall within the range of aquatic environment of their extant relatives7. This new proxy may therefore profit studies in ecology, archaeology and palaeontology.

 

1 M. T. Clementz, A. Goswami, P. D. Gingerich and P. L. Koch, Journal of Vertebrate Paleontology, 2006, 26, 355–370.

2  J. Fischer, S. Voigt, M. Franz, J. W. Schneider, M. M. Joachimski, M. Tichomirowa, J. Götze and H. Furrer, Palaeogeography, Palaeoclimatology, Palaeoecology, 2012, 353–355, 60–72.

3 J. Goedert, C. Lécuyer, R. Amiot, F. Arnaud-Godet, X. Wang, L. Cui, G. Cuny, G. Douay, F. Fourel, G. Panczer, L. Simon, J.-S. Steyer and M. Zhu, Nature, 2018, 558, 68–72.

4 J. Goedert, R. Amiot, D. Berthet, F. Fourel, L. Simon and C. Lécuyer, Sci Nat, 2020, 107, 10.

5 L. Kocsis, A. Ősi, T. Vennemann, C. N. Trueman and M. R. Palmer, Palaeogeography, Palaeoclimatology, Palaeoecology, 2009, 280, 532–542.

6 B. Schmitz, S. L. Ingram, D. T. Dockery and G. Åberg, Chemical Geology, 1997, 140, 275–287.

7 F. Thibon, J. Goedert, N. Séon, L. Weppe, J. E. Martin, R. Amiot, S. Adnet, O. Lambert, P. Bustamante, C. Lécuyer and N. Vigier, Earth and Planetary Science Letters, 2022, 599, 117840.

How to cite: Thibon, F., Goedert, J., Séon, N., Weppe, L., Martin, J. E., Amiot, R., Adnet, S., Lambert, O., Bustamante, P., Telouk, P., Lécuyer, C., and Vigier, N.: Lithium isotopes potential in (paleo)ecology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1093, https://doi.org/10.5194/egusphere-egu23-1093, 2023.

Posters on site: Thu, 27 Apr, 08:30–10:15 | Hall A

A.188
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EGU23-14655
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ECS
Franziska M. Stamm, Andre Baldermann, Daniel A. Frick, Dorothee Hippler, and Martin Dietzel

Silicate mineral weathering and the resulting formation of clay minerals are key processes at the Earth’s surface. Reverse weathering reactions regulate the evolution of ocean pH and chemistry, atmospheric carbon dioxide budget, soil formation and associated nutrient and element transfer within local and global biogeochemical cycles. A key element released during weathering processes is silicon (Si), that enters the biogeochemical cycle as dissolved silicic acid (Si(OH)4 or DSi). During the transport of DSi through the hydro-, bio-, pedo- and lithosphere towards the ocean, DSi is involved in the formation of new silicate minerals (i.e., clay minerals) in particular in the critical zone (CZ). Here, DSi is frequently precipitating as gel-like, amorphous phases such as short range ordered hydroxyaluminosilicate phases (HAS: e.g., allophane) or as hydrous ferric silicates (HFS: e.g., hisingerite). These highly reactive minerals are known precursors to the formation of important soil clay minerals e.g., within the smectite group.

The individual reaction pathways and the environmental controls underlying HAS and HFS formation within the CZ are not yet well constrained. Si isotope fractionation throughout HAS and HFS formation is a key tool to decode and assess such enigmatic reaction mechanisms. Therefore, a series of allophane-hisingerite precipitation experiments has been performed to investigate Si isotope fractionation to potentially resolve mechanisms and conditions underlying the formation of HAS and HFS phases. Kinetic and equilibrium Si isotope fractionation between reactive fluid and solid phases are studied at high temporal resolution. Isotope exchange mechanisms are investigated using the three-isotope method, which is a novel proxy to trace the direction and the progress of low-temperature water-mineral/rock interactions.

How to cite: Stamm, F. M., Baldermann, A., Frick, D. A., Hippler, D., and Dietzel, M.: Unravelling the formation paths of amorphous hydroxyaluminosilicates and hydrous ferric silicates: Evidence from silicon isotopes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14655, https://doi.org/10.5194/egusphere-egu23-14655, 2023.

A.189
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EGU23-2055
Ales Vaněk, Maria Vaňková, Martin Mihaljevič, and Vojtěch Ettler

Silver isotopic fractionation(s) during metallurgical processes (or other high-T processes) and its fate in mining- and smelter-affected environments, Ag-contaminated soils/sediments remain entirely unknown. Regarding the Ag-ore processing (roasting/smelting technologies), it can be assumed that, similarly to other isotopic systems (e.g., Tl and Cu), the isotopically lighter Ag (enriched in 107Ag) enters the smelter emissions (fly ash), and isotopically heavier Ag (enriched in 109Ag) remains in the residual metal phase. Our preliminary data from the Ag-contaminated soils around a former primary Ag-smelter at Příbram (Czech Rep.) indicate an apparent variability of ε109Ag within soil profiles. The identified ε109Ag values presented for the forest soil profile (Fig. 1) recorded ~0 in the organic horizon (O), ~0-+1 in the organo-mineral horizons (A1 and A2), and up to +7 in the mineral horizon (B). This finding suggests both the isotopically-lighter Ag fraction present in the topsoil (Fig. 1), as well as enhanced 107Ag release during high-T smelting operations, i.e., relative to geogenic Ag (local ore). However, the question that clearly remains to date is to which degree the alteration/chemical processes in soil could have produced the Ag isotopic fractionation and which type of mechanisms (e.g. Ag(I)→Ag0) could represent the key geochemical controls.

Fig. 1 An example of vertical evolution of Ag isotopic signatures (ε109Ag, relative to the NIST SRM 978a Ag standard) and Ag concentrations in a forest soil profile, ~1 km away from a former primary Ag-smelter (Příbram, Czech Rep.). The Ag isotopic data are assigned an estimated error of ±0.7 ε109Ag (2sigma), which is based on our long-term reproducibility of multiple separate analyses (n = 6) of NIST SRM 2782 (Industrial Sludge) (Vaněk et al., unpublished data).

How to cite: Vaněk, A., Vaňková, M., Mihaljevič, M., and Ettler, V.: Silver stable isotopic ratios determined in contaminated soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2055, https://doi.org/10.5194/egusphere-egu23-2055, 2023.

A.190
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EGU23-9366
Michael E. Böttcher, Cátia M. Ehlert von Ahn, Christoph Malik, Julia Westphal, Benjamin Rach, Carla Nantke, Anna-Kathrina Jenner, Rhodelyn Saban, Vera Winde, and Iris Schmiedinger

The flow path of a river draining a lowland in the southern Baltic Sea, the Warnow River, was investigated to evaluate its freshwater composition as a source of dissolved substances to regional coastal waters. A spatial study was carried out once to follow the variations from the source to the estuary. A temporal study in the composition as a function of the season, during 6 years (2017-2022), was carried out at a site just before the river reaches the estuary. Surface water was sampled to analyze major and tracer elements, stable (H, C, O, S), and unstable (Ra) isotopes. The results show that the composition of the Warnow River along the flow path is controlled by a complex interplay between in-situ processes, exchange with the atmosphere, diffuse groundwater, and surface water inlets. On a temporal scale, pH, nutrient, and redox sensitive trace element concentrations are strongly impacted by pelagic primary production in spring. During summer and autumn, influences occurred by benthic microbial activity, associated diffusive release from soils/sediments, and surface water inlets. Throughout the investigation period, the Warnow River was a source of isotopically light CO2 to the atmosphere and DIC to the estuarine waters. The delivered DIC concentrations seem to vary with the season due to changes in biological pelagic and benthic activity. DOC was derived from a mixture of C3 organic sources and fertilizers. From concentration-discharge relationships, examples of dilution, mobilization, and chemostasis trends were found. Discharge-controlled seasonal trends are superimposed by system-internal processes and the hydrological consequences of river and drainage management. Our analysis thus provides new insights into the controls on the variations of water and solutes in a managed river at the land-sea interface as part of the regional hydrological cycle of a lowland catchment coastal water system.

 

The study was supported by the DFG research training group BALTIC TRANSCOAST, DAAD ,  and the BMBF project CARBOSTORE/COOLSTYLE

How to cite: Böttcher, M. E., Ehlert von Ahn, C. M., Malik, C., Westphal, J., Rach, B., Nantke, C., Jenner, A.-K., Saban, R., Winde, V., and Schmiedinger, I.: Temporal and spatial variations in the isotope hydrobiogeochemistry (H, C, O, S, Ra) of a managed river draining a lowland towards the Baltic Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9366, https://doi.org/10.5194/egusphere-egu23-9366, 2023.

A.191
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EGU23-10139
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ECS
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Highlight
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Danielle Jones, Dulcinea Groff, and David Beilman

Low-elevation coastal ecosystems of the northern Antarctic Peninsula are responding to rapid climate change by expansion of ice-free areas and increased vegetation cover. The record-setting air temperatures during the austral summer of 2020 provided an opportunity to evaluate the sensitivity of peat-forming and carbon accumulating mosses to conditions with high evaporative demand, a departure from the typical wet, cold, and windy conditions of the northern Antarctic Peninsula. Mosses are sensitive to environmental change and have been used as archives of paleoclimate information where few terrestrial records exist. However, it is still unclear how much of an influence increased evaporative demand, source water variation, or microclimate can have on moss leaf cellulose. We collected environmental waters and moss surface samples to explore the influence of microclimate, persistent snowmelt, and evaporation on moss tissue waters and cellulose for two moss species, Chorisodontium aciphyllum and Polytrichum strictum. Using the δ18O isotope values of moss tissue waters and leaf alpha-cellulose from surface samples, we compared the enrichment found in mosses to the enrichment in local water sources.  

δ2H and δ18O isotopes of moss tissue waters ranged from -87 to -23 and -10.6 to 0.6, respectively, indicating significant enrichment relative to our environmental water samples (δ2H values of -98.3 to -48.7 and δ18O values of -12.1 to -5.21) and long-term summer precipitation (February, δ2H values of -66.9 ± 15.9‰ and δ18O values of -8.11 ± 2.20‰, Vernadsky Station). Negative correlations between δ18O of moss water and the daily average and maximum relative humidity (on the day of collection) (r = -0.44; p < 0.001 and r = -0.62; p < 0.001, respectively) suggest that conditions resulting in high evaporative demand may have a dominant effect and imprint on the δ18O of moss cellulose. We found no relationship between δ13C values of alpha-cellulose, which reflect changes in CO2 diffusivity with moisture conditions, and microclimate variables or average temperature and relative humidity on the day of sample collection. This is possibly the result of discrepancies between peak growth and seasonal peaks in cumulative evaporation leading the tissues to incorporate multiple seasons. The isotope values of moss waters likely reflect the anomalously warm summer on the Antarctic Peninsula, as indicated by the divergence from moss water lines published in non-record-setting summers. Although the δ18O values of moss tissue waters are a proxy for the composition of δ18O values of summer precipitation, it is critical to consider the imprint of high evaporative conditions, in addition to summer precipitation composition, when reconstructing past environmental conditions using cellulose from peat archives. 

How to cite: Jones, D., Groff, D., and Beilman, D.: The influence of high evaporative conditions on peat-forming mosses in the Antarctic Peninsula, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10139, https://doi.org/10.5194/egusphere-egu23-10139, 2023.

A.192
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EGU23-5210
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ECS
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Noémy Chénier, Paul M. Magyar, Lukas Emmenegger, Moritz F. Lehmann, and Joachim Mohn

The isotopic composition of nitrous oxide (N2O) reveals valuable information on the biological production sources that contribute to N2O accumulation in the atmosphere, i.e. denitrification, nitrifier-denitrification and nitrification1. Isotopic fingerprints for each of these microbial pathways have been identified in past work, however, overlapping signatures of co-occurring N2O production processes2, and limitations in the robustness of associated fractionation factors under varying growth/environmental conditions3 still pose significant challenges.

We will present data from the initial phase of our project, where we study N2O production and associated N and O isotopic fractionation by the denitrifier Pseudomonas aureofaciens, grown in the laboratory under different growth conditions and thus different reaction kinetics. N2O production was quantified on-line by Fourier-Transformation IR-spectroscopy (FTIR), and the isotopic composition of produced N2O was determined by quantum cascade-laser-absorption spectroscopy (QCLAS). The combination of N2O production and isotope data (continuously measured) allowed us to elucidate changes in N and O isotope fractionation in response to changing reaction kinetics.

In a later phase of the project, we will expand our isotope-analytical capability by including also the doubly-substituted molecules of N2O, 15N15N16O (556), 14N15N18O (458) and 15N14N18O (548). More specifically, we will interrogate the symmetry of N – N bond formation, verify combinatorial effects during N2O production, and we will test whether Δ556, Δ458, and Δ548 (and the preference for 15N substitution in the central/terminal N-position) can be applied as proxies for reaction kinetics4, 5.  

1 Yu, L., Harris, E., Lewicka Szczebak, D., Barthel, M., Blomberg, M.R., Harris, S.J., Johnson, M.S., Lehmann, M.F., Liisberg, J., Müller, C. and Ostrom, N.E., 2020. What can we learn from N2O isotope data?–Analytics, processes and modelling. Rapid Communications in Mass Spectrometry, 34(20), p.e8858.

2 Kantnerová, K., Tuzson, B., Emmenegger, L., Bernasconi, S.M. and Mohn, J., 2019. Quantifying isotopic signatures of N2O using quantum cascade laser absorption spectroscopy. Chimia, 73(4), pp.232-232.

3 Haslun, J.A., Ostrom, N.E., Hegg, E.L. and Ostrom, P.H., 2018. Estimation of isotope variation of N2O during denitrification by Pseudomonas aureofaciens and Pseudomonas chlororaphis: implications for N2O source apportionment. Biogeosciences, 15(12), pp.3873-3882.

4 Yeung, L.Y., 2016. Combinatorial effects on clumped isotopes and their significance in biogeochemistry. Geochimica et Cosmochimica Acta, 172, pp.22-38.

5 Kantnerová, K., Hattori, S., Toyoda, S., Yoshida, N., Emmenegger, L., Bernasconi, S.M. and Mohn, J., 2022. Clumped isotope signatures of nitrous oxide formed by bacterial denitrification. Geochimica et Cosmochimica Acta, 328, pp.120-129.

How to cite: Chénier, N., Magyar, P. M., Emmenegger, L., Lehmann, M. F., and Mohn, J.: Fractionation of nitrogen and oxygen isotopic composition in N2O produced by bacterial denitrification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5210, https://doi.org/10.5194/egusphere-egu23-5210, 2023.

A.193
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EGU23-7883
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ECS
Sebastian Floßmann, Kaiyu Lei, Sigrid van Grinsven, Jörg Völkel, Ingrid Kögel-Knabner, and Michael Dannenmann

Optimized slurry management targeted to increase nitrogen use efficiency (NUE) can be fundamental for limiting fertilizer N losses from agricultural grasslands causing eutrophication of ground- and surface water as well as air pollution. However, a holistic assessment of both agronomic and environmental impacts including key soil functions is still missing. This study aims to provide such information by assessing the impacts of traditional vs. modern slurry application techniques on NUE, hydrological and gaseous N losses, productivity and fodder quality, soil organic nitrogen formation and total N balances.  In a plot-scale grassland experiment 15N enriched slurry was applied after the first cut in early summer. The application treatments included: (1) traditional slurry broadcast spreading under dry weather; (2) application like (1) followed by a heavy rainfall event to increase slurry infiltration into soil; (3) broadcast spreading of slurry diluted with water; (4) injection of slurry into soil via shallow slits; and (5) injection of slurry into soil via deep slits. Variant (4) and (5) represent modern trailing shoe injections requiring extensive machinery. Fates of fertilizer N such as plant uptake, immobilization in soil and microbial biomass as well as NO3 leaching were investigated by 15N tracing approaches in order to create full N balances. For this, biomass harvest and soil sampling were conducted after a growth period of 2 months. Here, we will present first results of our work that are expected to provide highly relevant decision support for grassland management. 

How to cite: Floßmann, S., Lei, K., van Grinsven, S., Völkel, J., Kögel-Knabner, I., and Dannenmann, M.: Effects of different slurry application techniques on Nitrogen Use Efficiency (NUE) in an extensive grassland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7883, https://doi.org/10.5194/egusphere-egu23-7883, 2023.

A.194
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EGU23-16361
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ECS
Vanessa Russnak, Sophie Kache, Maren Voss, and Kirstin Dähnke

Estuaries are important biogeochemical reactors that can remove dissolved inorganic nitrogen (DIN, mostly nitrate) from the water column, but can also generate nitrate via remineralization and subsequent nitrification of organic matter in the water column. To assess this regeneration of nitrate, an important nutrient source for phytoplankton that contributes to eutrophication, various isotope-based laboratory methods are in use.

In this study, we compare two commonly used stable-isotope-based techniques to measure nitrification in estuarine water, the isotope dilution method and the addition of 15N-ammonium. Both measure the isotope enrichment in nitrate but have drastically different incubation times. We apply both methods in the estuary of the Elbe River and evaluate the drawbacks and advantages of each method to develop application recommendations.

Our results indicate that nitrification measurements using isotope dilution are less variable between stations, but suggest that rates are overestimated at high phytoplankton activity. On the counter side, the addition of 15N-ammonium as a tracer apparently overestimates nitrification in heterotrophic settings, probably because substrate addition stimulates nitrification. The adequate measurement technique must this be carefully chosen depending on the selected study site.

Funding information  - This study has been carried out and was financially supported by the BMBF “Blue-Estuaries” project (grant no. 03F0864C)

How to cite: Russnak, V., Kache, S., Voss, M., and Dähnke, K.: Comparison of two isotope-based methods to quantify nitrification rates in estuaries, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16361, https://doi.org/10.5194/egusphere-egu23-16361, 2023.

A.195
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EGU23-13490
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ECS
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Alessandra Mazzoli, Cameron M. Callbeck, Tim J. Paulus, Claudia Frey, Jakob Zopfi, Sergei Katsev, Donald E. Canfield, and Moritz F. Lehmann

Marine and lacustrine benthic habitats represent hotspots of nitrogen (N) turnover, with many N transformation processes occurring simultaneously, and at high rates. More specifically, sedimentary microbial reduction of nitrate to dinitrogen (N2), and other modes of N2 production (e.g., anammox), are the most important sinks of fixed N in aquatic environments. Natural abundance stable isotope ratio measurements can be employed to help disentangling N-loss mechanisms, given that the isotope effects (e) associated with each process are well studied.

In our study, we surveyed an array of lacustrine and marine benthic environments and assessed the isotopic composition (d15N and d18O) of porewater ammonium (NH4+) and nitrate (NO3-) – the key substrates driving the various N transformation pathways, such as denitrification, anammox, nitrification and dissimilatory nitrate reduction to ammonium (DNRA). We further examined how benthic N isotope dynamics vary in different sedimentary regimes, with distinct organic matter (OM) mineralization rates. We expect the relative importance of the various N pathways to affect the NH4+ and/or NO3- isotope pools differentially.

Our preliminary porewater d15N-NH4+ results suggest a diverse pattern with regard to the isotope enrichments between oligotrophic lakes, characterized by relatively strong depth gradients, and eutrophic lakes, where no significant depth gradient was observed. We hypothesize that this distinction could be attributed to different organic N availability and/or anammox contributions in the surveyed environments. Furthermore, we compared rate measurements (based on 15N addition experiments) to the N and O nitrate isotopic signatures to quantify processes, such as denitrification and DNRA. To investigate N isotope fractionation within the narrow nitracline, we employed the whole-core squeeze method, which provided a high-resolution porewater NO3-profile. Using this method, we found that the calculated community nitrate consumption e values at the nitracline (eelim_porewater) showed strong variability among the surveyed settings. We attribute such variation to shifts in the relative importance of denitrification versus DNRA and anammox between lakes of different trophic states. Overall, eelim_porewater wasconsiderably below 25‰ – the value generally reported for the biological isotope effect of denitrification (edenit).

How to cite: Mazzoli, A., Callbeck, C. M., Paulus, T. J., Frey, C., Zopfi, J., Katsev, S., Canfield, D. E., and Lehmann, M. F.: New insights into benthic nitrogen cycling using natural-abundance stable isotopes measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13490, https://doi.org/10.5194/egusphere-egu23-13490, 2023.

A.196
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EGU23-16727
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ECS
Tim J. Paulus, Alessandra Mazzoli, Claudia Frey, Jakob Zopfi, Cameron Callbeck, and Moritz F. Lehmann

Aquatic sediments play a critical role in moderating the availability of fixed nitrogen (N) in the biosphere. Microbial N cycling processes, such as denitrification and anammox, contribute to fixed‑N removal as N2 gas from lakes and the ocean. N‑isotopic measurements of dissolved inorganic N (e.g., nitrate (NO3-)) can provide insights into the different sources, sinks, and pathways of N, if the associated N isotope signatures/effects are constrained. While substantial work has been done to resolve N‑loss using microbial metagenomic‑based approaches and rate measurements, how nitrogen‑loss processes imprint natural‑abundance isotopes of 15N and 18O of NO3- remains largely understudied in freshwater lake sediments. Current marine evidence suggests that water column denitrification involves high NO3- isotope effects (>20‰). In contrast, the marine NO3- isotope effect of sedimentary denitrification is suppressed at the level of the sediment‑water interface (apparent N/O isotope effect, εapp <5‰). How anammox affects εapp in either marine or freshwater systems, is completely unknown. This study aims to achieve a deeper understanding of NO3- N and O isotope fractionation during benthic N transformation and sedimentary N‑loss (including anammox), and its ultimate expression in the water column of freshwater lakes. We also investigate how εapp values may vary with environmental conditions (e.g., trophic state) that affect the reactivity and amount of organic matter in the sediments, as well as the balance between benthic N‑cycle reactions.

The two study sites Lake Baldegg (eutrophic) and Sarnen (oligotrophic), were chosen because of their contrasting trophic states. We conducted a suite of experiments with sediment cores collected at different times of the year to assess the sedimentary εapp in these two lakes. More specifically, we carried out whole‑core incubations under oxic/anoxic conditions and examined the change 15N/14N and 18O/16O of NO3- with net NO3- depletion in the overlying water. We integrated natural‑abundance N and O isotope measurements with 15N‑label based N transformation‑rate measurements, to understand how the phenology and differential combination of the different N transformation pathways may modulate εapp. We demonstrate that nitrification, DNRA, denitrification, anammox and organic matter remineralization overlap spatially in the sediments of Lake Sarnen. In contrast, these processes are, in parts, spatially decoupled in Lake Baldegg. Moreover, the relative importance of anammox versus denitrification is significantly greater in Lake Baldegg. In both lakes the net N isotope effect of sedimentary NO3- consumption is strongly underexpressed at the ecosystem level, with NO3-‑N εapp values systematically below 4‰. In Lake Sarnen the NO3- N‑vs.‑O isotope signature followed a 1:1 trend, whereas in Lake Baldegg a systematically higher ratio was observed. This suggests that, while in Lake Sarnen, the NO3- N and O isotope signatures are dominated by NO3- reduction, NO3- regeneration (e.g., by nitrification or anammox) overprints the NO3- isotopic signature of net denitrification under the more eutrophic conditions in Lake Baldegg.

How to cite: Paulus, T. J., Mazzoli, A., Frey, C., Zopfi, J., Callbeck, C., and Lehmann, M. F.: The isotope effect of benthic N removal in two Swiss lakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16727, https://doi.org/10.5194/egusphere-egu23-16727, 2023.

A.197
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EGU23-16065
Angel Fernandez-Cortes, Tamara Martin-Pozas, Soledad Cuezva, Fernando Gazquez, David Benavente, Juan Carlos Cañaveras, Cesareo Saiz-Jimenez, and Sergio Sanchez-Moral

Water plays a key role in the colonization and ecology of microorganisms in natural subterranean environments since it controls their metabolic activity and the microbe-mineral interactions. In turn, microbial activity leaves its signature in groundwater. For instance, the aerial hyphae of Actinobacteria colonizing cave-rock surfaces and sediments act as vapour condensation nuclei and facilitate the retention of interstitial water in the porous system of rocks, sediments and speleothems.

Here, we present the results of exhaustive monitoring of cave waters from Pindal Cave (northern Spain), a shallow and well-ventilated cave on the coastline, with extensive microbial colonization on the sediments and walls surfaces. This study sheds light on the detection of biosignatures based on stables isotopes of cave waters and controlled by underground-dwelling microorganisms. The isotopic composition (d18O and dD) of seepage waters (fast drips, soda straws and gours) and condensation droplets were determined by cavity ring-down spectroscopy (CRDS). An analytical set-up based on a CRDS analyser coupled to an Induction Module enabled the step-heating of moonmilk deposits and other substrates with distinct degrees of microbial colonization, and the selective release of their interstitial water for isotope analyses.

The δ18O and δD values of the in-cave waters correlate linearly (δD=7.71·δ18O+12.28; R2=0.97), like the Local Meteoric Water Line (δD=7.00·δ18O+6.77; R2=0.92, based on the of isotopes composition of regional rain). The offset of d-excess between the cave waters and the LMWL would indicate a significant recharge by occult precipitation linked to local coastal fogs. The isotopic composition of dripping water,  -5.7‰ [-6.7 to -4.8 ‰] for  d18O and -31.3‰ [-39.0 to -22.7 ‰] for dD, agree with the water samples from the gours located far from the cave entrance, with some slight deviations indicating evaporation processes or sudden contributions of meteoric water due to flash-flood events.

The δ18O and δD values of condensation water collected from non-colonized surfaces are -5.4‰ [-5.9 to -4.5 ‰] and –28.4‰ [-32.9 to -21.3 ‰], respectively. This isotope composition is similar to the contemporary infiltration water, which suggests that condensation mostly comes from autochthonous vapour generated from in-cave waters. The Actinobacteria mats quickly absorb the condensation water and it is then retained during long periods on the rock surface. Thus, in the case of microbial induced deposits like moonmilk, condensate water becomes a reservoir of interstitial water exposed to isotopes fractionation linked to the microbial metabolism processes. Indeed, the δ18O and δD values of this interstitial water correlate linearly with the rest of the in-cave waters but show a distinctive composition with higher δ18O and δD values compared to the infiltration water and condensation droplets; -3.6‰ [-4.4 to -2.0 ‰] and –18.9‰ [-27.2 to -10.8 ‰], respectively. These findings lead to exploring the potential use of water isotopes as a tool for indirectly assessing microbial activity and its role in the water balances in underground ecosystems.

Research funded by PID2019-110603RB-I00 – SUBSYST and PID2020-114978GB-I00 projects

How to cite: Fernandez-Cortes, A., Martin-Pozas, T., Cuezva, S., Gazquez, F., Benavente, D., Cañaveras, J. C., Saiz-Jimenez, C., and Sanchez-Moral, S.: Isotopic tracking of condensation, infiltration and interstitial water to detect microbial activity in caves, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16065, https://doi.org/10.5194/egusphere-egu23-16065, 2023.