Publications

Dissolved organic carbon (DOC) is the largest reduced carbon reservoir in modern oceans^1,2. Its dynamics regulate marine communities and atmospheric CO₂ levels^3,4, whereas ¹³C compositions track ecosystem structure and autotrophic metabolism⁵. However, the geologic history of marine DOC remains largely unconstrained^6,7, limiting our ability to mechanistically reconstruct coupled ecological and biogeochemical evolution. Here we develop and validate a direct proxy for past DOC signatures using co-precipitated organic carbon in iron ooids. We apply this to 26 marine iron ooid-containing formations deposited over the past 1,650 million years to generate a data-based reconstruction of marine DOC signals since the Palaeoproterozoic. Our predicted DOC concentrations were near modern levels in the Palaeoproterozoic, then decreased by 90−99% in the Neoproterozoic before sharply rising in the Cambrian. We interpret these dynamics to reflect three distinct states. The occurrence of mostly small, single-celled organisms combined with severely hypoxic deep oceans, followed by larger, more complex organisms and little change in ocean oxygenation and finally continued organism growth and a transition to fully oxygenated oceans^8,9. Furthermore, modern DOC is ¹³C-enriched relative to the Proterozoic, possibly because of changing autotrophic carbon-isotope fractionation driven by biological innovation. Our findings reflect connections between the carbon cycle, ocean oxygenation and the evolution of complex life.

Cavity ring-down spectroscopy (CRDS) is rapidly becoming an invaluable tool to measure hydrogen (δ²H) and oxygen (δ¹⁸O) isotopic compositions in water, yet the long-term accuracy and precision of this technique remain relatively underreported. Here, we critically evaluate one-year performance of CRDS δ²H and δ¹⁸O measurements at ETH Zurich, focusing on high throughput (~200 samples per week) while maintaining required precision and accuracy for diverse scientific investigations. We detail a comprehensive methodological and calibration strategy to optimize CRDS reliability for continuous, high-throughput analysis using Picarros \u201cExpress\u201d mode, an area not extensively explored previously. Using this strategy, we demonstrate that CRDS achieves long-term precision better than ±0.5 for δ¹⁸O and ±1.0 for δ²H (±1σ) on three United States Geological Survey (USGS) reference materials treated as unknowns.¹⁸ Specifically, reported results for each reference material over this one-year period are: (i) USGS W-67444: δ2H = −399.32±0.96, δ18O = −51.07±0.45(n=30), (ii) USGS W-67400:δ2H = 2.55±0.49, δ18O =−1.85±0.13(n=140), and (iii) USGS-50: δ2H = 33.68±0.91, δ18O = 5.03±0.04 (n=21). We also address challenges such as aligning our analytical uncertainties with the narrower uncertainties of International Atomic Energy Agency reference materials, and mitigating inherent CRDS issues like memory and matrix effects when analyzing environmental samples. Our review provides a practical framework for CRDS applications in hydrology, paleoclimatology, and biogeochemistry, underscoring the importance of continuous evaluation and methodological refinement to ensure accuracy and precision in δ²H and δ¹⁸O analyses.¹⁸

We introduce a novel high-precision method for oxygen-isotope analysis of iron (oxyhydr)oxides using high-temperature conversion isotope ratio mass spectrometry (HTC-IRMS). In this approach, a finely ground mixture of iron (oxyhydr)oxide and graphite is heated at 1450 °C in a helium flow environment, converting oxygen to CO gas with nearly 100% yield. Continuous-flow IRMS analysis of the liberated CO yields a precision of ±0.15 (1σ, n = 28) and shows excellent agreement with (and improved precision over) traditional fluorination methods. This practical and safe technique expands access to oxygen-isotope measurements of iron oxides, thereby enhancing their utility in Earth and environmental sciences.

Carbon efflux from soils is the largest terrestrial carbon source to the atmosphere, yet it is still one of the most uncertain fluxes in the Earths carbon budget. A dominant component of this flux is heterotrophic respiration, influenced by several environmental factors, most notably soil temperature and moisture. Here, we develop a mechanistic model from micro to global scale to explore how changes in soil water content and temperature affect soil heterotrophic respiration. Simulations, laboratory measurements, and field observations validate the new approach. Estimates from the model show that heterotrophic respiration has been increasing since the 1980s at a rate of about 2% per decade globally. Using future projections of surface temperature and soil moisture, the model predicts a global increase of about 40% in heterotrophic respiration by the end of the century under the worst-case emission scenario, where the Arctic region is expected to experience a more than two-fold increase, driven primarily by declining soil moisture rather than temperature increase.

Traditionally, nuclear spin is not considered to affect biological processes. Recently, this has changed as isotopic fractionation that deviates from classical mass dependence was reported both in vitro and in vivo. In these cases, the isotopic effect correlates with the nuclear magnetic spin. Here, we show nuclear spin effects using stable oxygen isotopes (16O, 17O, and 18O) in two separate setups: an artificial dioxygen production system and biological aquaporin channels in cells. We observe that oxygen dynamics in chiral environments (in particular its transport) depend on nuclear spin, suggesting future applications for controlled isotope separation to be used, for instance, in NMR. To demonstrate the mechanism behind our findings, we formulate theoretical models based on a nuclear-spin-enhanced switch between electronic spin states. Accounting for the role of nuclear spin in biology can provide insights into the role of quantum effects in living systems and help inspire the development of future biotechnology solutions.

Iron oxides and oxyhydroxides show promise as superconductor materials and as repositories of paleo-environmental information. However, there are no microscale non-destructive analytical techniques to characterize their combined mineralogy, chemical composition, and crystal properties. We address this by developing cathodoluminescence mounted on a scanning electron microscope (SEM-CL) as an in situ, non-destructive method for the crystallographic and petrographic study of iron oxides and oxyhydroxides. We show that goethite, hematite, and magnetite display different SEM-CL spectra, which may be used for mineral identification. We further show that different formation pH, manganese substitution for iron in goethite and hematite, and titanium substitution for iron in magnetite cause shifts in the SEM-CL spectra of these minerals. These spectral shifts are not always detectable as a change in the emission color but are easily discernable by quantitative analysis of the spectra. Together with subtle but observable variations in the SEM-CL spectra of natural goethite and hematite, we suggest that these dependences of the SEM-CL spectra on pH and chemical composition may be used as a means of identifying multiple episodes of mineralization and recrystallization. We apply the newly developed SEM-CL methods to two polished sections of natural samples and show that quantitative analysis of the spectra obtained allows the identification of differences between varieties of the same mineral that are not observable by other means. Like the application of SEM-CL to geologic samples in this study, we suggest that this approach may be used to explore the in situ chemistry and crystallinity of various natural and manufactured iron oxides and oxyhydroxides.

The triple-oxygen isotopic fractionation associated with freezing is a fundamental property of water, knowledge of which is essential for reconstructions of the hydrological cycle from the triple-oxygen isotopic composition of natural materials. We constrained this isotopic fractionation, in freshwater and seawater, in a series of freezing experiments over a range of temperatures and freezing rates. The freshwater freezing experiments with the lowest freezing rates, which we consider closest to isotopic equilibrium, yield 18O/16O, 17O/16O and 2H/1H fractionations of 2.82±0.12, 1.49 ± 0.07 and 20.05 ± 0.72, respectively. The slowest-freezing seawater experiments yield 18O/16O, 17O/16O and 2H/1H fractionations of 2.92 ± 0.08, 1.55 ± 0.03 and 21.18 ± 1.85, respectively. The 18O/16O and 2H/1H fractionation estimates in freshwater and seawater are within error of each other and in broad agreement with past estimates. Our newly determined 17O/16O fractionations constrain the triple-oxygen mass dependence of water freezing to be ≈0.528, but with large uncertainty. If this mass dependence is accurate, then ice formation and melting processes in the hydrological cycle are expected to generate variability that is on the Global Meteoric Water Line. Similar ice-freshwater and ice-seawater near-equilibrium isotopic fractionations.The ice-freshwater and ice-seawater 2H/1H fractionation is ≈21±2.The ice-freshwater and ice-seawater 18O/16O fractionation is ≈2.9±0.1.With large uncertainty, the oxygen isotope mass dependence of freezing is ≈0.528.Ice formation/melting should produce isotopic variability along the GMWL.

Earths surface has undergone a protracted oxygenation, which is commonly assumed to have profoundly affected the biosphere. However, basic aspects of this history are still debatedforemost oxygen (O2) levels in the oceans and atmosphere during the billion years leading up to the rise of algae and animals. Here we use isotope ratios of iron (Fe) in ironstonesFe-rich sedimentary rocks deposited in nearshore marine settingsas a proxy for O2 levels in shallow seawater. We show that partial oxidation of dissolved Fe(II) was characteristic of Proterozoic shallow marine environments, whereas younger ironstones formed via complete oxidation of Fe(II). Regardless of the Fe(II) source, partial Fe(II) oxidation requires low O2 in the shallow oceans, settings crucial to eukaryotic evolution. Low O2 in surface waters can be linked to markedly low atmospheric O2likely requiring less than 1% of modern levels. Based on our records, these conditions persisted (at least periodically) until a shift toward higher surface O2 levels between ca. 900 and 750 Ma, coincident with an apparent rise in eukaryotic ecosystem complexity. This supports the case that a first-order shift in surface O2 levels during this interval may have selected for life modes adapted to more oxygenated habitats.

The oxygen isotope composition (d ¹⁸O) of marine sedimentary rocks has increased by 10 to 15 per mil since Archean time. Interpretation of this trend is hindered by the dual control of temperature and fluid d ¹⁸O on the rocks' isotopic composition. A new d ¹⁸O record in marine iron oxides covering the past ~2000 million years shows a similar secular rise. Iron oxide precipitation experiments reveal a weakly temperature-dependent iron oxide-water oxygen isotope fractionation, suggesting that increasing seawater d ¹⁸O over time was the primary cause of the long-term rise in d ¹⁸O values of marine precipitates. The ¹⁸O enrichment may have been driven by an increase in terrestrial sediment cover, a change in the proportion of high- and low-temperature crustal alteration, or a combination of these and other factors.

The electronic structure of cobalt-phthalocyanine (CoPc) molecules adsorbed on Ag(100) is investigated by photoemission spectroscopy. The results are compared to first-principles electronic structure calculations, based on many-body perturbation theory in the GW approximation. The photoemission data, obtained from both multilayer and monolayer films of CoPc, show that charge transfer occurs between the first molecular layer and the metal surface. Varying the photon energy, to tune the photoionization cross sections, reveals that the charge-transfer-related interface states mainly involve the Co 3d atomic orbitals of the Co central atom. GW calculations for the neutral CoPc molecule and its anion compare well with the experimental observations for a multilayer and a monolayer CoPc film, respectively. They confirm the major role played by the Co atom in the charge-transfer process and elucidate the complex energy rearrangement of the molecular electronic levels upon metal adsorption.

Spin injection efficiency is shown to strongly depend on the interfacial structure between Fe contacts and Al_xGa_1-xAs in spin-based light emitting diodes. Both the magnitude and sign of the injected carriers are dependent on the atomic structure of the contacts and can be controlled through changes in temperature both during and following growth. We propose that the observed dependence is due to phase formation resulting from Fe/GaAs interfacial reactions. This proposed mechanism is consistent with electronic structure calculations, which show that thin layers of DO₃ Fe₃Ga at the Fe/GaAs interface can produce the observed sign reversals in the spin polarization of injected carriers.

The combination of photoelectron spectroscopy and density functional theory is an important technique for clarifying a material's electronic structure. So far, however, it has been difficult to predict when the spectrum of occupied Kohn-Sham eigenvalues obtained from commonly used (semi-)local functionals bears physical relevance and when not. We demonstrate that a simple criterion based on evaluating each orbital's self-interaction allows prediction of the physical reliability of the eigenvalue spectrum. We further show that a self-interaction correction significantly improves the interpretability of eigenvalues also in difficult cases such as organic semiconductors where (semi-)local functionals fail.

Fibroblast growth factors (FGFs) and vascular endothelial growth factor (VEGF) play a pivotal role in the multistep pathway of tumor progression, metastasis, and angiogenesis. We have identified a porphyrin analogue, 5,10,15,20-tetrakis(methyl-4-pyridyl)-21H,23H-porphine-tetrap-tosylate salt (TMPP), as a potent inhibitor of FGF2 and VEGF receptor binding and activation. TMPP demonstrated potent inhibition of binding of soluble FGF receptor 1 (FGFR1) to FGF2 immobilized on heparin at submicromolar concentrations. TMPP inhibits binding of radiolabeled FGF2 to FGFR in a cell- free system as well as to cells genetically engineered to express FGFR1. Furthermore, TMPP also inhibits the binding of VEGF to its tyrosine kinase receptor in a dose-dependent manner. In an in vitro angiogenic assay measuring the extent of endothelial cell growth, tube formation, and sprouting, TMPP dramatically reduced the extent of the FGF2-induced endothelial cell outgrowth and differentiation. In a Lewis lung carcinoma model, mice receiving TMPP showed a marked inhibition of both primary tumor progression and lung metastases development, with nearly total inhibition of the metastatic phenotype upon alternate daily injections of TMPP at 25 μg/g of body mass. Finally, novel meso-pyridylium-substituted, nonsymmetric porphyrins, as well as a novel corrole-based derivative, with >50-fold increase in activity in vitro, had a significantly improved efficacy in blocking tumor progression and metastasis in vivo.