Effects of experimental warming and carbon addition on nitrate reduction and respiration in coastal sediments

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Biogeochemistry 125 (2015): 81-95, doi:10.1007/s10533-015-0113-4.Climate change may have differing effects on microbial processes that control coastal N availability. We conducted a microcosm experiment to explore effects of warming and carbon availability on nitrate reduction pathways in marine sediments. Sieved continental shelf sediments were incubated for 12 weeks under aerated seawater amended with nitrate (~50 μM), at winter (4°C) or summer (17°C) temperatures, with or without biweekly particulate organic C additions. Treatments increased diffusive oxygen consumption as expected, with somewhat higher effects of C addition compared to warming. Combined warming and C addition had the strongest effect on nitrate flux across the sediment water interface, with a complete switch early in the experiment from influx to sustained efflux. Supporting this result, vial incubations with added 15N-nitrate indicated that C addition stimulated potential rates of dissimilatory nitrate reduction to ammonium (DNRA), but not denitrification. Overall capacity for both denitrification and DNRA was reduced in warmed treatments, possibly reflecting C losses due to increased respiration with warming. Anammox potential rates were much lower than DNRA or denitrification, and were slightly negatively affected by warming or C addition. Overall, results indicate that warming and C addition increased ammonium production through remineralization and possibly DNRA. This stimulated nitrate production through nitrification, but without a comparable increase in nitrate consumption through denitrification. The response to C of potential DNRA rates over denitrification, along with a switch to nitrate efflux, raises the possibility that DNRA is an important and previously overlooked source of internal N cycling in shelf sediments.This material is based upon work supported by the National Science Foundation by OCE- 0852289 to JJR and OCE-0852263 and OCE-0927400 to AEG, and Rhode Island Sea Grant to JJR

Similar temperature responses suggest future climate warming will not alter partitioning between denitrification and anammox in temperate marine sediments

Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Global Change Biology 23 (2017): 331-340, doi:10.1111/gcb.13370.Removal of biologically available nitrogen (N) by the microbially mediated processes denitrification and anaerobic ammonium oxidation (anammox) affects ecosystem N availability. Although few studies have examined temperature responses of denitrification and anammox, previous work suggests that denitrification could become more important than anammox in response to climate warming. To test this hypothesis, we determined whether temperature responses of denitrification and anammox differed in shelf and estuarine sediments from coastal Rhode Island over a seasonal cycle. The influence of temperature and organic C availability was further assessed in a 12-week laboratory microcosm experiment. Temperature responses, as characterized by thermal optima (Topt) and apparent activation energy (Ea), were determined by measuring potential rates of denitrification and anammox at 31 discrete temperatures ranging from 3 to 59°C. With a few exceptions, Topt and Ea of denitrification and anammox did not differ in Rhode Island sediments over the seasonal cycle. In microcosm sediments, Ea was somewhat lower for anammox compared to denitrification across all treatments. However, Topt did not differ between processes, and neither Ea nor Topt changed with warming or carbon addition. Thus, the two processes behaved similarly in terms of temperature response, and this response was not influenced by warming. This led us to reject the hypothesis that anammox is more cold-adapted than denitrification in our study system. Overall, our study suggests that temperature responses of both processes can be accurately modeled for temperate regions in the future using a single set of parameters, which are likely not to change over the next century as a result of predicted climate warming. We further conclude that climate warming will not directly alter the partitioning of N flow through anammox and denitrification.This material is based upon work supported by the National Science Foundation under Grant No. OCE-0852289 to JJR and OCE-0852263, OCE-0927400 and OCE1238212 to AEG, and Rhode Island Sea Grant to JJR.2017-05-2

Dynamic behavior of magnetic avalanches in the spin-ice compound Dy2_2Ti2_2O7_7

Avalanches of the magnetization, that is to say an abrupt reversal of the magnetization at a given field, have been previously reported in the spin-ice compound Dy$_{2}$Ti$_{2}$O$_{7}$. This out-of-equilibrium process, induced by magneto-thermal heating, is quite usual in low temperature magnetization studies. A key point is to determine the physical origin of the avalanche process. In particular, in spin-ice compounds, the origin of the avalanches might be related to the monopole physics inherent to the system. We have performed a detailed study of the avalanche phenomena in three single crystals, with the field oriented along the [111] direction, perpendicular to [111] and along the [100] directions. We have measured the changing magnetization during the avalanches and conclude that avalanches in spin ice are quite slow compared to the avalanches reported in other systems such as molecular magnets. Our measurements show that the avalanches trigger after a delay of about 500 ms and that the reversal of the magnetization then occurs in a few hundreds of milliseconds. These features suggest an unusual propagation of the reversal, which might be due to the monopole motion. The avalanche fields seem to be reproducible in a given direction for different samples, but they strongly depend on the initial state of magnetization and on how the initial state was achieved.Comment: 11 pages, 14 figure

Design and development of a low temperature, inductance based high frequency ac susceptometer

We report on the development of an induction based low temperature high frequency ac susceptometer capable of measuring at frequencies up to 3.5 MHz and at temperatures between 2 K and 300 K. Careful balancing of the detection coils and calibration have allowed a sample magnetic moment resolution of $5\times10^{-10} Am^2$ at 1 MHz. We will discuss the design and characterization of the susceptometer, and explain the calibration process. We also include some example measurements on the spin ice material CdEr$_2$S$_4$ and iron oxide based nanoparticles to illustrate functionality

Vacancies, disorder-induced smearing of the electronic structure, and its implications for the superconductivity of anti-perovskite MgC0.93_{0.93}Ni2.85_{2.85}

The anti-perovskite superconductor MgC$_{0.93}$Ni$_{2.85}$ was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20\% reduction in the Fermi level density-of-states would lead to a significant ($\sim 70\%$) suppression of the superconducting $T_c$ for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller $T_c$ reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations.Comment: 11 pages, 3 figure

Stabilization of single-electron pumps by high magnetic fields

We study the effect of perpendicular magnetic fields on a single-electron system with a strongly time-dependent electrostatic potential. Continuous improvements to the current quantization in these electron pumps are revealed by high-resolution measurements. Simulations show that the sensitivity of tunnel rates to the barrier potential is enhanced, stabilizing particular charge states. Nonadiabatic excitations are also suppressed due to a reduced sensitivity of the Fock-Darwin states to electrostatic potential. The combination of these effects leads to significantly more accurate current quantization

Experimental measurement of the isolated magnetic susceptibility

The isolated susceptibility $\chi_{\rm I}$ may be defined as a (non-thermodynamic) average over the canonical ensemble, but while it has often been discussed in the literature, it has not been clearly measured. Here, we demonstrate an unambiguous measurement of $\chi_{\rm I}$ at avoided nuclear-electronic level crossings in a dilute spin ice system, containing well-separated holmium ions. We show that $\chi_{\rm I}$ quantifies the superposition of quasi-classical spin states at these points, and is a direct measure of state concurrence and populations.Comment: 9 pages, & figure

Nuclear spin assisted quantum tunnelling of magnetic monopoles in spin ice

Extensive work on single molecule magnets has identified a fundamental mode of relaxation arising from the nuclear-spin assisted quantum tunnelling of nearly independent and quasi-classical magnetic dipoles. Here we show that nuclear-spin assisted quantum tunnelling can also control the dynamics of purely emergent excitations: magnetic monopoles in spin ice. Our low temperature experiments were conducted on canonical spin ice materials with a broad range of nuclear spin values. By measuring the magnetic relaxation, or monopole current, we demonstrate strong evidence that dynamical coupling with the hyperfine fields bring the electronic spins associated with magnetic monopoles to resonance, allowing the monopoles to hop and transport magnetic charge. Our result shows how the coupling of electronic spins with nuclear spins may be used to control the monopole current. It broadens the relevance of the assisted quantum tunnelling mechanism from single molecular spins to emergent excitations in a strongly correlated system