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545 results on '"Thoms, S."'

1. Dynamical mechanism of antifreeze proteins to prevent ice growth

Author

Kutschan, B., Morawetz, K., and Thoms, S.

Subjects
Physics - Biological Physics, Condensed Matter - Soft Condensed Matter, Physics - Chemical Physics
Abstract

The fascinating ability of algae, insects and fishes to survive at temperatures below normal freezing is realized by antifreeze proteins (AFPs). These are surface-active molecules and interact with the diffusive water/ice interface thus preventing complete solidification. We propose a new dynamical mechanism on how these proteins inhibit the freezing of water. We apply a Ginzburg-Landau type approach to describe the phase separation in the two-component system (ice, AFP). The free energy density involves two fields: one for the ice phase with a low AFP concentration, and one for liquid water with a high AFP concentration. The time evolution of the ice reveals microstructures resulting from phase separation in the presence of AFPs. We observed a faster clustering of pre-ice structure connected to a locking of grain size by the action of AFP, which is an essentially dynamical process. The adsorption of additional water molecules is inhibited and the further growth of ice grains stopped. The interfacial energy between ice and water is lowered allowing the AFPs to form smaller critical ice nuclei. Similar to a hysteresis in magnetic materials we observe a thermodynamic hysteresis leading to a nonlinear density dependence of the freezing point depression in agreement with the experiments.

Published
2014
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2. Formation of brine channels in sea-ice

Author

Morawetz, K., Thoms, S., and Kutschan, B.

Subjects
Nonlinear Sciences - Pattern Formation and Solitons, Condensed Matter - Soft Condensed Matter
Abstract

Liquid salty micro-channels (brine) between growing ice platelets in sea ice are an important habitat for $CO_2$ - binding microalgaea with great impact on polar ecosystems. The structure formation of ice platelets is microscopically described and a phase field model is developed. The pattern formation during solidification of the two-dimensional interstitial liquid is considered by two coupled order parameters, the tetrahedricity as structure of ice and the salinity. The coupling and time-evolution of these order parameters are described by a consistent set of three model parameters. They determine the velocity of the freezing process and the structure formation, the phase diagram, the super-cooling and super-heating region, and the specific heat. The model is used to calculate the short-time frozen micro-structures. The obtained morphological structure is compared with the vertical brine pore space obtained from Xray computed tomography.

Published
2014
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3. Stability of 3D Cubic Fixed Point in Two-Coupling-Constant \phi^4-Theory

Author

Kleinert, H., Thoms, S., and Schulte-Frohlinde, V.

Subjects
Quantum Physics, Condensed Matter, High Energy Physics - Theory
Abstract

For an anisotropic euclidean $\phi^4$-theory with two interactions $[u (\sum_{i=1^M {\phi}_i^2)^2+v \sum_{i=1}^M \phi_i^4]$ the $\beta$-functions are calculated from five-loop perturbation expansions in $d=4-\varepsilon$ dimensions, using the knowledge of the large-order behavior and Borel transformations. For $\varepsilon=1$, an infrared stable cubic fixed point for $M \geq 3$ is found, implying that the critical exponents in the magnetic phase transition of real crystals are of the cubic universality class. There were previous indications of the stability based either on lower-loop expansions or on less reliable Pad\'{e approximations, but only the evidence presented in this work seems to be sufficently convincing to draw this conclusion., Comment: Author Information under http://www.physik.fu-berlin.de/~kleinert/institution.html . Paper also at http://www.physik.fu-berlin.de/~kleinert/kleiner_re250/preprint.html

Published
1996
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4. Resummation of anisotropic quartic oscillator. Crossover from anisotropic to isotropic large-order behavior

Author

Kleinert, H. and Thoms, S.

Subjects
Quantum Physics
Abstract

We present an approximative calculation of the ground-state energy for the anisotropic anharmonic oscillator Using an instanton solution of the isotropic action $\delta = 0$, we obtain the imaginary part of the ground-state energy for small negative $g$ as a series expansion in the anisotropy parameter $\delta$. From this, the large-order behavior of the $g$-expansions accompanying each power of $\delta$ are obtained by means of a dispersion relation in $g$. These $g$-expansions are summed by a Borel transformation, yielding an approximation to the ground-state energy for the region near the isotropic limit. This approximation is found to be excellent in a rather wide region of $\delta$ around $\delta = 0$. Special attention is devoted to the immediate vicinity of the isotropic point. Using a simple model integral we show that the large-order behavior of an $\delta$-dependent series expansion in $g$ undergoes a crossover from an isotropic to an anisotropic regime as the order $k$ of the expansion coefficients passes the value $k_{{\rm cross} \sim 1/ |{\delta}|$., Comment: Paper and .eps files. Author Information at http://www.physik.fu-berlin.de/~kleinert/institution.html . Paper at http://www.physik.fu-berlin.de/~kleinert/kleiner_re243/preprint.html

Published
1996
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5. Large-Order Behavior of Two-coupling Constant $\phi^4$-Theory with Cubic Anisotropy

Author

Kleinert, H. and Thoms, S.

Subjects
High Energy Physics - Theory
Abstract

For the anisotropic $[u (\sum_{i=1^N {\phi}_i^2)^2+v \sum_{i=1^N \phi_i^4]$-theory with {$N=2,3$} we calculate the imaginary parts of the renormalization-group functions in the form of a series expansion in $v$, i.e., around the isotropic case. Dimensional regularization is used to evaluate the fluctuation determinants for the isotropic instanton near the space dimension 4. The vertex functions in the presence of instantons are renormalized with the help of a nonperturbative procedure introduced for the simple $g{\phi^4$-theory by McKane et al., Comment: LaTeX file with eps files in src. See also http://www.physik.fu-berlin.de/~kleinert/institution.html

Published
1995
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6. Erratum: “Comparative analysis of void-containing and all-semiconductor 1.5 µm InP-based photonic crystal surface-emitting laser diodes” [AIP Adv. 11, 065315 (2021)]

Author

Bian, Z., primary, Rae, K. J., additional, King, B. C., additional, Kim, D., additional, Li, G., additional, Thoms, S., additional, Childs, D. T. D., additional, Gerrard, N. D., additional, Babazadeh, N., additional, Reynolds, P., additional, Grant, J., additional, McKenzie, A. F., additional, Orchard, J. R., additional, Taylor, R. J. E., additional, and Hogg, R. A., additional

Published
2022
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10. Conduction in n+-GaAs Wires

Author

Main, P. C., Taylor, R. P., Eaves, L., Thoms, S., Beaumont, S. P., Wilkinson, C. D. W., Beaumont, Steven P., editor, and Torres, Clivia M. Sotomayor, editor

Published
1990
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11. Comparative analysis of void-containing and all-semiconductor 1.5 µm InP-based photonic crystal surface-emitting laser diodes

Author

Bian, Z., primary, Rae, K. J., additional, King, B. C., additional, Kim, D., additional, Li, G., additional, Thoms, S., additional, Childs, D. T. D., additional, Gerrard, N. D., additional, Babazadeh, N., additional, Reynolds, P., additional, Grant, J., additional, McKenzie, A. F., additional, Orchard, J. R., additional, Taylor, R. J. E., additional, and Hogg, R. A., additional

Published
2021
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12. Modeling silicate–nitrate–ammonium co-limitation of algal growth and the importance of bacterial remineralization based on an experimental Arctic coastal spring bloom culture study

Author

Vonnahme, T.R., Leroy, M., Thoms, S., van Oevelen, D., Harvey, H.R., Kristiansen, S., Gradinger, R., Dietrich, U., Völker, C., Vonnahme, T.R., Leroy, M., Thoms, S., van Oevelen, D., Harvey, H.R., Kristiansen, S., Gradinger, R., Dietrich, U., and Völker, C.

Abstract

Arctic coastal ecosystems are rapidly changing due to climate warming. This makes modeling their productivity crucially important to better understand future changes. System primary production in these systems is highest during the pronounced spring bloom, typically dominated by diatoms. Eventually the spring blooms terminate due to silicon or nitrogen limitation. Bacteria can play an important role for extending bloom duration and total CO2 fixation through ammonium regeneration. Current ecosystem models often simplify the effects of nutrient co-limitations on algal physiology and cellular ratios and simplify nutrient regeneration. These simplifications may lead to underestimations of primary production. Detailed biochemistry- and cell-based models can represent these dynamics but are difficult to tune in the environment. We performed a cultivation experiment that showed typical spring bloom dynamics, such as extended algal growth via bacterial ammonium remineralization, reduced algal growth and inhibited chlorophyll synthesis under silicate limitation, and gradually reduced nitrogen assimilation and chlorophyll synthesis under nitrogen limitation. We developed a simplified dynamic model to represent these processes. Overall, model complexity in terms of the number of parameters is comparable to the phytoplankton growth and nutrient biogeochemistry formulations in common ecosystem models used in the Arctic while improving the representation of nutrient-co-limitation-related processes. Such model enhancements that now incorporate increased nutrient inputs and higher mineralization rates in a warmer climate will improve future predictions in this vulnerable system.

Published
2021

17. Void engineering in epitaxially regrown GaAs-based photonic crystal surface emitting lasers by grating profile design

Author

McKenzie, A. F., primary, King, B. C., additional, Rae, K. J., additional, Thoms, S., additional, Gerrard, N. D., additional, Orchard, J. R., additional, Nishi, K., additional, Takemasa, K., additional, Sugawara, M., additional, Taylor, R. J. E., additional, Childs, D. T. D., additional, MacLaren, D. A., additional, and Hogg, R. A., additional

Published
2021
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28. Modeling Photoprotection at Global Scale: The Relative Role of Nonphotosynthetic Pigments, Physiological State, and Species Composition

Author

Álvarez, E., Thoms, S., Bracher, A., Liu, Y., Völker, C., Álvarez, E., Thoms, S., Bracher, A., Liu, Y., and Völker, C.

Abstract

Microalgae are capable of acclimating to dynamic light environments, as they have developed mechanisms to optimize light harvesting and photosynthetic electron transport. When absorption of light exceeds photosynthetic capacity, various physiological protective mechanisms prevent damage of the photosynthetic apparatus. Xanthophyll pigments provide one of the most important photoprotective mechanisms to dissipate the excess light energy and prevent photoinhibition. In this study, we coupled a mechanistic model for phytoplankton photoinhibition with the global biogeochemical model Regulated Ecosystem Model version 2. The assumption that photoinhibition is small in phytoplankton communities acclimated to ambient light allowed us to predict the photoprotective needs of phytoplankton. When comparing the predicted photoprotective needs to observations of pigment content determined by high‐performance liquid chromatography, our results showed that photoprotective response seems to be mediated in most parts of the ocean by a variable ratio of xanthophyll pigments to chlorophyll. The variability in the ratio appeared to be mainly driven by changes in phytoplankton community composition. Exceptions appeared at high latitudes where other energy dissipating mechanisms seem to play a role in photoprotection and both taxonomic changes and physiological acclimation determine community pigment signature. Understanding the variability of community pigment signature is crucial for modeling the coupling of light absorption to carbon fixation in the ocean. Insights about how much of this variability is attributable to changes in community composition may allow us to improve the match between remotely sensed optical data and the underlying phytoplankton community.

Published
2019

29. Raman scattering of coupled longitudinal optical phonon-plasmon modes in dryetched n+-GaAs._

Author

Wang, P.D., Foad, M.A., Sotomayor-Torres, C.M., Thoms, S., Watt, M., Cheung, R., Wilkinson, C.D.W., and Beaumont, S.P.

Subjects
Raman effect -- Usage, Semiconductors -- Etching, Gallium arsenide semiconductors -- Research, Physics
Abstract

The damage caused by dry etching on GaAs was studied using Raman scattering wherein the heavily doped n+-GaAs (2-3 to the 18th power super -3) analyzes thedry etch-induced damage through the coupled longitudinal optical phonon-plasmonmode as a probe. Conventional radio frequency etching, ion beam etching and electron cyclotron radio frequency reactive ion etching (ECR-RIE) were used andresults show that ECR-RIE produces the least damage.

Published
1992

36. List of Contributors

Author

Altshuler, B., primary, Arney, S.C., additional, Ashoori, R.C., additional, Averin, D.V., additional, Baldwin, K.W., additional, Barbier, E., additional, Bauer, S., additional, Beaumont, S.P., additional, Beenakker, C.W.J., additional, Behringer, R.E., additional, Benisty, H., additional, Berger, S.D., additional, Bhat, R., additional, Bigelow, N.P., additional, Böhm, G., additional, Bouchard, A.M., additional, Bryant, G.W., additional, Camarda, R.M., additional, Caro, J., additional, Chakraborty, T., additional, Chin, A., additional, Christen, J., additional, Colas, E., additional, Cullis, A.G., additional, Cunningham, J.E., additional, Datta, S., additional, deGraaf, C., additional, de Lozanne, P.G.N., additional, de Vegvar, A.L., additional, Demeester, P., additional, Demel, T., additional, Demmerle, W., additional, Eaves, L., additional, Ehrichs, E.E., additional, Eisenstein, J.P., additional, Farrow, R.C., additional, Ferry, D.K., additional, Foad, M.A., additional, Fowler, A.B., additional, Foxman, E.B., additional, Foxon, C.T., additional, Frost, J.E.F., additional, Fulton, T.A., additional, Gebhardt, W., additional, Gibson, J.M., additional, Goldman, V.J., additional, Goodnick, S.M., additional, Gornik, E., additional, Grambow, P., additional, Grubin, H.L., additional, Halonen, V., additional, Harmans, C.J.P.M., additional, Hasslacher, B., additional, Haviland, D.B., additional, Hayes, D.G., additional, Heitmann, D., additional, Henini, M., additional, Hess, K., additional, Heyers, K., additional, Higgs, A.W., additional, Hirayama, Y., additional, Hirler, F., additional, Hong, J.M., additional, Horikoshi, Y., additional, Huggins, H.A., additional, Hughes, O.H., additional, Hwang, D.M., additional, Iafrate, G.J., additional, Imry, Y., additional, Johnson, A.T., additional, Jones, G.A.C., additional, Kapon, E., additional, Kastner, M.A., additional, Kelly, M.J., additional, Kern, K., additional, Kim, K.W., additional, Kirk, W.P., additional, Kouwenhoven, L.P., additional, Kraus, J.S., additional, Kriman, A.M., additional, Kuhn, W., additional, Kumar, A., additional, Kuzmin, L.S., additional, Lake, R., additional, Law, V.J., additional, Leiderer, H., additional, Leitch, W.E., additional, Lent, C.S., additional, Lévy, L.P., additional, Liddle, J.A., additional, Likharev, K.K., additional, Littlejohn, M.A., additional, Lootens, D., additional, Luscombe, J.H., additional, Ma, J., additional, Mabesoone, M., additional, MacDonald, L., additional, Martin-Moreno, N.C., additional, McEuen, P.L., additional, Meirav, U., additional, Menschig, A., additional, Mickevicius, R., additional, Mitin, V.V., additional, Mooij, J.E., additional, Niu, Q., additional, Palm, E.C., additional, Paris, E., additional, Pease, M., additional, Pepper, R.F., additional, Pfeiffer, L.N., additional, Ploog, K., additional, Popovíc, D., additional, Prentiss, M.G., additional, Radelaar, S., additional, Randall, J.N., additional, Razeghi, M., additional, Reed, M.A., additional, Register, L.F., additional, Ritchie, D.A., additional, Roukes, M.L., additional, Saku, T., additional, Schiavone, L.M., additional, Seabaugh, A.C., additional, Silsbee, R.H., additional, Simmonds, P.E., additional, Sivan, U., additional, Skolnick, M.S., additional, Smart, A.P., additional, Smith, G.W., additional, Smith, W.F., additional, Smoliner, J., additional, Sols, F., additional, Sotomayor-Torrés, C.M., additional, Spector, J., additional, Stanley, C.R., additional, Staring, A.A.M., additional, Stormer, H.L., additional, Stroscio, M.A., additional, Su, S., additional, Tarucha, S., additional, Tewordt, M., additional, Thoms, S., additional, Timmering, C.E., additional, Timp, G., additional, Tokura, Y., additional, Umbach, C.P., additional, Van Daele, P., additional, van der Vaart, N.C., additional, van Houten, H., additional, Wagner, P.H., additional, Wallace, H., additional, Wang, P.D., additional, Washburn, S., additional, Watt, M., additional, Weimann, G., additional, Weisbuch, C., additional, Weiss, D., additional, Weisshaar, A., additional, West, K.W., additional, White, C.R.H., additional, Whitehouse, C.R., additional, Wilkinson, C.D.W., additional, Williams, G.M., additional, Williamson, J.G., additional, Wind, S.J., additional, Wu, J.C., additional, Wybourne, M.N., additional, Yacoby, A,, additional, Yao, J.J., additional, Yindeepol, W., additional, and Zhang, Y.H., additional

Published
1992
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37. Impact of seawater [Ca2+] on the calcification and calciteMg / Ca of Amphistegina lessonii

Author

Mewes, A., Langer, G., Thoms, S., Nehrke, G., Reichart, G.-J., de Nooijer, L. J., and Bijma, J.

Subjects
lcsh:Geology, lcsh:QH501-531, lcsh:QH540-549.5, lcsh:QE1-996.5, lcsh:Life, lcsh:Ecology
Abstract

Mg / Ca ratios in foraminiferal tests are routinely used as paleotemperature proxies, but on long timescales, they also hold the potential to reconstruct past seawater Mg / Ca. The impact of both temperature and seawater Mg / Ca on Mg incorporation in Foraminifera has been quantified by a number of studies. The underlying mechanism responsible for Mg incorporation in foraminiferal calcite and its sensitivity to environmental conditions, however, has not been fully identified. A recently published biomineralization model (Nehrke et al., 2013) proposes a combination of transmembrane transport and seawater leakage or vacuolization to link calcite Mg / Ca to seawater Mg / Ca and explains inter-species variability in Mg / Ca ratios. To test the assumptions of this model, we conducted a culture study in which seawater Mg / Ca was manipulated by varying [Ca2+] and keeping [Mg2+] constant. Foraminiferal growth rates, test thickness and calcite Mg / Ca of newly formed chambers were analyzed. Results showed optimum growth rates and test thickness at Mg / Ca closest to that of ambient seawater. Calcite Mg / Ca is positively correlated to seawater Mg / Ca, indicating that it is not absolute seawater [Ca2+] and [Mg2+] but their ratio that controls Mg / Ca in tests. These results demonstrate that the calcification process cannot be based only on seawater vacuolization, supporting the mixing model proposed by Nehrke et al. (2013). Here, however, we suggest transmembrane transport fractionation that is not as strong as suggested by Nehrke et al. (2013).

Published
2015

38. Raman Scattering and Photoluminescence of GaAs-Based Nanostructures

Author

Torres, C. M. Sotomayor, primary, Watt, M., additional, Arnot, H. E. G., additional, Glew, R., additional, Leitch, W. E., additional, Kean, A. H., additional, Cornet, R. Cusco, additional, Kerr, T. M., additional, Thoms, S., additional, Beaumont, S. P., additional, Johnson, N. P., additional, and Stanley, C. R., additional

Published
1990
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39. Conduction in n+-GaAs Wires

Author

Main, P. C., primary, Taylor, R. P., additional, Eaves, L., additional, Thoms, S., additional, Beaumont, S. P., additional, and Wilkinson, C. D. W., additional

Published
1990
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40. Terahertz oscillations in an In0.53Ga0.47As submicron planar Gunn diode.

Author

Khalid, Ata, Dunn, G. M., Macpherson, R. F., Thoms, S., Macintyre, D., Li, C., Steer, M. J., Papageorgiou, V., Thayne, I. G., Kuball, M., Oxley, C. H., Montes Bajo, M., Stephen, A., Glover, J., and Cumming, D. R. S.

Subjects
GUNN diodes, TERAHERTZ spectroscopy, ELECTRON-phonon interactions, MONTE Carlo method, FERROMAGNETIC material fluctuations
Abstract

The length of the transit region of a Gunn diode determines the natural frequency at which it operates in fundamental mode-the shorter the device, the higher the frequency of operation. The long-held view on Gunn diode design is that for a functioning device the minimum length of the transit region is about 1.5 μm, limiting the devices to fundamental mode operation at frequencies of roughly 60 GHz. Study of these devices by more advanced Monte Carlo techniques that simulate the ballistic transport and electron-phonon interactions that govern device behaviour, offers a new lower bound of 0.5 μm, which is already being approached by the experimental evidence that has shown planar and vertical devices exhibiting Gunn operation at 600 nm and 700 nm, respectively. The paper presents results of the first ever THz submicron planar Gunn diode fabricated in In0.53Ga0.47As on an InP substrate, operating at a fundamental frequency above 300 GHz. Experimentally measured rf power of 28 μW was obtained from a 600 nm long × 120 μm wide device. At this new length, operation in fundamental mode at much higher frequencies becomes possible-the Monte Carlo model used predicts power output at frequencies over 300 GHz. [ABSTRACT FROM AUTHOR]

Published
2014
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43. Exploring foraminiferal Sr/Ca as a new carbonate system proxy

Author

Keul, N., Langer, G., Thoms, S., de Nooijer, L.J., Reichart, G.-J., Bijma, J., Keul, N., Langer, G., Thoms, S., de Nooijer, L.J., Reichart, G.-J., and Bijma, J.

Abstract

In present day paleoclimate research one of the most pressing challenges is the reconstruction of atmospheric CO2 concentrations. A variety of proxies for several components of the marine inorganic carbon system have been developed in this context (e.g. B isotopes, B/Ca, U/Ca) to allow reconstruction of past seawater pH, HCO3− and CO32− and thereby facilitate estimates of past atmospheric pCO2. Based on culture experiments using the benthic foraminifera Ammonia sp. we describe a positive correlation between Sr/Ca and the carbonate system, namely DIC/bicarbonate ion concentration. Foraminiferal Sr/Ca ratios provide potentially additional constraints on the carbonate system proxy, because the analysis of foraminiferal carbonate Sr/Ca is straightforward and not easily contaminated. Applying our calibration to a published dataset of paleo-Sr/Ca suggests the validity of Sr/Ca as a carbonate system proxy. Furthermore, we explore how our data can be used to advance conceptual understanding of the foraminiferal biomineralization mechanism.

Published
2017

46. InAs FinFETs with Hfin = 20 nm fabricated using a top-down etch process

Author

Oxland, R., Li, X., Chang, S.W., Wang, T., Vasen, P., Ramvall, R., Contreras-Guerrero, R., Rojas-Ramirez, J., Holland, M., Doornobos, G., Chang, Y.S., Macintyre, D.S., Thoms, S., Droopad, R., Yeo, Y.-C., Diaz, C.H., Thayne, I.G., and Passlack, M.

Abstract

We report the first demonstration of InAs FinFETs with fin width Wfin in the range 25–35 nm, formed by inductively coupled plasma etching. The channel comprises defect-free, lattice-matched InAs with fin height Hfin = 20 nm controlled by the use of an etch stop layer incorporated into the device heterostructure. For a gate length Lg = 1 nm, peak transconductance gm,peak = 1430 µS/µm is measured at Vd = 0.5 V demonstrating that electron transport in InAs fins can match planar devices.

Published
2016

47. Sr partitioning in the benthic foraminifera Ammonia aomoriensis and Amphistegina lessonii

Author

Langer, G., Sadekov, A., Thoms, S., Keul, N., Nehrke, G., Mewes, A., Greaves, M., Misra, S., Reichart, G.-J., de Nooijer, L.J., Bijma, J., Elderfield, H., Stratigraphy and paleontology, Stratigraphy & paleontology, Greaves, Mervyn [0000-0001-8014-8627], Apollo - University of Cambridge Repository, Stratigraphy and paleontology, and Stratigraphy & paleontology

Subjects
Biomineralization, food.ingredient, 010504 meteorology & atmospheric sciences, sub-01, Analytical chemistry, Mineralogy, Foraminifera, Minor element partitioning, 010502 geochemistry & geophysics, Mass spectrometry, 01 natural sciences, chemistry.chemical_compound, food, Foraminifera [hole bearers], Geochemistry and Petrology, 0105 earth and related environmental sciences, Calcite, biology, Geology, biology.organism_classification, Amphistegina, Partition coefficient, chemistry, Benthic zone, Seawater, Inductively coupled plasma
Abstract

The shallow water benthic foraminifera Ammonia aomoriensis and Amphistegina lessonii were grown at different seawater Sr/Ca and the test Sr/Ca ratio was determined by Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry. A. aomoriensis test Sr/Ca is positively correlated with seawater Sr/Ca. The linear regression has a slope of 0.17, representing the overall Sr partitioning coefficient KDSr. The slope remains unchanged, if seawater Ca concentration is changed in order to change the seawater Sr/Ca. In the case of A. lessonii, the slopes of the linear regressions representing KDSr differ from one another i.e., 0.16 if the Sr concentration is changed and 0.32 if the Ca level is changed. This difference in KDSr can be explained by the, relative to A. aomoriensis, high Mg content of A. lessonii (ca. 40 mmol/mol), distorting the calcite lattice and weakening the discrimination against Sr. The Mg content of A. aomoriensis is too low (ca. 4 mmol/mol) to observe the influence on Sr partitioning. All data are compatible with a recently developed model for minor element partitioning in foraminifera (Nehrke et al., 2013; Mewes et al., 2015a). On the whole, our data confirm the model (applicability of the model to Sr and dependence of the calcite Sr/Ca on seawater Sr/Ca, as opposed to seawater Sr concentration), and improve our understanding of the model (influence of calcite Mg/Ca on Sr partitioning).

Published
2016

48. Impact of seawater [Ca]2+ on the calcification and calcite Mg / Ca of Amphistegina lessonii

Author

Mewes, A., Langer, G., Thoms, S., Nehrke, G., Reichart, G. J., de Nooijer, L. J., Bijma, J., Organic geochemistry, and Organic geochemistry & molecular biogeology

Subjects
Ecology, Evolution, Behavior and Systematics, Earth-Surface Processes
Abstract

Mg / Ca ratios in foraminiferal tests are routinely used as paleotemperature proxies, but on long timescales, they also hold the potential to reconstruct past seawater Mg / Ca. The impact of both temperature and seawater Mg / Ca on Mg incorporation in Foraminifera has been quantified by a number of studies. The underlying mechanism responsible for Mg incorporation in foraminiferal calcite and its sensitivity to environmental conditions, however, has not been fully identified. A recently published biomineralization model (Nehrke et al., 2013) proposes a combination of transmembrane transport and seawater leakage or vacuolization to link calcite Mg / Ca to seawater Mg / Ca and explains inter-species variability in Mg / Ca ratios. To test the assumptions of this model, we conducted a culture study in which seawater Mg / Ca was manipulated by varying [Ca2+] and keeping [Mg2+] constant. Foraminiferal growth rates, test thickness and calcite Mg / Ca of newly formed chambers were analyzed. Results showed optimum growth rates and test thickness at Mg / Ca closest to that of ambient seawater. Calcite Mg / Ca is positively correlated to seawater Mg / Ca, indicating that it is not absolute seawater [Ca2+] and [Mg2+] but their ratio that controls Mg / Ca in tests. These results demonstrate that the calcification process cannot be based only on seawater vacuolization, supporting the mixing model proposed by Nehrke et al. (2013). Here, however, we suggest transmembrane transport fractionation that is not as strong as suggested by Nehrke et al. (2013).

Published
2015

49. Raman scattering of coupled longitudinal optical phonon-plasmon modes in dry etched n+-GaAs.

Author

Wang, P. D., Foad, M. A., Sotomayor-Torres, C. M., Thoms, S., Watt, M., Cheung, R., Wilkinson, C. D. W., and Beaumont, S. P.

Subjects
RAMAN effect, PHONONS, PLASMONS (Physics), GALLIUM arsenide
Abstract

Reports on Raman investigations of the coupled longitudinal optical phonon-plasmon scattering on heavily doped n[sup+]-GaAs. Background to the study; Theoretical considerations; Details of the dry etching and experimental technique; Results and discussion.

Published
1992
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50. Sr partitioning in the benthic foraminifera Ammonia aomoriensis and Amphistegina lessonii

Author

Stratigraphy and paleontology, Stratigraphy & paleontology, Langer, G., Sadekov, A., Thoms, S., Keul, N., Nehrke, G., Mewes, A., Greaves, M., Misra, S., Reichart, G.-J., de Nooijer, L.J., Bijma, J., Elderfield, H., Stratigraphy and paleontology, Stratigraphy & paleontology, Langer, G., Sadekov, A., Thoms, S., Keul, N., Nehrke, G., Mewes, A., Greaves, M., Misra, S., Reichart, G.-J., de Nooijer, L.J., Bijma, J., and Elderfield, H.

Published
2016

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