Eocene to Miocene Magnetostratigraphy, Biostratigraphy, and Chemostratigraphy at ODP Site 1090 (Sub-Antarctic South Atlantic)

At Ocean Drilling Program (ODP) Site 1090 (lat 42854.89S, long 8854.09E) locatedin a water depth of 3702 m on the Agulhas Ridge in the sub-Antarctic South Atlantic, ~300 m of middle Eocene to middle Miocenesediments were recovered with the advancedpiston corer (APC) and the extendedcore barrel (XCB). U-channel samplesfrom the 70–230 meters composite depth(mcd) interval provide a magnetic polaritystratigraphy that is extended to 380 mcd byshipboard whole-core and discrete sampledata. The magnetostratigraphy can be interpretedby the fit of the polarity-zone patternto the geomagnetic polarity time scale(GPTS) augmented by isotope data andbioevents with documented correlation tothe GPTS. Three normal-polarity subchrons(C5Dr.1n, C7Ar.1n, and C13r.1n),not included in the standard GPTS, are recordedat Site 1090. The base of the sampledsection is correlated to C19n (middleEocene), although the interpretation is unclearbeyond C17r. The top of the sampledsection is correlated to C5Cn (late earlyMiocene), although, in the uppermost 10 m of the sampled section, a foraminifer (Globorotaliasphericomiozea) usually associatedwith the Messinian and early Pliocene hasbeen identified. 87Sr/86Sr, d13C, and d18Ovalues measured on foraminifera, includingthe d18O and d13C shifts close to the Eocene/Oligocene boundary, support the correlationto the GPTS. For the interval spanningthe Oligocene/Miocene boundary, benthicd13C, d18O, and 87Sr/86Sr records from Site1090 can be correlated to isotope recordsfrom ODP Site 929 (Ceara Rise), providing support for the recently-published Oligocene/Miocene boundary age (22.92 Ma) of Shackleton et al

Magnetic record of deglaciation using FORC-PCA, sortable-silt grain size, and magnetic excursion at 26 ka, from the Rockall Trough (NE Atlantic)

Core MD04-2822 from the Rockall Trough has apparent sedimentation rates of ∼ 1 m/kyr during the last deglaciation (Termination I). Component magnetization directions indicate a magnetic excursion at 16.3 m depth in the core, corresponding to an age of 26.5 ka, implying an excursion duration of ∼350 years. Across Termination I, the mean grain size of sortable silt implies reduced bottom-current velocity in the Younger Dryas and Heinrich Stadial (HS)−1A, and increased velocities during the Bølling-Allerød warm period. Standard bulk magnetic parameters imply fining of magnetic grain size from the mid-Younger Dryas (∼12 ka) until ∼ 8 ka. First-order reversal curves (FORCs) were analyzed using ridge extraction to differentiate single domain (SD) from background (detrital) components. Principal component analysis (FORC-PCA) was then used to discriminate three end members corresponding to SD, pseudo-single domain (PSD), and multidomain (MD) magnetite. The fining of bulk magnetic grain size from 12 to 8 ka is due to reduction in concentration of detrital (PSD + MD) magnetite, superimposed on a relatively uniform concentration of SD magnetite produced by magnetotactic bacteria. The decrease in PSD+MD magnetite concentration from 12 to 8 ka is synchronized with increase in benthic δ13C, and with major (∼70 m) regional sea-level rise, and may therefore be related to detrital sources on the shelf that had reduced influence as sea level rose, and to bottom-water reorganization as Northern Source Water (NSW) replaced Southern Source Water (SSW)

Enhanced ionization in small rare gas clusters

A detailed theoretical investigation of rare gas atom clusters under intense short laser pulses reveals that the mechanism of energy absorption is akin to {\it enhanced ionization} first discovered for diatomic molecules. The phenomenon is robust under changes of the atomic element (neon, argon, krypton, xenon), the number of atoms in the cluster (16 to 30 atoms have been studied) and the fluency of the laser pulse. In contrast to molecules it does not dissappear for circular polarization. We develop an analytical model relating the pulse length for maximum ionization to characteristic parameters of the cluster

Magnetic unmixing of first-order reversal curve diagrams using principal component analysis

We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first-order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single-domain (SD), pseudo-single-domain (PSD), and multi-domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end-member (EM) FORC diagrams, located at user-defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt-sized MD component, a fine silt-sized PSD component, and a mixed clay-sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA-based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four-EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi-component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved

Calcareous nannofossil data and magnetostratigraphy from the Atlantic and Tethys Oceans - An integrated approach to approximate the Jurassic/Cretaceous (J/K) boundary in low-latitudinal pelagic and hemipelagic sequences.

The Tithonian \u2013 Early Berriasian interval is characterized by a major calcareous nannofossil speciation event: several Cretaceous genera and species first appear and rapidly evolve (Bralower et al., 1989). Progressive increases in diversity, abundance and degree of calcification (Nannofossil Calcification Event \u2013 NCE; Bornemann et al., 2003) have also been documented. Integrated magneto- and calcareous nannofossil biostratigraphy across the Jurassic/Cretaceous (J/K) boundary have been independently investigated in Tethyan land sections (Torre de Busi and Foza, Southern Alps) and at Atlantic Ocean DSDP sites (534A, Blake Bahama Basin and 105, Hatteras Basin). Calcareous nannofossil biostratigraphy, absolute and relative abundances have been obtained using three different techniques: random settling slides (Geisen et al., 1999), simple smear slides and ultra-thin sections (7-8 \ub5m thick). Similar variations in nannofloral abundance and composition, including the NCE, have been documented in both Atlantic and Tethys oceans (low latitude associations). All known calcareous nannofossil Zones and corresponding Subzones, following the biostratigraphic scheme of Bralower et al. (1989), have been recognized: NJ-19b; NJ-20a, NJ-20b; NJK-A, NJK-B, NJK-C across J/K boundary, NJK-D; NK-1 In the Middle Tithonian the nannoliths taxa C. mexicana minor, C. mexicana mexicana, and P. beckmannii increase significantly in abundance (Bornemann et al., 2003; Tremolada et al., 2006): the maximum relative abundance is reached between the calcareous nannofossil Zone NJ-20B and early NJK-A (Atlantic Ocean) or NJK-B (Tethys Ocean), followed by a decrease through NJK-A and NJK-B. Nannoconids appear and rapidly evolve across the J/K boundary reaching high relative abundances in the lowermost Berriasian (from calcareous nannofossil Subzone NJK-C to NK-1). Quantitative and morphometric studies have identified new potential events. Relative abundances of the placolith genera Watznaueria and the nannolith genera Conusphaera show opposite trends, while morphometric analysis show a size increase of placoliths, nannoliths and nannoconids during NCEs both in the Atlantic and Tethyan sections: calibration with magnetostratigraphy indicate that these trends are useful as additional bio-horizons for locating the J/K boundary. Calcareous nannofossil zonations and abundance variations of tethyan Torre de Busi section have also been correlated with calpionellid biostratigraphy, which has been investigated on the same samples used for the calcareous nannofossil study. It has been possible to identify the Chitinoidella, Crassicollaria and Calpionella Zones across the J/K boundary (Remane, 1986;Pop, 1994b and Reh\ue1kov\ue1 and Michal\uedk, 1997). Six polarity chrons (from CM22 to CM17) have been identified in DSDP site 534A, and in the tethyan land sections. The speciation of highly-calcified and dissolution resistant calcareous nannofossil forms, and related remarkable abundance and size increases, and the relative trends between genera Watznaueria and Conusphaera could provide new reliable stratigraphic tools for the approximation of the J/K boundary in low latitudinal pelagic and hemipelagic sequences in the Atlantic and Tethyan Oceans. In conclusion integrated stratigraphy, derived from the correlation among several calcareous nannofossils events, capionellid zonation and magnetostratigraphic events, can be used to characterize the J/K boundary interval, and is believed essential for defining the Jurassic/Cretaceous boundary particularly in the absence of orthostratigraphic markers (e.g. ammonites). References: Bornemann, A., Aschwer, U. and Mutterlose, J., 2003. The impact of calcareous nannofossils on the pelagic carbonate accumulation across the Jurassic-Cretaceous boundary. Palaeogeography Palaeoclimatology Palaeoecology, 199(3-4): 187-228. Bown, P.R. and Cooper, M.K.E., 1998. Jurassic. In: P.R. Bown (Editor), Calcareous nannofossil stratigraphy. British Micropalaeontological Society Publications Series. Kluver Academic Publishers, Dordrecht, Boston, London, pp. 34-85. Bown, P.R., Lees, J.A. and Young, J.R., 2004. Calcareous nannoplankton evolution and diversity through time. In: H. Thierstein and J.R. Young (Editors), ), Coccolithophores - From Molecular Processes to Global Impact. Springer, Berlin, pp. 481-508. Bralower, T.J., Monechi, S. and Thierstein, H.R., 1989. Calcareous nannofossil Zonation of the Jurassic-Cretaceous Boundary Interval and Correlation with the Geomagnetic Polarity Timescale. Marine Micropaleontology, 14: 153-235. Geisen, M., Bollmann, J., Herrle, J.O., Mutterlose, J. and Young, J.R., 1999. Calibration of the random settling technique for calculation of absolute abundances of calcareous nannoplankton. Micropaleontology, 45(4): 437-442. Erba, E. and Quadrio, B., 1989. Biostratigrafia a Nannofossili Calcarei, Calpionellidi e Foramminiferi planctonici della Maiolica (Titoniano superiore - Aptiano) nelle Prealpi Bresciane (Italia settentrionale). Riv. It. Paleont. Strat. 93(1): 3-108 Danelian, T. and Johnson, K.G., 2001. Patterns of biotic changes in Middle Jurassic to Early Cretaceous Tethyan radiolaria. Marine Micropaleontology 43: 239-260 Pop, G., 1994b. Calpionellid evolutive events and their use in biostratigraphy. Rom. J. Stratigraphy, 76: 7-24. Reh\ue1kov\ue1, D. and Michal\uedk, J., 1997: Evolution and distribution of calpionellids- the most characteristic constituents of Lower Cretaceous Tethyan microplankton. Cretaceous Research, 18: 493-504 Remane, J., 1986: Calpionellids and the Jurassic-Cretaceous boundary. Acta Geologica Hungarica, 29: 15-26 Rais, P., 2007. Ph.D. Thesis Roth, P.H., 1983. Jurassic and Lower Cretaceous calcareous nannofossils in the western North Atlantic (site 534): biostratigraphy, preservation, and some observation on biogeography and paleoceanography. Init. Rep. DSDP 76: 587-621 Tremolada, F., Bornemann, A., Bralower, T.J., Koeberl, C. and van de Schootbrugge, B., 2006. Paleoceanographic changes across the Jurassic/Cretaceous boundary: The calcareous phytoplankton response. Earth and Planetary Science Letters, 241(3-4): 361-371. Weissert, H. and Channell, J.E.T., 1989. Tethyan carbonate carbon isotope stratigraphy across the Jurassic/Cretaceous boundary: an indicator of decelerated global carbon cycling?. Paleoceanography 4(4): 483-49

A consideration of the challenges involved in supervising international masters students

This paper explores the challenges facing supervisors of international postgraduate students at the dissertation stage of the masters programme. The central problems of time pressure, language difficulties, a lack of critical analysis and a prevalence of personal problems among international students are discussed. This paper makes recommendations for the improvement of language and critical thinking skills, and questions the future policy of language requirements at HE for international Masters students

Magnetic unmixing of first-order reversal curve diagrams using principal component analysis

We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first-order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single-domain (SD), pseudo-single-domain (PSD), and multidomain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end-member (EM) FORC diagrams, located at user-defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt-sized MD component, a fine silt-sized PSD component, and a mixed clay-sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA-based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four-EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi-component magnetic mixtures, a fundamental task of rock magnetic studies.The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC grant agreement 320750.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/2015GC00590

Mechanical Systems with Symmetry, Variational Principles, and Integration Algorithms

This paper studies variational principles for mechanical systems with symmetry and their applications to integration algorithms. We recall some general features of how to reduce variational principles in the presence of a symmetry group along with general features of integration algorithms for mechanical systems. Then we describe some integration algorithms based directly on variational principles using a discretization technique of Veselov. The general idea for these variational integrators is to directly discretize Hamilton’s principle rather than the equations of motion in a way that preserves the original systems invariants, notably the symplectic form and, via a discrete version of Noether’s theorem, the momentum map. The resulting mechanical integrators are second-order accurate, implicit, symplectic-momentum algorithms. We apply these integrators to the rigid body and the double spherical pendulum to show that the techniques are competitive with existing integrators