Julie A. Bowles

Astronomical pacing of late Palaeocene to early Eocene global warming events

At the boundary between the Palaeocene and Eocene epochs, about 55 million years ago, the Earth experienced a strong global warming event, the Palaeocene–Eocene thermal maximum. The leading hypothesis to explain the extreme greenhouse conditions prevalent during this period is the dissociation of 1,400 to 2,800 gigatonnes of methane from ocean clathrates, resulting in a large negative carbon isotope excursion and severe carbonate dissolution in marine sediments. Possible triggering mechanisms for this event include crossing a threshold temperature as the Earth warmed gradually, comet impact, explosive volcanism or ocean current reorganization and erosion at continental slopes, whereas orbital forcing has been excluded. Here we report a distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge, which we term the Elmo horizon. Using orbital tuning, we estimate deposition of the Elmo horizon at about 2 million years after the Palaeocene–Eocene thermal maximum. The Elmo horizon has similar geochemical and biotic characteristics as the Palaeocene–Eocene thermal maximum, but of smaller magnitude. It is coincident with carbon isotope depletion events in other ocean basins, suggesting that it represents a second global thermal maximum. We show that both events correspond to maxima in the ∼405-kyr and ∼100-kyr eccentricity cycles that post-date prolonged minima in the 2.25-Myr eccentricity cycle, implying that they are indeed astronomically paced.

Paleointensity applications to timing and extent of eruptive activity, 9°-10°N East Pacific Rise

Placing accurate age constraints on near-axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges. Geomagnetic paleointensity of submarine basaltic glass (SBG) holds promise for placing quantitative age constraints on near-axis flows. In one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful SBG paleointensity estimates from 189 near-axis (<4 km) sites at the East Pacific Rise, 9°–10°N. Paleointensities range from 6 to 53 μT and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field. Samples within and adjacent to the axial summit trough (AST) have values approximately equal to or slightly higher than the present-day. Samples out to 1–3 km from the AST have values higher than the present-day, and samples farther off axis have values lower than the present-day. The on-axis samples (<500 m from the AST) provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique. Results from samples collected near a well-documented eruption in 1991–1992 suggest there may be a small negative bias in the paleointensity estimates, limiting resolution of the dating technique. Possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain; anomalously high magnetic unblocking temperatures, leading to a small cooling rate bias; and/or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories. Paleointensity remains useful in approximating age differences in young flows, and a clear along-axis paleointensity contrast near 9°50′N is suggestive of a ∼150–200 year age difference. Paleointensity values of off-axis samples are generally consistent with rough age interpretations based on side scan data. Furthermore, spatial patterns in the paleointensity suggest extensive off-axis flow emplacement may occur infrequently, with recurrence intervals of 10–20 kyr. Results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows, with flow size linked to time between eruptions.

Inferred time- and temperature-dependent cation ordering in natural titanomagnetites

Despite years of efforts to quantify cation distribution as a function of composition in the magnetite-ulvöspinel solid solution, important uncertainties remain about the dependence of cation ordering on temperature and cooling rate. Here we demonstrate that Curie temperature in a set of natural titanomagnetites (with some Mg and Al substitution) is strongly influenced by prior thermal history at temperatures just above or below Curie temperature. Annealing for 10(-1) to 10(3) h at 350-400 °C produces large and reversible changes in Curie temperature (up to 150 °C). By ruling out oxidation/reduction and compositional unmixing, we infer that the variation in Curie temperature arises from cation reordering, and Mössbauer spectroscopy supports this interpretation. Curie temperature is therefore an inaccurate proxy for composition in many natural titanomagnetites, but the cation reordering process may provide a means of constraining thermal histories of titanomagnetite-bearing rocks. Further, our theoretical understanding of thermoremanence requires fundamental revision when Curie temperature is itself a function of thermal history.

Deconvolution of u channel magnetometer data: Experimental study of accuracy, resolution, and stability of different inversion methods

We explore the effects of sampling density, signal/noise ratios, and position-dependent measurement errors on deconvolution calculations for u channel magnetometer data, using a combination of experimental and numerical approaches. Experiments involve a synthetic sample set made by setting hydraulic cement in a 30-cm u channel and slicing the hardened material into ∼2-cm lengths, and a natural lake sediment u channel sample. The cement segments can be magnetized and measured individually, and reassembled for continuous u channel measurement and deconvolution; the lake sediment channel was first measured continuously and then sliced into discrete samples for individual measurement. Each continuous data set was deconvolved using the ABIC minimization code of Oda and Shibuya (1996) and two new approaches that we have developed, using singular-value decomposition and regularized least squares. These involve somewhat different methods to stabilize the inverse calculations and different criteria for identifying the optimum solution, but we find in all of our experiments that the three methods converge to essentially identical solutions. Repeat scans in several experiments show that measurement errors are not distributed with position-independent variance; errors in setting/determining the u channel position (standard deviation ∼0.2 mm) translate in regions of strong gradients into measurement uncertainties much larger than those due to instrument noise and drift. When we incorporate these depth-dependent measurement uncertainties into the deconvolution calculations, the resulting models show decreased stability and accuracy compared to inversions assuming depth-independent measurement errors. The cement experiments involved varying directions and uniform intensities downcore, and very good accuracy was obtained using all of the methods when the signal/noise ratio was greater than a few hundred and the sampling interval no larger than half the length scale of magnetization changes. Addition of synthetic noise or reduction of sampling density decreased the resolution and accuracy of all the methods equally. The sediment-core experiment involved uniform (axial) magnetization direction and strongly varying intensities downcore. Intensity variations are well resolved and directions are accurate to within about 5 degrees, with errors attributable to omission and/or inaccurate calibration of cross terms in the instrument response function.

Eruptive timing and 200 year episodicity at 92°W on the hot spot‐influenced Galapagos Spreading Center derived from geomagnetic paleointensity

Eruptive timing in mid-ocean ridge systems is relatively poorly constrained, despite being an important variable in our understanding of many mid-ocean ridge processes, including volcanic construction; magma recharge, flux, and storage; and the stability of hydrothermal systems and biological communities. Only a handful of absolute eruption chronologies exist, yet they are essential in understanding how eruptive timing varies with important controlling variables. To construct an eruptive history at one location on the Galapagos Spreading Center, we present age determinations derived from geomagnetic paleointensity. To aid interpretation of the paleointensity data, we also present results from on-bottom magnetic anomaly measurements and forward modeling of topographic-induced magnetic anomalies. Anomalies may lead to a 1–2 µT bias in flow-mean paleointensities, which does not significantly affect the overall interpretation. Paleointensity results for the three youngest sampled units are indistinguishable, consistent with the flows being emplaced in relatively rapid succession. Comparisons with models of geomagnetic field behavior suggest these flows were erupted sometime in the past 100–200 years. The fourth sampled unit has a significantly higher paleointensity, consistent with an age of roughly 400 years. The possible bias in paleointensity data allows for ages as young as ∼50 years for the youngest three flows and 200–400 years for the oldest flow. This age distribution demonstrates an episodicity in the emplacement of the largest flows at this location, with a 200–300 year period of relative quiescence between emplacement of the oldest unit and the three youngest units.

Timing of magnetite formation in basaltic glass: Insights from synthetic analogs and relevance for geomagnetic paleointensity analyses

Absolute paleointensity estimates from submarine basaltic glass (SBG) typically are of high technical quality and accurately reflect the ambient field when known. SBG contains fine-grained, low-Ti magnetite, in contrast to the high-Ti magnetite in crystalline basalt, which has lead to uncertainty over the origin of the magnetite and its remanence in SBG. Because a thermal remanence is required for accurate paleointensity estimates, the timing and temperature of magnetite formation is crucial. To assess these factors, we generated a suite of synthetic glasses with variable oxygen fugacity, cooling rate, and FeO* content. Magnetic properties varied most strongly with crystallinity; less crystalline specimens are similar to natural SBG and have weaker magnetization, a greater superparamagnetic contribution, and higher unblocking temperatures than more crystalline specimens. Thellier-type paleointensity results recovered the correct field within 1σ error with 2 (out of 10) exceptions that likely result from an undetected change in the laboratory field. Unblocking and ordering temperature data demonstrate that low-Ti magnetite is a primary phase, formed when the glass initially quenched. Although prolonged heating at high temperatures (during paleointensity experiments) may result in minor alteration at temperatures < 580°C, this does not appear to impact the accuracy of the paleointensity estimate. Young SBG is therefore a suitable material for paleointensity studies.

Full vector low-temperature magnetic measurements of geologic materials

GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS, VOL. ???, XXXX, DOI:10.1029/, Full vector low-temperature magnetic measurements of geologic materials Joshua M. Feinberg 1 , Peter A. Solheid 1 , Nicholas L. Swanson-Hysell 1,2 , Mike J. Jackson 1 , and Julie A. Bowles 1,3 The magnetic properties of geologic materials offer insights into an enormous range of important geophysical phenomena ranging from inner core dynamics to pale- oclimate. Often it is the low-temperature behavior (<300 K) of magnetic minerals that provides the most useful and highest sensitivity information for a given problem. Con- ventional measurements of low-temperature remanence are typically conducted on instru- ments that are limited to measuring one single axis component of the magnetization vec- tor and are optimized for measurements in strong fields. These instrumental limitations have prevented fully optimized applications and have motivated the development of a low-temperature probe that can be used for low-temperature remanence measurements between 17 and 300K along three orthogonal axes using a standard 2G Enterprises SQuID rock magnetometer. In this contribution, we describe the design and implementation of this instrument and present data from five case studies that demonstrate the probe’s con- siderable potential for future research: a polycrystalline hematite sample, a polycrystalline hematite and magnetite mixture, a single crystal of magnetite, a single crystal of pyrrhotite and samples of Umkondo Large Igneous Province diabase sills. Abstract. Measurement System (MPMS), most often with the inten- tion of revealing information about the dominant magnetic mineral phases and grain size distribution. While these in- struments are adept at a range of low-temperature exper- iments, understanding the full behavior of a rock’s natu- ral remanence at low-temperature is hampered by the mea- surement capabilities being limited to a single axis and the instrument not providing an ultra-low field environment. If low-temperature measurements of a natural remanence (NRM) are desired using such instrumentation, great care must be taken to align the NRM with the measurement axis, and any directional change during thermal cycling will not be captured. In such an instrument, deviation from the single-axis will result in a measured magnetization that is less than the specimen’s actual total magnetization. In this contribution, we describe a low-temperature probe developed for use with superconducting rock magnetometers (SRM) at the Institute for Rock Magnetism (IRM ), Uni- versity of Minnesota. This instrument allows for three-axis full-vector measurements of magnetic remanence at temper- atures between 300 and 17 K in low-field environments (<10 nT). It was developed with different engineering, but the same intent, as a low-temperature insert that has previously been implemented at the University of Rochester Paleomag- netic Laboratory [Smirnov and Tarduno, 2011]. 1. Introduction Magnetic behavior at low temperatures (<300 K) is one of the most sensitive indicators of the iron mineral phases and their concentrations and grain size distributions in nat- ural samples. Changes in magnetocrystalline anisotropy and crystallographic structure give rise to low-temperature tran- sitions that are diagnostic of specific mineral phases. The Morin transition of hematite (at ∼262 K; Morin [1950]), the Verwey transition of magnetite (at ∼122 K; Verwey [1939]) and the Besnus transition of pyrrhotite (at ∼32 K, Besnus and Meyer [1964]) are all diagnostic of common magnetic minerals that carry remanence at Earth surface tempera- tures. Other phases that acquire remanence at low tem- perature, such as siderite (with a Ne´el temperature of 38 K; Frederichs et al. [2003]) and superparamagnetic grains [Worm and Jackson, 1999], can also be readily identified through their low-temperature behavior. In addition to the utility of low-temperature data as a diagnostic tool for mag- netic mineral identification and characterization, irreversible changes in remanence that are associated with cycling to low temperatures are often used as a tool in paleomagnetic stud- ies. Low-temperature steps in paleodirectional and paleoin- tensity study are applied in some protocols with the goal of preferentially removing magnetic remanence held by mul- tidomain grains and thereby isolating magnetizations held by single-domain grains [e.g., Schmidt [1993]; Dunlop [2003]; Yamamoto et al. [2003]]. Low-temperature remanence experiments are routinely conducted on the Quantum Designs Magnetic Properties 2. Instrument design In order to develop the capacity to make three-axis mea- surements of remanence in an ultra-low-field environment, a cryostat insert was developed at the IRM in coopera- tion with ColdEdge Technologies (Allentown, PA) (Figure 1). This low-temperature instrument (IRM-LTI) allows for three-axis measurements to be made between room temper- ature and ∼17 K using horizontal-loading SRMs. There are many advantages to outfitting a superconducting rock mag- netometer for measurements at low-temperatures. First, these instruments are specifically designed for three-axis re- manence measurements, and ambient fields are minimized using a superconducting lead shield. Nulling fields are ap- plied by external coils while the shield cools to superconduct- ing temperatures, ultimately trapping a ∼2-3 nT field along 1 Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA 2 Department of Earth and Planetary Science, University of California, Berkeley, California, USA 3 Department of Geosciences, University of Wisconsin, Milwaukee, WI, USA Copyright 2015 by the authors.

Curie temperatures of titanomagnetite in ignimbrites: Effects of emplacement temperatures, cooling rates, exsolution, and cation ordering

Pumices, ashes, and tuffs from Mt. St. Helens and from Novarupta contain two principal forms of titanomagnetite: homogeneous grains with Curie temperatures in the range 350–500°C and oxyexsolved grains with similar bulk composition, containing ilmenite lamellae and having Curie temperatures above 500°C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement temperatures exceeded 600°C, and in laboratory experiments, heating to over 600°C in air causes the homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured during cooling, but little or no change is observed in room temperature susceptibility. We interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although we cannot conclusively rule out an alternative interpretation involving fine-scale subsolvus unmixing. Short-range ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the Curie temperature have important implications for the acquisition, stabilization, and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites.

Geomagnetic paleointensity in historical pyroclastic density currents: Testing the effects of emplacement temperature and postemplacement alteration

Thellier-type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 Novarupta (NV) and 1980 Mt. St. Helens (MSH) pyroclastic density currents (PDCs) with the intention of evaluating their suitability for geomagnetic paleointensity studies. PDCs are common worldwide, but can have complicated thermal and alteration histories. We attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of PDCs. Results demonstrate two types of nonideal behavior: unstable remanence in multidomain (MD) titanomagnetite, and nonideal behavior linked to fumarolic and vapor phase alteration. Emplacement temperature indirectly influences MSH results by controlling the fraction of homogenous MD versus oxyexsolved pseudo-single domain titanomagnetite. NV samples are more directly influenced by vapor phase alteration. The majority of NV samples show distinct two-slope behavior in the natural remanent magnetization—partial thermal remanent magnetization plots. We interpret this to arise from a (thermo)chemical remanent magnetization associated with vapor phase alteration, and samples with high water content (>0.75% loss on ignition) generate paleointensities that deviate most strongly from the true value. We find that PDCs can be productively used for paleointensity, but that—as with all paleointensity studies—care should be taken in identifying potential postemplacement alteration below the Curie temperature, and that large, welded flows may be more alteration-prone. One advantage in using PDCs is that they typically have greater areal (spatial) exposure than a basalt flow, allowing for more extensive sampling and better assessment of errors and uncertainty.

Effects of open and closed system oxidation on texture and magnetic response of remelted basaltic glass

As part of an experimental and observational study of the magnetic response of submarine basaltic glass (SBG), we have examined, using ion backscattering spectrometry (RBS), transmission and scanning electron microscopy, energy dispersive X-ray spectrometry, and surface X-ray diffraction, the textures wrought by the controlled, open and closed system oxidation of glasses prepared by the controlled environment remelting and quenching of natural SBG. Initial compositions with ∼9 wt % FeO* were melted at 1430°C with the oxygen fugacity buffered at fayalite-magnetite-quartz; melts were cooled at a rate of 200°C min−1 near the glass transition (Tg = 680°C). In open system experiments, where chemical exchange is allowed to occur with the surrounding atmosphere, polished pieces of glass were reheated to temperatures both below and above Tg for times 1–5000 h; undercooled melts were oxidized at 900°C and 1200°C for 18 and 20 h, respectively. RBS demonstrates unequivocally that the dynamics of open system oxidation involves the outward motion of network-modifying cations. Oxidation results in formation of a Fe-, Ca-, and Mg-enriched surface layer that consists in part of Ti-free nanometer-scale ferrites; a divalent-cation-depleted layer is observed at depths >1 μm. Specimens annealed/oxidized above Tg have magnetizations elevated by 1–2 orders of magnitude relative to the as-quenched material; this does not appear to be related to the surface oxidation. Quenched glass (closed system, i.e., no chemical exchange between sample and atmosphere) exhibits very fine scale chemical heterogeneities that coarsen with time under an electron beam; this metastable amorphous immiscibility is the potential source for the nucleation of ferrites with a wide range of Ti contents, ferrites not anticipated from an equilibrium analysis of the bulk basalt composition.