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The Hamburg meteorite fell on January 16, 2018, near Hamburg, Michigan, after a fireball event widely observed in the U.S. Midwest and in Ontario, Canada. Several fragments fell onto frozen surfaces of lakes and, thanks to weather radar data, were recovered days after the fall. The studied rock fragments show no or little signs of terrestrial weathering. Here, we present the initial results from an international consortium study to describe the fall, characterize the meteorite, and probe the collision history of Hamburg. About 1kg of recovered meteorites was initially reported. Petrology, mineral chemistry, trace element and organic chemistry, and O and Cr isotopic compositions are characteristic of H4 chondrites. Cosmic ray exposure ages based on cosmogenic 3He, 21Ne, and 38Ar are ~12Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic 10Be concentration point to a small 40-60cm diameter meteoroid. An 40Ar-39Ar age of 4532±24Ma indicates no major impact event occurring later in its evolutionary history, consistent with data of other H4 chondrites. Microanalyses of phosphates with LA-ICPMS give an average Pb-Pb age of 4549±36Ma. This is in good agreement with the average SIMS Pb-Pb phosphate age of 4535.3±9.5Ma and U-Pb Concordia age of 4535±10Ma. The weighted average age of 4541.6±9.5Ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system.

It has been proposed that all L chondrites resulted from an ongoing collisional cascade of fragments that originated from the formation of the similar to 500 Ma old asteroid family Gefion, located near the 5:2 mean-motion resonance with Jupiter in the middle Main Belt. If so, L chondrite pre-atmospheric orbits should be distributed as expected for that source region. Here, we present contradictory results from the orbit and collisional history of the October 24, 2015, L6 ordinary chondrite fall at Creston, CA (here reclassified to L5/6). Creston's short 1.30 +/- 0.02 AU semimajor axis orbit would imply a long dynamical evolution if it originated from the middle Main Belt. Indeed, Creston has a high cosmic ray exposure age of 40-50 Ma. However, Creston's small meteoroid size and low 4.23 +/- 0.07 degrees inclination indicate a short dynamical lifetime against collisions. This suggests, instead, that Creston originated most likely in the inner asteroid belt and was delivered via the nu(6) resonance. The U-Pb systematics of Creston apatite reveals a Pb-Pb age of 4,497.1 +/- 3.7 Ma, and an upper intercept U-Pb age of 4,496.7 +/- 5.8 Ma (2 sigma), circa 70 Ma after formation of CAI, as found for other L chondrites. The K-Ar (age similar to 4.3 Ga) and U,Th-He (age similar to 1 Ga) chronometers were not reset at similar to 500 Ma, while the lower intercept U-Pb age is poorly defined as 770 +/- 320 Ma. So far, the three known L chondrites that impacted on orbits with semimajor axes a AU all have high (>3 Ga) K-Ar ages. This argues for a source of some of our L chondrites in the inner Main Belt. Not all L chondrites originate in a continuous population of Gefion family debris stretching across the 3:1 mean-motion resonance.

Meteorites have been found on the small Misfits Flat dry lakebed near Stagecoach, Nevada (119.382W, +39.348N). Since the first find on Sept. 22, 2013, a total of 58 stones of weathering stage W2/3 with a combined mass of 339 g have been collected in 19 visits to the area. This small (3.3 × 3.6 km) lakebed is now a newly designated dense collection area (DCA). Most meteorites were found in a small 350 × 180 m area along the north shore and most are fragments of several broken individual stones. Three of these fragments were classified as an LL4/5 of shock stage S2, now named Misfits Flat 001, one of which (stone MF33) fell 8.1 ± 1.3 ka ago based on the 14C terrestrial age, assuming it came from a 20–80 cm diameter meteoroid. In addition, a small darkly crusted meteorite MF34, now named Misfits Flat 002, was found 820 m WSW from the main mass. This meteorite is classified as an LL5 ordinary chondrite with shock stage S4/5. The meteorite is saturated in 14C at 63 dpm kg−1, suggesting it originated from the center of a 0.5 m diameter meteoroid, or deep inside a ~1.0 m meteoroid, less than 300 yr ago. Accounts exist of a fireball seen at 13:15 UT on March 2, 1895, that are consistent with the find location of Misfits Flat 002.

A new record of the Mono Lake excursion (MLE) is reported from the Summer Lake Basin of Oregon, USA. Sediment magnetic properties indicate magnetite as the magnetization carrier and imply suitability of the sediments as accurate recorders of the magnetic field including relative paleointensity (RPI) variations. The magnitudes and phases of the declination, inclination, and RPI components of the new record correlate well with other coeval but lower resolution records from western North America including records from the Wilson Creek Formation exposed around Mono Lake. The virtual geomagnetic pole (VGP) path of the new record is similar to that from another high-resolution record of the MLE from Ocean Drilling Program (ODP) Site 919 in the Irminger Basin between Iceland and Greenland but different from the VGP path for the Laschamp excursion (LE), including that found lower in the ODP-919 core. Thus, the prominent excursion recorded at Mono Lake, California, is not the LE but rather one that is several thousands of years younger. The MLE VGP path contains clusters, the locations of which coincide with nonaxial dipole features found in the Holocene geomagnetic field. The clusters are occupied in the same time progression by VGPs from Summer Lake and the Irminger Basin, but the phase of occupation is offset, a behavior that suggests time-transgressive decay and return of the principal field components at the beginning and end of the MLE, respectively, leaving the nonaxial dipole features associated with the clusters dominant during the excursion.

[1] We synthesize environmental magnetic results for sediments from the Victoria Land Basin (VLB), which span a total stratigraphic thickness of 2.6 km and a ~17 Myr age range. We assess how magnetic properties record paleoclimatic, tectonic, and provenance variations or mixtures of signals resulting from these processes. The magnetic properties are dominated by large-scale magnetite concentration variations. In the late Eocene and early Oligocene, magnetite concentration variations coincide with detrital smectite concentration and crystallinity variations, which reflect paleoclimatic control on magnetic properties through influence on weathering regime; high magnetite and smectite concentrations indicate warmer and wetter climates and vice versa. During the early Oligocene, accelerated uplift of the Transantarctic Mountains gave rise to magnetic signatures that reflect progressive erosion of the Precambrian-Mesozoic metamorphic, intrusive, and sedimentary stratigraphic cover succession associated with unroofing of the adjacent Transantarctic Mountains. From the early Oligocene to the early Miocene, a consistent fining upward of magnetite particles through the recovered composite record likely reflects increased physical weathering with glacial grinding contributing to progressively finer grained Ferrar Dolerite-sourced magnetite. After 24 Ma, the magnetic properties of VLB sediments are primarily controlled by the weathering and erosion of McMurdo Volcanic Group rocks; increased volcanic glass contents contribute to the fining upward of magnetite grain size. Overall, long-term magnetic property variations record the first-order geological processes that controlled sedimentation in the VLB, including paleoclimatic, tectonic, provenance, and volcanic influences.

A sediment core taken from the western edge of the Bonneville Basin has provided high-resolution proxy records of relative lake-size change for the period 45.1e10.5 calendar ka (hereafter ka). Age control was provided by a paleomagnetic secular variation (PSV)-based age model for Blue Lake core BL04-4. Continuous records of d 18 O and total inorganic carbon (TIC) generally match an earlier lake-level envelope based on outcrops and geomorphic features, but with differences in the timing of some hydrologic events/states. The Stansbury Oscillation was found to consist of two oscillations centered on 25 and 24 ka. Lake Bonneville appears to have reached its geomorphic highstand and began spilling at 18.5 ka. The fall from the highstand to the Provo level occurred at 17.0 ka and the lake intermittently overflowed at the Provo level until 15.2 ka, at which time the lake fell again, bottoming out at w14.7 ka. The lake also fell briefly below the Provo level at w15.9 ka. Carbonate and d

We consider a local least-squares criterion for aligning multiple time series fragments differing by locations and show the consistency of the time-lag estimator and the asymptotic normality of the location estimator. We apply the criterion to the problem of aligning 50 glacial varve fragments and construct a 3000-year surrogate for global temperature. Copyright 2007, Oxford University Press.

We conducted detailed rock magnetic investigations on 36 m of drill core collected during Ocean Drilling Program Leg 195 at Hole 1202B (24°48.24′N, 122°30.00′E), in the Southern Okinawa Trough, with the goal of extracting a reliable paleointensity signal with centennial resolution. An age-depth model was established from a chronology obtained by accelerator mass spectromety 14C dating. The sedimentary section spans almost the entire Holocene (0–9.4 kyr) and exhibits sedimentation rates close to 400 cm/kyr. The magnetic properties are dominated by stable, pseudo-single domain magnetite. High-field hysteresis data and the grain-size sensitive ratio of anhysteretic remanent magnetization (ARM) to low field magnetic susceptibility indicate a narrow range of grain sizes and concentrations. Magnetic parameters vary by a factor of 4 thereby fulfilling the usual criteria for a relative paleointensity study. The relative geomagnetic paleointensity was obtained by normalizing the intensity of natural remanent magnetization (NRM) by the ARM and the low field magnetic susceptibility. Both normalizations yield nearly identical results (r = 0.89). Spectral analysis indicates that the record is not significantly affected by local environmental conditions. Comparison of this West Pacific paleointensity curve with other curves suggests a geomagnetic origin for the signal. Millennial-scale features of our record correlate to variations of the archeomagnetic dipole moment. This suggest that the sediments at Hole1202B recorded changes of the geomagnetic field over the studied time interval.

SUMMARY A mixed-polarity zone, representing alternations between remagnetized and non-remagnetized strata, has been documented within the lower few metres of the CRP-1 core (Ross Sea, Antarctica). Detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single-domain particles of a ferrimagnetic iron sulphide mineral, while the reversed-polarity magnetization of non-remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions. Scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite (Fe3S4). Based on microtextural relationships, it is not possible to determine the relative timing of formation for much of the greigite. However, a significant proportion of the greigite has grown on the surface of authigenic siderite (FeCO3) grains that occur as microconcretions and as cement surrounding detrital matrix grains. In such cases, microtextural relationships indicate that siderite post-dates early diagenetic pyrite and that greigite post-dates the siderite. Siderite usually forms in environments with abundant dissolved iron and carbonate, but without dissolved pore water H2S. This set of geochemical conditions occurs in methanic settings below the sulphate reduction zone (in which early diagenetic pyrite forms). We interpret the observed remagnetization of the lower part of the CRP-1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide, thereby causing precipitation of greigite. The distribution of siderite (and associated greigite) in the lower part of the CRP-1 core is patchy, which accounts for the apparent alternation of polarities. This study is part of a growing catalogue of remagnetizations involving greigite, which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies.

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today1, the Antarctic ice sheets may have been unstable2, 3, 4, 5, 6, 7. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles8, 9, 10. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets11. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event5).

We have conducted mineral magnetic investigations of samples of modern dune sand, pristine loess and present-day loess along a NW-SE transect from the northern to the central loess plateau in China. The methods used include the temperature-dependence of susceptibility (TDS) before and after citrate-bicarbonate-dithionite (CBD) treatment as well as X-ray diffraction (XRD) analysis. The XRD analyses demonstrate that magnetite and hematite both exist in the Chinese loess-paleosol sequence and its modem source area, but the TDS measurements show that magnetite is the predominant contributor to magnetic susceptibility. Maghemite is present in the pristine loess and the present-day loess due to pedogenesis. The pedogenic processes that produce the maghemite are closely linked to paleoclimate, and for this transect, precipitation appears to be the most important climatic variable. We suggest that TDS can be used as a useful method for the analysis of pedogenesis and climatic change.

SUMMARY Magnetic susceptibility variations in the Chinese loess/palaeosol sequences have been used extensively for palaeoclimatic interpretations. The magnetic signal of these sequences must be divided into lithogenic and pedogenic components because the palaeoclimatic record is primarily reflected in the pedogenic component. In this paper we compare two methods for separating the pedogenic and lithogenic components of the magnetic susceptibility signal: the citrate-bicarbonate-dithionite (CBD) extraction procedure, and a mixing analysis. Both methods yield good estimates of the pedogenic component, especially for the palaeosols. The CBD procedure underestimates the lithogenic component and overestimates the pedogenic component. The magnitude of this eect is moderately high in loess layers but almost negligible in palaeosols. The mixing model overestimates the lithogenic component and underestimates the pedogenic component. Both methods can be adjusted to yield better estimates of both components. The lithogenic susceptibility, as determined by either method, suggests that palaeoclimatic interpretations based only on total susceptibility will be in error and that a single estimate of the average lithogenic susceptibility is not an accurate basis for adjusting the total susceptibility. A long-term decline in lithogenic susceptibility with depth in the section suggests more intense or prolonged periods of weathering associated with the formation of the older palaeosols. The CBD procedure provides the most comprehensive information on the magnitude of the components and magnetic mineralogy of loess and palaeosols. However, the mixing analysis provides a sensitive, rapid, and easily applied alternative to the CBD procedure. A combination of the two approaches provides the most powerful and perhaps the most accurate way of separating the magnetic susceptibility components.

Sediments recovered from the most seaward site of the Baja Transect of the California Margin (ODP Leg 167) yielded a high-fidelity magnetostratigraphic record spanning the last 6 Myr. Using a long-core cryogenic magnetometer, we made NRM measurements of 59 u-channel samples, which cover the interval from 0 to 78.5 mcd (metres composite depth) of the spliced section of Holes 1010C and 1010E. Stepwise AF demagnetization between 20 and 60 mT showed that the remanence is essentially composed of a single stable component, which was not affected by a coring-induced magnetization. The investigated section can be divided into at least 25 polarity intervals, which are assigned to the chrons from C3A.n2 to the Brunhes (C1n). The sedimentation rate was nearly constant at about 13 m Myr−1, except for a reduction to 9 m Myr−1 between 4 and 5 Ma. ARM and IRM measurements of the u-channels revealed that the sediments below 23 mcd exhibit sharp drops in magnetic concentration at several horizons, but that the upper part has a relatively uniform magnetic concentration, containing magnetite of pseudo-single-domain size. The upper interval yielded consistent relative intensity records when normalized with ARM, IRM and shipboard susceptibility data. Several peaks in the relative intensity variations correspond to drops in the S ratio, in particular at 0.1 T, or to anomalous peaks in the IRM and magnetic susceptibility profiles, suggesting that the normalized intensity record is partly influenced by episodic changes in the flux of magnetic grains. The normalized intensity for the intervals younger than 1.2 Ma shows similarities with published palaeointensity records from the equatorial Pacific, including results from ODP Leg 138. In particular, the intensity minima at 0.05, 0.11, 0.18 and 0.29 Ma and those associated with the onset and termination of the Jaramillo subchron are well correlated.

SUMMARY The onset of continent-wide glaciation in Antarctica is still poorly understood, despite being one of the most important palaeoclimatic events in the Cenozoic. The Eocene/ Oligocene boundary interval has recently been recognized as a critical time for Antarctic climatic evolution, and it may mark the preglacial‐glacial transition. Magnetic susceptibility, intensity of natural and artificial remanences, hysteresis parameters and magnetic anisotropy of the lower half (late Eocene/early Oligocene) of the CIROS-1 core (from McMurdo Sound, Antarctica) reveal alternating intervals of high and low magnetic mineral concentrations that do not correspond to lithostratigraphic units in the core. Pseudo-single-domain magnetite is the main magnetic mineral throughout the sequence, and sharp changes in magnetite concentration match changes in clay mineralogy beneath and at the Eocene/Oligocene boundary. The detrital magnetite originated from weathering of the Ferrar Group (which comprises basic extrusive and intrusive igneous rocks). Weathering processes and input of magnetite to the Victoria Land Basin were intense during periods when the Antarctic climate was warmer than today, but during intervals when the climate was relatively cool, chemical weathering of the Ferrar Group was suppressed and input of detrital magnetite to the Victoria Land Basin decreased. Our results also indicate that a cold and dry climate was not established in Antarctica until the Eocene/Oligocene boundary, with major ice sheet growth occurring at the early/late Oligocene boundary. Some earlier cold intervals are identified, which indicate that climate had begun to deteriorate by the middle/late Eocene boundary.

A previously unsampled outcrop of gently dipping or flat-lying Upper Cretaceous sedimentary strata in the Vancouver Island region, which contains unaltered aragonitic mollusk fossils, yielded a stable remanent magnetization that is biostratigraphically consistent with Cretaceous magnetochrons 33R, 33N, and 32R. These results, characterized by shallow inclinations, indicate an Upper Cretaceous paleolatitude of about 25 ± 3 degrees north, which is equivalent to that of modern-day Baja California. These findings are consistent with the Baja–British Columbia hypothesis, which puts the Insular Superterrane well south of the Oregon-California border in the Late Cretaceous.

Deep Impact? On 15 February 2013, the Russian district of Chelyabinsk, with a population of more than 1 million, suffered the impact and atmospheric explosion of a 20-meter-wide asteroid—the largest impact on Earth by an asteroid since 1908. Popova et al. (p. 1069 , published online 7 November; see the Perspective by Chapman ) provide a comprehensive description of this event and of the body that caused it, including detailed information on the asteroid orbit and atmospheric trajectory, damage assessment, and meteorite recovery and characterization.

Authigenic greigite (Fe 3 S 4 ) has been identified in several horizons of lake beds in a 102-m core from Butte Valley, northern California, using mineral magnetic methods and x-ray diffraction analysis. The presence of greigite has several implications for the paleoenvironmental record from Butte Valley. First, its occurrence in 2.5-3.0 Ma strata confirms that greigite can persist in the geological record for long periods of time. Second, the detrital mineral magnetic record may be partially obscured by the presence of authigenic greigite and care must be taken in interpreting magnetic variations in the greigite-bearing zones as paleoclimate proxies. Third, differences in the timing of remanence acquisition for authigenic and detrital phases may compromise studies of high-frequency geomagnetic field variations. Fourth, greigite may also be significant as a paleoenvironmental indicator of lake and sediment chemistry. The magnetic detection of greigite may therefore provide important information about paleolimnological conditions.

SUMMARY We have studied the magnetic properties of 69 loess and palaeosol samples from the loess plateau in China. Our methodology involves the combination of differential dissolution of secondary iron oxides and oxyhydroxides using citrate-bicarbonatedithionite (CBD) with measurements of magnetic parameters. Because most of these parameters are additive, we can use pre- and post-CBD values to determine the mineral magnetic properties of the CBD-soluble material. We can also calculatc the partitioning of iron atnong various phases. Approximately two-thirds of the total iron in our samples exists as paramagnetic iron in silicate minerals. Antiferromagnetic material (haematite) constitutes almost all of the remainder. However, the magnetic susceptibility record of the loess/palaeosol sequence arises primarily lrom a small amount of iron in ferrimagnetic phases. This iron consists of a CBD-resistant component which rcpresents multidomain grains inherited from the parent material of the loess, and a CBD-soluble component which represents grains near the superparamagnetic/single domain boundary. The first component appears in nearly uniform amounts throughout the loess column and is the dominant ferrimagnetic phase in the magnetically less-enhanced loess. Accumulation of the second component in the palaeosols gives rise to the magnetic susceptibility enhancement of the loess column. The magnetic properties of the CBD-soluble component indicate that it formed as a result of in sitir pedogenesis, which confirms our earlier conclusion that the palaeoclimate record of the loess/palaeosol sequence at the sites that we have sampled is primarily a record of pedogenesis.

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583 ) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutter's Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed.

We studied sixteen basalt samples from a landslide in the Columbia River Gorge to determine if they had acquired a thermoviscous remanent magnetism (TVRM) since the slide was emplaced about 800 years ago. All samples were thermally demagnetized at 20 heating steps until 360°C, where a large change in susceptibilit y was noted. Analysis of the directional changes during demagnetization indicates that the samples contain up to four TVRM components, in addition to an NRM component. The TVRM components with the lowest blocking temperatures are tightly clustered around the present field direction while the NRM directions are consistent with a random distribution, as expected for a landslide deposit. Measurements of hysteresis parameters and thermomagnetic analyses of the samples demonstrate that the dominant magnetic mineral in the basalt is single- domain magnetite. The temperature at which the first TVRM component was removed ranges from 70°C to 100°C. This result can be compared to nomographs that relate the time and temperature used to demagnetize a TVRM with the time and temperature at which the TVRM was acquired. Our results are more consistent with the nomograph of Pullaiah et al. (1975) than with the nomograph of Middleton & Schmidt (1982).

SUMMARY Detailed palaeomagnetic and rock magnetic studies of a 15 m succession of MiddlelLate Pleistocene lacustrine sediments from ancient Lake Chewaucan, southern Oregon, western United States, indicate that the remanence-bearing grains are sufficiently uniform to be applicable to relative palaeointensity studies. We have used ARM, SIRM and x for normalization of the NRM. All three parameters give essentially identical results in their relative stratigraphic variations, which indicates that the normalizations efficiently remove the effects of variation in magnetic mineral concentration. Patterns in grain-size variation, as indicated by small-scale quasi-cyclic fluctuations in hysteresis parameters, may be due to environmental changes such as lake-level variation. However, these fluctuations are within the acceptable range of grain sizes for palaeointensity studies and cannot be correlated with any of the features of the normalized remanence record. We therefore conclude that the large-scale variations in the normalized remanence record are due to geomagnetic palaeointensity fluctuations. Parts of the normalized remanence record, where firm chronological constraints exist, may correlate with features of relative palaeointensity records from deep-sea sediments. Our results also confirm the observation that low geomagnetic field intensities dominate during geomagnetic excursions. Further studies of relative palaeointensity of the geomagnetic field may enable the development of an independent time-scale which would make possible the direct correlation of palaeoclimate records from deep-sea and continental environments.

Natural samples of oxidized and unoxidized magnetite have been used to study the effect of low-temperature oxidation on magnetic domain state. All of the magnetites studied were large multi-domain (MD) grains, ranging in size from 53 µm to at least 250 µm. Hysteresis data typical of MD grains are obtained from unoxidized magnetite samples while data from partially oxidized samples are characteristic of pseudo-single domain (PSD) grains. Our results demonstrate that low-temperature oxidation can seriously affect the magnetic properties of magnetite by significantly increasing the PSD-MD threshold size. The presence of a suppressed Verwey transition at 118 K in the oxidized samples indicates that the oxidation may only be surficial and that there may exist a core of magnetite under the maghemitized surface shell. We suggest three possible mechanisms for the observed PSD-like behavior. First, the magnetite core could be reduced sufficiently in volume to make it a PSD grain, independent of the surficial maghemite. Second, internal stress in the composite grain due to lattice mismatches between the magnetite core and maghemite rim will increase the domain wall energy and make it more difficult to add walls for a given grain size. Coupled with a reduced volume of the magnetite core, this could give rise to PSD-like behavior. Third, the composite grain could be a mixture of SD maghemite and MD magnetite which gives rise to bulk PSD-like properties. Our results have potentially important implications for paleomagnetic records because they provide a mechanism whereby a stable chemical remanent magnetization can be acquired by MD grains in sediments and igneous rocks.

We have analyzed paleointensity data from five polarity transitions and geomagnetic excursions recorded by lava flows. We find a significant correlation between the logarithm of the virtual dipole moment and the angle between the virtual geomagnetic pole and the Earth's rotation axis. This correlation implies that any physical model for geomagnetic field behavior during transitions and excursions must address the question of how changes in direction are linked to changes in intensity. In addition, large changes in paleointensity may occur even when the virtual geomagnetic pole is still at high latitude so the concept of transitional direction must be redefined.

Support for the astronomically calibrated geomagnetic polarity time scale is provided by new 40Ar/39Ar dating and magnetostratigraphic data from the Hadar Formation, Ethiopia. Three tuffs and a basalt flow at Hadar span the Mammoth subchron of the Gauss chron, and range in age from 3.18±0.01 to 3.40 ±0.03 Ma. Interpolation of sediment accumulation rates in two magnetostratigraphic sections yields an estimated age range of 3.30±0.02 to 3.21±0.01 Ma for the Mammoth subchron, closely coinciding with orbitally based predictions of 3.33 to 3.22 Ma rather than with previous estimates of 3.15 to 3.05 Ma.

Paleomagnetic measurements of samples from the Mio-Pliocene Purisima Formation demonstrate that the Pomponio tectonic block of central coastal California has rotated clockwise by approximately 35° to 55° within the last 2.5 million years. The most likely interpretation of this data is that the Pomponio block is broken into several small blocks which have rotated by various amounts. The data suggest that rotations contribute to vertical deformation and secondary faulting within the central San Andreas Fault System, and that they play an important role in the accommodation of shear along the fault system.

The authors report the successful application of magnetic techniques to the study of rock varnish, a surficial coating that forms on rocks in arid environments. Using thin wafers of varnished and unvarnished greywacke sandstone, they have detected a stable remanent magnetization that can be unambiguously attributed to the rock varnish. They have also measured several rock magnetic properties of the rock varnish and have shown that the primary magnetic carrier is either magnetite or maghemite. These results demonstrate the feasibility of applying magnetic techniques to the study of geomorphic surfaces. Subsequent rock magnetic and paleomagnetic studies of rock varnish should provide new information about the geochemical processes involved in the formation of rock varnish and about the validity of a new chronological technique known as rock varnish dating.

[1] Recently silica-mediated dissolution has been recognized as a potentially important factor influencing magnetic studies of marine and lacustrine sediments. Although direct evidence for the dissolution of magnetic particles in silica-rich environments is lacking, the process is expected to produce changes in the magnetic grain-size distribution, a hypothesis that is tested in this study on sediments from Lake Tanganyika, East Africa, using First Order Reversal Curves (FORCs). Results from different magnetic intensity zones within the studied samples clearly show changes in the grain-size distribution of magnetic minerals. In particular, zones with high biogenic silica content (BSi) correlated with depletion in fine-grained magnetic material, whereas zones with lower BSi showed no depletion. These results are consistent with the idea that silica-mediated dissolution results in the preferential removal of fine-grained magnetic material, and indicate that FORC diagrams are effective in characterizing silica-mediated dissolution in sediments.

[1] Chondrules in chondritic meteorites record the earliest stages of formation of the solar system, potentially providing information about the magnitude of early magnetic fields and early physical and chemical conditions. Using first-order reversal curves (FORCs), we map the coercivity distributions and interactions of 32 chondrules from the Allende, Karoonda, and Bjurbole meteorites. Distinctly different distributions and interactions exist for the three meteorites. The coercivity distributions are lognormal shaped, with Bjurbole distributions being bimodal or trimodal. The highest-coercivity mode in the Bjurbole chondrules is derived from tetrataenite, which interacts strongly with the lower-coercivity grains in a manner unlike that seen in terrestrial rocks. Such strong interactions have the potential to bias paleointensity estimates. Moreover, because a significant portion of the coercivity distributions for most of the chondrules is

[1] To retrieve reliable long-term paleoclimatic signals from the Chinese loess/paleosol sequences deposited over the past 2.6 Myr, multiparameter mineral magnetic investigations have been conducted on the Jiaodao section on the Chinese central Loess Plateau. First, the lithogenic magnetic components and the pedogenic overprinting were chemically separated by the citrate-bicarbonate-dithionite (CBD) method. The magnetic properties of the least pedogenically altered loess units and post-CBD loess/paleosols are dominated by coarse-grained lithogenic magnetite. Increasing degree of pedogenesis increases both the concentration and grain size of pedogenic magnetite/maghemite. Long-term changes of the pre-CBD magnetic properties suggest a long-term decrease in summer monsoon intensity and a long-term increase in winter monsoon intensity from the late Pliocene to the late Pleistocene. Long-term variations of the post-CBD magnetic properties suggest an increase in both concentration and grain size of coarse-grained lithogenic PSD/MD magnetite grains, and hence indicate a long-term increase in winter monsoon intensity over that period. The coercivity decrease of the lithogenic component for the loess unit L15 may be related to the accelerated uplift of the Tibetan Plateau and its adjacent regions during the period of L15 deposition. From paleosol S5 to loess L1, higher values of coercivity of the lithogenic component occur in typical loess and paleosols, and lower values are found at the loess/paleosol transition. These coercivity variations are thought to be related to significant changes in the source region as well as dust deposition in the Loess Plateau caused by changes in ice/snow or vegetational cover. These new results lead to a better understanding of the relationships between the effects of tectonic, paleoclimatic, and paleoenvironmental processes in northwestern China.

[1] We have conducted a multiparameter investigation of 15 loess-paleosol couplets (S0/L1 to S14/L15) from the Jiaodao section in the central loess plateau of China using environmental magnetic approaches coupled with soil science techniques. The magnetic parameters display systematic variations that seem to be closely related to paleoclimate variations and intensity of pedogenesis. High-temperature susceptibility curves of paleosols show a generally decreasing trend in reversibility from the base of the Lishi Formation to the Holocene black loam, possibly indicating a decrease in weathering intensity. This may reflect a long-term increase in aridity and/or a general long-term cooling trend of the interior of the Asian continent from 1.2 Ma to the present. Several samples display wasp-waisted hysteresis loops. These are most pronounced in moderately enhanced paleosols, less pronounced in the practically unaltered loess, and subdued in the well-developed paleosols, but wasp waistedness reappears in the most developed paleosols. This wasp-waistedness sequence suggests that the composition, concentration, and grain size of magnetic minerals all contribute to the hysteresis behavior of samples from the studied loess-paleosol sequence, but each factor has a different effect at different stages of pedogenesis.

We report results demonstrating a singularity in the hysteresis of magnetic materials, the reversal-field memory effect. This effect creates a nonanalyticity in the magnetization curves at a particular point related to the history of the sample. The microscopic origin of the effect is associated with a local spin-reversal symmetry of the underlying Hamiltonian. We show that the presence or absence of reversal-field memory distinguishes two widely studied models of spin glasses (random magnets)., Comment: 3 pages, 5 figures. Proceedings of "2002 MMM Conferece", Tampa, FL

We report a novel singularity in the hysteresis of spin glasses, the reversal-field memory effect, which creates a non-analyticity in the magnetization curves at a particular point related to the history of the sample. The origin of the effect is due to the existence of a macroscopic number of "symmetric clusters" of spins associated with a local spin-reversal symmetry of the Hamiltonian. We use First Order Reversal Curve (FORC) diagrams to characterize the effect and compare to experimental results on thin magnetic films. We contrast our results on spin glasses to random magnets and show that the FORC technique is an effective "magnetic fingerprinting" tool., Comment: 4 pages, 6 figures

The Acheulean site of Gesher Benot Ya'aqov in the Dead Sea Rift of Israel documents hominin movements and technological development on a corridor between Africa and Eurasia. New age data place the site at 780,000 years ago (oxygen isotope stage 19), considerably older than previous estimates. The archaeological data from the site portray strong affinities with African stone tool traditions. The findings also reflect adroit technical skills and in-depth planning abilities, more advanced and complex than those of earlier archaeological occurrences in the Levant.

[1] It was never my intention to claim that postmodernism wasthe explanation for the climate change debate. Rather, I was trying to provide a somewhat different perspective on this controversial topic in hopes of broadening the discussion and lowering the volume. I am well aware of the scenario that Hapke lays out; in fact, one could describe similar scenarios for other issues where science intersects with public policy and where people hold strong opinions, such as evolution and creationism, the development of new energy resources, and the safety of genetically modified organisms. The key statement in Hapke's comment is that “similar messages are being heard daily by millions of people, and…[apparently] many believe them.” What we as scientists have not done but need to do is gain a better understanding of why so many people believe these messages. Only then will we be able to figure out an effective response to them. My observation about the pervasiveness of postmodernism in modern society was meant to be a small contribution toward gaining that understanding.

Like many readers of Eos, I have found it hard to understand the persistence of climate doubters and climate skeptics. How can they not accept the science? An important clue can be found in an editorial by Daniel Henninger in the Wall Street Journal that made a connection between climate science and postmodernism [Henninger, 2009]. Postmodernism is a concept that permeates the humanities and the social sciences. In its simplest terms, it postulates that truth is a relative concept. Facts exist, but their interpretation is determined as much by society, culture, politics, and economics as by scientific objectivity. From this perspective, any interpretation is as valid as any other. So, for instance, Herman Melville's Moby Dick can be seen as a novel equally about morality, homosexuality, the repression of the masses, the quest for God, or the killing of whales in the nineteenth century. All interpretations are valid, and all truth is relative.

The human impacts of the 1815 eruption of Tambora volcano in eastern Indonesia are well known. The sulfur injected into the stratosphere by that eruption produced a veil of sunlight-reflecting sulfate aerosols. The following year, often called the “year without a summer,” was marked by significant global cooling, resulting in crop failures, famine, and social unrest as well as by a broad array of climatic impacts [Oppenheimer, 2003]. Over the past 10,000 years, there have been many other volcanic eruptions of comparable (and greater) magnitude than Tambora, and at least some of these must have had similar effects. However, Tambora is the only major eruption whose global climatic and human impacts have been comprehensively documented.

Information about the magnetic field direction and intensity in sediment samples is important for understanding past climate change and the behavior of Earth9s geomagnetic field. In his Perspective, Verosub discusses the recent development of the long-core magnetometer, an instrument that permits great improvements in the speed with which paleomagnetic data can be acquired.

Lake Tahoe ranks among the largest, oldest, and deepest lakes in North America. In addition, the lake is located at a major tectonic boundary These factors make the Lake Tahoe basin an exciting natural laboratory for studying the interaction between tectonics and climate in a high-altitude temperate setting. A recent meeting to explore the potential benefits of a comprehensive program of scientific coring in the Lake Tahoe basin attracted 67 researchers from 28 institutions. The meeting was supported by a grant from the Drilling, Observations, and Sampling of the Earth's Continental Crust (DOSECC) consortium with additional funding provided by the John Muir Institute for the Environment and the Tahoe Environmental Research Center at the University of California, Davis, the Institute for Geophysics and Planetary Physics at the University of California, San Diego, the Desert Research Institute in Reno, Nev, the Academy for the Environment of the University of Nevada, Reno, and the U.S. Geological Survey.

I'm tired of seeing letters from faculty at small liberal arts colleges (for example, [18 Oct., p. 326][1]) piously proclaiming that they, and they alone, are the only ones capable of reforming the way science is taught at colleges and universities. Although many faculty at research universities do not place much value on teaching, there is a growing cadre of faculty at such institutions who are strongly committed to undergraduate teaching and to the improvement of science education. In many departments and even across entire campuses, these groups are beginning to reach the critical mass needed to bring about substantial change. Although there are many reasons why reform at research institutions has lagged behind change at small liberal arts colleges, one of the most important is that the magnitude of the problems at large universities dwarfs that at small colleges. Indeed, it is not all that difficult to teach effectively when one is dealing with a class of 20 students, carefully selected for their homogeneity. It is another matter completely to deal with 100 to 250 students with a full range of abilities, backgrounds, and learning styles. If faculty at small, liberal arts colleges truly want to contribute to the improvement of science education on the national level, they should join with faculty at larger institutions to find ways of adapting the successes of small institutions to the realities of large ones. [1]: /lookup/doi/10.1126/science.274.5286.324c

In the Chinese loess-paleosol sequences from Luochuan, Haanxi province, China, variations in the magnetic susceptibility have been accepted as an excellent proxy for paleoclimate, and the standard interpretation is that climatic processes have enhanced the magnetic susceptibility of the paleosols beyond the base-line level of the loess. By using mineral magnetic properties and a soil-chemistry extraction procedure, we show that most of the magnetic susceptibility signal in both the paleosols and the loess is due to pedogenesis. In effect, loess deposition and pedogenesis are competing processes at all times, and the presence of a paleosol simply indicates that the latter process was predominant. These results imply that any interpretation of the paleoclimate record of loess-paleosol sequences must recognize the pedogenic nature of the magnetic susceptibility signal and that the focus of future research must be on the critical role of climate in the development of the paleosols.

Three meters of sediment from Blue Lake, Idaho, have provided a 3500-year paleomagnetic record with low scatter and good serial correlation. Comparison of the Blue Lake record with the record from Fish Lake, Oregon, reveals differences of up to 963 years for secular variation extrema based on the separate radiocarbon chronologies for Blue Lake and Fish Lake. To achieve a common time scale, the Blue Lake record was redated using the Fish Lake ages for five major inclination extrema. With this adjusted time scale, the excellent correspondence between the declination records confirms the extremal correlations and demonstrates that paleomagnetic correlations may provide more accuracy for relative stratigraphy than radiocarbon dating. The morphological agreement between the smoothed records documents secular variation features with wavelengths of 1000-2000 years and amplitudes of 10°-20° in inclination and 20°-30° in declination. Examination of the unsmoothed records indicates that features with wavelengths of less than a few hundred years and amplitudes of less than 5° are probably noise.

Summary A new method for modelling a geomagnetic field has been developed. After the directional data has been analysed in terms of spherical harmonics, a representation of the field sources is obtained as a combination of a geocentric axial dipole and an eccentric (not necessarily axial) dipole. The free parameters are the location of the eccentric dipole and the relative proportions of the field arising from the geocentric axial dipole and from the eccentric dipole. Previous models for field source configurations are included as special cases. The new method has the particular advantage that it emphasizes the choice among models rather than the fit to a pre-selected model. Analyses of synthetic data sets demonstrate the stability and accuracy of the method. A regionally-averaged set of late Cenozoic palaeomagnetic data can be best represented by a combination of 75-80 per cent geocentric dipole and 25-20 per cent eccentric dipole. The eccentric component of this configuration is displaced toward the North Pacific.

Comparison of the paleomagnetic directions recorded by varved sediments in western New England has provided a means of determining the inherent variability of the paleomagnetic recorder of depositional detrital remanent magnetism. Although a given varve has a stable, well-defined direction of magnetization, successive sampling horizons at the same site as well as the same sampling horizon at different sites may show variations of as much as 8° in inclination and 15° in declination. This variability is attributed not to the secular variation of the geomagnetic field but rather to the physical factors which can influence the magnetic direction recorded by a sediment. Since the time scale for changes in these factors is comparable to the time scale for changes of the geomagnetic field, it may not always be possible to distinguish the true variation of the geomagnetic field from the variability of the paleomagnetic recorder. In certain circumstances correlation of sedimentary sequences on the basis of apparent secular variation will thus be impossible.

The catalog of historical oriental aurorae compiled by Keimatsu lists 116 aurorae from China and 31 aurorae from Japan in the period 801 to 1400 AD. The two listings have no events in common. If they are considered as independent random samples from the same population, the lack of overlapping events can be used to determine the probabilities of different medieval auroral occurrence frequencies over the China-Japan quarter. We find that there is a 95 percent probability that the average rate exceeded the inferred present rate by more than a factor of 20. This corresponds to a geomagnetic pole located in the Eastern Hemisphere displaced more than 9 degrees from the geographic pole.