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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 1 kg 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 ~12 Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic 10Be concentration point to a small 40-60 cm diameter meteoroid. An 40Ar-39Ar age of 4532 ± 24 Ma 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 ± 36 Ma. This is in good agreement with the average SIMS Pb-Pb phosphate age of 4535.3 ± 9.5 Ma and U-Pb Concordia age of 4535 ± 10 Ma. The weighted average age of 4541.6 ± 9.5 Ma 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 ~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° 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 ν 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 (2σ), circa 70 Ma after formation of CAI, as found for other L chondrites. The K-Ar (age ~4.3 Ga) and U,Th-He (age ~1 Ga) chronometers were not reset at ~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 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.

Author(s): Tim R. Naish (corresponding author) [1]; Ken J. Woolfe [2, 3]; Peter J. Barrett [4]; Gary S. Wilson [5]; Cliff Atkins [4]; Steven M. Bohaty [6]; Christian J. Bücker [...]

A practical method for measuring and calculating accurate first order reversal curve diagrams is presented. The technique is investigated using samples of highly dispersed magnetic particles which leads to a more detailed understanding of interactions in fine magnetic particle systems. The method allows for a more precise measure of the strength of the interactions and can be employed in the decoupling of the effects of the mean interaction field from the effects of local interaction field variance.

In 1986, cores were obtained to a depth of 702 m (with 98% recovery) from the CIROS-1 drill hole beneath the Ross Sea on the Victoria Land margin. Glaciogene sediments identified near the base of the hole mark the earliest known record of Antarctic glaciation. Initial biostratigraphic analysis indicated that the lower 336 m of the core is early Oligocene in age, and that the upper 366 m is of late Oligocene-early Miocene age. Recently, the chronology of the CIROS-1 core has been questioned. We developed a magnetostratigraphy for the lower 400 m of the CIROS-1 core to clarify the chronology. Our magnetobiostratigraphic results indicate that the base of the CIROS-1 core is early-late Eocene in age (corresponding to Chron C16r; ca. 36.5 Ma). We identify the Eocene-Oligocene boundary at about 410-420 m, within a 20-m-thick, poorly stratified, bioturbated sandy mudstone. This makes the CIROS-1 core the highest latitude site (77.1 [degrees] S) from which this datum event has been recognized. At 366 m, a 4 m.y. hiatus, which lies immediately beneath fluvial sediments, accounts for most of Chrons C11 and C12. We recognize three major climatic episodes in the CIROS-1 core: (1) the late Eocene (34.5-36.5 Ma, 430-702 m), when relatively warm conditions dominated and there were high sedimentation rates and some glacial activity; (2) the late Eocene-early Oligocene boundary interval (28.5-34.5 Ma, 340-430 m), which was a transition from relatively warm to cooler conditions that coincided with glacial intensification, sea-level fall, and subaerial erosion of the shelf; and; (3) the late Oligoceneearly Miocene (22-28.5 Ma, 50-340 m), when large-scale glaciation dominated the region and glaciers grounded across the continental shelf. From correlation with global oxygen isotope and sea-level records, we infer that the Antarctic climate and surrounding oceans cooled after separation of Australia and Antarctica and development of deep-water circulation between them. This marked the onset of the Eocene-Oligocene transition at ca. 34.5 Ma. A major East Antarctic ice sheet did not develop until the early-late Oligocene boundary, toward the end of the Eocene-Oligocene transition (ca. 28.5 Ma). Outlet glaciers did not breach the Transantarctic Mountains and ground across the Ross Sea Shelf until 0.5 m.y. later (ca. 28 Ma).

Paleomagnetic examination of samples gathered from dikes exposed in the walls of the Hess Deep Rift reveals more information regarding the structure of the crust formed at the East Pacific Rise. Residual magnetic inclinations of dike samples are different from near-horizontal inclinations, suggesting a tectonic rotation of dikes since their intrusion. Structural complexity may be a local feature and tectonic rotations might have developed due to the opening of the Hess Deep Rift.

Pedogenesis can be studied by analyzing the sequence of loess and interbedded paleosols of north-central China. The paleosols, which have accumulated over many years, can be differentiated from unaltered parent material zones by their higher low-field mass magnetic susceptibility.

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.

The use of magnetic susceptibility (MS) measurements to determine inhomogeneity in soil chronosequences was evaluated. This method is based on the presence of MS enhancements in soils with vertical morphologic variance and that these enhancements usually conform to a specific pattern. Deviations from the expected pattern would signify inhomogeneity. All tests done under field conditions showed that MS alone could be used in detecting inhomogenous layers and the measurements were valid basis for excluding a pedon from a chronosequence.