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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.

This study investigates the expected cosmic‐ray exposure (CRE) of meteorites if they were to be ejected by a near‐Earth object, that is, from an object already transferred to an Earth‐crossing orbit by an orbital resonance. Specifically, we examine the CRE ages of CI and CM carbonaceous chondrites (CCs), which have some of the shortest measured CRE ages of any meteorite type. A steady‐state near‐Earth carbonaceous meteoroid probability density function is estimated based on the low‐albedo near‐Earth asteroid population, including parameters such as the near‐Earth dynamic lifetime, the impact probability with the Earth, and the orbital parameters. This model was then compared to the orbits and CRE ages of the five CC falls with precisely measured orbits: Tagish Lake, Maribo, Sutter's Mill, Flensburg, and Winchcombe. The study examined two meteoroid ejection scenarios for CI/CM meteoroids: Main Belt collisions and ejections in near‐Earth space. The results indicated that applying a maximum physical lifetime in near‐Earth space of 2–10 Myr to meteoroids and eliminating events evolving onto orbits entirely detached from the Main Belt (Q < 1.78 au) significantly improved the agreement with the observed orbits of carbonaceous falls. Additionally, the CRE ages of three of the five carbonaceous falls have measured CRE ages one to three orders of magnitude shorter than expected for an object originating from the Main Belt with the corresponding semi‐major axis value. This discrepancy between the expected CRE ages from the model and the measured ages of three of the carbonaceous falls indicates that some CI/CM meteoroids are being ejected in near‐Earth space. This study proposes a nuanced hypothesis involving meteoroid impacts and tidal disruptions as significant contributors to the ejection and subsequent CRE age accumulation of CI/CM chondrites in near‐Earth space. [ABSTRACT FROM AUTHOR]

In situ cosmogenic 14 C (in situ 14 C) in quartz provides a recently developed tool to date exposure of bedrock surfaces of up to ∼ 25 000 years. From outcrops located in east-central Sweden, we tested the accuracy of in situ 14 C dating against (i) a relative sea level (RSL) curve constructed from radiocarbon dating of organic material in isolation basins and (ii) the timing of local deglaciation constructed from a clay varve chronology complemented with traditional radiocarbon dating. Five samples of granitoid bedrock were taken along an elevation transect extending southwestwards from the coast of the Baltic Sea near Forsmark. Because these samples derive from bedrock outcrops positioned below the highest postglacial shoreline, they target the timing of progressive landscape emergence above sea level. In contrast, in situ 14 C concentrations in an additional five samples taken from granitoid outcrops above the highest postglacial shoreline, located 100 km west of Forsmark, should reflect local deglaciation ages. The 10 in situ 14 C measurements provide robust age constraints that, within uncertainties, compare favourably with the RSL curve and the local deglaciation chronology. These data demonstrate the utility of in situ 14 C to accurately date ice sheet deglaciation, and durations of postglacial exposure, in regions where cosmogenic 10 Be and 26 Al routinely return complex exposure results. [ABSTRACT FROM AUTHOR]

Determining the timing and extent of Quaternary glaciations around the globe is critical to understanding the drivers behind climate change and glacier fluctuations. Despite synchronous ice-volume and extent change across hemispheres, evidence from the southern mid-latitudes indicates that local glacial maxima occurred earlier in the glacial cycle, preceding the global Last Glacial Maximum (LGM), implying that feedbacks in the climate system or ice dynamics played a role beyond the underlying orbital parameters. To shed light on these processes, we investigated the glacial landforms shaped by the Lago Argentino glacier (50° S), an outlet lobe of the former Patagonian Ice Sheet in southern Argentina, during the last two glacial cycles. We mapped geomorphological features on the landscape and dated moraine boulders and outwash sediments using 10Be cosmogenic nuclides and feldspar infrared stimulated luminescence (IRSL) to constrain the chronology of glacial advance and retreat. We, therefore, provide the first published age constraints on the middle-to-late Pleistocene glaciations at Lago Argentino, and report that this outlet lobe expanded during Marine Isotope Stage 6, at 153.0 ± 14.7 ka, and during Marine Isotope Stage 3, culminating at 44.5 ± 8.0 ka and at 36.6 ± 1.0 ka. Our results indicate that the most recent was its most extensive advance during the last glacial period, and hypothesize that this was a result of longer and colder winters, as well as increased precipitation delivered by a northward migration of the Southern Westerly Winds belt, highlighting the role of local and regional climate feedbacks in driving ice mass changes in the southern mid-latitudes. [ABSTRACT FROM AUTHOR]

In situ 14C in quartz provides a recently developed tool to date exposure of bedrock surfaces up to ~25 000 years. From outcrops located in east-central Sweden, we test the accuracy of in situ 14C dating against (i) a relative sea level (RSL) curve constructed from radiocarbon dating of organic material in isolation basins, and (ii) the timing of local deglaciation constructed from a clay varve chronology complemented with radiocarbon dating. Five samples of granitoid bedrock were taken along an elevation transect extending southwestwards from the Baltic Sea coast near Forsmark. Because these samples derive from bedrock outcrops positioned below the highest postglacial shoreline, they target the timing of progressive landscape emergence above sea level. In contrast, in situ 14C concentrations in an additional five samples taken from granitoid outcrops above the highest postglacial shoreline, located 100 km west of Forsmark, should reflect local deglaciation ages. The ten in situ 14C measurements provide robust age constraints that, within uncertainties, compare favorably with the RSL curve and with the local deglaciation chronology. These data demonstrate the utility of in situ 14C to accurately date ice sheet deglaciation, and durations of postglacial exposure, in regions where cosmogenic 10Be and 26Al routinely return complex exposure results. [ABSTRACT FROM AUTHOR]

There is unambiguous evidence that glaciers have retreated from their 19th century positions, but it is less clear how far glaciers have retreated relative to their long-term Holocene fluctuations. Glaciers in western North America are thought to have advanced from minimum positions in the Early Holocene to maximum positions in the Late Holocene. We assess when four North American glaciers, located between 38–60 ∘ N, were larger or smaller than their modern (2018–2020 CE) positions during the Holocene. We measured 26 paired cosmogenic in situ 14 C and 10 Be concentrations in recently exposed proglacial bedrock and applied a Monte Carlo forward model to reconstruct plausible bedrock exposure–burial histories. We find that these glaciers advanced past their modern positions thousands of years apart in the Holocene: a glacier in the Juneau Icefield (BC, Canada) at ∼2 ka, Kokanee Glacier (BC, Canada) at ∼6 ka, and Mammoth Glacier (WY, USA) at ∼1 ka; the fourth glacier, Conness Glacier (CA, USA), was likely larger than its modern position for the duration of the Holocene until present. The disparate Holocene exposure–burial histories are at odds with expectations of similar glacier histories given the presumed shared climate forcings of decreasing Northern Hemisphere summer insolation through the Holocene followed by global greenhouse gas forcing in the industrial era. We hypothesize that the range in histories is the result of unequal amounts of modern retreat relative to each glacier's Holocene maximum position, rather than asynchronous Holocene advance histories. We explore the influence of glacier hypsometry and response time on glacier retreat in the industrial era as a potential cause of the non-uniform burial durations. We also report mean abrasion rates at three of the four glaciers: Juneau Icefield Glacier (0.3±0.3 mm yr -1), Kokanee Glacier (0.04±0.03 mm yr -1), and Mammoth Glacier (0.2±0.2 mm yr -1). [ABSTRACT FROM AUTHOR]

The East Antarctic Ice Sheet (EAIS) is the largest potential contributor to sea-level rise. However, efforts to predict the future evolution of the EAIS are hindered by uncertainty in how it responded to past warm periods, for example, during the Pliocene epoch (5.3 to 2.6 million years ago), when atmospheric carbon dioxide concentrations were last higher than 400 parts per million. Geological evidence indicates that some marine-based portions of the EAIS and the West Antarctic Ice Sheet retreated during parts of the Pliocene.sup.1,2, but it remains unclear whether ice grounded above sea level also experienced retreat. This uncertainty persists because global sea-level estimates for the Pliocene have large uncertainties and cannot be used to rule out substantial terrestrial ice loss.sup.3, and also because direct geological evidence bearing on past ice retreat on land is lacking. Here we show that land-based sectors of the EAIS that drain into the Ross Sea have been stable throughout the past eight million years. We base this conclusion on the extremely low concentrations of cosmogenic .sup.10Be and .sup.26Al isotopes found in quartz sand extracted from a land-proximal marine sediment core. This sediment had been eroded from the continent, and its low levels of cosmogenic nuclides indicate that it experienced only minimal exposure to cosmic radiation, suggesting that the sediment source regions were covered in ice. These findings indicate that atmospheric warming during the past eight million years was insufficient to cause widespread or long-lasting meltback of the EAIS margin onto land. We suggest that variations in Antarctic ice volume in response to the range of global temperatures experienced over this period--up to 2-3 degrees Celsius above preindustrial temperatures.sup.4, corresponding to future scenarios involving carbon dioxide concentrations of between 400 and 500 parts per million--were instead driven mostly by the retreat of marine ice margins, in agreement with the latest models.sup.5,6.Analysis of cosmogenic isotopes from a marine sediment core shows that much of the land-based East Antarctic Ice Sheet has remained stable for the past eight million years, including during the warm Pliocene epoch., Author(s): Jeremy D. Shakun [sup.1] , Lee B. Corbett [sup.2] , Paul R. Bierman [sup.2] , Kristen Underwood [sup.3] , Donna M. Rizzo [sup.3] , Susan R. Zimmerman [sup.4] , [...]