The reliability of Arctic climate predictions is currently hampered by insufficient knowledge of natural climate variability in the past. A sediment core from Lake El'gygytgyn in northeastern (NE) ...

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2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia

Newly exposed cave sediments at the Malapa site include a flowstone layer capping the sedimentary unit containing the Australopithecus sediba fossils. Uranium-lead dating of the flowstone, combined with paleomagnetic and stratigraphic analysis of the flowstone and underlying sediments, provides a tightly constrained date of 1.977 ± 0.002 million years ago (Ma) for these fossils. This refined dating suggests that Au. sediba from Malapa predates the earliest uncontested evidence for Homo in Africa.

https://science.sciencemag.org/content/sci/333/6048/1421.full.pdf

Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo

http://www.archaeomagnetism.com/wp-content/uploads/2011/03/herries-and-shaw-2011-web.pdf

Palaeomagnetic analysis of the Sterkfontein palaeocave deposits: implications for the age of the hominin fossils and stone tool industries.

/pdf/psv10-a-global-data-set-for-0-10-ma-time-averaged-field-and-10yi9imydc.pdf

PSV10: A Global Data Set for 0–10 Ma Time‐Averaged Field and Paleosecular Variation Studies

Understanding the time scales of magmatic differentiation, storage, and eruption of large-volume silicic magmas is a primary goal of igneous petrology. Within the Huckleberry Ridge Tuff (HRT; Idaho, USA), representing the earliest and largest caldera-forming eruption associated with Yellowstone volcanic activity, zircon morphological zoning patterns coupled to strongly correlated changes in Ti-in-zircon thermometry and trace element indicators of progressive differentiation provide a proxy record for the evolution of the HRT member B magma body. Tandem in situ and isotope dilution U-Pb dating of single zircon crystals demonstrates an absence of pre-Pleistocene xenocrysts, but reveals the presence of antecrysts recycled from pre-caldera rhyolites in the HRT magma. The petrochronologic interpretation of autocrystic zircon thermal, chemical, and temporal characteristics suggests that HRT member B differentiated over ∼10 k.y. prior to eruption at 2.0794 ± 0.0046 Ma as defined by new astronomically calibrated, single-crystal total fusion 40 Ar/ 39 Ar sanidine analyses. This refined eruption age demonstrates that the transitional polarity preserved by HRT member B does not record the Reunion subchron, but rather a separate, younger geomagnetic event. Our novel approach places the thermal and chemical regime of silicic magmas within a temporal context and demonstrates the rapid evolution of a large volume of silicic magma.

Rapid magma evolution constrained by zircon petrochronology and 40Ar/39Ar sanidine ages for the Huckleberry Ridge Tuff, Yellowstone, USA

We present new radiometric ages obtained at the type locality in La Reunion Island where palaeomagnetic records of the Reunion events have first been identified. Seven dated lava flows from the Riviere Saint Denis section, which recorded a reverse-to-normal transition, display ages from 2.12 ± 0.03 to 2.17 ± 0.03 Ma, with a mean age of 2.15 ± 0.02 Ma. Two significantly younger flows from this section, interpreted here as valley fill flows from trace elements compositions and Pb isotopic data, have been dated at 2.06 ± 0.03 and 2.08 ± 0.03 Ma. Within the Grande Chaloupe section, where a normal-to-reverse transition is recorded, two coherent ages of 2.05 ± 0.03 and 2.03 ± 0.03 Ma have been obtained. When a direct comparison was possible, our new K-Ar ages performed on separated groundmass show a rather good coherence with previous ages from La Reunion Island. When considered with continuous palaeomagnetic sedimentary records in the 2.2-2.0 Ma interval, these new results suggest that two distinct events are recorded in La Montagne lava flows at La Reunion Island, with ages of 2.15 ± 0.02 and 2.04 ± 0.02 Ma. Following recent nomenclature, the former is the Reunion event s.s., while the latter can be related to the Huckleberry Ridge event. Globally distributed volcanic and sedimentary records show that the first (Reunion s.s.; RU-1) is associated with a large dipole intensity decrease at 2.15 ± 0.02 Ma, and hence is recorded in many sequences. On the other hand, the dipole intensity decrease was not as pronounced at 2.04 ± 0.02 Ma, when the Huckleberry Ridge (RU-2) palaeomagnetic event occurred. Consequently, it is not present as a full directional change in many sections worldwide, but rather appears as a geomagnetic excursion during an episode of increased secular variation. Finally, the use of the Reunion event for magnetostratigraphic studies is recommended, while the clear identification of the Huckleberry Ridge excursion might often be difficult, preventing its use as a reliable time marker.

https://hal.archives-ouvertes.fr/hal-00509777/document

New K-Ar ages from La Montagne massif, Réunion Island (Indian Ocean), supporting two geomagnetic events in the time period 2.2–2.0 Ma

SUMMARY A palaeomagnetic study has been conducted from Indonesian lava flows and dykes. Sampling was concentrated to the Merapi and Merbabu volcanoes (central Java), the Bromo‐Tengger volcanic complex, the Lurus volcano (both east of Java), and the Bawean Island. Numerical dating indicates that the rock units sampled have ages that are well distributed within the Brunhes chron. Only a few reverse polarity flows were probably emplaced during the late Matuyama chron. Alternating field (AF) and thermal demagnetization treatments were routinely used for all sites. Results from both methods were generally in good agreement, demonstrating the reliability and the reproducibility of the palaeomagnetic determinations obtained from 36 independent sites. The large dispersion of the directions and the age distribution indicate that a large spectrum of the secular variation has been sampled. Our best estimate of the mean Brunhes field direction in Java is characterized by a zero declination and an inclination anomaly of −4.7 ◦ (with a 95 per cent confidence angle of 5.3 ◦ ). This value agrees with an axial dipolar field model, or a model to which a persisting zonal quadrupolar term of a few per cent of the geocentric axial dipole is added. This study adds new constraints to recent results which suggested that a large zone of negative inclination anomalies may have persisted in the southwestern Pacific ocean over the past 500 Kyr, and possibly extended toward the eastern Indian ocean. The present results do not support such an extension. They also suggest that the inclination anomaly may have varied over 100 Kyr, and that it was more pronounced in recent times.

H. Bouquerel

Papers

New K-Ar ages from La Montagne massif, Réunion Island (Indian Ocean), supporting two geomagnetic events in the time period 2.2–2.0 Ma

Palaeosecular variation in Java and Bawean Islands (Indonesia) during the Brunhes chron