Gathering recognizes contributions of former Section President

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94955/1/eost13498.pd

Assessment of the usefulness of lithic clasts from pyroclastic deposits for paleointensity determination

Paleomagnetic and rock magnetic measurements were carried out on lithic clasts found within pyroclastic deposits to assess their potential for paleointensity determinations. The use of multiple lithologies in a single paleointensity determination would provide confidence that the result is not biased by alteration within one lithology. Lithic clasts were sampled from three historically active volcanoes: Láscar in the Chilean Andes, Mt. St. Helens, United States, and Vesuvius, Italy. At Láscar, triple heating paleointensity experiments allow development of new selection criteria for lithic clasts found within pyroclastic deposits. Using these criteria, the Láscar data yield a mean paleointensity of 24.3 ± 1.3 μT (1σ, N = 26), which agrees well with the expected value of 24.0 μT. This indicates that pyroclastic rocks have promise for paleointensity determinations. Pyroclastics, however, still suffer from the range of problems associated with conventional paleointensity experiments on lava flows. Samples from Mt. St. Helens are strongly affected by multidomain (MD) behavior, which results in all samples failing to pass the paleointensity selection criteria. At Vesuvius, MD grains, magnetic interactions, and chemical remanent magnetizations contributed to failure of all paleointensity experiments. Rock magnetic analyses allow identification of the causes of failure of the paleointensity experiments. However, in this study, they have not provided adequate preselection criteria for identifying pyroclastics that are suitable for paleointensity determination.NERC, Royal Societ

High-resolution record of the Laschamp geomagnetic excursion at the Blake-Bahama Outer Ridge

Geomagnetic excursions are brief deviations of the geomagnetic field from behaviour expected during ‘normal secular’ variation. The Laschamp excursion at ?41?ka was one such deviation. Previously published records suggest rapid changes in field direction and a concurrent substantial decrease in field intensity associated with this excursion. Accurate dating of excursions, and determination of their durations from multiple locations, is vital to our understanding of global field behaviour during these deviations. We present here high-resolution palaeomagnetic records of the Laschamp excursion obtained from two Ocean Drilling Program (ODP) Sites, 1061 and 1062 on the Blake-Bahama Outer Ridge (ODP Leg 172). High sedimentation rates (?30–40?cm?kyr?1) at these locations allow determination of transitional field behaviour during the excursion. Palaeomagnetic measurements of discrete samples from four cores reveal a single excursional feature, across an interval of 30?cm, associated with a broader palaeointensity low. We determine the age and duration of the Laschamp excursion using a stratigraphy linked to the ?18O record from the Greenland ice cores. This chronology dates the Laschamp excursion at the Blake Ridge to 41.3?ka. The excursion is characterized by rapid transitions (less than 200?yr) between stable normal polarity and a partially reversed polarity state. The palaeointensity record is in good agreement between the two sites, revealing two prominent minima. The first minimum is associated with the Laschamp excursion at 41?ka and the second corresponds to the Mono Lake excursion at ?35.5?ka. We determine that the directional excursion during the Laschamp at this location was no longer than ?400?yr, occurring within a palaeointensity minimum that lasted 2000?yr. The Laschamp excursion at this location is much shorter in duration than the Blake and Iceland Basin excursions

Reduced plate motion controlled timing of Early Jurassic Karoo-Ferrar large igneous province volcanism

This is the final version. Available on open access from the American Association for the Advancement of Science via the DOI in this recordData and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.Past large igneous province (LIP) emplacement is commonly associated with mantle plume upwelling and led to major carbon emissions. One of Earth’s largest past environmental perturbations, the Toarcian oceanic anoxic event (T-OAE; ~183 Ma), has been linked to Karoo-Ferrar LIP emplacement. However, the role of mantle plumes in controlling the onset and timing of LIP magmatism is poorly understood. Using global plate reconstruction models and Lower Toarcian sedimentary mercury (Hg) concentrations, we demonstrate (i) that the T-OAE occurred coevally with Karoo-Ferrar emplacement and (ii) that timing and duration of LIP emplacement was governed by reduced Pangean plate motion, associated with a reversal in plate movement direction. This new model mechanistically links Earth’s interior and surficial processes, and the mechanism is consistent with the timing of several of the largest LIP volcanic events throughout Earth history and, thus, the timing of many of Earth’s past global climate change and mass extinction events.National Natural Science Foundation of ChinaShell International Exploration and Production B.V.Natural Environment Research Council (NERC)SFI Research Centre in Applied Geosciences (iCRAG)European Research Council (ERC)International Continental Scientific Drilling Programme (ICDP

Improved determination of marine sedimentation rates using 230Thxs

Measurements of excess 230Th (230Thxs) have proved to be a useful tool in constraining changes in sedimentation rate, and improving our understanding of the fluxes of other components into marine sediments. To obtain the initial activity of 230Thxs (230Thxs0) in sediment: the total measured 230Th must be corrected for the presence of 230Th associated with detrital minerals, for ingrowth from uranium-bearing authigenic phases and then also corrected for the decay of 230Thxs since deposition. We describe a number of improvements in the way these corrections are applied to obtain more accurate determinations of 230Thxs0. We present a new method for the determination of a local estimate for the detrital 238U/232Th activity ratio; suggest more appropriate values for the isotopic composition of authigenic uranium; and question the assumption of secular equilibrium in detrital material. We also present a new, freely-available MATLAB® script called ‘XSage’ that can calculate 230Thxs0, from user-supplied datasets of uranium and thorium isotope activities from sedimentary samples following the theoretical approach described. ‘XSage’ can determine variations in sedimentation rate between stratigraphic horizons of known age and thus produce high-resolution age models. Using a Monte Carlo approach, the program calculates uncertainties for these age models and on the durations of intervals between tie-points. An example of the application of the XSage program using a previously published record is provided

Writing in Britain and Ireland, c. 400 to c. 800

No abstract available

Vertical axis rotation (or lack thereof) of the eastern Mongolian Altay Mountains: implications for far-field transpressional mountain building

The Altay Mountains of Western Mongolia accommodate 10–20% of the current shortening of the India-Asia collision in a transpressive regime. Kinematic models of the Altay require faults to rotate anticlockwise about a vertical axis in order to accommodate compressional deformation on the major strike slip faults that cross the region. Such rotations should be detectable by palaeomagnetic data. Previous estimates from the one existing palaeomagnetic study from the Altay, on Oligocene and younger sediments from the Chuya Basin in the Siberian Altay, indicate that at least some parts of the Altay have experienced up to 39 ± 8° of anticlockwise rotation. Here, we present new palaeomagnetic results from samples collected in Cretaceous and younger sediments in the Zereg Basin along the Har-Us-Nuur fault in the eastern Altay Mountains, Mongolia. Our new palaeomagnetic results from the Zereg Basin provide reliable declinations, with palaeomagnetic directions from 10 sites that pass a fold test and include magnetic reversals. The declinations are not significantly rotated with respect to the directions expected from Cretaceous and younger virtual geomagnetic poles, suggesting that faults in the eastern Altay have not experienced a large degree of vertical axis rotation and cannot have rotated >7° in the past 5 m.y. The lack of rotation along the Har-Us-Nuur fault combined with a large amount of rotation in the northern Altay fits with a kinematic model for transpressional deformation in which faults in the Altay have rotated to an orientation that favours the development of flower structures and building of mountainous topography, while at the same time the range widens at the edges as strain is transferred to better oriented structures. Thus the Har-Us-Nuur fault is a relatively young fault in the Altay, and has not yet accommodated significant rotation