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  • English  (6)
  • 2020-2022  (6)
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  • English  (6)
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  • 1
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    Constraints on the Upper Mantle Structure Beneath the Pacific From 3‐D Anisotropic Waveform Modeling (2021)
    In:  Journal of Geophysical Research: Solid Earth
    Details
    Publication Date: 2021-09-20
    Description: Seismic radial anisotropy is a crucial tool to help constrain flow in the Earth's mantle. However, Earth structure beneath the oceans imaged by current 3-D radially anisotropic mantle models shows large discrepancies. Here, we provide constraints on the radially anisotropic upper mantle structure beneath the Pacific by waveform modeling and subsequent inversion. Specifically, we objectively evaluate three 3-D tomography mantle models which exhibit varying distributions of radial anisotropy through comparisons of independent real data sets with synthetic seismograms computed with the spectral-element method. The data require an asymmetry at the East Pacific Rise (EPR) with stronger positive radial anisotropy ξ = urn:x-wiley:21699313:media:jgrb54831:jgrb54831-math-0001 = 1.13–1.16 at ∼100 km depth to the west of the EPR than to the east (ξ = 1.11–1.13). This suggests that the anisotropy in this region is due to the lattice-preferred orientation of anisotropic mantle minerals produced by shear-driven asthenospheric flow beneath the South Pacific Superswell. Our new radial anisotropy constraints in the Pacific show three distinct positive linear anomalies at ∼100 km depth. These anomalies are possibly related to mantle entrainment at the Nazca-South America subduction zone, flow at the EPR and from the South Pacific Superswell and shape-preferred orientation (SPO) of melt beneath Hawaii. Radial anisotropy reduces with lithospheric age to ξ 〈 1.05 in the west at ∼100 km depth, which possibly reflects a deviation from horizontal flow as the mantle is entrained with subducting slabs, a change in temperature or water content that could alter the anisotropic olivine fabric or the SPO of melt.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 2
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    The Evolution of Mantle Plumes in East Africa (2020)
    In:  Journal of Geophysical Research: Solid Earth
    Details
    Publication Date: 2021-09-20
    Description: Global tomography models show a large low-velocity anomaly extending from the core-mantle boundary (CMB) beneath South Africa to the upper mantle in East Africa. Although it is believed that this anomaly is linked to mantle upwellings that control key surface features of the African continent, its origin and evolution are still debated. Here we assemble geochemical and seismological constraints along with information from new seismic analyses and geodynamic laboratory experiments to propose that presently there are at least two different plume heads beneath Afar and Kenya that originated at the CMB. A third plume between Kenya and Afar may have caused the Ethiopia-Yemen traps 30 Ma, now merging with the Afar plume. We infer that the Afar plume is presently detached from the CMB probably because of an interaction with the subducted Tethyan slab and that it is likely a dying plume. This may imply that rifts along the Main Ethiopian Rift would fail by the loss of thermal sources, which consequently hampers continental breakup.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 3
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    Substantial hysteresis in emergent temperature sensitivity of global wetland CH4 emissions (2021)
    In:  Nature Communications
    Details
    Publication Date: 2021-12-22
    Description: Wetland methane (CH4) emissions (FCH4) are important in global carbon budgets and climate change assessments. Currently, FCH4 projections rely on prescribed static temperature sensitivity that varies among biogeochemical models. Meta-analyses have proposed a consistent FCH4 temperature dependence across spatial scales for use in models; however, site-level studies demonstrate that FCH4 are often controlled by factors beyond temperature. Here, we evaluate the relationship between FCH4 and temperature using observations from the FLUXNET-CH4 database. Measurements collected across the globe show substantial seasonal hysteresis between FCH4 and temperature, suggesting larger FCH4 sensitivity to temperature later in the frost-free season (about 77% of site-years). Results derived from a machine-learning model and several regression models highlight the importance of representing the large spatial and temporal variability within site-years and ecosystem types. Mechanistic advancements in biogeochemical model parameterization and detailed measurements in factors modulating CH4 production are thus needed to improve global CH4 budget assessments.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 4
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    The Low-Latitude Plasma Irregularities after Sunrise from Multiple Observations in Both Hemispheres during the Recovery Phase of a Storm. (2020)
    In:  Remote Sensing
    Details
    Publication Date: 2020-12-10
    Description: The daytime plasma density disturbances in the low-latitude ionosphere, referred to as plasma irregularities, mainly occur during the nighttime and are an unusual phenomenon. Based on the observations from multiple low Earth orbiting (LEO) satellites, e.g., the Defense Meteorological Satellite Program (DMSP) F13 and F15, the first Satellite of the Republic of China (ROCSAT-1), the Gravity Recovery and the Climate Experiment (GRACE), and Challenging Mini-satellite Payload (CHAMP) satellites, as well as the ground-based Global Positioning System (GPS) receivers, we report a special event of low-latitude plasma irregularities that were observed after sunrise in the Pacific longitudes on 18 August, 2003, following a moderate geomagnetic storm. Observations from three ground-based GPS stations in both hemispheres showed remarkable total electron content (TEC) disturbances during 20:00 to 21:00 UT (around local sunrise), agreeing well with the in situ plasma density irregularities recorded by the nearby flying LEO satellites. The plasma irregularities observed by these LEO satellites showed quite different depletion intensities at different altitudes. We suggest that the plasma irregularities were freshly generated near sunrise hours due to the disturbance of the dynamo electric field (DDEF), evolving into the post-sunrise and morning sector, but were not the remnant of the plasma irregularities generated during the previous nighttime.
    Description: Introduction; Data Description; Geophysical Conditions during 16-20 August 2003; Results; Discussions; Conclusions; References
    Language: English
    Type: info:eu-repo/semantics/article
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  • 5
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    Plasma and field observations in the magnetospheric source region of a stable auroral red (SAR) arc by the Arase satellite on 28 March 2017. (2020)
    In:  Journal of Geophysical Research: Space Physics
    Details
    Publication Date: 2021-04-01
    Description: The conjugate measurements of a detached SAR arc on 28 March 2017 are analyzed using observations from the Arase satellite. Swarm and GNSS‐TEC data show that the electron density decreased and the electron temperature increased in the ionosphere above the SAR arc. The observed plasmas and electromagnetic fields suggest that Coulomb collision is the most plausible mechanism for the SAR‐arc generation.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 6
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    Major element stream water chemistry, compiled 10Be erosion rates, and analyses of weathering across an erosion-rate gradient in in southern Taiwan (2021)
    GFZ Data Services
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    Publication Date: 2021-03-03
    Description: This dataset was used to analyse the link between chemical weathering and erosion rates across the southern tip of Taiwan. The weathering of silicate minerals is a key component of Earth’s long-term carbon cycle, and it stabilises Earth’s climate by sequestering carbon dioxide (CO2) from the atmosphere – thereby balancing CO2-emissions from the mantle. Conversely, the weathering of accessory carbonate and sulphides acts as a CO2 source. Chemical weathering is fundamentally dependent on the exposure of fresh minerals by erosion. With these data we investigated the link between the exposure of rocks by erosion and the chemical weathering of silicates, carbonates, and sulphides across a landscape with a significant erosion-rate gradient and comparatively little variation in runoff and lithology. This dataset includes new major element chemistry and water isotopes of river waters collected from across the southern tip of Taiwan as well as associated topographic and lithologic data (tab 1 in the excel table). Moreover, the data include a compilation of published 10Be-derived erosion rates from a subset of the sampled rivers (tab 2 in the excel file) and available major element chemistry from hotsprings in the region (tab 3 in the excel file). Using a mixing model, we derived the cation contributions from silicate and carbonate weathering as well as from hotspring and cyclic sources. Further, we estimated the erosion rates for each sample from the compiled 10Be data and the steepness of river channels, and we estimated saturation and pH in the weathering zone. For more information please refer to the associated data description file and especially to Bufe et al. (2021).
    Language: English
    Type: info:eu-repo/semantics/workingPaper
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