Long-lived low Th/U Pacific-type isotopic mantle domain

2021 ◽  
Author(s):  
Xijun Liu ◽  
Zhiguo Zhang ◽  
Pengde Liu ◽  
Yujia Song ◽  
Yao Xiao
Keyword(s):  
Upper Mantle ◽  
Plate Tectonics ◽  
Large Scale ◽  
Mafic Rock ◽  
Global Scale ◽  
Data Set ◽  
Provenance Data ◽  
The Pacific ◽  
Long Time ◽  
Material Circulation

<p>    The presence of Pacific-type and Indian-type mid-ocean ridge (MORB) isotopic source domains in the upper mantle is a clear manifestation of global-scale mantle compositional heterogeneities. The Indian-type mantle domain is a long-lived feature that can be traced back to, at least, the Palaeozoic Tethyan mantle domain. Little temporal constraints currently exist, however, regarding the longevity of Pacific-type mantle domain. The extinct Paleo-Asia Ocean (PAO), a subsidiary ocean of the Panthalassic Ocean that formed during the breakup of the Rodinia Supercontinent in Mesoproterozoic to Neoproterozoic, can provide a solution to this dilemma. Here, we report the first complete geochemical and Sr, Nd and high-precision Pb isotopic data set for representative mafic rock samples from ophiolites representing remnants of the PAO basement ranging in age from 275 to 624Ma to constrain the composition of their mantle provenance. Data suggest that the sub-PAO mantle has a similar long time-integrated, high Sm/Nd ratio as the global depleted upper mantle, but also shows typical Pacific MORB-like Pb isotopic compositions with lower <sup>207</sup>Pb/<sup>204</sup>Pb<sub>(t) </sub>and <sup>208</sup>Pb/<sup>204</sup>Pb<sub>(t)</sub> for given <sup>206</sup>Pb/<sup>204</sup>Pb<sub>(t)</sub> ratios, and low radiogenic <sup>208</sup>Pb*/<sup>206</sup>Pb*, indicating a long time-integrated, low Th/U ratios. Thus, the Pacific-type mantle domain, like the Indian-type mantle domain, is a long-lived secular mantle domain that can be traced back to early Paleozoic or even to the Neoproterozoic. Data further indicate that the Nd and Pb isotopic distinction between such two large-scale and long-term mantle domains is due to the different evolutionary and tectonic histories of the circum-Pacific (PAO, Paleo- and modern Pacific) and sub-Tethys-Indian oceanic mantle realms. The Panthalassic-Pacific ocean realm had remarkable permanency existing as a big ocean at lease throughout the Phanerozoic, that implies that continental materials were limit to recycle into underlying mantle, thus the underlying mantle was relative free of the continental material contamination and then produce the low time-integrated Th/U Pacific-type mantle domain. In contrast, the break-up of the Gondwana supercontinent makes the Tethys realms to experience repeated opening and closures, which transferred large volume of continental materials into the underlying mantle and then produce the high Th/U Indian-type mantle domain. Our results indicate that the high Sm/Nd and low Th/U ratio of Pacific-type mantle domain most likely are an inherited, long-standing intrinsic feature of the depleted upper mantle derived from the Earth's primordial mantle with less contamination of continental materials. In contrast, the large-scale and long-lived Indian-type mantle heterogeneity is produced by plate tectonic-driven continental material circulation in the upper mantle. Such a genetic link between plate tectonics and mantle chemical geodynamics is crucial to our understanding of how the Earth system works.</p><p>    This study was financially supported by the National Natural Science Foundation of China (92055208，41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).</p>

The formation of viscous anisotropy in the asthenosphere and its effect on plate tectonics

2020 ◽  
Author(s):  
Agnes Kiraly ◽  
Clinton P. Conrad ◽  
Lars N. Hansen ◽  
Menno Fraters
Keyword(s):  
Upper Mantle ◽  
Effective Viscosity ◽  
Plate Tectonics ◽  
Large Scale ◽  
Plate Motion ◽  
Shear Direction ◽  
Viscosity Change ◽  
Macro Scale ◽  
Geodynamic Processes ◽  
Anisotropic Viscosity

<p>Developing an appropriate characterization of upper mantle viscosity structure presents one of the biggest challenges for understanding geodynamic processes in the upper mantle. This is because different creep mechanisms become activated depending on depth, accumulated strain, and applied stress, and other factors such grain size and anisotropic fabric can change as the deformation develops, changing the effective viscosity. Here we focus on the relationship between anisotropic fabric development and viscous anisotropy.</p><p>Under applied shear, olivine crystals, which form a large proportion of the asthenosphere, rotate towards the shear direction and accumulate a lattice preferred orientation (LPO) parallel to the macroscopic deformation. On a large scale, LPO can be observed through the propagation of seismic waves because of the anisotropic elastic properties of olivine. As olivine is anisotropic in its viscous properties, this developing texture within the asthenosphere can affect the macro-scale viscosity of the asthenosphere. This behavior has been detected in rock mechanics measurements on pure olivine aggregates, showing more than an order magnitude of viscosity change between shear parallel to the olivine aggregate’s LPO versus shear across this fabric (Hansen et al., EPSL 2016a, JGR 2016b).</p><p>Here, we use numerical models developed first in MATLAB and then implemented into the mantle convection code ASPECT. These models incorporate both anisotropic fabric development and anisotropic viscosity, both calibrated according to laboratory measurements on slip system activities of olivine aggregates (Hansen et al., JGR 2016b), to better understand the coupling between the large-scale formation of LPO textures and changes in asthenospheric viscosity.</p><p>The modeling results allows us to discuss the role of anisotropic viscosity on the processes of plate tectonics. An asthenosphere with a well-developed LPO becomes weak parallel to its texture, allowing for increasing plate velocity at the surface, for a given applied driving force.  On the other hand, this fabric resists abrupt changes in the direction of plate motion because the effective viscosity is elevated for shear perpendicular to the developed LPO. This increased resistance to fabric-perpendicular shear also decreases strain rates, which slows texture development. This means that asthenospheric fabric can impede changes in plate motion direction for periods of over 10 Myrs. However, the same well-developed texture in the asthenosphere could enhance the initiation of subduction or lithospheric gravitational instabilities as vertical deformation is favored across a sub-lithospheric olivine fabric, and the sheared fabric can quickly rotate into a vertical LPO. These end-member cases examining shear-deformation across a formed asthenospheric fabric illustrate the importance of olivine fabrics, and their associated viscous anisotropy, for a variety of geodynamic processes.</p>

Global and hemispheric climate variations affecting the Southern Ocean

2004 ◽  
Vol 16 (4) ◽  
pp. 401-413 ◽  
Author(s):  
IAN SIMMONDS ◽  
JOHN C. KING
Keyword(s):  
Southern Ocean ◽  
Fresh Water ◽  
Large Scale ◽  
Mechanical Energy ◽  
Large Fraction ◽  
World Ocean ◽  
Southern Annular Mode ◽  
Reanalysis Data ◽  
Data Set ◽  
The Pacific

The hemispheric and regional atmospheric circulation influences the Southern Ocean in many and profound ways, including intense air-sea fluxes of momentum, energy, fresh water and dissolved gases. The Southern Ocean ventilates a large fraction of the world ocean and hence these influences are spread globally. We use the NCEP-2 reanalysis data set to diagnose aspects of the large-scale atmospheric structure and variability and explore how these impact on the Southern Ocean. We discuss how the ‘Southern Annular Mode’ and the ‘Pacific-South American’ pattern influence the Southern Ocean, particularly in the eastern Pacific. We review the importance of atmospheric eddies in Southern Ocean climate, and the role they play in the transport of mechanical energy into the ocean. The fluxes of fresh water across the air-sea boundary influence strongly the processes of water mass formation. It is shown that climatological precipitation exceeds evaporation over most of the Southern Ocean. When averaged over the ocean from 50°S to the Antarctic coast the annual mean excess is 0.80 mm day−1. The magnitude of the flux displays only a small measure of seasonality, and its largest value of 0.92 mm day−1 occurs in summer.

scholarly journals Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

2016 ◽  
Vol 113 (38) ◽  
pp. 10503-10506 ◽  
Author(s):  
Lars N. Hansen ◽  
Chao Qi ◽  
Jessica M. Warren
Keyword(s):  
Upper Mantle ◽  
Plate Tectonics ◽  
Large Scale ◽  
Flow Model ◽  
Seismic Anisotropy ◽  
Current Debate ◽  
Tectonic Plates ◽  
Midocean Ridges ◽  
Wide Range ◽  
Lithospheric Plates

Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.

Measuring gravity on ice: An example from Wanapitei Lake, Ontario, Canada

Geophysics ◽  
2006 ◽  
Vol 71 (3) ◽  
pp. J23-J29 ◽  
Author(s):  
Hernan A. Ugalde ◽  
Elizabeth L’Heureux ◽  
Richard Lachapelle ◽  
Bernd Milkereit
Keyword(s):  
Large Scale ◽  
Field Measurements ◽  
Noise Sources ◽  
Large Lakes ◽  
Data Set ◽  
Long Time ◽  
Calm Wind ◽  
Regional Gravity ◽  
The Impact

Large lakes have always represented a problem for regional gravity databases; the difficulty of access means gaps or coarse spacing in the sampling. Satellite, airborne, and shipborne gravity techniques are options, but the resolution and/or cost of these systems make them impractical or inaccurate for exploration or environmental studies, where the required resolution is [Formula: see text]. In this study, the feasibility of a ground gravity survey over a frozen lake where ice moves because of windy conditions is assessed. Lake Wanapitei, widely accepted as resulting from the impact of a meteorite 37 million years ago, is one of these cases in which the necessity of expanding coverage over poorly sampled regions arose from a significant gap between surface and airborne geophysical maps. Two gravity surveys were completed on the ice of Lake Wanapitei in the winters of 2003 and 2004. To study the structure, longtime series of gravity field measurements were recorded for 98 stations, allowing for improved control over the noise sources in the data. Final processing and integration with an existing regional data set in the area and the application of terrain corrections reduced the amplitude of the circular anomaly from 15 to [Formula: see text] and its diameter from 11 to [Formula: see text]. The feasibility of gravity surveys on ice was assessed, and we determined that for large-scale studies such as this one, the quality of the data, even under noisy conditions, was acceptable. However, for more detailed mapping, calm wind conditions and long time series are required.

scholarly journals Earth evolution and dynamics—a tribute to Kevin Burke

2016 ◽  
Vol 53 (11) ◽  
pp. 1073-1087 ◽  
Author(s):  
Trond H. Torsvik ◽  
Bernhard Steinberger ◽  
Lewis D. Ashwal ◽  
Pavel V. Doubrovine ◽  
Reidar G. Trønnes
Keyword(s):  
Plate Tectonics ◽  
Large Scale ◽  
Plate Boundaries ◽  
Mantle Plumes ◽  
The Past ◽  
The Pacific ◽  
Igneous Provinces ◽  
After Effect ◽  
Essential Contribution ◽  
Lowermost Mantle

Kevin Burke’s original and thought-provoking contributions have been published steadily for the past 60 years, and more than a decade ago he set out to resolve how plate tectonics and mantle plumes interact by proposing a simple conceptual model, which we will refer to as the Burkian Earth. On the Burkian Earth, mantle plumes take us from the deepest mantle to sub-lithospheric depths, where partial melting occurs, and to the surface, where hotspot lavas erupt today, and where large igneous provinces and kimberlites have erupted episodically in the past. The arrival of a plume head contributes to continental break-up and punctuates plate tectonics by creating and modifying plate boundaries. Conversely, plate tectonics makes an essential contribution to the mantle through subduction. Slabs restore mass to the lowermost mantle and are the triggering mechanism for plumes that rise from the margins of the two large-scale low shear-wave velocity structures in the lowermost mantle, which Burke christened TUZO and JASON. Situated just above the core–mantle boundary, beneath Africa and the Pacific, these are stable and antipodal thermochemical piles, which Burke reasons represent the immediate after-effect of the moon-forming event and the final magma ocean crystallization.

scholarly journals Towards Global-Scale Seagrass Mapping and Monitoring Using Sentinel-2 on Google Earth Engine: The Case Study of the Aegean and Ionian Seas

2018 ◽  
Vol 10 (8) ◽  
pp. 1227 ◽  
Author(s):  
Dimosthenis Traganos ◽  
Bharat Aggarwal ◽  
Dimitris Poursanidis ◽  
Konstantinos Topouzelis ◽  
Nektarios Chrysoulakis ◽  
...  
Keyword(s):  
Large Scale ◽  
Anthropogenic Impacts ◽  
Global Scale ◽  
Google Earth ◽  
Support Vector ◽  
Learning Approaches ◽  
Data Set ◽  
Independent Field ◽  
Google Earth Engine ◽  
Sentinel 2

Seagrasses are traversing the epoch of intense anthropogenic impacts that significantly decrease their coverage and invaluable ecosystem services, necessitating accurate and adaptable, global-scale mapping and monitoring solutions. Here, we combine the cloud computing power of Google Earth Engine with the freely available Copernicus Sentinel-2 multispectral image archive, image composition, and machine learning approaches to develop a methodological workflow for large-scale, high spatiotemporal mapping and monitoring of seagrass habitats. The present workflow can be easily tuned to space, time and data input; here, we show its potential, mapping 2510.1 km2 of P. oceanica seagrasses in an area of 40,951 km2 between 0 and 40 m of depth in the Aegean and Ionian Seas (Greek territorial waters) after applying support vector machines to a composite of 1045 Sentinel-2 tiles at 10-m resolution. The overall accuracy of P. oceanica seagrass habitats features an overall accuracy of 72% following validation by an independent field data set to reduce bias. We envision that the introduced flexible, time- and cost-efficient cloud-based chain will provide the crucial seasonal to interannual baseline mapping and monitoring of seagrass ecosystems in global scale, resolving gain and loss trends and assisting coastal conservation, management planning, and ultimately climate change mitigation.

scholarly journals A validation of river routing networks for catchment modelling from small to large scales

2012 ◽  
Vol 44 (5) ◽  
pp. 917-925 ◽  
Author(s):  
Chantal Donnelly ◽  
Jörgen Rosberg ◽  
Kristina Isberg
Keyword(s):  
Catchment Area ◽  
Large Scale ◽  
Global Scale ◽  
Hydrological Modelling ◽  
Data Sets ◽  
Data Set ◽  
Point Of Interest ◽  
Basin Scale ◽  
Scale Modelling ◽  
River Routing

Underpinning all hydrological simulations is an estimate of the catchment area upstream of a point of interest. Locally, the delineation of a catchment and estimation of its area is usually done using fine scale maps and local knowledge, but for large-scale hydrological modelling, particularly continental and global scale modelling, this level of detailed data analysis is not practical. For large-scale hydrological modelling, remotely sensed and hydrologically conditioned river routing networks, such as HYDRO1k and HydroSHEDS, are often used. This study evaluates the accuracy of the accumulated upstream area in each gridpoint given by the networks. This is useful for evaluating the ability of these data sets to delineate catchments of varying scale for use in hydrological models. It is shown that the higher resolution HydroSHEDS data set gives better results than the HYDRO1k data set and that accuracy decreases with decreasing basin scale. In ungauged basins, or where other local catchment area data are not available, the validation made in this study can be used to indicate the likelihood of correctly delineating catchments of different scales using these river routing networks.

scholarly journals Woce Radiocarbon IV: Pacific Ocean Results; P10, P13N, P14C, P18, P19 & S4P

Radiocarbon ◽  
2002 ◽  
Vol 44 (1) ◽  
pp. 239-392 ◽  
Author(s):  
Robert M Key ◽  
Paul D Quay ◽  
Peter Schlosser ◽  
A P McNichol ◽  
KF von Reden ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
Ocean Circulation ◽  
Large Scale ◽  
World Ocean ◽  
Mass Spectrometry Analysis ◽  
Data Set ◽  
Spectrometry Analysis ◽  
The Pacific ◽  
World Ocean Circulation Experiment ◽  
The Pacific Ocean

The World Ocean Circulation Experiment, carried out between 1990 and 1997, provided the most comprehensive oceanic survey of radiocarbon to date. Approximately 10,000 samples were collected in the Pacific Ocean by U.S. investigators for both conventional large volume p counting and small volume accelerator mass spectrometry analysis techniques. Results from six cruises are presented. The data quality is as good or better than previous large-scale surveys. The 14C distribution for the entire WOCE Pacific data set is graphically described using mean vertical profiles and sections, and property-property plots.

scholarly journals Madden-Julian oscillation winds excite an intraseasonal see-saw of ocean mass that affects Earth’s polar motion

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
M. Afroosa ◽  
B. Rohith ◽  
Arya Paul ◽  
Fabien Durand ◽  
Romain Bourdallé-Badie ◽  
...  
Keyword(s):  
Large Scale ◽  
Tropical Indian Ocean ◽  
Polar Axis ◽  
Global Scale ◽  
Pacific Basin ◽  
Ocean Bottom ◽  
Madden Julian Oscillation ◽  
Ocean Mass ◽  
The Pacific ◽  
The Pacific Ocean

AbstractStrong large-scale winds can relay their energy to the ocean bottom and elicit an almost immediate intraseasonal barotropic (depth independent) response in the ocean. The intense winds associated with the Madden-Julian Oscillation over the Maritime Continent generate significant intraseasonal basin-wide barotropic sea level variability in the tropical Indian Ocean. Here we show, using a numerical model and a network of in-situ bottom pressure recorders, that the concerted barotropic response of the Indian and the Pacific Ocean to these winds leads to an intraseasonal see-saw of oceanic mass in the Indo-Pacific basin. This global-scale mass shift is unexpectedly fast, as we show that the mass field of the entire Indo-Pacific basin is dynamically adjusted to Madden-Julian Oscillation in a few days. We find this large-scale ocean see-saw, induced by the Madden-Julian Oscillation, has a detectable influence on the Earth’s polar axis motion, in particular during the strong see-saw of early 2013.

scholarly journals Global-scale distribution of ozone in the remote troposphere from the ATom and HIPPO airborne field missions

2020 ◽  
Vol 20 (17) ◽  
pp. 10611-10635 ◽  
Author(s):  
Ilann Bourgeois ◽  
Jeff Peischl ◽  
Chelsea R. Thompson ◽  
Kenneth C. Aikin ◽  
Teresa Campos ◽  
...  
Keyword(s):  
Tropospheric Ozone ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Global Climate ◽  
Global Scale ◽  
Ozone Production ◽  
Commercial Aircraft ◽  
The Pacific ◽  
Vertical Distributions ◽  
Remote Troposphere

Abstract. Ozone is a key constituent of the troposphere, where it drives photochemical processes, impacts air quality, and acts as a climate forcer. Large-scale in situ observations of ozone commensurate with the grid resolution of current Earth system models are necessary to validate model outputs and satellite retrievals. In this paper, we examine measurements from the Atmospheric Tomography (ATom; four deployments in 2016–2018) and the HIAPER Pole-to-Pole Observations (HIPPO; five deployments in 2009–2011) experiments, two global-scale airborne campaigns covering the Pacific and Atlantic basins. ATom and HIPPO represent the first global-scale, vertically resolved measurements of O3 distributions throughout the troposphere, with HIPPO sampling the atmosphere over the Pacific and ATom sampling both the Pacific and Atlantic. Given the relatively limited temporal resolution of these two campaigns, we first compare ATom and HIPPO ozone data to longer-term observational records to establish the representativeness of our dataset. We show that these two airborne campaigns captured on average 53 %, 54 %, and 38 % of the ozone variability in the marine boundary layer, free troposphere, and upper troposphere–lower stratosphere (UTLS), respectively, at nine well-established ozonesonde sites. Additionally, ATom captured the most frequent ozone concentrations measured by regular commercial aircraft flights in the northern Atlantic UTLS. We then use the repeated vertical profiles from these two campaigns to confirm and extend the existing knowledge of tropospheric ozone spatial and vertical distributions throughout the remote troposphere. We highlight a clear hemispheric gradient, with greater ozone in the Northern Hemisphere, consistent with greater precursor emissions and consistent with previous modeling and satellite studies. We also show that the ozone distribution below 8 km was similar in the extra-tropics of the Atlantic and Pacific basins, likely due to zonal circulation patterns. However, twice as much ozone was found in the tropical Atlantic as in the tropical Pacific, due to well-documented dynamical patterns transporting continental air masses over the Atlantic. Finally, we show that the seasonal variability of tropospheric ozone over the Pacific and the Atlantic basins is driven year-round by transported continental plumes and photochemistry, and the vertical distribution is driven by photochemistry and mixing with stratospheric air. This new dataset provides additional constraints for global climate and chemistry models to improve our understanding of both ozone production and loss processes in remote regions, as well as the influence of anthropogenic emissions on baseline ozone.

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