African Anomaly Research Articles

Instability of the African large low-shear-wave-velocity province due to its low intrinsic density

Seismic observations have revealed two seismic anomalies in the lowermost mantle, one beneath Africa and the other beneath the Pacific Ocean, named large low-shear-wave-velocity provinces. These structures are generally considered to be intrinsically dense thermochemical piles that influence mantle and core processes. However, the controls on their morphology, including their relative height difference and their stability, remain unclear. Here we analyse published global shear-wave tomography models, which show that the African anomaly is about 1,000 km greater in height than the Pacific anomaly. With our numerical simulations, we find that the maximum height a thermochemical pile can reach is more controlled by its density and the surrounding mantle viscosity, and less so by its own viscosity and volume. Comparing these findings suggests that the African anomaly has a relatively lower density and thus may be less stable than the Pacific anomaly, implying the two anomalies have different compositions, dynamics and evolution histories. The seismic anomaly in the lowermost mantle beneath Africa is greater in height and less stable than its Pacific counterpart because of its lower density, according to numerical simulations of the anomalies as thermochemical piles.

Widespread Small-Scale Anisotropic Structure in the Lowermost Mantle beneath the North American Continent and Northeastern Pacific

Abstract We constrain D″ anisotropy beneath the North American continent and northeastern Pacific using two approaches: (1) joint splitting analysis of SKS and SKKS phase pair for a common event, in which we obtain 158 pairs exhibiting discrepant splitting results and 791 pairs nondiscrepant splitting results; and (2) group splitting analysis of SKS (or SKKS) phase from neighboring events recorded at a common station, in which we observe 109 2°×2° grids with consistent splitting parameters, and 164 grids with abrupt changes from splitting to no splitting within 30–100 km. The seismic data from both analyses indicate that small-scale variations of D″ anisotropy are widespread beneath the studied regions, with a lateral scale up to tens of kilometers. For portion of the data recorded at the stations of simple upper-mantle anisotropy, we correct for the effects of upper-mantle anisotropy and obtain the splitting parameters of D″ anisotropy. The inferred D″ anisotropy exhibits a changing geographic pattern and lateral transition of anisotropy to a lateral scale of tens of kilometers. Such a length scale of changing anisotropy is also confirmed by synthetics modeling of the seismic data. We suggest that the inferred small-scale anisotropies could be best explained by the shape preferred orientation of widespread small-scale partial melt pockets derived by a composition change produced early in the Earth’s history, a similar compositional origin that was invoked to explain the African anomaly in the lower mantle.

Eight new Lepadellidae (Rotifera, Monogononta) from the Congo bring to levelendemism in Africa's rotifers.

While patterns in diversity and biogeography of macroscopic organisms may be fairly well-documented, this is much less the case for microscopic organisms, for which mechanisms working at entirely different scales may be relevant. Data deficiency remains a major obstacle to the study of these mechanisms, and this situation is exacerbated in regions for which accessibility is not self-evident. We here report on the Lepadellidae rotifers in zooplankton samples collected during the Boyekoli ebale Congo 2010 expedition and a subsequent visit to Yangambi, DR Congo in June 2012. The material contained eight hitherto unknown species amongst 33 Lepadellidae taxa. This remarkable result illustrates a previously unknown and largely endemic microfauna of the Congo Basin and refutes the "African anomaly" hypothesis on the purported poverty of the Central African rotifer fauna. We describe the new species, viz. Colurella asymmetrica n. sp., Lepadella hanneloreae n. sp., L. jingruae n. sp., L. weijiai n. sp., L.wilungulai n. sp., L. yangambi n. sp., Squatinella curviseta n. sp. and S. longipila n. sp., and comment on the diversity and biogeographical relationships of this fauna. Two of the new species appear to have a South American congener as their closest relative, potentially constituting two more examples of rare African-South American vicariant species-pairs in Rotifera.

Ultra-low velocity zone heterogeneities at the core\u2013mantle boundary from diffracted PKKPab waves

Diffracted waves around Earth’s core could provide important information of the lowermost mantle that other seismic waves may not. We examined PKKPab diffraction waves from 52 earthquakes occurring at the western Pacific region and recorded by USArray to probe the velocity structure along the core–mantle boundary (CMB). These diffracted waves emerge at distances up to 10° past the theoretical cutoff epicentral distance and show comparable amplitudes. We measured the ray parameters of PKKPab diffraction waves by Radon transform analysis that is suitable for large-aperture arrays. These ray parameters show a wide range of values from 4.250 to 4.840 s/deg, suggesting strong lateral heterogeneities in sampling regions at the base of the mantle. We further estimated the P-wave velocity variations by converting these ray parameters and found the CMB regions beneath the northwestern edge of African Anomaly (Ritsma et al. in Science 286:1925–1928, 1999) and southern Sumatra Islands exhibit velocity reductions up to 8.5% relative to PREM. We suggest that these low velocity regions are Ultra-low velocity zones, which may be related to partial melt or iron-enriched solids.Graphical abstract.

Ecologies of relation: post-slavery, post-apartheid, and rethinking race across the Atlantic in Zakes Mda’s Cion

This article will argue that Zakes Mda’s 2007 novel Cion stages a dialog, one where two “Souths” – South Africa and the American South – speak to one another and give a critical voice to an under-acknowledged history of transatlantic discursive exchange on race and racial governance. Mda’s fictional South African critique, of an America still struggling with the cultural and political legacies of slavery, gestures towards a history of exchange between the two countries that in many ways is representative of a more global dialog on racial segregation during the first half of the twentieth century – of which both southern (US) segregation and apartheid are seminal examples. Moreover, this article explores various conceptualizations of race as well as the governance of racial relations as they have been articulated through ecological imaginaries, and especially between South Africa and the Southern United States over the course of the twentieth century. In this article, I argue that not only can apartheid (as well as pre-apartheid segregation) be rethought of as part of a global conversation on race and thus less as a South African anomaly, but also that the United States through its examples of various racialist technologies was highly influential across the colonial and apartheid worlds.

Geographic boundary of the “Pacific Anomaly” and its geometry and transitional structure in the north

We determine the geographical boundary and average shear velocity structure of the Pacific Anomaly at the base of the mantle based on travel time analysis of ScSH‐SH and ScS2 (ScSScS)‐SS phases and waveform modeling results. We further constrain the detailed geometry of the northern Anomaly around (20°N, −170°E) and its transition to the surrounding high velocity region along three perpendicular cross sections on the basis of forward waveform modeling of the observed direct S and ScS phases. The observed differential travel‐time residuals and waveform modeling results allow the whole geographic boundary of the Anomaly to be delineated and the area of the base of the Anomaly is estimated to be 1.9 × 107 km2. The maximum shear velocity perturbation inside the Anomaly reaches −5% in the lowermost 500 km of the mantle. Waveform analysis suggests that the northern Anomaly reaches 450 km above the CMB with both steeply and shallowly dipping edges and its basal layer extends beneath the surrounding mantle with the degree of extension changing rapidly across a small distance. The inferred characteristics of the Anomaly support the previous suggestion that the Pacific Anomaly represents a chemical anomaly. However, unlike the inferred features of the African Anomaly pointing to an ancient compositionally distinct and geologically stable anomaly, the existence of several separated piles extending into the mid‐lower mantle, the complex morphology of the piles with both steeply and shallowly dipping edges and the presence of many ultra‐low velocity zones at its base suggest that the Pacific Anomaly likely possesses varying intrinsic compositions and exhibits complex interaction with the surrounding mantle.

Correction to “Geometry and P and S velocity structure of the “African Anomaly””

Correction to “Geometry and P and S velocity structure of the “African Anomaly””

Geometry and P and S velocity structure of the “African Anomaly”

We constrain the geometry and P and S velocity structure of a low‐velocity anomaly in the lower mantle beneath southern Africa (we term it the “African Anomaly”) on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the East Pacific Rise to the Japan Sea. Our collected data set consists of direct S, direct P, Sdiff, ScS, PcP, SKS, and SKKS phases recorded by three temporary broadband PASSCAL seismic arrays deployed in Africa between 1994 and 2002, the Tanzania seismic array (1994–1995), the Kaapvaal seismic array (1997–1999), and the Ethiopia/Kenya seismic array (2000–2002) for earthquakes occurring in the East Pacific Rise, Drake Passage, South Sandwich islands, Iran, Hindu Kush, Xinjiang, and the Japan Sea. The seismic data provide excellent sampling of the African Anomaly in the lower mantle along the specific great arc. In order to accurately account for the contributions from the African Anomaly, we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the Global Seismographic Network and correct for the contributions from the seismic heterogeneities outside the African Anomaly. The seismic observations suggest that the African Anomaly locally extends 1300 km above the core‐mantle boundary beneath southern Africa (around −25°N, 27°E) and exhibits a “bell‐like” geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth. The base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions. The seismic data can best be explained by a shear velocity structure with average velocity decreases of −5% in the base and −2% to −3% in the mid‐lower mantle above the base, and a compressional velocity structure with a uniform S to P velocity perturbation ratio of 3:1 for the entire African Anomaly. These geometric and velocity features suggest that the mid‐lower mantle portion of the African Anomaly is an integral component of the very low velocity province and the African Anomaly is compositionally distinct and geologically stable.

Thermochemical structures beneath Africa and the Pacific Ocean

Large low-velocity seismic anomalies have been detected in the Earth's lower mantle beneath Africa and the Pacific Ocean that are not easily explained by temperature variations alone. The African anomaly has been interpreted to be a northwest-southeast-trending structure with a sharp-edged linear, ridge-like morphology. The Pacific anomaly, on the other hand, appears to be more rounded in shape. Mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes. It has not been shown, however, that such models can lead to thermochemical structures that satisfy the geometrical constraints, as inferred from seismological observations. Here we present numerical models of thermochemical convection in a three-dimensional spherical geometry using plate velocities inferred for the past 119 million years. We show that Earth's subduction history can lead to thermochemical structures similar in shape to the observed large, lower-mantle velocity anomalies. We find that subduction history tends to focus dense material into a ridge-like pile beneath Africa and a relatively more-rounded pile under the Pacific Ocean, consistent with seismic observations.

Drifts and Intensity Variations of the Geomagnetic Field

AbstractThe Briggs' method of correlation analysis of moving random pattern is modified. The characteristics of the westward drift and intensity variations of the non‐dipole geomagnetic field and 6 planetary‐scale geomagnetic anomalies are studied. During 1900‐2000, the non‐dipole field drifted westward at an average speed of 0:15°/α, and intensified by 29%. Six planetary‐scale magnetic anomalies exhibited different characteristics in the drift. The African Anomaly (AF) drifts westward at an average speed of 0:26°/α. The Australia Anomaly (AUS) drifts at an average speed of 0:23°/α. The slowest drift occurs in the Eurasia Anomaly (EA) at an average speed of 0:09°/α. As drifting westward, most anomalies drift northward slowly. During 1945‐1955, the anomalies EA, NAM, NAT in the northern hemisphere and AF near the equator changed its drift directions, from the westward to northwestward; a little later, the anomalies AUS and SAT in the southern hemisphere slowed down. In the 20 th century, the intensities of AUS, SAT, AF and EA, the largest in area and greatest in intensity anomalies, were increased, while the intensities of other two anomalies were decreased.

Secular Variations of the Planetary‐Scale Geomagnetic Anomalies

AbstractThe characteristics of a planetary‐scale geomagnetic anomaly are described by using “unsigned magnetic flux” through the anomaly region. The eighth generation of the international geomagnetic reference filed (IGRF) is analyzed to study the secular variations of 5 planetary‐scale geomagnetic anomalies during the 1900‐2000 period. The results show that the magnetic fluxes through the southern Atlantic anomaly (SAT), Australian anomaly (AUS), and African anomaly (AF) have increased by more than 200 MWb, the increase of the flux through the Eurasian anomaly (EA) is smaller (157MWb), while the flux through the North America anomaly (NAM) has decreased by 50MWb. On the other hand, the variations of the anomaly areas are within 10%. The westward drift at the core‐mantle boundary (CMB) is less than 0.1°/a, being much slower than that at the Earth's surface (0.2°/a). This contrast may be attributed to the different westward drift velocities of the various harmonics in the geomagnetic field: the westward drift at the Earth's surface is controlled by the dominant low‐degree harmonics, while the westward drift at the CMB largely depends on high‐degree harmonics. Consequently, it should be careful to take the westward drift velocity at the Earth's surface as the characteristic velocity of the fluid flow in the outer core.

The three-dimensional shear velocity structure of the mantle from the inversion of body, surface and higher-mode waveforms

SUMMARY We present a 3-D model of shear heterogeneity (Vsh) in the whole mantle, derived from the inversion of hand-picked body, surface and higher-mode waveforms. The forward and inverse problems are formulated using the non-linear asymptotic coupling theory, the zeroth-order asymptotic expansion of a Born seismogram computed by normal mode summation and including coupling across mode branches. We invert iteratively for mantle heterogeneity and for centroid moment tensors until convergence. Our model, SAW24B16, is parametrized laterally in spherical harmonics up to degree 24 and radially in 16 cubic b-splines with knots spaced to reflect the data sampling with depth. The power spectrum of the model is dominated by heterogeneity in the boundary layers at low degrees: degrees 5 and 6 near the surface and degree 2 near the core–mantle boundary. The rest of the lower mantle is dominated by degree 3. We find significant heterogeneity up to spherical harmonic degree 24 in the transition zone and the uppermost part of the lower mantle. Our model displays three slow domains extending continuously between uppermost and lowermost mantle, one originating under Africa and two in the Pacific. The results of our modelling also suggest an interaction near the surface between the anomalies in the Pacific and the network of mid-ocean ridges in the East Pacific, and between the African anomaly and the low-velocity zone in the Red Sea, the East African Rift, the Mid-Indian Ridge and the Mid-Atlantic Ridge. These anomalies appear to ascend into the upper mantle without altering their shape near the 670 km discontinuity. Fast anomalies accumulate near the 670 km discontinuity under the Southern Kurile, the Japan trench and the Izu arc, with little or no penetration into the lower mantle, in agreement with results from a variety of regional studies. Cross-sections through subduction zones such as the Marianas, Tonga–Kermadec, Java, the palaeosubduction zone beneath North America and the Peru–Chile trench show continuous fast structures penetrating into the lower mantle, with the depth of penetration varying from region to region. The distribution of both fast and slow anomalies near 670 km thus suggests that the discontinuity does not act as a strong barrier to upgoing and downgoing flow between the upper and the lower mantle.

Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume

We explore the hypothesis that southern Africa is actively being uplifted by a large‐scale, positively buoyant structure within the mid‐lower mantle. Using a new formulation in which dynamic topography and uplift rate are jointly used, we place constraints on mantle density and viscosity. The solution of the momentum equation is coupled with the advection of the density field to solve for the surface uplift rate in both an axisymmetric and fully spherical geometry. We demonstrate how dynamic topography and its rate of change depend on density and lateral and radial variations in viscosity. In the full spherical models the geometry of mantle density is derived by scaling a tomographic shear velocity model. Using a variety of geologic observations, we estimate residual topography (i.e., the topography remaining after shallow sources of density are removed) and an average Cenozoic uplift rate to be 300–600 m and 5–30 m/Myr, respectively, for southern Africa. We are able to satisfy these constraints with a mantle model in which the mid‐lower mantle beneath southern Africa is 0.2% less dense and has a viscosity of ∼ 1022 Pa s. In addition, if the continental lithosphere is thick beneath southern Africa, as suspected from seismic inversions, and has a high effective viscosity, then we find that southern Africa can be further elevated owing to increased coupling between the deep mantle and surface. We show that recent estimates of mantle density, suggesting that the lowest parts of the African anomaly may be anomalously dense are compatible with geologic constraints. We conclude that uplift rate, when combined with estimates of present‐day dynamic topography, provides a powerful tool to constrain the properties of the deep mantle.

Global Changes of the Non-Dipole Magnetic Field for 1900-2000

Based on the seventh generation of International Geomagnetic Reference Field(IGRF), the non-dipole magnetic fields for each 5 years during the period of 1900–2000 were calculated and analyzed. The location and intensity of each magnetic anomaly center at different epoch were computed and analysed. There are five magnetic anomalies with large spatial coverage and strong intensity, they are Eastern Asian positive magnetic anomaly, Oceanian negative magnetic anomaly, Southern Atlantic positive magnetic anomaly, African negative magnetic anomaly, and Northern American positive magnetic anomaly. The intensity of Southern Atlantic positive anomaly is the biggest. The intensity of Eastern Asian positive anomaly is the second before 1990.The intensity of African negative anomaly has exceeded Eastern Asian positive anomaly and becomes the second since 1990. The increasing rate of magnetic anomaly intensity (absolute value) of African negative anomaly is the largest(average annual variation rate is 67.1nT/a.), that of Southern Atlantic positive anomaly is the second (64.8nT/a.), that of Oceanian negative anomaly is the third (49.6nT/a.). The changes of magnetic anomaly centers of African and Southern Atlantic anomaly are the fastest compared with other magnetic anomalies.

Interpreting regional geomagnetic anomalies and their secular variation with the aid of radial dipoles and current loops.

By adjusting the depth and strength of a radial magnetic dipole so that the intensity of the field at the focal point of Z and the distance between the positive and negative focal points of the X and Y isolines correspond to those around a regional anomaly obtained from non-dipole maps, it is demonstrated that the dipole is located at a radial distance of 0.25 Re from the geocentre. A circular current loop at the core-mantle boundary corresponding to this dipole has a radius of 0.35 Re.Using a conducting Earth model, it was found that the effect of the conductivity of the mantle is negligible if the period of the source dipole is more than 50 years.The secular change in Europe, Asia, Africa, and part of North America was found to follow the change of two radial dipoles, one lying under the Asian anomaly and the other under the African anomaly. As interpreted by this two-dipole model, the strong centres of the secular variation observed in the Atlantic Ocean are caused by the rapid westerly drift of the African anomaly. In Asia, the change of the large positive Z-cell to a negative one during the last 30 years was interpreted as having been caused by changes in the intensity of the Asian anomaly.The observed secular change at 29 observatories in Eurasia and part of North America for the epoch 1965.0 can be explained by a two-dipole model with an rms error of 7.1nT/y, which is clearly better than the rms error of the IGRF (14.5nT/y), and roughly the same as that of the definitive model for 1965 (6.1nT/y) obtained.

Particle precipitation in the South Atlantic anomaly deduced from VLF propagation path measurements

The VLF propagation path NWC to Sao Paulo, Brazil, passes through both the Brazilian and South African Anomalies. Particle produced ionization in the South African Anomaly is identified by its expected poststorm continuation up to 3–4 days. By comparing it with another mid-latitude path outside the anomaly it is shown that there is no D-region enhancement in the South African Anomaly. Any ionization enhancement in the Brazilian Anomaly should increase to a new level just after a major magnetic storm but no evidence for such VLF effects could be found.

Diurnal variation in trapped electron intensities atL= 2.0

In a recent article [Williams and Kohl, 1965] we presented an observed longitudinal dependence of trapped energetic (Ee ≥ 280 kev) electron intensities at high B values and at middle latitudes as observed on the dayside hemisphere near the noon meridian. Plots of the fractional transmission of these electrons through the South African anomaly were presented as a function of B value on L shells from L = 1.6 to L = 3.2. Each point observed in (B, L) space in the northern hemisphere that displayed a longitudinal asymmetry yielded a minimum in the trapped-electron intensities when the corresponding southern-hemisphere (B, L) pointwas in the South African anomaly. Furthermore, the spatial extent of this effect could be predicted by considering the relative behavior of the L dependence of Bmin at 100 km and the L dependence of the B values sampled in the 1100-km polar orbit employed (see Figure 4 of Williams and Kohl [1965]).