Limits...
Antiquity of the South Atlantic Anomaly and evidence for top-down control on the geodynamo.

Tarduno JA, Watkeys MK, Huffman TN, Cottrell RD, Blackman EG, Wendt A, Scribner CA, Wagner CL - Nat Commun (2015)

Bottom Line: Directions change relatively rapidly at ca. 1300 AD, whereas intensities drop sharply, at a rate greater than modern field changes in southern Africa, and to lower values.We propose that the recurrence of low field strengths reflects core flux expulsion promoted by the unusual core-mantle boundary (CMB) composition and structure beneath southern Africa defined by the African large low shear velocity province (LLSVP).Because the African LLSVP and CMB structure are ancient, this region may have been a steady site for flux expulsion, and triggering of geomagnetic reversals, for millions of years.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA. [2] Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA. [3] School of Geological Sciences, University of KwaZulu-Natal, Durban 4000, South Africa.

ABSTRACT
The dramatic decay of dipole geomagnetic field intensity during the last 160 years coincides with changes in Southern Hemisphere (SH) field morphology and has motivated speculation of an impending reversal. Understanding these changes, however, has been limited by the lack of longer-term SH observations. Here we report the first archaeomagnetic curve from southern Africa (ca. 1000-1600 AD). Directions change relatively rapidly at ca. 1300 AD, whereas intensities drop sharply, at a rate greater than modern field changes in southern Africa, and to lower values. We propose that the recurrence of low field strengths reflects core flux expulsion promoted by the unusual core-mantle boundary (CMB) composition and structure beneath southern Africa defined by the African large low shear velocity province (LLSVP). Because the African LLSVP and CMB structure are ancient, this region may have been a steady site for flux expulsion, and triggering of geomagnetic reversals, for millions of years.

No MeSH data available.


Related in: MedlinePlus

Vote map of cluster analysis of shear wave profiles.(1,000–2,800 km depth). Map from ref. 31 shown with the edge of African LLSVP at CMB from refs 27, 28. The cluster analyses evaluate five global tomographic models3233343536, with the colour-coded voting map representing the number of models, which assign a lower than average shear wave velocity to the pixel. The average spacing of the pixels is 4°. The voting map highlights the consistency between the global models in defining the African LLSVP, as well as the similar, but more spatially complex, Pacific LLSVP. Because of the inherent averaging in the cluster analyses, the voting map should not be used to comment on the sharpness of the African anomaly, which is better constrained by regional dense data coverage such as that used in refs 27, 28.
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f4: Vote map of cluster analysis of shear wave profiles.(1,000–2,800 km depth). Map from ref. 31 shown with the edge of African LLSVP at CMB from refs 27, 28. The cluster analyses evaluate five global tomographic models3233343536, with the colour-coded voting map representing the number of models, which assign a lower than average shear wave velocity to the pixel. The average spacing of the pixels is 4°. The voting map highlights the consistency between the global models in defining the African LLSVP, as well as the similar, but more spatially complex, Pacific LLSVP. Because of the inherent averaging in the cluster analyses, the voting map should not be used to comment on the sharpness of the African anomaly, which is better constrained by regional dense data coverage such as that used in refs 27, 28.

Mentions: The CMB beneath South Africa today is characterized by a low seismic wave anomaly26272829 called the African large low shear velocity province (LLSVP)30 (Fig. 3), which is a robust feature of different tomographic models313233343536 (Fig. 4). There also appears to be a growing consensus that the African LLSVP is bounded by unusual steep sides262728293738. Here we note that the steep-sided border of the anomaly falls close to the prominent reversed flux patch beneath South Africa that has been linked to the SAA (arrow, Fig. 3). The sharp structural gradients that the core flow encounters can be expected to stimulate the formation of small-scale structures (or vortices) in the flow. We propose that core flow into the region near the LLSVP will develop an upward component on such small scales and that RM is sufficiently reduced to allow reversed polarity flux bundles to leak upward.


Antiquity of the South Atlantic Anomaly and evidence for top-down control on the geodynamo.

Tarduno JA, Watkeys MK, Huffman TN, Cottrell RD, Blackman EG, Wendt A, Scribner CA, Wagner CL - Nat Commun (2015)

Vote map of cluster analysis of shear wave profiles.(1,000–2,800 km depth). Map from ref. 31 shown with the edge of African LLSVP at CMB from refs 27, 28. The cluster analyses evaluate five global tomographic models3233343536, with the colour-coded voting map representing the number of models, which assign a lower than average shear wave velocity to the pixel. The average spacing of the pixels is 4°. The voting map highlights the consistency between the global models in defining the African LLSVP, as well as the similar, but more spatially complex, Pacific LLSVP. Because of the inherent averaging in the cluster analyses, the voting map should not be used to comment on the sharpness of the African anomaly, which is better constrained by regional dense data coverage such as that used in refs 27, 28.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525173&req=5

f4: Vote map of cluster analysis of shear wave profiles.(1,000–2,800 km depth). Map from ref. 31 shown with the edge of African LLSVP at CMB from refs 27, 28. The cluster analyses evaluate five global tomographic models3233343536, with the colour-coded voting map representing the number of models, which assign a lower than average shear wave velocity to the pixel. The average spacing of the pixels is 4°. The voting map highlights the consistency between the global models in defining the African LLSVP, as well as the similar, but more spatially complex, Pacific LLSVP. Because of the inherent averaging in the cluster analyses, the voting map should not be used to comment on the sharpness of the African anomaly, which is better constrained by regional dense data coverage such as that used in refs 27, 28.
Mentions: The CMB beneath South Africa today is characterized by a low seismic wave anomaly26272829 called the African large low shear velocity province (LLSVP)30 (Fig. 3), which is a robust feature of different tomographic models313233343536 (Fig. 4). There also appears to be a growing consensus that the African LLSVP is bounded by unusual steep sides262728293738. Here we note that the steep-sided border of the anomaly falls close to the prominent reversed flux patch beneath South Africa that has been linked to the SAA (arrow, Fig. 3). The sharp structural gradients that the core flow encounters can be expected to stimulate the formation of small-scale structures (or vortices) in the flow. We propose that core flow into the region near the LLSVP will develop an upward component on such small scales and that RM is sufficiently reduced to allow reversed polarity flux bundles to leak upward.

Bottom Line: Directions change relatively rapidly at ca. 1300 AD, whereas intensities drop sharply, at a rate greater than modern field changes in southern Africa, and to lower values.We propose that the recurrence of low field strengths reflects core flux expulsion promoted by the unusual core-mantle boundary (CMB) composition and structure beneath southern Africa defined by the African large low shear velocity province (LLSVP).Because the African LLSVP and CMB structure are ancient, this region may have been a steady site for flux expulsion, and triggering of geomagnetic reversals, for millions of years.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA. [2] Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA. [3] School of Geological Sciences, University of KwaZulu-Natal, Durban 4000, South Africa.

ABSTRACT
The dramatic decay of dipole geomagnetic field intensity during the last 160 years coincides with changes in Southern Hemisphere (SH) field morphology and has motivated speculation of an impending reversal. Understanding these changes, however, has been limited by the lack of longer-term SH observations. Here we report the first archaeomagnetic curve from southern Africa (ca. 1000-1600 AD). Directions change relatively rapidly at ca. 1300 AD, whereas intensities drop sharply, at a rate greater than modern field changes in southern Africa, and to lower values. We propose that the recurrence of low field strengths reflects core flux expulsion promoted by the unusual core-mantle boundary (CMB) composition and structure beneath southern Africa defined by the African large low shear velocity province (LLSVP). Because the African LLSVP and CMB structure are ancient, this region may have been a steady site for flux expulsion, and triggering of geomagnetic reversals, for millions of years.

No MeSH data available.


Related in: MedlinePlus