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

Archaeomagnetic directional data for southern Africa.Mean directional data (Supplementary Table 1, ref. 9) from burnt floors with 95% confidence intervals. Green arrows highlight rate of change between observations. Model path CALS3k.4 from ref. 11.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4525173&req=5

f1: Archaeomagnetic directional data for southern Africa.Mean directional data (Supplementary Table 1, ref. 9) from burnt floors with 95% confidence intervals. Green arrows highlight rate of change between observations. Model path CALS3k.4 from ref. 11.

Mentions: Alternating field and thermal demagnetization data from the burnt structures define linear trajectories towards the origin of orthogonal vector plots (Supplementary Figs 3 and 4). Directions of similar age are well grouped. We average directions from floors for which ages are indistinguishable, and compare these mean directions (Fig. 1, Supplementary Tables 1–5) with predictions from the CALS3k.4 (ref. 11) model that is based on a compilation of results from archaeomagnetic, sediment and lava samples. Considering the dearth of SH archaeomagnetic data in the CALS3k.4 database, broad similarities between the predictions and the new data are notable. In detail, however, rates of directional change differ markedly. Although only four time windows are available, the directional data show some clear trends. The observations show modest changes (0.06° per year ) from 1030 to 1225 AD, followed by a time of rapid change (0.11° to 0.12° per year) from 1225 to ∼1550 AD. For reference, the rate of field change from 1840 to present at our South African sites constrained by historical measurements is about 0.07° per year.


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)

Archaeomagnetic directional data for southern Africa.Mean directional data (Supplementary Table 1, ref. 9) from burnt floors with 95% confidence intervals. Green arrows highlight rate of change between observations. Model path CALS3k.4 from ref. 11.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Archaeomagnetic directional data for southern Africa.Mean directional data (Supplementary Table 1, ref. 9) from burnt floors with 95% confidence intervals. Green arrows highlight rate of change between observations. Model path CALS3k.4 from ref. 11.
Mentions: Alternating field and thermal demagnetization data from the burnt structures define linear trajectories towards the origin of orthogonal vector plots (Supplementary Figs 3 and 4). Directions of similar age are well grouped. We average directions from floors for which ages are indistinguishable, and compare these mean directions (Fig. 1, Supplementary Tables 1–5) with predictions from the CALS3k.4 (ref. 11) model that is based on a compilation of results from archaeomagnetic, sediment and lava samples. Considering the dearth of SH archaeomagnetic data in the CALS3k.4 database, broad similarities between the predictions and the new data are notable. In detail, however, rates of directional change differ markedly. Although only four time windows are available, the directional data show some clear trends. The observations show modest changes (0.06° per year ) from 1030 to 1225 AD, followed by a time of rapid change (0.11° to 0.12° per year) from 1225 to ∼1550 AD. For reference, the rate of field change from 1840 to present at our South African sites constrained by historical measurements is about 0.07° per year.

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