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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

Field strength versus time for southern Africa.(a) Palaeointensity based on multispecimen technique12 data (Supplementary Table 6) applied to ceramics. Uncertainties are from jack-knife-1 analyses (1σ). (b) Palaeointensity based on Thellier–Coe17 and Shaw18 palaeointensity data meeting selection criteria from ceramics and burnt floors (Supplementary Tables 7 and 8, ref. 9). Palaeointensity means (large circles) shown with 1σ uncertainties. Model prediction CALS3k.4 from ref. 11 and from gufm1 from ref. 15.
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f2: Field strength versus time for southern Africa.(a) Palaeointensity based on multispecimen technique12 data (Supplementary Table 6) applied to ceramics. Uncertainties are from jack-knife-1 analyses (1σ). (b) Palaeointensity based on Thellier–Coe17 and Shaw18 palaeointensity data meeting selection criteria from ceramics and burnt floors (Supplementary Tables 7 and 8, ref. 9). Palaeointensity means (large circles) shown with 1σ uncertainties. Model prediction CALS3k.4 from ref. 11 and from gufm1 from ref. 15.

Mentions: To further investigate the nature of the field, we first consider the coeval record of Iron Age ceramics. We conducted a survey of palaeointensity using the multispecimen technique described in ref. 12. These data (Supplementary Table 6, Supplementary Fig. 5) define a sharp decrease in field strength after 1250 AD. Lowest values in our data are recorded at ca. 1370 AD, and are followed by an increase in field strength thereafter (Fig. 2). Models predict a dip in palaeointensity 100 to 200 years after that seen in the available data. However, the closest Southern Hemisphere (SH) palaeointensity observation used in the model is thousands of kilometres from southern Africa. In general, the models better match predictions in the North Hemisphere; however, they still fail to capture some centennial-scale features13. Overall, the spatial and temporal resolution of the model prediction for southern Africa is crude relative to that of the new data (see Supplementary Methods).


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)

Field strength versus time for southern Africa.(a) Palaeointensity based on multispecimen technique12 data (Supplementary Table 6) applied to ceramics. Uncertainties are from jack-knife-1 analyses (1σ). (b) Palaeointensity based on Thellier–Coe17 and Shaw18 palaeointensity data meeting selection criteria from ceramics and burnt floors (Supplementary Tables 7 and 8, ref. 9). Palaeointensity means (large circles) shown with 1σ uncertainties. Model prediction CALS3k.4 from ref. 11 and from gufm1 from ref. 15.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Field strength versus time for southern Africa.(a) Palaeointensity based on multispecimen technique12 data (Supplementary Table 6) applied to ceramics. Uncertainties are from jack-knife-1 analyses (1σ). (b) Palaeointensity based on Thellier–Coe17 and Shaw18 palaeointensity data meeting selection criteria from ceramics and burnt floors (Supplementary Tables 7 and 8, ref. 9). Palaeointensity means (large circles) shown with 1σ uncertainties. Model prediction CALS3k.4 from ref. 11 and from gufm1 from ref. 15.
Mentions: To further investigate the nature of the field, we first consider the coeval record of Iron Age ceramics. We conducted a survey of palaeointensity using the multispecimen technique described in ref. 12. These data (Supplementary Table 6, Supplementary Fig. 5) define a sharp decrease in field strength after 1250 AD. Lowest values in our data are recorded at ca. 1370 AD, and are followed by an increase in field strength thereafter (Fig. 2). Models predict a dip in palaeointensity 100 to 200 years after that seen in the available data. However, the closest Southern Hemisphere (SH) palaeointensity observation used in the model is thousands of kilometres from southern Africa. In general, the models better match predictions in the North Hemisphere; however, they still fail to capture some centennial-scale features13. Overall, the spatial and temporal resolution of the model prediction for southern Africa is crude relative to that of the new data (see Supplementary Methods).

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