Questions surrounding the chronology, place, and character of the initial human colonization of the Americas are a long-standing focus of debate. Interdisciplinary debate continues over the timing of ...entry, the rapidity and direction of dispersion, the variety of human responses to diverse habitats, the criteria for evaluating the validity of early sites, and the differences and similarities between colonization in North and South America. Despite recent advances in our understanding of these issues, archaeology still faces challenges in defining interdisciplinary research problems, assessing the reliability of the data, and applying new interpretative models. As the debates and challenges continue, new studies take place and previous research reexamined. Here we discuss recent exploratory excavation at and interdisciplinary data from the Monte Verde area in Chile to further our understanding of the first peopling of the Americas. New evidence of stone artifacts, faunal remains, and burned areas suggests discrete horizons of ephemeral human activity in a sandur plain setting radiocarbon and luminescence dated between at least ~18,500 and 14,500 cal BP. Based on multiple lines of evidence, including sedimentary proxies and artifact analysis, we present the probable anthropogenic origins and wider implications of this evidence. In a non-glacial cold climate environment of the south-central Andes, which is challenging for human occupation and for the preservation of hunter-gatherer sites, these horizons provide insight into an earlier context of late Pleistocene human behavior in northern Patagonia.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
•Relating the SAA to RFPs is not straightforward.•SAA longitudes dictated by prominent geomagntic flux patches.•SAA latitudes is affected by the strength of the axial dipole and patches.•SAA path can ...be explained by frozen-flux SV scenarios.•SAA intensity decrease requires diffusion.
The South Atlantic Anomaly (SAA) is a region of weak geomagnetic field intensity at the Earth’s surface, which is commonly attributed to reversed flux patches (RFPs) on the core-mantle boundary (CMB). While the SAA is clearly affected by the reversed flux region below the South Atlantic, we show that the relation between the intensity minimum at Earth’s surface and RFPs is not straightforward. We map a field-dependent intensity kernel (Constable, 2007a) to study the relation between the radial geomagnetic field at the CMB and the field intensity at Earth’s surface. Synthetic tests highlight the role of specific patches (reversed and normal) in determining the location of the surface intensity minimum and demonstrate that the SAA can indeed be explained by a few intense patches. We show that the level of axial dipolarity of the field determines the stability of the relation between the SAA minimum and RFPs. The present position of the SAA minimum is determined by the interplay among several robust geomagnetic flux patches at the CMB. The longitude of the SAA minimum appears near the longitude of the Patagonia RFP due to the low-latitude normal flux patches (NFPs) near Africa and mid-Atlantic which diminish the effect of the Africa RFPs. The latitude of the SAA minimum is lower than the Patagonia RFP latitude due to the South Pacific high-latitude NFP and the axial dipole effect. The motion of the SAA minimum is explained by the motions and changes in intensity of these robust geomagnetic flux patches. Simple secular variation (SV) scenarios suggest that while the SAA path can be explained by advection, its intensity decrease requires magnetic diffusion. In addition these SV scenarios provide some speculative predictions for the SAA.
Investigations into long‐term geomagnetic variations provide useful information regarding paleomagnetic field behavior. In this study, we assess the latitudinal structure of paleosecular variation ...(PSV) and the time‐averaged field (TAF) for the Brunhes normal and Matuyama reverse chrons, and for the 0–10 Ma period, from an updated and reviewed paleodirectional database spanning the past 10 Myr. The new database comprises 2,543 paleomagnetic sites from igneous rocks, providing improvements in the geographic and temporal distributions of high‐quality data relative to previous compilations. In addition, the new data collection differs considerably in application of strict selection criteria. Statistical analysis of the virtual geomagnetic pole (VGP) dispersion curve of Model G reveals a low latitudinal dependence of PSV for the last 10 Myr. For this period, we present a zonal TAF model based on the latitudinal distribution of inclination anomaly data. The best estimates found for axial quadrupole and octupole components were about 3% and 1% relative to axial dipole component, respectively. The new statistical models for the Brunhes and Matuyama chrons have different patterns in both PSV and TAF, in compliance with earlier studies. Our quantitative assessments indicate an apparent hemispheric PSV asymmetry, particularly in the Brunhes chron, with a stronger latitudinal signature in the southern hemisphere compared to the north. These findings suggest that equatorial PSV asymmetry, that has previously been found in modern, historical and millennial scale geomagnetic models, has persisted over the past 0.78 Ma.
Key Points
An updated 0–10 Ma database provides improvements in the geographic and temporal distributions of high‐quality data
We propose a zonal time‐averaged field model for the past 10 Myr
Quantitative analyses suggest equatorial asymmetry of Brunhes chron paleosecular variation
The diminishing strength of the Earth’s magnetic dipole over recent millennia is accompanied by the increasing prominence of the geomagnetic South Atlantic Anomaly (SAA), which spreads over the South ...Atlantic Ocean and South America. The longevity of this feature at millennial timescales is elusive because of the scarcity of continuous geomagnetic data for the region. Here, we report a unique geomagnetic record for the last ∼1500 y that combines the data of two well-dated stalagmites from Pau d’Alho cave, located close to the present-day minimum of the anomaly in central South America. Magnetic directions and relative paleointensity data for both stalagmites are generally consistent and agree with historical data from the last 500 y. Before 1500 CE, the data adhere to the geomagnetic model ARCH3K.1, which is derived solely from archeomagnetic data. Our observations indicate rapid directional variations (>0.1°/y) from approximately 860 to 960 CE and approximately 1450 to 1750 CE. A similar pattern of rapid directional variation observed from South Africa precedes the South American record by 224 ± 50 y. These results confirm that fast geomagnetic field variations linked to the SAA are a recurrent feature in the region. We develop synthetic models of reversed magnetic flux patches at the core–mantle boundary and calculate their expression at the Earth’s surface. The models that qualitatively resemble the observational data involve westward (and southward) migration of midlatitude patches, combined with their expansion and intensification.
Archaeomagnetic field models cover longer timescales than historical models and may therefore resolve the motion of geomagnetic features on the core–mantle boundary (CMB) in a more meaningful ...statistical sense. Here we perform a detailed appraisal of archaeomagnetic field models to infer some aspects of the physics of the outer core. We characterize and compare the identification and tracking of reversed flux patches (RFPs) in order to assess the RFPs robustness. We find similar behaviour within a family of models but differences among different families, suggesting that modelling strategy is more influential than data set. Similarities involve recurrent positions of RFPs, but no preferred direction of motion is found. The tracking of normal flux patches shows similar qualitative behaviour confirming that RFPs identification and tracking is not strongly biased by their relative weakness. We also compare the tracking of RFPs with that of the historical field model gufm1 and with seismic anomalies of the lowermost mantle to explore the possibility that RFPs have preferred locations prescribed by lower mantle lateral heterogeneity. The archaeomagnetic field model that most resembles the historical field is interpreted in terms of core dynamics and core–mantle thermal interactions. This model exhibits correlation between RFPs and low seismic shear velocity in co-latitude and a shift in longitude. These results shed light on core processes, in particular we infer toroidal field lines with azimuthal orientation below the CMB and large fluid upwelling structures with a width of about 80° (Africa) and 110° (Pacific) at the top of the core. Finally, similar preferred locations of RFPs in the past 9 and 3 kyr of the same archaeomagnetic field model suggest that a 3 kyr period is sufficiently long to reliably detect mantle control on core dynamics. This allows estimating an upper bound of 220–310 km for the magnetic boundary layer thickness below the CMB.
South America contributes only a small fraction of the global archeomagnetic data. Recent work in the region has expanded significantly the previous database with new data being generated from Chile, ...Argentina, Northeast and Southeast Brazil. We report here new results from Jesuit Missions in South Brazil, at the triple border with Argentina and Paraguay. Our archeological collection comprises a total of 24 fragments of baked clay construction materials from three Jesuit Missions, São Luiz Gonzaga 1657–1687 AD (3 fragments), São João Batista 1667–1697 AD (4 fragments) and Santo Ângelo 1676–1706 AD (17 fragments). Archeointensity determinations were performed with the double-heating technique in its modified form, with pTRM checks and pTRM tail checks. Measurements were complemented by anisotropy and cooling-rate corrections. A total of 24 specimens (11 fragments) passed strict quality selection, corresponding to a success rate of 45%. We also performed an experimental test for the 6-specimen average anisotropy correction technique and we show that it does not correct for the effects of TRM anisotropy. Results were similar within error for the three missions: São Luiz Gonzaga (40.2±2.4 μT), São João Batista (39.1±1.6 μT) and Santo Ângelo (41.1±2.0 μT). These data were then compared with the most reliable data from South America, after a critical assessment of the current database. According to our analysis, only 39 intensity data for the continent can be considered as high-quality, most within the last 700 years; only three data were retained for older periods (800–1100 AD). The filtered data match reasonably well the available models for the past five centuries. A combined curve for South and Southeast Brazil plus Argentina plot systematically below relocated data from NE Brazil and Chile. These differences are likely due to complexities in the geometry of the field in South America not appropriately accounted for by a simple axial dipole. Our analysis highlights the need for high-quality data for the continent.
•Three high-quality archeointensity data are presented for South America.•The South American database is reassessed to retain only high-quality entries.•The filtered results are coherent and define a well-behaved evolution of the field.•We prove the 6-specimen average technique does not correct for anisotropy effects.
The South American Monsoon System (SAMS) is generally considered to be highly sensitive to Northern Hemisphere (NH) temperature variations on multi-centennial timescales. The direct influence of ...solar forcing on moisture convergence in global monsoon systems on the other hand, while well explored in modeling studies, has hitherto not been documented in proxy data from the SAMS region. Hence little is known about the sensitivity of the SAMS to solar forcing over the past millennium and how it might compete or constructively interfere with NH temperature variations that occurred primarily in response to volcanic forcing. Here we present a new annually-resolved oxygen isotope record from a 1500-year long stalagmite recording past changes in precipitation in the hitherto unsampled core region of the SAMS. This record details how solar variability consistently modulated the strength of the SAMS on centennial time scales during the past 1500 years. Solar forcing, besides the previously recognized influence from NH temperature changes and associated Intertropical Convergence Zone (ITCZ) shifts, appears as a major driver affecting SAMS intensity at centennial time scales.
SUMMARY
Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement ...surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolution. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti-rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites.
Time evolution of the South Atlantic Magnetic Anomaly Hartmann, Gelvam A.(Universidade de São Paulo Instituto de Astronomia, Geofísica e Ciências Atmosféricas Departamento de Geofísica); Pacca, Igor G.(Universidade de São Paulo Instituto de Astronomia, Geofísica e Ciências Atmosféricas Departamento de Geofísica)
Anais da Academia Brasileira de Ciências,
06/2009, Letnik:
81, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The South Atlantic Magnetic Anomaly (SAMA) is one of the most outstanding anomalies of the geomagnetic field. The SAMA secular variation was obtained and compared to the evolution of other anomalies ...using spherical harmonic field models for the 1590-2005 period. An analysis of data from four South American observatories shows how this large scale anomaly affected their measurements. Since SAMA is a low total field anomaly, the field was separated into its nondipolar, quadrupolar and octupolar parts. The time evolution of the non-dipole/total, quadrupolar/total and octupolar/total field ratios yielded increasingly high values for the South Atlantic since 1750. The SAMA evolution is compared to the evolution of other large scale surface geomagnetic features like the North and the South Pole and the Siberia High, and this comparison shows the intensity equilibrium between these anomalies in both hemispheres. The analysis of non-dipole fields in historical period suggests that SAMA is governed by (i) quadrupolar field for drift, and (ii) quadrupolar and octupolar fields for intensity and area of influence. Furthermore, our study reinforces the possibility that SAMA may be related to reverse fluxes in the outer core under the South Atlantic region.
A Anomalia Magnética do Atlântico Sul (SAMA) é uma das maiores anomalias do campo geomagnético. A variação secular da SAMA foi obtida e comparada com a evolução de outrasanomalias usando modelos de campo por harmônicos esféricos para o período de 1590-2005. Uma análise dos dados de quatro observatórios da América do Sul mostra como esta anomalia de grande escala afetou suas medidas. Como a SAMA é uma anomalia de campo total baixo, o campo foi separado nas componentes não-dipolar, quadrupolar e octupolar. A evolução temporal das razões dos campos não-dipolar/total, quadrupolar/total e octupolar/total mostram valores elevados para o Atlântico Sul desde 1750. A evolução da SAMA é comparada com a evolução de outras grandes feições geomagnéticas de superfície como os pólos Norte e Sul e o Alto da Sibéria, e sua comparação mostra o equilíbrio de intensidade entre estas anomalias em ambos os hemisférios. A análise dos campos não-dipolares no período histórico sugere que a SAMA é regida (i) pelo campo quadrupolar para a deriva, e (ii) pelos campos quadrupolar e octupolar para a intensidade e área de influência. Além disso, este estudo reforça a possibilidade de que a SAMA possa estar relacionada aos fluxos reversos no núcleo externo sob a região do Atlântico Sul.
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