Gelvam A. Hartmann

New Archaeological Evidence for an Early Human Presence at Monte Verde, Chile

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.

Time evolution of the South Atlantic Magnetic Anomaly

A Anomalia Magnetica do Atlântico Sul (SAMA) e uma das maiores anomalias do campo geomagnetico. A variacao secular da SAMA foi obtida e comparada com a evolucao de outrasanomalias usando modelos de campo por harmonicos esfericos para o periodo de 1590-2005. Uma analise dos dados de quatro observatorios da America do Sul mostra como esta anomalia de grande escala afetou suas medidas. Como a SAMA e uma anomalia de campo total baixo, o campo foi separado nas componentes nao-dipolar, quadrupolar e octupolar. A evolucao temporal das razoes dos campos nao-dipolar/total, quadrupolar/total e octupolar/total mostram valores elevados para o Atlântico Sul desde 1750. A evolucao da SAMA e comparada com a evolucao de outras grandes feicoes geomagneticas de superficie como os polos Norte e Sul e o Alto da Siberia, e sua comparacao mostra o equilibrio de intensidade entre estas anomalias em ambos os hemisferios. A analise dos campos nao-dipolares no periodo historico sugere que a SAMA e regida (i) pelo campo quadrupolar para a deriva, e (ii) pelos campos quadrupolar e octupolar para a intensidade e area de influencia. Alem disso, este estudo reforca a possibilidade de que a SAMA possa estar relacionada aos fluxos reversos no nucleo externo sob a regiao do Atlântico Sul.

Centennial-scale solar forcing of the South American Monsoon System recorded in stalagmites.

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.

First archeointensity results from Portuguese potteries (1550-1750 AD)

Geomagnetic field variations at archeomagnetic timescales can be obtained from well-dated heated structures and archeological potsherds. Here, we present the first archeointensity results obtained on Portuguese ceramics (1550 to 1750 AD) collected at Brazilian archeological sites. The results are compared to those obtained from Western Europe and currently available geomagnetic field models. Continuous thermomagnetic and IRM acquisitions curves indicate that Ti-poor titanomagnetite is responsible for the remanence in these ceramic fragments. Five fragments (24 samples) out of twelve analyzed yielded reliable intensity estimates. The row archeointensity data were corrected for TRM anisotropy and cooling rate effect. The mean dipole moments are obtained for three different age intervals: 1550±30 AD, 1600±30 AD and 1750±50 AD. Mean intensities vary from 37.9±4.2 μT to 54.8±7.6 μT in agreement with the previously reported data for 1550 AD and 1750 AD. Relatively weaker, but still highly dispersed, values were obtained for 1600 AD ceramics.

The time dependence of reversed archeomagnetic flux patches

Archeomagnetic field models may provide important insights to the geodynamo. Here we investigate the existence and mobility of reversed flux patches (RFPs) in an archeomagnetic field model. We introduce topological algorithms to define, identify, and track RFPs. In addition, we explore the relations between RFPs and dipole changes and apply robustness tests to the RFPs. In contrast to previous definitions, patches that reside on the geographic equator are adequately identified based on our RFPs definition. Most RFPs exhibit a westward drift and migrate toward higher latitudes. Undulations of the magnetic equator and RFPs oppose the axial dipole moment (ADM). Filtered models show a tracking behavior similar to the nonfiltered model, and surprisingly new RFPs occasionally emerge. The advection and diffusion of RFPs have worked in unison to yield the decrease of the ADM at recent times. The absence of RFPs in the period 550–1440 A.D. is related to a low in intermediate degrees of the geomagnetic power spectrum. We thus hypothesize that the RFPs are strongly dependent on intermediate spherical harmonic degrees 4 and above.

Paleointensity data from Early Cretaceous Ponta Grossa dikes (Brazil) using a multisample method

Definition of the long-term variation of the geomagnetic virtual dipole moment requires more reliable paleointensity results. Here, we applied a multisample protocol to the study of the 130.5 Ma Ponta Grossa basaltic dikes (southern Brazil) that carry a very stable dual-polarity magnetic component. The magnetic stability of the samples was checked using thermomagnetic curves and by monitoring the magnetic susceptibility evolution through the paleointensity experiments. Twelve sites containing the least alterable samples were chosen for the paleointensity measurements. Although these rocks failed stepwise double-heating experiments, they yielded coherent results in the multisample method for all sites but one. The coherent sites show low to moderate field intensities between 5.7±0.2 and 26.4±0.7 μT (average 13.4±1.9 μT). Virtual dipole moments for these sites range from 1.3±0.04 to 6.0±0.2 × 1022 A m2 (average 2.9±0.5 × 1022 A m2). Our results agree with the tendency for low dipole moments during the Early Cretaceous, immediately prior to the Cretaceous Normal Superchron (CNS). The available paleointensity database shows a strong variability of the field between 80 and 160 Ma. There seems to be no firm evidence for a Mesozoic Dipole Low, but a long-term tendency does emerge from the data with the highest dipole moments occurring at the middle of the CNS.

The Fate of Carbon in Sediments of the Xingu and Tapajós Clearwater Rivers, Eastern Amazon

The Xingu and Tapajos rivers in the eastern Amazon are the largest clearwater systems of the Amazon basin. Both rivers have “fluvial rias” (i.e., lake-like channels) in their downstream reaches as they are naturally impounded by the Amazon mainstem. Fluvial rias are widespread in the Amazon landscape and most of the sedimentary load from the major clearwater and blackwater rivers is deposited in these channels. So far, little is known about the role of Amazon rias as a trap and reactor for organic sediments. In this study, we used organic and inorganic geochemistry, magnetic susceptibility, diatom, and pollen analyses in sediments (suspended, riverbed, and downcore) of the Xingu and Tapajos rias to investigate the effects of hydrologic variations on the carbon budget in these clearwater rivers over the Holocene. Ages of sediment deposition (~100 to 5,500 years) were constrained by optically stimulated luminescence and radiocarbon. Major elements geochemistry and concentration of total organic carbon (TOC) indicate that seasonal hydrologic variations exert a strong influence on riverine productivity and on the input and preservation of organic matter in sediments. Stable carbon isotope data (δ13C from -31.04 to -27.49‰) and pollen analysis indicate that most of the carbon buried in rias is derived from forests. In the Xingu River, diatom analysis in bottom sediments revealed 65 infrageneric taxa that are mostly well-adapted to slack oligotrophic and acidic waters. TOC values in sediment cores are similar to values measured in riverbed sediments and indicate suitable conditions for organic matter preservation in sediments of the Xingu and Tapajos rias at least since the mid-Holocene, with carbon burial rates varying from about 84 g m-2 yr-1 to 169 g m-2 yr-1. However, redox-sensitive elements in sediment core indicate alternation between anoxic/dysoxic and oxic conditions in the water-sediment interface that may be linked to abrupt changes in precipitation. The variation between anoxic/dysoxic and oxic conditions in the water-sediment interface controls organic matter mineralization and methanogenesis. Thus, such changes promoted by hydrological variations significantly affect the capacity of Amazon rias to act either as sources or sinks of carbon.

Correction: New Archaeological Evidence for an Early Human Presence at Monte Verde, Chile

The images for Figs ​Figs77 and ​and88 have been incorrectly swapped. Please view the correct Figs ​Figs77 and ​and88 here. Fig 7 Serpentine pebble tool from Unit 17, MV-I, showing bifacially knapped and retouched edge. Serpentine is a raw material available in the coastal cordillera west of Monte Verde. Fig 8 Basalt wedge showing seven facets on obverse face (one of which is cortex) and three on the reverse face.

Possible relationship between the Earth's rotation variations and geomagnetic field reversals over the past 510 Myr

The Earth’s rotation can change as a result of several internal and external processes, each of which is at a different timescale. Here, we present some possible connections between the Earth’s rotation variations and the geomagnetic reversal frequency rates over the past 120 Myr. In addition, we show the possible relationship between the geomagnetic field reversal frequency and the δ18O oscillations. Because the latter reflects the glacial and interglacial periods, we hypothesize that it can be used as a possible indicator to explain the length of day (LOD) variations and consequently the reversal field frequency over the past 510 Myr. Therefore, our analysis suggests that the relationships between the geomagnetic reversal frequency rates and the Earth’s rotation changes during the Phanerozoic. However, more reversal data are required for periods before the KRS to strengthen the perspective of using the geomagnetic reversal data as a marker for the LOD variations through geological times.

Preliminary Results of Magnetic Archaeointensity Measurements in Brazil

Variations in intensity and direction of the Earth’s magnetic field oscillate according to time scales ranging from seconds to millions of years. Variations in time period of 102–103 years are referred to as “archaeomagnetic variations” and may be used as a dating tool due to their stochastic behavior. Archaeomagnetic dating is based on the comparison of the magnetic record of archaeological material of unknown age with master-curves for the variation of the magnetic field in a given region of the planet (e.g. Le Goff et al. 2002).