Cabral A.R.,Rhodes University |
Cabral A.R.,Clausthal University of Technology |
Lehmann B.,Clausthal University of Technology |
Tupinamba M.,State University of Rio de Janeiro |
And 2 more authors.
Journal of Geochemical Exploration
The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaço, Minas Gerais, Brazil, are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often comprise Au-Pd-Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline and titaniferous hematite (up to ~6wt.% TiO2). The latter minerals connect the alluvial mineralisation to the rutile-hematite-quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvial gravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasive B metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongs to the alkali group, with Mg/(Mg+Fe) and X/(X+Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29, respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from ~1 to -10.4% δ11B. The B-isotope range, in conjunction with the Na-Mg-rich tourmaline composition, and the widespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, under oxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggest episodic vein emplacement over a temperature range from around 500°C to ~350°C. Cross-cutting relationships and episodic vein emplacement indicate a late-Brasiliano age. © 2011 Elsevier B.V. Source
Usoskin I.G.,University of Oulu |
Kromer B.,Curt Engelhorn Center for Archaeometry |
Ludlow F.,Harvard University |
Beer J.,Eawag - Swiss Federal Institute of Aquatic Science and Technology |
And 5 more authors.
Astronomy and Astrophysics
Aims. Miyake et al. (2012, Nature, 486, 240, henceforth M12) recently reported, based on 14C data, an extreme cosmic event in about AD775. Using a simple model, M12 claimed that the event was too strong to be caused by a solar flare within the standard theory. This implied a new paradigm of either an impossibly strong solar flare or a very strong cosmic ray event of unknown origin that occurred around AD775. However, as we show, the strength of the event was significantly overestimated by M12. Several subsequent works have attempted to find a possible exotic source for such an event, including a giant cometary impact upon the Sun or a gamma-ray burst, but they are all based on incorrect estimates by M12. We revisit this event with analysis of new datasets and consistent theoretical modelling. Methods. We verified the experimental result for the AD775 cosmic ray event using independent datasets including 10Be series and newly measured 14C annual data. We surveyed available historical chronicles for astronomical observations for the period around the AD770s to identify potential sightings of aurorae borealis and supernovae. We interpreted the 14C measurements using an appropriate carbon cycle model. Results. We show that: (1) The reality of the AD775 event is confirmed by new measurements of 14C in German oak; (2) by using an inappropriate carbon cycle model, M12 strongly overestimated the event's strength; (3) the revised magnitude of the event (the global 14C production Q = (1.1 - 1.5) × 108 atoms/cm2) is consistent with different independent datasets (14C, 10Be, 36Cl) and can be associated with a strong, but not inexplicably strong, solar energetic particle event (or a sequence of events), and provides the first definite evidence for an event of this magnitude (the fluence >30 MeV was about 4.5 × 1010 cm-2) in multiple datasets; (4) this interpretation is in agreement with increased auroral activity identified in historical chronicles. Conclusions. The results point to the likely solar origin of the event, which is now identified as the greatest solar event on a multi-millennial time scale, placing a strong observational constraint on the theory of explosive energy releases on the Sun and cool stars. © 2013 ESO. Source
Manning S.W.,Cornell University |
Kromer B.,University of Heidelberg |
Kromer B.,Curt Engelhorn Center for Archaeometry
The debate over the dating of the Santorini (Thera) volcanic eruption has seen sustained efforts to criticize or challenge the radiocarbon dating of this time horizon. We consider some of the relevant areas of possible movement in the 14C dating-and, in particular, any plausible mechanisms to support as late (most recent) a date as possible. First, we report and analyze data investigating the scale of apparent possible 14C offsets (growing season related) in the Aegean-Anatolia-east Mediterranean region (excluding the southern Levant and especially pre-modern, pre-dam Egypt, which is a distinct case), and find no evidence for more than very small possible offsets from several cases. This topic is thus not an explanation for current differences in dating in the Aegean and at best provides only a few years of latitude. Second, we consider some aspects of the accuracy and precision of 14C dating with respect to the Santorini case. While the existing data appear robust, we nonetheless speculate that examination of the frequency distribution of the 14C data on short-lived samples from the volcanic destruction level at Akrotiri on Santorini (Thera) may indicate that the average value of the overall data sets is not necessarily the most appropriate 14C age to use for dating this time horizon. We note the recent paper of Soter (2011), which suggests that in such a volcanic context some (small) age increment may be possible from diffuse CO2 emissions (the effect is hypothetical at this stage and has not been observed in the field), and that "if short-lived samples from the same stratigraphic horizon yield a wide range of 14C ages, the lower values may be the least altered by old CO2." In this context, it might be argued that a substantive "low" grouping of 14C ages observable within the overall 14C data sets on short-lived samples from the Thera volcanic destruction level centered about 3326-3328 BP is perhaps more representative of the contemporary atmospheric 14C age (without any volcanic CO2 contamination). This is a subjective argument (since, in statistical terms, the existing studies using the weighted average remain valid) that looks to support as late a date as reasonable from the 14C data. The impact of employing this revised 14C age is discussed. In general, a late 17th century BC date range is found (to remain) to be most likely even if such a late-dating strategy is followed-a late 17th century BC date range is thus a robust finding from the 14C evidence even allowing for various possible variation factors. However, the possibility of a mid-16th century BC date (within ~1593- 1530 cal BC) is increased when compared against previous analyses if the Santorini data are considered in isolation. © 2012 by the Arizona Board of Regents on behalf of the University of Arizona. Source
Fehren-Schmitz L.,University of California at Santa Cruz |
Fehren-Schmitz L.,University of Gottingen |
Llamas B.,University of Adelaide |
Lindauer S.,Curt Engelhorn Center for Archaeometry |
And 16 more authors.
The discovery of human remains from the Lauricocha cave in the Central Andean highlands in the 1960's provided the first direct evidence for human presence in the high altitude Andes. The skeletons found at this site were ascribed to the Early to Middle Holocene and represented the oldest known population of Western South America, and thus were used in several studies addressing the early population history of the continent. However, later excavations at Lauricocha led to doubts regarding the antiquity of the site. Here, we provide new dating, craniometric, and genetic evidence for this iconic site. We obtained new radiocarbon dates, generated complete mitochondrial genomes and nuclear SNP data from five individuals, and re-analyzed the human remains of Lauricocha to revise the initial morphological and craniometric analysis conducted in the 1960's. We show that Lauricocha was indeed occupied in the Early to Middle Holocene but the temporal spread of dates we obtained from the human remains show that they do not qualify as a single contemporaneous population. However, the genetic results from five of the individuals fall within the spectrum of genetic diversity observed in pre-Columbian and modern Native Central American populations. © 2015 Fehren-Schmitz et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Source
Stable (206Pb, 207Pb, 208Pb) and radioactive (210Pb) lead isotopes in 1year of growth of Sphagnum moss from four ombrotrophic bogs in southern Germany: Geochemical significance and environmental implications
Shotyk W.,University of Heidelberg |
Shotyk W.,University of Alberta |
Kempter H.,University of Heidelberg |
Kempter H.,Curt Engelhorn Center for Archaeometry |
And 4 more authors.
Geochimica et Cosmochimica Acta
The surfaces of Sphagnum carpets were marked with plastic mesh and 1year later the production of plant matter was harvested in four ombrotrophic bogs from two regions of southern Germany: Upper Bavaria (Oberbayern, OB) and the Northern Black Forest (Nordschwarzwald, NBF). Radioactive, 210Pb was determined in solid samples using ultralow background gamma spectrometry while total Pb concentrations and stable isotopes (206Pb, 207Pb, 208Pb) were determined in acid digests using ICP-SMS. Up to 12 samples (40×40cm) were collected per site, and 6-10 sites investigated per bog. The greatest variations within a given sampling site were in the range 212-532Bqkg-1 for 210Pb activity, whereas 206Pb/207Pb and 208Pb/206Pb varied less than 1%. The median values of all parameters for the sites (6-10 per bog) were not significantly different. The median activities of 210Pb (Bqkg-1) in the mosses collected from the bogs in NBF (HO=372±56, n=55; WI=342±58, n=93) were slightly less from those in OB (GS=394±50, n=55; KL=425±58, n=24). However, the mosses in the NBF bogs exhibited much greater productivity (187-202gm-2a-1) compared to those of OB (71-91gm-2a-1), and this has a profound impact on the accumulation rates of 210Pb (Bqm-2a-1), with the bogs in the NBF yielding fluxes (HO=73±30; WI=65±20) which are twice those of OB (GS=29±11; KL=40±13). Using the air concentrations of 210Pb measured at Schauinsland (SIL) in the southern Black Forest and average annual precipitation, the atmospheric fluxes of 210Pb at SIL (340Bqm-2a-1) exceeds the corresponding values obtained from the mosses by a factor of five, providing the first quantitative estimate of the net retention efficiency of 210Pb by Sphagnum. When the 210Pb activities of all moss samples are combined (n=227), a significant decrease with increasing plant production rate is observed; in contrast, total Pb concentrations show the opposite trend. The contrasting behaviour of 210Pb and total Pb in the mosses may reflect differences in the particle size distribution of the corresponding aerosols, their physical and chemical properties, the extent of their interaction with plant surfaces, or some combination of these factors. The 206Pb/207Pb ratios from NBF (HO=1.159±0.002, n=19; WI=1.157±0.003, n=48) and OB (GS=1.157±0.003, n=28; KL=1.159±0.003, n=15) are uniform and indicate that both regions are impacted by Pb from predominately anthropogenic sources. Although Sphagnum moss represents an established receptor in monitoring atmospheric Pb deposition, the physical characteristics (size, morphology, composition) of the three predominant kinds of Pb-bearing aerosols considered here, namely 210Pb (adsorbed onto aerosol surfaces, following decay of 222Rn), anthropogenic Pb (sub-micron aerosols from high temperature combustion processes) and lithogenic Pb soil-derived mineral dusts (tens of microns, from chemical weathering of crustal rocks), are fundamentally different and these have consequences for the retention efficiency of the three kinds of particles. © 2015 Elsevier Ltd. Source