Content uploaded by A. E. Sheffield
Author content
All content in this area was uploaded by A. E. Sheffield on Aug 27, 2014
Content may be subject to copyright.
A preview of the PDF is not available
Microchemical and Molecular Dating
Abstract and Figures
Important knowledge has been gained through "direct' (sequential) and "indirect' (parallel) links between microchemistry and ¹⁴C measurement. The former is illustrated by ¹⁴C measurements on specific amino acids and on the polycyclic aromatic hydrocarbon class of compounds. Isolation of the respective molecular fractions from far greater quantities of extraneous carbon held the key to valid dating and source apportionment respectively. Parallel data on ¹⁴C and molecular patterns promises new knowledge about the identity of sources of environmental carbon at the nanogram level through multivariate techniques such as principal component analysis and multiple linear regression. -from Authors
Figures - uploaded by A. E. Sheffield
Author content
All figure content in this area was uploaded by A. E. Sheffield
Content may be subject to copyright.
... For a subset of samples 14 C speciation was applied, showing the elemental carbon fraction to be generally more fossil in character (41%) than the total extracted organic matter (18%) (midranges, Albuquerque; Klouda et al. 1988). The combination of 14 C and mutagenicity testing on the extracted organic matter showed that, on average, the concentration of carbonaceous aerosol from woodburning (16 µg C m −3 ) was approximately twice that from motor vehicles, but that the mutagenic potency of the latter (3.4 rev mg −1 aerosol) was greater by about a factor of 4 (Currie et al. 1989, and references therein). Using PM10 particulate matter samples collected in the Boise phase of the IACP, Benner and coworkers demonstrated the ability of dimethylphenanthrene isomers to distinguish residential wood combustion from mobile source emissions; excellent correlation was obtained with 14 C data derived from the same field samples (Benner et al. 1995). ...
... A comparative study of the fundamental limiting factors for AMS and low-level β counting for environmental 14 C research showed that: 1) for AMS, the isotopic-chemical blank constitutes the most important limitation, outweighing the machine background by 1 to 2 orders of magnitude, but 2) for β counting, the situation is reversed, with the typical blank being quite negligible compared to the counter background (Currie et al. 1989). (See Figure 2, where limits are expressed in terms of modern carbon equivalent mass.) ...
... Other laboratories, utilizing "graphite" targets, have been generally successful in measuring samples containing less than 100 µg carbon, but 20 µg is the typical lower limit (Vogel et al. 1987(Vogel et al. , 1989Pearson et al. 1998). More recent efforts to overcome the 10 µg barrier using "dilution AMS" with on-line purity monitoring appear to be successful for pre-combustion samples containing as little as 1-2 µg carbon ). Figure 2 For aerosol research the minimum amount of modern carbon required for AMS measurement is about 3 orders of magnitude smaller than that needed for miniature counter decay counting (llc) (Currie et al. 1989). The switch in the fundamental limiting factor, from detector background to the isotopic-chemical blank, has a profound effect. ...
A review is given of some critical events in the development of radiocarbon aerosol science, and the profound influence of radiocarbon accelerator mass spectrometry (AMS) on its current application and future prospects. The birth of this discipline occured shortly after the initial development of 14C dating. Unlike dating, which is founded on the continual decay of 14C and the resulting full range of 14C/12C ratios in once-living matter, 14C applications to atmospheric aerosol research relate primarily to the determination of mixing ratios of fossil and biomass components. Such determinations have come to have major importance in work ranging from the resolution of woodburning and motor vehicle components of urban particulate pollution, to the apportionment of radiation-forcing (black) particulate carbon from natural wildfires and anthropogenic regional plumes. The development of this area has paralleled that of AMS itself, with the one or the other alternately serving as the driving force, in a sort of counterpoint. The remarkable million-fold improvement in sensitivity made possible by AMS has become critical in meeting rapidly emerging societal concerns with the origins and effects of individual carbonaceous species on health and climate.
... Samples were analyzed for bulk 14 C using conventional graphite preparation at the laboratory of ion beam physics, ETH Zürich. CSRA of BPCAs was performed in triplicate, with resulting data used to apportion fossil (F 14 C = 0) and modern (F 14 C~1) sources of combustion products according to Currie et al. (1989). To account for different possible modern sources we used two end members: F 14 C contemporary, which is the year of sampling (annual production) and F 14 C biomass integrating 30 years of biomass growth (perennial production) similar to Xu et al. (2012). ...
... Combustion-derived carbon can be apportioned to fossil and contemporary sources by F 14 C measurements Currie et al. 1989;Szidat et al. 2006). ...
Fire is a combustion process and as such has been an integral driver of the natural biogeochemical cycles on our planet Earth since its first occurrence about four hundred million years ago. Humans have interacted with fire for thousands of years, yet industrialization and with it the accompanied utilization of fossil fuels marked the beginning of a new epoch, the Anthropocene. Combustion processes are incomplete and cause the formation of pyrogenic carbon (PyC). The term describes a continuum of condensed aromatic structures originating from biomass burning and fossil fuel combustion. They are ubiquitous in the environment.
The scope of this thesis was to reconstruct combustion history from environmental archives foremost aquatic sediments of the northeastern United States (U.S.) from the preindustrial era and throughout industrialization until the early 2000s. A sedimentary sequence in high temporal resolution facilitated the development and the comparison of quantitative PyC records in high temporal resolution. The results showed that sediments chronicle different modes of PyC production: condensation products and residuals of pyrolysis and their temporal trends are clearly decoupled. To decipher the origins of combustion products, isotopic records
of radiocarbon (14C) measurements facilitated source apportionment to either biomass burning or fossil fuel combustion. A comprehensive error analysis was carried out for the specific markers of combustion residues, benzene polycarboxylic acids (BPCAs), aiming at yielding robust molecular 14C data. Triplicate measurements of BPCAs 14C on four standard reference materials provided new benchmark 14C values on these environmental samples. Temporal 14C records for BPCAs and the precursors of combustion condensates, polycyclic aromatic
hydrocarbons (PAHs), allowed for unravelling the origins and the transport trajectory of sedimentary PyC from a sub-urban and a remote catchment in the U.S. The majority of PyC stems from local sources and is supplied to ‘ultimate’ sites of burial on different time scales: decadal and millennial. Whereas a small portion stems from contemporary sources, the majority of PyC entails a time lag because it was retained in catchment soils and underwent temporary storage before deposition. Additionally, BPCAs (soot) and PAHs (precursors of soot) trace
fossil fuel-derived PyC and both coincide with historical records on the consumption of fossil fuels in U.S. yet never account for more than 19 % of total PyC.
In this thesis it could be shown that sedimentary profiles chronicle combustion history on local and regional scale and it is necessary to use complementary approaches coupled with qualitative measures such as 14C to unambiguously allocate sources of PyC in the Anthropocene. With the onset of industrialization, long-range atmospheric transport and deposition started to supply fossil fuel-derived PyC to sub-urban and remote (pristine) locations. The temporal resolution of sedimentary sequences thus facilitate the reconstruction of past combustion practices to elucidate the human impact on natural biogeochemical cycles in the modern environment.
... Samples were analyzed for bulk 14 C using conventional graphite preparation at the laboratory of ion beam physics, ETH Zürich. CSRA of BPCAs was performed in triplicate, with resulting data used to apportion fossil (F 14 C = 0) and modern (F 14 C~1) sources of combustion products according to Currie et al. (1989). To account for different possible modern sources we used two end members: F 14 C contemporary, which is the year of sampling (annual production) and F 14 C biomass integrating 30 years of biomass growth (perennial production) similar to Xu et al. (2012). ...
... Combustion-derived carbon can be apportioned to fossil and contemporary sources by F 14 C measurements (Currie and Murphy 1977;Currie et al. 1989;Currie 2000;Szidat et al. 2006). In reference to the growth phase of biomass (May to August), we calculated the mean atmospheric F 14 C using the extended IntCal13 14 CO 2 concentration of the Northern Hemisphere (Levin and Kromer 2004;Levin et al. 2013). ...
Compound-specific radiocarbon analysis (CSRA) of benzene polycarboxylic acids (BPCAs) yields molecular-level, source-specific information necessary to constrain isotopic signatures of pyrogenic carbon. However, the purification of individual BPCAs requires a multistep procedure that typically results in only microgram quantities of the target analyte(s). Such small samples are highly susceptible to contamination by extraneous carbon, which needs to be minimized and carefully accounted for in order to yield accurate results. Here, we undertook comprehensive characterization and quantification of contamination associated with molecular radiocarbon ( ¹⁴ C) BPCA analyses through systematic processing of multiple authentic standards with both fossil and modern ¹⁴ C signatures at various concentrations. Using this approach, we precisely apportion the contribution of extraneous carbon with respect to the four implemented subprocedures. Assuming a constant source and quantity of extraneous carbon we correct and statistically evaluate uncertainties in resulting ¹⁴ C data. Subsequently, we examine the results of triplicate analyses of reference materials representing four different environmental matrices (sediment, soil, aerosol, riverine natural organic matter) and apportion their BPCA sources in terms of carbon residues derived from biomass or fossil fuel combustion. This comprehensive approach to CSRA facilitates retrieval of robust ¹⁴ C data, with application in environmental studies of the continuum of pyrogenic carbon.
... All data are openly provided in spreadsheets in the Supplementary Information section of the Hua et al. study. The symbols F 14 C and Δ 14 C denote the so-called "fraction modern" and "the per mil difference of the normalized sample/modern-carbon ratio from unity," respectively, and are defined in [76][77][78]. As we consistently refer to atmospheric CO2, and since both quantities express ratios, the symbols Δ 14 C and Δ 14 CO2 are used here interchangeably (and likewise for F 14 C). ...
Reservoir routing has been a routine procedure in hydrology, hydraulics and water management. It is typically based on the mass balance (continuity equation) and a conceptual equation relating storage and outflow. If the latter is linear, then there exists an analytical solution of the resulting differential equation, which can directly be utilized to find the outflow from known inflow and to obtain macroscopic characteristics of the process, such as response and residence times, and their distribution functions. Here we refine the reservoir routing framework and extend it to find approximate solutions for nonlinear cases. The proposed framework can also be useful for climatic tasks, such as describing the mass balance of atmospheric carbon dioxide and determining characteristic residence times, which have been an issue of controversy. Application of the theoretical framework results in excellent agreement with real-world data. In this manner, we easily quantify the atmospheric carbon exchanges and obtain reliable and intuitive results, without the need to resort to complex climate models. The mean residence time of atmospheric carbon dioxide turns out to be about four years, and the response time is smaller than that, thus opposing the much longer mainstream estimates.
... ábra). A módszert számos kutatásban alkalmazták sikeresen, és segítségével az aeroszol összes széntartalmának, illetve al-frakcióinak pontosabb forrásbeazonosítása vált lehetővé (Clayton et al., 1955;Currie et al., 1989;2000;Szidat et al., 2004Szidat et al., , 2009). ...
Based on the results of climate investigations, the various types of carbonaceous material present in the atmosphere have a significant influence on the local-global changes of climate. The rising concentration of atmospheric CO2 is getting larger scientific, social and economic attention due to its role in global climate change. Investigations of atmospheric aerosol are becoming increasingly significant due to the serious haze events forming around large cities. Carbonaceous aerosol particles are dominant constituents of the atmosphere which also influence climate change and the quality of the air, moreover, have an adverse effect on the human health. The radiocarbon-based (14C) investigation of atmospheric CO2 gas and fine-fraction carbonaceous aerosol can help in identifying local and regional contributors and, moreover, support understanding the transport processes.
In the course of our research, we initiated the quantitative investigations of the fossil and modern fractions of atmospheric CO2 gas and PM2.5 carbonaceous aerosol. The primary aim of our investigation was the development of sample preparation systems and methods by which the collected atmospheric CO2 and carbonaceous aerosol samples can be converted to a form suitable for carbon isotopic measurements. Using these methods, our other goal was to estimate the contribution of fossil and modern fractions being present in the CO2 gas collected in the urban environment of Debrecen and at two levels at a background site (115 m and 10 m, Hegyhátsál) and to evaluate their short-term seasonality and long-term trends. We had the same purposes in case of the atmospheric carbonaceous aerosol samples collected in Debrecen.
... In aerosol science, the fraction of modern (fM) is widely used. As underlined by Eriksson Stenström et al. (2011), it is not always clear if fM has been corrected for decay since 1950 as in Currie et al. (1989). To avoid any confusion in our paper, all measurements will be expressed in F 14 C as defined by Reimer et al. (2004). ...
Atmospheric particulate matter (PM) affects the climate in various ways and has a negative impact on human health. In populated mountain valleys from Alpine regions, emissions from road traffic contribute to carbonaceous aerosols, but residential wood burning can be another source of PM during the winter. We determine the contribution of fossil and non-fossil carbon sources by measuring radiocarbon in aerosols using the recently installed AixMICADAS facility. The accelerator mass spectrometer is coupled to an elemental analyzer (EA) by means of a gas interface system directly connected to the gas ion source. This system provides rapid and accurate radiocarbon measurements for small samples (10–100 µgC) with minimal preparation from the aerosol filters. We show how the contamination induced by the EA protocol can be quantified and corrected for. Several standards and synthetic samples are then used to demonstrate the precision and accuracy of aerosol measurements over the full range of expected 14C/12C ratios ranging from modern carbon to fossil carbon depleted in 14C. Aerosols sampled in Chamonix and Passy (Arve Valley, French Alps) from November 2013 to August 2014 are analyzed for both radiocarbon (124 analyses in total) and levoglucosan, which is commonly used as a specific tracer for biomass burning. NOx concentration, which is expected to be associated with traffic emissions, is also monitored. Based on 14C measurements, we can show that the relative fraction of non-fossil carbon is significantly higher in winter than in summer. In winter, non-fossil carbon represents about 85 % of total carbon, while in summer this proportion is still 75 % considering all samples. The largest total carbon and levoglucosan concentrations are observed for winter aerosols with values up to 50 and 8 µg m−3, respectively. These levels are higher than those observed in many European cities, but are close to those for other polluted Alpine valleys. The non-fossil carbon concentrations are strongly correlated with the levoglucosan concentrations in winter samples, suggesting that almost all of the non-fossil carbon originates from wood combustion used for heating during winter. For summer samples, the joint use of 14C and levoglucosan measurements leads to a new model to quantify separately the contributions of biomass burning and biogenic emissions in the non-fossil fraction. The comparison of the biogenic fraction with polyols (a proxy for primary soil biogenic emissions) and with the temperature suggests a major influence of the secondary biogenic aerosols. Significant correlations are found between the NOx concentration and the fossil carbon concentration for all seasons and sites, confirming the relation between road traffic emissions and fossil carbon. Overall this dual approach combining radiocarbon and levoglucosan analyses strengthens the conclusion concerning the impact of biomass burning. Combining these geochemical data both serves to detect and quantify additional carbon sources. The Arve Valley provides a first illustration of this model to aerosols.
... Since all of the total 14 C content of fossil fuels has already decayed, their f M value is 0. However, current biological aerosols are formed by recent biological sources; thus, their f M values are equal to that of the atmosphere and the current biological materials (f M ~ 1). As the majority of the currently combusted firewood was growing during the period of the 14 C-bomb peak caused by the nuclear weapon tests (between 1960-1990) (Currie et al. 1989), the aerosol particles containing carbon from wood combustion have a 14 C activity slightly higher than that of the present atmosphere. According to Heal et al. (2011), the 14 C level of atmospheric carbonaceoius aerosol is on average 1.08 times (weighted average of 1.05 and 1.15 after Szidat et al. 2006Szidat et al. , 2009 as high as that before the bomb effect, which was taken into correction. ...
Radiocarbon investigation of atmospheric PM2.5 aerosol synchronized with 14CO2 observations began in Debrecen in the winter of 2010. The aim of the study was to determine the contemporary and fossil carbon fractions in the aerosol and to set them against the fossil CO2 excess data referring to the same period. The mass of the collected PM2.5 mode on prebaked quartz filters was determined gravimetrically, while its total carbon mass was calculated from the pressure of CO2 gas produced after the combustion of the filters. As a result of the applied sampling and preparation method, the stable, nonvolatile carbon forms were principally studied. 14C measurements of the tiny aerosol bulk samples were performed using the EnvironMICADAS accelerator mass spectrometer at ATOMKI. The sample preparation method was tested using several blanks, standards, and real samples. Test results showed good reproducibility for the applied aerosol sample preparation and accelerator mass spectrometry (AMS) 14C analyses. Atmospheric fossil CO2 excess data were calculated according Levin et al. (2003), using the 14C results of collected CO2 samples measured by the gas proportional counting system at ATOMKI. Mass concentration of PM10 involving the PM2.5 mode in the city air exceeded the daily average of 50 μg/m3 (24-hr limit value in the EU) several times in 2011, mainly during the winter. The results showed that recently derived carbon most likely from domestic wood burning was causing the elevated carbon mass concentration of PM2.5 in Debrecen at the time. In the course of the 1-yr-long continuous and systematic comparison of fossil carbon mass concentration of PM2.5 mode and mole fraction of fossil excess of atmospheric CO2, similar and synchronous trends were observed during the studied period in Debrecen. © 2015 by the Arizona Board of Regents on behalf of the University of Arizona.
... EC is basically equivalent to black carbon (BC) denoting optically absorptive carbonaceous substances (Bond et al., 2013). The 14 C analysis enables a distinction between fossil fuel and non-fossil components and is widely used in source apportionment studies (Currie et al., 1989;Currie et al., 2002;Gelencsér et al., 2007;Yttri et al., 2011;Lemire et al., 2002;Szidat et al., 2004a;Szidat et al., 2004b;Szidat et al., 2006;Szidat et al., 2009). Ceburnis et al. (2011) demonstrated that the dual carbon isotope analysis method had the ability to discern contribution of three principal sources to atmospheric aerosols: marine, continental non fossil and fossil fuel. ...
Abstract Carbonaceous aerosol sources were investigated by measuring the stable carbon isotope ratio (δ13CTC) in size segregated aerosol particles. The samples were collected with a micro-orifice uniform deposit impactor (MOUDI) in 11 size intervals ranging from 0.056 μm to 18 μm. The aerosol particle size distribution obtained from combined measurements with a scanning mobility particle sizer (SMPS; TSI 3936) and an aerosol particle sizer (APS; TSI 3321) is presented for comparison with MOUDI data. The analysis of δ13CTC values revealed that the total carbonaceous matter in size segregated aerosol particles significantly varied from -23.4 ± 0.1 ‰ in a coarse mode to -30.1 ± 0.5 ‰ in a fine mode. A wide range of the δ13CTC values of size segregated aerosol particles suggested various sources of aerosol particles contributing to carbonaceous particulate matter. Therefore, the source mixing equation was applied to verify the idea of mixing of two sources: continental non-fossil and fossil fuel combustion. The obtained δ13CTC value of aerosol particles originating from fossil fuel combustion was -28.0 — -28.1 ‰, while the non-fossil source δ13CTC value was in the range of -25.0 — -25.5 ‰. The two source mixing model applied to the size segregated samples revealed that the fossil fuel combustion source contributed from 100 % to 60 % to the carbonaceous particulate matter in the fine mode range (Dp < 1 μm). Meanwhile the second, continental non-fossil, source was the main contributor in the coarse fraction (Dp > 2 μm). The particle range from 0.5 to 2.0 μm was identified as a transition region where two sources almost equally contributed to carbonaceous particulate matter. The proposed mixing model offers an alternative method for determining major carbonaceous matter sources where radiocarbon analysis may lack the sensitivity (as in size segregated samples).
Atmospheric particulate matter (PM) affects the climate in various ways and has a negative impact on human health. In populated mountain valleys in Alpine regions, emissions from road traffic contribute to carbonaceous aerosols, but residential wood burning can be another source of PM during winter.
We determine the contribution of fossil and non-fossil carbon sources by measuring radiocarbon in aerosols using the recently installed AixMICADAS facility. The accelerator mass spectrometer is coupled to an elemental analyzer (EA) by means of a gas interface system directly connected to the gas ion source. This system provides rapid and accurate radiocarbon measurements for small samples (10–100 µgC) with minimal preparation from the aerosol filters. We show how the contamination induced by the EA protocol can be quantified and corrected for. Several standards and synthetic samples are then used to demonstrate the precision and accuracy of aerosol measurements over the full range of expected ¹⁴C ∕ ¹²C ratios, ranging from modern carbon to fossil carbon depleted in ¹⁴C.
Aerosols sampled in Chamonix and Passy (Arve River valley, French Alps) from November 2013 to August 2014 are analyzed for both radiocarbon (124 analyses in total) and levoglucosan, which is commonly used as a specific tracer for biomass burning. NOx concentration, which is expected to be associated with traffic emissions, is also monitored.
Based on ¹⁴C measurements, we can show that the relative fraction of non-fossil carbon is significantly higher in winter than in summer. In winter, non-fossil carbon represents about 85 % of total carbon, while in summer this proportion is still 75 % considering all samples. The largest total carbon and levoglucosan concentrations are observed for winter aerosols with values up to 50 and 8 µg m⁻³, respectively. These levels are higher than those observed in many European cities, but are close to those for other polluted Alpine valleys.
The non-fossil carbon concentrations are strongly correlated with the levoglucosan concentrations in winter samples, suggesting that almost all of the non-fossil carbon originates from wood combustion used for heating during winter.
For summer samples, the joint use of ¹⁴C and levoglucosan measurements leads to a new model to separately quantify the contributions of biomass burning and biogenic emissions in the non-fossil fraction. The comparison of the biogenic fraction with polyols (a proxy for primary soil biogenic emissions) and with the temperature suggests a major influence of the secondary biogenic aerosols.
Significant correlations are found between the NOx concentration and the fossil carbon concentration for all seasons and sites, confirming the relation between road traffic emissions and fossil carbon.
Overall, this dual approach combining radiocarbon and levoglucosan analyses strengthens the conclusion concerning the impact of biomass burning. Combining these geochemical data serves both to detect and quantify additional carbon sources. The Arve River valley provides the first illustration of aerosols of this model.
Many factors influence the preparation and quality of graphite targets for ¹⁴ C accelerator mass spectrometry (AMS). We identified four factors (sample size, H 2 pressure, catalyst temperature and pretreatment time) as potentially critical, and investigated their effects on two particular characteristics: the integrated rates of CO 2 reduction (to graphite) and methane production. We used a 2-level fractional factorial experimental design and determined chemical reduction yield rates through manometry and partial pressure monitoring of residual gases by mass spectrometry.
Chemical reduction yield rates ranged from 0.2% to 6.2% per hour. With respect to their influence on percent yield rate, the factors we studied were ordered as: sample size > level of hydrogen > pretreatment of the catalyst. The temperature of the catalyst, and the sample size × hydrogen (2-factor) interaction, were only marginally influential. Other interactions did not appear to be significantly important. We estimated uncertainty in the order of influence and magnitudes of the effects by the Monte Carlo method of error propagation.
We observed significant methane production in only one experiment, which suggests that methane originates from indigenous carbon in untreated iron catalyst only in the presence of hydrogen and only at thermodynamically favorable temperatures. This exploratory investigation indicates that factorial design techniques are a useful means to investigate multivariate effects on the preparation and quality of AMS graphite targets.
A long-term mountain station series of tropospheric 14 C data for the period 1959 to 1984 is presented. This series is considered representative of the higher altitude 14 C level over central Europe. Even tree-ring 14 C levels from a rural ground level site in southern Germany are consistently lower (by Δ 14 C depression = −15‰ if compared with the mountain station summer average in atmospheric CO 2 ). The rural tree-ring series is considered to represent the additional continental Suess effect at ground level without local contamination. This Suess effect decreases gradually with the distance from the ground ( ie , source) level. We therefore estimate the additional continental Suess effect in the vegetation period to be Δ 14 C depression = −5‰ for the mountain station and −20‰) for a rural ground level site, respectively. Based on this assumption, yearly mean tropospheric 14 C levels corrected for fossil fuel contamination and representative of the Northern Hemisphere are provided for use in global carbon cycle models.
Atmospheric gas samples (0.1m 3 ) were collected at ground level during January/February 1984 in Las Vegas, Nevada for 14 C/ 13 C accelerator mass spectrometry and total abundance measurements of CO and CH 4 . During winter months in this locale, CO concentrations can occur at 10 to 100 times background, occasionally exceeding the National Ambient Air Quality Standard (NAAQS). Methane concentrations show a slight enhancement (∼24%) above the background (non-urban troposphere) level. A comparison of CO and CH 4 concentrations shows a good linear correlation which may indicate a common source. Preliminary 14 C/ 13 C results of the two species suggest that fossil emissions are the predominant source of excess CO and CH 4 in the samples taken. Estimates of anthropogenic CO and CH 4 are important for source apportionment of combustion emissions. In addition, this information is valuable for understanding the global CO and CH 4 cycles and, therefore, human impact on climate and the stratospheric ozone layer.
Miniature gas counters have been in use since the early 1960s for the measurement of 14 C but were for a long time seen as suitable for providing approximate indications of activity rather than measurements for more precise dates. In recent years the need for better measurements of small samples has posed a continuing challenge for the 14 C laboratories. This paper examines how the challenge has been met across the world using conventional beta decay counting techniques and proportional gas counters of 50ml volume or less. A survey is made of the rise of these techniques and attention paid to the solution through modern technology of earlier problems. Some practical systems, now in routine use, are described and consideration is given to the future for miniature counter measurements. Such systems have several attractive features that will guarantee their usefulness in 14 C measurements for the future.
Briefly reviews selected localities in the US that have provided an opportunity for rigorous stratigraphic radiocarbon control, showing large mammal extinctions were completed by 10 500 BP. There is stratigraphic coincidence of the first visible evidence of hunters (Clovis culture) and the last skeletons of late Pleistocene megafauna. Considerable stress may also have been caused to large mammals by the loss of streams and watering holes in the early part of the interglacial. -T.M.Kennard
The authors investigate the sources of different chemical fractions in fine particles. They used oxidation with nitric acid to isolate the elemental carbon fraction from selected fine-particle samples and analyzed both elemental and total carbon for 14C. The results show that: (1) as expected, the fossil (motor vehicle) contribution was highest for the high-traffic intersection in the daytime and was quite low for the residential sites at night; and (2) the elemental carbon fraction had a higher fossil contribution than the corresponding total carbon in all cases. This result suggests that elemental carbon may be more useful than total or organic carbon for tracing mobile sources. In a second Albuquerque study conducted in December 1985, the authors isolated the polycyclic aromatic hydrocarbon (PAH) fraction from fine-particle samples. The behavior of two PAH, dehydroabietic acid (DHA) and benzo(ghi)perylene (BGP), is presented.
Accelerator radiocarbon dates on shell, charcoal, and human bone samples are an indication that human activity at the Del Mar site dates to 5000 B.P. and possibly from 7000 B.P. The shell and charcoal fragments were dislocated stratigraphically by extensive bioturbation and provide evidence that accelerator dates on extremely small samples should be used with caution. Certain depositional environments will require multiple dating within a site because single-sample dating will not yield definitive ages. Dating of individual amino acids from the Del Mar sphenoid yielded an average age of 4900 ± 40 B.P. and conclusively establishes the age for the Del Mar skull. The previously dated tibia was found to be contaminated with exogenous amino acids, thereby making its age uncertain.
Changing fuel patterns and increased awareness of health effects from combus-tion aerosols have generated considerable interest in the use of 14C as a biogenic-fossil aerosol source discriminator. Prior studies in the US demonstrated the importance of 14C measure-ment for estimating the wood-burning contribution to urban aerosols. The present work treats a specific air-pollution problem in the town of Elverum, Norway where large winter-time concentrations of aerosol carbon and polycyclic aromatic hydrocarbons (PAH) were sus-pected to come from residential woodl combustion (RWC). The problem was significant in that up to 50µg/m3[C] and 490ng/m3[PAH] were found during pollution episodes. Samples col-lected during two winters were analyzed for C, C, PAH, and several elements in the fine fraction (<3µm) aerosol. Source apportionment based on these species indicated an average of ca 65% RWC-carbon (14C), ca 5% fine particle mass from motor vehicles (Pb), but negligible contributions from heavy fuel oil (Ni, V). Patterns of 14C and total C, examined as a function of temperature and PAH, indicated large increases in RWC aerosol on the coldest days, and a major RWC contribution to the PAH fraction. Patterns with inorganic species implied multi-ple tracer sources, and one important case of long-range transport.