Time‐consuming and expensive radiometric dating techniques limit the number of dates available to construct absolute chronologies for high‐resolution paleoclimate reconstructions. A recently ...developed rapid‐screen 14C dating technique reduces sample preparation time and per sample costs by 90%, but its accuracy has not yet been tested on shallow‐water corals. In this study, we test the rapid‐screen 14C dating technique on shallow‐water corals by comparing 44 rapid‐screen 14C dates to both high‐precision 14C dates and U/Th dates from mid‐ to late‐Holocene fossil corals collected from the central tropical Pacific (2–4°N, 157–160°W). Our results show that 42 rapid‐screen 14C and U/Th dates agree within uncertainties, confirming closed‐system behavior and ensuring chronological accuracy. However, two samples that grew ∼6500 years ago have calibrated 14C ages ∼1000 years younger than the corresponding U/Th ages, consistent with diagenetic alteration as indicated by the presence of 15–23% calcite. Mass balance calculations confirm that the observed dating discrepancies are consistent with 14C addition and U removal, both of which occur during diagenetic calcite recrystallization. Under the assumption that aragonite‐to‐calcite replacement is linear through time, we estimate the samples' true ages using the measured 14C and U/Th dates and percent calcite values. Results illustrate that the rapid‐screen 14C dates of Holocene‐aged fossil corals are accurate for samples with less than 2% calcite. Application of this rapid‐screen 14C method to the fossil coral rubble fields from Kiritimati Island reveal significant chronological clustering of fossil coral across the landscape, with older ages farther from the water's edge.
Key Points:
Rapid 14C dating is accurate for Holocene‐aged fossil corals
Both 14C and U/Th coral dates are sensitive to secondary calcite
Concordant 14C and U/Th dates confirm closed system behavior
We present a status report of the accelerator mass spectrometry (AMS) facility at the University of California, Irvine, USA. Recent spectrometer upgrades and repairs are discussed. Modifications to ...preparation laboratory procedures designed to improve sample throughput efficiency while maintaining precision of 2–3‰ for 1-mg samples (Santos et al. 2007c) are presented.
Radiocarbon measurements at LAC-UFF: Recent performance Linares, Roberto; Macario, Kita D.; Santos, Guaciara M. ...
Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms,
10/2015, Letnik:
361
Journal Article
Recenzirano
In 2012 a single stage accelerator mass spectrometer from NEC was installed at the Radiocarbon Laboratory of Universidade Federal Fluminense (LAC-UFF), Niterói, Brazil. Here, we present a status ...report of our facility. We discuss some modifications applied to our combustion protocol in an attempt to reduce our procedural blank, mostly to processed organic samples. Measurements of reference materials indicate low precision and accuracy that are partially related to beam optics through the acceleration tube. We observed that once the beam current intensity increases the measured 13C+/12C+ becomes erratic. Therefore, in order to maintain the AMS-δ13C values within reasonable values, so that fractionation corrections using the spectrometer 13C+/12C+ values does not affect the final 14C results, we are forced to limit the 12C− beam intensity to ⩽30μA. This requirement was confirmed during our accuracy tests, when measuring selected annual tree-rings wood samples from a Parana pine (Araucaria angustifolia) between 1927 and 1997 previously measured at the Keck Carbon Cycle AMS Facility (KCCAMS), at the University of California, Irvine (UCI). At the LAC-UFF tree-ring wood samples were processed and measured in 4 different batches during a period of about 5months. The 14C results were later compared to the high-precision data obtained at KCCAMS/UCI and reached a good agreement. Recently a problem associated with graphitization yield were finally identified and new measurements with secondary standards are promising.
In temperate climates, tree growth dormancy usually ensures the annual nature of tree rings, but in tropical environments, determination of annual periodicity can be more complex. The purposes of the ...work are as follows: (1) to generate a reliable tree‐ring width chronology for Prioria copaifera Griseb. (Leguminoceae), a tropical tree species dwelling in the Atrato River floodplains, Colombia; (2) to assess the climate signal recorded by the tree‐ring records; and (3) to validate the annual periodicity of the tree rings using independent methods. We used standard dendrochronological procedures to generate the P. copaifera tree‐ring chronology. We used Pearson correlations to evaluate the relationship of the chronology with the meteorological records, climate regional indices, and gridded precipitation/sea surface temperature products. We also evaluated 24 high‐precision 14C measurements spread over a range of preselected tree rings, with assigned calendar years by dendrochronological techniques, before and after the bomb spike in order to validate the annual nature of the tree rings. The tree‐ring width chronology was statistically reliable, and it correlated significantly with local records of annual and October–December (OND) streamflow and precipitation across the upper river watershed (positive), and OND temperature (negative). It was also significantly related to the Oceanic Niño Index, Pacific Decadal Oscillation, and the Southern Oscillation Index, as well as sea surface temperatures over the Caribbean and the Pacific region. However, 14C high‐precision measurements over the tree rings demonstrated offsets of up to 40 years that indicate that P. copaifera can produce more than one ring in certain years. Results derived from the strongest climate–growth relationship during the most recent years of the record suggest that the climatic signal reported may be due to the presence of annual rings in some of those trees in recent years. Our study alerts about the risk of applying dendrochronology in species with challenging anatomical features defining tree rings, commonly found in the tropics, without an independent validation of annual periodicity of tree rings. High‐precision 14C measurements in multiple trees are a useful method to validate the identification of annual tree rings.
We use dendrochronological procedures to generate a reliable tree ring‐width chronology for Prioria copaifera Griseb. This tree‐ring chronology correlated significantly with local records of annual and October–December (OND) of streamflow and precipitation across the upper river watershed (positive), and OND temperature (negative). However, 24 high‐precision 14C measurements spread over a range of preselected tree rings, with assigned calendar years by dendrochonological techniques, before and after the bomb spike demonstrated offsets of up to 40 years that indicates that P. copaifera can produce more than one ring in certain years.
The coastal region of Cabo Frio, in the Brazilian state of Rio de Janeiro, is characterized by xeric vegetation. Surrounded by humid forest, it is considered as a phytogeographical enclave. The ...vegetation is adapted to the dry conditions which are present locally, mainly due to the Cabo Frio coastal upwelling. Although Quaternary changes in the intensity of the upwelling were reconstructed from coastal lagoons and oceanic sediment records, the lack of continental deposits has precluded vegetation reconstructions. The objective of this study was to investigate the pedogenic features of two soil profiles developed under patches of dry forest in the Cabo Frio area and to assess their potential for inferring past changes in tree cover density, from their phytolith assemblages. For this purpose, field and petrographical observations, C content, C/N ratio, phytolith content and phytolith assemblages were investigated. Soil phytolith assemblages were compared to modern phytolith assemblages. The deepest soil organic carbon (SOC) samples were analyzed in 14C-AMS. Related 14C mean age values were interpreted as the youngest ages of the oldest SOC. These data suggest a first soil development phase, occurring after 13 ka cal BP followed by erosive and depositional episodes and by a second soil development phase. Given the complexity of the studied soil sequences, an attempt to quantify the sources of SOC and phytoliths would require investigation of many more soil samples than the ones collected. This prevents interpretation of the phytolith sequences as continuous chronological sequences. However, for both profiles phytolith indices D/P from A, Ab and bottom horizons can be compared, assuming a bicompartmental distribution of phytoliths in relation with each of the soil development phases. D/P values range from 0.8 to 4, in agreement with what would be expected for dry forest D/P values, and do not substantially change in both profiles. This comparison suggests that the tree cover density of the successive vegetation sources did not suffer considerable change over the period under analysis (last 13 ka for profile A) and never reached the tree density of the humid forest currently widespread in the Rio de Janeiro state.
Fossil fuel-derived CO2 (Cff) emission patterns and their point sources across the Rio de Janeiro megacity and state were estimated from a single regional-scale Δ14C distribution map based on ...isotopic measurements of ipê leaves (Tabebuia, a popular flowering deciduous perennial tree). Data from multi-year sampling (i.e., 2014–2016) was renormalized to reflect 14C signatures of the 2015 calendar year. Spatial variability in Δ14C ranges from a maximum of 27.1 ± 0.4‰ (city of Petrópolis, a higher-elevation municipality) to a minimum of −43.6 ± 1.4‰ (i.e., approximately 27.6 ± 1 ppm of Cff — Santo Cristo, a district within the Rio de Janeiro city). Overall, higher Δ14C values correlate well with green habitats and high elevation areas, while lower values are associated with Cff emissions in densely populated areas with higher industrial and traffic footprints. Cff emissions are higher where local air circulation is poor, such as the area surrounding Guanabara Bay. Other areas with significantly higher Cff emissions were the Paraíba Valley and Mountain regions. These results may be explained by atmospheric transport of CO2 from neighboring states, such as São Paulo and Minas Gerais, and by the predominant west winds and the limited regional air flow created by large topographic features. Lower Cff emissions were observed in the Northwest and Lakes regions, which are dominated by agriculture and tourism activities. Our results highlight the potential of directly estimating Cff for studying urban landscapes in the southern region of Brazil through 14C time-integrated distribution mapping of ipê leaves. The method could also be used to augment greenhouse gas (GHG) emissions inventory studies trends in partitioning Cff from CO2 of bio-template sustainable sources.
•Ipê leaves have been successfully used to assess the emission of 14C-depleted CO2ff.•CO2ff was associated with traffic footprint, industrial processes and prevalent winds.•Deciduous tree leaf analysis is important for mitigating local CO2ff emissions.
Black carbon (BC) and organic carbon (OC) aerosols are important components of fine particulate matter (PM2.5) in polluted urban environments. Quantifying the contribution of fossil fuel and biomass ...combustion to BC and OC concentrations is critical for developing and validating effective air quality control measures and climate change mitigation policy. We used radiocarbon (14C) to measure fossil and contemporary biomass contributions to BC and OC at three locations in Salt Lake City, Utah, USA, during 2012–2014, including during winter inversion events. Aerosol filters were analyzed with the Swiss_4S thermal‐optical protocol to isolate BC. We measured fraction modern (fM) of BC and total carbon in PM2.5 with accelerator mass spectrometry and derived the fM of OC using isotope mass balance. Combined with 14C information of end‐member composition, our data set of 31 14C aerosol measurements provided a baseline of the fossil and contemporary biomass components of carbonaceous aerosol. We show that fossil fuels were the dominant source of carbonaceous aerosol during winter, contributing 88% (80–98%) of BC and 58% (48–69%) of OC. While the concentration of both BC and OC increased during inversion events, the relative source contributions did not change. The sources of BC also did not vary throughout the year, while OC had a considerably higher contemporary biomass component in summer at 62% (49–76%) and was more variable. Our results suggest that in order to reduce PM2.5 levels in Salt Lake City to meet national standards, a more stringent policy targeting mobile fossil fuel sources may be necessary.
Key Points
Black carbon aerosols primarily had a fossil fuel origin, while OC had a significant biogenic and biomass burning fraction that was higher in summer
The sources of BC and OC observed in Salt Lake City were consistent with measurements from other large metropolitan areas outside the U.S.
Using radiocarbon to monitor BC and OC aerosol sources may provide information about the success of different mitigation policies
The caldera-forming Rotoiti eruption from Okataina volcano was one of the largest rhyolite events of the last 100ka in the Taupo Volcanic Zone, New Zealand. Its associated widespread tephra layer ...(Rotoehu tephra) is a significant time marker in terrestrial–marine paleoclimate correlation studies in the SW Pacific. However, the accurate and precise age of this eruption, and of the subsequent, smaller volume Earthquake Flat (EQF) eruption, has been the subject of controversy despite numerous studies. We have applied combined 238U/230Th disequilibrium and (U–Th)/He dating of zircon from Rotoiti and EQF deposits, and obtained overlapping ages of 45.1(7)±3.3ka and 45.1(6)±2.9ka, respectively. These results are supported by new and published high-precision radiocarbon data bracketing the age of the Rotoiti eruption between 44.8±0.3 and 47.5±2.1ka cal BP. These age data are also in good agreement with a range of previously published estimates based on paleoclimate (palynology), luminescence dating of enclosing sediment, and sedimentation rates in terrestrial and marine settings. However, these results are at variance with a commonly quoted age of ∼60ka, largely constrained by a single 40Ar/39Ar age of an overlying glassy lava flow at a distal tephra site. This study demonstrates the potential for combined 238U/230Th disequilibrium and (U–Th)/He dating of zircon in tephra and other volcanic deposits to provide age control in the 40–100ka time interval, a period difficult to constrain using more traditional radiocarbon, K/Ar and 40Ar/39Ar methods.
► New ages for key Late Pleistocene eruptions in Taupo Volcanic Zone, New Zealand. ► Three independent dating methods are applied to key eruptions and cross validated. ► Zircon U/Th+(U–Th)/He eruption ages are in excellent agreement with 14C ages. ► Revised age for the coeval Rotoiti/EQF eruptions is ∼45ka. ► Zircon U/Th+(U–Th)/He dating proved reliable at late Quaternary time scale.
Over the last few decades, radiocarbon laboratories have used different procedures for measuring a broad range of carbonaceous materials. To produce reliable results, the processes employed for ...sample processing, graphite target production, and spectrometer measurement must be rigorous, well tested, and reproducible. Most of the procedures have been developed, improved, and published as part of the laboratories’ quality control and research programs, and can be frequently found in the literature. Nevertheless, there are suites of laboratory techniques (or “small useful skills”), products, and other resources that either have never been described in publications, or have been somewhat hidden in much larger scientific articles and reports. We feel that with the rapid rise of newer laboratories and facilities, a set of resourceful suggestions might come in handy. Here we gathered these skill sets that can be used in all aspects of 14C sample processing, with the intention to simplify and expedite procedures, from glass-tube making to graphitization and measurements. We also included some miscellaneous items to help in laboratory setup.
Abstract
Over the last few decades, radiocarbon laboratories have used different procedures for measuring a broad range of carbonaceous materials. To produce reliable results, the processes employed ...for sample processing, graphite target production, and spectrometer measurement must be rigorous, well tested, and reproducible. Most of the procedures have been developed, improved, and published as part of the laboratories’ quality control and research programs, and can be frequently found in the literature. Nevertheless, there are suites of laboratory techniques (or “small useful skills”), products, and other resources that either have never been described in publications, or have been somewhat hidden in much larger scientific articles and reports. We feel that with the rapid rise of newer laboratories and facilities, a set of resourceful suggestions might come in handy. Here we gathered these skill sets that can be used in all aspects of
14
C sample processing, with the intention to simplify and expedite procedures, from glass-tube making to graphitization and measurements. We also included some miscellaneous items to help in laboratory setup.
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