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The vast majority of radiocarbon measurement results ( 14 C/ 12 C isotopic ratios or sample activities) are corrected for isotopic fractionation processes (measured as 13 C/ 12 C isotopic ratios) that occur in nature, in sample preparation and measurement. In 1954 Harmon Craig suggested a value of 2.0 for the fractionation ratio b that is used to correct 14 C/ 12 C ratios for shifts in the 13 C/ 12 C ratios and this value has been applied by the radiocarbon community ever since. While theoretical considerations suggest moderate deviations of b from 2.0, some measurements have suggested larger differences (e.g. b = 2.3, measured by Saliège and Fontes in 1984). With the high precision attained in radiocarbon measurements today (±2‰), even a relatively small deviation of b from 2.0 can impact the accuracy of radiocarbon data, and it is, therefore, of interest to re-evaluate the fractionation corrections. In the present study, the fractionation ratio b was determined by independent experiments on the chemical reduction of carbon dioxide (CO 2 ) to elemental carbon (graphitization reaction) and on the photosynthetic uptake of CO 2 by C 3 and C 4 plants. The results yielded b = 1.882 ± 0.019 for the reduction of CO 2 to solid graphite and b = 1.953 ± 0.025 for the weighted mean of measurements involving C 3 and C 4 photosynthesis pathways. In addition, the analysis of over 9600 full-sized OX-I and OX-II normalizing standards measured between 2002 and 2012 confirms b values lower than 2.0. The obtained values are in good agreement with quantum mechanical estimates of the equilibrium fractionation and classic kinetic fractionation as well as with results from other light three-isotope systems (oxygen, magnesium, silicon and sulfur). While the value of the fractionation ratio varies with the relative importance of kinetic and equilibrium fractionation, the values obtained in the present study cluster around b = 1.9. Our findings suggest that a significant fraction of all samples (“unknowns”) would be shifted by 2‰ (16 radiocarbon years) or more due to this effect: for example, for b = 1.882, between 16.8% and 25.9% of almost 60,000 radiocarbon values measured at the Keck Carbon Cycle AMS facility between 2002 and 2012 would be affected. The implications for radiocarbon dating and its accuracy are discussed. [ABSTRACT FROM AUTHOR]

Rigorously studied and dated Late Quaternary paleoenvironmental reconstructions from Ranch La Brea (RLB) and the Los Angeles Basin are scarce. Here, we use data from AMS radiocarbon dated insect fragments to infer local climates over the past 50,000 years. Our results indicate: 1) Quaternary insect remains can be located with great accuracy in radiocarbon time, and 2) well-dated and documented climate indicator beetle species are sensitive proxies for environmental change in the Los Angeles Basin. A total of 182 extant RLB ground and darkling beetle species (Coleoptera: Carabidae, Tenebrionidae) were radiocarbon dated. The resulting radiocarbon dates form a semi-continuous range from ∼50 to 28, 16–7.5, and 4 kcal yrs BP to the present. Associated insect climate ranges indicate past conditions consistent with, or very similar to, the current Los Angeles Basin Mediterranean climate. Importantly, these insect data suggest higher temperatures and aridity than inferred previously from other RLB proxies. Furthermore, wider-than-assumed dating spreads for some deposits emphasize the lack of biostratigraphy for RLB, and challenge inferences based on limited sets of radiocarbon dates and assumptions about stratigraphic integrity. Our results demonstrate the necessity to independently radiocarbon date each taxon. The insect paleoclimate interpretations were compared to regional pollen data, primarily from various southern Californian sites including Lake Elsinore and Santa Barbara Basin. These comparisons reveal an important difference in climate interpretations for the last Glacial: the RLB insect data suggest climate similar to the current one, while the regional pollen data have been interpreted as indicating a climate wetter than present. [ABSTRACT FROM AUTHOR]

Ilopango volcano (El Salvador) erupted violently during the Maya Classic Period (250–900 CE) in a densely-populated and intensively-cultivated region of the southern Maya realm, causing regional abandonment of an area covering more than 20,000 km2. However, neither the regional nor global impacts of the Tierra Blanca Joven (TBJ) eruption in Mesoamerica have been well appraised due to limitations in available volcanological, chronological, and archaeological observations. Here we present new evidence of the age, magnitude and sulfur release of the TBJ eruption, establishing it as one of the two hitherto unidentified volcanic triggers of a period of stratospheric aerosol loading that profoundly impacted Northern Hemisphere climate and society between circa 536 and 550 CE. Our chronology is derived from 100 new radiocarbon measurements performed on three subfossil tree trunks enveloped in proximal TBJ pyroclastic deposits. We also reassess the eruption magnitude using terrestrial (El Salvador, Guatemala, Honduras) and near-shore marine TBJ tephra deposit thickness measurements. Together, our new constraints on the age, eruption size (43.6 km3 Dense Rock Equivalent of magma, magnitude = 7.0) and sulfur yield (∼9–90 Tg), along with Ilopango's latitude (13.7° N), squarely frame the TBJ as the major climate-forcing eruption of 539 or 540 CE identified in bipolar ice cores and sourced to the tropics. In addition to deepening appreciation of the TBJ eruption's impacts in Mesoamerica, linking it to the major Northern Hemisphere climatic downturn of the mid-6th century CE offers another piece in the puzzle of understanding Eurasian history of the period. Image 1 • Major eruption of Ilopango volcano, El Salvador occurred in the first half of the 6th century. • Ilopango eruption is consistent with 'mystery' eruption of 540 CE that caused global cooling. • Magnitude 7 event ranks as one of the 10 largest on Earth in past 7000 years. • Impacts on the Maya of Central America were severe, including estimated 100,000 + fatalities. [ABSTRACT FROM AUTHOR]

Several recent studies have used records of the radiocarbon ( 14 C) bomb peak in speleothems to inversely model the soil a 14 CO 2 and the age distribution of soil organic material (SOM) above caves, in part to investigate the potential of speleothems as sensitive records of past SOM dynamics. The results of these modeling studies have suggested that soil CO 2 at karst sites is derived primarily from the decomposition of SOM with turnover times on the order of decades to centuries. This result is in stark contrast with observations of soil a 14 CO 2 at non-karst sites, which indicate that soil CO 2 is derived primarily from root respiration and decomposition of SOM with much shorter turnover times. This discrepancy suggests that SOM in karst settings may have a very different age distribution than sites that have been studied previously and/or that soil CO 2 is not the main source of speleothem carbon. To help resolve this discrepancy, we present an improved inverse model which we use to estimate the age of CO 2 above several caves. We also present results from a detailed case study of soil carbon dynamics at Heshang Cave, China. This work demonstrates that SOM in karst sites may be much older than SOM in non-karst soils that have been studied previously, but that CO 2 produced in the shallow soil zone is unlikely to be the main source of speleothem carbon. A review of the literature suggests that the most likely explanation for the aforementioned discrepancy is that decomposition of down-washed SOM in the vadose zone is the dominant source of speleothem carbon. [ABSTRACT FROM AUTHOR]

We developed and are testing a system for extracting in situ produced 14C from quartz. 14C is liberated from quartz matrix using step-wise heating during which time a spiked carrier gas consisting O2–CO–CO2–He is flowed through the high-temperature chamber continuously. The total 14C background is reproducible and typically (2.3 ± 0.2) × 106 atoms, and the recovery is consistently greater than 90%. To validate the performance of the system and determine the blank level, we are using quartz samples taken from the Homestake mine (1600 m below the surface), South Dakota. To determine the 14C release pattern and recovery, we used samples taken from the Transantarctic Mountains, Antarctica. [Copyright &y& Elsevier]

Dissolved beryllium-10 concentration profiles in sea water of the East China Sea and the Okinawa Trough in 1993 autumn and 1994 summer have been investigated. The results show that robe concentrations in this area are mainly controlled by surface biological productivity, particle remineralization, and the degree of mixing with the Yangtze River and the Kuroshio waters. During the sampling periods (summer and autumn), the East China Sea was well stratified. Generally, the [sup 10]Be water depth profiles can be divided into three layers: the surface mixed layer, the particulate [sup 10]Be regeneration layer, and the bottom layer. Surface water robe concentrations increase gradually towards the Kuroshio and increase sharply at the edge of the Kuroshio Current. Vertical distributions of [sup 10]Be show that in the summer robe is enriched in the bottom water near the Yangtze River estuary and the bottom water in the middle of the continental shelf. The two enriched areas are separated, probably by an intrusion of the Continental Coastal Water. In the autumn, [sup 10]Be bottom enrichment only occurred in the western part of the East China Sea. This phenomenon is consistent with the seasonal circulation pattern change of currents induced by monsoon winds. The influence on robe by the Kuroshio branch intrusion in the southern East China Sea northeast of Taiwan may be more significant than the Kuroshio main flow. Simple box model results indicate that [sup 10]Be input from the Kuroshio Current is more important than Yangtze River input and atmospheric precipitation. About 81% of [sup 10]Be input to the East China Sea is scavenged into the sediments and 19% of [sup 10]Be flows out of the East China Sea by currents and water exchange. The [sup 10]Be sedimentation flux in the East China Sea is nearly five times of the average global robe production rate, suggesting that the East China Sea is an important sink for [sup 10]Be. [ABSTRACT FROM AUTHOR]

Abstract Relatively little is known about the amount of time that lapses between the photosynthetic fixation of carbon by vascular land plants and its incorporation into the marine sedimentary record, yet the dynamics of terrestrial carbon sequestration have important implications for the carbon cycle. Vascular plant carbon may encounter multiple potential intermediate storage pools and transport trajectories, and the age of vascular plant carbon accumulating in marine sediments will reflect these different pre-depositional histories. Here, we examine down-core 14C profiles of higher plant leaf wax-derived fatty acids isolated from high fidelity sedimentary sequences spanning the so-called "bomb-spike", and encompassing a ca. 60-degree latitudinal gradient from tropical (Cariaco Basin), temperate (Saanich Inlet), and polar (Mackenzie Delta) watersheds to constrain integrated vascular plant carbon storage/transport times ("residence times"). Using a modeling framework, we find that, in addition to a "young" (conditionally defined as < 50 y) carbon pool, an old pool of compounds comprises 49 to 78 % of the fractional contribution of organic carbon (OC) and exhibits variable ages reflective of the environmental setting. For the Mackenzie Delta sediments, we find a mean age of the old pool of 28 ky (±9.4, standard deviation), indicating extensive pre-aging in permafrost soils, whereas the old pools in Saanich Inlet and Cariaco Basin sediments are younger, 7.9 (±5.0) and 2.4 (±0.50) to 3.2 (±0.54) ky, respectively, indicating less protracted storage in terrestrial reservoirs. The "young" pool showed clear annual contributions for Saanich Inlet and Mackenzie Delta sediments (comprising 24% and 16% of this pool, respectively), likely reflecting episodic transport of OC from steep hillside slopes surrounding Saanich Inlet and annual spring flood deposition in the Mackenzie Delta, respectively. Contributions of 5–10 year old OC to the Cariaco Basin show a short delay of OC inflow, potentially related to transport time to the offshore basin. Modeling results also indicate that the Mackenzie Delta has an influx of young but decadal material (20–30 years of age), pointing to the presence of an intermediate reservoir. Overall, these results show that a significant fraction of vascular plant C undergoes pre-aging in terrestrial reservoirs prior to accumulation in deltaic and marine sediments. The age distribution, reflecting both storage and transport times, likely depends on landscape-specific factors such as local topography, hydrographic characteristics, and mean annual temperature of the catchment, all of which affect the degree of soil buildup and preservation. We show that catchment-specific carbon residence times across landscapes can vary by an order of magnitude, with important implications both for carbon cycle studies and for the interpretation of molecular terrestrial paleoclimate records preserved in sedimentary sequences. [ABSTRACT FROM AUTHOR]

A refined relative sea level (RSL) history spanning the past 14,300 calendar years is described for the Quadra Island area in the northern Strait of Georgia on the Pacific coast of Canada. Here marine shorelines dating to the time of earliest post-glacial emergence are at least 197 m above present-day sea-level at 14,300 years ago. RSL fell rapidly, reaching two to three metres above present-day by 12,000 years ago. A series of raised marine terraces at ca. 4, 10 and 30 m above present day high tide level suggest the rapid fall in RSL during early post-glacial time may have been briefly interrupted by factors such as regional ice advances and recessions and global meltwater pulses generated by climatic variations. A possible minor sea-level transgression of 1–2 m around 12,000 to 11,400 years ago was followed by slow regression to modern levels. This sea-level reconstruction is providing critical input for efficient discovery and cataloging of late Pleistocene and early Holocene archaeological sites on ancient raised shorelines in the region. Integration of the sea-level history with LiDAR imagery has proven successful in locating a number of archaeological sites on these ancient shorelines. [ABSTRACT FROM AUTHOR]

Fluctuations in oceanic circulation and upwelling associated with the North Pacific Gyre Oscillation (NPGO) are the largest source of salinity and nutrient concentration variability across the Pacific basin. Recent observations suggest NPGO-like variability is intensifying, but longer, “pre-instrumental” records are required to improve our understanding of NPGO amplitude and phase change. Here, using measurements of coral skeletal chemistry from San Benedicto Island in the Eastern Tropical North Pacific (ETNP), we assess this region's suitability for reconstructing NPGO behavior. We find that coral geochemical proxy measurements of ETNP salinity and dissolved inorganic carbon radiocarbon (Δ 14 C) content reflect NPGO-driven gyre circulation and regional coastal upwelling. These results provide the basis for reconstructing NPGO-related ocean conditions hundreds of years prior to the modern observational record. [ABSTRACT FROM AUTHOR]

The response of the Northern Hemisphere cryosphere to the monotonic decline in summer insolation and variable radiative forcing during the Holocene has been one of irregular expansion culminating in the Little Ice Age, when most glaciers attained their maximum late Holocene dimensions. Although periods of intervening still-stand or ice-retreat can be reconstructed by direct dating of ice-recessional features, defining times of Neoglacial ice growth has been limited to indirect proxies preserved in distal archives. Here we report 45 precise radiocarbon dates on in situ plants emerging from beneath receding glaciers on Svalbard that directly date the onset of snowline descent and glacier expansion, entombing the plants. Persistent snowline lowering occurred between 4.0 and 3.4 ka, but with little additional persistent lowering until early in the first millennium AD. Populations of individual 14 C calendar age results and their aggregate calendar age probabilities define discrete episodes of vegetation kill and snowline lowering 240–340 AD, 410–540 AD and 670–750 AD, each with a lower snowline than the preceding episode, followed by additional snowline lowering between 1000 and 1220 AD, and between 1300 and 1450 AD. Snowline changes after 1450 AD, including the maximum ice extent of the Little Ice Age are not resolved by our collections, although snowlines remained lower than their 1450 AD level until the onset of modern warming. A time-distance diagram derived from a 250-m-long transect of dated ice-killed plants documents ice-margin advances ∼750, ∼1100 and after ∼1500 AD, concordant with distributed vegetation kill ages seen in the aggregate data set, supporting our central thesis that vegetation kill ages provide direct evidence of snowline lowering and cryospheric expansion. The mid- to late-Holocene history of snowline lowering on Svalbard is similar to ELA reconstructions of Norwegian and Svalbard cirque glaciers, and consistent with a cryospheric response to the secular decline of regional summertime insolation and stepped changes in nearby surface ocean environments. The widespread exposure of entombed plants dating from the first millennium AD suggests that Svalbard's average summer temperatures of the past century now exceed those of any century since at least 700 AD, including medieval times. [ABSTRACT FROM AUTHOR]

Abstract: The Chinese national radiocarbon standard Chinese Sugar Carbon (CSC), or often called the “Chinese Sucrose Charcoal,” was investigated for its homogeneity and its suitability as a radiocarbon standard for Accelerator Mass Spectrometer (AMS). We randomly chose ten new CSC bottles and two opened bottles and measured their carbon isotope compositions. δ13C results obtained by Isotope Ratio Mass Spectrometer (IRMS) showed relatively large variation: from −15.9 to −22.2‰ with an average of −19.0±1.2‰ (n =49), indicating CSC grains in the mg range are not isotopically homogeneous. However, 14C results of CSC, expressed as Fraction Modern (FM), are well constrained to 2.4‰ after fractionation correction to −25‰. Measurements from all 12 bottles gave an average FM value of 1.3533±0.0034 (n =55). This value is about 7‰ higher that the FM value of 1.346±0.002, converted from the originally reported R value of 1.362±0.002 (n =14, with δ13C of −19.32, n =9) [1,2]. The R value of 1.362 is the ratio of 14C activity of CSC to 95% of OXI, not an FM value. The online simultaneous AMS 13C/12C measurement that allows for correction of all isotopic fractionation, including manufacture, graphitization and machine induced fractionation, is the key for obtaining consistent 14C measurements on this standard. Without further homogenization, CSC may not be a good choice of standard for an AMS that does not have the capability of measuring online AMS δ13C. Further study is needed to see if mixing/grinding can reduce the inhomogeneity. Re-measurement is suggested with more laboratories involved in order to get a better defined consensus value. [Copyright &y& Elsevier]

Abstract: The increased detection sensitivity offered by AMS has dramatically expanded the utility of 10Be. As these applications become more sophisticated attention has focused on the accuracy of the 10Be standards used to calibrate the AMS measurements. In recent years it has become apparent that there is a discrepancy between two of the most widely used 10Be AMS standards, the ICN 10Be standard and the NIST 10Be standard. The ICN (ICN Chemical & Radioisotope Division) 10Be AMS standard was calibrated by radioactive decay counting. Dilutions, ranging from 5×10−13 to 3×10−11 10Be/Be, have been prepared and are extensively used in many AMS laboratories. The NIST 10Be standard, prepared at the National Institute of Standards and Technology (NIST), is calibrated by mass spectrometric isotope ratio measurements. To provide an independent calibration of the 10Be standards we implanted a known number of 10Be atoms in both Si detectors and Be foil targets. The 10Be concentrations in these targets were measured by AMS. The results were compared with both the ICN and NIST AMS standards. Our 10Be measurements indicate that the 10Be/9Be isotopic ratio of the ICN AMS standard, which is based on a 10Be half-life of 1.5×106 yr, is 1.106±0.012 times lower than the nominal value. Since the decay rate of the ICN standard is well determined, the decrease in 10Be/9Be ratio requires that the 10Be half-life be reduced to (1.36±0.07)×106 yr. The quoted uncertainty includes a ±5% uncertainty in the activity measurement carried out by ICN. In a similar fashion, we determined that the value of the NIST 10Be standard (SRM4325) is (2.79±0.03)×10−11 10Be/9Be, within error of the certified value of (2.68±0.14)×10−11. The Lawrence Livermore National Laboratory (LLNL) internal standards were also included in this study. We conclude that the 9Be(n,γ) neutron cross section is 7.8±0.23mb, without taking into account the uncertainty in the neutron irradiation. [Copyright &y& Elsevier]