Calculation of the solar activity effect on the production rate of cosmogenic radiocarbon in polar ice

The propagation of cosmic rays in the Earth??s atmosphere is simulated. Calculations of the omnidirectional differential flux of neutrons for different solar activity levels are illustrated. The solar activity effect on the production rate of cosmogenic radiocarbon by the nuclear-interacting and muon components of secondary cosmic radiation in polar ice is studied. It has been obtained that the ¹⁴C production rates in ice by the cosmic ray nuclear-interacting component are lower or higher than the average value by 30% during periods of solar activity maxima or minima, respectively. Calculations of the altitudinal dependence of the radiocarbon production rate in ice by the cosmic radiation components are illustrated.     

Depth-dependence of the production rate of in situ 14C in quartz from the Leymon High core,Spain

To constrain the depth-dependence of in situ ¹⁴C production we measured the cosmogenic ¹⁴C concentration of quartz separates along a quartzite core from the Leymon High site in northwest Spain. A total of 16 quartz samples were measured over a depth range of 1–1545 cm (3–4017 g cm⁻²). The obtained ¹⁴C profile was modeled using a neutron production rate, exponentially decreasing with depth, and a fast and negative muon production parameterized as a function of the local muon flux as derived by (Heisinger et al., 2002a, 2002b). This model yields a total negative muon capture probability of 1.72 (+0.22/−0.56) × 10⁻² and a fast muon reaction cross section of 0 (+11.8/−0.00) μb. Rescaled to sea level high latitude using a Lal/Stone scaling scheme, these estimates yield a surface muon production rate of 3.31 (+0.43/−1.07) and 0 (+0.42/−0.00) at·g⁻¹ yr⁻¹ for negative muon capture and fast muons, respectively. This is the first muon production estimate for in situ ¹⁴C from a natural setting and is within uncertainty of the previous experimental estimates. The present contribution also provides new long-term blank and standard (PP-4, CRONUS-A & CRONUS-N) in situ ¹⁴C data from the ETH Zürich ¹⁴C extraction line.     

Radiocarbon in polar ice: The mechanism by which radiocarbon is preserved in firn grains

This work is devoted to the problem of localizing the ¹⁴C cosmogenic radionuclide in a firn layer covering glaciers. The data on ¹⁴C in ice samples from the GISP2 ice core drilled on a Greenland ice dome (Summit) are analyzed. It has been indicated that experimental values of the ¹⁴C concentration are systematically smaller than theoretically calculated values, which indicates that firn grains partially lose ¹⁴C. Diffusion of cosmogenic ¹⁴C in firn grains and hydration of ¹⁴CO₂ in a disordered ice layer, which is formed on a firn grain surface and at the boundary between ice monocrystals, are considered. It has been indicated that these processes are among the main ones responsible for the level of radiocarbon concentration in firn and ice samples.     

Can in-situ cosmogenic 14C be used to assess the influence of clast recycling on exposure dating of ice retreat in Antarctica?

Cosmogenic nuclide exposure dating of glacial clasts is becoming a common and robust method for reconstructing the history of glaciers and ice sheets. In Antarctica, however, many samples exhibit cosmogenic nuclide ‘inheritance’ as a result of sediment recycling and exposure to cosmic radiation during previous ice free periods. In-situ cosmogenic ¹⁴C, in combination with longer lived nuclides such as ¹⁰Be, can be used to detect inheritance because the relatively short half-life of ¹⁴C means that in-situ ¹⁴C acquired in exposure during previous interglacials decays away while the sample locality is covered by ice during the subsequent glacial. Measurements of in-situ ¹⁴C in clasts from the last deglaciation of the Framnes Mountains in East Antarctica provide deglaciation ages that are concordant with existing ²⁶Al and ¹⁰Be ages, suggesting that in this area, the younger population of erratics contain limited inheritance.     

Cosmic ray flux variations, modulated by the solar and earth’s magnetic fields, and climate changes. 1. Time interval from the present to 10–12 ka ago (the Holocene Epoch)

Direct and indirect data on variations in cosmic rays, solar activity, geomagnetic dipole moment, and climate from the present to 10–12ka ago (the Holocene Epoch), registered in different natural archives (tree rings, ice layers, etc.), have been analyzed. The concentration of cosmogenic isotopes, generated in the Earth’s atmosphere under the action of cosmic ray fluxes and coming into the Earth archives, makes it possible to obtain valuable information about variations in a number of natural processes. The cosmogenic isotopes ¹⁴C in tree rings and ¹⁰Be in ice layers, as well as cosmic rays, are modulated by solar activity and geomagnetic field variations, and time variations in these concentrations gives information about past solar and geomagnetic activities. Since the characteristics of natural reservoirs with cosmogenic ¹⁴C and ¹⁰Be vary with climate changes, the concentrations of these isotopes also inform about climate changes in the past. A performed analysis indicates that cosmic ray flux variations are apparently the most effective natural factor of climate changes on a large time scale.     

14C content of ten meteorites measured by tandem accelerator mass spectrometry

¹⁴C has been measured in three North American and seven Antarctic meteorites with the Chalk River MP tandem accelerator. In most cases cosmogenic¹⁴C, which is tightly bound, was separated from absorbed atmospheric radiocarbon by stepwise heating extractions. Terrestrial ages obtained by comparing cosmogenic¹⁴C in the meteorite to that in Bruderheim are (7.2 ± 0.6) × 10³ years for Yamato 7304, (11.6 ± 0.4) × 10³ years for Estacado, and range from (32.7 ± 0.5) × 10³ to (41.0 ± 0.8) × 10³ years for six meteorites recovered at Allan Hills and its vicinity. The present upper limit to age determination by the accelerator method varies from 50 × 10³ to 70 × 10³ years depending upon mass and carbon content of the sample. The natural limit caused by cosmic ray production of¹⁴C in silicate rocks at 2000 m elevation is estimated to be (55 ± 5) × 10³ years. “Weathering ages” were estimated for the Antarctic meteorites from the specific activity of loosely-bound CO₂ considered to be absorbed from the terrestrial atmosphere on weathering. The accelerator measurements are in accordance with previous low-level counting measurements but have higher precision and sensitivity.     

A New Approach in Coal Mine Exploration Using Cosmic Ray Muons

Muon radiography is a technique that uses cosmic ray muons to image the interior of large scale geological structures. The muon absorption in matter is the most important parameter in cosmic ray muon radiography. Cosmic ray muon radiography is similar to X-ray radiography. The main aim in this survey is the simulation of the muon radiography for exploration of mines. So, the production source, tracking, and detection of cosmic ray muons were simulated by MCNPX code. For this purpose, the input data of the source card in MCNPX code were extracted from the muon energy spectrum at sea level. In addition, the other input data such as average density and thickness of layers that were used in this code are the measured data from Pabdana (Kerman, Iran) coal mines. The average thickness and density of these layers in the coal mines are from 2 to 4 m and 1.3 gr/c³, respectively. To increase the spatial resolution, a detector was placed inside the mountain. The results indicated that using this approach, the layers with minimum thickness about 2.5 m can be identified.     

Explosive eruptive record in the Katmai region,Alaska Peninsula: an overview

At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin and from Shelikof Strait to Bristol Bay (∼8,500 km²). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy Mountain summit dome (<250 ¹⁴C years B.P.). East Mageik has erupted most frequently during Holocene time with seven explosive events (9,400 to 2,400 ¹⁴C years B.P.) preserved as tephra layers. Mount Martin erupted entirely during the Holocene, with lava coulees (>6 ka), two tephras (∼3,700 and ∼2,700 ¹⁴C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time (mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 ¹⁴C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite ash from Kaguyak’s caldera-forming event (5,800 ¹⁴C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 ¹⁴C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500 ¹⁴C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 ¹⁴C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available.     

Zenith angular distributions of atmospheric high-energy neutrinos

In this paper, we present a new calculation of the atmospheric neutrino flux in the energy range 10–10⁷ GeV, which reveals sizable differences in muon neutrino flux predictions obtained with known hadronic models. The calculation is based on the method of solving nuclear cascade equations in the atmosphere, which takes into account the nonscaling behavior of inclusive cross sections for particle production, the increase in the total inelastic hadron-nucleus cross sections, and the non-power-law character of the primary cosmic ray spectrum. The efficiency of the method was recently tested in atmospheric muon flux calculations. The results of neutrino spectrum calculations have been compared with Frejus, AMANDA-II, and IceCube measurement data.     

Reconstructions of the heliospheric modulation potential and Wolf numbers based on the content of the <Superscript>14</Superscript>C isotope in tree rings during the Maunder and Spörer minimums

Data on variations in the content of the 14C cosmogenic isotope in tree rings and the Earth’s atmosphere (Δ¹⁴C) make it possible to study the behavior of solar activity (SA) in previous centuries and millenniums. The latter is related to the fact that SA temporal variations result in a change in the IMF (Interplanetary Magnetic Field) parameters and, as a consequence, in the galactic cosmic ray (GCR) flux, under the action of which the ¹⁴C isotope is produced in the Earth’s atmosphere. This makes it possible to study SA history based on data on the ¹⁴C isotope content in tree rings. However, in this case we have several difficulties related to climate change. Climate changes result in carbon redistribution between natural reservoirs, which is reflected in radiocarbon data and results in solar signal distortion. The effect of variations in the global temperature and carbon dioxide concentration on the reconstruction of the heliospheric modulation potential and Wolf numbers from the late 14th century to the early 19th century is considered. It has been shown that the radiocarbon data do not make it possible to conclude that SA during the Maunder minimum was extremely low as compared to SA during the Dalton minimum.     

Reduction of thermohaline circulation during deglaciation: the effect on atmospheric radiocarbon and CO2

A two-? ocean reservoir model is employed to examine the combined effects of vertical ocean circulation, organic matter extraction and cosmic ray production on the¹⁴C/¹²C in the atmosphere, ocean and sediment. In this model, dissolution of deep-sea calcium carbonate sediment is assumed to respond to the supply of particulate carbonate from the surface ocean and to the dissolved carbonate-ion concentration of the deep-sea.If the vertical ocean circulation decreased by 50% during the maximum rate of deglaciation, the atmospheric¹⁴C/¹²C would have increased concurrently by 10% relative to the pre-bomb present. Further, if cosmic ray production of¹⁴C was 50% greater than present at about 7800 years B.P. as suggested by archeomagnetic measurements, a double maximum of 10% occurs. The first is at about 10.5 ka and the second at 6.5 ka. This result is similar to the variation of¹⁴C/¹²C over the last 10,000 years calculated from the¹⁴C dates measured by Stuiver on the varved Lake of the Clouds. The result is not sensibly altered if 10¹⁷ moles of organic carbon is extracted from the ocean during sea-level rise.Reduction of the thermohaline ocean circulation by 50% over a one- to two-thousand-year interval would not be sensibly detected in the dating of deep-sea sediment. If Broecker's organic nutrient extraction occurs in conjunction with reduction of vertical circulation, a decrease in the atmospheric P_CO2 will precede the eventual rise.     

Cosmogenic isotopes and their role in present-day solar paleoastrophysics

Present-day data on ¹⁴C and ¹⁰Be concentration in natural archives have been statistically analyzed. It has been established that it is difficult to extract information about solar activity variations on long (several Myr and longer) and, especially, short (to 30 years) time scales using radiocarbon data. It has been indicated that beryllium series bear reliable information about short-term, secular, and, probably, 1000-year variations in solar activity. Moreover, ¹⁰Be concentration in polar ice can also be used to study the internal dynamics of solar activity. It has been concluded that beryllium data are more promising than radiocarbon ones from the viewpoint of solar paleoastrophysics.     

Progress in refining the global radiocarbon calibration curve using New Zealand kauri (Agathis australis) tree-ring series from Oxygen Isotope Stage 3

Ancient kauri (Agathis australis) buried in bogs across Northland, New Zealand, provide potential for a continuous high-precision radiocarbon calibration curve that extends to the full chronologic range of the dating method. Here we report four new tree-ring series which in addition to previous work represent some 5312 years across Oxygen Isotope Stage 3, a period during which human evolution, migration and extinction took place against a backdrop of extreme and abrupt global climate and environmental change. We compare our radiocarbon-dated series against three different calibration datasets and find considerable differences in the wiggle-matched solutions, demonstrating the difficulty in undertaking high-precision correlation between ice, marine and terrestrial sequences. One way around the apparent impasse is the use of ¹⁰Be in the Greenland and Antarctic ice cores to precisely link to kauri ¹⁴C from which a decadally/bidecadally-resolved floating ‘calibration’ dataset can be generated.     

Ice core record of Be over the past millennium from Dome Fuji, Antarctica: A new proxy record of past solar activity and a powerful tool for stratigraphic dating

The cosmogenic nuclide ¹⁰Be was analyzed by using accelerator mass spectrometry on an ice core drilled at the Dome Fuji station, inland Antarctica, for 700–1900 yr CE. The measured concentration of ¹⁰Be in the Dome Fuji ice core and the derived ¹⁰Be flux show similar fluctuations, with both increasing at known solar-activity minima over the last millennium in agreement with earlier observations of ¹⁰Be and ¹⁴C. Based on the similar nature of the ¹⁰Be flux to the reconstructed ¹⁴C production rate patterns, a ¹⁰Be–¹⁴C correlation age model for the Dome Fuji ice core was successfully constructed. This age model agrees well with the initial version of the tephrochronology of the core. The ¹⁰Be-flux record contains information on variability in the amount of cosmic radiation incident on the atmosphere, which is mainly attributable to high-frequency change in solar activity and low-frequency background intensity adjustment of the geomagnetic field. High-resolution ¹⁰Be analyses of the Dome Fuji ice cores promise to provide potentially important information on the history of cosmic radiation intensity over the past several hundred thousand years.     

Hydrological control of the dead carbon fraction in a Holocene tropical speleothem

Over the past decade, a number of speleothem studies have used radiocarbon (¹⁴C) to address a range of palaeoclimate problems. These have included the use of the bomb pulse ¹⁴C to anchor chronologies over the last 60 years, the combination of U-Th and ¹⁴C measurements to improve the radiocarbon age-calibration curve, and linking atmospheric ¹⁴C variations with climate change. An issue with a number of these studies is how to constrain, or interpret, variations in the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that reduces radiocarbon concentrations in speleothems. In this study, we use ¹⁴C, stable-isotopes, and trace-elements in a U-Th dated speleothem from Flores, Indonesia, to examine DCF variations and their relationship with above-cave climate over the late Holocene and modern era. A strong association between the DCF and hydrologically-controlled proxy data suggests that more dead carbon was being delivered to the speleothem during periods of higher cave recharge (i.e. lower δ¹⁸O, δ¹³C and Mg/Ca values), and hence stronger summer monsoon. To explore this relationship, we used a geochemical soil-karst model coupled with ¹⁴C measurements through the bomb pulse to disentangle the dominant components governing DCF variability in the speleothem. We find that the DCF is primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than kinetic fractionation and/or variations in the age spectrum of soil organic matter above the cave. Therefore, we infer that periods of higher rainfall resulted in a higher DCF because the system was in a more closed state, which inhibited carbon isotope exchange between the karst water dissolved inorganic carbon and soil-gas CO₂, and ultimately led to a greater contribution of dead carbon from the bedrock.     

Production rate calculations for cosmic-ray-muon-produced 10Be and 26Al benchmarked against geological calibration data

First, I benchmark existing methods of calculating subsurface ²⁶Al, ¹⁰Be, and ¹⁴C production rates due to cosmic-ray muons against published calibration data from bedrock cores and mine excavations. This shows that methods based on downward propagation of the surface muon energy spectrum fit calibration data adequately. Of these methods, one that uses a simpler geographic scaling method based on energy-dependent attenuation of muons in the atmosphere appears to fit calibration data better than a more complicated one that uses the results of a global particle transport model to estimate geographic variation in the surface muon energy spectrum. Second, I show that although highly simplified and computationally much cheaper exponential function approximations for subsurface production rates are not globally adequate for accurate production rate estimates at arbitrary location and depth, they can be used with acceptable accuracy for many exposure-dating and erosion-rate-estimation applications.     

Towards radiocarbon calibration beyond 28 ka using speleothems from the Bahamas

We present a new speleothem record of atmospheric Δ¹⁴C between 28 and 44 ka that offers considerable promise for resolving some of the uncertainty associated with existing radiocarbon calibration curves for this time period. The record is based on a comprehensive suite of AMS ¹⁴C ages, using new low-blank protocols, and U–Th ages using high precision MC-ICPMS procedures. Atmospheric Δ¹⁴C was calculated by correcting ¹⁴C ages with a constant dead carbon fraction (DCF) of 22.7 ± 5.9%, based on a comparison of stalagmite ¹⁴C ages with the IntCal04 (Reimer et al., 2004) calibration curve between 15 and 11 ka. The new Δ¹⁴C speleothem record shows similar structure and amplitude to that derived from Cariaco Basin foraminifera (Hughen et al., 2004, 2006), and the match is further improved if the latter is tied to the most recent Greenland ice core chronology (Svensson et al., 2008). These data are however in conflict with a previously published ¹⁴C data set for a stalagmite record from the Bahamas — GB-89-24-1 (Beck et al., 2001), which likely suffered from ¹⁴C analytical blank subtraction issues in the older part of the record. The new Bahamas speleothem ?¹⁴C data do not show the extreme shifts between 44 and 40 ka reported in the previous study (Beck et al., 2001). Causes for the observed structure in derived atmospheric Δ¹⁴C variation based on the new speleothem data are investigated with a suite of simulations using an earth system model of intermediate complexity. Data-model comparison indicates that major fluctuations in atmospheric ?¹⁴C during marine isotope stage 3 is primarily a function of changes in geomagnetic field intensity, although ocean–atmosphere system reorganisation also played a supporting role.     

Radiocarbon: A chronological tool for the recent past

The past few hundred years have seen large fluctuations in atmospheric ¹⁴C concentration. In part, these have been the result of natural factors, including the climatic changes of the Little Ice Age, and the Spörer and Maunder solar activity minima. In addition, however, changes in human activity since the middle of the 19th century have released ¹⁴C-free CO₂ to the atmosphere. Moreover, between c. 1955 and c. 1963, atmospheric nuclear weapon testing resulted in a dramatic increase in the concentration of ¹⁴C in the atmosphere. This was followed by a significant decrease in atmospheric ¹⁴C as restrictions on nuclear weapon testing began to take effect and as rapid exchange occurred between the atmosphere and other carbon reservoirs. The large fluctuations in atmospheric ¹⁴C that occurred prior to 1955 mean that a single radiocarbon date may yield an imprecise calibrated age consisting of several possible age ranges. This difficulty may be overcome by obtaining a series of ¹⁴C dates from a sequence and either wiggle-matching these dates to a radiocarbon calibration curve or using additional information on dated materials and their surrounding environment to narrow the calibrated age ranges associated with each ¹⁴C date. For the period since 1955 (the bomb-pulse period), significant differences in atmospheric ¹⁴C levels between consecutive years offer the possibility of dating recent samples with a resolution of from one to a few years. These approaches to dating the recent past are illustrated using examples from peats, lake and salt marsh sediments, tree rings, marine organisms and speleothems.     

Radiocarbon age anomalies of land snail shells in the Chinese Loess Plateau

In the Chinese Loess Plateau, land snail shells are often the only material available for dating in paleoenvironmental and archaeological research. However, the geochronological suitability of land snail shells is limited because of poor knowledge about their deposition dynamics, particularly with regards to the incorporation of inorganic carbonate and the resulting age anomalies. To evaluate the factors controlling these age anomalies, radiocarbon and stable carbon analyses were carried out on surface soils, as well as the shells and organic bodies of different modern snail species from different ecological habitats. The results showed that all specimens were depleted in ¹⁴C, indicating the influence of inorganic, radiocarbon-free carbonate on the ¹⁴C-activity of the snail shells. The apparent ¹⁴C-deficiencies and the resultant age anomalies of both the Cathaica and the Bradybaena snail shells were within close ranges across the Chinese Loess Plateau, indicating that the shells of these species could, after corrections for radiocarbon anomalies, provide reliable age estimates. The apparent ¹⁴C-deficiencies were closely associated with the ecological habitats of the snails. The shells of the ground-dwelling Bradybaena had the smallest age offsets (533 ± 150 a), followed grass-dwelling Cathaica (1107 ± 138 a) and Cathaica living on trees (1550 ± 345 a). These results suggest that the availability of calcium in the respective ecological habitats is an important factor in explaining the apparent ¹⁴C-deficiencies. The influence of carbonate on the stable carbon isotope composition of shells is overwhelmed by the organic diets of snails, making δ¹³C unsuitable for identifying and correcting shell age anomalies. The radiocarbon activities of surface soils (A_calc) increase with weathering intensity. Thus, a significant uncertainty could be caused by assuming that A_calc is zero when estimating the proportions of different carbon sources in shells, as has been the case in most previous studies.     

Comparison between luminescence and radiocarbon dating of late Quaternary loess from the Ili Basin in Central Asia

Dust depositions are critical archives for understanding interior aridification and westerly climatic changes in Central Asia. Accurate and reliable dating of loess is very important for interpreting and correlating environmental records. There remains a disparity between luminescence ages and radiocarbon dating of late Quaternary loess from the Ili Basin in Central Asia. In this study, we establish a closely spaced quartz optically stimulated luminescence (OSL) chronology for the 20.5-m-thick Nilka loess section in the Ili Basin. Based on OSL ages, two intervals of higher mass accumulation rate occurred at 49–43 ka and 24–14 ka. We further compare these OSL ages with 23 accelerator mass spectrometry (AMS) ¹⁴C ages of bulk organic matter. The results indicate that the OSL and radiocarbon ages agree well for ages younger than ca. 25 ¹⁴C cal ka BP. However, beyond 30 cal ka BP, there is no consistent increase in AMS ¹⁴C age with depth, while the OSL ages continue to increase. These differences confirm the observation that the AMS ¹⁴C ages obtained using conventional acid–base–acid (ABA) pretreatment are severely underestimated in other terrestrial deposits in Central Asia, which could be due to 2–4% modern carbon contamination. However, OSL dating is applicable for constructing an accurate chronology beyond 30 cal ka BP. We suggest caution when interpreting paleoenvironmental changes based on radiocarbon ages older than 25 cal ka BP.