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.     

Cosmic ray flux variations,modulated by the solar and terrestrial magnetic fields,and climate changes. Part 2: The time interval from ∼10000 to ∼100000 years ago

A joint analysis of paleodata on variations in cosmic ray fluxes, solar activity, geomagnetic field, and climate during the period from ～10000 to ～100000 years ago has been performed. Data on the time variations in the concentration of ¹⁴C and ¹⁰Be cosmogenic isotopes, which are generated in the Earth’s atmosphere under the action of cosmic ray fluxes modulated by solar activity and geomagnetic field variations, were used to detect variations in solar activity and the geomagnetic dipole. Information about climate changes has been obtained mainly from variations in the concentration of stable isotopes in the natural archives. A performed analysis indicates that the variations in cosmic ray fluxes under the action of variations in the geomagnetic field and solar activity are apparently one of the most effective natural factors of long-term climate changeability on a large time scale.     

Reconstructions of the <Superscript>14</Superscript>С cosmogenic isotope content from natural archives after the last glacial termination

Data on the content of the ¹⁴C cosmogenic isotope in tree rings, which were obtained as a result of laboratory measurements, are often used when solar activity (SA) is reconstructed for previous epochs, in which direct observations are absent. However, these data contain information not only about SA variations but also about changes in the Earth climatic parameters, such as the global temperature and the CO₂ content in the Earth’s atmosphere. The effect of these variations on the ¹⁴C isotope content in different natural reservoirs after the last glacial termination to the middle of the Holocene is considered. The global temperature and the CO₂ content increased on this time interval. In this case the ¹⁴C absolute content in the atmosphere increased on this time interval, even though the ¹⁴С to ¹²С isotope concentration ratio (as described by the Δ¹⁴С parameter) decreased. These variations in the radiocarbon absolute content can be caused by its redistribution between natural reservoirs. It has been indicated that such a redistribution is possible only when the rate of carbon exchange between the ocean and atmosphere depends on temperature. The values of the corresponding temperature coefficient for the 17–10 ka BC time interval, which make it possible to describe the carbon redistribution between the ocean and atmosphere, have been obtained.     

The geochemical cycle of 9Be: a reconnaissance

An initial survey has been made of the geochemistry of ⁹Be in natural waters with a view to utilizing the isotope with the cosmogenic species ¹⁰Be (t_1/2 = 1.5 m.y.) in geochronological studies of sedimentary accumulation. The mobility of⁹Be in continental waters is a strong function of pH with acid streams (pH < 6) being strongly enriched as compared to alkaline carbonate rivers. Large-scale scavenging of beryllium occurs in the estuaries of rivers containing more than about 200 pM of the element. In alkaline systems of lower concentration the element appears to behave conservatively. The net flux of beryllium to the ocean is estimated as that equivalent to an average effective river concentration of about 150 pM. Beryllium is strongly enriched in ridge crest hydrothermal solutions over ambient seawater. The flux is approximately 70% of that from the continents. The deep water concentrations of ⁹Be vary by only 20% between the North Atlantic and North Pacific demonstrating that the “chemical saltation” produced by scavenging from the deep water column and release from the sediments predominates over simple advective concentration along flow lines. Isotopic homogenization of ¹⁰Be and ⁹Be is probably achieved in the ocean. The situation in continental waters appears much less promising.     

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.     

Greenland iceBe concentrations and average precipitation rates north of 40°N to 45°N

A model of¹⁰Be deposition within the subarctic and arctic is developed based on the behavior of⁹⁰Sr in the troposphere. Measured¹⁰Be fluxes, based on analyses of¹⁰Be in one year's snow fall (1979–1980) from the Dye-3 site in southwest Greenland and on published data, and predicted¹⁰Be fluxes, based on the¹⁰Be deposition model, agree. These results indicate that with regard to¹⁰Be, the troposphere north of 40°N to 45°N presently behaves as if it is well-mixed and that the average precipitation rate within that reservoir controls in large part the concentration of¹⁰Be in Greenland ice. Inversion of the Greenland ice core¹⁰Be concentration record with the aid of the model indicates: (1) that the average precipitation rate in the subarctic and arctic was lower than the present rate during the Maunder minimum of solar activity, and higher than the present rate during the Wolf and Sporer solar activity minimums; and (2) that during the Wisconsin-Holocene transition the average precipitation rate in the subarctic and arctic was about one third the present precipitation rate.     

Tracing environmental change in southern Patagonia using beryllium isotopes,Laguna Potrok Aike,Argentina

We have examined the relationship between beryllium isotopes and the hydrological record of Laguna Potrok Aike, a maar lake in southern Argentina for the past 16,000 cal BP. Our study shows that sedimentary ¹⁰Be and ⁹Be records of Laguna Potrok Aike are associated with the hydrological balance, when compared to other proxies such as Ca, Ti and total inorganic carbon (TIC). During drier periods, the level of ⁹Be is decreased. ¹⁰Be follows this trend from 16,000 to 8000 cal BP, for younger samples, the concentration of ¹⁰Be increases at about 5000 cal BP and in recent times, but is otherwise relatively constant. At 13,000 cal BP total beryllium (⁹Be) was relatively low, but the ¹⁰Be/⁹Be ratio was the highest for the entire record studied. Our study shows that beryllium isotopes can be used for tracing climatic signals associated with lake level changes, i.e., dry or wet conditions at Laguna Potrok Aike.     

Long-term solar cycles according to data on the cosmogenic beryllium concentration in ice of central Greenland

Data on the concentrations of the cosmogenic ¹⁰Be isotope and Na⁺ and Ca²⁺ ions, as well as on ice accumulation rates in central Greenland, obtained by the GISP2 collaboration were studied in a time interval spanning about 40 000 recent years. Joint analysis of the indicated glacial and chemical paleoseries demonstrated that in a period from 12 008 to 24 008 years ago, the ¹⁰Be flux variations were most free of influence of climate and changes in the geomagnetic dipole moment. As a consequence, this time interval is most suitable for studying periodicities of purely solar nature. Statistical analysis of the ¹⁰Be flux data has revealed significant variations with periods of 500–800 years and 1300–1600 years in the time interval from 12 008 to 17 008 years ago. This evidence favors the existence of the appropriate cycles of solar activity in the mentioned epoch. Similar periodicities are also revealed in the time interval from 3288 to 8008 years ago.     

Long-term solar activity as a controlling factor for global warming in the 20th century

Such high-resolution indirect data on solar activity as the ¹⁴C and ¹⁰Be cosmogenic isotopes have been considered. The long-term solar activity cyclicity during the last millennium with periods of approximately 90 and 210 years, which can be related to substantial climatic warming and cooling events in this millennium, has been established based on an analysis of these data. It has been indicated that long-term recent climate warming can result from the effect of the ∼90- and ∼210-year solar cycles on the climatic system, which is characterized by the nonlinear dynamics.     

Be in a deep-sea core: implications regardingBe production changes over the past 420 ka

The influx of¹⁰Be into a globigerinid ooze core (CH72-02) from the eastern North Atlantic has been studied. This core contains a depositional record of the first 11 δ¹⁸O stages covering the last 423 ka. It is shown that the marine deposition of¹⁰Be is strongly influenced by the sedimentation of clays. Clay particles appear 10 times more efficient than the carbonate component as a carrier in bringing¹⁰Be to the bottom sediments. In core CH72-02, the deposition rates of¹⁰Be averaged over each oxygen-isotope stage for the past 11 stages show a scatter of ±40% about the mean value of 6.6 × 10⁸ atoms cm⁻² ka⁻¹. However, after correction for changes in lithology, the data show that the production rate of¹⁰Be over the same period has varied no more than ±25%, and the variations are not systematic in that high or low¹⁰Be production appear to be associated with either cold or warm climates. On the time scale of this investigation (intervals of ca. 50 ka over the last 420 ka, with resolutions as fine as 10 ka for portions of the record), it is unlikely that the shielding effect of the solar wind has deviated by more than ±25% or the geomagnetic field intensity has deviated by more than a factor of 1.6 from their long-term averages.     

Distribution ofBe andBe in the Pacific Ocean

The vertical distributions of¹⁰Be and⁹Be at three locations in the Pacific (25°N, 170°E; 17°N, 118°W; 3°S, 117°W) are presented. The results show that both isotopes exhibit nutrient-like profiles. From the surface to the bottom, the increase for¹⁰Be is two- to threefold and that for⁹Be is about fivefold. While the inter-station variations in surface water concentrations may reach a factor of two, deep-water values tend to be much more uniform averaging about 2000 atoms/g for¹⁰Be and 30 pM for⁹Be. A similar situation applies to the¹⁰Be/⁹Be ratio; it varies approximately from 1 to 3 × 10⁻⁷ (atom/atom) at shallow depths but tends toward a value close to 1.1 × 10⁻⁷ in the deep ocean. The variation of¹⁰Be/⁹Be can be viewed as resulting from the fact that¹⁰Be in a given parcel of water consists of two components: recycled and primary. The recycled component is that part of¹⁰Be which has reached tracer equilibrium with⁹Be, as opposed to the primary component which, upon entering the sea from the atmosphere, has yet to equilibrate with⁹Be through particle cycling and mixing processes. It is estimated that 70% to nearly 100% of¹⁰Be at the three stations are being recycled, and the recycled beryllium bears an atomic ratio of¹⁰Be/⁹Be close to 1 × 10⁻⁷. The oceanic residence time of Be is of the order of 1000–4000 years, comparable to or slightly longer than the ocean mixing time.     

Secular variation in aerosol transparency of the atmosphere as the possible link between long-term variations in solar activity and climate

Information about variations in solar activity and climate on the time intervals from 130 years to four–five last centuries, including results of instrumental measurements (Wolf numbers, actinometry, thermometry) and indirect indicators (ice core acidity, NO ₃ ^? ion concentration in polar ice, temperature tree-ring reconstructions), has been analyzed for the Northern Hemisphere and its high-latitude part. It has been obtained that the observed relation between secular variations in solar activity and near-Earth temperature resulted from the effect of the corresponding variation in aerosol transparency of the stratosphere on terrestrial climate. It has been also indicated that long-term variations in the aerosol content of the stratosphere can, in turn, be related to secular cycles in atmospheric ionization caused by variations in fluxes of ionizing cosmic particles.     

Long-term variations in the flux of cosmogenic isotope 10Be over the last 10000 years: Variations in the geomagnetic field and climate

A spectral analysis of data on the flux of cosmogenic ¹⁰Be in ice core samples from the Central Greenland (project GRIP) over the last 10 thousand years have been carried out. It has been shown that the ¹⁰Be flux varies cyclically; the most significant cycle is of about 2300 years. Variations in the position of the virtual geomagnetic pole over 8000 years have been analyzed. Significant components, pointing to the cyclic variation in the position of the geomagnetic pole with a period of about 2300 years, have been revealed in a periodogram of the virtual geomagnetic pole longitude. In addition to the nearly 2300-year-long cycle, some lines are observable in the ¹⁰Be flux periodogram, which can be considered as a manifestation of the 1000-year-long cycle of the ¹⁰Be deposition rate on the ice surface. The relationship between the cyclicity of the geomagnetic pole position and the ¹⁰Be flux is discussed.     

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.     

10Be dating of North Pacific sediment cores up to 2.5 million years B.P.

The¹⁰Be method of dating of marine sediment cores is applied to five North Pacific cores. Assuming a constant¹⁰Be precipitation rate and varying sedimentation rates with time during the past 2.5 m.y. dating confirms to that obtained from paleomagnetic stratigraphy. The¹⁰Be concentration variations with depth in the cores are primarily due to changes in sediment dilution and do not reflect cosmic ray intensity or global climate variations. The limits of¹⁰Be deposition rate variation in the investigated cores are less than ± 10% for periods of (2–7) × 10⁵ years and less than ±30% for periods of 1 × 10⁵ years. The data set gives a half-life of¹⁰Be is 1.50 × 10⁶ years. The latitudinal effect of¹⁰Be concentrations and¹⁰Be/⁹Be ratios relates to a frequency of particulate matter occurrence (detrital and biological particles) in the oceans and to oceanic circulation.     

Influence of climatic factors on the past atmospheric content of the C-14 isotope

Reconstructions of solar activity in the past epochs based on information on the past atmospheric content of the cosmogenic ¹⁴C isotope are nowadays actively discussed. The ¹⁴C isotope is generated in the atmosphere of the Earth under the influence of cosmic rays, and its concentration in annual tree rings carries information on the past solar activity. However, the concentration of this isotope in annual tree rings may also be influenced by climatic factors. In the present work, the possible correlation between variations in the ¹⁴C atmospheric content and in the Earth’s global temperature from the late 14th century to the middle of the 19th century is studied. It is shown that variations in global temperature may produce changes in the ¹⁴C atmospheric content and consequently have to be taken into account in reconstructions of the past solar activity.     

Regional beryllium-10 production rate calibration for late-glacial northeastern North America

The major uncertainty in relating cosmogenic-nuclide exposure ages to ages measured by other dating methods comes from extrapolating nuclide production rates measured at globally scattered calibration sites to the sites of unknown age that are to be dated. This uncertainty can be reduced by locating production rate calibration sites that are similar in location and age to the sites to be dated. We use this strategy to reconcile exposure age and radiocarbon deglaciation chronologies for northeastern North America by compiling ¹⁰Be production rate calibration measurements from independently dated late-glacial and early Holocene ice-marginal landforms in this region. ¹⁰Be production rates measured at these sites are 6–12% lower than predicted by the commonly accepted global ¹⁰Be calibration data set used with any published production rate scaling scheme. In addition, the regional calibration data set shows significantly less internal scatter than the global calibration data set. Thus, this calibration data set can be used to improve both the precision and accuracy of exposure dating of regional late-glacial events. For example, if the global calibration data set is used to calculate exposure ages, the exposure-age deglaciation chronology for central New England is inconsistent with the deglaciation chronology inferred from radiocarbon dating and varve stratigraphy. We show that using the regional data set instead makes the exposure age and radiocarbon chronologies consistent. This increases confidence in correlating exposure ages of ice-marginal landforms in northeastern North America with glacial and climate events dated by other means.     

Transport ofBe andBe in the ocean

Beryllium isotopes (¹⁰Be and⁹Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of¹⁰Be and⁹Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 10⁹ and 6 × 10¹⁶ atoms g⁻¹ of¹⁰Be and⁹Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The¹⁰Be/⁹Be ratio in trap samples varies from 3 to 20 × 10⁻⁸. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of¹⁰Be into the traps at about 1500 m are estimated as 9 × 10⁵ and 4 × 10⁵ atoms cm⁻² a⁻¹ at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 10⁵ atoms cm⁻² a⁻¹ at both locations. Good correlations exist between¹⁰Be,⁹Be and²⁷Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.     

Beryllium-10 in continental sediments

The concentration of¹⁰Be has been measured in 10 samples taken from a transect of surface sediments beginning in the Atchafalaya River and extending across the Bay 136 km into the Gulf of Mexico. If corrected for a lower retentivity of sand for Be, they have a concentration that is constant within 13%. This concentration is about an order of magnitude smaller than that of deep ocean sediments. For comparison, measurements of¹⁰Be in rainwater, in a sample of soil and in a deep ocean core were made.     

Regional Be production rate calibration for the past 12 ka deduced from the radiocarbon-dated Grøtlandsura and Russenes rock avalanches at 69° N, Norway

Two rock avalanches in Troms County – the Grøtlandsura and Russenes – were selected as CRONUS-EU natural cosmogenic ¹⁰Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring ⁹Be (typically < 1.5 ppb); and (c) have reliable minimum radiocarbon ages of 11,424 ± 108 cal yr BP and 10,942 ± 77 cal yr BP (1σ), respectively. Quartz samples (n = 6) from these two sites contained between 4.28 × 10⁴ and 5.06 × 10⁴ at ¹⁰Be/g using the 1.387 Myr ¹⁰Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced ¹⁰Be concentrations by 10%. Using the ¹⁰Be half-life of 1.387 Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total ¹⁰Be production rate of 4.12 ± 0.19 at/g/yr (1σ). A corresponding error-weighted mean spallogenic ¹⁰Be production rate is 3.96 ± 0.16 at/g/yr (1σ), respectively. These are in agreement within uncertainty with other ¹⁰Be production rates in the literature, but are significantly, statistically lower than the global average ¹⁰Be production rate. This research indicates, like other recent studies, that the production of cosmogenic ¹⁰Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total ¹⁰Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Hølen avalanche) to be 7.5 ± 0.3 kyr (1σ). This age suggests that the rock avalanche occurred shortly after the 8.2 kyr cooling event, just as the radiocarbon ages of the Grøtlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation.