Research on codaQ distribution in Beijing and its surrounding regions

Based on the single scattering model of seismic coda waves, we have calculated theQ-factor in Beijing and its surrounding regions by means of calculating the power density spectrum in frequency domain with a fixed time window. The digital seismic data of 69 earthquakes from Beijing Telemetered Seismographic Network are used. These earthquakes were recorded from January 1, 1989 to December 31, 1990 at 20 stations. This paper shows the variations of the codaQ-factors in the studied region with different sites, frequency and lapse time, and the temporal change of the codaQ-factors in these two years. The results indicate that codaQ-factor depends strongly on the lapse time and frequency. It is assumed that whenQ _C=Q ₀f^η, for the three time windows of 15–30s, 30–60s and 60–90s, the average values ofQ ₀ are 48, 115 and 217; and the average values ofη are 0. 89, 0.91 and 0.74, respectively.     

241Pu/239+240Pu ratios in polar glaciers

The analyses of plutonium isotopes in dated strata of polar ice sheets indicate that the²⁴¹Pu/^239+240Pu activity ratio produced in the U.S.-dominated nuclear tests during the 1950s was about 26, while that in the U.S.S.R.-dominated weapons tests in the early 1960s was between 12 and 14. This difference provides time horizons for sedimentary deposits. Further, the²³⁹Pu/²⁴⁰Pu ratio may show a similar difference in fallout values from these two periods of weapons testing and may provide an additional, and perhaps more sensitive, chronology for sediments.     

Elemental and stable isotope variations of organic matter from a terrestrial sequence containing the Cretaceous/Tertiary boundary at York Canyon,New Mexico

Elemental and isotopic composition of organic matter from a terrestrial sequence including the palynological Cretaceous/Tertiary (K-T) boundary together with an Ir anomaly at York Canyon, New Mexico, record information about paleoclimatological and environmental conditions. Six layers of coal, carbonaceous shale and mudstone with high contents of organic material were selected for analysis. A Late Cretaceous coal bed 10 m below the K-T boundary and an Early Tertiary coal bed containing the K-T iridium anomaly at its base were sampled intensively. In the lower bed, the isotopic ratios¹³C/¹²C,¹⁵N/¹⁴N,andD/H and the elemental ratiosC/N andN/H, all varied sympathetically with one another over depth. In contrast, in the upper coal layer, only theD/H,C/N,andN/H ratios showed some coupling. Immediately above the K-T boundary, theδ¹³C values displayed a long-term shift of 1.8‰ to more negative values, while the hydrogen isotope ratios in these samples did not change significantly. We interpret the covariations in both coal layers as sympathetic responses of the isotopic and elemental ratios to climatic and environmental changes, as have been observed for younger sedimentary organic matter. The long-termδ¹³C shift during the early Tertiary is similar to the trend observed forδ¹³C values of marine carbonates. Our data thus support the proposal that the carbon cycle was perturbed globally by the effects of a drastically decreased marine bioproductivity along the K-T transition. The uncoupling of variations in the climatically sensitive isotopic and elemental ratios seen in the Early Tertiary coal bed provides evidence for major geochemical and environmental changes in the York Canyon area at the end of the Mesozoic. On the other hand, the constancy of δD values in the organic matter deposited at and above the K-T boundary gives no indication of significant changes in the hydrologic/climatic regimes as recorded in theD/H ratios at the site for several thousand years following the event which produced the high Ir concentrations. Our results provide constraints on models that have been advanced to explain that event and its consequences.     

Turnover of Eastern Caribbean deep water from 14C measurements

A¹⁴C balance for the Eastern Caribbean deep water indicates the average inflow of Atlantic water into the basin to be 2.3 × 10⁵ m³/sec (±30%), or about 2–4 times the values estimated previously. The balance uses a model representation of the deep-water turnover, and is based on¹⁴C concentrations at a station in the Venezuelan Basin which average Δ¹⁴C= 89‰ below 800 m depth with a total range of only 9‰, as well as on a¹⁴C concentration of the Atlantic inflow of Δ¹⁴C= ?71%. as obtained from measurements outside the Antilles Arch. The turnover time of the basin water below 2500 m depth is 55 years, which corresponds to an average upwelling velocity at this depth of about 35 m/year. With such upwelling, the temperature profile below 1800 m (the depth of the sill determining the inflow of new water) requires a vertical eddy diffusivity of about 5 cm²/sec. The oxygen consumption, and silica and CO₂ regeneration, rates below 2500 m depth are obtained as ?0.18, + 0.08, and + 0.2 μmole kg^?1 yr^?1, respectively. The CO₂ regeneration has but a negligible effect on the¹⁴C balance.     

Research on coda Q distribution in Beijing and its surrounding regions

Based on the single scattering model of seismic coda waves, we have calculated theQ-factor in Beijing and its surrounding regions by means of calculating the power density spectrum in frequency domain with a fixed time window. The digital seismic data of 69 earthquakes from Beijing Telemetered Seismographic Network are used. These earthquakes were recorded from January 1, 1989 to December 31, 1990 at 20 stations. This paper shows the variations of the codaQ-factors in the studied region with different sites, frequency and lapse time, and the temporal change of the codaQ-factors in these two years. The results indicate that codaQ-factor depends strongly on the lapse time and frequency. It is assumed that whenQ _C=Q ₀f^η, for the three time windows of 15–30s, 30–60s and 60–90s, the average values ofQ ₀ are 48, 115 and 217; and the average values ofη are 0. 89, 0.91 and 0.74, respectively. Contribution No. 95A0009, Institute of Geophysics, SSB, China. This work is a contract subject 85-04-01-02 of the State Seismological Bureau, China.     

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.     

Estimation of the percentage of annual groundwater recharge with bomb tritium using a cumulative mass balance method

The bomb tritium (³H) distribution patterns in the aquifer beneath an abandoned landfill at the Canadian Forces Base (CFB) Borden, Ontario, and in a sandy aquifer at Whiteshell Nuclear Research Establishments (WNRE) Pinawa, Manitoba, all in Canada, were delineated in great detail. A sampling and monitoring network of multilevel samplers and bundle piezometers were used. The directions of groundwater flow were established, and the boundary between the tritiated and non-tritiated zones of the two aquifers were closely demarcated. Using a cumulative mass balance method, the³H input mass into the aquifers was compared with the³H mass in groundwater storage to estimate the percentages of annual groundwater recharge from 1953 to 1978. Two recharge calculations for theeffective recharge zone and thetotal recharge area of the aquifers as established from the flownet analysis, and the distributions of dissolved geochemical constitutents showed that theeffective recharge zone calculations gave higher values of 30.6 cm/yr for CFB Borden and 20.1 cm/yr for WNRE while thetotal recharge areas gave lower values of 19.1 and 10.1 cm/yr for the Borden and WNRE aquifers respectively. The two recharge values provide possible minimum and maximum recharge estimates for the two study areas.     

Thermoluminescence and the terrestrial age of meteorites: Some recent results

This paper reports the results of an examination of the thermoluminescence (TL) of 23 meteorites which were observed to fall, and 17 meteorite finds which have had their terrestrial age determined by the¹⁴C method. The terrestrial ages of the observed falls range from 1 to 205 years, whilst the¹⁴C terrestrial ages range from 1200 ± 2000 to >20,000 years. A statistically significant correlation has been observed between the natural TL — as expressed as the ratio of the intensity of the low-temperature TL peak to that of the high-temperature peak — and the terrestrial age of the 40 meteorites. Furthermore, peak height ratios in excess of 3.0 are only observed in falls which fell within the last 250 years, suggesting that finds with peak height ratios as large as this, such as Allan Hills A77003 and Plainview (1917), fell within the last few hundred years. The present results are consistent with evidence that meteorite TL decay is a non-first-order process. The implications of the results for estimates of the terrestrial ages of 8 meteorite finds, for which there are no¹⁴C data, are also discussed.     

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.     

Al andBe production in iron meteorites

We have measured by accelerator mass spectrometry the²⁶Al contents of 20 and the¹⁰Be contents of 14 iron meteorites. The²⁶Al contents are typically 30% or more lower than values obtained by counting techniques; the¹⁰Be contents are 10–15% lower. The production rates (P) of these nuclides decrease by more than a factor of two as the⁴He/²¹Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimateP₂₆(Fe) as 3–4 dpm/kg andP₁₀(Fe) as 4–5 dpm/kg. The average P/₁₀P₂₆ activity ratio is close to 1.5. Exposure ages calculated from²¹Ne/²⁶Al ratios cannot be calibrated so as to agree with both⁴⁰KK/ ages and ages based on the shorter-lived nuclides³⁹Ar and³⁶Cl. If agreement with the latter is forced, then the disagreement with⁴⁰KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 10⁷ a.     

Lower Groundwater 14C Age by Atmospheric CO2 Uptake During Sampling and Analysis

Uptake of atmospheric CO₂ during sample collection and analysis, and consequent lowering of estimated ages, has rarely been considered in radiocarbon dating of groundwater. Using field and laboratory experiments, we show that atmospheric CO₂ can be easily and rapidly absorbed in hyperalkaline solutions used for the extraction of dissolved inorganic carbon, resulting in elevated ¹⁴C measurements. Kinetic isotope fractionation during atmospheric CO₂ uptake may also result in decrease of δ¹³C, leading to insufficient corrections for addition of dead carbon by geochemical processes. Consequently, measured ¹⁴C values of groundwater should not be used for age estimation without corresponding δ¹³C values, and historical ¹⁴C data in the range of 1 to 10% modern Carbon should be re‐evaluated to ensure that samples with atmospheric contamination are recognized appropriately. We recommend that samples for ¹⁴C analysis should be collected and processed in the field and the laboratory without exposure to the atmosphere. These precautions are considered necessary even if ¹⁴C measurements are made with an accelerator mass spectrometer.     

Penetration of bomb radiocarbon in the tropical Indian Ocean measured by means of accelerator mass spectrometry

Radiocarbon measurements performed on seawater samples by means of accelerator mass spectrometry (AMS) enable to reduce by a factor of 2000 the water sample size needed for the¹⁴C measurements. Therefore no chemical treatment on board the oceanographic vessel is required. Seventy-four AMS¹⁴C determinations on samples collected in the tropical-equatorial Indian Ocean during the second leg of the INDIGO program (1986) are presented and compared with the β-counting results obtained during the same campaign and the GEOSECS program (1978). A pronounced reduction of the equatorial¹⁴C deficit suggests that substantial amounts of bomb-¹⁴C are associated with the westward flowing Pacific water which enters the Indian Ocean via passages through the Indonesia archipelago and/or to meridional mixing with¹⁴C-rich water of the southern subtropical gyre.     

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.     

Organic carbon release by phytoplankton and bacterial reassimilation

The release of organic carbon by phytoplankton and its reassimilation by bacteria were studied in Lake Geneva during four daily kinetics, using¹⁴C techniques. Gentamycin was used to suppress bacterial activity. Size fractionation was used to measure¹⁴C fixation in 2 particulate fractions (≥1 μm and 0.2 to 1 μm) and in a third fraction: dissolved organic carbon. Measurements of bacterial biomass and¹⁴C glucose uptake as the H¹⁴CO ₃ ⁻ uptake by samples prefractionated before incubation showed that the whole algae were retained on 1 μm pore size Nuclepore filters, but bacteria were retained on 1 μm and 0.2 μm filters. Concentration of 20 μg.ml⁻¹ gentamycin resulted in incomplete bacterial inhibition while phytoplankton was affected. Phytoplankton released less of 20% of its photoassimilated carbon of which a large proportion was utilized by bacterioplankton.     

Ancient solar wind in lunar microbreccias

The Apollo 11 soil breccias are samplers of the ancient lunar environment due to their history in the regolith and their efficient closure to addition of recent solar wind upon compaction. These breccias contain the lowest¹⁵N/¹⁴N isotopic ratio yet reported for any lunar sample (in fact, for any natural sample). This extends the range of variation of¹⁵N/¹⁴N of the solar wind to greater than 30%, from a δ¹⁵N of ?190‰ in the past to +120‰ at present. No mechanism is yet known that is capable of accounting for such a large change in the¹⁵N/¹⁴N ratio without producing a substantial concomitant change in the¹³C/¹²C ratio, although some sort of nuclear reaction in the sun appears to be required. Apollo 11 soil breccias and 15086 are all formed by meteoritic impacts which compact the lower regolith against the basement rock without much heating. Rock 15086 formed from the layer of regolith between 100 and 200 cm depth, as shown by the close agreement between the nitrogen content and isotopic ratios of 15086 and those of the Apollo 15 deep drill core. Cosmic ray exposure ages, based on spallation-produced¹⁵N, are 2.3 ± 0.4 b.y. for Apollo 11 breccias. This age is much greater than the estimate from cosmogenic²¹Ne, presumably due to diffusive loss of neon.     

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.     

Radium and thorium isotopes in the surface waters of the East Pacific and coastal Southern California

A fiber extraction technique is used to concentrate Ra and Th isotopes from 1000 liters or more of seawater. Natural²²⁶Ra and²³⁴Th are used as yield tracers. In the equatorial Pacific the²²⁸Ra activity of surface water varies from 20 to 1 dpm/1000 kg and generally decreases away from continental shelf areas. Across the Peru Current System, this decrease is modeled as one-dimensional diffusion and indicates the possibility of two flow regimes with distinct characteristic mixing lengths and apparent eddy diffusivities of 10⁵ and 10⁷ cm²/s. The perturbing effects of advection and equatorial upwelling west of the Galapagos Islands are noted. Off the coast of Southern California a vertical²²⁸Ra distribution gives an apparent diffusivity of 1.6 cm²/s for the upper thermocline.²²⁶Ra concentrations near the coast appear to be higher than the open ocean values at comparable depths, which may reflect supply of this isotope from continental shelf sediments and/or upwelling. The insoluble daughter/soluble parent activity ratios²²⁸Th/²²⁸Ra and²³⁴Th/²³⁸U in the equatorial Pacific surface water display latitudinal trends which may be correlated with productivity variations. Near the coast of California these ratios reflect the differing oceanographic conditions north and south of Pt. Conception indicating a mean chemical removal time constant on the order of 4 months for Th and other highly reactive elements within the Southern California Bight. The²³²Th content of seawater sampled is less than 0.1 μg/1000 1; most of the published values for seawater²³²Th could well be too high. A comparison of the two methods of determining²²⁸Ra (via²²⁸Ac and via²²⁸Th) made on 64 seawater samples shows that the time delay required by the²²⁸Th method is more than compensated by its better analytical simplicity and precision.     

Later Pleistocene/Holocene climate conditions of Qinghai-Xizhang Plateau (Tibet) based on carbon and oxygen stable isotopes of Zabuye Lake sediments

We present a time series of carbon and oxygen stable isotope records of the last 30?000 ¹⁴C years throughout the last glacial-postglacial cycle from western Qinghai-Xizhang (Tibet) Plateau. A 20-m core drilled in the south basin of Zabuye Salt Lake was analyzed for inorganic and organic carbon and total sulfur contents, δ¹³C and δ¹⁸O values of carbonates. Our results indicate that climatic changes have led to a drastic negative shift of stable isotope ratios at the transition between the Last Full Glacial and the postglacial phase during Later Pleistocene times (∼16.2 kyr BP), and a rapid positive shift at the transition from Pleistocene to Holocene (∼10.6 kyr BP). The first shift is marked by the drop of δ¹⁸O_carb values of about 10‰ (from +2 to −8‰) and δ¹³C_carb values of about 3‰ (from 5 to 2‰). The second shift which occurred at the transition from Pleistocene to Holocene was of similar magnitude but in the opposite direction. Isotope data, combined with total organic and inorganic carbon contents and the lithological composition of the core, suggest this lake was an alluvial pre-lake environment prior to ca. 28 ¹⁴C kyr BP. During ca. 28-16.2 ¹⁴C kyr BP, Zabuye Lake was likely a moderately deep lake with limited outflow. The cool and arid glacial climate led the lake level to drop drastically. Extended residence time overwhelmed the lower temperature and caused a steady increase of δ¹³C_carb and δ¹⁸O_carb values and total inorganic carbon content in the sediments. During ca. 16.2-10.6 ¹⁴C kyr BP, this lake probably overflowed and received abundant recharge from melting glaciers when the deglaciation was in its full speed. A spike of markedly enhanced δ¹³C_carb and δ¹⁸O_carb is seen at ∼11.5 kyr BP, probably due to the isotopic effects left behind by the short but severe Younger Dryas (YD) event. After ca. 10.6 ¹⁴C kyr BP, Zabuye Lake probably closed its surface outflow, due to strong desiccation and drastic climate warming. The Early and Middle Holocene were characterized by unstable climatic conditions with alternating warmer/cooler episodes as indicated by the severe fluctuations of total organic carbon, δ¹³C and δ¹⁸O values. A hypersaline salt lake environment was finally formed at Zabuye after ∼5 ¹⁴C kyr BP when the mirabilite and halite concentrations steadily increased and became the dominant minerals in the sediments. Severe imbalance of inflow/outflow resulted in the drastic increase of total sulfur, δ¹³C_carb and δ¹⁸O_carb values and dominance of halite in the lake since ca. 3.8 kyr BP to present.     

Excess40Ar in metamorphic rocks from Broken Hill,New South Wales: implications for40Ar/39Ar age spectra and the thermal history of the region

⁴⁰Ar/³⁹Ar age spectrum analyses of samples from Broken Hill, New South Wales, indicate that the region has experienced a complex thermal history following high-grade metamorphism, 1660 Ma ago. The terrain cooled slowly (～3°C Ma^?1) until about 1570 Ma ago, when the temperature fell below about 500°C. Following granitoid emplacement ～1500 Ma ago, the region remained relatively cold until affected by a thermal pulse 520±40Ma ago, causing temperatures to rise to～350°C in some places. During this event, accumulated⁴⁰Ar was released from minerals causing a significant Ar partial pressure to develop. Laboratory Ar solubility data combined with the⁴⁰Ar/³⁹Ar age spectra gives a local estimate of this partial pressure of ～10^?4atm. The region finally cooled below 100°C about 280 Ma ago.⁴⁰Ar/³⁹Ar age spectrum analyses of hornblende, plagioclase and clinopyroxene containing excess⁴⁰Ar are characterized by saddle-shaped age spectra. Detailed analysis of plagioclase samples reveals a complex diffusion behaviour, which is controlled by exsolution structures. This effect, in conjunction with the presumed different lattice occupancy of excess⁴⁰Ar with respect to radiogenic⁴⁰Ar, appears to be responsible for the saddle-shaped age spectra.     

Radiocarbon reservoir corrections on the Brazilian coast from pre-bomb marine shells

Despite the great extent of the Brazilian coast, there are very few studies on the marine radiocarbon reservoir correction local offset (ΔR) for this region.In the present study, we used marine shells from the malacological collection of the National Museum of Brazil to obtain values of ΔR for different locations on the Brazilian coast. According to the museum records, the mollusks were collected alive from the coasts of Rio de Janeiro, Rio Grande do Sul, São Paulo and Bahia, prior to the ¹⁴C bomb-peak. The exact year of collection for each shell is known.The samples were chemically prepared and measured at the Radiocarbon Laboratory of the Universidade Federal Fluminense (LAC-UFF) in Brazil. The conventional ages were calibrated using the Marine13 curve and the ΔR values were calculated using the OxCal software. The values range from 17 ± 56 ¹⁴C yr in São José do Norte, Rio Grande do Sul to 96 ± 48 ¹⁴C yr in Cabo Frio, Rio de Janeiro.