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1.
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.  相似文献   

2.
The concentrations of7Be have been measured in Pacific and Atlantic ocean water for the past several years to determine the deposition velocity of aerosol particles on the ocean surface.7Be is produced at a relatively constant rate in the atmosphere by spallation reactions of cosmic rays with atmospheric nitrogen and oxygen. Immediately after its formation7Be becomes attached to aerosol particles, and therefore can serve as tracers of the subsequent behavior of these particles. Isopleths of7Be surface water concentrations,7Be inventory in the ocean, and deposition velocity have been prepared for the Pacific Ocean from 30°S to 60°N and for the Atlantic Ocean from 10°N to 55°N. The concentrations, inventories and deposition velocities tended to be higher in regions where precipitation was high, and generally increased with latitude. The average flux of7Be across the ocean surface was calculated to be 0.027 atoms cm?2 s?1 which is probably not significantly greater than the worldwide average7Be flux across land and ocean surfaces of 0.022 atoms cm?2 s?1 calculated by Lal and Peters. The average deposition velocity was calculated to be 0.80 cm s?1. This value may be 10–50% too low, since it was calculated using atmospheric7Be concentrations which were measured at continental stations. Measurements of atmospheric7Be concentrations at ocean stations suggest that the concentrations at the continental stations averaged 10–50% higher than the concentrations over the ocean.  相似文献   

3.
Short‐lived fallout isotopes, such as beryllium‐7 (7Be), are increasingly used as erosion and sediment tracers in watersheds. 7Be is produced in the atmosphere and delivered to the Earth's surface primarily in precipitation. However, relatively little has been published about the variation in 7Be wet deposition caused by storm type and vegetation cover. Our analysis of precipitation, throughfall, and sediments in two forested, headwater catchments in the mid‐Atlantic USA indicates significant variation in isotope deposition with storm type and storm height. Individual summer convective thunderstorms were associated with 7Be activity concentrations up to 5.0 Bq l?1 in precipitation and 4.7 Bq l?1 in throughfall, while single‐event wet depositional fluxes reached 168 Bq m?2 in precipitation and 103 Bq m?2 in throughfall. Storms originating from the continental USA were associated with lower 7Be activity concentrations and single‐event wet depositional fluxes for precipitation (0.7–1.2 Bq l?1 and 15.8–65.0 Bq m?2) and throughfall (0.1–0.3 Bq l?1 and 13.5–98.9 Bq m?2). Tropical systems had relatively low activity concentrations, 0.2–0.5 Bq l?1 in precipitation and 0.2–1.0 Bq l?1 in throughfall, but relatively high single‐event depositional fluxes due to large rainfall volumes, 32.8–67.6 Bq m?2 in precipitation and 25.7–134 Bq m?2 in throughfall. The largest sources of 7Be depositional variation were attributed to storm characteristics including precipitation amount and maximum storm height. 7Be activity associated with fluvial suspended sediments also exhibited the highest concentration and variability in summer (175–1450 Bq kg?1). We conclude the dominant source of variation on event‐level 7Be deposition is storm type. Our results illustrate the complex relationships between 7Be deposition in precipitation and throughfall and demonstrate event‐scale relationships between the 7Be in precipitation and on suspended sediment. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

4.
The presence of cosmogenic10Be (t1/2 = 1.5 × 106 y) in island-arc volcanic rocks has been interpreted as indicating that sediments are subducted to the depths of island-arc magma genesis [1,2]. We have measured10Be and9Be (the stable common isotope) in phenocryst and groundmass fractions separated from four Aleutian volcanic rocks.10Be and9Be covary linearly in three of the rocks proving that the10Be was incorporated prior to the formation of phenocrysts. In the other rock10Be and9Be do not covary; the lack of covariation indicates either that the rock incorporated10Be during weathering or that9Be-rich xenocrysts were incorporated by mechanical mixing. Our results strongly support the hypothesis of Brown et al. [1] and Tera et al. [2] that sediments containing10Be are an integral part of island-arc magmatic systems. As we cannot identify the depth at which the10Be was incorporated we still cannot completely rule out the possibility of assimilation of sediments or altered crust near the Earth's surface as a source of10Be.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
The reasons why53Mn (a cosmogenic radionuclide with a half-life of 3.7 × 106 y) appears as one of the best indicators of the presence of interplanetary dust are summarized. This paper reports the detection of53Mn in pre-1952 snow samples collected on the Eastern Antarctic Plateau in the vicinity of Plateau Station. The measurements were carried out by neutron activation and X-ray spectrometry on three samples weighing a few hundred kg and covering each the time interval 1935–1950. The specific activity of53Mn was found to be (0.82 ± 0.17) disint.min?1/103 tons of snow, corresponding to a deposition rate at Plateau Station of (2.2 ± 0.5) × 10?5 disint. min?1 m?2 y?1. The mean global deposition rate would be three times higher if53Mn were assumed to behave in the same way as stratospheric90Sr. By comparing this figure with existing data on the meteorite flux reaching the earth and with the galactic and solar production rates of53Mn, it is concluded that the bulk of the53Mn found at Plateau Station is associated with interplanetary dust in which it had been produced by the action of solar protons on iron. The deposition rate of extra-terrestrial dust-borne iron must be between 1.3 × 10?5 and 1.3 × 10?4 g m?2 y?1 at Plateau Station. These results support jointly with other studies the concept of an interplanetary zodiacal cloud of dust with a chemical composition and density not essentially different from chondritic meteorites, with a relatively ‘flat’ grain size distribution and a mass influx to the earth of the order of 105 tons/y.  相似文献   

8.
Atmospheric chloride deposition in continental Spain   总被引：1，自引：0，他引：1  
F. J. Alcalá  E. Custodio 《水文研究》2008,22(18):3636-3650
The atmospheric bulk deposition rate of chloride in continental Spain was studied to get basic information in order to help in the evaluation of diffuse recharge to aquifers through an environmental chemical balance. Both new, recent data and bibliographic data have been used. Most sampling records are less than 5 years long and often only 1 year long. This means that the calculated mean yearly bulk deposition rate of chloride is quite uncertain by 30% on average, and larger than the values derived form records up to 15 years long. A map of atmospheric bulk deposition of chloride has been drawn using ordinary kriging. The mean bulk deposition rate of chloride varies from 1 to 30 g m?2 year?1 in coastal areas, with strong negative landward gradients between 0·1 and 1 g m?2 year?1 km?1. In the centre of the Iberian Peninsula, chloride deposition rates vary from 0·2 to 0·5 g m?2 year?1, with gradients around or less than 5 × 10?3 g m?2 year?1 km?1. The coefficient of variation of the mean bulk atmospheric deposition rate of chloride, for any place, ranges from 0·1 to 1. Values larger than ～0·5 are not a good indicator of natural uncertainty for this series of data that has a skewed distribution. The map of bulk deposition rate and its error is one of the terms needed for aquifer recharge estimation by means of the chloride ion balance. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

9.
The production rate of cosmogenic 3He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against 10Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have 10Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals 3He correlates with 10Be and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, 3He/10Be systematics imply apparent sea-level high-latitude (SLHL) apparent production rates for 3He of 226 atoms g? 1 yr? 1 in zircon, 254 atoms g? 1 yr? 1 in apatite, 177 atoms g? 1 yr? 1 in kyanite, and 153 atoms g? 1 yr? 1 in garnet. These production rates are unexpectedly high compared with rates measured elsewhere in the world, and also compared with proposed element-specific production rates. For apatite and zircon, the data are sufficient to conclude that the 3He/10Be ratio increases with elevation. If this reflects different altitudinal scaling between production rates for the two isotopes then the SLHL production rates estimated by our approach are overestimates. We consider several hypotheses to explain these observations, including production of 3He via thermal neutron capture on 6Li, altitudinal variations in the energy spectrum of cosmic-ray neutrons, and the effects of snow cover. Because all of these effects are small, we conclude that the altitudinal variations in production rates of cosmogenic 3He and 10Be are distinct from each other at least at this location over the last ～ 10 kyr. This conclusion calls into question commonly adopted geographic scaling laws for at least some cosmogenic nuclides. If confirmed, this distinction may provide a mechanism by which to obtain paleoelevation estimates.  相似文献   

10.
Increases in the production rate of cosmogenic radionuclides associated with geomagnetic excursions have been used as global tie-points for correlation between records of past climate from marine and terrestrial archives. We have investigated the relative timing of variations in 10Be production rate and the corresponding palaeomagnetic signal during one of the largest Pleistocene excursions, the Iceland Basin (IB) event (ca. 190 kyr), as recorded in two marine sediment cores (ODP Sites 1063 and 983) with high sedimentation rates. Variations in 10Be production rate during the excursion were estimated by use of 230Thxs normalized 10Be deposition rates and authigenic 10Be/9Be. Resulting 10Be production rates are compared with high-resolution records of geomagnetic field behaviour acquired from the same discrete samples. We find no evidence for a significant lock-in depth of the palaeomagnetic signal in these high sedimentation-rate cores. Apparent lock-in depths in other cores may sometimes be the result of lower sample resolution. Our results also indicate that the period of increased 10Be production during the IB excursion lasted longer and, most likely, started earlier than the corresponding palaeomagnetic anomaly, in accordance with previous observations that polarity transitions occur after periods of reduced geomagnetic field intensity prior to the transition. The lack of evidence in this study for a significant palaeomagnetic lock-in depth suggests that there is no systematic offset between the 10Be signal and palaeomagnetic anomalies associated with excursions and reversals, with significance for the global correlation of climate records from different archives.  相似文献   

11.
The problem of identifying areas of accelerated erosion in a dynamic landscape is complicated. The limited history of sediment yield measurements makes this task difficult even if geomorphic evidence is available. Beryllium-10, a cosmogenic isotope produced by cosmic rays interacting with the earth's atmosphere and surface, has chemical and physical properties that make it useful as a tracer for erosion and sediment transport processes. The rarity of the stable isotope, 9Be, allows 10Be to be detected with accelerator mass spectrometry in natural materials at extremely low levels. Backgrounds for rocks and sediments below 105 atom per g are now attainable, a value to be compared with an average deposition rate of 1.3 × 106 atom cm?2 yr?1. The affinity of Be for the components of soil and sediment is sufficiently high that it is effectively immobilized on contact, thereby allowing 10Be to function as a tracer of sediment transport. To a good approximation all the 10Be transport out of a drainage basin is on the sediment leaving it. The number of 10Be atoms passing the gauging station can be determined by measuring the concentration of the isotope in the sediment, if the annual sediment load is known. The ratio of the 10Be carried from the basin by the sediment to that incident upon it, called the erosion index, has been determined for 48 drainage basins within the same physiographic province, which allows them to be reasonably compared, all of which have sediment yield data. Basins located in the Atlantic coastal plain have an average index of 0.3 with the maximum observed being 0.9. Basins located between the fall line and the mountains, a region called the Piedmont, have an average value of 2.2 with individual values ranging from 0.6 to 6.7; this marked difference is thought to result from two centuries of farming on land of moderate gradient. Basins in the highland regions reflect local conditions with low indices for those in grass and timber and high indices associated with destructive land use. The data allow an estimate of the erosion index for the pre-colonial Piedmont, which then allows the pre-colonial sediment yield to be calculated. A number of basins have also been examined world wide with similar conclusions derived. An important deviation from the rule is noted for rivers that erode large regions of loess, such as the Mississippi, Hwang Ho, and Yangtze. Large aeolian deposits were laid down during the ice age in these basins, deposits that brought inherited 10Be with them and that are easily eroded.  相似文献   

12.
Terrestrial cosmogenic nuclides (TCN) have widely been used as proxies in determining denudation rates in catchments. Most studies were limited to samples from modern active streams, thus little is known about the magnitude and causes of TCN variability on millennial time scales. In this work we present a 6 kyrs long, high resolution record of 10Be concentrations (n = 18), which were measured in sediment cores from an alluvial fan delta at the outlet of the Fedoz Valley in the Swiss Alps. This record is paired with a 3‐year time series (n = 4) of 10Be measured in sediment from the active stream currently feeding this fan delta. The temporal trend in the 10Be concentrations after correction for postdepositional production of 10Be was found to be overall constant and in good agreement with the modern river 10Be concentration. The calculated mean catchment‐wide denudation rate amounts to 0.73 ± 0.18 mm yr?1. This fairly constant level of 10Be concentrations can be caused by a constant denudation rate over time within the catchment or alternatively by a buffered signal. In this contribution we suggest that the large alluvial floodplain in the Fedoz Valley may act as an efficient buffer on Holocene time scales in which sediments with different 10Be signatures are mixed. Therefore, presumable variations in the 10Be signals derived from changes in denudation under a fluctuating Holocene climate are only poorly transferred to the catchment outlet and not recorded in the 10Be record. However, despite the absence of high frequency signals, we propose that the buffered and averaged 10Be signal could be meaningfully and faithfully interpreted in terms of long‐term catchment‐averaged denudation rate. Our study suggests that alluvial buffers play an important role in regulating the 10Be signal exported by some alpine settings that needs to be taken into account and further investigated. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

13.
Well-dated records of tropical glacier fluctuations are essential for developing hypotheses and testing proposed mechanisms for past climate changes. Since organic material for radiocarbon dating is typically scarce in low-latitude, high-altitude environments, surface exposure-age dating, based on the measurement of in situ produced cosmogenic nuclides, provides much of the chronologic information on tropical glacier moraines. Here, we present a locally calibrated 10Be production rate for a low-latitude, high-altitude site near Quelccaya Ice Cap (∼13.95°S, 70.89°W, 4857 m asl) in the southeastern Peruvian Andes. Using an independent age (12.35 +0.2, −0.02 ka) of the late glacial Huancané IIa moraines based on thirty-four bracketing radiocarbon ages and twelve 10Be concentrations of boulders on the moraines, we determine a local production rate of 43.28 ± 2.69 atoms gram−1 year−1 (at g−1 yr−1). Reference 10Be production rates (i.e., production rates by neutron spallation appropriate for sea-level, high-latitude sites) range from 3.97 ± 0.09 to 3.78 ± 0.09 at g−1 yr−1, determined using scaling after Lal (1991) and Stone (2000) and depending on our assumed boulder surface erosion rate. Since our boulder surface erosion rate estimate is a minimum value, these reference production rates are also minimum values. A secondary control site on the Huancané IIIb moraines suggests that the 10Be production rates are at least as low as, or possibly lower than, those derived from the Huancané IIa moraines. These sea-level, high-latitude production rates are at least 11–15% lower than values derived using the traditional global calibration dataset, and they are also lower than those derived from the late glacial Breque moraine in the Cordillera Blanca of Peru. However, our sea-level, high-latitude production rates agree well with recently published, locally calibrated production rates from the Arctic, New Zealand, and Patagonia. The production rates presented here should be used to calculate 10Be exposure ages in low-latitude, high-altitude locations, particularly in the tropical Andes, and should improve the ability to compare the results of studies using 10Be exposure-age dating with other chronological data.  相似文献   

14.
The concentration profiles of 9Be, 10Be, 230Th, 232Th, 231Pa (via 227Th) and 238U have been measured in three manganese nodules, one each from the North Pacific (A47-16(4)), the South Pacific (TF-5) and the Indian Ocean (R/V Vitiaz). In addition the 10Be concentration in deep water from the GEOSECS reoccupation station 500 of the North Pacific, and in ? cores raised from the manganese nodule field in the North Pacific have been measured. The 10Be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the Yale Tandem Van de Graaff accelerator.The concentrations of 10Be, 230Thexc and 231Paexc and ratios of 10Be/9Be and 230Thexc/232Th all decrease with depth in the nodules. This decrease, interpreted in terms of nodule growth, yields “average” growth rates of a few millimeters per million years for the nodules. The growth rates of the nodules exhibit temporal variations, both on short time (～ 50,000 years) and long time (several million years) scales. Of the three nodules studied, only in TF-5 is the short-term average growth rate based on 230Thexc in the top 0–0.5 mm the same as the long-term average rate based on Be isotope data for the 0.5–17 mm interval. For the other two nodules, the recent average growth rates based on230Thexc data differ significantly from the long-term average growth rates based on Be isotopes. In A47-16(4) the 10Be based rate is less than the 230Thexc rate and in R/V Vitiaz the 10Be based rate is greater than the 230Thexc rate. This observation, coupled with measurable changes in growth rates even during the past few hundred thousand years, suggests, but does not prove, that the discordant growth rates deduced from 230Thexc and 10Be profiles document changes in nodule growth rate with time rather than mixing effects on 230Thexc profiles.The 10Be concentration in the GEOSECS North Pacific deep water is 6100±1200 atoms/g. This value coupled with the average surface 10Be/9Be ratio of North Pacific nodules predicts a 9Be concentration within the limits of measured values. The inventory of 10Be and 230Thexc in the nodules is only ～ 10% of the total, the remaining being in sediments. The 10Be concentrations in the upper portions of two adjacent cores studied are nearly the same, but the deposition fluxes of both 10Be and 230Th based on 230Th dating vary by a factor of two. This difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation.  相似文献   

15.
210Pb- and137Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with210Pb/210Po and revealed sediment accumulation rates of (0.055±0.015) g·cm?2·y?1 and (0.09±0.03) g·cm?2·y?1, respectively. A comparable rate of (0.07±0.01) g·cm?2·y?1 has been obtained from137Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm?2·y?1. Constant210Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct137Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of7Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant210Pb activities. Remobilization may also be the reason for an incomplete210Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the210Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.  相似文献   

16.
Detailed soil erosion studies bene?t from the ability to quantify the magnitude of erosion over time scales appropriate to the process. An inventory balance for 7Be was used to calculate sediment erosion in a 30·73 m2 plot during a series of runoff‐producing thunderstorms occurring over three days at the Deep Loess Research Station in Treynor, Iowa, USA. The inventory balance included determination of the pre‐ and post‐storm 7Be inventories in the soil, the atmospheric in?ux of 7Be during the event, and pro?les of the 7Be activity in the soil following the atmospheric deposition. The erosion calculated in the plot using the 7Be inventory balance was 0·058 g cm?2, which is 23 per cent of the annual average erosion determined using 137Cs inventories. The calculated erosion from the mass balance is similar to the 0·059 g cm?2 of erosion estimated from the amount of sediment collected at the outlet of the 6 ha ?eld during the study period and the delivery ratio (0·64). The inventory balance of 7Be provides a new means for evaluating soil erosion over the time period most relevant to quantifying the prediction of erosion from runoff. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

17.
Cosmogenic7Be(t1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess210Pb and/or239,240Pu have provided limits on rates of sediment accumulation in these environments and indices of the efficiency of the sediments as collectors of reactive nuclides over longer time scale.In sediment cores from Long Island Sound (marine) and Lake Whitney (fresh-water)7Be was measurable in the top 2–3 cm. Diffusion-analog particle-mixing coefficients calculated from these data are in the range of 10?7 cm2/s. For Long Island Sound the coefficients are lower by factors of 3–6 than those estimated from the depth distributions of excess234Th at the same stations [14]. For Lake Whitney the calculated mixing coefficient is an upper limit because of the possibility of a sampling artifact.Measurements of total (wet + dry) atmospheric deposition of7Be in New Haven give an average flux of 0.07 dpm/cm2 day during March-November, 1977; this is equivalent to a steady-state inventory of 5.4 dpm/cm2 in a perfect collector. Sediment cores from Long Island Sound contain about half this7Be inventory, consistent with either a mean residence time for7Be in the water column of about one half-life or with post-depositional loss of7Be from Long Island Sound sediments. The Lake Whitney cores contain about 5 dpm/cm2, much nearer the atmospheric delivery. A higher inventory of7Be in fresh-water, as compared to marine, sediments could be due either to a shorter mean residence time for7Be in fresh water or to lateral transport processes in the lake or its catchment. High inventories of excess210Pb and239,240Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.  相似文献   

18.
Cosmogenic 10Be concentrations in exposed bedrock surfaces and alluvial sediment in the northern Flinders Ranges reveal surprisingly high erosion rates for a supposedly ancient and stable landscape. Bedrock erosion rates increase with decreasing elevation in the Yudnamutana Catchment, from summit surfaces (13·96 ± 1·29 and 14·38 ± 1·40 m Myr?1), to hillslopes (17·61 ± 2·21 to 29·24 ± 4·38 m Myr?1), to valley bottoms (53·19 ± 7·26 to 227·95 ± 21·39 m Myr?1), indicating late Quaternary increases to topographic relief. Minimum cliff retreat rates (9·30 ± 3·60 to 24·54 ± 8·53 m Myr?1) indicate that even the most resistant parts of cliff faces have undergone significant late Quaternary erosion. However, erosion rates from visibly weathered and varnished tors protruding from steep bedrock hillslopes (4·17 ± 0·42 to 14·00 ± 1·97 m Myr?1) indicate that bedrock may locally weather at rates equivalent to, or even slower than, summit surfaces. 10Be concentrations in contemporary alluvial sediment indicate catchment‐averaged erosion at a rate dominated by more rapid erosion (22·79 ± 2·78 m Myr?1), consistent with an average rate from individual hillslope point measurements. Late Cenozoic relief production in the Yudnamutana Catchment resulted from (1) tectonic uplift at rates of 30–160 m Myr?1 due to range‐front reverse faulting, which maintained steep river gradients and uplifted summit surfaces, and (2) climate change, which episodically increased both in situ bedrock weathering rates and frequency–magnitude distributions of large magnitude floods, leading to increased incision rates. These results provide quantitative evidence that the Australian landscape is, in places, considerably more dynamic than commonly perceived. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

19.
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.  相似文献   

20.
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.  相似文献   

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