14C age and vertical mixing of deep-sea sediments

Vertical mixing models applied to the radiocarbon age stratigraphy of sediments have to make allowance for the presumably non-vanishing zero-age of the sedimentary particulates freshly arriving at the sediment surface. The appropriate modifications of the simple box model recently presented by Berger and Johnson are outlined. For seven deep-sea cores, zero-age values are found varying between 0.1 and 1.4 kyr. A high zero-age is likely to be indicative for redistribution processes occurring at the sea floor. The box model has been extended to make allowance for a change of sedimentation rate in the depositional interval under study.     

The 14C distribution in West Atlantic abyssal waters

¹⁴C specific activities in the western Atlantic show aging of about 160 years between 42°N and 30°S for southward-moving North Atlantic Deep Water. Most of the aging occurs in the North Atlantic, with a small increase in¹⁴C level for abyssal water near the equator.The northward-flowing Antarctic Bottom Water component ages about 80 years between 50°S and the equator. The rate of Atlantic bottom water formation is estimated at 18 Sverdrups; the rate of flow for northward-moving Antarctic Bottom Water at about 6 Sverdrups.     

The abundances of3H and 14C in the solar wind

Tritium is measured as a function of depth in a Surveyor 3 sample. The upper limit for solar-wind-implanted tritium gives a³H/¹H limit for the solar wind of 1 × 10^?11. The temperature release patterns of¹⁴C from lunar soils are measured. The¹⁴C release patterns from surface soils differ from a trench bottom soil and gives evidence for the presence of¹⁴C in the solar wind with a¹⁴C/¹H ratio of approximately 4 × 10^?11. The implications of these radio nuclide abundances in the solar wind are discussed.     

210Po and 210Pb distributions in ocean water profiles from the Eastern South Pacific

Two ocean profiles from the Peru Basin from regions with different surface productivities were analyzed for total²¹⁰Pb and²⁰¹Po to evaluate the influence of particulates in the water column on their distribution. Comparison with a published²²⁶Ra profile for the region was made. The profile closest to the coast, where upwelling and productivity are high, shows depletion of²¹⁰Pb relative to²²⁶Ra at all depths, with particularly marked excursions from radioactive equilibrium at the surface and in the bottom water.²¹⁰Po appears to be deficient relative to²¹⁰Pb at depth as well. Mean residence times in the deep water, relative to particulate removal from the water column to the sediments, of about 100 years for²¹⁰Pb and about two years for²¹⁰Po are indicated. The profile northwest of the upwelling region shows the²²⁶Ra²¹⁰Pb²¹⁰Po system close to equilibrium at all depths to 1500 m (except for the effect of atmospheric²¹⁰Pb input seen at the surface.     

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.     

REVIEW ON IN-SITU COSMOGENIC 14C DATING AND POTENTIAL APPLICATION IN PALEOEARTHQUAKE

Over the last two decades, in-situ cosmogenic ¹⁴C dating has become an import tool in Quaternary geology and is recognized to geoscientists because of its potential to provide information on exposure age and process rate estimates for geomorphic studies. The in-situ cosmogenic radiocarbon has a relatively short half-life(5730 yr)and is substantially more sensitive than all the other cosmogenic nuclides used so far. It is therefore particularly useful to determine surface-exposure ages of Holocene landforms and quantify erosion rates in rapidly denuding landscapes during the past few tens of thousands of years. Moreover, in situ ¹⁴C is produced in quartz which is both highly resistant to weathering and common in nature, so it can be used in combination with other in-situ cosmogenic nuclides such as ³He, ¹⁰Be,²¹Ne,²⁶Al, and ³⁶Cl to constrain complex exposure histories involving burial and/or erosion occurring over the past 25ka. The age and slip rate of Holocene normal fault have been undoubtedly a challenge for seismologists to be faced with as result from lack of appropriate late Quaternary sediment. Recently, the cosmogenic nuclides such as ³⁶Cl of preserved, seismically exhumed normal fault scarps were used to identify the last few major earthquakes and recover their ages and displacements through the modeling of the content of ³⁶Cl in the scarp rocks. This paper mainly summarizes the development of in-situ ¹⁴C dating, including its research history, production rate estimate, production mechanism, chemical behavior and experimental method. The potential application of in-situ ¹⁴C dating to recovering past earthquakes, their timing, and the regularity of their recurrence for preserved, seismically exhumed normal fault scarps is also introduced.     

中国新疆博斯腾湖现代碳库效应研究

碳库效应存在影响了¹⁴C测年的准确性,制约了沉积物在湖泊研究中的应用。本文研究位于典型西风环流带的新疆博斯腾湖现代碳库效应,结合同位素地球化学、水化学等方法,探讨了博斯腾湖碳库效应的影响机制。研究结果表明,博斯腾湖最大现代碳库年龄为3535年,最小为现代碳,集中在670~945年。而过去碳库效应集中在1033~2200年。深水区表现为碳库年龄较小且稳定;在入湖口碳库效应最大,富水生植物浅水区碳库效应最小。与过去碳库年龄相比较,现代碳库效应整体表现更为年轻。研究发现,博斯腾湖口受流域外源“死”碳影响,深水区受湖水与大气CO₂交换率差异影响,富水生植物浅水区可能受水生植物光合作用影响。此外,核爆效应也对博斯腾湖现代碳库效应产生了影响,可能导致最高约1000年的年代误差。利用深水区放射性碳比活度(pMC)平均值与大气碳比活度差值(~18%),得出深水区沉积物存在约846年现代碳库效应,再经核爆效应校正后得出博斯腾湖存在最高约1800年碳库效应。     

13C/12C ratio variations in Pinus longaeva (bristlecone pine) cellulose during the last millennium

δ¹³C values are presented for cellulose samples prepared from two dendrochronologically dated Pinus longaeva (bristlecone pine) trees which grew during the last 1000 years. δ¹³C variations for these lower forest border trees are similar to upper tree line ring-width variations for the same species and English high summer temperature variations for the same time period. However, the δ¹³C variations appear to be unrelated to lower forest border ring-width variations and cellulose δ D variations for the same specimens.     

First-order14C dating of Holocene molluscs

The investigation of many problems in geology requires radiometric ages which need not be precise provided they are reliable and preferably cheap and quick. Such first-order ages can be determined on Holocene molluscs at 1/100 the cost of conventional¹⁴C ages and in the space of a few hours by trapping CO₂ from the shell carbonate in an organic base for assay in a liquid scintillation counter. Using standard laboratory glassware and a counter with preset channels the technique gives ages for the last 8000 years with a standard error (2σ) of 1500 years or less.     

Land-air boundary environment as recorded by the18O/16O and13C/12C isotope ratios in the shells of land snails

A yearly cycle of carbon and oxygen isotope composition of shells of the Israeli land snailXeropicta vestalis is presented. The¹⁸O/¹⁶O values indicate that the snails use water from the land-air boundary zone. The¹⁸O/¹⁶O ratio of the shells is in isotopic equilibrium with the water condensate from the vapour during the winter months. During the summer months a contribution to the above water from soil water migrating upwards due to evaporation is noticeable. The δ¹³C values indicate that as in marine molluscs, the carbon isotopic composition in land snails is controlled mainly by the aqueous carbonate compound which is in equilibrium with the land-air boundary CO₂.     

18O and 13C in the shells of freshwater molluscs and their environments

Carbon and oxygen isotope analyses on shells of freshwater molluscs and their habitat are presented. The data obtained reconfirm the usefulness of such ¹⁸O analyses for paleoenvironmental and paleohydrological studies.The ¹³C analyses on freshwater molluscs from lakes in southwestern Ontario, specimens grown under laboratory conditions and a comparison with the ¹³C contents of the dissolved inorganic carbon in their habitat show that the ¹³C contents in mollusc shells are primarily controlled by the aqueous carbonate species. Vital effects and food control appear to have only minor importance. The significance of this observation on the usufulness of mollusc shells for ¹⁴C dating is discussed.     

Evidence for a secular variation in the13C/12C ratio of carbon implanted in lunar soils

A plot of δ¹³C against δ¹⁵N for all lunar soils and breccias for which both values are available indicates that the 30% change in δ¹⁵N previously observed is accompanied by a change in δ¹³C about one-tenth as large. The correlation remains when the data are broken down into sub-sets according to soil maturity. The correlation therefore is not due to maturation effects or non-selective sample contamination and must represent concurrent changes in the isotope ratios of both elements at their source. A rough calculation of the relative production rates of¹³C and¹⁵N indicates that spallation reactions in the sun could lead to the observed ratio of the δ¹³C to δ¹⁵N variations.     

Excess 3He in deep water on the East Pacific Rise

Helium isotope measurements show that water on the crest and flanks of the East Pacific Rise has the highest enrichment in ³He so far observed in the oceans; the ³He/⁴He ratio anomaly relative to atmospheric helium is + 32% at the mid-depth maximum in the profiles. The corresponding ³He solubility anomaly relative to saturation with atmospheric helium is +50%. These data indicate that active sea-floor spreading sites on the crests of the mid-ocean rises are the sources of primordial helium injected into the ocean from the earth's interior. The ³He/⁴He ratio in this flux is approximately 1.6 × 10^?5, about 11 times the atmospheric ratio of 1.4 × 10^?6. The total flux of ³He into the atmosphere is 4.6 atoms cm^?2 earth-surface sec^?1, most of which (4.0 atoms cm^?2 sec^?1) is supplied by the oceanic flux. The corresponding atmospheric residence time for ³He is 10⁶ years, which, within the large uncertainties of supply and demand (thermal escape), is consistent with the requirement for a steady state.     

The 3He distribution in deep water over the Mid-Atlantic Ridge

³He/⁴He ratios in dissolved helium at GEOSECS stations 115, 117, 30, and 120 provide an east-west section across the Mid-Atlantic Ridge at 30°N. Below 1500 m depth, the³He/⁴He profiles show little structure and have values within a few percent of the atmospheric ratio, indicating that the Mid-Atlantic Ridge is not a significant source of injected³He at this latitude. Mean³He/⁴He ratios calculated for the deep water at each station show that the³He/⁴He ratio in the western Atlantic at 30°N is 2–3% higher than in the eastern basin, probably due to mixing between a³He-rich boundary current in the western basin and low-³He deep water to the east.     

13C/12C ratios of rocks and inclusions in popping rocks of the Mid-Atlantic Ridge and their bearing on the problem of isotopic composition of deep-seated carbon

CO₂-rich inclusions recovered from “popping” and related tholeiitic rocks from the Mid-Atlantic Ridge have δ¹³C values of ?7.6 ± 0.5%. relative to PDB. δ¹³C values of total carbon in the same rocks range from ?12 to ?13.7‰. These values are discussed in the light of the known δ¹³C variations in rocks of deep-seated origin. The ?7.6‰ value is interpreted as a reasonable estimate of the primary value of δ¹³C of deep-seated carbon in the ridge area.     

Atmospheric14C changes resulting from fossil fuel CO2 release and cosmic ray flux variability

A high-precision tree-ring record of the atmospheric¹⁴C levels between 1820 and 1954 is presented. Good agreement is obtained between measured and model calculated 19th and 20th century atmospheric Δ¹⁴C levels when both fossil fuel CO₂ release and predicted natural variations in¹⁴C production are taken into account. The best fit is obtained by using a ?-diffusion model with an oceanic eddy diffusion coefficient of 3 cm²/s, a CO₂ atmosphere-ocean gas exchange rate of 21 moles m^?2 yr^?1 and biospheric residence time of 60 years.For trees in the state of Washington the measured 1949–1951 atmospheric Δ¹⁴C level was20.0±1.2%. below the 1855–1864 level. Model calculations indicate that in 1950 industrial CO₂ emissions are responsible for at least 85% of the Δ¹⁴C decline, whereas natural variability accounts for the remaining 15%.     

Reworking effects in the Holocene Zhuye Lake sediments:A case study by pollen concentrates AMS 14C dating

Transported by wind and water,the relatively old sediments can deposit in the terminal lake of an inland drainage basin.The reworking effect can affect the lake sediments 14 C dating and explanations for proxies.The Zhuye Lake is the terminal lake of the Shiyang River Basin.Previous studies indicated that sediments in different locations of the lake basin showed different climatic change patterns.And then,some radiocarbon dates were inverted for some Late Pleistocene sections.Whether this phenomenon is related with the reworking effect? The pollen concentrates 14 C dating can avoid the reservoir effect,which is an ideal method for studying the reworking effect.In this study,we used the pollen concentrates as dating materials and dated five Holocene sections in the Zhuye Lake Basin.Based on the 14 C dates comparison between the pollen concentrates,organic matter,and shells,the pollen concentrates dates are relatively older than other dating materials.Based on the result,the reworking effect worked in the Zhuye Lake Basin during the Holocene;however,in different locations of the lake basin the reworking effects were in different levels.Furthermore,the Holocene lacustrine deposits were formed mostly during the early and middle Holocene.This study provided clues for reworking effect studies of other lakes in arid China.