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.     

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.     

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.     

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.     

Measurements of 36Cl in Antarctic meteorites and Antarctic ice using a Van de Graaff accelerator

Cosmic-ray produced ³⁶Cl(t_1/2 = 3.0 × 10⁵ years) has been measured in four Antarctic meteorites and one sample of Antarctic ice using a tandem Van de Graaff accelerator as an ultrasensitive mass spectrometer with the extremely low background level of ³⁶Cl/Cl< 2 × 10^?16. Results from this ion counting technique (applied here to extraterrestrial materals for the first time) are used to support a two-stage irradiation model for the Yamato-7301 and Allan Hills-76008 meteorites and to show a long terrestrial age (0.7 ± 0.1 m.y.) for Allan Hills-77002. Yamato-7304 has a terrestrial age of less than 0.1 m.y. The ³⁶Cl content of the Antarctic ice sample from the Yamato Mountain area implies that the age of the ice cap at this site is less than one ³⁶Cl half-life.     

87Rb-87Sr chronology of enstatite meteorites

A⁸⁷Rb-⁸⁷Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ⁸⁷Rb= 1.42 × 10^?11yr^?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the⁸⁷Sr/⁸⁶Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the⁸⁷Rb-⁸⁷Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the⁸⁷Rb-⁸⁷Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.     

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.     

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.     

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.     

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。     

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.     

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.     

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.     

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.     

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.     

36Cl and53Mn in Antarctic meteorites and10Be-36Cl dating of Antarctic ice

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been measured in twenty Antarctic meteorites by neutron activation analysis.³⁶Cl (t_1/2 = 3.0 × 10⁵years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.¹⁴C (t_1/2 = 5740years) and²⁶Al (t_1/2 = 7.2 × 10⁵years) data. The terrestrial ages range from 3 × 10⁴ to 5 × 10⁵ years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,¹⁰Be (t_1/2 = 1.6 × 10⁶years) and³⁶Cl have been measured in six Antarctic ice samples. The first measurements of¹⁰Be/³⁶Cl ratios in the ice core samples demonstrate a new dating method for ice.