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1.
Determining geologic ages of fossils by amino acid racemization techniques is often difficult because of the uncertainties in assumptions about diagenetic temperatures. Two kinetic model methods have been employed. Method 1, used by us, assumes that racemization of amino acids in the bivalve mollusk Saxidomus giganteus from Willapa Bay, Washington, follows linear kinetics. Ages are calculated by means of first-order kinetic equations. Method 2, used by Wehmiller, involves an empirical non-linear kinetic model Method 1 is simpler in concept and more easily applied. Wehmiller claims that ambiguities in paleotemperature arise when method 1 is used and that these ambiguities can be reconciled by the use of method 2. We show that application of method 1 can also provide reasonable temperature histories and leads to age estimates that are consistent with the geologic history of the sedimentary deposits at Willapa Bay. 相似文献
3.
The stratigraphic record in the James and Hudson Bay Lowlands indicates that the sequence of glacial events at the geographical center of the 12.6 × 10 6 km 2 Laurentide Ice Sheet may have been more complex than hitherto imagined. Isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups. Two alternative explanations of the data are offered, of which we strongly favor the first. Hypothesis 1: Setting the age of the “last interglacial” marine incursion, the Bell Sea, at 130,000 yr B.P. results in a long-term average diagenetic temperature for the lowlands of +0.6°C. Using this temperature enables us to predict the age of shells intermediate in age between the “last interglaciation” and the incursion of the Tyrrell Sea 8000 yr ago. Between these two interglacial marine inundations, Hudson Bay is predicted to have been free of ice along its southern shore about 35,000, 75,000, and 105,000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments. These results suggest (1) that traditional concepts of ice-sheet build-up and decay must be reexamined; (2) that “high” sea levels may have occurred during the Wisconsin Glaciation; and (3) that a critical reappraisal is required of the open ocean δ 18O record as a simple indicator of global ice volume. An alternative, Hypothesis 2, is also examined. It is based on the assumption that the 35,000-yr-old deposits calculated on the basis of Hypothesis 1 date from the “last interglaciation”; this, in effect, indicates that the Missinaibi Formation, commonly accepted as sediments of the “last interglaciation,” are about 500,000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130,000 yr has been close to ?6°C. We argue for rejection of this alternative hypothesis. 相似文献
4.
Humic acids and fulvic acids were extracted from six paleosols in Southern Italy. Humic acids (HAs) constituted between 96.5 and 99.2% of the total extracts; the remaining materials consisted of fulvic acids (FAs). Radiocarbon ages of the HAs ranged from about 6,000 to close to 29,000 years B.P., δ13C values averaged The HAs were characterized by chemical (elemental and functional group analyses) and spectroscopic (IR, ESR, 13CNMR, E 4/E 6 ratios) methods. FAs were characterized by chemical methods, E 4/E 6 ratios and IR spectra.The chemical and spectroscopic analyses showed practically no differences in the chemical structure and composition of the six HAs and FAs, so that age appeared to have little effect on these parameters. The paleosols were found to be closed systems with low polysaccharide and protein contents, thus providing unfavorable substrates for microbial activity. The preservation of the humic materials in the paleosols may have been due to low biological activity and/or to retention by amorphous minerals. The HAs did not appear to be affected by temperatures higher than 170–200°C over the 23,000 year period which we observed. 相似文献
5.
Drill cores of Enewetak Atoll, Marshall Islands, reveal six stratigraphic intervals, numbered in downward sequence, which represent vertical coral growth during Quaternary interglaciations. Radiocarbon dates indicate that the Holocene sea transgressed the emergent reef platform by about 8000 yr B.P. The reef grew rapidly upward (about 5 to 10 mm/yr) until about 6500 yr B.P. Afterward vertical growth slowed to about 0.5 mm/yr, then lateral development became dominant during the last several thousand years. The second interval is dated at 131,000 ± 3000 yr B.P. by uranium series. This unit correlates with oxygen-isotope substage 5e and with terrace VIIa of Huon Peninsula, New Guinea, and of Main Reef-2 terrace at Atauro Island. The third interval is not dated because corals were recrystallized and it is tentatively correlated with either oxygen-isotope stages 7 or 9. The age of the fourth interval is estimated at 454,000 ± 100,000 yr B.P. from measured activity ratios. This unit is correlated with either oxygen-isotope stage 9, 11, or 13. 相似文献
6.
Analytical techniques have been developed for using a secondary ion mass spectrometer, the ion microprobe mass analyzer (IMMA), to determine, in situ, and U/Pb ages on approximately 10-μm areas of individual mineral phases containing relatively abundant radiogenic Pb isotopes. Standard samples of known age and U, Th and Pb contents, together with the Andersen-Hinthorne local thermal equilibrium (LTE) model for predicting ionization parameters are used to establish a semi-empirical relationship for correcting observed U/Pb intensities to atom ratios. Measurements of isotope standards show that mass fractionation corrections are not required and that the accuracy and precision of analysis are generally limited by Poisson counting statistics.Many U-rich accessory minerals yield spectra which consist only of Pb at masses 204, 206, 207 and 208; thus the measurement of ages is accomplished by simply measuring the intensities of these peaks and the background. An excellent correspondence of the ages determined using the IMMA with those from more conventional techniques is demonstrated for terrestrial, lunar and meteoritic phases. For example, the following ages were obtained from polished thin sections of crystalline lunar samples: 10047, 3.75 ± 0.05 (2 σ) Ga; 14310, 3.96 ± 0.03 Ga; and 15555, 3.36 ± 0.06 Ga. From small U-rich phases in CaAl-rich inclusions in the Allende carbonaceous chondrite, seven ages were obtained, averaging 4.60 ± 0.09 (2 σ) Ga.A method of correcting low-resolution Pb-isotope data for molecular ion interferences in zircon and baddeleyite is presented and shown to be practical through the analysis of terrestrial and lunar samples. 相似文献
7.
Four glacial drifts that are interstratified with lava flows and tephra layers on the upper slopes of Mauna Kea demonstrate that an ice cap formed repeatedly at the summit of the volcano during the middle and late Pleistocene. The oldest drift (Pohakuloa Formation) probably was deposited shortly after eruption of a lava flow having a age of 278,500 ± 68,500 yr. Drift of the Waihu Formation, marked by a belt of subdued end moraines, is correlated with hyaloclastite cones and associated lava flows that were erupted beneath an ice cap about 170,000–175,000 yr ago. One of four younger subglacially erupted lavas at the crest of the volcano has a age of 41,300 ± 8300 yr. Tephra layers that antedate the last glaciation are about 29,700 to 37,200 14C yr old and underlie dune sand that is believed to correlate with drift of the Makanaka Formation deposited during the last ice advance. The late Makanaka ice cap, which covered an area of about 70 km 2 and was as much as 100 m thick, is reconstructed from end moraines and limits of erratic stones that encircle the summit region. The ice cap disappeared from the summit before about 9080 yr ago. Postglacial lavas and tephra overlie the youngest drift on the upper south flank of the mountain and buried a widespread post-Makanaka soil on the lower south rift zone about 4500 14C yr ago. The island of Hawaii is subsiding isostatically due to crustal loading by Quaternary volcanic rocks, with subsidence near the midpoint of Mauna Kea estimated as about 2.5 ± 0.5 mm/yr. A curve depicting an inferred long-term subsidence rate has been used to adjust equilibrium-line altitudes (ELAs) of former ice caps that are calculated on the basis of reconstructed glacier topography and an assumed accumulation-area ratio of 0.6 ± 0.05. The results indicate that ELA depression was greatest during Waihu glaciation, least during Pohakuloa glaciation, and that the ELA during late Makanaka glaciation was somewhat lower than that of the early Makanaka advance. Available radiometric dates show that late Makanaka glaciation correlates with stage 2 of the marine oxygen-isotope record, and suggest that early Makanaka, Waihu, and Pohakuloa glaciations correlate, respectively, with isotope stages 4, 6, and 8. Because ice caps could have formed on Mauna Kea only after the snowline was lowered many hundreds of meters below its inferred present level, episodes of Hawaiian glaciation probably were restricted to times of maximum ice volume on the continents. The asymmetry of the late Makanaka ice cap and the southeast-descending gradient of its equilibrium line are consistent with a southeast (tradewinds) source of precipitation during the last glaciation. Although departures of glacial-age temperature and precipitation from present values are difficult to assess quantitatively, growth of former ice caps on Mauna Kea most likely was due to enhanced winter snowfall and to reduced ablation rates brought about by lower air temperature and increased cloudiness. 相似文献
8.
The Vermilion district of northerneastern Minnesota is a classic example of a lower Precambrian greenstone-granite terrane. It is a complex volcanic-sedimentary pile, characterized by repeated periods of volcanism and the presence of intercalated pyroclastic, volcanoclastic and epiclastic rocks. The volcanic-sedimentary pile is surrounded and intruded by contemporaneous granitic batholiths. Several rock units from the district have been dated by the whole-rock Rb-Sr method. The isochron ages and the corresponding initial Sr 87/Sr 86 ratios (= I) are: | | | | Ely Greenstone | | | | Newton Lake Formation | | | | Granitic pebbles | | | 相似文献
9.
The sequence of Quaternary deposits beneath the floor of San Francisco Bay includes four to seven noncontemporaneous estuarine units intercalated with alluvium and dune sand. Units L (0–10,000 B.P.), M (>40,000 B.P., probably ca. 80,000–140,000 B.P.), and N (older than unit M) are distinctly superposed. The dominant molluscan fossil in each of these three units is Ostrea lurida Carpenter, the native oyster along much of the pacific Coast of North America. Despite a lamellar structure that suggests vulnerability to contamination, O. lurida shells generally yield amino acid enantiomeric ratios that are analytically reproducible and stratigraphically consistent. The kinetics of racemization in O. lurida conceivably resembles that of Protothaca and Saxidomus, other bivalves whose kinetics of racemization are relatively well understood. Assuming such a resemblance, enantiomeric ratios in O. lurida imply that (1) unit M is the same approximate age as estuarine terrace deposits bordering San Pablo Bay and Carquinez Strait, providing that the terrace deposits have been at diagenetic temperatures 1°-2°C warmer than unit M; and (2) the age of unit N is about four times greater than that of unit M, providing that both units have been at the same approximate diagenetic temperature. 相似文献
10.
Various models have been suggested concerning the origin and evolution of the earth's atmosphere. An estimate of the nitrogen content of the mantle could further constrain atmospheric models. Total nitrogen content was determined by thermal neutron activation analysis via 14N(n,p) 14C. The 14C was converted to carbon dioxide and counted in miniature low level proportional counters. The total nitrogen content of U.S.G.S. standards BCR-1 and G-2 as determined by different laboratories is variable, probably due to atmospheric adsorption by the finely ground samples. Total nitrogen content was determined in deep sea basalt glasses from three regions: East Pacific Rise ( , and ), Mid-Atlantic Rift (FAMOUS Region: , and ) and the Juan de Fuca Ridge ( ). Matrix material from the same samples as the glasses was available from the East Pacific Rise ( , and ) and the Mid-Atlantic Rift ( ) which are about 50 to 100% greater than the associated glasses. The increased matrix abundance may be due to incorporation of chemically bound nitrogen from sea water rather than dissolved molecular nitrogen. The nitrogen content of the FAMOUS samples are inconsistent with the model of Langmuir et al. (1977) for petrogenesis based on trace element data. Factors which can affect the observed nitrogen content in the basalts and the interpretation in terms of the mantle nitrogen abundance are discussed ( e.g. partial melting and degassing of the basalts). A lower limit of about 2 ppm N in the mantle can be estimated. 相似文献
11.
Constant-temperature laboratory culture experiments of the planktonic foraminiferal species Globigerinoides sacculifer (Brady) suggest that the ratios of Li and Sr to Ca in the shells are a function of these ratios in the culture solutions. and in the shells did not vary with changes of these ratios in the culture solution. These are the first direct determinations of the relationship between foraminiferal shell chemistry and solution composition.The possibility of temperature dependence for the minor elemental composition of foraminiferal shells was also investigated in the laboratory and by analysis of several planktonic and one benthic foraminiferal species from sediment trap and sediment core samples. The , , and ratios in the natural samples roughly correlate with calcification temperature, whereas differences in the Li/Ca ratios are small and not systematically related to temperature. However, laboratory culture experiments at 20°C and 30°C showed no variation in the , , , and ratios with calcification temperature for the planktonic foraminifera G. sacculifer and Orbulina universa. Therefore, observed differences in the , , and ratios for the sediment trap and core foraminiferal samples cannot be ascribed to direct effects of calcification temperature, but may be due to some other environmental factor which is correlated with temperature. 相似文献
12.
The inert gases have been measured in six size fractions covering the range below 500 μm, in a single feldspathic fragment weighing 523 μg, and in an agglutinate particle weighing 465 μg. The two size fractions between 125 and 250 μm as well as 250 and 500 μm were separated into magnetic and non-magnetic portions, which were measured separately. Like the Apollo and Luna 16 fines, the terra fines represented by Luna 20 are very rich in trapped solar-wind gases, but they contain relatively less He 4 and Ne 20, which is revealed by their average ratio of 35 and ratio of 2.9. Obviously the terra materials are less retentive for solar-wind He and Ne than typical mare fines such as 10084. Whether this is due to the relatively small TiO 2 or the relatively large plagioclase content of the former is not resolved. ( and ( ratios are relatively large; the average values are 2800 and 14400, respectively. The apparent Ne 21 radiation ages of all the size fractions are in the range 209–286 × 10 6 yr; the average is 260 × 10 6 yr. This is in the range of values known for the Apollo and Luna 16 fines. The feldspathic fragment has a much greater apparent Ne c21 age of 780 × 10 6 yr. The Ar 40-Ar 36 systematic reveals the presence of two Ar 40 components, because Ar40 = (1.41 ± 0.076) Ar36 + (0.490 ± 0.130) × 10 ?4 (cm 3 STP/g). The slope of 1.41 is not inconsistent with an origin of the sample from a relatively old terra region. 相似文献
13.
The reef-crest coral Acropora palmata from late Pleistocene reefs on Barbados has recorded the same global variations in oxygen isotopes as planktonic and benthonic foraminifera. Although the record of oxygen isotopes in Acropora palmata is discontinuous, it offers several advantages over the isotope records from deep-sea sediments: (1) the coral grows at water depths of less than 5 m; (2) the samples are unmixed; (3) specimens may be sampled from various elevations of paleo-sea level; and (4) aragonitic corals are suitable for and dating techniques. The latter advantage means that direct dating of the marine oxygen isotope record is possible. Oxygen isotope stage 5e corresponds to Barbados III, dated at 125,000 ± 6000 yr BP. Petrographic and geochemical evidence from five boreholes drilled into the south coast of Barbados indicates a major eustatic lowering (greater than 100 m below present sea level) occurred between 180,000 and 125,000 yr BP. The age and isotopic data suggest correlation of this change in sea level to Emiliani's oxygen isotope stage 6. Acropora palmata deposited at various elevations of sea level during oxygen isotope stage 6 vary by 0.11 ‰ δ 18O for each 10 m of change in sea level. We further hypothesize a minimum drop of 2°C in the average temperature occurred during the regressive phase of oxygen isotope stage 6. These data indicate that temperature lowering of surface water near Barbados lagged behind a major glacial buildup during this time period. Using the δ 18O vs sea level calibration herein derived, we estimate the relative height of sea stands responsible for Barbados coral reef terraces in the time range 80,000 to 220,000 yr BP. 相似文献
14.
239 + 240Pu activities of 100–450dpm/kg are found down to 15–18 cm in anoxic Saanich Inlet sediments, with a subsurface maximum in undisturbed deposits. Integrated 239 + 240Pu inventories which overlap delivery estimates are present both in two cores of anoxic sediments from Saanich Inlet and in one core of oxic sediments 65 km away in Dabob Bay, Washington. ratios in Saanich Inlet sediments overlap ratios in unfractionated midnorthern latitude fallout, in oxic sediments from the Washington continental shelf, and in anoxic sediments from two basins off southern California and Mexico. The ratios in three intervals of Saanich Inlet sediments are also in agreement with ratios previously reported for oxic coastal marine sediments. The Pu inventories, the and ratios, and the Saanich Inlet Dabob Bay comparison all argue that Pu is not rapidly remobilized in anoxic sediments.The subsurface 239 + 240Pu activity maximum is not in agreement with the historical record of peak Pu fallout in 1963–1964 unless our 210Pb-derived sedimentation rates are incorrectly high. However, they are in good agreement with previous 210Pb and varve chronologies in Saanich Inlet, and also give reasonable dates for times when 239 + 240Pu and SNAP-9A supplied 238Pu first appear in the sediments. We conclude they properly date the maximum in sedimentary 239 + 240Pu activity at 1970–1973, and seek explanations for the 7–10yr time lag after peak fallout. 239 + 240Pu inventories in one core from the eastern basin of the Cariaco Trench and in two cores from Golfo Dulce. an anoxic basin off the Pacific coast of Costa Rica, are also in reasonable agreement with fallout delivery to these latitudes when excess 210Pb inventories and fluxes are used to verify recovery of at least a major fraction of the most recently deposited sediments. 相似文献
15.
The concentrations of 238Pu, 239 + 240Pu, 241Am and 137Cs were determined in rain samples collected at Monaco in the course of 1978–1979. Based on these data, the annual deliveries of these radionuclides to the Mediterranean by rain are computed to be 0.18 ± 0.01 pCim?2 for 238Pu, 8.1 ± 0.1 pCim?2 for 239 + 240Pu, 0.58 ± 0.02 pCim?2 for 241Am and 351 ± 4 pCim?2 for 137Cs.Comparing the delivery data with the mixed layer inventories of 239 + 240Pu and 241Am in the Mediterranean, the upper limits of the mean residence time of these radionuclides in the mixed layer were estimated to be 12.3 yr for 239 + 240pu and 2.9 yr for 241Am. These values are consistent with the conclusion deduced from the vertical distribution pattern of these transuranic elements in the Mediterranean.Based on delivery values, the annual activity ratios for , and are found to be 0,022, 0.072 and 0.023 respectively. The and activity ratios vary within relatively narrow ranges with time, while a much wider variation was observed for the activity ratio. The cause of the wider variation of the ratio may be related to the difference in the mean age of fallout brought down in different seasons. 相似文献
16.
Eighteen basalts and some volcanic gases from the submarine and subaerial parts of Kilauea volcano were analyzed for the concentration and isotope ratios of sulfur. By means of a newly developed technique, sulfide and sulfate sulfur in the basalts were separately but simultaneously determined. The submarine basalt has 700 ± 100 ppm total sulfur with δ34SΣs of . The sulfate/sulfide molar ratio ranges from 0.15 to 0.56 and the fractionation factor between sulfate and sulfide is . On the other hand, the concentration and δ34SΣs values of the total sulfur in the subaerial basalt are reduced to 150 ± 50 ppm and , respectively. The sulfate to sulfide ratio and the fractionation factor between them are also smaller, 0.01 to 0.25 and +3.0‰, respectively. Chemical and isotopic evidence strongly suggests that sulfate and sulfide in the submarine basalt are in chemical and isotopic equilibria with each other at magmatic conditions. Their relative abundance and the isotope fractionation factors may be used to estimate the and temperature of these basalts at the time of their extrusion onto the sea floor. The observed change in sulfur chemistry and isotopic ratios from the submarine to subaerial basalts can be interpreted as degassing of the SO 2 from basalt thereby depleting sulfate and 34S in basalt.The volcanic sulfur gases, predominantly SO 2, from the 1971 and 1974 fissures in Kilauea Crater have δ34S values of 0.8 to 0.9%., slightly heavier than the total sulfur in the submarine basalts and definitely heavier than the subaerial basalts, in accord with the above model. However, the δ34S value of sulfur gases (largely SO 2) from Sulfur Bank is 8.0%., implying a secondary origin of the sulfur. The δ34S values of native sulfur deposits at various sites of Kilauea and Mauna Loa volcanos, sulfate ions of four deep wells and hydrogen sulfide from a geothermal well along the east rift zone are also reported. The high δ34S values (+5 to +6%. o) found for the hydrogen sulfide might be an indication of hot basaltseawater reaction beneath the east rift zone. 相似文献
18.
A study was undertaken to determine the chronology of a pristine granite clast (1062) from Apollo 14 breccia 14321 using Rb-Sr, Sm-Nd and 39Ar- 40Ar methods. The genesis of the granite as constrained by the isotopic results and trace element characteristics is discussed. Chronology: The Rb-Sr internal isochron is slightly disturbed and yields an age of 4.09 ± 0.11 AE ( ) and an imprecise initial I( Sr) = 0.702 ? .008. If two data are excluded, the age becomes 4.13 ± 0.03 AE and I( Sr) = 0.698 ? .003. The whole rock and mineral separates are extremely radiogenic; they yield model ages which are relatively well-defined. The average model age is 4.12 ± 0.03 AE (relative to BABI = 0.69898). The Sm-Nd internal isochron is also slightly disturbed and gives an age of 4.11 ± 0.20 AE ( ?1). The 39Ar- 40Ar average age of the non-magnetic fraction of the sample yields a slightly younger age of 3.88 ± 0.03 AE (K-Ar constants from Steiger and , 1977). The concordancy of Rb-Sr and Sm-Nd internal isochrons with the Rb-Sr model age strongly suggests that the granitic clast formed at 4.1 AE ago in the shallow crust and was later excavated and brecciated about 3.88 AE ago. Petrogenesis: Isotopic and trace element data of the lunar granite show large K/La and Rb/Sr fractionations, small Sm/Nd fractionation and the distinct V-shaped REE distribution pattern at the time of crystallization. A two-stage model involving crystal fractionation followed by silicate liquid immiscibility (SLI) is proposed for lunar granite genesis. We propose that the granite can be the immiscible acidic liquid produced by SLI from a residual liquid which underwent fractionation of ca, 3% of phases with REE distribution coefficients similar to those of phosphate minerals from a highly evolved parental magma with REE contents about twice those of the 15405,85 quartz monzodiorite (QMD).The extreme scarcity of lunar granitic samples and their young formation ages suggest that they are probably not directly crystallized from the differentiation of the primordial magma ocean. Our isotopic results and trace elements data from other workers suggest that granites, QMD and probably Mggabbronorites may be genetically related and may have formed in a plutonic environment similar to gabbro-granophyre associations in terrestrial layered intrusions such as the Skaergaard Intrusions. 相似文献
19.
Rare gas data are presented from step-wise heatings of lunar breccias 14066 and 14318 and from an interlaboratory cross-calibration of five standards used in 40Ar- 39Ar dating. Four samples of 14066 all show depressed 401Ar/ 391Ar ratios at high temperatures, thus making age interpretation uncertain. While different in detail, the Ar release patterns in the four samples yield indistinguishable plateau ages of b.y. and > 400°C total ages of b.y. Concentrations of K, Ca, Ba, Br, U and I are given for 14318 and 14066. We also present an updating of all of the 40Ar- 39Ar ages and trace element concentrations previously published by this laboratory. 40Ar- 39Ar dating standards from Menlo Park, Pasadena, Stony Brook, Toronto and Berkeley are calibrated against each other and the internal homogeneity of their 401Ar/K ratios is tested. The Berkeley standard (from the St. Severin meteorite) has an age of b.y. from this intercalibration. 80Kr from capture of lunar neutrons is detected in 14318. A comparison of the release pattern of the 80Kr produced by lunar neutrons with the 80,82Kr produced by pile neutrons in 14318, indicates that 14318 has lost approximately 35 per cent of the 80Kr produced by lunar neutrons. 相似文献
20.
Thirty-eight fossil coral samples of known age from late Pleistocene uplifted reef terraces have been analyzed for amino acid composition and for the degree of racemization of selected amino acids. Particular attention has been given to the epimerization (racemization about the α-carbon) of isoleucine. The ratios observed in many of these samples do not conform to the concept of increasing racemization being associated with increasing fossil age. It is shown that sixteen of the samples demonstrate concordance between their known age and the age estimated from racemization data. Racemization ages are based upon a kinetic model derived from the kinetics observed in foraminifera in marine sediments and published estimates of the temperature dependence of the isoleucine epimerization reaction. Lack of concordance for the remaining samples is explained by either of two separate diagenetic phenomena: extensive leaching of free amino acids from the fossil, or contamination by ‘young’ amino acids. Ambiguities observed in many of the results may be due to the fact that coralline organic matter can have a complex history because of variations in its source and in its relation to the mineral phase. Neither of these factors has been observed to exert such a significant effect on racemization kinetics observed in fossil foraminifera or molluscs. Fossil corals are of only modest value as specimens for amino acid geochronological studies. 相似文献
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