Pumice from thera (santorini) identified from a greek mainland archeological excavation

Pumice fragments recovered from an archeological excavation on the Greek mainland have been correlated by means of index of refraction measurements with the Late Bronze Age volcanic eruption on the island of Thera (Santorini) in the Aegean Sea. Pottery from the strata containing the pumice dates from the 15th century B.C.     

Chronology for the Aegean Late Bronze Age 1700-1400 B.C

Radiocarbon (carbon-14) data from the Aegean Bronze Age 1700-1400 B.C. show that the Santorini (Thera) eruption must have occurred in the late 17th century B.C. By using carbon-14 dates from the surrounding region, cultural phases, and Bayesian statistical analysis, we established a chronology for the initial Aegean Late Bronze Age cultural phases (Late Minoan IA, IB, and II). This chronology contrasts with conventional archaeological dates and cultural synthesis: stretching out the Late Minoan IA, IB, and II phases by approximately 100 years and requiring reassessment of standard interpretations of associations between the Egyptian and Near Eastern historical dates and phases and those in the Aegean and Cyprus in the mid-second millennium B.C.     

Bang! Month-scale eruption triggering at Santorini volcano

The time lag between intrusion of fresh, hot magma and an ensuing eruption is of critical importance in both understanding the triggering and mitigating the consequences of volcanic eruptions. This work looks at material erupted during 1925-28 at the Nea Kameni volcanic center in Santorini, Greece, to determine this time scale. By exploiting Fe-Mg diffusion in olivine crystals, we constrained the intrusion-to-eruption time lag to between 3 and 10 weeks. These techniques have potential application at many volcanic centers; previously erupted material can be used to calibrate records of the short-time scale processes common to many volcanic centers.     

Redefining the age of Clovis: implications for the peopling of the Americas

The Clovis complex is considered to be the oldest unequivocal evidence of humans in the Americas, dating between 11,500 and 10,900 radiocarbon years before the present (14C yr B.P.). Adjusted 14C dates and a reevaluation of the existing Clovis date record revise the Clovis time range to 11,050 to 10,800 14C yr B.P. In as few as 200 calendar years, Clovis technology originated and spread throughout North America. The revised age range for Clovis overlaps non-Clovis sites in North and South America. This and other evidence imply that humans already lived in the Americas before Clovis.     

Radioisotope dating with a cyclotron

By considering radioisotope dating as a problem in trace element detection, and by using the cyclotron as a high-energy mass spectrometer for this purpose, we have shown that one can greatly increase the maximum age that can be determined while simultaneously reducing the size of the sample required. The cyclotron can be used to detect atoms or simple molecules that are present at the 10(-16) level or greater. For (14)C dating one should be able to go back 40,000 to 100,000 years with 1- to 100-mg carbon samples; for (10)Be dating, 10 to 30 million years with 1-mm(3) to 10-cm(3) rock samples; for tritium dating, 160 years with a 1-liter water sample. The feasibility of the technique has been demonstrated experimentally by measuring the tritium/deuterium ratio in a sample 24 years old. For samples many half-lives old, the fractional error in the age is small even if rates of production or deposition of the isotopes. Although cyclotrons are expensive to build, their operating costs are relatively low. If several samples are dated per hour the cost per date may not be substantially higher than it is today for decay dating. There are already more than 50 cyclotrons in operation which have the potential to do radioisotope dating, and their application to important problems of dating and trace element analysis should prove very fruitful.     

Eruption of soufriere volcano on st. Vincent island, 1971-1972

The Soufrière volcano in St. Vincent erupted from October 1971 to March 1972, as 80 x 10(6) m(3) of basaltic andesite lava was quietly extruded inside the mile-wide crater. The eruption was largely subaqueous, taking place in the 180-m-deep crater lake, and resulted in the emergence of a steep-sided island. The mild character of the eruption and the absence of seismic activity stand in direct contrast to the highly explosive character of the eruption of 1902 to 1903.     

40Ar/39Ar Dating of the Brunhes-Matuyama Geomagnetic Field Reversal

Magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale (GPTS) to date events in the range 0 to 5 million years ago. A critical tie point on the GPTS is the potassium-argon age of the most recent (Brunhes-Matuyama) geomagnetic field reversal. Astronomical values for the forcing frequencies observed in the oxygen isotope record in Ocean Drilling Project site 677 suggest that the age of this last reversal is 780 ka (thousand years ago), whereas the potassium-argon-based estimate is 730 ka. Results from 4039; Ar incremental heating studies on a series of lavas from Maui that straddle the Brunhes-Matuyama reversal give an age of 783 + 11 ka, in agreement with the astronomically derived value. The astronomically based technique appears to be a viable tool for dating young sedimentary sequences.     

Mechanisms of Argon Retention in Clays Revealed by Laser 40Ar-39Ar Dating

A method for dating clays is important for studies of weathering, diagenesis, hydrocarbon migration, and the formation of major metalliferous deposits. However, many attempts have produced imprecise or inaccurate results. Data from shales show that, contrary to expectations, the (40)Ar-(39)Ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because (39)Ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic (40)Ar in nature, rather than by direct recoil as is generally assumed.     

Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo

Newly exposed cave sediments at the Malapa site include a flowstone layer capping the sedimentary unit containing the Australopithecus sediba fossils. Uranium-lead dating of the flowstone, combined with paleomagnetic and stratigraphic analysis of the flowstone and underlying sediments, provides a tightly constrained date of 1.977 ± 0.002 million years ago (Ma) for these fossils. This refined dating suggests that Au. sediba from Malapa predates the earliest uncontested evidence for Homo in Africa.     

Probing the accumulation history of the voluminous Toba magma

The age and compositional zonation in crystals from the Youngest Toba Tuff record the prelude to Earth's largest Quaternary eruption. We used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150,000 years of magma storage and evolution. The dominant subvolcanic magma was relatively homogeneous and thermally stagnant for approximately 110,000 years. In the 35,000 years before eruption, the diversity of melts increased substantially as the system grew in size before erupting 75,000 years ago.     

Ionium dating of igneous rocks

Local fractionation of uranium and thorium, between minerals within a sample of igneous rock at the time of crystallization, makes it possible to date its solidification by use of ionium and uranium. Results on samples of granite, pumice, and lava suggest that this method of dating is reliable.     

Measurements of the imaginary part of the refractive index between 300 and 700 nanometers for mount st. Helens ash

The absorption properties, expressed as a wavelength-dependent imaginary index of refraction, of the Mount St. Helens ash from the 18 May 1980 eruption were measured between 300 and 700 nanometers by diffuse reflectance techniques. The measurements were made for both surface and stratospheric samples. The stratospheric samples show imaginary index values that decrease from approximately 0.01 to 0.02 at 300 nanometers to about 0.0015 at 700 nanometers. The surface samples show less wavelength variation in imaginary refractive index over this spectral range.     

不同生态区播期对直播稻生育期及温光利用的影响

【目的】探讨不同生态区直播条件下播期对不同类型水稻品种生育期及温光利用的影响,为品种及其适宜播期的科学区划布局提供初步理论依据。【方法】苏南太湖稻区常熟（N 31.4°）以迟熟中粳、早熟晚粳和中熟晚粳为材料,苏中里下河稻区姜堰（N 32.3°）以中熟中粳、迟熟中粳和早熟晚粳为材料,苏北淮北稻区东海（N 34.5°）以早熟中粳、中熟中粳和迟熟中粳为材料,通过分期播种试验,比较研究不同播期对直播稻生育期及温光利用的影响。【结果】随着播期的推迟,各类型品种的拔节期、抽穗期和成熟期相应推迟,生育期明显缩短,总体表现为播期每推迟1 d,生育期缩短0.6 d左右。同一品种类型,随纬度的升高,拔节期、抽穗期和成熟期逐渐延迟,生育期依次变长。同一试验点,随播期的推迟一般晚粳品种较中粳品种生育期天数缩短略多。生育期随播期推迟而缩短主要表现在营养生长期和营养生长与生殖生长并进期,变化范围为2-24 d,生殖生长阶段则相对稳定,变化范围仅为-2-1 d。各类型品种生育期积温和光照时数均表现出随播期的推迟呈显著或极显著的减少趋势,其差异主要源于抽穗前积温与光照时数的不同,其中尤以拔节前的差异最为明显,达到显著或极显著水平。同一试验点,各类型品种间生育期的积温与光照时数均表现为晚粳品种大于中粳品种;同一类型品种,随试验点纬度升高,积温呈依次减少的趋势,光照时数除杂交品种以外均呈依次增加的趋势。随播期的推迟,各类型品种的温光利用率均呈显著或极显著的下降趋势。播种至抽穗阶段和生育期的天数随积温的升高而线性增加,随日照时数的增加而线性减小;抽穗至成熟阶段的天数随积温的升高或日照时数的增加不同处理间表现不尽一致。【结论】播期对直播稻生育期及温光的利用有较大的影响。据此,对不同类型水稻品种直播适宜布局范围进行了初步区划,并综合分析后给出其能够安全成熟的可能播期,为大面积生产提供参考。     

Middle holocene age of the sunnyvale human skeleton

A morphologically modern human skeleton from Sunnyvale, California, previously dated by aspartic acid racemization to be approximately 70,000 years old and by uranium series isotopic ratios to be 8300 and 9000 years old, appears to be younger when dated by the carbon-14 method. Four carbon-14 determinations made by both decay and direct counting on three organic fractions of postcranial bone support a middle Holocene age assignment for the skeleton, probably in the range of 3500 to 5000 carbon-14 years before the present. This dating evidence is consistent with the geologic, archeological, and anthropometric relationships of the burial as well as previously determined carbon-14 determinations on associated materials.     

Potassium-argon geochronology of deep-sea sediments

A potassitlim-argon dating method applicable to small quantities of volcanic minerals anid glass has been developed and used to determine the ages of North Pacific sediments. Tertiary sedimentation rates range from less than 1.0 millimeter per 10(3) years for deep-sea "red clay" to 1 centimeter per 10: years for calcareotus-siliceous ooze necrer the continent. The potassium-argon ages obtained from different minerals are concordaat, and in samples withl associated fossils, these ages are compatible with the paleontologic evidence.     

Deep earthquakes beneath mount st. Helens: evidence for magmatic gas transport?

Small-magnitude earthquakes began beneath Mount St. Helens 40 days before the eruption of 20 March 1982. Unlike earlier preeruption seismicity for this volcano, which had been limited to shallow events (less than 3 kilometers), many of these earthquakes were deep (between 5 and 11 kilometers). The location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 March eruption than prior to any of the earlier dome-building eruptions. The depth-time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir.     

Calcium-41 concentration in terrestrial materials: prospects for dating of pleistocene samples

Calcium-41 has been suggested as a new tool for radiometric dating in the range of 10(5) to 10(6) years. The concentration of cosmogenic calcium-41 in natural samples of terrestrial origin has now been determined by high-sensitivity accelerator mass spectrometry after pre-enrichment in calcium-41 with an isotope separator. Ratios of calcium-41 to total calcium between 2 x 10(-14) and 3 x 10(-15) were measured for samples of contemporary bovine bone and from limestone deposits. Some prospects for the use of calcium-41 for dating Middle and Late Pleistocene bone and for other geophysical applications are discussed.     

Radiative climate forcing by the mount pinatubo eruption

Radiative flux anomalies derived from the National Aeronautics and Space Administration (NASA) spaceborne Earth Radiation Budget Experiment were used to determine the volcanic radiative forcing that followed the eruption of Mount Pinatubo in June 1991. They are the first unambiguous, direct measurements of large-scale volcanic forcing. The volcanic aerosols caused a strong cooling effect immediately; the amount of cooling increased through September 1991 as shortwave forcing increased relative to the longwave forcing. The primary effects of the aerosols were a direct increase in albedo over mostly clear areas and both direct and indirect increases in the albedo of cloudy areas.     

土壤绝对年龄测定方法研究进展

土壤年龄与各种成土因素密切相关,对于土壤生物地球化学循环、土壤环境变化、地质学评估、考古研究均有着重要意义。由于土壤样品所处环境复杂,干扰因素较多,因此在土壤年龄测定方面有很多困难。目前,土壤绝对年龄的主要研究方法涉及土壤学、地质学、考古学等多种学科,主要包括放射性同位素测年法、释光测年法、电子自旋共振测年法、古地磁法以及氨基酸外消旋测年法等。对常用测年方法的原理和研究进展进行了总结与分析,从全局的角度对各种方法的优劣进行了比较：放射性同位素测年所用元素广泛,可选土壤材料与时间尺度也各不相同;释光定年和电子自旋共振法以土壤中石英矿物为测定对象,时间尺度均为中等至长时间;氨基酸外消旋定年适用于有机质含量较高的土壤;富含磁性矿物如磁铁矿的土壤定年可以采用古地磁法,该方法时间尺度较长。认为土壤测年应重点从土壤粘土矿物和有机物着手,根据土壤样品理化特性进行对症分析,并通过多种方法综合评价,从而对土壤年龄进行更准确评价,并为土壤成因、环境气候变化以及长期碳氮循环等相关研究提供有力证据。     

The age of parana flood volcanism, rifting of gondwanaland, and the jurassic-cretaceous boundary

The Paraná-Etendeka flood volcanic event produced approximately 1.5 x 10(6) cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New (40)Ar/(39)Ar data combined with earlier paleomagnetic results indicate that Paraná flood volcanism in southern Brazil began at 133 +/- 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paraná flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.