Progress on I-129 analysis and its application in environmental and geological researches

Iodine-129 is a naturally generated isotope, but anthropogenic releases are the dominated source of I-129 in the present environment. Among many measurement techniques, neutron activation analysis (NM) and accelerator mass spectrometry (AMS) are only methods used for measurement of I-129 in environmental level. Based on its source terms, chemical properties and environmental behaviors, I-129 can be applied for geological dating in a range of 2-80 Ma, investigation of formation and migration of hydrocarbon, circulation of ocean water, atmospheric process of iodine, as well as reconstruction of dispersion and migration of short-lived radioisotopes of iodine released from nuclear accidents. This article aims to summarize and critically compare the analytical techniques used for I-129 measurement and chemical methods for separation of iodine from various sample matrices, purification from the interferences, as well as preparation of suitable target for AMS measurement. The major applications in environmental and geological researches are reviewed, which mainly focus on the new progress and potential development in the future. The application of I-129 in the investigation of radioactive contamination from the Fukushima accident is discussed.</p

Asynchronous records of Brunhes/Matuyama reversal in marine sediments and Chinese loess: Review and discussion

Geomagnetic polarity reversals are generally considered to occur synchronously around the world, and are commonly used as time markers to correlate events between different sediment archives. They also play an indispensable role for understanding the mechanisms which generate the Earth&#39;s geomagnetic field. Nevertheless, in the case of the most recent reversal, the Brunhes-Matuyama (B/M) reversal (similar to 780 ka), comparison of paleomagnetic studies in Chinese loess-paleosol sequences versus marine cores reveal a marked discrepancy in timing of this event. This timing offset has led to uncertainties in correlating the Chinese loess-paleosol sequences with other global climate records. This paper briefly reviews the history of study on the B/M reversal both in marine and Chinese loess sediments, and reviews several contentious explanations of the aforementioned B/M boundary offset. Finally, the most recent progress of paleogeomagnetic field variation studies using cosmogenic Be-10 in Chinese loess is introduced, with the purpose of suggesting a potential way to address this B/M boundary timing offset issue.</p

Proxy value of n-alkan-2-ones in the Hongyuan peat sequence to reconstruct Holocene climate changes on the eastern margin of the Tibetan Plateau

n-Alkan-2-ones in the Hongyuan peat core from the eastern margin of the Tibetan Plateau were investigated to assess the paleoclimate proxy value of variations in their compositions over the last 13,500 cal years. A homologous series of these compounds that ranged from C(19) to C(31) was identified throughout the peat sequence, maximizing at C(23) and showing a strong odd-over-even carbon chain length predominance. To help evaluate the possible paleoclimatic value of the n-alkan-2-ones in the Hongyuan peat core, we compared our molecular results with the pollen records from the same sequence. Results show that past changes in monsoon precipitation amounts as revealed by the ratio of Abies/Cyperaceae (firs/sedges) are also recorded in the n-alkan-2-one (C(23) + C(25))/(C(27) + C(29) + C(31))-KET ratio and CPI-KET (Carbon Preference Index) value. Stratigraphic variations of n-alkane and n-alkanoic acid ratios suggest that changes in the moisture-sensitive microbial alpha-oxidation of odd-chain n-alkanes and decarboxylation of even-chain length n-alkanoic acids is recorded in the abundance of n-alkan-2-ones. Changes in the monsoon rain intensity on millennial scales are also evident in the n-alkan-2-one proxies, indicating that their variations in the peat sequence are correlative with changes in microbial activity controlled by the Asian monsoon. (C) 2011 Published by Elsevier B.V.</p

The last 130 ka precipitation reconstructionfrom Chinese loess 10Be

Cosmogenic 10Be is a promising precipitation index, because its fallout flux in sediments is mainly controlled by wet precipitation after its production in the atmosphere. Here we report on a new study for reconstructing precipitation during the last 130 ka using 10Be measurements from Chinese loess, with multivariable linear regression to remove the geomagnetic field modulation and dust flux dilution effects from the loess 10Be record. The broad similarity between our result and speleothem &delta;18O indicates that the new precipitation record is robust. It also records an interesting increase in precipitation that occurred during Marine Isotope Stage 3 (MIS 3), exhibiting a similar rainfall amount with that of MIS 5, suggesting that MIS 3 is a special period with strengthened summer Monsoon intensity. By comparison with a stacked marine isotope record and a summer insolation record, our precipitation data clearly show a close correspondence with Northern Hemisphere summer (June, July, and August) solar insolation changes on orbital timescales. During MIS 3, our record follows the insolation differential between 30&deg;N and 30&deg;S, suggesting that rising rainfall changes during MIS 3 are a response to the interhemispheric summer insolation differential forcing.</p

C-14 DATING OF SOIL ORGANIC CARBON (SOC) IN LOESS-PALEOSOL USING SEQUENTIAL PYROLYSIS AND ACCELERATOR MASS SPECTROMETRY (AMS)

The chemical extraction of soil organic carbon (SOC) fractions from soils often does not produce satisfactory results for radiocarbon dating. In this study, a sequential pyrolysis technique was investigated. The soil was pyrolyzed at temperatures of 200, 400, 600, and 800 degrees C to partition organic carbon into pyrolytic volatile (Py-V) and pyrolytic residue (Py-R) fractions. The preliminary results show that the C-14 dates of both fractions become progressively older as the pyrolysis temperature is increased. In addition, the ages of the Py-V fractions are consistently younger than the corresponding Py-R fractions extracted at the same temperature. Experimental results of known-age paleosol samples indicate that the Py-V fractions obtained between 600 and 800 degrees C yield the most reliable ages. This technique provides a new approach to improve the accuracy of C-14 dating of loess-paleosol sequences.</p

The n-alkanol paleoclimate records in two peat deposits: a comparative study of the northeastern margin of the Tibetan Plateau and Northeast China

A number of studies have revealed that the climate in the eastern margin of the Tibetan Plateau and Northeast China is sensitive to postglacial changes. Unfortunately, the link of the past climate evolution between the two regions is not well understood. In this study, two cores are analyzed to determine this link directly. The high-resolution n-alkanol distribution patterns from two typical peat sequences covering the past 16,000 cal years in the northeastern margin of the Tibetan Plateau and Northeast China, respectively, are closely examined by gas chromatograph-mass spectrometry analysis. In combination with other palaeoclimatic proxies, it is proposed that the n-alkanol average chain length and (C(22) + C(24))/(C(26) + C(28)) ratio could reflect past climate changes in the two peat sequences. The n-alkanol proxies reveal several climatic intervals in the period from the last deglaciation through the Holocene. A comparison of n-alkanol records between the northeastern margin of the Tibetan Plateau and Northeast China indicates that the start and end of the warm Holocene Optimum differed at the two locations. The spatially asynchronous pattern of climatic change is possibly a result of different responses to change in solar radiation. The evolution of the Holocene paleoclimate is more consistent with changes in Northern Hemisphere solar radiation in Northeast China than on the Tibetan Plateau. The Holocene Optimum began and terminated earlier in Northeast China than in the northeastern margin of the Tibetan Plateau. Thus, the two n-alkanol proxies provide valuable insights into the regional Holocene climate and local environmental conditions.</p

Analysis of low-level I-129 in brine using accelerator mass spectrometry

An improved solvent extraction procedure for iodine separation from brine samples has been applied at Xi&#39;an Accelerator Mass Spectrometry (AMS) center. Oil in the brine sample has to be removed to avoid appearance of the third phase during solvent extraction and to improve the chemical yield of iodine. The small amount of oil remained in the water phase was first removed by phase separation through settling down sufficiently based on their immiscibility, and then by filtration through a cellulose filter, on which oil was absorbed and removed. After oil removed, extraction recovery of iodine could achieve more than 90 %. The sodium bisulfite as an effective reductant should be added before acidification to avoid loss of iodine by formation of I-2 in sample via reaction of iodate and iodide at pH 1-2, and then pH was adjusted to 1-2 to reduce the iodate to iodide followed by oxidation of iodide to I-2 and solvent extraction to separate all inorganic iodine. As a pre-nuclear era sample, I-129/I-127 ratio in brine is normally more than two orders of magnitude lower than that in present surface environmental samples, so prevention of cross-contamination and memory effect in apparatus during processing procedure are very critical for obtaining reliable results, and monitoring the procedure blank is very important for analytical quality of I-129. The I-129/I-127 isotopic ratio in the brine samples and procedure blank of iodine reagents were measured to be (1.9-2.7) x 10(-13) and 2.08 x 10(-13), respectively, 3-4 orders of magnitudes lower than that in environmental samples in Xi&#39;an, and the result of procedure blank is in the same level as the previous experiments in past 3 years, indicating contamination is not observed in our method.</p

MEASUREMENT OF Be-10 CONCENTRATION OF MODERN FALLING DUST IN NORTHERN CHINA

Paleoenvironmental and paleogeomagnetic tracing studies from Chinese loess Be-10 have progressed in recent years (Zhou et al. 2007a,b, 2010). In this approach, Be-10 flux determined from sediment concentration and accumulation rate may be used to recover information about paleomonsoon rainfall rates as well as past variations in the geomagnetic field strength. However, these methods require that a correction be made for residual undecayed Be-10 in remobilized dust. To better understand the feature of the Be-10 signals related to the remobilized dust, we report the first observational study on Be-10 concentration of modern falling dust using the 3MV multi-element accelerator mass spectrometer (AMS) in the Xi&#39;an AMS Center. Ten samples collected at Ansai observation station (109 degrees 19&#39;E, 36 degrees 51&#39;N) in northern China from May 2008 to June 2009 are measured along with 3 chemical blanks. The results clearly show that the Be-10 content of modern falling dust is relatively uniform, with a mean value of 1.21 x 10(8) atoms/g, a measurement similar to that of Chinese loess (Zhou et al. 2007a) and to the value found in the study by Shen et al. (2009) on dust Be-10 falling near Dingbian, China (1.25 +/- 0.06 x 10(8) atoms/g). Despite the fact that modern dust flux is much higher in spring relative to summer in northern China, Be-10 concentration in falling dust remains fairly constant. In addition, we find that dust Be-10 concentration is roughly independent of the local precipitation changes. This feature might be considered as an analogue to improve our understanding on the fundamental information of the source component contained in loess Be-10 records and its spatial/temporal distribution features.</p

Observations of Atmospheric Δ14CO2 at the Global and RegionalBackground Sites in China: Implication for Fossil Fuel CO2 Inputs

Six months to more than one year of atmospheric &Delta;14CO2 were measured in 2014-2015 at one global background site in Waliguan (WLG) and four regional background sites at Shangdianzi (SDZ), Lin&#39;an (LAN), Longfengshan (LFS) and Luhuitou (LHT), China. The objectives of the study are to document the &Delta;14CO2 background levels at each site and to trace the variations in fossil fuel CO2 (CO2ff) inputs at regional background sites. &Delta;14CO2 at WLG varied from 7.1&plusmn;2.9&permil; to 32.0&plusmn;3.2&permil; (average 17.1&plusmn;6.8&permil;) in 2015, with high values generally in autumn/summer and low values in winter/spring. During the same period, &Delta;14CO2 values at the regional background sites were found to be significantly (p &lt; 0.05) lower than those at WLG, indicating different levels of CO2ff inputs at those sites. CO2ff concentrations at LAN (12.7&plusmn;9.6 ppm) and SDZ (11.5&plusmn;8.2 ppm) were significantly (p &lt; 0.05) higher than those at LHT (4.6&plusmn;4.3 ppm) in 2015. There were no significant (p &gt; 0.05) seasonal differences in CO2ff concentrations for the regional sites. Regional sources contributed in part to the CO2ff inputs at LAN and SDZ, while local sources dominated the trend observed at LHT. These data provide a preliminary understanding of atmospheric &Delta;14CO2 and CO2ff inputs for a range of Chinese background sites.</p

n-Alkan-2-one distributions in a northeastern China peat core spanning the last 16 kyr

Most research on long chain methyl ketones has focused on their origins and distributions. Their application in paleoclimate studies is less common than that of other n-alkyl lipids. The goal of this research was to explore this potential by studying n-alkan-2-ones from the Hani peat sequence in northeastern China. They were identified using gas chromatography-mass spectrometry (GC-MS) and showed a distribution ranging from C(19) to C(31) with a strong odd/even predominance. This type of distribution is considered to derive from Sphagum and microbial oxidation of n-alkanes. Comparison with climate sensitive indicators and macrofossil analysis shows that microbial oxidation of n-alkanes derived from higher plants was enhanced during the warm early Holocene period. This led us to develop three n-alkan-2-one proxies - C(27)/Sigma C(23-31) (C(27)/HMW-KET), carbon preference index (CPI(H)-KET) and average chain length (ACL((27-31))-KET) - as possible indicators of paleoclimate in the peat-forming environment. These proxies, in combination with C(27) n-alkane delta D values and peat cellulose delta(18)O records, might allow examination of paleo-ecosystem behavior during climatic evolution in northeastern China over the past 16,000 yr.</p