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Cosmogenic nuclides contained in the annual layers of natural materials record temporal variations in the galactic cosmic ray flux. 14C in tree rings and 10Be in ice cores from polar regions are often used to retrieve accurate histories of galactic cosmic rays. However, the variation in 14C produced in the atmosphere is strongly attenuated in the carbon cycle and the amplitude of decadal scale variations in tree rings is approximately 0.2‰. In the case of 10Be in ice cores, the variation remains at a relatively large amplitude; however, the records are accompanied by dating uncertainties. Another material that contains 10Be is sediment in continental areas. Terrestrial carbonate deposits sometimes form clear annual layers; however, detailed analyses on such deposits have not yet been widely conducted. Therefore, we evaluate the possibility of retrieving information concerning the galactic cosmic ray flux from the terrestrial carbonate deposits. In this paper, we report our preliminary results obtained from the measurements of 10Be in travertine and tufa samples from South China. We find that the 10Be flux obtained from the travertines is comparable to the level produced by galactic cosmic rays and that travertines might have potential as a tool for investigating past cosmic-ray variations with annual resolution.

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth’s atmosphere. Several rapid 14C increases in the past, such as the 775 CE and 994CE 14C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid 14C increase, also known as the ca. 660 BCE event in German oak tree rings as well as increases of 10Be and 36Cl in ice cores, was presumed similar to the 775 CE event; however, as the 14C increase of approximately 10‰ in 660 BCE had taken a rather longer rise time of 3–4 years as compared to that of the 775 CE event, the occurrence could not be simply associated to an extreme SPE. In this study, to elucidate the rapid increase in 14C concentrations in tree rings around 660 BCE, we have precisely measured the 14C concentrations of earlywoods and latewoods inside the annual rings of Japanese cedar for the period 669–633 BCE. Based on the feature of 14C production rate calculated from the fine measured profile of the 14C concentrations, we found that the 14C rapid increase occurred within 665–663.5 BCE, and that duration of 14C production describing the event is distributed from one month to 41 months. The possibility of occurrence of consecutive SPEs over up to three years is offered.

To explore the possibility of using ground ice archives for studies of the cosmogenic radionuclide 10Be, we analyzed the beryllium isotopes in ice wedges exposed in the Cold Regions Research and Engineering Laboratory (CRREL) Permafrost Tunnel and the Barrow Permafrost Tunnel in Alaska. We determined the concentrations of 10Be and 9Be in samples pretreated following two procedures: acidification before (procedure A) and after (procedure B) removal of particles. The 10Be and 9Be concentrations spanned wide ranges. Concentrations in procedure A samples were higher than those in procedure B samples. The 10Be/9Be ratios fell within a narrow range, and values from CRREL (about 8.5 × 10−9) and Barrow (about 7 × 10−9) were of the same order of magnitude. Further studies are needed to validate our findings and assess the feasibility of using the 10Be/9Be ratio of syngenetic ice wedges for reconstruction of 10Be variations due to cosmogenic and environmental changes, and radiometric dating of ice-wedge sequences that contain very old (beyond 1 Ma) ice.

Cosmogenic radionuclides have been widely used to decipher Earth surface processes. At the University of Tokyo Accelerator Mass Spectrometry facility (5 MV Tandem accelerator), we have been measuring both in-situ and meteoric cosmogenic 10Be and 26Al. In this paper, we report some development of methodology, including how to prepare target samples chemically to reduce their background. We then introduce some examples using the method to provide further insights into earth surface processes. These studies include: i) exposure dating to identify the growths and decays of the Antarctic ice sheet, ii) understanding ice shelf collapse history, iii) paleomagnetic excursion history reconstructions using ice cores, iv) understanding the erosion rates using depth profiles of mid latitude outcrops, v) identifying the timing of impact crater formation.

The accurate reconstruction of past solar activity is a prerequisite for assessing the role of solar forcing on climate variations. Cosmogenic 10Be, produced by galactic cosmic rays and stored in natural archives such as ice cores and lake sediments, has been shown to be the most reliable as an indicator of past solar activity, although reconstructions at a higher, annual, resolution have remained elusive due to dating imprecision and uncertainties concerning the effect of depositional processes. Here, we employ a methodology that facilitates the annual reconstruction of past cosmic-ray and solar activity variations. An annual 10Be record is assembled from an endogenic travertine in Baishuitai, China, covering the period 2001–2016 CE. We demonstrate that the effect of depositional processes when 10Be is incorporated into travertine can be accounted for using the associated 9Be and potassium (K) contents, and that the resulting corrected 10Be content significantly correlates with the modeled global 10Be production rates with a time lag of 1.5–2.5 years. Our results from the Baishuitai endogenic travertine suggest that the 10Be record in such carbonate sediments has considerable potential as a high resolution proxy for past cosmic-ray and solar activity variations.

For atmospheric gases and aerosols, snow and ice provide a useful archive of paleoenvironmental history. In the northern hemisphere, this archive has been largely limited to Greenland ice, but promising pioneering work has been done recently using chronology of an ice wedge in Barrow, northern Alaska. Here, we investigate past aerosols at the same ice-wedge sampling site, reconstructing the sea-ice fluctuations in the adjacent Beaufort Sea during the Bolling/Allerod (BA) and Younger Dryas (YD) periods. We confirm the integrity of methanesulfonate ion (MS−) concentrations in the BIWS as marine proxies, and then find that the ice wedge has a high MS− concentration through the beginning (coldest) YD periods. The high MS− concentration indicates that even during the coldest YD periods (12,900–12,700 yrBP), the near-shore region in the Alaskan Beaufort Sea near Barrow may not have been completely filled by permanent sea ice.

The Sun exhibits centennial-scale activity variations and sometimes encounters grand solar minimum when solar activity becomes extremely weak and sunspots disappear for several decades. Such an extreme weakening of solar activity could cause severe climate, causing massive reductions in crop yields in some regions. During the past decade, the Sun’s activity has tended to decline, raising concerns that the Sun might be heading for the next grand minimum. However, we still have an underdeveloped understanding of solar dynamo mechanisms and hence precise prediction of near-future solar activity is not attained. Here we show that the 11-year solar cycles were significantly lengthened before the onset of the Maunder Minimum (1645–1715 CE) based on unprecedentedly high-precision data of carbon-14 content in tree rings. It implies that flow speed in the convection zone is an essential parameter to determine long-term solar activity variations. We find that a 16 year-long cycle had occurred three solar cycles before the onset of prolonged sunspot disappearance, suggesting a longer-than-expected preparatory period for the grand minimum. As the Sun has shown a tendency of cycle lengthening since Solar Cycle 23 (1996–2008 CE), the behavior of Solar Cycle 25 can be critically important to the later solar activity.

129I in natural archives, such as ice cores, can be used as a proxy for human nuclear activities, age marker, and environmental tracer. Currently, there is only one published record of 129I in ice core (i.e., from Fiescherhorn Glacier, Swiss Alps) and its limited time resolution (1–2 years) prevents the full use of 129I for the mentioned applications. Here we show 129I concentrations in an ice core from SE-Dome, Greenland, covering years 1956–1976 at a time resolution of ∼6 months, the most detailed record to date. Results revealed 129I bomb peaks in years 1959, 1962, and 1963, associated to tests performed by the former Soviet Union, one year prior, in its Novaya Zemlya test site. All 129I bomb peaks were observed in winter (1958.9, 1962.1, and 1963.0), while tritium bomb peaks, another prominent radionuclide associated with nuclear bomb testing, were observed in spring or summer (1959.3, and 1963.6; Iizuka et al., 2017). These results indicate that 129I bomb peaks can be used as annual and seasonal age markers for these years. Furthermore, we found that 129I recorded nuclear fuel reprocessing signals and that these can be potentially used to correct timing of estimated 129I releases during years 1964–1976. Comparisons with other published records of 129I in natural archives showed that 129I can be used as common age marker and tracer for different types of records. Most notably, the 1963 129I bomb peak can be used as common age marker for ice and coral cores, providing the means to reconcile age models and associated trends from the polar and tropical regions, respectively.

We introduce the method of multiple cross-wavelet algorithm, hereafter also as Einstein’s cross functions, for the time-frequency study of solar activity records or any astronomical and geophysical time series in general. The main purpose of this algorithm is to allow the simultaneous examination of the time-frequency information contents in n > 2 time series available. Previous cross-wavelet algorithm only permit the study of two time series at a time and was not extended to the generalized n > 2 problems until now. Furthermore, our new work lifted the restriction from the original formulation that are valid only for stationary processes. We applied our new algorithm to several of the solar activity proxies available in order to demonstrate the broad and powerful utility of this technique. We have used solar activity proxy records that are obtained under different geophysical archives and time periods which are, in turn, suitable for studying both the statistical and physical properties for solar variations valid on timescales of multi-century, millennium to several millennia. We focus on documenting the methodology in this paper rather than any elaborate interpretation of the results.

The research investigated three iron carbonate (siderite) sedimentary concretions from Nagykovacsi, Uri and Delegyhaza, Hungary. To identify possible source rocks and effects of the glaze-like exposed surface of the concretions, we carried on comparative petrological, mineralogical, geochemical and isotopic studies. The samples were microbially mediated siderite concretions with embedded metamorphous and igneous mineral clasts, and had specific rim belts characterized by semi-concentric outer Fe-oxide layers, fluffy pyrite-rich outer belts and siderite inner parts. We investigated the cross section of the Fe-carbonate concretions by independent methodologies in order to identify their rim effects. Their surficial oxide layers showed evidence of degassing of the exposed surface caused most probably by elevated temperatures. The inner rim pyrite belt in the concretions excluded the possibility of a prolonged wet surface environment. Microtextural and mineralogical features did not support desert varnish formation. 10Be nuclide values of the Nagykovacsi and Uri concretions were far above the level of terrestrial in-situ cosmogenic nuclides, but they were consistent with the lowest levels for meteorites. Though the data were not conclusive to confirm any kind of known origin, they are contradictary, and open possibilities for a scenario of terrestrial meteorite origin.

The Ninth Symposium on Polar Science/Interdisciplinary sessions: [IF] Toward Future Plan of the Arctic and Antarctic Science, Thu. 6 Dec. / Entrance Hall (1st floor), National Institute of Polar Research

The Western Pacific Warm Pool plays a significant role in large-scale atmospheric circulation and global hydrology. We conducted an environmental magnetic study of two late Pleistocene sediment cores from the western equatorial Pacific Ocean offshore of New Guinea in order to better constrain climatic and oceanographic variability, particularly spatiotemporal ocean productivity variations. Magnetic property measurements and transmission electron microscopy reveal that the magnetic mineral assemblages in the studied sediments are a mixture of biogenic and terrigenous magnetite. Variations in the acid soluble sediment component, interpreted as carbonate content, and the proportion of biogenic to terrigenous magnetite estimated from the ratio of anhysteretic to saturation remanent magnetizations are in-phase with northern hemisphere summer insolation variations. We interpret that ocean productivity increased during insolation maxima, which induced higher populations of magnetotactic bacteria through a larger nutrient supply to the seafloor. This interpretation assumes that magnetotactic bacterial populations are greatest in sediments just below the seafloor. Precessional frequencies in magnetic mineral concentration variations are suppressed after correction for carbonate dilution, whereas cyclic changes with a ∼100 kyr periodicity remain in carbonate-free magnetic concentration variations. Glacial/interglacial changes in bottom water currents may have influenced transportation and deposition of magnetic minerals. We demonstrate the usefulness of magnetic proxies for paleoceanographic studies, particularly of biogenic magnetite proxies for estimating paleoproductivity variations. This article is protected by copyright. All rights reserved.

Cosmogenic 10Be is a proxy of cosmic-ray flux, and its natural records provide vital information about the past intensity variability of the geomagnetic field and solar activity. 10Be records also serve as powerful tools for global synchronization among a variety of paleoarchives and for elucidating sedimentary processes on natural remanent magnetization acquisition. However, high-resolution (multi-decadal to multi-centennial) records of 10Be are scarce, especially those older than several tens of thousands of years. Here we present multiple high-resolution 10Be records of the Iceland Basin geomagnetic excursion interval (ca. 170–200 kyr ago) obtained from sediment cores (authigenic 10Be/9Be ratio) and an ice core (atmospheric 10Be flux). Comparing sedimentary 10Be records with relative paleointensity from the same cores, we found differences in the magnetic lock-in depth, even between adjacent cores. The 10Be-proxy records from the sediment and ice cores exhibit common characteristics: an asymmetric large-scale variation, a ∼7-kyr quasi-plateau around the maximum with a characteristic mid-term depression, and multi-millennial fluctuations in cosmic-ray flux during this interval. Minimal-synchronized and stacked 10Be records show that maximum cosmic-ray flux occurred 188.5–190.0 kyr ago and was double the present flux. A wavelet analysis of the stacked curve reveals dominant 4-kyr and secondary 8-kyr periodicities, both of which can be interpreted as intrinsic geomagnetic cycles. The wavelet spectrum of the high-resolution ice-core record shows a periodicity of 1.7 kyr and somewhat intermingled multi-centennial cycles around the maxima of 10Be, which likely represent solar cycles in this period. High-resolution 10Be records from multiple paleoarchives provide both a robust proxy record of cosmic-ray flux and a valuable tool for detailed global synchronization based on cosmic-ray variations.

Target materials were exposed to a muon beam with an energy of 160 GeV/c at the COMPASS experiment line in CERN-SPS to measure the production cross-sections for muon-induced radionuclides. A muon imager containing four nickel plates, each measuring 100 mm×100 mm, exposed to the IP plate successfully detected the muon beam image during an irradiation period of 33 days. The contrasting density rate of the nickel plate was (5.2±0.7)×10–9 PSL/muon per one-day exposure to IP. The image measured 122 mm and 174 mm in horizontal and vertical lengths, respectively, in relation to the surface of the base, indicating that 50±6% of the muon beam flux is confined to an area of 18% of the whole muon beam. The number of muons estimated from the PSL value in the total beam image area (0.81±0.1)×1013 was comparable to the total muon counts of the ion-chamber at the M2 beam line in the CERN-SPS. The production cross-sections of Cr-51, Mn-54, Co-56, Co-57, and Co-58 in nickel were 0.19±0.08, 0.34±0.06, 0.5±0.05, 3.44±0.07, 0.4±0.03 in the unit of mb, respectively, reducing muon associated particles effects. They are approximately 10 times smaller than that a proceeding study by Heisinger et al.

A 10Be analysis using only 10 μg of commercially available Be carrier was explored using Accelerator Mass Spectrometry (AMS) at the Micro Analysis Laboratory, Tandem accelerator, The University of Tokyo. A combination of sample pretreatment and AMS analysis dedicated to this limited-carrier experiment enabled us to achieve reproducible measurements linear up to 10−14 (10Be/9Be ratio). Using this method we determined an order of 105 atoms g−1 10Be concentrations in sub-gram natural quartz samples taken from low (0.5–1 km) altitudes in Asian mid to high latitudes, and with apparent 10Be ages of 10–60 ka. Concentrations determined with the limited-carrier analysis were fairly consistent with those of previous ordinary analysis with tens of grams of quartz. Therefore, 10Be analysis using 10 μg of Be carrier was demonstrated for natural samples with an order of 105 atoms g−1, with an error of about 10%.

14C content in tree rings and 10Be concentration records in polar ice core provide information about past cosmic ray intensities. The A.D. 774–775 cosmic ray event has been identified by 14C measurement in several tree rings from all over the world. Although the quasi-decadal 10Be Dome Fuji data in the Antarctic ice core also shows a sharp peak around A.D. 775, annual 10Be variations in the Dome Fuji core or in other cores have not been revealed. We have measured quasi-annual 10Be concentrations from approximately A.D. 763–794 in the Dome Fuji ice core, and detected a clear increase (~80% above the baseline) in 10Be concentration around A.D. 775. However, an accurate height of this increase is not straightforwardly estimated due to the background variation in 10Be concentration. The 10Be increase can be due to the same cosmic ray event as shown in the 14C content in A.D. 774–775.

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850-2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python., PAGES, a core project of Future Earth, is supported by the U.S. and Swiss National Science Foundations. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Some of this work was conducted as part of the North America 2k Working Group supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the U.S. Geological Survey. B. Bauer, W. Gross, and E. Gille (NOAA National Centers for Environmental Information) are gratefully acknowledged for helping assemble the data citations and creating the NCEI versions of the PAGES 2k data records. We thank all the investigators whose commitment to data sharing enables the open science ethos embodied by this project.

Stable nitrogen isotope ratios (δ 15 N) in sediment cores recovered from two Siberian lakes (Baikal and Hovsgol) are determined in order to evaluate the biologically mediated nitrogen cycle from the Last Glacial period to the postglacial period. The ranges of δ 15 N values from the two lakes are similar, varying from +2.8‰ to +6.7‰. The high values of δ 15 N (around +6‰) during the Last Glacial Maximum (LGM, 26.5–19 cal ka BP) indicate that a larger fraction of the available nitrate in both lakes was utilised by phytoplankton. A cool and dry climate during the LGM may have limited the input of terrestrial nitrate, resulting in the use of a large fraction of the nitrate, and low productivity, in both lakes. The use of a smaller fraction of the nitrate by phytoplankton in both lakes after the LGM (19–12 cal ka BP) is reflected in the lower δ 15 N values (down to +3.0‰). The increase in summer insolation after the LGM caused increased vertical mixing of the lake water column, which transported nitrate upwards from the deeper waters and into the photic zone. The reduced rate of nitrogen consumption after the LGM indicates that nitrate was more readily available to phytoplankton at this time and did not limit their activities. This change suggests that other factors limited phytoplankton activity during this period. At the climatic transition between Oxygen Isotope Stages 2 and 1 (around 11.5 cal ka BP), the rapid increase in δ 15 N (from +3.0‰ to +6.0‰) reflected a rapid increase in nitrate utilization. At this transition, increases in precipitation and lake levels, which were associated with the increased intensity of the East Asian Monsoon, may have brought a large influx of nutrients required by the phytoplankton into the lakes from the watershed. This hydrological change led to increased phytoplankton activity in both lakes, resulting in increased nitrate consumption. The δ 15 N data obtained here from the sediments of lakes Baikal and Hovsgol suggest that the nitrogen cycle in the Siberian region is strongly linked to hydrological changes across the Asian continent, which are related to global climate change.

The research investigated three iron carbonate (siderite) sedimentary concretions from Nagykovácsi, Úri and Délegyháza, Hungary. To identify possible source rocks and effects of the glaze-like exposed surface of the concretions, we carried on comparative petrological, mineralogical, geochemical and isotopic studies. The samples were microbially mediated siderite concretions with embedded metamorphous and igneous mineral clasts, and had specific rim belts characterized by semi-concentric outer Fe-oxide layers, fluffy pyrite-rich outer belts and siderite inner parts. We investigated the cross section of the Fe-carbonate concretions by independent methodologies in order to identify their rim effects. Their surficial oxide layers showed evidence of degassing of the exposed surface caused most probably by elevated temperatures. The inner rim pyrite belt in the concretions excluded the possibility of a prolonged wet surface environment. Microtextural and mineralogical features did not support desert varnish formation.

Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets., SCOPUS: ar.j, info:eu-repo/semantics/published

Multi-proxy records (geochemistry, clay mineralogy and paleontology) from three sediment cores from Lake Hovsgol, northwest Mongolia, provide paleoenvironmental changes in continental Asia from the last glacial to the present (during the last 29 cal ka BP). This paper presents the first continuous δ 13 C and δ 18 O dataset for ostracod shells ( Cytherissa lacustris ) from Lake Hovsgol sediments. A positive shift in ostracod δ 13 C values (from + 0.5‰ to + 3.2‰) began at ca. 21 cal ka BP, and was associated with a decrease in δ 13 C values of total organic carbon (from − 24.7‰ to − 27.3‰) and an increase in the mass accumulation rate of total organic carbon (MAR-TOC). These results suggest that increased lake productivity led to the 13 C-enrichment of dissolved inorganic carbon in Lake Hovsgol. In addition, decreases in clay content and the illite crystallinity index (full width at half maximum of the 10 A peak) began at ca. 21 cal ka BP, indicating an increase in surface runoff and a change in sources of clastic materials in the watershed. The highest δ 18 O ostracod values (up to − 4.8‰) at 16–15 cal ka BP indicate the increases in precipitation from the East Asian monsoon. The total pollen amount increased, and Betula , Alnus , and Salix were dominant (up to 53%, 30%, and 6%, respectively) after 16–15 cal ka BP. These findings indicate that broad-leaved trees expanded around Lake Hovsgol. In addition, fossil diatom valves appeared after 16–15 cal ka BP, suggesting climate amelioration, and increased input of soil-derived nutrients and dissolved silica. Between 10 and 5 cal ka BP (Holocene climate optimum), productivity was high in and around the lake (MAR-TOC, up to 47.3 mg/cm 2 kyr; MAR-diatoms, up to 14.4 × 10 8 valves/cm 2 kyr; total pollen concentration, up to 9.0 × 10 5 grains/cm 3 ). Environmental changes during the last 29 kyr in and around Lake Hovsgol are clearly associated with fluctuations in summer insolation and East Asian monsoon intensity.

We studied organic components in the X106 sediment core (length 130.3 cm, water depth 236 m, 50°53′01″N, 100°21′22″E) from Lake Hovsgol to elucidate the biological production, source of organic components, and paleoenvironmental and paleolimnological changes during the last 27 kyr in northwest Mongolia. Total organic carbon (TOC) contents (0.20–0.70%) in the core of the last glacial period increased dramatically and attained 3.16–5.85% in the postglacial period (Holocene), together with the increase of the contribution of terrestrial organic matter. Biological production (both terrestrial and aquatic production) based on the TOC contents in the Holocene was 14 times higher than that in the last glacial period. The Bolling-Allerod warm period and Younger Dryas cool period were both observed at depths of 55–50 cm (ca. 15–13 cal kyr BP) and 50–45 cm (ca. 13–11 cal kyr BP), respectively. We propose here a terrestrial/aquatic index (TAI) for organic matter in lake sediments. The TAI values suggest that terrestrial organic matter in the bottom of the core was less than 10%, increased to 48% in the Bolling-Allerod warm period, decreased abruptly to 20% in the Younger Dryas cool period, and again increased to 30–40% in the Holocene. Normal-C31 alkane (a biomarker of herbaceous land plants) and n-C18 alkanoic acid (marker of plankton) decreased from the last glacial period to the Holocene, whereas n-C23 alkane and n-C22 alkanoic acid (a marker of higher vascular plants) increased from the last glacial period to the Holocene. Scarce herbaceous plant vegetation, such as Artemisia spp. of the lake basin in the last glacial period, changed into an abundance of higher woody plant vegetation (e.g., Pinus spp., Betula spp. and/or Larix spp.) in the Holocene. Stanol/sterol ratios suggest that relatively high oxygen tension of the lake bottom in ca. 27–22 cal kyr BP decreased from this age to the present, though benthic organisms are still abundant.

Changes in sulfur cycles in Lake Hovsgol (northwest Mongolia) are discussed on the basis of total organic carbon (TOC), total sulfur (TS) concentrations and stable sulfur isotope ratios (δ 34 S) in two sediment cores (X104 and X106) covering the last glacial to the present. Correlation plots of TOC and TS (C–S plots) of the sediment cores indicate dissolved sulfate enrichment in sediment pore water and high sulfate reducing bacteria (SRB) activity during the climate transition from the last glacial to postglacial period (LG–PG transition, from ca. 21 to 11.6 cal ka BP). In addition, positive shifts of δ 34 S values (from + 3.7‰ to + 32.6‰) were observed during the LG–PG transition. These results suggest a large influx of 34 S-rich dissolved sulfate into the lake during this period. The 34 S-rich dissolved sulfate may have accumulated on the shore (wetlands and marshes) of Lake Hovsgol during the last glacial period. Owing to the increase in atmospheric precipitation and lake level rise during the LG–PG transition, the 34 S-rich dissolved sulfate may have been supplied from the shore to the lake. Increased sulfate influx during the LG–PG transition has also been observed in a Lake Baikal sediment core. Our new sulfur geochemical data shows past-environmental changes, particularly moisture changes, within continental Asia during the climate transition.

This paper reports the results of trial measurements of the concentration of cosmogenic 36Cl in an ice core recovered from Dome Fuji Station, Antarctica. Ten ice samples were extracted from depths corresponding to the Last Glacial Maximum and the early Holocene, with each sample comprising ∼150 g for a core length of 0.5 m (∼30–40 yr resolution). Analyses of 36Cl concentrations by accelerator mass spectrometry were successfully performed within ±10% precision, even for samples with low 36Cl/Cl values (∼3 × 10–14). The average 36Cl concentrations were 16 × 103 atoms g–1 for the Last Glacial Maximum, and 7 × 103 atoms g–1 for the early Holocene. The apparent 36Cl depositional flux at Dome Fuji Station during these periods was estimated to be approximately 2.5–3.5 × 104 atoms cm–2 yr–1.

Soil samples from an 85-cm-long continuous section (PY608ES) were collected from an island in Lake Pumoyum Co (southeastern Tibetan Plateau, ∼5020 m asl) in August 2006. To estimate past environmental conditions of Lake Pumoyum Co during the Holocene, we analyzed radiocarbon ages, stable carbon isotope compositions, and total organic carbon/total nitrogen (TOC/TN) atomic ratios of the soil samples. The 14C measurements were performed with the Tandetron accelerator mass spectrometry system at the Center for Chronological Research, Nagoya University. The 14C concentration in the surface layer (101 pMC; 5–10 cm soil depth) was nearly modern. A 14C chronology of the sequence indicated that continuous soil development began on the island in Lake Pumoyum Co at ∼5800 cal BP (at 63 cm soil depth, the top of a gravel layer). These results may reflect a decrease in the lake level in the middle Holocene. The age of the obvious lithologic boundary (∼5800 cal BP) corresponds to the end of Holocene climate optimum.

High-time resolution 14 C dating of Lake Baikal sediment cores indicates negative and positive anomalies of calculated linear sedimentation rate (LSR; 1.1 and 35.6 cm/ka, respectively) during the period of climate transition from the last glacial to Holocene. The timing of the Lake Baikal apparent LSR anomalies is consistent with that of the changes in the atmospheric radiocarbon concentration (Δ 14 C) during Younger Dryas rapid cooling event. 14 C dating of lipids in the Lake Baikal surface sediments revealed that the sources of sedimentary lipids were different in each basin. In the Northern Basin of Lake Baikal, the 14 C age of total lipids from the surface sediment (4.0 14 C ka) was found to be older than that of TOC (1.6 14 C ka). By contrast, the 14 C age of total lipids in the Southern Basin was younger than that of the TOC by ca. 0.7–3.0 ka. In the Lake Hovsgol sediment cores, ages of the main lithologic boundaries during the last glacial–interglacial transition were estimated based on new 14 C data sets. TOC concentration in the cores started to rapidly increase at 13.8 ± 0.3 14 C ka at the base of the basinwide finely laminated layer deposited during Bolling/Allerod. The base of the layer diatomaceous mud corresponds to the end of Younger Dryas event (10.6 ± 0.1 14 C ka).

The cosmogenic nuclide 10Be was analyzed by using accelerator mass spectrometry on an ice core drilled at the Dome Fuji station, inland Antarctica, for 700–1900 yr CE. The measured concentration of 10Be in the Dome Fuji ice core and the derived 10Be flux show similar fluctuations, with both increasing at known solar-activity minima over the last millennium in agreement with earlier observations of 10Be and 14C. Based on the similar nature of the 10Be flux to the reconstructed 14C production rate patterns, a 10Be–14C correlation age model for the Dome Fuji ice core was successfully constructed. This age model agrees well with the initial version of the tephrochronology of the core. The 10Be-flux record contains information on variability in the amount of cosmic radiation incident on the atmosphere, which is mainly attributable to high-frequency change in solar activity and low-frequency background intensity adjustment of the geomagnetic field. High-resolution 10Be analyses of the Dome Fuji ice cores promise to provide potentially important information on the history of cosmic radiation intensity over the past several hundred thousand years.

We present preliminary data of our 10Be analysis of an ice core retrieved from Dome Fuji station, Eastern Antarctica for 1500–1810 yr AD. The concentration of 10Be from 1500-1810 yr AD ranged between 7.0 × 104 and 13.0 × 104 atoms g−1 and a prominent peak was observed in the period 1645–1715 yr AD (i.e., the Maunder Minimum period). An increase in the concentration was also observed in the periods before 1540 yr AD and near to 1800 yr AD. A comparison of our 10Be record with the South Pole 10Be record shows a clear similarity in their temporal fluctuations. On the other hand, our record shows ∼2.5 times higher concentration of 10Be than in the South Pole record attributable to the difference in local snow accumulation rates. These observations suggest a direct (uncomplicated) fallout of atmospheric 10Be onto inland Antarctica, which in turn, leads to a detailed 10Be stratigraphy throughout this region. We believe that the 10Be profiles of the ice cores from the Dome Fuji station are therefore likely to be a good proxy indicator of changes in solar activity.

We have determined the radiocarbon ages for 40-yr-interval tree rings in 2 fossil trees of the Towada Hachinohe buried forest, northeastern Honshu Island, Japan. The 14C ages range from 13.0 to 13.3 kyr BP (about 15.5 cal kyr BP). The weighted average of the 14C age of the outermost 5 rings is 13,133 ± 33 BP, which can be calibrated to 15,363–15,679 cal BP by using the IntCal04 standard curve (Reimer et al. 2004). The estimated δ14C values range between 265 and 300% and show approximately sinusoidal fluctuation of an indicated ∼200-yr cycle, perhaps reflecting contemporary solar activity change. Comparison between the tree 14C profile and the Cariaco Basin 14C record provides further information on the accurate date of the Towada Hachinohe buried forest and the eruption that produced it. 14C analysis of tree rings from the buried forest may contribute to the construction of a better 14C calibration curve and the elucidation of solar activity change during the last glacial period, as well as possible global and regional impacts of the huge eruption from Towada Volcano.

We analyzed photosynthetic pigments, total organic carbon (TOC) and total nitrogen (TN) in two radiocarbon-dated sediment cores of Lake Hovsgol, NW Mongolia, covering the period of the last glacial/post-glacial transition since 23 14C ka. The sedimentary profiles of total chlorophyll-a derivatives (TChl-a), TOC and TN reflect productivity increase during the last glacial/post-glacial transition between 15 and 10 14C ka in response to regional climate warming and enhanced precipitation. Correlation between TChl-a and TOC was high ( R 2 = 0.934 , n = 74 ) with a zero intercept of a regression line suggests that total organic carbon in Lake Hovsgol sediments is of autochthonous origin. The depth profiles of TOC/TN ratio showed increases toward the core top in parallel with other paleoproductivity proxies, serving as an indicator of climatic transition, quite similar to Lake Baikal sedimentary record. The primary reason for the shifts of this parameter is still not clear, however. Chlorophyll-b derivatives and some algal carotenoids such as lutein, zeaxanthin diatoxanthin and canthxanthin, were detected, although in concentrations lower than that of TChl-a. These biomarkers indicate that green algae, cyanobacteria and diatoms were present in phytoplankton during the post-glacial period, significantly pre-dating the beginning of the Holocene. Abrupt drops of biomarker profiles at ca. 1–2 and 10 14C ka due to decreased productivity in the lake or enhanced degradation of the biomarkers likely record short-term climatic events.

Cosmogenic 10Be and 26Al dating of three erratic boulders in the southern coastal area of Lake Baikal was performed using accelerator mass spectrometry (AMS) at MALT (MicroAnalysis Laboratory, Tandem accelerator) of the University of Tokyo. We examined the accuracy of our 10Be AMS measurements and the differences in scaling factors for the 10Be and 26Al production rates. The measured 10Be and 26Al ages are 15–13 and 17–11.5 kyr, respectively. The 10Be ages suggest that release of the erratic boulders from a melting glacier occurred during the Bolling/Allerod warm period, when the land and lacustrine environments of the Baikal region changed dramatically.

MALT (Micro-Analysis Laboratory, Tandem accelerator) at The University of Tokyo is a research and service facility for elemental and isotopic analysis using a tandem accelerator for AMS, PIXE and NRA. Among these, AMS is the most promising technique at MALT. After the AMS-8 conference in 1999, the overall performance was greatly improved for 10Be-, 26Al- and 14C-AMS. For 10Be-AMS, the negative ion current was considerably increased by a new Cs-ionizer system and the replacement of the compound powder (Nb instead of Ag) for the BeO target. More than 3 μA current of BeO− ions can be extracted while it had been typically 1 μA before. For 26Al-AMS, by examining the sample ignition process, the negative ion gain was increased more than 200 nA (typically 100 nA before), which greatly improved the overall efficiency. For 14C-AMS, the 12C− current dependency problem has been moderated substantially by installing a fine controllable Cs oven and careful beam tuning. As a consequence, the precision is now better than 0.5% for modern carbon. In addition, systems for 36Cl-AMS and 129I-AMS are currently being developed for future research programs at MALT.

A 150 ky record of 10 Be found in the sediments of Lake Baikal is compared with the two geochemical stacks indicating warm and cold climates (“warm stack” and “cold stack”). The stacks were calculated from the concentrations of elements which were determined by Synchrotron Radiation X-Ray Fluorescent Analysis and Neutron Activation Analysis. The profiles of 10 Be concentrations and flux suggest that the main reason of the fluctuations was the changing extents of dilution with terrigenous matter and diatom algae in layers belonging to warm and cold periods marked by the peaks of geochemical stacks. Contrary to the explanations proposed for other records, the rate of production of cosmogenic 10 Be is not the principal reason of 10 Be fluctuations in the sediments of Lake Baikal; however, the minor peak of 10 Be flux at ca. 40 ky BP may be partly due to the global increase in the production of 10 Be around the Laschamp excursion. The concentrations of 10 Be in the terrigenous fraction, as well as values of the warm geochemical stack, abruptly decreased at the wake of the cold stages, suggesting an increased rate of production of clastic matter which was the source of the terrigenous fraction of the lake sediments. Hence, they are candidates for the proxy of the accumulation rate of terrigenous materials in Lake Baikal sediments.

Sedimentary 10Be records covering the last 150 kyr were obtained from three cores collected at the Academician Ridge (BDP-96/hole2 core and VER96/st.3 core) and at the Buguldeika Saddle (BDP-93/hole2 core) in Lake Baikal. The 10Be concentrations of the three cores varied between 0.5×109 and 1.5×109 atoms/g, and coincidently dropped at the stratigraphic intervals of marine oxygen isotope stages (MIS) 2, 4, 5d and 6. The depositional fluxes of 10Be, on the other hand, generally rose in those stages having an increase in the dry bulk densities and sediment accumulation rates. These results are consistent with previous work (Horiuchi et al., 1999), suggesting that the dilution effects of low-10Be-concentration particles principally controlled the fluctuations of the 10Be concentrations of Lake Baikal sediments. Low-10Be-concentration particles have been intensively produced by mechanical weathering and physical erosion under the cold and dry climatic conditions during the peak glaciation period, and have been directly brought from the source areas into the lake as a result of the thin vegetative cover of the watershed.

A sediment core recovered from Lake Baikal was studied in an attempt to elucidate the palaeoenvironmental history of an intra-continental lake under the control of last-glacial to post-glacial climatic fluctuations. The lake conditions were characterised by subdued primary production and increased turbidity conditions during the last glacial maximum. Terrestrial vegetation and the development of soil layers were reduced in the drainage areas. The deglaciation, suggested by increased terrigenous sand influx and biological productivity, took place at around 18 000 yr B.P., responding to the increased insolation in the Northern Hemisphere high latitude. The onset of wet and warm conditions at ca. 12 000 yr B.P. brought on a maximum of aquatic production and forestal expansion between 7 000 and 6 000 yr B.P. Since 5 000 yr B.P., enhanced blooming of diatoms has occurred due to an increased nutrient supply and dissolved silica input into the lake.

10Be concentrations, sediment composition and sediment accumulation rates were determined in a sediment core from the Academician Ridge of Lake Baikal. The 10Be concentrations vary from 5.07 × 108 atoms/g in the last glacial section to 11.25 × 108 atoms/g in the section representing the post-glacial climate stage. 10Be fluxes, on the other hand, increase toward the last-glacial stage coinciding with an increase of the sediment accumulation rate. These facts imply that climate-related sedimentation is related to both flux and concentration of the nuclide in the sediment core. During the last-glacial stage, arid conditions in the lake's watershed caused a marked increase in flux of clay particles with low 10Be content to the lake, resulting in an increased flux but lowered concentration of 10Be in the sediments. It is suggested that the 10Be record of the bottom sediments of Lake Baikal can be used as a proxy indicator of climate change in central Asia.

Biogenic siliceous deposits are a significant component of Neogene stratigraphic sequences of the northeast Japan arc. Field observations demonstrate that the siliceous rocks are classified into five lithologic types: diatomite, opaline porcelanite, opaline chert, quartzose porcelanite, and black shale. Stratigraphic data suggest that the siliceous formations were deposited continuously from the Middle Miocene to Early Pliocene. Diatomite occurs throughout the sedimentary sequences and changes laterally into chert and porcelanite. Multivariate statistical analysis of geochemical data indicates that lithification and mineralogical transformation of silica-rich deposits was governed not only by compaction and cementation during burial but also by sorting of grains by biological sediment-seawater mixing. Diatomite is composed of frustules without fine ornamentations and delicate structures. Fine and delicate tests were more susceptible to dissolution than coarse and solid ones, and dissolved in the course of biological sediment disturbance. Ubiquitous occuerences of diatomite in the stratigraphic sequences are attributed to residual frustules which were more resistant to the diagenetic dissolution-precipitation reaction of silica. Biogenic silica is the main source of silica to form chert. Diagenetic precipitation of silica in diatomaceous beds through progressive sediment burial formed porcelanitic rocks, with opaline porcelanite being the precursor of quartzose porcelanite. Chert was not subjected to biological sediment reworking, and fragile shells were preserved during accumulation. Silicification of precursor deposits of chert was promoted by intraformational dissolution-precipitation reaction of silica with increasing sediment compaction and dehydration.

PII of originalarticle S0031-0182(99)00156-X.* Corresponding author.Department of Earthand EvironmentalSciences Facultyof Scienceand Technology, HirosakiUni-versityBunkyo-chou,Hirosaki,Aomori, 036-8561 Japan. Tel.: +81-172-39-3547; Fax: +81-172-39-3547.E-mail address: kh@cc.hirosaki-u.ac.jp(K. Horiuchi).PALAEO288930-5-02Palaeogeography, Palaeoclimatology, Palaeoecology 183 (2002)181^183www.elsevier.com/locate/palaeo

第１回極域科学シンポジウム「極域大気圏を通して探る地球規模環境変動」12月1日（水） 国立極地研究所 ２階大会議室

[1] In this article, we aim to determine frequency ranges and intervals of time in which the solar signal dominates in different cosmogenic isotope data. From a 14C-based reconstruction of cosmic ray intensity over the last millennia, we computed expected 10Be variations in two Antarctic sites (Dom Fuji and South Pole) and two Greenland sites (Dye-3 and GISP-2) and compared them with the actually measured 10Be abundance at the sites. By applying different methods of analysis, such as bivariate correlation, conventional FFT coherence, and wavelet coherence, we found the following: (1) The modeled series, on the basis of 14C data, are in good agreement with the measured 10Be data sets, on different timescales and at different locations, confirming the existence of a common solar signal in both isotope data. (2) The 10Be data are driven by the solar signal on timescales from about 100 years up to 1000 years or even to multimillennial scales (at the longer scales, paleomagnetism plays an increasingly important role). (3) The local climate dominates the 10Be data mostly on short (