Hiroyuki Matsuzaki

Variations of East Asian summer monsoon since the last deglaciation based on Mg/Ca and oxygen isotope of planktic foraminifera in the northern East China Sea

[1]xa0Variations in Mg/Ca-based sea surface temperature and oxygen isotope ratio (δ18O) of the surface water in the northern East China Sea (ECS) were reconstructed with high resolution during the last 18 kyr using planktic foraminifera. Millennial-scale variations between warmer, more saline surface water and cooler, less saline surface water were recognized during the early deglacial period and the Holocene, suggesting changes in the mixing ratio between the Kuroshio Water and the Changjiang Diluted Water. Stronger East Asian summer monsoon (EASM) precipitation events in south China are identified at 10.5, 8.8, 7.0, 5.3, 4.7, 2.9, 1.7, and 0.5 ka, based on sea surface salinity (SSS) records of the northern ECS. Weaker EASM precipitation events are also detected at 9.3, 8.3, 7.3, 6.0, 3.3, 2.3, 0.7, and 0.4 ka during the Holocene. These events agree with the maxima in δ18O records of stalagmites from various parts of the Changjiang (Yangtze) River drainage. This agreement supports that our SSS record properly captures the millennial-scale dry (less EASM precipitation) events over the drainage basin of the Changjiang River during the Holocene. These dry events are also in good agreement with North Atlantic ice-rafted events, suggesting a teleconnection between North Atlantic climate and the EASM during the Holocene.

Analysis of 129I in the soils of Fukushima Prefecture: preliminary reconstruction of 131I deposition related to the accident at Fukushima Daiichi Nuclear Power Plant (FDNPP).

Iodine-131 is one of the most critical radionuclides to be monitored after release from reactor accidents due to the tendency for this nuclide to accumulate in the human thyroid gland. However, there are not enough data related to the reactor accident in Fukushima, Japan to provide regional information on the deposition of this short-lived nuclide (half-lifexa0=xa08.02xa0d). In this study we have focused on the long-lived iodine isotope, (129)I (half-life of 1.57xa0×xa010(7)xa0y), and analyzed it by accelerator mass spectrometry (AMS) for surface soil samples collected at various locations in Fukushima Prefecture. In order to obtain information on the (131)I/(129)I ratio released from the accident, we have determined (129)I concentrations in 82 soil samples in which (131)I concentrations were previously determined. There was a strong correlation (R(2)xa0=xa00.84) between the two nuclides, suggesting that the (131)I levels in soil samples following the accident can be estimated through the analysis of (129)I. We have also examined the possible influence from (129m)Te on (129)I, and found no significant effect. In order to construct a deposition map of (131)I, we determined the (129)I concentrations (Bq/kg) in 388 soil samples collected from different locations in Fukushima Prefecture and the deposition densities (Bq/m(2)) of (131)I were reconstructed from the results.

Developing Ultra Small-Scale Radiocarbon Sample Measurement at the University of Tokyo

We have developed accelerator mass spectrometry (AMS) measurement techniques for ultra small-size samples ranging from 0.01 to 0.10 mg C with a new type of MC-SNICS ion source system. We can generate 4 times higher ion beam current intensity for ultra-small samples by optimization of graphite position in the target holder with the new ionizer geometry. CO2 gas graphitized in the newly developed vacuum line is pressed to a depth of 1.5 mm from the front of the target holder. This is much deeper than the previous position at 0.35 mm depth. We measured 12C4? beam currents generated by small standards and ion beam currents (15-30 μA) from the targets in optimized position, lasting 20 min for 0.01 mg C and 65 min for 0.10 mg C. We observed that the measured 14C/12C ratios are unaffected by the difference of ion beam currents ranging from 5 to 30 μA, enabling measurement of ultra-small samples with high precision. Examination of the background samples revealed 1.1 μg of modern and 1 μg of dead carbon contaminations during target graphite preparation. We make corrections for the contamination from both the modern and background components. Reduction of the contamination is necessary for conducting more accurate measurement.

Tsunami recurrence revealed by Porites coral boulders in the southern Ryukyu Islands, Japan

Information about past tsunami hazards, such as their recurrence interval and magnitude, is needed for future disaster prediction and mitigation. We examined radiocarbon ages of the surfaces of massive coral boulders cast ashore by past tsunamis in the southern Ryukyu Islands, Japan, where few historical and geological records of past tsunamis are available. We selected only non-eroded Porites coral boulders along the shoreline, because their characteristics make it possible to determine the probable timing of their deposition by tsunamis, and we applied a dating method that uses the cumulative probability distributions of large numbers of radiocarbon measurements of those boulders to determine the timing of past tsunamis. The results demonstrate that the southern Ryukyu Islands have repeatedly experienced tsunami events since at least 2400 yr ago, with a recurrence interval of ∼150–400 yr. The largest Porites tsunami boulder that we studied (long axis, 9 m), which is probably the largest single-colony tsunami boulder in the world, was displaced by the A.D. 1771 Meiwa tsunami. Although the 1771 Meiwa tsunami was likely the largest event in at least the past 700 yr, calculations of current velocity show that all identified tsunamis occurring before 1771 were probably large enough to cause considerable damage to human-built structures and loss of life. This study demonstrates that by reliably dating large numbers of selected coastal boulders it is possible to ascertain the timing, recurrence interval, and magnitude of past tsunamis in a location where few adequate survey sites of sandy tsunami deposits exist.

Mid-Holocene palaeoceanography of the northern South China Sea using coupled fossil-modern coral and atmosphere-ocean GCM model

[1]xa0High-resolution records of past environments of the South China Sea (SCS) could provide important information to better understand the mechanisms of El Nino Southern Oscillation (ENSO) and East Asian monsoon evolution since SCS is located between Pacific Ocean and Eurasian continent. SCS plays a key role as the moisture source area of monsoon precipitation that affects the terrestrial climate of Asia. Fossil and modern corals were obtained from South China Sea (SCS) to study changes in oceanographic conditions when the northern hemisphere experienced perihelion during the early to mid-Holocene and thermal contrast between SCS and the Asian continent was larger. The fossil coral is 6600 years old and XRD and SEM investigations confirmed pristine nature of this sample. Oxygen isotope measurement of modern coral yielded an average value of ca. −6‰, whereas the fossil coral showed ca. −5.5‰. Given that previously reported alkenone SST thermometry and foraminiferal SST reconstruction indicate little changes in SST (<0.5°C) throughout the Holocene in SCS, we consider the possibility of changes in δ18O of seawater to be due to local sea surface salinity (SSS). Coral data from the present study (6600 years old coral) as well as previously published record (4400 years old) showed higher SSS during the mid-Holocene. Coral data were then compared with the coupled Ocean-Atmosphere GCM (MIROC3.2). Higher SSS during the mid-Holocene time was also seen in AOGCM experiments. We observed northward shift of inter tropical convergent zone (ITCZ) in the experiments that produced increased precipitation on the Asian continent. The cause of increase in salinity was, therefore, due to less precipitation in SCS and increased continental precipitation inland of Asia.

Atmospheric Fallout of 129I in Japan before the Fukushima Accident: Regional and Global Contributions (1963–2005)

Atmospheric (129)I deposition was studied in different locations of Japan (Akita, Tsukuba, Tokyo, and Ishigaki Island) with samples collected between 1963 and 2005 in order to understand the distribution and sources of this nuclide and provide a reference deposition level prior to the Fukushima accident. Over this time period, the deposition pattern of (129)I in Tsukuba and Tokyo (on the Pacific side) differed from that of Akita (on the Japan Sea side). The primary source of deposition in Tsukuba and Tokyo is related to the (129)I discharge from domestic reprocessing in Tokai-mura. In contrast, the time-series pattern of deposition in Akita seems to have been influenced by (129)I discharges from reprocessing facilities in Europe and the transport of this radionuclide by westerly winds to coastlines of the Japan Sea. The (129)I deposition in Ishigaki (one of the southernmost islands in Japan) is influenced primarily by oceanic air masses (easterly winds), and deposition was 1 order of magnitude lower than that observed in Tsukuba and Tokyo. Cumulative (129)I deposition in Tokyo before the Fukushima accident was estimated at 13 mBq/m(2). The results of this study on deposition contribute to understanding the deposition levels of (129)I prior to the accident.

Monsoon hydrography and productivity changes in the East China Sea during the past 100,000 years: Okinawa Trough evidence (MD012404)

[1]xa0We analyzed the high-resolution foraminifer isotope records, total organic carbon (TOC), and opal content from an Okinawa Trough core MD012404 in order to estimate the monsoon hydrography and productivity changes in the East China Sea (ECS) of the tropical western Pacific over the past 100,000 years. The variability shown in the records on orbital time scales indicates that high TOC intervals coincide with the increases of boreal May–September insolation driven by precession cycles (∼21 ka), implying a strong connection to the variations in monsoons. We also observed possibly nearly synchronous, millennial-scale changes of the ECS surface hydrography (mainly driven by salinity changes but also by temperature effects) and productivity coincident with monsoon events in the Hulu/Dongge stalagmite isotope records. We found that increased freshening and high productivity correlate with high monsoon intensity in interstadials. This study suggests that the millennial-scale changes in monsoon hydrography and productivity in the ECS are remarkable and persistent features over the past 100,000 years.

Variations of 129I in the atmospheric fallout of Tokyo, Japan: 1963–2003

Atmospheric fallout samples collected from Tokyo between 1963 and 2003 were analyzed using accelerator mass spectrometry (AMS) in order to determine (129)I/(127)I ratios and to examine the deposition rate of (129)I and its secular variation in Tokyo. The (129)I/(127)I ratios in the atmosphere during 1963-1977 ranged from 1 × 10(-8) to 2 × 10(-8). This is roughly 4 orders of magnitude higher than pre-atomic levels, possibly due to atmospheric nuclear weapons tests. The calculated monthly atmospheric deposition rates of (129)I differed from those produced by nuclear fallout of (90)Sr and (137)Cs, indicating that the variations in (129)I deposition are not influenced exclusively by either nuclear bomb testing or by the Chernobyl accident. After 1978, high (129)I depositions (up to 0.13 mBq/m(2)/month) were observed. The (129)I depositions started to increase markedly at the latter half of the 1970s. The secular variation of the estimated annual (129)I deposition in Tokyo showed a close relationship between the annual atmospheric discharge of (129)I from the Tokai Reprocessing plant. Therefore, the atmospheric fallout collected from Tokyo after the late 1970s is influenced primary by the (129)I discharge from the Tokai Reprocessing plant.

Depth profile and mobility of 129I and 137Cs in soil originating from the Fukushima Dai-ichi Nuclear Power Plant accident

The (129)I derived from the FDNPP accident were clearly identified near the surface and showed a trend of rapid decrease with depth. The FDNPP (129)I and (137)Cs was 51.6xa0±xa01.7xa0mBqxa0cm(-2) and 88.2xa0±xa027.1xa0kBqxa0cm(-2) (average of four cores inventory) respectively. On average, 91% of the FDNPP (129)I existed within the top 5xa0gxa0cm(-2) and 98% within the top 10xa0gxa0cm(-2) and average of 100% of the FDNPP (137)Cs existed within the top 5xa0gxa0cm(-2). From the observation of the temporal variation of depth profiles from the same upland field (Kawauchi village, 20xa0km away from the FDNPP to the southwest direction), downward migration rates of 0.81xa0±xa00.32xa0gxa0cm(-2)xa0yr(-1) for the FDNPP (129)I and 0.19xa0±xa00.17xa0gxa0cm(-2) yr(-1) for the FDNPP (137)Cs were estimated. A simple diffusion model was introduced to evaluate the downward mobility of the FDNPP-derived (129)I and (137)Cs. The apparent diffusion coefficients D of 0.0086xa0±xa00.0034 and 0.0011xa0±xa00.0010xa0g(2)xa0cm(-)(4)xa0d(-)(1) were obtained for (129)I and (137)Cs, respectively. These values might be representative for Haplic Gray lowland soils in near the steady state under humid temperate climate.

Compound-Specific 14C Dating of IODP Expedition 318 Core U1357A Obtained off the Wilkes Land Coast, Antarctica

This study applied compound-specific radiocarbon analysis (CSRA) to a 186-m-long sediment core (U1357A) taken from Adelie Basin located on the continental shelf off Wilkes Land, East Antarctica. The CSRA targeted C 16 fatty acid as well as C 16:1 fatty acid and cyclopheophorbide- a -enol isolated from the sediment. Due to their high degradation rate, these compounds are expected to occur in low abundances in relict organic matter deposited at this site. Twelve compound-specific (CS) 14 C ages were obtained that are mostly consistent with their stratigraphic order. The CS 14 C results of all samples are Holocene in age (9800 to 440 cal BP). These results suggest that significant sedimentation started ~10,000 cal BP. Moreover, the data suggest that 14 C measurements of C 16:1 fatty acid and cyclopheophorbide- a -enol are useful for dating sediments from the Southern Ocean. DOI:xa0 10.2458/56.17773