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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

The Mekong River Delta (MRD) is an essential agricultural area for the worldwide rice supply. Floods and droughts triggered by El Niño southern oscillation (ENSO) have been threatening sustenance in the MRD. Sustainable food supplies require understanding the response of the MRD hydrology to the changing ENSO behaviour in recent decades. Here, we reconstructed the annual rainfall maxima in the MRD using the oceanic paleoclimate proxy from coral skeletons and compared them with ENSO indexes. Annual minima of coral-based seawater oxygen isotope (δ¹⁸O_sw) correlated with annual rainfall maxima, which allowed to extend rainfall data from 1924 to the recent. The annual rainfall maxima based on δ¹⁸O_sw negatively correlated with the central Pacific El Niño index. This suggested that La Niña and central Pacific El Niño events lead to heavy and light rainy seasons. The heavy rainy season had more serious impacts in recent decades, which likely increases the flood risk. In contrast, the frequency and rainfall amount of the light rainy season has not changed significantly, although a catastrophic drought has hit the MRD. Our finding concludes that the impact of the ENSO event on MRD hydrology is inconsistent in the past century.

DOI： 10.1038/s41598-022-20597-7

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Other Link： https://www.nature.com/articles/s41598-022-20597-7

Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Coral Reef Society  

DOI： 10.3755/jcrs.24.29

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CiNii Research

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The barium/calcium ratio (Ba/Ca) in coral skeletons has been used as a proxy to understand river floods and oceanic upwelling in the tropics and subtropics for the past centuries. Here, we establish a new method for Ba/Ca analysis using inductively coupled plasma optical emission spectrometry (ICP-OES), which has been widely used for coral proxy analyses and can save analysis time compared to other methods. We analyze Ba/Ca by combining an ultrasonic nebulizer with the intensity ratio calibration method to improve the signal intensity of Ba. Our method can determine Ba/Ca in samples with a Ca concentration of 9 mg/L with excellent analytical precision (<0.17 mu mol/mol) using ICP-OES.

DOI： 10.1029/2021GC009646

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Upwelling in the Arabian Sea driven by the Indian summer monsoon pumps deep, cold, and eutrophic seawater to the sea surface every summer. The Indian summer monsoon and the Arabian Sea upwelling were expected to intensify with global warming, following the hypothesis that the Eurasian landmass would warm faster than the Indian Ocean. Contrary to expectations, the northern Indian Ocean currently warms faster than the Indian subcontinent. A weakening of the Indian summer monsoon circulation is reported, which possibly weakens the Arabian Sea upwelling. However, a lack of observations limits understanding of current and historical changes of the Arabian Sea upwelling. Here, we reconstruct the Arabian Sea upwelling over the past millennium using modern and fossil corals. Our coral records show that the Arabian Sea upwelling intensity was very stable over the last millennium and unprecedentedly declines at present. Our finding implies anthropogenic forcing likely weakens the Arabian Sea upwelling.

DOI： 10.1029/2021GL092432

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

To assess the potential of long-lived freshwater bivalve shells as a proxy for river environments, we examined daily and annual growth increments and analysed trace elements in the shells of Margaritifera laevis collected alive from the Shiribetsu River, located in the central western region of Hokkaido, Japan. The sum of the daily growth lines within a single annual increment corresponded to the days when the average daily water temperature was greater than 9 degrees C, suggesting that shell deposition occurred from spring to autumn. The growing degree days (GDD) are correlated with the maximum snow depth in winter and the average river discharge in spring. We conducted trace element microanalysis across daily and annual growth increments by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and compared the results with in situ environmental data. Sharp peaks in the barium/calcium ratio (Ba/Ca) observed during spring river discharge also suggest that the growth of Margaritifera shells is influenced by winter snow volume and meltwater. A 67-year profile of the standard growth index (SGI) estimated from twelve individual annual growth histories correlates to the annual snowfall accumulation. Distinct decadal variability is observed in the SGI and synchronized with the North Pacific Index and Pacific Decadal Oscillation. Sclerochronological approaches using Margaritifera shells could be valuable for reconstructing river and atmospheric conditions during the winter season, which is influenced by the Asian winter monsoon in the central Hokkaido region.

DOI： 10.1016/j.palaeo.2021.110346

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

The ratios of barium to calcium (Ba/Ca) in reef coral skeletons have been used as a proxy to reconstruct temporal change in barium concentration in surface ocean. To reduce the uncertainty of the coral Ba/Ca proxy, the Ba distribution coefficients between seawater and coral skeletons were determined under different light conditions. Coral planulae (Acropora tenuis) were metamorphosed and settled in dishes with commercial peptide Hym-248 in the laboratory. Juvenile corals were infected with symbiotic algae (Durusdinium sp.) and cultured in 5 Ba concentrations (5, 10, 20, 35, and 150 mol/mol) for 2 months. Half of the dishes were placed under low light conditions (10-33 mu mol m(-2) s(-1)), and another half of the dishes were placed under high light conditions (67-84 mu mol m(-2) s(-1)). The Ba/Ca ratios in the coral skeletons were highly correlated with those in ambient seawater under both high and low conditions (R-2 > 0.8). The Ba distribution coefficients between seawater with 5-35 mol/mol of Ba concentration and coral skeletons were 0.72 and 2.83 under high and low light conditions, respectively. These results suggest that biological processes related to light availability could influence to Ba/Ca-coral.

DOI： 10.1016/j.chemgeo.2020.119911

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Strontium/calcium ratios (Sr/Ca) in reef coral skeletons have been used for sea surface temperature (SST) proxy. Because skeletons of reef corals are accumulated throughout more than 100 years, SST records from coral skeletons were important archives to reveal climate mechanisms. Here, we improved the coral Sr/Ca analytical method using an inductively coupled plasma optical emission spectrometer which method has advantages in terms of initial/running cost and analysis time. We checked variations of Sr and Ca sensitivities as increases of Ca concentrations using various wavelengths of Sr and Ca. Our results suggested that sensitivities of Sr/Ca in a range of coral skeletons were modulated by selections of wavelengths and sensitivity changes of Sr and Ca. A combination of Sr(421.5 nm) and Ca(373.6 nm) is stable for the Sr/Ca analysis at a low Ca concentration. A SD of Sr/Ca(421.5/373.6 nm) was 0.006 mmol mol(-1) (1 sigma, n = 40) based on the replicate measurement of the reference material (JCp-1) using our new method. By applying a Porites colony from Kikai island, Japan, a precision of reconstructed SST was over +/- 0.3 degrees C at weekly resolution.

DOI： 10.1002/lom3.10357

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/lom3.10357

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The annual banded skeletons of reef corals potentially record past earthquakes events. We examined cores of five living Porites coral heads in Simeulue Island, Indonesia, near the epicenter of the 2004 Sumatra-Andaman and the 2005 Nias-Simeulue earthquakes. These sites showed 0.4-1.4 m of uplift. We measured skeletal strontium, magnesium, and calcium; carbon and oxygen isotopic ratios; and skeletal density, extension, and calcification rates, from 1994 to 2010. Coral geochemistry fluctuates more than are expected from strictly environmental causes; however, stress bands, reduced growth rates, and changed skeletal delta C-13 appear to reflect the tsunami and seismic uplift (the step change in skeletal delta C-13 results equated to 0.31 +/- 0.10 parts per thousand/m in response to the 2004 uplift and 0.23 +/- 0.03 parts per thousand/m to the 2005 uplift). (C) 2020 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.gca.2020.01.037

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOLOGICAL SOC AMER, INC  

The Akkadian Empire was the first united empire in Mesopotamia and was established at 4.6 kyr B.P. (where present is A.D. 1950). The empire abruptly collapsed in 4.2 +/- 0.2 kyr B.P. Seasonal-scale climatic dynamics behind this collapse have not yet been resolved. Here, we present monthly climatic parameters (temperature and hydrology) inferred from fossil Omani corals that lived between 4.5 and 2.9 kyr B.P. Winter temperatures derived from a modern Omani coral correlate with winter shamal (western Asian dust storm) frequency. A fossil coral from 4.1 kyr B.P. shows a prolonged winter shamal season with frequent shamal days. This likely caused agricultural failures in Mesopotamia and contributed to the Akkadian Empire collapse, as this region depends on winter rainfall.

DOI： 10.1130/G46604.1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The Indian Ocean Dipole (IOD) is an interannual mode of climate variability in the Indian Ocean that has intensified with 20th century global-warming. However, instrumental data shows a global-warming hiatus between the late-1990s and 2015. It is presently not clear how the global-warming hiatus affects modes of climate variability such as the IOD, and their basin-wide ocean-atmosphere teleconnections. Here, we present a 26-year long, biweekly record of Sr/Ca and delta O-18 from a Porites coral drilled in the Gulf of Oman. Sea surface temperature (SSTanom) is calculated from Sr/Ca ratios, and seawater delta O-18 (delta O-18(sw-anom)) is estimated by subtracting the temperature component from coral delta O-18. Our delta(18)Osw-anom record reveals a significant regime shift in 1999, towards lower mean delta O-18(sw) values, reflecting intensified upwelling in the western Indian Ocean. Prior to the 1999 regime shift, our SSTanom and delta O-18(sw-anom) show a clear IOD signature, with higher values in the summer of positive-IOD years due to weakened upwelling. The IOD signature in SSTanom and delta O-18(sw-anom) disappears with the overall intensification of upwelling after the 1999 regime shift. The inferred increase in upwelling is likely driven by an intensified Walker circulation during the global-warming hiatus. Upwelling in the Western Indian Ocean uncouples from the IOD.

DOI： 10.1038/s41598-018-38429-y

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DOI： 10.1007/978-3-319-92735-0_36

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOCHEMICAL SOC JAPAN  

The offshore flow of the Mekong River is strongly governed by the East Asian monsoon (EAM). Monthly-resolved skeletal carbonate Sr/Ca, Ba/Ca and oxygen isotope (delta O-18(c)) data from 1980 to 2005 of a Porites coral from Con Dao Island, similar to 90 km from the Mekong River mouth, were analyzed for capturing the past flood events and the signal of East Asian winter monsoon. Ba/Ca time series is characterized with intra-annual double peaks, the first large one in March during the dry season and relative small one in August during the wet season. The low values of seawater delta O-18 (delta O-18(sw)), deduced from both the Sr/Ca ratio and delta O-18(c), in the wet season resulted from precipitation and/or freshwater input from the Mekong River. The difference in the seasonal characteristics of Ba/Ca and delta O-18(sw) between flood and no-flood years could be attributed to the seasonal reversal of the regional ocean current derived from the EAM.

DOI： 10.2343/geochemj.2.0552

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

To validate the usability of the giant clam shell as a recorder of short-term environmental changes such as typhoons, we collected a live Tridacna maxima from Okinotori Island, Japan, on 15 June 2006. Growth increment thickness, stable isotope ratio (O-18(shell), C-13(shell)), and the barium/calcium ratio (Ba/Ca) in the T.maxima shell sample were measured and compared to Okinotori Island instrumental environmental data. In the outer layer of the shell sample, there were 3656 growth increments per year, as estimated by the O-18(shell) profile compared with sea surface temperature. The growth increments in the specimen were formed daily, and thus, we can determine the date of the sampling points of O-18(shell), C-13(shell) and the Ba/Ca ratio by counting growth increments. After typhoon approach, there is a decrease in increment thickness and some disturbed growth increments. The positive peaks in the shell Ba/Ca ratio and O-18(shell) corresponded to lower sea surface temperature caused by typhoons. These results indicated that the microstructural and geochemical record in Tridacna maxima shells could be useful for detecting past typhoon events.

DOI： 10.1029/2017JG004082

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

We used a high-resolution oxygen isotope (delta O-18(coral)), carbon isotope (delta O-18(coral)) and Sr/Ca ratios measured in the skeleton of a reef-building coral, Porites sp., to reveal seasonal-scale upwelling events and their interannual variability in the Gulf of Oman. Our delta C-13(coral) record shows sharp negative excursions in the summer, which correlate with known upwelling events. Using delta C-13(coral) anomalies as a proxy for upwelling, we found 17 summer upwelling events occurred in the last 26 years. These anomalous negative excursions of delta C-13(coral) result from upwelled water depleted in C-13 (dissolved inorganic carbon) and decreased water-column transparency. We reconstructed biweekly SSTs from coral Sr/Ca ratios and the oxygen isotopic composition of seawater (delta O-18(SW)) by subtracting the reconstructed Sr/Ca-SST from delta O-18(coral). Significant delta O-18(SW) anomalies occur during major upwelling events. Our results suggest delta C-13(coral) anomalies can be used as a proxy for seasonal upwelling intensity in the Gulf of Oman, which, driven by the Indian/Arabian Summer Monsoon, is subject to interannual variability.

DOI： 10.1038/s41598-017-04865-5

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Language：Japanese   Publisher：海洋出版  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOCHEMICAL SOC JAPAN  

Understanding seasonal to interannual characteristics of the climate during the transition from the "Holocene Optimum" (7.0-5.0 kyr BP) to mid-Holocene cold and dry period (4.6-4.0 kyr BP) is important since it is related to the evolution and collapse of human civilization in East Asia. To investigate those characteristics, we reconstructed a seasonal scale sea surface temperature (SST) and measured oxygen isotope ratios (delta O-18) in seawater (delta O-18(seawater)) using modern and fossil (4.9 kyr) corals from Kikai Island. Larger seasonal amplitudes observed among the reconstructed SST values and delta O-18(seawater) change at 4.9 kyr suggest that the East Asian Monsoon (EAM) circulation might be stronger than the present-day. Our compiled coral records, along with the previous studies from Kikai Island, also suggest that the largest SST amplitude during the Holocene Optimum was recorded at 4.9 kyr and an abrupt cold climate shift occurred during the Holocene Optimum and the Pulleniatina Minimum Event (PME) in the north-western Pacific.

DOI： 10.2343/geochemj.2.0482

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Language：Japanese   Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

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DOI： 10.14862/geochemproc.64.0_98

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

In this study, a sea-surface water Delta C-14 record of AD 1966-2000 (i.e., after the atmospheric nuclear-bomb testing period of the mid-1950s to early 1960s) was reconstructed from a coral sample collected from Iki Island, western Japan. The island is located in the Tsushima Strait where the Tsushima Current flows from the East China Sea (ECS) continental shelf into the Sea of Japan, indicating a strong influence of the ECS shelf water on the island. It is widely accepted that the Tsushima Current originates in the area between the ECS shelf break and the Nansei Islands further offshore as a branch of the Kuroshio Current, although another possible origin is the Taiwan-Tsushima Current System. The Delta C-14 record from Iki Island shows the following evidence of a response to the atmospheric nuclear testing: (1) an increase from similar to 55aEuro degrees in 1966 to similar to 133aEuro degrees in 1970, (2) a plateau ranging between similar to 123 and similar to 142aEuro degrees during the 1970s to the late 1980s, and (3) a gradual decrease from similar to 115aEuro degrees in 1990 to similar to 83aEuro degrees in 2000. Comparison of this record with coral Delta C-14 records from the Nansei Islands (Okinawa Island, Ishigaki Island and Kikai Island), located similar to 160-280 km off the ECS shelf break and little influenced by the shelf water, suggests that the surface-water Delta C-14 around Iki Island was similar to 30-45aEuro degrees lower than that of the Nansei Islands from the mid-1960s to late 1970s, and that the Delta C-14 difference between Iki Island and the Nansei Islands decreased from the end of the 1970s to similar to 0-15aEuro degrees in the mid-1980s to 2000. The lower Delta C-14 around Iki Island can be explained as follows: (1) in contrast to the Nansei Islands area, the ECS shelf area is a vertically mixed, highly concentrated carbon reservoir significantly connected to subsurface and deeper waters outside the shelf area, strongly suggesting that the surface-water Delta C-14 of the shelf area (perhaps excepting very shallow innermost shelf areas) was significantly less sensitive to the atmospheric nuclear-bomb C-14 spike than that of the Nansei Islands area; (2) the shelf-surface water is conveyed from the northern ECS to Iki Island by the Tsushima Current. Thus, it can be suggested that the post-bomb coral Delta C-14 record from Iki Island reflects oceanographic conditions of the northern ECS shelf.

DOI： 10.1007/s00367-016-0456-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The Kuroshio Current is a major global ocean current that drives the physical ocean-atmosphere system with heat transport from tropical to temperate zones in the North Pacific Ocean. We reconstructed the variability of the Kuroshio transport over the past 150years using coral skeletal nitrogen isotopic composition (N-15(coral)). A 150year N-15(coral) record (1859-2008 A.D.) is 4 times the length of the observational record (1971 to present) and could provide a direct comparison with global climate change, such as the Pacific Decadal Oscillation (PDO) index and El-Nino-Southern Oscillation (ENSO), through recent global warming. Coral cores from Porites were collected from Tatsukushi Bay in 2008 on the Pacific coast of Japan, which is located on the northern front of the Kuroshio Current. N-15(coral) was used as a proxy to record the N-15 of nitrate (N-15(nitrate)) controlled by the upwelling of subtropical subsurface water (N-15(nitrate); similar to+2-+3), and N-15(coral) was negatively correlated with observations of the Kuroshio transport (R=-0.69, P&lt;0.001) and the 2year lagged PDO index (R=-0.63, P&lt;0.005) from 1972 to 2007. The 150year record of N-15(coral) suggested that the Kuroshio transport varied with similar to 25year cycle, and the amplitude became more stable, and the volume was intensified through the twentieth century. The Kuroshio transport was intensified by the La Nina state in the early 1900s and by the El Nino-PDO state after the 1920s. Our results suggested that the Kuroshio transport was influenced by the combined climate modes of ENSO and PDO during the last century.

DOI： 10.1002/2015PA002880

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DOI： 10.14862/geochemproc.63.0_190

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We analysed the nitrogen isotopes in two coral cores (delta N-15(coral)) from the mouth of the Todoroki River, Ishigaki Island, Japan, to examine whether the delta N-15(coral) reflects the run-off of nitrate related to the land use in the river catchment. The two coral cores were used to examine the seasonal variation in delta N-15(coral) for 14 years (CORE1; 1993-2007) and the annual variation of delta N-15(coral) for 52 years (CORE2; 1958-2010). In CORE1, the 5-month running mean of delta N-15(coral) was positively correlated with that of monthly precipitation, excluding all strong precipitation events (&gt; 150 mm d(-1)). In CORE2, the delta N-15(coral) mean in the earlier period (1958-1980) was 1.0 aEuro degrees greater than that in the later period (1981-2010). The annual averages of delta N-15(coral) are positively correlated with the total precipitation in the rainy season (May-June) for both time periods. The difference in the delta N-15(coral) between the earlier and later periods is probably caused by the land use changed from paddy fields with N-15-rich manure to sugar cane fields in the early 1980s. Although some uncertainties still remain regarding the precision of delta N-15 coral proxy records, this study emphasises the clear potential for their use in reconstructing terrestrial nitrate discharge records from corals.

DOI： 10.1007/s00338-014-1235-1

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Manganese (Mn) and iron (Fe) concentrations were measured in coral skeletons (Porites spp.) collected from the Todoroki River on Ishigaki Island, Japan, to reconstruct the history of land use in the river catchment area. We prepared (1) five bulk samples to investigate the present spatial distribution and (2) micro-samples from two long cores to study the temporal variability of sediment loading from the Todoroki River. The existing state of the elements Mn and Fe in bulk coral skeleton samples was examined by a chemical cleaning experiment. The results of the experiment suggested that Fe was not incorporated into the crystal lattice of the coral skeleton but that Mn was incorporated, as previously reported. The bulk sample data, with and without chemical cleaning, indicated that the spatial distribution of both elements in corals collected along a sampling line from the river mouth toward the reef crest was complex and most likely reflected salinity changes and the amount of suspended particulate matter. The temporal variation of Mn and Fe, in particular the variation of baseline/background levels, mainly reflected the history of land development on Ishigaki Island. In addition, Mn showed clear seasonal variability that appeared to be controlled by a combination of temperature, primary productivity, and precipitation. The results of the present study suggest that Mn may be a useful proxy for river discharge or biological activity depending on local marine conditions, if the specific behavior of Mn at the coral growth site is known.

DOI： 10.1007/s00338-014-1128-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The Pacific Decadal Oscillation (PDO) is a dominant climate mode in the Pacific Ocean and thought to be related to seasonal to decadal changes in sea surface conditions. Colonies of long-living Porites coral, widely used to reconstruct monthly to century-scale tropical sea surface temperature and sea surface salinity records, were discovered near Koshiki Island, Japan (31 degrees N, 129 degrees E). A monthly resolved, 106 year 18O record revealed that distinct decadal-scale variability was significantly correlated with the PDO index. Our comparison showed 1 to 3 years lead-lag correlation of summer coral 18O with the winter PDO index, suggesting that the East Asian summer monsoon (EASM) may act as the driving force of winter PDO variability over the last 100 years. Cross-spectral analysis between the winter PDO index and summer coral 18O suggested that recent and future global warming may lead to a more frequent and/or stronger teleconnection between EASM and PDO.

DOI： 10.1002/2014GL060037

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Language：Japanese   Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

本研究では北大西洋の熱帯域に位置するカリブ海ケイマン諸島に生息する造礁サンゴ骨格から過去90年間の窒素固定の変遷を復元した。サンゴ骨格中に保存されている有機物の窒素同位体比を過去90年間分年単位で分析した結果、カリブ海の窒素固定量は過去90年間で減少傾向にあること、そして海水温に伴って数十年規模で変動しており、高水温時に窒素固定が増大してきたことが推定された。本講演では復元された窒素固定の変遷と温暖化および数十年規模の大気-海洋相互作用との関係を議論する。

DOI： 10.14862/geochemproc.61.0_189

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

To assess the preservation of the nitrogen isotope composition in reef corals, nitrogen isotopes in a well-preserved Pliocene fossil coral (located in the Tartaro formation on Luzon Island, Philippines (14 degrees N, 121 degrees E)) and in a modern coral (Kochi, Japan (32 degrees N, 132 degrees E)) were analysed using stepwise heating methods. The thermal decomposition of aragonite triggered the largest release of nitrogen at 700 degrees C for the modern coral and 550 degrees C for the Pliocene coral. The highest rate of nitrogen gas emission occurred at the aragonite collapse temperature, indicating that organic nitrogen was bound within the intra-crystals of coralline aragonites in both corals. After the aragonite collapsed in both corals, the nitrogen isotope ratios increased due to fractionation and then decreased to values similar to those observed in bulk samples of the modern (+10.1%) and Pliocene (+4.4%) corals. These results suggested that fresh organic nitrogen was released due to the decomposition of the internal skeletal structure at higher temperatures (900-1000 degrees C). Nitrogen isotopes in coral skeletons were preserved in intra-crystal aragonite, even in a Pliocene fossil, and stepwise heating methods were shown to be useful for determining the preservation of coralline nitrogen isotopes. (c) 2013 The Authors. Published by Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemgeo.2013.05.024

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To demonstrate the utility of coral skeletons as a recorder of nitrate dynamics in the surface ocean, we collected coral skeletons of Porites lobata and determined their nitrogen isotope composition (δ15N_[coral]) from 2002 to 2006. Skeletons were collected at Okinotori Island in southwestern Japan, far from any sources of terrestrial nitrogen. Nitrogen isotope compositions along the growth direction were determined at 800 μm intervals (∼1 month resolution) and compared against the skeletal carbon isotope composition (δ13C_[coral-carb]), barium/calcium ratio (Ba/Ca), and Chlorophyll-a conce...

DOI： 10.1029/2011JG001697

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Understanding tropical nutrient dynamics is essential for quantifying marine productivity. In tropical and subtropical oceans, however, the spatial and continuous observation of nutrients has been scarce because of low nutrient concentration. The nitrogen isotopes of organic nitrogen in coral skeletons (delta(15)Ncoral) could be used to record nitrogenous nutrient origins at oceanic surfaces. Here, we show the intra- and inter-reef variations of delta(15)Ncoral in the western Pacific. The zonal distribution of delta(15)Ncoral was found inside a coral reef corresponding with delta(15)N of seawater nitrate (delta(15)N(nitrate)). The extended analysis of delta(15)Ncoral among various coral reefs also shows a latitudinal gradient from tropical to temperate in the western Pacific. The delta(15)Ncoral records high-resolution dynamics of nitrogenous nutrients through the geologic time scale. Citation: Yamazaki, A., T. Watanabe, and U. Tsunogai (2011), Nitrogen isotopes of organic nitrogen in reef coral skeletons as a proxy of tropical nutrient dynamics, Geophys. Res. Lett., 38, L19605, doi:10.1029/2011GL049053.

DOI： 10.1029/2011GL049053

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Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

サンゴ礁への陸起源の栄養塩の負荷はサンゴ礁を衰退させる原因のひとつとして考えられている。本研究では、造礁性サンゴ骨格に含まれる微量の有機物中の窒素同位体比がサンゴ礁に流入した陸起源物質の指標となるか検討するため、石垣島白保サンゴ礁轟川河口の造礁性サンゴ骨格の窒素同位体比の分布を調べ、礁内の海水および河川水の硝酸の窒素同位体比分布と比較した。その結果、造礁性サンゴと硝酸の窒素同位体比が一致することを発見した。次に、同研究地点で採取したサンゴ骨格コアの窒素同位体比の季節変動を復元した。その季節特性は石垣島の降水量の季節変化と同じパターンを示しており、また硝酸の窒素同位体比の季節変動とも一致していた。本研究の結果から、造礁性サンゴの窒素同位体比はサンゴ礁に流入する陸起源硝酸の変動を復元する指標に利用可能であることが示唆された。

DOI： 10.14862/geochemproc.58.0.13.0

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Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

高知県西南部に生息する造礁性サンゴ骨格の窒素同位体比分析から黒潮による海洋環境の変動が造礁性サンゴ骨格にどのように記録されているかを検証した。サンゴ骨格の窒素同位体比と観測により求められる黒潮流量を比較したところ，サンゴ骨格の窒素同位体比変動は黒潮による栄養塩濃度の変化を示し，黒潮の影響が強いときには貧栄養のため、窒素固定生物による窒素固定が行われている様子が示唆された。本研究の結果から造礁性サンゴ骨格の窒素同位体比は黒潮流量の変動を復元する唯一の指標となり得ることが期待される。

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