The North Equatorial Current (NEC) bifurcates at the eastern coast of the Philippines and moves northward as the Kuroshio, a North Pacific western boundary current. The NEC bifurcation point and Kuroshio variability are known to be affected by changes in climate such as the El Niño–Southern Oscillation and the pacific decadal oscillation. However, observational data are not sufficient to examine the mechanisms of decadal fluctuation. Here, we report seasonal radiocarbon data recorded from 1968 to 1995 in coral skeletons northwest of Luzon Island. The data suggest that the East Asian winter monsoon is a dominant factor in the seasonal fluctuations in water mass northwest of Luzon Island. Compared with other coral records reported for Guam, Ishigaki, Con Dao, and Hon Tre Island, the data suggest that the area of the Kuroshio loop current through the Luzon Strait decreased from the 1970s to 1980s as a result of the change in Kuroshio transport and the migration of the NEC bifurcation latitude after a regime shift in 1976.
The oxygen isotope ratio in seawater (δ 18 O sw ) is an important indicator of past hydroclimatic conditions in the tropics. In this study, we apply a dual proxy approach using δ 18 O and Sr/Ca in coral skeletons to estimate δ 18 O sw . Due to a lack of long‐term continuous observational data for δ 18 O sw , a simplified one‐dimensional box model of the ocean surface associated with historical surface atmospheric data is used for comparison with coral‐derived δ 18 O sw record. The atmospheric data consist of the precipitation and evaporation values, together with their δ 18 O compositions from the global isotope reanalysis data. The model successfully reproduces a multidecadal time series for δ 18 O sw at southeastern Luzon Island, the Philippines, for the period 1979–2001 ( r = 0.57, p < 0.01). The result suggests that the coral records are an accurate indicator of hydrological cycle changes at the site. A discrepancy between the reproduction and the coral record in the wet season (December–February) may be caused by seasonal variation in the depth of the mixed layer and upwelling, since these factors are not accounted for in the model. The reproduction is in good agreement with the coral records in the dry season (April–June), implying that δ 18 O sw estimated using the coral dual proxy method is more robust at this time of year.
ABSTRACT Several thousand kilometers of high resolution seismic reflection data were acquired from the east Texas continental shelf to map the major depositional systems formed since the last glacial -ecstatic lowstand. This work was followed by more detailed surveys of selected depositional systems using a variety of seismic sources and aimed at characterizing seismic facies of sandy systems. Several hundred sediment cores and oil company platform borings provided ground truth of seismic facies interpretations. Several types of sand bodies were investigated, including incised fluvial valleys, fluvial deltas, transgressive sand bodies, tidal inlet/tidal delta deposits, and shoreface deposits. Seismic reflection character, and therefore eismic facies, are controlled mainly by the scale of sedimentary structures within the deposit, the nature of bounding surfaces, and by lateral and stratigraphic variability in lithology. Distributary channels of shelf margin deltas and tidal inlet/tidal deltas complexes have large-scale cross stratification that is imaged with most high resolution sound sources. The main exception to this is fluvial sands that are buried beneath estuarine and marine muds within incised valleys. Delta mouth bars are characterized by a chaotic reflector pattern. Shelf sand bodies and sho reface sands have small-scale sedimentary structures that are imaged only with high frequency (3.5 kHz) sound sources. Attempts to estimate the size, distribution, and volume of sand bodies based on seismic data alone met with poor success. However, when good quality seismic data and reasonable core coverage are coupled with sedimentation odels, good results are obtained. INTRODUCTION Clastic sand and gravel deposits on low latitude (non-glaciated) continental shelves vary greatly in size, shape, thickness and internal stratification. One of the key problems in exploiting these sand resources is locating them, predicting their subsurface expression, and estimating their volume. High resolution seismic reflection profiling provides an invaluable tool for accomplishing these tasks. Good quality seismic data can be used to identify seismic facies which, when combined with the proper sedimentation models and sediment cores, provide a powerful tool for predicting the size and shape, and therefore the volume, of a sand body. In any seismic investigation the objective is to get the best possible resolution while penetrating deep enough to image features of interest. But there is always a trade-off between improved resolution and depth of penetration. It is important to know what is being imaged in terms of depth of burial, thickness, and the resolution required to image diagnostic features so that the right tools and acquisition parameters can be selected. The seismic expression of a sand body is controlled largely by internal stratification, which occurs at a variety of scales. The type of stratification is largely determined by the sedimentation processes that form the sand body, and therefore its depositional environment. There is a wealth of outcrop information on internal stratification for virtually every type of sand body and this information can be used to refine seismic facies models.
Historical variations of surface temperature in relation to anthropogenic warming has been extensively studied to understand and explain changes in the contemporary climate and to estimate future impacts of climate．Inoue and others (in press) reported 228-year records of SST and salinity based on Sr/Ca and d18O analyses with monthly time resolution in Porites coral collected from Bicol, the south of Luzon, Philippines. From the record, we investigated the relationship between the reconstructed temperature and the volcanic eruptions in late 18th and early 19th centuries. There were three great famines during the Edo period (1603-1868), almost corresponding to the Little Ice Age in Japan. Of these, the two were Tenmei-famine in 1782-88 and Tempo-famine in 1833-1837(1839). Both famines killed more than one million people out of a population of 30 million at the time. Our reconstructed SST anomaly fluctuated between -1.5 degree and 1.0 degree. The age model may have the age error of 1 to 3 years before around 1885. Large minima occurred in 1785-1789, 1815-1819, 1822-25, 1827-1830, 1834-1835, and 1843-45. Although Laki eruption, Iceland in 1783 has not been described as large eruption in previous studies, their impact on climatic conditions around the Northern Hemisphere and the globe was widely reported. Local eruption of Asama, Japan in 1783 released volcanic ash over eastern part of Japanese islands, In addition, El Nino event, which often cooled down Japanese islands, occurred around those days. These factors could have been responsible for the coldest anomaly in 1785-1789 recorded in our coral samples. After Tambora eruption in 1815, sharp cooling of around 2.0˚C was observed in our coral sample and almost all over the world. However, this world-scale cooling event have no or little influence on the climate in Japanese islands based upon the historical documents and agriculture records. This indicates that there are areas that do not become exceptionally cold, even by major volcanic eruptions. Large eruption of Galunggung in 1822 brought appreciable degree of cooling anomaly in our coral record. Just after Agung exploded largely in 1843, reconstructed SST significantly dropped. This might be also influenced by another large eruption of Cosiguina in Nicaragua, central America. Cold climate was reported in Japan, New York in USA, Copenhagen, UK in 1840s. It was most likely global in scale in the northern hemisphere.
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