A large meander of the Kuroshio was generated in the region off the southern coast of Japan in August 2004 and continued until approximately July 2005. The formation and decay of the large-meander (LM) path was observed by bottom pressure (BP) sensors installed on inverted echo sounders (PIESs) and a seismic observing system off Shikoku. The variation in BP was examined focusing on the development, persistence, and decay of the LM path. The BP was found to be depressed associated with a Kuroshio path disturbance, called a small meander, and this BP depression led the sea surface height (SSH) depression by up to approximately two months. The temporal phase shift between the sea surface and deep disturbances was significantly greater than those of other small meanders that did not develop into large meanders. After the formation of the LM path, the BP beneath the Kuroshio increased with a lag of approximately two months behind the SSH elevation along with the upward displacement of the main thermocline. The increase in BP is associated with that of the positive southward BP gradient anomaly, i.e., the eastward deep Kuroshio current anomaly, which suggests an enhancement of the topographic steering and stability of the LM path. This is consistent with the fact that no small meanders occurred in the early LM period from late July 2004 to late January 2005. 相似文献
Subduction of lithosphere, involving surficial materials, into the deep mantle is fundamental to the chemical evolution of the Earth. However, the chemical evolution of the lithosphere during subduction to depth remains equivocal. In order to identify materials subjected to geological processes near the surface and at depths in subduction zones, we examined B and Li isotopes behavior in a unique diamondiferous, K-rich tourmaline (K-tourmaline) from the Kokchetav ultrahigh-pressure metamorphic belt. The K-tourmaline, which includes microdiamonds in its core, is enriched in 11B relative to 10B (δ11B = −1.2 to +7.7) and 7Li relative to 6Li (δ7Li = −1.1 to +3.1). It is suggested that the K-tourmaline crystallized at high-pressure in the diamond stability field from a silicate melt generated at high-pressure and temperature conditions of the Kokchetav peak metamorphism. The heavy isotope signature of this K-tourmaline differs from that of ordinary Na-tourmalines in crustal rocks, enriched in the light B isotope (δ11B = −16.6 to −2.3), which experienced isotope fractionation through metamorphic dehydration reactions. A possible source of the heavy B-isotope signature is serpentine in the subducted lithospheric mantle. Serpentinization of the lithospheric mantle, with enrichment of heavy B-isotope, can be produced by normal faulting at trench-outer rise or trench slope regions, followed by penetration of seawater into the lithospheric mantle. Serpentine breakdown in the lithospheric mantle subducted in subarc regions likely provided fluids with the heavy B-isotope signature, which was acquired during the serpentinization prior to subduction. The fluids could ascend and cause partial melting of the overlying crustal layer, and the resultant silicate melt could inherit the heavy B-isotope signature. The subducting lithospheric mantle is a key repository for modeling the flux of fluids and associated elements acquired at a near the surface into the deep mantle. 相似文献
The impact of quasi-decadal (QD: 8 to 18 years) variability in the tropical Pacific on ENSO events is investigated. It is found that there is a significant difference in the behavior of ENSO events between the phases of positive and negative anomalies of the QD Niño-3.4 index. During the period of negative QD-scale Niño-3.4 index, ENSO events, especially La Niña events, occur more frequently, and larger amplitudes of thermal anomalies related to El Niño events appear over the central to eastern equatorial Pacific. Furthermore, propagations of upper ocean heat content anomaly and a phase relationship between upper ocean heat content and Niño-3 index in the equatorial Pacific, which have been pointed out by previous studies, are clearly detected during the period of negative QD Niño-3.4 index. 相似文献
The n-alkane C31/(C29 + C31) ratios from surface sediments in the eastern equatorial Pacific (EEP) exhibit higher values to the north and lower values to the south across the southern edge (2–4°N) of the Intertropical Convergence Zone (ITCZ). Since plants tend to synthesize longer chain length n-alkanes in response to elevated temperature and/or aridity, the higher C31/(C29 + C31) ratios at northern sites suggest a higher contribution of vegetation under hot and/or dry conditions. This is consistent with the observation that northern sites receive higher levels of plant waxes transported by northeasterly trade winds from northern South America, where hot and dry conditions prevail. Furthermore, from a sediment core covering the past 750 ka (core HY04; 4°N, 95°W) we found that C31/(C29 + C31) ratios exhibit a long-term decrease from MIS (marine oxygen isotope stage) 17 to 13. During this period, the zonal SST (sea-surface temperature) gradient in the equatorial Pacific increased, suggesting an increase in Walker circulation. Such intensified Walker circulation may have enhanced moisture advection from the equatorial Atlantic warm pool to the adjacent northern South America, causing arid regions in northern South America to contract, which may explain long-term decrease in n-alkane chain lengths. 相似文献
Two-dimensional 18O/16O isotopic analysis of the Vigarano matrix was conducted by secondary ion-imaging using a novel two-dimensional ion-imager. Quantitative oxygen-isotope images (isotopographs) of the Vigarano matrix show that 16O-rich micrograins are scattered within 16O-poor matrix. This heterogeneous O-isotopic distribution indicates that matrix is composed of different O-isotopic components that formed in different locations and/or at different times. However, the O-isotopic composition of groundmass in the matrix is the same as the bulk isotopic composition of the matrix within ±5‰ uncertainty. The spatial resolution and isotopic precision of our technique should allow submicron-size objects (>0.2 μm) with extreme O-isotopic anomalous characteristics (δ18OSMOW ∼250‰) to be detectable in isotopographs. Because the mean grain size of the matrix is ∼0.2 μm, the inability to detect such O-isotopic anomalous objects indicates that isotopically anomalous micrograins (e.g., presolar grains) are extremely rare in the Vigarano matrix and that most objects in the matrix were formed in the solar nebula or in the parent body. 相似文献
Zircon is resistant to alteration over a wide range of geological environments, and isotopic ratios within the mineral provide constraints on ages and their parental magmas. Trace element compositions in zircon are also expected to reflect those of their parent magmas, and have a potential as essential indicators for their host rocks. Because most detrital zircons that accumulate at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. This study employs the conventional classification scheme of granites (I‐, S‐, M‐, and A‐types). To clarify geochemical characteristics of zircons in A‐type granites, trace element compositions of zircons extracted from the A‐type Ashizuri granitoids were examined. Zircons from the Ashizuri granitoids commonly show enrichments of heavy rare earth elements and positive Ce anomalies, indicating that these zircons were igneous in origin. In addition, zircons in these A‐type granites are characterized by enrichments of Nb, Y, Ta, Th, and U and strong negative Eu anomalies, which exhibit good positive correlations with those in their whole rocks. This fact indicates that these signatures in zircons reflect well those in their parental bodies and are useful in identifying zircons derived from A‐type granite. Based on compilations of available data, zircons from A‐type granites can be clearly discriminated from other‐types of granites within Nb/Sr–Eu anomaly, U/Sr–Eu anomaly, Nb/Sr–U/Sr, and Nb/Sr–Ta/Sr cross‐plots. All indices used in these diagrams were selected based on the geochemical features of both zircon and whole rock of A‐type granites. Application of these discrimination diagrams to detrital zircons will likely provide further insights. For example, some Hadean detrital zircons plot in similar fields to A‐type granites, implying the existence of A‐type magmatism in the Earth's earliest history. 相似文献
Since September 2017, the Kuroshio has taken a large-meander (LM) path in the region south of Japan. We examined characteristics of the 2017–present LM path in comparison with previous LM paths, using tide gauge, altimetric sea surface height, and bottom pressure data. The 2017–present LM path was formed from a path passing through a channel south of Hachijo-jima Island, while a typical LM path originated from a path through a channel north of Miyake-jima Island. The meander trough of this atypical path was found to be shifted far to the east and to vary on a timescale of months. These characteristics are different from those of a typical LM path but they are similar to those of the 1981–1984 LM path. Therefore, we identified two types of LM path; a stable and unstable LM paths. The 2017–present unstable type large meander has a zonal scale greater than that of the 2004–2005 stable type large meander and protrudes from the eastern boundary of the Shikoku Basin, i.e., Izu-Ogasawara Ridge. No significant bottom pressure depression was observed, associated with the formation of the 2017–present LM path, indicating that baroclinic instability was not important in the formation of this LM path. Due to no significant bottom steering, even during the 2017–present LM period, a mesoscale current path disturbance occurred southeast of Kyushu, propagated eastward, and amplified the offshore displacement of the Kuroshio.