Northwest Pacific Margin Research Articles

Deciphering anthropogenic uranium sources in the equatorial northwest Pacific margin

This work reports the first high-resolution deposition records of anthropogenic uranium (236U and 233U) in a sediment core taken at the continental slope of the Philippine Sea off Mindanao Island in the equatorial northwest Pacific Ocean. Two notable peaks were observed in both profiles of 236U and 233U concentrations, with a narrower peak in 1951–1957 corresponding to close-in Pacific Proving Grounds (PPG) signal, and a broader peak in 1960s–1980s corresponding to the global fallout from nuclear weapons testing. 236U and 233U areal cumulative inventories in the studied sediment core are (2.79 ± 0.20) ∙ 1012 atom ∙ m−2 and (3.12 ± 0.41) ∙ 1010 atom ∙ m−2, respectively, about 20–30% of reported 233U and 236U inventories from the direct global fallout deposition. The overall 233U/236U atomic ratios obtained in this work vary within (0.3–3.5) ∙ 10−2, with an integrated 233U/236U atomic ratio of (1.12 ± 0.17) ∙ 10−2. The contribution from global fallout and close-in PPG fallout to 236U in the sediment core is estimated to be about 69% and 31%, respectively. We believe the main driving process for anthropogenic uranium deposition in the Philippine sediment is continuous scavenging of dissolved 236U from the surface seawater by sinking particles.

Enhanced climate variability during the last millennium recorded in alkenone sea surface temperatures of the northwest Pacific margin

Previous studies on surface temperature reconstructions for the last 2000 years (2 k) revealed a long-term cooling trend for the last millennium in comparison to the previous millennium. However, knowledge on the decadal- to centennial-scale variability in sea surface temperature and the underlying governing mechanisms throughout the period is limited. We reconstructed high-resolution continuous sea surface temperature changes over the last 2 k in the northwest Pacific margin based on the alkenone unsaturation index. Our alkenone temperature record revealed enhanced and more rapidly changing climate variability during the last millennium (approximately 1200–1850 Common Era) than during the previous millennium. Cold and hot extremes also occurred more frequently during the last millennium. The enhanced and rapidly changing climate variability appears to be associated with frequent volcanic eruptions and grand solar minima. The reconstructed surface temperature variability tends to be associated with variations in the East Asia summer monsoon and the Pacific Decadal Oscillation, implying that these variations are also enhanced in the last millennium than in the previous millennium.

U‐Pb Dating and Lu‐Hf Isotopes of Detrital Zircons From the Southern Sikhote‐Alin Orogenic Belt, Russian Far East: Tectonic Implications for the Early Cretaceous Evolution of the Northwest Pacific Margin

AbstractThe Sikhote‐Alin orogenic belt in Russian Far East is comprised of several N‐S trending belts, including the Late Jurassic to Early Cretaceous accretionary prisms and turbidite basin which are now separated by thrusts and strike‐slip faults. The origin and collage of the belts have been studied for decades. However, the provenance of the belts remains unclear. Six sandstone samples were collected along a 200 km long east‐west traverse across the major belts in the southern Sikhote‐Alin for U‐Pb dating and Lu‐Hf isotope analysis to constrain the provenance and evaluate the evolution of the northwest Pacific margin at this time. The result reveals that the sediments from the main Samarka belt was mainly from the adjacent Bureya‐Jiamusi‐Khanka Block (BJKB); the eastern Samarka belt and the Zhuravlevka turbidite basin were supplied by detritus from both the North China Craton (NCC) and the BJKB; the Taukha belt was mainly fed by sediments from the NCC; whereas the data from the Sergeevka nappes are insufficient to resolve their provenance. In the Late Jurassic to Early Cretaceous, collision and subduction was important in the initial collage of most belts in Sikhote‐Alin. However, merely E‐W trending collage cannot explain the increasing importance of the NCC provenance from west to east. It is proposed that the main Samarka belt was located adjacent to the BJKB when deposited, whereas the other belts were farther south to accept the materials from the NCC. Sinistral strike‐slip faulting transported the eastern belts northward after their initial collage by thrusting.

Middle Pleistocene Ostracods from the Naganuma Formation, Sagami Group, Central Japan: Significance of the Occurrence for the Bay Fauna Along the Northwest Pacific Margin

Abstract. The ostracod fauna from the middle Pleistocene Naganuma Formation (ca. 0.5 Ma) in the Sagami Group near Tokyo Bay, central Japan, was investigated. This study reports for the first time an ostracod fauna around 0.5 Ma in the marine oxygen isotope stage (MIS) 13 of Japan. This fauna consists of 58 species, and three assemblages were defined by Q-mode cluster analysis. The depositional environment of each assemblage can be defined as (1) outer bay, influenced by the open sea with relatively high salinity, (2) inner bay, with relatively low salinity, and (3) central bay, with intermediate salinity level between those of (1) and (2). A decrease in water depth occurred from the outer bay through the central bay to the inner bay during the depositional period of the studied horizons. The three most abundant species of each assemblage are Krithe japonica, Bicornucythere bisanensis and Amphileberis nipponica. The fossil fauna does not include the shallow-bay species Neomonoceratina delicata, currently f...

ENSO signals and their spatial-temporal variation characteristics recorded by the sea-level changes in the northwest Pacific margin during 1965–2005

Sea-level is closely linked to the Earth’s climate and its change is important as a metric for global and regional climate change. Identifying, extracting, and revealing such information through detailed analysis is the prerequisite for understanding the mechanisms of sea-level change. The monthly- average tide-change records reported and examined in this paper are from 10 tide gauge stations distributed in the northwest margin of the Pacific Ocean, registered during the period 1965-2005. In particular, we have utilized the Second Order Blind Identification (SOBI) method to identify and the Empirical Mode Decomposition (EMD) method to extract the El Nino-Southern Oscillation (ENSO) signals imprinted in those tide gauge records; and subsequently, we have investigated the spatial-temporal characteristics of the extracted ENSO signals using wavelet analysis methods. The following results are obtained: (1) the ENSO events recorded by each tide gauge series are of different types and intensity, which show considerable temporal-spatial variation characteristics, with sea-level responses to ENSO signals remarkably stronger in low latitude areas than in medium-high latitude areas; (2) due to the influences of ocean currents, topographical conditions, and other factors, there exist variations in the type of relationship between the sea-level changes and the recorded ENSO events at different latitudes; (3) sea-level changes can also denote scale-variation characteristics of ENSO events, and particularly, since 1980s, all the tide gauges located south of Kanmen show intense responses to ENSO and the timescale of ENSO events extended gradually from around 4 years to 2–8 years, reflecting variations in the intensity and frequency of ENSO events; and (4) reverse changes of certain scale were noted before and after strong ENSO events recorded by sea-level changes, supporting the research findings about the interaction (mutual coupling and superimposition) between the probability of occurrence of strong ENSO events and their time scale.

240Pu/ 239Pu atom ratios in seawater from Sagami Bay, western Northwest Pacific Ocean: sources and scavenging

Seawater samples were collected in Sagami Bay, western Northwest Pacific Ocean, and their 239+240Pu activities and 240Pu/ 239Pu atom ratios were determined by α-spectrometry and sector field high-resolution ICP-MS. A few samples also were analyzed for 137Cs activities. The 239+240Pu inventory of 41.1 Bq m −2 was equivalent to the expected cumulative deposition density of atmospheric global fallout at the same latitude and this inventory was considerably lower than inventories in the underlying sediment columns. This result indicated that a significant amount of 239+240Pu has been removed into the underlying sediments through enhanced scavenging from the water column by the high fluxes of particles in this region. The atom ratio of 240Pu/ 239Pu showed no notable variation from the surface to the bottom; the average value was 0.234 ± 0.004. This atom ratio was significantly higher than the mean global fallout ratio of 0.18, proving the existence of close-in fallout plutonium originating from the Pacific Proving Grounds (PPG). The relative contributions of the global stratospheric fallout and the PPG close-in fallout were evaluated by using the two end-member mixing model. The contribution of the PPG close-in fallout was estimated to be 15.2 Bq m −2, which corresponded to 37% of the 239+240Pu inventory in the water column. Thus 239Pu and 240Pu from the two sources of global fallout and close-in fallout have been homogenized in the water masses in the western Northwest Pacific margin during the past three decades.

Geology and stratigraphy of forearc basin sediments in Hokkaido, Japan: Cretaceous environmental events on the north-west Pacific margin

Litho-, bio-, and chemostratigraphy of the Cretaceous forearc basin sediments exposed in Hokkaido, northern Japan allow a synthesis of the faunal, sedimentological, and environmental history of the north-west Pacific margin. Although the succession, named the Yezo Group, has yielded an abundant record of mid- to late Cretaceous invertebrates, monotonous lithologies of sandstone and mudstone, showing occasional lateral facies changes, have caused confusion regarding the lithostratigraphic nomenclature. Based on our wide areal mapping of the sequence, and analysis of litho- and biofacies, a new lithostratigraphic scheme for the Yezo Group is proposed. In ascending order, the scheme is as follows: the Soashibetsugawa Formation (Lower Aptian mudstone unit); the Shuparogawa Formation (Lower Aptian–lower Upper Albian sandstone-dominant turbidite unit); the Maruyama Formation (lower Upper Albian tuffaceous sandstone unit); the Hikagenosawa Formation (Upper Albian–Middle Cenomanian mudstone-dominant unit); the Saku Formation (Middle Cenomanian–Upper Turonian sandstone-common turbidite unit); the Kashima Formation (Upper Turonian–Lower Campanian mudstone-dominant unit); and the Hakobuchi Formation (Lower Campanian–Paleocene shallow-marine sandstone-conglomerate unit). In addition, we designate two further lithostratigraphic units, the Mikasa Formation (Upper Albian–Turonian shallow-marine sandstone-dominated unit) and the Haborogawa Formation (Middle Turonian–Campanian shelf mudstone/sandstone unit), which correspond in age to the shallower facies of the Saku and Kashima formations, respectively. Despite a lack of so-called “black shales”, because of siliciclastic dilution, our stratigraphic integration has revealed the horizons of oceanic anoxic events (OAEs) in the Yezo Group. The OAE1a horizon in the Soashibetsugawa Formation is characterized by a lack of foraminifers, macrofossils and bioturbation, and a prominent positive excursion of δ 13C org. A significant hiatus during the late Aptian and early Albian removed the OAE1b horizon. The OAE1c horizon in the Maruyama Formation shows a distinct negative excursion of δ 13C org with a concomitant high productivity of radiolarians. The OAE1d horizon in the middle part of the Hikagenosawa Formation consists of weakly laminated, pyrite-rich mudstone. Planktonic and calcareous benthic foraminifers are absent, whereas radiolarians are abundant above the OAE1d horizon. The mid-Cenomanian event (MCE) horizon is identified at the top of the Hikagenosawa Formation. Stepwise extinction of calcareous benthic foraminifers and a decrease in radiolarian diversity become apparent above the MCE horizon. In the study area, the OAE2 horizon has been well documented, and is placed in the middle part of the Saku Formation.

Vertical Profiles, Inventories, and Activity Ratios of 239+240Pu and 137Cs in Sediments from Sagami Bay, Western Northwest Pacific Margin

Marine sediment cores were taken from Sagami Bay of the western Northwest Pacific and analyzed for 239+240Pu and 137Cs activities. A significant correlation was observed between 239+240Pu and 137Cs activities in sediment cores, i.e., their vertical profiles. The measured activities which were used to obtain the inventories of 239+240Pu and 137Cs in the sediment column, showed large variations, ranging from 36 to 474 MBq/km2 for 239+240Pu and 49 to 652 MBq/km2 for 137Cs. 239+240Pu inventories in the sediment from one region in the Bay, Sagami Nada were 5–10 times greater than those predicted from atmospheric global fallout 42 MBq/km2 at the same latitude. On the other hand, 137Cs inventories, with a mean value of 297±168 MBq/km2, were much less than predicted, 2040 MBq/km2. The activity ratios of 239+240Pu/137Cs obtained, with a mean of 0.65±0.17, were significantly greater than the ratio predicted from fallout, 0.021. Furthermore, there was a clear relationship between both 239+240Pu and 137Cs inventories and the water content, indicating that higher inventories may be attributed principally to concentration in, and scavenging by, fine-grained particles.

A new model of back‐arc spreading in the Parece Vela Basin, northwest Pacific margin

Abstract Swath bathymetric data, single‐channel seismic reflection profiles, magnetic and gravity anomalies in the northern part of the Parece Vela (West Mariana) Basin were obtained by comprehensive surveys conducted by the Hydrographic Department of Japan. The central zone of the Parece Vela Basin is characterized by the north‐south trending chain of depressions in a right‐stepping en echelon alignment. The morphology of these depressions is diamond shaped and bordered by steep escarpments of 1000‐1500 m relative height. These fault escarpments extend northeastward and southwestward from the depressions into the surrounding basin floor and then gradually fade out. These escarpments have an S‐shaped trend, and their geometry seems to be symmetric about the depressions. Minor ridges and troughs trending orthogonal to these escarpments are recognized. It is concluded that these depressions and escarpments are the topographic expression of extinct spreading axes and S‐shaped transform faults, respectively. The age of the central depressions seems to be young, although details of tectonic processes forming them remains unsolved. The western province of the basin floor and basement is extremely rugged and characterized by minor ridges and troughs trending in a north‐south direction. Although magnetic anomalies of the basin are very weak, magnetic lineations trending parallel to the topographic trend are recognizable in the central and western parts of the basin. Based on updated geomorpholog‐ical features and magnetic anomalies revealed by the present survey, together with the previously published data including drilling results, it is proposed that the evolution of Parece Vela Basin took place in four stages of opening and tectonic activity: rifting, east‐west spreading, northeast‐southwest spreading with counter‐clockwise rotation of spreading axes, and post‐spreading deformation and volcanism. This proposed spreading model of the Parece Vela Basin is similar to that of the adjacent Shikoku Basin. The spreading axes of both basins were segmented and gradually rotated counter‐clockwise in a later phase of the basin evolution, after the cessation of relatively uniform spreading nearly in an east‐west direction.

Mesozoic terranes of the northwest Pacific continental margin (Russia): Radiolarian ages and sedimentary environments

Abstract Geological mapping using detailed tectonic and complex radiolarian analysis revealed significant northward displacement of a number of Russian Far and Northeast Asia terranes. It was recorded that some terranes possibly crossed the equator. Terranes of north‐east Russia were composed of different allochthonous formations, ranging in age from Middle Triassic to Maestrichtian‐Paleocene and accumulated from the margin to oceanic basins. The Middle to Upper Triassic interval included two formations: (i) volcanogenic, consisting of typical volcanic rocks of the island arcs (up to 800 m thick); and (ii) a chert‐limestone‐terrigenous one composed of marginal sandstone, siltstone, limestone and tuffic chert (about 400 m). Lower Jurassic allochthonous formations are represented by chert‐terrigenous (about 300 m) and jasper‐alkaline‐basaltic (WPB‐type) seamount deposits (about 100 m). Middle Jurassic to Hauterivian allochthonous terranes from the northern part of the Koryak‐Kamchatka region include five formations: jasper (bedding jaspers with condensed limestone lenses with Buchias, 80 m), jasper‐basalt (with MORB, 100‐150 m), ferrotitanic basalt (WPB with lenses of jasper mainly composed of genus Parvicingula, about 75%, 150 m), terrigenous‐volcanic (with MORB, IAT, CA basalts and olistostrome, 600 m), tuffic‐jasper‐basalt (MORB and deposits of arc‐trench system, about 500 m) with the same age according to radiolarian data. Aptian? Albian‐Maestrichtian ones are predominantly terrigenous‐tuffaceous‐siliceous. Moreover, the Early and Middle Jurassic faunas of the northwest Pacific margin contain many boreal elements similar to those of New Zealand (Southern Hemisphere), Japan, ODP Site 801. The Late Jurassic faunas of the Koryak and Kamchatka region are mainly North Tethyan and seldom Central Tethyan and are very closely related to those of the Americas. The Tithonian to Early Cretaceous radiolarian are predominantly Central Tethyan and Equatorial in contrast to Boreal Late Cretaceous. The combining in the same region at 60°N Pacific margin of the formations accumulated in different tectonic paleoenvironments and paleoclimatic provinces, is good evidence for the possible significant northward displacement of some terranes in the northwestern Pacific.

Accretionary Tectonics of Kamchatka

The current status of investigations of Kamchatka, a critical area for reconstruction of the tectonic history of the northwest Pacific margin, is summarized in this paper, with special attention gi...

A deep section of accretionary complex: Susunai Complex in Sakhalin Island, Northwest Pacific Margin

Abstract A deep section of accretionary complex, the metamorphosed Susunai Complex, is observed on Sakhalin Is., Russia. High pressure part of pumpellyite‐actinolite facies metavolcanics, metacherts and metapelites are well exposed and constitute a tectonic pile preserving primary structures related to underplating of the oceanic crust. Three stages of deformation, D1 through D3, suggest successive deformation during subduction, underplating and exhumation of the complex. Oceanic material in the complex is more abundant than other well documented ancient accretionary complexes (e.g. the Shimanto Belt in southwest Japan and the Ghost Rocks Formation in Alaska), which were shallowly underplated. At Susunai, deep down‐stepping of a décollément has scraped off the upper part of the oceanic crust, primarily the pillowed basalt horizon. This down‐stepping results from crustal weakening as overpressured water is released from the fractured oceanic crust during metamorphism.

Right‐lateral shear along the Northwest Pacific Margin and the India‐Eurasia Collision

Right‐lateral shear along the eastern margin of Asia, from the Eocene to the Present has led to the opening of pull‐apart basins, intracontinental such as the Bohai basin, or oceanic such as the Japan Sea. We suggest in this paper that this right‐lateral shear is a consequence of indentation of Asia by India. As in small‐scale analog experiments, we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left‐lateral shear zones (Tien Shan‐Baikal‐Stanovoy for the northern one, and Qin Ling for the southern one). We discuss the compatibility of this mechanism, which involves a rather small amount of extrusion, with the fast eastward expulsion described for southeast Asia. We re‐emphasize the role played in the opening of marginal basins by the Pacific subduction as a free boundary to the east.

Neotectonic transverse structures of mobile belts: A study of an example of their deep tectonic relationships in the Koryak region, northwest Pacific margin

The type transverse structures that trend across mobile belts and into the platform of eastern Europe have in the past been investigated by traditional geological and geophysical methods. Now, however, these methods are becoming integrated with recently developed neotectonic studies that use aerial and Earth-satellite photographs and morphometrically and morphographically processed topographic maps. These integrated studies, both traditional and neotectonic, have revealed deep-seated transverse tectonic features that extend down to and include the basement. Thus, starting from geomorphic anomalies at the Earth9s surface, such methods have proved useful in deciphering the deep as well as shallow transverse structures of the Koryak region. This region includes not only transverse Paleozoic and early Mesozoic basement structures but also late Mesozoic and Cenozoic longitudinal structures in which relics of the older transverse structures survive. It also includes new transverse structures that have appeared since the longitudinal Koryak mobile belt evolved.