Toshitsugu Yamazaki

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Subducting seamounts and deformation of overriding forearc wedges around Japan

Abstract Two subducting seamounts under inner trench slopes have been identified around Japan on the basis of magnetic anomalies, morphology and geological structure. The first one is located under the foot of the inner trench slope at the junction between the Japan Trench and the Kuril Trench. Another one occurs beneath the slope slightly seaward of the Tosabae (the basement high at the trench slope break along the Nankai Trough off Shikoku). The magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i.e., a swell landward and a depression seaward. The seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions, which suggests that the subducting seamounts have kept roughly their original shapes. The morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward. Deformation of the forearc wedge by the subducting seamount extends to the forearc basin. The seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes. Two other examples of deformed inner trench slopes around Japan which can be explained by subduction of topographic highs are presented. One example is a depression on the foot of the inner trench slope northeast of the junction between the Kyushu-Palau Ridge and the Nankai Trough. Another one is an area of complex morphology of the inner trench slope along the Japan Trench around the Daiichi-Kashima Seamount.

Evolution of backarc rifting: Mariana Trough, 20°–24°N

The Mariana Trough is an actively opening backarc basin in the western Pacific. The trough formed by extension which longitudinally split an earlier arc massif creating a crescent-shaped basin between the remnant West Mariana Ridge and eastern active volcanoes of the Mariana Arc. Opening increases southward from the Volcano Islands near 24°N where the two arcs join. At 18°N in the central Mariana Trough the basin is widest and may be opening primarily by seafloor spreading. We present a synthesis of closely spaced shipboard gravity, magnetic, and bathymetry measurements from the northern basin (20°–24°N), an area that undergoes a significant progressive southward increase in extension. We have identified three stages in the evolution of rifting in this area: (1) asymmetric rifting from 24°N to 22°15′N where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin, (2) a “localization” of rifting from 22°15′N to ∼21°N where the primary zone of rifting separates from the active volcanic arc, and (3) a further concentration of rifting from 21°N to 20°N leading to the formation of deep tectonic grabens near the center of the basin. This last stage may be a precursor to (or incipient) seafloor spreading. We describe a new type of magnetic lineation resolved in a three-dimensional seafloor magnetization inversion of the trough between 20° and 24°N. The detailed character of these lineations, their association with tectonic structures, and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting, less magnetic rifted arc crust. The development of these well-defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough, although tectonic deformation appears to be more widely distributed. In addition, the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward.

Long-term secular variation of the geomagnetic field during the last 200 kyr recorded in sediment cores from the western equatorial Pacific

We have conducted a paleomagnetic study of five hemipelagic-clay cores of 4–7 m in length obtained from the West Caroline Basin, western equatorial Pacific. The purpose of this study is to clarify the long-term secular variation of the geomagnetic field during the last 200 kyr, using both intensity and directional changes. One core was dated using oxygen isotope ratios (δ18O), and characteristic magnetic susceptibility variations of about a factor of four allowed precise correlation between all cores. The magnetic mineralogy is estimated to be dominated by magnetite of single-domain (SD) and/or pseudo-single-domain (PSD) state. The magnetic mineralogy and grain size are uniform throughout the cores, which enabled us to obtain relative paleointensity variations as well as directional changes. Remanent intensity normalized by anhysteretic remanent magnetization (ARM) indicates that sharp intensity drops (to ≤ 20% of the present field) occurred at about 40 and 190 kyr B.P., with a broader decrease around 110 kyr B.P. The former two are accompanied by anomalous remanent directions, suggesting short polarity reversals or excursions. Their ages are close to those of the previously reported Laschamp excursion and Biwa I event. A prominent intensity peak occurred immediately preceding the possible excursion at 40 kyr B.P. Our results are in general similar to the recently reported datasets spanning the last 80–140 kyr from the Somali Basin, the Mediterranean, and the Sulu Sea [1–3]. The inclination record shows long-term cyclic changes of several degrees in amplitude. The intervals of the recurring variation are 40–50 kyr, which is longer than the cores memory and is close to the Milankovitch frequency associated with the change in obliquity of Earths rotational axis. We infer that an external force such as orbital forcing could be a cause of the long-term secular variation.

Rock-magnetic changes with reduction diagenesis in Japan Sea sediments and preservation of geomagnetic secular variation in inclination during the last 30,000 years

A rock-magnetic and paleomagnetic study was conducted on a sediment core of about 4.4 m long taken from the northeastern part of the Japan Sea. The core covers the last about 30 kyrs, which was dated by nineteen radiocarbon (14C) ages. Remanent magnetization is carried dominantly by magnetite. Reductive dissolution of magnetic minerals occurs between 1.2 and 1.6 m in depth (about 5–8 ka in age). A rapid downcore decrease of anhysteretic remanent magnetization (ARM) begins at the shallowest depth. Saturation isothermal remanent magnetization (SIRM) follows, and a decrease of magnetic susceptibility (k) takes place at the deepest. Within this zone, coercivity of natural remanent magnetization (NRM) and the ratios of ARM to k and SIRM to k also decreases with depth. These observations indicate that finer magnetic grains were lost earlier than larger grains. A decrease of S ratios, wasp-waisted hysteresis curves, and a deviation from a mixing trend of single-domain and multi-domain grains in a Day plot occur as the dissolution proceeds, which suggests that high coercivity minerals like hematite are more resistive to dissolution than low coercivity minerals like magnetite. The start of the dissolution at 1.2 m in depth is synchronous with increases in organic-carbon and total-sulfur contents, but the horizon does not coincide with the present Fe-redox boundary at about 0.02 m below the sediment-water interface. From low-temperature magnetometry, it is estimated that magnetites with maghemite skin are reduced to pure magnetites prior to dissolution. There is no evidence for precipitation of secondary magnetic phases and acquisition of chemical remanent magnetization (CRM). Neither pyrrhotite nor greigite was detected. Information of paleomagnetic directions have survived the reductive dissolution. Inclination variations of this core resembles closely to the secular variation records available around Japan. Well-dated records older than 10 ka are still very rare, and hence our new record could be useful for establishing regional secular variations.

Provenance of the north Pacific sediments and process of source material transport as derived from Rb–Sr isotopic systematics

Abstract Rb–Sr isotopic systematics of 111 samples of sediments in nine cores from the north Pacific of Quaternary and Pliocene ages have been investigated. They provide information on the provenance, the process of particle transport and temporal variation in the flux of source material. The Rb–Sr isotopic systematics of the core sediments show well-correlated pseudo isochrons. The pseudo isochrons reflect the mixing of two types of material, i.e., the Asian continental material with high 87 Rb / 86 Sr ratios (4.5–6.5) and high 87 Sr / 86 Sr ratios (0.723–0.727) and the volcanic material with low 87 Rb / 86 Sr ratios and low 87 Sr / 86 Sr ratios, from island–arc volcanics such as the Izu–Ogasawara–Mariana and the Japanese Islands and oceanic islands such as the Hawaiian Islands (0.0–2.0; 0.703–0.708). The clearness of the pseudo isochrons implies that the Rb–Sr isotopic composition of the weathering products derived from the Asian continental crust is extremely homogenized. The fine fraction (a few μm) with a high 87 Rb / 86 Sr ratio (5.0–6.5) is widely transported into the north Pacific by the middle-latitude westerlies and contributes largely to pelagic sediments. The coarser fraction with a lower 87 Rb / 86 Sr ratio (

Environmental rock‐magnetism of pelagic clay: Implications for Asian eolian input to the North Pacific since the Pliocene

We conducted a rock-magnetic study of pelagic sediments in order to document variations of Asian eolian input to the North Pacific since the Pliocene. The materials studied consist of five pelagic-clay (red-clay) cores of several meters long taken from the central North Pacific and surface sediments of box cores obtained at 40 sites along two lines, a N-S line along 175°E and an E-W line along 20°N. The magnetic susceptibility (concentration of magnetic minerals) and S ratio (relative abundance of low-coercivity magnetic minerals) of the five pelagic-clay cores have decreased synchronously since about 2.5 Ma. This age coincides with the onset of the northern hemisphere glaciation and Chinese loess deposition. Geographical distribution of S ratios measured on the surface sediments revealed that they are remarkably low in the central North Pacific. These facts suggest that S ratio can be used as a proxy for Asian eolian dust. Mossbauer spectroscopy showed that hematite dominates the high-coercivity minerals. Observation with a transmission electron microscope suggests two sources of magnetic minerals in the pelagic clay: biogenic magnetite produced in situ and detrital magnetite and hematite. The variations of S ratio and magnetic susceptibility with time and region can be explained by a model that Asian eolian dust has higher (lower) hematite (magnetite) concentration than other sources of magnetic minerals and that its input to the central North Pacific has increased since 2.5 Ma. We recognized subtle S-ratio fluctuations of about 400 kyr in recurring intervals superimposed on the decreasing trend since 2.5 Ma. Rock-magnetic parameters diagnostic of magnetic grain size, on the other hand, showed small or no variations with age, although eolian grain size is generally thought to have increased significantly since the Pliocene based on sedimentological studies.

The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km2 and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?

Relative paleointensity of the geomagnetic field during Brunhes Chron recorded in North Pacific deep-sea sediment cores : orbital influence?

Abstract Relative paleointensity records of the geomagnetic field during the Brunhes Chron were obtained from two sediment cores in the North Pacific near the Hess Rise. The cores, taken below the carbonate compensation depth (CCD), showed an oxidized environment, and accompany little paleoclimatically induced lithological changes. Homogeneity of magnetic mineralogy and magnetic grain size, which are prerequisites for sedimentary paleointensity, were thoroughly examined using various techniques including low-temperature magnetometry, room-temperature hysteresis parameters, and S ratio. Isothermal remanent magnetization (IRM) was used as a normalizer of the natural remanent magnetization (NRM). The appropriateness of the normalization was checked by the absence of correlation between the normalized intensity (NRM/IRM) and the normalizer (IRM). The relative paleointensity records obtained are generally similar to the datasets from the east and central equatorial Pacific. Spectral analysis showed significant power at the orbital eccentricity (100 kyr) frequency in the relative intensity record, but not for the normalizer. This suggests orbital influence on the geomagnetic field intensity, but the possibility of paleoclimatic contamination through rock-magnetic changes should be examined further using cores from different sedimentary regime.

Organic carbon flux controls the morphology of magnetofossils in marine sediments

Magnetotactic bacteria produce chains of magnetite crystals within a cell. Bacterial magnetites have characteristic morphologies and sizes that are strictly biologically controlled. We examined morphologies of fossil bacterial magnetites (magnetofossils) preserved in Pacific deep-sea sediments and their relations to organic carbon fluxes. Isotropic crystals dominate magnetofossils in sediments in relatively oxidized conditions, and anisotropic crystals predominate in more reduced conditions. Our finding has important implications for biomineralization processes and demonstrates the potential of magnetofossil morphology as a paleoenvironmental indicator.