Nobuaki Niitsuma

Latest Paleocene benthic foraminiferal extinction and environmental changes at Tawanui, New Zealand

A major extinction of intermediate-water (500–1000 m) benthic foraminiferal species coincided with a major decrease in δ13C (2.8‰) of terrestrial organic matter (n-C29 alkane) and δ34S (20‰) of whole rock sulfide in a continuous siltstone sequence in the Tawanui Section (46°S paleolatitude) along the Akitio River, southeastern North Island, New Zealand, in the middle part of the uppermost Paleocene nannofossil zone (CP8). The benthic extinction (25% of species) occurred over ∼3 kyr at ∼55.5 Ma. Increases in kaolinite/illite and kaolinite/smectite ratios and in terrestrial organic carbon percentages started ∼3 kyr before the major benthic extinctions, lasted over ∼40 kyr, and probably reflect warmer climate and increased rainfall. The productivity of planktonic foraminifera and calcareous nannoplankton decreased ∼3 kyr prior to the major extinctions and recovered at the time of benthic extinctions. These events that started ∼3 kyr before the extinction can be best explained by warming, increased rainfall, reduced salinity of surface waters, and increased influence of warm saline deep water (WSDW). Benthic foraminiferal oxygen indices indicate a strong decrease in dissolved oxygen levels within the intermediate water from low oxic (1.5–3.0 mL/L O2) to suboxic (0.3–1.5 mL/L O2) conditions coinciding with the benthic extinctions. Increases in total organic carbon (TOC) and in the hydrocarbon-generating potential of kerogen (measured as the hydrogen index (HI)) agree with the interpretation of decreased dissolved oxygen levels of the intermediate water. The lowest oxygen conditions lasted ∼40 kyr and coincided with a decrease in calcareous benthic foraminiferal productivity, highest TOC levels, and lowest δ13C of terrestrial organic carbon. Dominant formation of WSDW or sluggish intermediate-water circulation caused by warming and high rainfall in high-latitude areas most likely led to the ∼3-kyr time lag between events on land and in surface waters preceeding the extinction and the development of dysaerobia in the sea, coinciding with the major benthic extinction and decrease in δ13C and δ34S in New Zealand. Global warming of deep and intermediate waters may have caused decomposition of methane hydrate in sediments, resulting in a strongly decreased δ13C of marine carbonates, promoting dysaerobia in the ocean, and warming global climate by increased methane concentrations in the atmosphere. Upwelling of WSDW, occurring soon after it became dominant in high-latitude areas, is likely responsible for the recovery of normal salinity and the concomitant recovery of planktonic foraminifera and calcareous nannoplankton productivity in high-latitude surface waters. Minor benthic foraminiferal extinctions (9% of species) occurred ∼40 kyr after the major extinctions, lasted ≤ ∼6 kyr, and coincided with the initiation of environmental recovery.

Detailed paleomagnetic records during the Brunhes-Matuyama geomagnetic reversal, and a direct determination of depth lag for magnetization in marine sediments

Abstract Detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the Brunhes-Matuyama geomagnetic reversal outcropping in the Boso Peninsula, central Japan. The records are based on three continuous sediment cores from three sections within a distance of 14 km. The stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the Brunhes-Matuyama reversal. The sedimentation rate, according to the oxygen isotope stratigraphy for the three sections, decreases eastward and is 3.70, 2.20 and 1.83 m ka−1. The depth lag of magnetization in the sediments was calculated directly as 42 cm, based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole (VGP) paths for the three sections. VGP movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability. The last swing of the VGP from the southern hemisphere to the northern hemisphere in the Brunhes-Matuyama geomagnetic reversal took only 38 yr. Intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one-fifth for about 5.4 ka including the geomagnetic reversal. The movement of the VGP longitude is 0.36° yr−1 westward and this westward drift can be related to the rotation of the non-dipole field.

Deep scientific dives in the Japan and Kuril Trenches

Abstract In the summer of 1985, during the French-Japanese Kaiko program, ten dives to depths of 6000 m in the Japan and Kuril Trenches were made in the newly launched submersible “Nautile”. The sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving Seabeam mapping, geomagnetic and gravimetric measurements, and single-channel seismic profiling. The results of the dives provide new constraints on the geodynamics of these subduction zones. In the Japan and Kuril Trenches huge slump scars were observed on the landward slopes of the trenches. Slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material. The slump scars allowed us to observe the internal structure of the margin; the monoclinal structure on the northern Japan Trench margin deduced from the seismic profiles and DSDP drilling was confirmed. Several dives on Kashima Seamount confirmed that this volcano has recently been split into two parts by a normal fault system. Comparisons of lithology and paleontology on the two separated parts of the seamount were made. Deep-sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches. It can be concluded that the whole margin is venting fluids from depths of 2–3 km which is consistent with the indications of overpressure observed in drill sites on the Japan Trench margin. The fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor.

Abyssal molluscan colony of Calyptogena in the Pliocene strata of the Miura Peninsula, Central Japan

Fossil Calyptogena are found as colonial occurrence in Pliocene abyssal sediments cropping out in the Miura Peninsula, Central Japan. The sediments containing Calyptogena deposited on a talus during drastic rearrangement of the sedimentary basins, caused by a collision process around a trench-trench-trench type triple junction. The population density of Calyptogena shells is 90–130 individuals/m2 which is comparable to the living colony of Calyptogena. The oxygen and carbon isotopes of the fossil shells indicate that the shells lived at abyssal depth and under the influence of seepage of connate water with a high concentration of organic carbon.

Facies belts of the Middle America Trench and forearc region, southern Mexico: Results from Leg 66 DSDP

Summary The Middle America Trench SE of Acapulco is flanked by a steep canyon-incised slope and narrow shelf, showing one of a variety of sedimentary facies patterns possible at convergent margins. Piston and drill cores from this region define eight facies belts including: (1) a pelagic facies of brown clay, (2) an outer slope mud facies, (3) a trench sand facies, (4) a foraminiferan-free facies on the lower slope, (5) a foraminiferan-bearing facies on the mid-slope, (6) a laminated mud facies on the upper slope, (7) a shelf facies of sand and mud, and (8) a canyon facies of sand and gravel. The superposition of trench and lower slope sediment during accretion results in a fining upward sequence reflecting a gradual uplift of the seafloor through the trench sediment-plume. The lower limit of the foraminiferan-bearing facies is defined by the absence of in situ calcareous foraminiferans and is controlled by the calcite compensation depth. The upper slope laminated mud facies probably reflects the depth range of the oxygen minimum zone. In the Leg 66 area sedimentation rates are high in the trench and on the outer and lower slope, decrease on the mid-slope, and increase again on the shelf. On the inner shelf, waves and currents concentrate sand which funnels through a prominent submarine canyon, bypassing the mud-dominated slope and accumulating in the trench. A terrigenous sediment-plume generated by trench turbidity flow causes accelerated sediment accumulation to about 500 m above and 40 km seaward of the trench. The volume of material transported by the trench sediment-plume is five or six times greater than that moved by the shelf sediment-plume which supplies detritus to the shelf, upper slope and mid-slope environments.

Carbon and oxygen isotope time series records of planktonic and benthic foraminifera from the Arabian Sea: Implications on upwelling processes

Abstract Oxygen and carbon isotopic analyses have been performed on the tests of Globigerina bulloides and Pulleniatina obliquiloculata (planktonic foraminifera) and Uvigerina excellens (benthic foraminifer) to study the evolution of surface and bottom water hydrographic changes associated with summer monsoon upwelling process at the Oman Margin over the last 19 kyr. Globigerina bulloides, P. obliquiloculata and U. excellens show sharp depleted δ18O excursions around 9 ka, ascribed to the distribution of melt water flux of Termination IB in this region. A synchronous δ18O shift in surface, subsurface and bottom water-living foraminifera around 9 ka reveals a rapid transfer of the Termination IB signal through the vertical circulation of the Arabian Sea in response to peak monsoon intensity. A steep increase in δ13C values of U. excellens between 9 and 8 ka reveals advection of Red Sea intermediate water into the Arabian Sea during peak monsoon intensity. For the first time a drastic climatic shift at ∼4 ka is noticed in the Arabian Sea as evidenced by the lowest δ18O values of both P. obliquiloculata (−1.10‰) and G. bulloides (−2.29‰). This points to reduced upwelling strength. Both planktonic foraminifer species δ18O records suggest the establishment of modern surface water conditions after 3 ka at the Oman Margin.

Further application of the deconvolution method of post-depositional DRM to the precise record of the Matuyama-Brunhes reversal in the sediments from the Boso Peninsula, Japan

The Matuyama-Brunhes geomagnetic reversal was continuously recorded in the massive siltstones of the Boso marine sediments in Japan with the time resolution better than 9 years. However the Boso sediments probably have the post-depositional detrital remanent magnetization (pDRM) depressing short-period variations due to the convolution of the geomagnetic field. The previous application of the deconvolution method of pDRM to the 900 year records gave the half fixing depth of 21 cm. Further application of this fixing depth to the 4100 year record clearly shows nearly 100 year variations in both of the inclination and declination.

Scientific Continental Drilling Project in Japan and Scientific Goals

The Japanese Island Arcs locate between the Eurasian continent and the Pacific Ocean, and the crustal structure represents both continental and oceanic characteristics. The Island Arc involves continental crust formed by the accretionary process. The continental crust is separated from the Eurasian Continent by the marginal seas with oceanic crust. Studies of the geologic history of the continental crust suggest that one should consider the continental crusts as being linked to the global tectonic events, but as an isolated event. Naturally, the optimum scientific goal of the proposed continental drilling is to focus on the ultimate understanding of the dynamics of the crust.