(ODP Site Survey) Geophysical and geological investigation of the Nankai Trough. Boso Triple Juction, Zenisu Ridge and Muroto Trough

航海番号: KH-86-5 ; 航海日程: November 22 - December 14, 198

(ODP Site Survey) Geophysical and geological investigation of the Nankai Trough. Boso Triple Juction, Zenisu Ridge and Muroto Trough

航海番号: KH-86-5 ; 航海日程: November 22 - December 14, 198

Magnetic properties and sand composition at DSDP Site 87-582 and Hole 87-583D

At Site 582, DSDP Leg 87, turbidites about 560 m thick were recovered from the floor of the Nankai Trough. A turbidite bed is typically composed of three subdivisions: a lower graded sand unit, an upper massive silt unit, and an uppermost Chondrites burrowed silt unit. The turbidites intercalate with bluish gray hemipelagic mud which apparently accumulated below the calcite compensation depth. In order to investigate the nature and provenance of the turbidites, we studied the grain orientation, based on magnetic fabric measurements and thin-section grain counting, and grain size, using a photo-extinction settling tube and detrital modal analysis. The following results were obtained: (1) grain orientation analysis indicates that the turbidity current transport parallels the trench axis, predominantly from the northeast; (2) Nankai Trough turbidites generally decrease in grain size to the southwest; (3) turbidite sands include skeletal remains indicative of fresh-water and shallow-marine environments; and (4) turbidites contain abundant volcanic components, and their composition is analogous to the sediments of the Fuji River-Suruga Bay area. Considering other evidence, such as physiography and geometry of trench fill, we conclude that the turbidites of Site 582 as well as Site 583 were derived predominantly from the mouth of Fuji River and were transported through the Suruga Trough to the Nankai Trough, a distance of some 700 km. This turbidite transport system has tectonic implications: (1) the filling of the Nankai Trough is the direct consequence of the Izu collision in Pliocene- Pleistocene times; (2) the accretion of trench fill at the trench inner slope observed in the Nankai Trough is controlled by collision tectonics; and (3) each event of turbidite deposition may be related to a Tokai mega-earthquake

(Table 1) Age determination of sediment core KH92-15cBX

A sediment core collected from Caroline Basin, western tropical Pacific was analyzed for lipid class compounds (aliphatic hydrocarbons, long-chain alkenones, fatty alcohols, sterols, and fatty acids) to reconstruct changes in paleoenvironments during the last 23.3 kyr. Around the boundary between the glacial and deglacial periods (19 ka), mass accumulation rates of terrestrial biomarkers, C25-C35 n-alkanes, C24-C28 fatty alcohols, and C23-C34 fatty acids, were found to decrease significantly and stayed in low levels during the deglaciation, suggesting a reduction of atmospheric transport of continental materials during that time. In the same period, mass accumulation rates of C17-C20 n-alkanes, pristane, cholesterol, and dinosterol which are thought to be mainly derived from marine organisms also decreased, suggesting a significant drop of marine biological productivity. The decreased biological productivity in the western tropical Pacific may be caused by a reduced supply of nutrients from upwelling which is associated with decreased wind velocity and/or caused by a shift of upwelling zone

(Table 2) Age control points and sedimentation rates for sediment cores of the eastern edge of Shikoku Basin

Geochemical analyses were performed on three cores of hemipelagic sediment from the northwest Pacific Ocean (eastern edge of Shikoku Basin) dating from the last interglacial and glacial periods. The objective of this work is to delineate the relations between excess sedimentary barium [Ba(ex)] content and biogenic opal, carbonate, and organic carbon contents. Calculated Ba(ex) values show variable degrees of correlation with biogenic sediment fractions. The mass accumulation rate (MAR) of Ba(ex) covaries with the MARs of biogenic fractions, and in particular, with the MAR of biogenic carbonate (r**2 = 0.68) and TOC (R**2 = 0.50). Variable relations between barium and biogenic fractions in hemipelagic sediment provide detailed insight into the behavior of sedimentary barium. It appears that a significant proportion of the barium is affiliated with the carbonate fractions. Based on this strong correlation, carbonate dissolution rates of the last glacial to interglacial periods are estimated. Assuming that the MARs of carbonate and Ba(ex) covary, variation of the Ba/Ca ratio in sediments reflects the extent of postdepositional carbonate dissolution. The record of sedimentary Ba/Ca ratios exhibits striking difference between sediment of the glacial and interglacial periods, with a higher positive correlation during glacial intervals and a lower correlation during interglacial intervals; this pattern is the result of enhanced carbonate dissolution during interglacial times. Sedimentary Ba/Ca ratios may, therefore, be a useful tool for estimating the relative extent of carbonate dissolution. Ba(ex) curves and patterns in sedimentary Ba/Ca ratios resemble the established Pacific carbonate dissolution record (i.e., enhanced dissolution during interglacial times and reduced dissolution during glacial times) as suggested by previous studies. Variations in sedimentary Ba/Ca ratios strongly support an interpretation of pronounced glacial-interglacial fluctuation in carbonate dissolution in the northwest Pacific Ocean. An especially pronounced dissolution event is evident at 75 kyr during the transition interval form interglacial to glacial conditions. This suggests that Ba(ex) may be used as a proxy for evaluating carbonate dissolution in hemipelagic settings, but further study of the association between barium content and carbonate dissolution will be required to confirm this