MILLENNIAL-SCALE VARIABILITY OF SURFACE PRODUCTIVITY IN THE JAPAN SEA BASED ON HIGH-RESOLUTION ANALYSIS USING XRF MICRO-SCANNER

Fine-grained sediments with distinct alternations of dark-light colored layers in the Japan Sea during the last glacial period were considered to reflect the drastic changes of paleoceanographic conditions in the sea in response to millennial-scale climatic changes known as Dansgaard-Oeschger (D-O) cycles. However, the detail characteristics of the millennial-scale variability are not clarified because of the limitation of resolution in previous analyses.

We tried to apply our new XRF micro-scanner method to wet sediment cores MD01-2407, 08 obtained from the Japan Sea to conduct high-resolution (1.25 cm interval) quantitative analysis of major elements. Our new method estimates the water content for the individual XRF scanning area, and corrected XRF intensities for absorption effect of interstitial water using the estimated water content. With this method, we can conduct quantitative analysis of Al, Si, K, Ca, Ti and Fe concentrations less than }2.7 wt%.

Biogenic silica, biogenic carbonate, and terrigenous material contents are estimated from Si, Ca, and Al concentrations assuming constant Si/Al and Ca/Al ratios, and constant Al concentrations in the terrigenous material. The concentrations of biogenic silica and carbonate show several hundred to thousand years scale variations during the last 150ky. Especially, the pattern of biogenic carbonate contents shows variability that closely match with D-O cycles with higher contents in the interstadial dark layers, whereas the biogenic silica and terrigenous material contents show higher contents in the stadial light layers during the last glacial period (MIS 2 to4). The relationship between the dark layers and carbonate content is reversed during MIS 1 and 5, whereas opal and terrigenous contents do not have consistent relationship with the dark and light layers.

Dark layers during the MIS3 probably reflect the increase in the carbonate productivity as a result of increased influx of the nutrient-rich East China Sea Coastal Water (Tada et al., 1999). Whereas low biogenic carbonate in the dark layers during the MIS5 probably reflects combination effects of the increased productivity of biogenic silica, the increased flux of terrigenous material, and dissolution of carbonate.