The number of diatom valves varies from 1.6 x 107/g, the minimum value at 8.00 mbsf, to 18.4 x 107/g, the maximum at 1.85 mbsf (average = 7.2 x 107/g) (Fig. F2). Large fluctuations occur, with secondary and smaller fluctuations. The high values are observed at ~1.50, 17.00, and 26.20 mbsf. The large decreases occur at ~7.50 and 20.20 mbsf.
Warm-water diatoms are overwhelmed by cold-water diatoms throughout the section. In the upper part, warm-water Fragilariopsis doliolus and Thalassiosira leptopus are slightly more abundant and show the maximum of 9.5% at 1.53 mbsf. Cold-water diatoms, such as Bacteriosira fragilis, Fragilariopsis cylindrus, Neodenticula seminae, Odontella aurita, Thalassiosira gravida, Thalassiosira hyalina, Thalassiosira nordenskioeldii, and Thalassiosira trifulta, generally increase in abundance through the whole section, but sharp decreases occur at ~21.59 and 9.16 mbsf.
Td value is very low and is punctuated in brief intervals by relatively larger fluctuations of 2.5%-5.0% throughout the section. The maximum value is 16.81% at 1.53 mbsf. These fluctuations represent changes in the surface water temperature, probably caused by the mixing of warm water and cold water in the frontal area.
Major fluctuations among secondary, smaller fluctuations in the abundance of sublittoral diatoms are recognized as declining through the whole section. The abundance of sublittoral diatoms remarkably decreases in the upper 0-5.00 mbsf. The increases in abundance of Delphineis surirella, Paralia sulcata, and Stephanopyxis turris occur in four intervals: 21.40-23.10, 15.80-19.60, 11.30-13.70, and 6.40-8.00 mbsf.
The increased frequencies of Aulacosira granulata, Eunotia spp., and Pinnularia spp. are recognized in two intervals: 21.60-23.50 and 6.00-11.90 mbsf.
The sawtoothed curve consisting of a triple increase in abundance is recognized throughout the section. The sawtooth pattern is remarkably sharp in the upper 0-10.00 mbsf. Melosira albicans and Pseudopodosira elegans are dominant throughout the section.
The six highest principal components, which were chosen by plotting the eigenvalues of 33 species, are significant for the species composition because they explain 51.1% of the total variance (Fig. F3; Table T3).
The first principal component is fairly stable and shows high positive scores for the upper 3.00 mbsf. It suggests inflow of the Tsugaru Warm Current because of the mixed assemblage consisting of cold-water, warm-water, and oceanic species. On the other hand, negative scores consisting of cold-water and sublittoral species indicate a coastal area nearshore in the subarctic ocean (Table T4).
The second component shows positive scores consisting of dominant sublittoral and extinct diatoms at 19.00-23.00 and 6.00-10.00 mbsf. Negative scores consisting of exclusively cold-water and oceanic species are dominant in the upper 6.00 mbsf.
The third component is negative in the lower part of the section studied but positive in the upper part. The positive scores represent dominant cold-water and oceanic species, and the negative scores represent cold-water and sublittoral diatoms.
The positive scores of the fourth factor occur in the lower 20.00-26.00 mbsf and in the upper 2.00-10.00 mbsf. They are composed of extinct and cold-water species. The fifth and sixth factors are unstable and keep fluctuating throughout the section.
Large-scale fluctuations in diatom abundance that are composed of smaller fluctuations are also recognized in the section studied from Site 1151. Diatoms are generally abundant in the lower part but indicate a decline in abundance from the bottom upward throughout the section (Fig. F4). That tendency is opposite to that recognized at Site 1150. The maximum value is 10.29 x 107/g at 18.40 and 21.40 mcd, and the minimum is 0.46 x 107/g at 1.85 mcd. The average value, 4.4 x 107/g, is ~60% of that at northern Site 1150.
Warm-water diatoms such as Alveus marinus, F. doliolus, Roperia tesselata, and T. leptopus are abundant at a maximum of 15% at 19.60 mcd in the lower part, but they are a minor component in the section. Cold-water diatoms such as B. fragilis, N. seminae, O. aurita, T. gravida, T. hyalina, T. nordenskioeldii, and T. trifulta increase with sharp fluctuations from the bottom to the top throughout the section.
The high Td value, 35.71% at 19.60 mcd in the lower part of the section, comes from the decrease of cold-water diatoms and from the increase of warm-water diatoms. The major increases occur at 19.60, ~13.00, 7.00, and 1.85 mcd.
The abundance curve of sublittoral diatoms is marked by three sharp drops occurring at ~15.40, 7.00, and 1.00 mcd. Remarkably high abundances of S. turris are present at 3.70 mcd. The variation in abundance is composed of secondary, smaller fluctuations.
The abundance of freshwater diatoms increases toward the top of the section, and high values of Cyclotella spp. and Stephanodiscus spp. are present at 1.85 mcd.
In the lower part of the section, the abundance of M. albicans, P. elegans, and T. nidulus increases upward and then drops at 12.80 mcd. In the upper section, the abundance gradually decreases with a peak at 5.20 mcd.
Five principal components were selected as significant based upon the eigenvalues of 34 species because they explain 46.7% of the total variance (Fig. F5; Table T5).
The first factor changes scores from high and positive in the bottom part to lower in the upper part with punctuations by several negative scores. The negative scores are remarkable in the upper 2.00 mbsf. The positive scores are composed of warm-water and oceanic diatoms, suggesting the influence of the Kuroshio Warm Current. On the other hand, the negative scores consist exclusively of cold-water and oceanic diatoms, which are dominant in this area. These fluctuations suggest an overall decline in sea-surface temperature toward the present day (Table T6).
Contrary to the first, the second factor shows negative scores in the bottom part and positive scores in the upper 9.50-5.00 mbsf. Sublittoral and cold-water diatoms represent the positive scores, and extinct diatoms and warm-water diatoms represent the negative scores.
The third factor fluctuates between negative and positive scores throughout the section. The amplitudes increase upward. The positive scores are represented exclusively by cold-water and extinct diatoms, but the negative scores are attributed to sublittoral and cold-water diatoms.
The fourth factor shows the negative scores at ~14.00 mbsf and positive scores in the upper part. The interval from 19.50 to 15.00 mbsf is occupied by positive scores, which are composed of both cold-water and warm-water diatoms and oceanic diatoms. The negative scores are represented by the mixed assemblage of sublittoral, oceanic, extinct, and cold-water diatoms, and no warm-water diatoms are included.
The fifth factor fluctuates throughout the section.