CONCLUSIONS

Results from this study indicate that the sedimentary organic matter at Sites 1150 and 1151 comes from marine photosynthetic organisms, as suggested by low C/N ratios and values of ¹³C_org typical of algae. However, diagenetic degradation of organic matter has affected the values for C/N and ¹³C_org, producing a decreasing trend of these two geochemical parameters with depth. Superimposed on this diagenetic trend, ¹³C_org values show ~1.5 fluctuations related to changes in the solubility of CO₂ in seawater.

Moderate values for organic carbon derived from primary producers coupled with the persistent presence of diatomaceous sediments at Leg 186 sites suggests relatively high rates of primary productivity. This condition is characteristic of the Oyashio Current, thereby suggesting that the Kuroshio Current rarely reached the latitude of Sites 1150 and 1151 throughout the Cenozoic. The relative high rates of primary productivity in the Oyashio Current significantly increased between 5 and 7 Ma, as indicated by higher MAR for carbonate and organic carbon at Leg 186 sites.

The results presented in this study are in agreement with previous reports for increased surface productivity during the late Miocene and early Pliocene. However, MAR values at Leg 186 sites could be somewhat affected by diagenetic formation of carbonate minerals and lower degradation rates of organic carbon. The extent of increase in MAR values is large enough that it is unlikely to be solely the result of diagenesis. Therefore, the interpretation that increased MAR values between 5 and 7 Ma are the result of eutrophic conditions in the Oyashio Current implies an ocean-wide increase in the delivery of nutrients. However, the source of these nutrients remains to be determined.