Paleoceanographic approach to discover causal relationship between marine biological production and nutrient availability in mixing layer synchronized with climatic change

3.00 p.m., September28, 2008

B-206, Zeng Cheng Kui Building

By Prof.Masso Minagawa, Vice Dean,Faculty of Environmental Earth Science, Hokkaido University, Japan

Abstract:

Marine biological production, its role, intensity, extent and the mechanism are undoubtedly important to know for understanding the role of the ocean system in the global carbon cycling. These subjects have been studied usually by biogeochemical researches mainly based on modern oceanographic observations. On the other hand, paleoceanographic approach using sediment core analysis has also give us useful information to discover environmental factors affecting local and global events, especially processes having relatively longer trends such as millennium or centennial time intervals. Here I introduce some case studies, that are directed eventually to reveal nitrogen supply and demand condition in mixed layer by means of natural isotopes and biomarker analyses for sediment cores.  Brief procedure of analysis is as following. Core age of each sediment depth was determined by the age model of benthic foraminiferal 18O and 14C. Sea surface temperature of each age were estimated by unsaturation index of biomarker molecule, as known UK37 and TEX86. TOC, opal and the concentration of biomarker such as alkenone are used as indicators of biological production. Nitrogen isotope composition of TN is used for a proxy of nitrogen fixation, denitrification and the nitrate availability in the surface seawater.  The comparative study of three piston cores from the Sulu sea, the east tropical Pacific and Southern Ocean give us details of temporal variations of nitrogen supply and productivity for last 80,000 to 350,000 years. These areas were far remote each other and have different environmental factors, but still some concomitant signals reflecting global excurtion of biogeochemical process were found in time series. Such results can be a promising proxy to interpret biological and hydrographic response of the ocean in glacial-interglacial climatic change.