ALBERT

Description: Deep-Earth convection can be understood by studying hotspot volcanoes that form where mantle plumes rise up and intersect the lithosphere, the Earth’s rigid outer layer. Hotspots characteristically leave age-progressive trails of volcanoes and seamounts on top of oceanic lithosphere, which in turn allow us to decipher the motion of these plates relative to “fixed” deep-mantle plumes, and their (isotope) geochemistry provides insights into the long-term evolution of mantle source regions. However, it is strongly suggested that the Hawaiian mantle plume moved ~15° south between 80 and 50 million years ago. This raises a fundamental ques- tion about other hotspot systems in the Pacific, whether or not their mantle plumes experienced a similar amount and direction of motion. Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamounts showed that the Louisville hotspot in the South Pacific behaved in a different manner, as its mantle plume remained more or less fixed around 48°S latitude during that same time period. Our findings demonstrate that the Pacific hotspots move independently and that their trajectories may be controlled by differ- ences in subduction zone geometry. Additionally, shipboard geochemistry data shows that, in contrast to Hawaiian volcanoes, the construction of the Louisville Seamounts doesn’t involve a shield-building phase dominated by tholeiitic lavas, and trace elements confirm the rather homoge- nous nature of the Louisville mantle source. Both observations set Louisville apart from the Hawaiian-Emperor seamount trail, whereby the latter has been erupting abundant tholeiites (char- acteristically up to 95% in volume) and which ex- hibit a large variability in (isotope) geochemistry and their mantle source components.

Description: The Japan Meteorological Agency has been conducting long-term magnetic observations at Kakioka (1913-), Memambetsu (1952-) and Kanoya (1958-), and archived the observations in magnetograms. We have already converted these magnetograms into digital images at 600 dpi for the periods 1924-1983 (Kakioka), 1963-1984 (Memambetsu) and 1964-1984 (Kanoya). In 1956, time marks in the magnetograms were changed from 'gap' to 'vertical lines'. For the period after 1956, we have been producing 1-minute and 7.5-second digital values by our self-developed automated tracing software. The digitization was finished for the periods 1956-1983 (Kakioka) and 1968-1984 (Memambetsu and Kanoya), except for times of some abrupt phenomena. As automatic reading by the software is not applicable to such time intervals, we keep working on them with manual reading software. Our ultimate goal is to complete digitization of all records prior to 1956, which involve a number of extreme events (PC, SFE, SC) including the 24 March 1940 event, the greatest SC event since 1868. 7.5-second data would be very useful resources for elucidating the Sun-Earth coupling processes. However, sharp peaks associated with SCs may not be adequately resolved even with those data. Additional information is desirably obtained by spotting apexes of individual peaks. Challenges to be addressed include: (1) difficulty in strict time identification due to the time mark gap, (2) difficulty in tracing lines due to dimness of the magnetograms, and (3) inaccuracy of the time axis due to paper distortion. New working tools and a certain amount of labor will be further required.