Japan Geoscience Union Meeting 2015

Presentation information


Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS34] Paleoclimatology and paleoceanography

Wed. May 27, 2015 6:15 PM - 7:30 PM Convention Hall (2F)

Convener:*Kazuyoshi Yamada(Museum of Natural and Environmental history, Shizuoka), Minoru Ikehara(Center for Advanced Marine Core Research, Kochi University), Tomohisa Irino(Faculty of Environmental Earth Science, Hokkaido University), Akira Oka(Atmosphere and Ocean Research Institute, The University of Tokyo), Yusuke Okazaki(Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University), Ikuko Kitaba(Research Centre for Palaleoclimatology, Ritsumeikan University), Akihisa Kitamura(Institute of Geosciences, Faculty of Science, Shizuoka University), Masaki Sano(Research Institute for Humanity and Nature), Takeshi Nakagawa(Ritsumeikan University), Akira Hayashida(Department of Environmental Systems Science, Doshisha University)

6:15 PM - 7:30 PM

[MIS34-P05] Biogenic opal changes in the Gulf of Alaska for the last 50 kyrs

*Sei-ichiro SUGA1, Hirofumi ASAHI2, Susumu KONNO3, Itsuki SUTO4, Yusuke OKAZAKI3 (1.Faculty of Sciences, Kyushu University, 2.Korea Polar Research Institute, 3.Graduate School of Sciences, Kyushu University, 4.Graduate School of Environmental Studies, Nagoya University)

Keywords:Biogenic opal, Gulf of Alaska, Last glacial period, IODP

Gulf of Alaska is known as high productivity area mainly by diatoms. Diatom is a phytoplankton with biogenic frustules and can be frequently preserved in sediments. Therefore, sedimentary biogenic opal content is used as a proxy for past diatom productivity (e.g. Ragueneau et al., 2000). During the Integrated Ocean Drilling Program (IODP) Expedition 341 held in 2013, Site U1418 was drilled near the continental margin of the Gulf of Alaska (58′46.6″N, 144′29.6″W, and 3,667 m water depth)(Expedition 341 Scientists, 2014). At Site U1418, continuous sediments for the past 1 Myrs with foraminifera and diatoms were recovered from five holes. Age model was constructed by oxygen isotope stratigraphy of planktonic foraminifera (Neogloboquadrina pachyderma sinistral) supported by paleogeomagnetic and microfossil datums (Asahi et al., 2014). In this study, biogenic opal contents for the last 50 kyrs were measured using 120 samples from upper 60 m core depths. Biogenic opal was analyzed by extracting with an alkaline solution (2M NaCO3) and molybdenum yellow colorimetric method (Mortlock and Froelich, 1989). In order to estimate paleo-flux of biogenic opal, accumulation rate (AR; g cm-2 kyr-1) was calculated based on wt% of biogenic opal, sedimentary density, and sedimentation rate between age control points. Averaged biogenic opal ARs at Site U1418 during the Marine Isotope Stages (MIS) 2 and 3 were 3.60 g cm-2 kyr-1 and 6.89 g cm-2 kyr-1, respectively. Note that most of Holocene sediments were not recovered at U1418. Because of low biogenic opal AR during MIS 2, it is possible to consider that diatom productivity was low during deep glacial period. The present Gulf of Alaska is known as high nutrient low chlorophyll (HNLC) region owing to lack of iron which phytoplankton needs in their production. It is also well-known that eolian dust increased in glacial periods and provided iron to the sea (e.g., Kohfeld and Harrison, 2001). Decreased biogenic opal AR during MIS 2 at U1418 does not support iron fertilization scenario which expects increased productivity by iron supply from eolian dust during glacial period. As these results, the decrease in diatom productivity may have been driven by increased light limitation due to expanded sea-ice cover in the Gulf of Alaska (de Vernal and Pedersen, 1997).

Asahi, H. et al., AGU fall meeting, PP21A-1286, 2014
de Vernal, A. and T.F. Pedersen, Paleoceanography 12, 821-830, 1997
Expedition 341 Scientists, IODP Exp. 341 Preliminary Report, 2014
Kohfeld, K.E. and S.P. Harrison, Earth-Sci. Rev. 54, 81?114, 2001
Mortlock, R.A. and P.N. Froelich, Deep-Sea Res. A, 36, 1415-1426, 1989
Ragueneau, O. et al., Global Planet. Change 26, 317?365, 2000