[M-IS19] Paleoclimatology and paleoceanography

[MIS19-P21] Silicoflagellate and Actiniscus response to meltwater during the last deglaciation in the Alaska margin

*Asuka Tomokawa¹, Yusuke Okazaki¹, Jonaotaro Onodera², Keiji Horikawa³ (1.Department of Earth and Planetary Sciences, Kyushu University, 2.Research and Development Center for Global Change, JAMSTEC, 3.Graduate School of Engineer and Science, University of Toyama)

Keywords:Silicoflgellate, Gulf of Alaska

Silicoflagellate is a phytoplankton with biogenic opal skeleton, and its fossil assemblage is used for reconstruction of paleoenvironment. There are two major genera for modern silicoflagellates: Dictyocha mainly dwelling in tropical to temperate zone and Stephanocha mainly dwelling in subarctic zone. The Dictyocha/Stephanocha ratio has been used as a paleotemperature proxy. Actiniscus pentasterias is a species of Dinoflagellata with biogenic opal skeleton, indicator of coastal and low salinity environments. Gulf of Alaska is located eastern North Pacific. The southern Alaska margin has fjord created by the Cordilleran Ice Sheet. The purpose of this study is to clarify the silicoflagellate and Actiniscus assemblages to meltwater input from the Cordilleran Ice Sheet at the last deglaciation in the southern Alaska margin. A piston core KH17-3 CL14 was obtained from the southern Alaska margin (59° 33.350’N, 144° 09.344’E, water depth: 695 m). Age model of core KH17-3 CL14 is established by planktic foraminiferal radiocarbon dates and the bottom of the core is estimated to be ~17 ka. We used 48 samples for Silicoflagellate and Actiniscus analyses. 100 silicoflagellate specimens were counted for each slide and all Actiniscus pentasterias specimens were counted for each slide under light microscope. 7 species and 2 subspecies of genus Dictyocha and 4 species of genus Stephanocha were identified. Relative abundances of genus Dictyocha accounted for greater than 50% since Younger Dryas (YD), whereas genus Stephanocha (particularly S. speculum) was abundant during Bolling-Allerod (BA). This suggests that Dictyocha/Stephanocha ratio didn’t simply reflect cold (YD) and warm (BA) events during the last deglaciation. Two Actiniscus pentasterias peaks were observed during the last deglaciation which were coincident with Meltwater Pulse 1A and 1B. This suggests salinity decline due to meltwater from the Cordilleran Ice Sheet. The meltwater into the subarctic Pacific may have leaded to enhanced productivity during BA. Because Stephanocha speculum is suggested to be an indicator of high productivity from modern sediment trap observation, high abundances of S. speculum in core KH17-3 CL14 during BA can be explained by the enhanced productivity driven by meltwater from the Cordilleran Ice Sheet.