Journal of Oceanography, Vol. 65 (No. 6), pp. 871-883, 2009
Shigeto Nishino1*, Koji Shimada2, Motoyo Itoh1 and Sanae Chiba1
1Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
2Department of Ocean Sciences, Faculty of Marine Science, Tokyo University of Marine Science and Technology, Tokyo 108-8477, Japan
(Received 21 January 2009; in revised form 30 July 2009; accepted 3 August 2009)
Abstract: The R/V Mirai conducted hydrographic surveys in the western Arctic Ocean during summer 2004 (Mirai04) over wide east-west ranges from Alaska to eastern Siberia, where sea-ice cover has been greatly reduced in recent summers. The obtained data reveal differences in silicate profiles between shelf slope areas east and west of the Chukchi Plateau, the ridge that divides the Canada Basin into the Alaskan and east Siberian sides. East of the plateau, a single silicate maximum was found in a layer of Pacific-origin winter water, as examined in many previous studies. In contrast, west of the plateau, we found vertical double silicate maxima, which are reported for the first time in this study. The shallower silicate maximum corresponded to an N** minimum, signaling denitrification at the shelf bottom. This suggests that the shallower silicate maximum was caused by the spreading of shelf water. In contrast, the deeper silicate maximum corresponded to an oxygen minimum and a maximum silicate/phosphate ratio (Si/P), suggesting that this deeper maximum resulted from the decomposition of opal-shelled organisms. We also compared a silicate profile from Mirai04 to a profile from the Arctic Ocean Section 94 (AOS94) expedition of 1994, a heavy ice year. The results suggest that sea-ice loss has enhanced biological activities, likely resulting in the appearance of the deeper silicate maximum.