Nd isotopic composition and REE concentrations in the Bering Sea and the North Pacific

Hajime Obata, The University of Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Japan, Takuya Hara, The University of Tokyo, Kashiwa, Japan and Hirofumi Tazoe, Hirosaki University, Aomori, Japan
Since neodymium isotopic composition (Nd IC) of seawater is an important chemical tracer for water mass and expected as a powerful proxy for paleoceanography, Nd IC in seawater has been reported in many oceanic regions. However, data on Nd IC in the Bering Sea and the North Pacific Ocean have not been obtained intensively. In this study, we investigated the distribution of Nd IC and rare earth element concentrations in seawater in the Bering Sea and the North Pacific.

Surface seawaters were collected from the equatorial to the subarctic Pacific and the Bering Sea during two R/V Hakuho-Maru cruises (KH-09-3 and KH-14-3) in 2009 and 2014. Vertical seawater samples were collected in the North Pacific and Bering Sea during KH-14-3 cruise with Niskin-X samplers deployed onto CTD-CMS.

Dissolved Nd concentrations in surface waters of the North Pacific systematically increased (3.3 - 9.8 pM) with latitude, while the Nd IC showed distinctive features in each surface water mass, Pacific Equatorial Water, North Pacific Central Water, North Pacific Transition Zone Water, Pacific Subarctic Upper Water. We observed relatively low epsilon Nd values at the mid-latitude, indicating the lateral transport process of unradiogenic Nd from the East China Sea via Kuroshio Extension. Dissolved Nd concentrations in surface waters of the Bering Sea ranged from 10.6 to 26.6 pM. The vertical profile of Nd in the continental shelf indicated an increasing trend toward the seafloor, implying the supply of Nd from the surface sediment to overlying seawater. In the Bering Sea, high epsilon Nd values were often found, which may be originally derived from volcanic rocks in the Aleutian Islands. We will mainly discuss the geochemical cycles of Nd in the subarctic Pacific and the Bering Sea.