During Heinrich Stadial 1 (HS1, 18.0–14.7 kyr ago), the Bering and Okhotsk Seas in the western Subarctic Pacific (SAP) exhibited boundary conditions that enhanced intermediate water convection. However, little is known about the quantitative contribution of the intermediate waters from the Bering and Okhotsk Sea (BSIW and OSIW) to ventilation in the open SAP intermediate–deep layers. In this study, we analyzed the neodymium (Nd) isotopes of fish debris (εNd-FD) from sediment cores in the Northwestern Pacific, and observed a reduction in εNd (up to −2.6) from the Last Glacial Maximum to HS1 at Sites MD01-2420 (2,101 m) and KH99-3 ES (2,388 m). The relevance of reduced HS1 εNd-FD shifts across the cores cannot be explained by the porewater influence, which generally changes authigenic εNd in opposite ways. Instead, as the εNd-FD shifts concurred with a reduction in the 14C ventilation age, HS1 εNd-FD shifts suggest the entrainment of younger and distinct εNd water mass sinking in the Okhotsk and/or Bering Seas. A mixing model applying Nd isotopes, their concentrations, and 14C ventilation ages of BSIW, OSIW, and the Lower Circumpolar Deep Water showed that during HS1, BSIW (36%–41%) was more conducive to the water mass at Site MD01-2420, compared to OSIW (4%–8%). We argue that the Bering Sea has wider and deeper connections (Kamchatka and Near Straits, ∼4,400–2,200 m) with the western SAP, and the deep conduit would enable BSIW to more effectively flow out into the open Northwestern Pacific deep layer.
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