We determined Zn isotope composition (δ 66 Zn) in the water column of the Northwestern Pacific Ocean (NWPO) and its marginal seas to investigate the processes driving vertical and spatial variations. Comparable to previous studies, dissolved δ 66 Zn was relatively low in the top 200 m, ranging from −0.91 to +0.24‰ and increased with depth toward an averaged value, +0.38±0.10‰, in deep water. We found that δ 66 Zn observed in the deep water of the NWPO was much lower than the Northeastern Pacific Ocean. Box model approaches suggest that spatial variations in the deep water may be attributed to isotopically light Zn originating from sedimentary input or anthropogenic aerosols. In the surface water, the fractionation factors derived from both closed or open system fractionation models are all less than one in the NWPO. However, both closed and open system models show a relatively poor fit to the measured data. We thus propose that external input is a major factor causing the variations of [Zn] and δ 66 Zn values in surface water, in addition to the effects of scavenging, physical mixing, and biological uptake. As elevated [Zn] and relatively light δ 66 Zn in the surface water were observed in high aerosol input regions, aerosol deposition may play a dominant role in regulating Zn elemental and isotopic composition in the surface ocean. Key Points: • We observed spatial variations of δ 66 Zn in the deep water of the NWPO and found considerably lighter δ 66 Zn than the NEPO • The sedimentary input may be a major cause of relatively light dissolved δ 66 Zn in the deep water of the NWPO • Aerosol Zn input is a major cause of the variations of [Zn] and δ 66 Zn in the surface water of the NWPO Supporting Information: • Supporting Information S1

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