Zn Isotope Composition in the Water Column of the Northwestern Pacific Ocean: The Importance of External Sources

Abstract

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

Pervasive sources of isotopically light zinc in the North Atlantic Ocean

“…These trends illustrate the fact that light δ 66 Zn values are mostly associated with an enrichment in Zn relative to Si or NO3 -(i.e., lower macronutrient/Zn ratios). This relationship suggests that the North Atlantic δ 66 Zn distribution is influenced by the addition of isotopically light Zn, similar to the recent inference of an isotopically light external source influencing the δ 66 Zn distribution of the Northwest Pacific (Liao et al, 2020). In Figure 7, Zn-poor upper-ocean samples, which plot towards the bottom left corner of each plot, tend to be more strongly affected, appearing to have both a stronger relative source (at lower macronutrient and Zn concentrations) and lower δ 66 Zn values.…”

Advantages of zinc isotopes as a new dietary proxy in marine ecology

“…The low geographical  66 Zn variability in P. hispida and U. maritimus bones implies that Arctic food-web baseline and/or low trophic level consumer  66 Zn values are more homogenous than for  15 N and  13 C values. This is remarkable considering the large surface water's isotopic variability observed for dissolved Zn across the Atlantic and Pacific of −1.1 to +0.9 ‰ and −0.9 to +0.2 ‰, respectively 33,42 .…”

“…Bulk marine Zn is enriched in 66 Zn relative to its major inputs from rivers and aeolian dust, which centre on the global crustal average of +0.3 ‰ 37 . Although most studies on cultured phytoplankton demonstrate a preferential uptake of light Zn into the cell relative to the bulk growth medium 38,39 , Atlantic and Pacific vertical Zn isotope profiles generally show lower  66 Zn values in surficial waters compared to that of the deep water 33,36,40,41,42 . These studies demonstrate that the isotopic composition of Zn is most variable within the surface water (< 500 m), often with higher values in the uppermost surface (< 20 m).…”

Bioactive trace metals and their isotopes as paleoproductivity proxies: An assessment using GEOTRACES-era data

“…The deep ocean is isotopically homogeneous, with a δ 66 Zn signature of about +0.45‰ (Fig. 6.;Conway & John, 2014b;Zhao et al, 2014;Conway & John, 2015a;Samanta et al, 2017;Takano et al, 2017;John et al, 2018b;Wang et al, 2019a;Vance et al, 2019;Sieber et al, 2020;Liao et al, 2020;Lemaitre et al, 2020), and isotopically heavier than the upper continental crust (UCC; δ 66 Zn +0.3 ‰; Moynier et al, 2017). Deviations to deep ocean δ 66 Zn compositions as light as -0.2 ‰ have been observed near sediments or hydrothermal Zn sources Lemaitre et al, 2020).…”

“…Elevated Zn in the deep Pacific has also been attributed to additional Zn input via, e.g., hydrothermalism , with a lighter than deep ocean δ 66 Zn signature (John et al, 2018b). Local and basin scale deviations towards lighter deep ocean δ 66 Zn compositions in both the Atlantic and Pacific have been attributed to sedimentary input (Conway & John, 2014b;John et al, 2017;Liao et al, 2020;Lemaitre et al, 2020), and could also reflect hydrothermalism (Conway & John, 2014b;John et al, 2018b;Lemaitre et al, 2020). Anthropogenic aerosol deposition is thought to supply significant Zn to regions of the surface ocean (e.g., Liao & Ho, 2018), with possible direct and indirect (via scavenging) regional impacts on upper ocean δ Zn values (Liao et al, 2020;Lemaitre et al, 2020).…”