Zn is an essential biological nutrient in the oceans, and its concentration is largely controlled by uptake by phytoplankton and remineralization. In addition to its critical role in many metalloenzymes (see Zinc Biology section), Zn is an important component of the carbonate shells of foraminifera and siliceous frustules in diatoms. The main inputs of Zn to the ocean are thought to be from rivers and dust. In some photic zones in the ocean, Zn is a limiting nutrient for phytoplankton, and thus its concentration in surface waters serves as one control on marine primary productivity. Zn concentrations are extremely low in the surface ocean (<0.1 nM) but are maximal at depth (~2 nM in the deep Atlantic; ~10 nM in the deep Pacific), indicating a deep regeneration cycle. The deep/surface ratio of Zn is typically on the order of 100, significantly larger than that observed for Cu.
A multitude of complex processes fractionate Zn isotopes in the marine environment. As seen with copper isotopes, the bulk isotopic composition of zinc in the oceans (δZn = +0.5‰) is heavier than that of the riverine input (δZn = +0.3‰), reflecting both equilibrium, biological and other processes that affect Zn isotope ratios in the ocean. In the surface ocean, phytoplankton preferentially uptake Zn, and as a result have average δZn values of ~+0.16‰ (i.e., 0.34‰ lighter than the bulk ocean). This preferential removal of Zn by photosynthetic marine organisms in the photic zone is most prominent in the spring and summer seasons when primary productivity is highest, and the seasonal variability of Zn isotope ratios is reflected in the δZn values of settling materials, which are heavier (e.g., by ~+0.20‰ in the Atlantic Ocean) during spring and summer than during the colder seasons. Additionally, the surface layers of FeMn-nodules are Zn enriched at high-latitudes (average δZn = +1‰), while δZn values of low-latitude samples are smaller and more variable (spanning +0.5 to +1‰). This observation has been interpreted as due to high levels of Zn consumption and preferential uptake of Zn above the seasonal thermocline at high latitudes during warmer seasons, and transfer of this heavy δZn signal to the settling sedimentary Fe-Mn hydroxides.
Sources and sinks for Zn isotopes are further highlighted in the vertical profile of Zn/Zn in the water column. In the upper 2,000 m of the Atlantic Ocean, δZn values are highly variable near the surface (δZn = +0.05 to +0.33‰) due to biological uptake and other surface processes, then gradually increase to ~+0.50‰ at 2,000 m depth. Potential sinks for light Zn isotopes, which enrich the residual bulk Zn isotope ratios in the ocean, include binding to and burial with sinking particulate matter, as well as Zn sulfide precipitation in buried sediments. As a result of preferential burial of Zn over the heavier Zn isotopes, sediments in the ocean are generally isotopically lighter than that of bulk seawater. For example, δZn values in 8 sedimentary cores from three different continental margins were depleted in Zn relative to the bulk ocean (δZn = -0.15 to +0.2‰), and furthermore the vertical profiles of δZn values in the cores showed no downcore isotopic variability, suggesting diagenesis does not significantly fractionate Zn isotopes.