The relationship between zinc, its isotopes, and the major nutrients in the North-East Pacific

Abstract

Trace metal micronutrients play key roles in photosynthesis by oceanic phytoplankton. Though they are required in much smaller amounts than the major nutrients (P, N, Si), their bioavailable forms are also present in the seawater solution at much lower levels. Relationships between the dissolved chemistry of the nutrient-type trace metals, their stable isotope variations, as well those of the major nutrients, have highlighted the importance of biological and physical processes in the Southern Ocean in controlling their oceanic biogeochemistry. However, the first-order Southern Ocean processes are overprinted by vertical cycling in other parts of the ocean, particularly upwelling regions remote from the Southern Ocean, with the North Pacific standing out in particular. Here we present new zinc (Zn) concentration and isotope, as well as major nutrient data for the NE Pacific, and couple these new data with a compilation of published data from across the region, with the objective of better understanding the impact of this important region on oceanic biogeochemical cycles.The new Zn isotope data for two stations along Line P (P04 and P26) show a large range in d 66 Zn in the upper ocean (-0.4‰ up to >1‰), associated with a very small isotope fractionation but extreme depletion of the dissolved pool during photic zone biological uptake, and the regeneration of this cellular Zn at very shallow depths (50m). Beneath this, the two profiles approach the d 66 Zn value of +0.5‰, seen throughout the deep ocean, by about 500m. The minimum d 66 Zn resulting from regeneration is associated with very high Zn concentrations, particularly at the marginal P04 station where diatoms dominate the phytoplankton ecology. Combining the new data with published Zn and major nutrient concentrations from across the North Pacific emphasises the role of vertical biological cycling in controlling regional biogeochemistry in the North Pacific, resulting in the partial overprinting of biogeochemical signatures transported out of the Southern Ocean by the ocean circulation. Zinc isotope data document the uptake of this metal into diatoms and the co-regeneration of Zn with phosphate in the upper water column. Silica in contrast is regenerated at greater depth, resulting in a decoupling of the Zn-Si correlation that is set in the Southern Ocean and that dominates the Atlantic. Previous work has suggested that the decoupling of Zn and Si in the subarctic North Pacific results from removal of Zn (and other metals) to water column particulate sulphide. In our dataset, and in the compilation of data documenting relationships between Zn and the major nutrients across the North Pacific, this decoupling is clearly due to the different lengthscales of regeneration for organic matter (Zn and P) and diatom opal (Si).

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

“…Although Zn is a biologically essential trace metal, dissolved δ 66 Zn mostly exhibits an isotopically light signature in the surface ocean (Conway & John, , ), in contrast to the distribution pattern of Cd isotope composition. However, some studies reported that dissolved δ 66 Zn may exhibit insignificant fractionation or even isotopically heavy signature in oceanic surface water, including the studies observed in the SO, the Tasman Sea, the SEPO, and the subarctic Northeastern Pacific Ocean (Samanta et al, ; Sieber et al, ; Vance et al, ; Wang et al, ; Zhao et al, ). Since culture studies showed that adsorbed Zn is isotopically heavy relative to intracellular Zn, scavenging has first been proposed to be the major process causing the relatively light Zn feature in the surface water (John et al, ; John & Conway, ).…”

“…Since culture studies showed that adsorbed Zn is isotopically heavy relative to intracellular Zn, scavenging has first been proposed to be the major process causing the relatively light Zn feature in the surface water (John et al, ; John & Conway, ). A recent study using model calculation approach suggested that physical mixing may be one of the processes causing δ 66 Zn variation in the surface water of the Northeastern Pacific Ocean, which may be up to 0.3‰ (Vance et al, ). In addition, Liao and Ho () found that particulate Zn concentrations are significantly correlated with aerosol Zn fluxes in the global ocean, implying the importance of aerosol deposition on Zn cycling in the surface ocean.…”

“…In the surface water of the SO, dissolved δ 66 Zn is comparable to or slightly isotopically heavier than the values observed in the deep water (Sieber et al, ; Wang et al, ; Zhao et al, ). A recent study in the subarctic Northeastern Pacific Ocean also observed some isotopically heavy signal in the surface water (Vance et al, ). The low dissolved δ 66 Zn observations are apparently at odds with the expectation that isotopically light Zn would be preferentially taken up by phytoplankton, leaving behind heavy Zn in dissolved pool.…”

“…Spatial distribution patterns of Zn isotope composition (δ 66 Zn) would provide further insights in understanding its sources, sinks, and internal cycling processes in marine water column (Moynier et al, ). Recent studies on dissolved δ 66 Zn have provided further insight for Zn cycling in different oceanic regions, including the North Atlantic Ocean (Conway & John, ), the Southern Ocean (SO; Zhao et al, ; Sieber et al, ; Wang et al, ), the Southeastern Pacific Ocean (SEPO; John et al, ), and the Pacific Ocean (Bermin et al, ; Conway & John, ; John & Conway, ; Samanta et al, ; Vance et al, ). Several mechanisms have been proposed to explain the distribution of dissolved Zn elemental and isotopic composition in marine water column globally.…”

“…Locations of sampling stations in this study (red circles) and stations from the literature (orange circles) in the North Pacific Ocean (Station J: John and Conway (); P04 and P26: Bermin et al () and Vance et al (); SAFe and San Pedro: Conway and John ()).…”