Pervasive sources of isotopically light zinc in the North Atlantic Ocean

Abstract

In this study, we report seawater dissolved zinc (Zn) concentration and isotope composition (δ66Zn) from the GEOTRACES GA01 (GEOVIDE) section in the North Atlantic. Across the transect, three subsets of samples stand out due to their isotopically light signature: those close to the Reykjanes Ridge, those close to the sediments, and those, pervasively, in the upper ocean. Similar to observations at other locations, the hydrothermal vent of the Reykjanes Ridge is responsible for the isotopically light Zn composition of the surrounding waters, with an estimated source δ66Zn of -0.42 ‰. This isotopically light Zn is then transported over a distance greater than 1000 km from the vent. Sedimentary inputs are also evident all across the trans-Atlantic section, highlighting a much more pervasive process than previously thought. These inputs of isotopically light Zn, ranging from -0.51 to +0.01 ‰, may be caused by diffusion out of Zn-rich pore waters, or by dissolution of sedimentary particles.The upper North Atlantic is dominated by low δ66Zn, a feature that has been observed in all Zn isotope datasets north of the Southern Ocean. Using macronutrient to Zn ratios to better understand modifications of preformed signatures exported from the Southern Ocean, we suggest that low upper-ocean δ66Zn results from addition of isotopically light Zn to the upper ocean, and not necessarily from removal of heavy Zn through scavenging. Though the precise source of this isotopically light upper-ocean Zn is not fully resolved, it seems possible that it is anthropogenic in origin. This view of the controls on upper-ocean Zn is fundamentally different from those put forward previously.