Hydrothermal venting as a globally relevant source of zinc to the oceans.

Hydrothermal venting along mid-ocean ridges exerts an important control on the chemical composition of the oceans by serving as a major source for a number of trace elements. Of these elements, zinc (Zn) has been largely neglected owing to a paucity of data coverage and the prevailing idea that dissolved Zn may be efficiently removed by rapid, near-vent precipitation and sedimentation processes. As such, Zn has historically been classified as a hydrothermally unimportant element. This is of great significance as Zn plays a key role in the productivity of the oceans due its vital requirement in phytoplankton physiology. Here we analyse and compare recent Zn data from the Mid-Atlantic Ridge (MAR; GA13) and Southeast Pacific Rise (SEPR; GP16), representing vent sites with differing geochemical settings, to highlight hydrothermal venting as a globally relevant source for Zn. By utilizing the conservative hydrothermal tracer ³He, our analysis indicates 1) the magnitude of hydrothermal Zn input is highly variable, even between MAR vent sites, 2) rapid, near-vent precipitation is the principle removal mechanism for hydrothermally derived Zn, and yet 3) dissolved Zn is stabilised and transported several hundred km off-axis from the MAR compared with several thousand km from the SEPR. Finally, we present a refinement of the oceanic Zn budget based on observations from multiple hydrothermal systems. Our findings challenge the prevailing view that considers hydrothermalism as a non-significant source of Zn to the oceans, and have important implications for the oceanic Zn cycle.