Long-range transport of hydrothermal dissolved Zn in the tropical South Pacific

摘要

Zinc (Zn) is one of the essential micronutrients that can regulate oceanic primary productivity due to its central roles as a co-factor in carbonic anhydrase and alkaline phosphatase. However, the sources of dissolved Zn to the ocean are poorly understood, mainly because of the difficulties of sample collection and analysis for Zn at very low concentrations in oceanic waters. The prevailing view considers rivers as the major source of dissolved Zn to the ocean. Here we report the dissolved Zn section along a similar to 4500-km transect at approximately 15 degrees S in the tropical South Pacific Ocean along the western segment of the US GEOTRACES EPZT cruise (the GP16 transect). Dissolved Zn exhibits a substantial enrichment emanating from the hydrothermal vents at the axis of the East Pacific Rise to the central South Pacific, similar to 4000 km away. Total dissolved Zn and mantle-derived He-3 show a correlation with R-2 = 0.71 at depths 2300-2800 m along the transect. It is likely that this correlation can be improved by accounting for the non-uniformity in the non-hydrothermal background of dissolved Zn. After subtracting the non-hydrothermal Zn imprint using Zn-Si relationship in the North Pacific, the correlation between "excess" dissolved Zn and mantle-derived He-3 improves significantly (R-2 = 0.87). The correlation leads to a global hydro thermal Zn flux of 1.75 +/- 035 g mol yr(-1) that is many-fold higher than the input fluxes estimated by other studies. These results suggest that mantle-derived dissolved Zn dominates the oceanic Zn inventory and that dissolved Zn residence time is much shorter (3000 +/- 600 years) than previous input-based estimates (11,000 and 50,000 years) and more consistent with previous removal-based estimate (3000-6000 years). The shorter residence time, the dominance of hydrothermal input and the negligible contribution of dust to the oceanic dissolved Zn inventory imply that changes in the efficiency of particle-associated removal according to changes in oceanic productivity through time may change the oceanic dissolved Zn inventory rapidly. This, in turn, may lead to a divergence between Zn and the nutrients whose oceanic inventories are more affected by dust which can change dramatically on glacial/interglacial timescales. (C) 2016 Published by Elsevier B.V.