The Cycling of Iron, Zinc and Cadmium in the North East Pacific Ocean – Insights from Stable Isotopes

Dissolved stable isotope ratios of the transition metals provide useful information, both for understanding the cycling of these bioactive trace elements through the oceans, and tracing their marine sources and sinks. Here, we present seawater dissolved Fe, Zn and Cd concentration and stable isotope ratio (δ ⁵⁶ Fe, δ ⁶⁶ Zn, and δ ¹¹⁴ Cd) profiles from two stations in the Pacific Ocean, the SAFe Station (30°N 140°W) in the subtropical North East Pacific from the GEOTRACES IC2 cruise, and the marginal San Pedro Basin (33.8°N 118.4°W) within the South California Bight. These data represent, to our knowledge, the first full-water column profiles for δ ⁶⁶ Zn and δ ⁵⁶ Fe from the open-ocean North Pacific, and the first observations of dissolved δ ⁶⁶ Zn and δ ¹¹⁴ Cd in a low-oxygen marginal basin. At the SAFe station, δ ⁵⁶ Fe is isotopically lighter throughout the water column (−0.6 to +0.1‰, relative to IRRM-014) compared to the North Atlantic, suggesting significant differences in Fe sources or Fe cycling between these two ocean basins. A broad minimum in δ ⁵⁶ Fe associated with the North Pacific oxygen minimum zone (OMZ; <75 μmol kg ⁻¹ dissolved oxygen; ∼550–2000 m depth) is consistent with reductive sediments along the California margin being an important source of dissolved Fe to the North Pacific. Other processes which may influence δ ⁵⁶ Fe at SAFe include biological cycling in the upper ocean, and input of Fe from hydrothermal vents and oxic sediments below the OMZ. Zn and Cd concentration profiles at both stations broadly match the distribution of the macronutrients silicate and phosphate, respectively. At SAFe, δ ¹¹⁴ Cd increases towards the surface, reflecting the biological preference for assimilation of lighter Cd isotopes, while negative Cd ^∗ (−0.12) associated with low oxygen waters supports the recently proposed hypothesis of water-column CdS precipitation. In contrast to δ ¹¹⁴ Cd, δ ⁶⁶ Zn at SAFe decreases towards the surface ocean, perhaps due to scavenging of isotopically heavy Zn, while at intermediate depths δ ⁶⁶ Zn provides further evidence of a mid-depth dissolved δ ⁶⁶ Zn maximum. We suggest this may be a global feature of Zn biogeochemistry related to either regeneration of heavy adsorbed Zn, or to ZnS formation and removal within the water column. Data from San Pedro shows that anoxic sediments can be a source of isotopically light Zn to the water column (δ ⁶⁶ Zn of ∼−0.3‰ relative to JMC Lyon), though evidence of this signal is not observed being transported to SAFe. Within North Pacific Intermediate Water at SAFe (NPIW; ∼500 m) elevated Cd ^∗ and Zn ^∗ and a focused minimum in δ ⁵⁶ Fe suggest possible transport of Fe, Zn, and Cd over thousands of km from subpolar waters, meaning that NPIW may have a strong influence on the subsurface distribution of trace metals throughout the North Pacific.