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

Authors

Tim M. Conway, University of South CarolinaFollow
Seth G. John, University of South Carolina

Document Type

Article

Publication Date

9-2015

Digital Object Identifier (DOI)

https://doi.org/10.1016/j.gca.2015.05.023

Abstract

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 (δ⁵⁶Fe, δ⁶⁶Zn, and δ¹¹⁴Cd) 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 δ⁶⁶Zn and δ⁵⁶Fe from the open-ocean North Pacific, and the first observations of dissolved δ⁶⁶Zn and δ¹¹⁴Cd in a low-oxygen marginal basin. At the SAFe station, δ⁵⁶Fe 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 δ⁵⁶Fe 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 δ⁵⁶Fe 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, δ¹¹⁴Cd 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 δ¹¹⁴Cd, δ⁶⁶Zn at SAFe decreases towards the surface ocean, perhaps due to scavenging of isotopically heavy Zn, while at intermediate depths δ⁶⁶Zn provides further evidence of a mid-depth dissolved δ⁶⁶Zn 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 (δ⁶⁶Zn 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 δ⁵⁶Fe 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.

Was this content written or created while at USF?

No

Citation / Publisher Attribution

Geochimica et Cosmochimica Acta, v. 164, p. 262-283

Scholar Commons Citation

Conway, Tim M. and John, Seth G., "The Cycling of Iron, Zinc and Cadmium in the North East Pacific Ocean – Insights from Stable Isotopes" (2015). School of Geosciences Faculty and Staff Publications. 1504.
https://digitalcommons.usf.edu/geo_facpub/1504