Thorium isotopes tracing the iron cycle at the Hawaii Ocean Time-series Station ALOHA
Thorium isotopes tracing the iron cycle at the Hawaii Ocean Time-series Station ALOHA
Date
2015-07
Authors
Hayes, Christopher T.
Fitzsimmons, Jessica N.
Boyle, Edward A.
McGee, David
Anderson, Robert F.
Weisend, Rachel
Morton, Peter L.
Fitzsimmons, Jessica N.
Boyle, Edward A.
McGee, David
Anderson, Robert F.
Weisend, Rachel
Morton, Peter L.
Linked Authors
Person
Person
Person
Person
Person
Alternative Title
Citable URI
As Published
Date Created
Location
DOI
Related Materials
Replaces
Replaced By
Keywords
Abstract
The role of iron as a limiting micronutrient motivates an effort to understand the supply and
removal of lithogenic trace metals in the ocean. The long-lived thorium isotopes (232 Th and
230 Th) in seawater can be used to quantify the input of lithogenic metals attributable to the partial
dissolution of aerosol dust. Thus, Th can help in disentangling the Fe cycle by providing an
estimate of its ultimate supply and turnover rate. Here we present time-series (1994-2014) data
on thorium isotopes and iron concentrations in seawater from the Hawaii Ocean Time-series
Station ALOHA. By comparing Th-based dissolved Fe fluxes with measured dissolved Fe
inventories, we derive Fe residence times of 6-12 months for the surface ocean. Therefore, Fe
inventories in the surface ocean are sensitive to seasonal changes in dust input. Ultrafiltration
results further reveal that Th has a much lower colloidal content than Fe does, despite a common
source. On this basis, we suggest Fe colloids may be predominantly organic in composition, at
least at Station ALOHA. In the deep ocean (>2 km), Fe approaches a solubility limit while Th,
surprisingly, is continually leached from lithogenic particles. This distinction has implications
for the relevance of Fe ligand availability in the deep ocean, but also suggests Th is not a good
tracer for Fe in deep waters. While uncovering divergent behavior of these elements in the water
column, this study finds that dissolved Th flux is a suitable proxy for the supply of Fe from dust
in the remote surface ocean.
Description
Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 169 (2015): 1-16, doi:10.1016/j.gca.2015.07.019.