Ironing Out Fe Residence Time in the Dynamic Upper Ocean

Type Article
Date 2020-09
Language English
Author(s) Black Ee1, 2, 3, Kienast Ss2, Lemaitre N4, Lam Pj5, Anderson Rf3, Planquette Helene6, Planchon Frederic6, Buesseler Ko1
Affiliation(s) 1 : Department of Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, 266 Woods Hole Road, MS25, Woods Hole, MA, 02543, USA.
2 : Department of Oceanography, Dalhousie University, 1355 Oxford St, PO Box 15000, Halifax, NS, B3H 4R2, Canada
3 : Division of Geochemistry, Lamont Doherty Earth Observatory, 61 Route 9W, PO Box 1000, Palisades, NY, 10964, USA
4 : Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH-Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
5 : Department of Ocean Sciences, University of California, Santa Cruz, CA, 95064, USA
6 : Univ Brest, CNRS, IRD, Ifremer, LEMAR, F-29280 Plouzané, France
Source Global Biogeochemical Cycles (0886-6236) (American Geophysical Union), 2020-09 , Vol. 34 , N. 9 , P. e2020GB006592 (17p.)
DOI 10.1029/2020GB006592
WOS© Times Cited 6
Keyword(s) thorium-234</AUTHOR_KEYWORD>, iron</AUTHOR_KEYWORD>, export</AUTHOR_KEYWORD>, GEOTRACES</AUTHOR_KEYWORD>, residence time</AUTHOR_KEYWORD>
Abstract

Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234Th‐ and sediment‐trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation‐based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0‐250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub‐annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond.

Plain Language Summary

Iron is a key micronutrient for organisms living in the upper ocean and thus, its availability is one of the key factors controlling the removal of carbon dioxide via phytoplankton growth in much of the global ocean. Until very recently, measurements of internal iron cycling were scarce. This includes estimates of how much iron leaves the surface ocean via sinking particles. Due to the lack of observations, models struggle to reproduce observed patterns in global surface iron distributions. For the first time, we constrain the rate of iron loss from the upper ocean along three basin‐wide transects and bring together all preexisting estimates to determine the timescales on which different forms of iron are retained in the upper ocean. Overall, our findings suggest that iron cycles more rapidly between the surface and the subsurface ocean than previously estimated and we encourage the modeling community to utilize the wealth of data presented here to explore the global consequences of these findings.

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