Ironing Out Fe Residence Time in the Dynamic Upper Ocean

• Published in: Global biogeochemical cycles 2020-09, Vol.34 (9), p.n/a
• Main Authors: Black, E. E., Kienast, S. S., Lemaitre, N., Lam, P. J., Anderson, R. F., Planquette, H., Planchon, F., Buesseler, K. O.
• Format: Article
• Language: English
• Subjects: Annual; Availability; Biodiversity and Ecology; Biogeochemistry; Carbon; Carbon cycle; Carbon dioxide; Carbon dioxide fixation; Carbon dioxide removal; Carbon sequestration; Climate models; Climate prediction; Core loss; Cycles; Drawing and ironing; Earth Sciences; Environmental Sciences; Estimates; export; Geochemistry; GEOTRACES; Iron; Ocean models; Ocean, Atmosphere; Oceanography; Oceans; Particulate flux; Phytoplankton; Removal; Residence time; Residence time distribution; Sciences of the Universe; Thorium; thorium‐234; Upper ocean
• ISSN: 0886-6236; 1944-9224; 1944-8224
• DOI: 10.1029/2020GB006592

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. Key Points The majority of the residence times determined for total Fe in the upper 250 m fall between 10 and 100 days Export‐based results suggest that dissolved Fe cycles sub‐annually to annually in the upper ocean, typically faster than previously thought