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Abstract Dissolved iron (dFe) plays an important role in regulating marine productivity. In high nutrient, low chlorophyll regions (>33% of the global ocean), iron is the primary growth limiting nutrient, and elsewhere iron can regulate nitrogen fixation by diazotrophs. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas or cellular Fe:C ratios. This ratio varies by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentrations in field observations. Representing Fe:C ratios within models is necessary to investigate how ocean carbon cycling will interact with perturbations to iron cycling in a changing climate. The Community Earth System Model ocean component was modified to simulate dynamic, group‐specific, phytoplankton Fe:C that varies as a function of ambient iron concentration. The simulated Fe:C ratios improve the representation of the spatial trends in the observed Fe:C ratios. The acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron, compared to a fixed Fe:C ratio. However, varying atmospheric soluble iron supply has first order impacts on global carbon and nitrogen fluxes and on nutrient limitation spatial patterns. Our results suggest that pyrogenic Fe is a significant dFe source that rivals mineral dust inputs in some regions. Changes in dust flux and iron combustion sources (anthropogenic and wildfires) will modify atmospheric Fe inputs in the future. Accounting for dynamic phytoplankton iron quotas is critical for understanding ocean biogeochemistry and projecting its response to variations in atmospheric deposition.
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Evolution of the South Pacific's Iron Cycle Over the Cenozoic
Iron (Fe) availability impacts marine primary productivity, potentially influencing the efficiency of the biological carbon pump. Stable Fe isotope analysis has emerged as a tool to understand how Fe is sourced and cycled in the water column; however its application to sediment records is complicated by overlapping isotope signatures of different sources and uncertainties in establishing chronologies. To overcome these challenges, we integrate Fe and osmium isotope measurements with multi‐element geochemical analysis and statistical modeling. We apply this approach to reconstruct the history of Fe delivery to the South Pacific from three pelagic clay sequences spanning 93 million years. Our analysis reveals five principal Fe sources—dust, distal background, two distinct hydrothermal inputs, and a magnesium‐rich volcanic ash. Initially, hydrothermal inputs dominated Fe deposition, but as the sites migrated away from their respective mid‐ocean ridges, other sources became prominent. Notably, from 66 to 40 million years ago (Ma), distal background Fe was the primary source before a shift to increasing dust dominance around 30 Ma. This transition implies that Fe in South Pacific seawater has been dust‐dominated since ≈30 Ma, despite extremely low dust deposition rates today. We speculate that the shift to episodic and low Fe fluxes in the South Pacific and Southern Ocean over the Cenozoic helped shape an ecological niche that favored phytoplankton that adapted to these conditions, such as diatoms. Our analysis highlights how Fe delivery to the ocean is driven by large‐scale tectonic and climatic shifts, while also influencing climate through its integral role in marine phytoplankton and Earth's biogeochemical cycles.
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- PAR ID:
- 10645770
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 40
- Issue:
- 7
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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