UC Riverside

Dissolved iron is an essential micronutrient for marine phytoplankton, and its availability has controlled patterns of primary productivity and carbon cycling throughout Earth history. Iron, although abundant in the Earth's crust, is present at low concentrations in seawater today and is a limiting nutrient for phytoplankton. Aeolian (wind blown) dust (loess) is a major source of this micronutrient to the ocean, and its deposition has important implications for the global CO2 budget. In this study, I explore distributions of potentially bioavailable Fe, the soluble fraction required by phytoplankton for photosynthesis and nitrogen assimilation, in deep-sea sediments in the North and South Atlantic Oceans. I used a state-of-the-art Fe speciation technique to characterize Fe inputs from different source regions, specifically North Africa and Patagonia, to address the patterns and their implications across spatial gradients and glacial-interglacial time scales.

In many open-ocean regions, the input of new iron to the surface waters is dominated by atmospheric deposition of soluble iron in aeolian dust. Multiple records have shown dust accumulation is correlated with glacial-interglacial cycles, with glacial periods being substantially dustier. Furthermore, the delivery of aeolian dust to the North and South Atlantic Oceans are from two very different source regions and soil types. With this framework, I analyzed a total of five IODP cores from these two regions, and my preliminary data show similar patterns of iron distribution from both the North and South Atlantic Oceans. Furthermore, while total dust accumulation varies dramatically on glacial-interglacial time scales, I have found no pattern in the reactivity of the dust-associated Fe across the same interval. I have also analyzed a range of sediment grain sizes to isolate the dust-dominated fraction and found no size effects in the distribution of bioavailable iron. There is, however, a trend of decreasing ratios of highly reactive (oxide iron that is/was potentially bioavailable) to total iron (FeHR/FeT) with greater distance from the source region. This trend might reflect increased reactivity (likely through prolonged atmospheric/cloud processing) during long-range transport and subsequent loss of soluble Fe in the water column. This lost iron could have simulated primary production in the surface ocean even (or preferentially) at great distances from the source region. If correct, these data suggest lower dust fluxes but with proportionally more reactive iron with increasing distance from the source. Remaining challenges include a better understanding of the role of deep-water dust dissolution and enhanced solubility linked specifically to low oxygen conditions in the water column and sediments. The latter could be a positive feedback tied to high primary production and associated oxygen demand.