Limited atmospheric iron availability increase during the Younger Dryas in the Northern Hemisphere

Iron (Fe) availability modulates phytoplankton blooms in High-Nutrient Low-Chlorophyll (HNLC) regions, i.e., ocean areas characterized by an abundance of major nutrients but low marine productivity. Fe can be delivered to the oceans through atmospheric dust deposition, making ice cores unique archives for reconstructing past changes in aeolian Fe deposition. However, while it is known that during dustier periods atmospheric Fe depositions increased, uncertainties remain regarding the fraction of Fe actually available to phytoplankton. Here, we present evidence from the EGRIP ice core (Greenland) during the Pleistocene-Holocene transition (10.3–13.0 ka), which provides insights into atmospheric aerosol deposition over the Fe-limited North Pacific Ocean. Results show that, in contrast to the 17-fold enhancement in total Fe determined by Inductively Coupled Plasma Mass Spectrometry, the iron fraction more likely available for phytoplankton (i.e., labile iron) increased only modestly (+29 %) during the Younger Dryas compared to the Early Holocene, likely due to prevailing alkaline aerosol conditions reducing its solubility. This finding supports the hypothesis that factors other than atmospheric Fe deposition (e.g., stronger water stratification, sea-ice extent, volcanic eruptions, iron remobilization from sediments), played a more relevant role in regulating marine net primary productivity in the HNLC North Pacific Ocean over the last glacial transition.