A test of the relative importance of iron fertilization from aeolian dust and volcanic ash in the stratified high-nitrate low-chlorophyll subarctic Pacific Ocean

Abstract

To test the biological effectiveness of iron fertilization by Asian aeolian dust and volcanic ash in the stratified high-nitrate, low-chlorophyll subarctic Pacific Ocean over long-time scales, we analysed sediments with ages between 2.8 and 0.9 Ma at ODP Site 885/886. This site lies far from the influence of other iron sources (e.g. icebergs or lateral iron transportation) and the water column has been stratified since intensification of northern hemisphere glaciation (iNHG) at ∼2.7 Ma, which limits nutrient upwelling and makes it an ideal location for testing iron bioavailability of different atmospheric sources that fell onto the surface ocean. We investigate diatom assemblages and opal export changes, and compare them with the properties of iron minerals in Asian aeolian dust and volcanic ash. Our results reveal that Asian aeolian dust contains mostly unreactive iron-bearing minerals that supplied limited bioavailable iron for diatom growth at Site 885/886. Diatom productivity changes were dominated by upwelling/stratification changes over long timescales, and by sporadic iron fertilization from volcanic ash inputs. Iron from volcanic ash particles is likely to have become bioavailable because such particles dissolved more readily due to their high porosity and ultra-fine size.