Presenter: Nicholas Hawco (University of Hawaiʻi at Mānoa)

Description:
We present a fully resolved Fe budget at Station ALOHA in the North Pacific Subtropical Gyre during June 2019, based on measurements from the water column, sediment traps, and incubations to constrain dissolved Fe uptake. Using low levels of ⁵⁷Fe–⁵⁸Fe double spike, measured with high precision by multi-collector inductively coupled plasma mass spectrometry, we find dFe turnover timescales on the order of 10–15 days in the mixed layer and 1–3 days near the deep chlorophyll maximum. With respect to external inputs, the dissolved Fe residence time is estimated to be 5 months in the upper 150 m while the particulate Fe residence time is 3–5 days. These descriptions agree with a thorium isotope mass balance conducted on the same cruise that tracks the input of dust. At Station ALOHA, the average iron atom may be recycled as many as 30 times before sinking below the euphotic zone (an ‘Fe-ratio’ equal to 0.03), a level of conservation that has only been documented in Fe-limited regions thus far. In contrast to current conceptual models, these results suggest that oligotrophic ecosystems are capable of recycling iron efficiently even when communities are not iron-limited.