An investigation into the exchange of iron and zinc between soluble, colloidal, and particulate size-fractions in shelf waters using low-abundance isotopes as tracers in shipboard incubation experiments

Abstract

A vertical mixing event was simulated in shipboard incubation experiments on the mid-continental shelf of the eastern Bering Sea to investigate Fe and Zn cycling between the soluble (< 0.03 μm or 200 kDa), colloidal (0.03–0.2 μm), and particulate (0.2–10 μm, > 10 μm) size-fractions. The particulate Fe and Zn were further separated into chemically labile (25% acetic acid-leachable) and refractory pools. The experiment employed 57Fe (+ 0.90 nM) and 68Zn (+ 0.99 nM) as stable, low-abundance isotope amendments to the soluble fraction, and the exchange of Fe and Zn between the different physico-chemical fractions was measured using high resolution-inductively coupled plasma-mass spectrometry (HR-ICP-MS). More than 50% of the added 57Fe partitioned to the colloidal fraction within 45 min of adding the tracer. Both the 57Fe and 56Fe colloidal fraction were removed from the dissolved phase at a faster rate than the soluble Fe fraction. In contrast, the colloidal 66Zn and 68Zn concentrations remained constant over the 5-day experiment, suggesting a unique removal mechanism for colloidal Fe. The net removal of dissolved 57Fe was observed to be 3 to 4 times more rapid than dissolved 56Fe, which can be attributed to the regeneration of particulate Fe. Using a simple first-order model, it was determined that the net removal of 2.0 nM of dissolved Fe during the experiment was a consequence of dynamic cycling, whereby 2.9 nM of particulate Fe was regenerated and contributed to an overall removal of 4.9 nM of Fe from the dissolved phase. The amended 68Zn tracer resided in the soluble fraction and was assimilated by the diatom biomass (> 10 μm size-fraction) at the same rate as 66Zn. This similarity in rates suggests that nearly all of the net removal of Zn was due to assimilation and that regeneration did not play a significant role in Zn cycling within the incubation experiment. This research demonstrates the advantage of using low-abundance isotopes as tracers and the importance of particulate and colloidal Fe in the overall biogeochemical cycling of Fe in ocean surface waters.