No abstract

The micronutrient iron is thought to limit primary productivity in large regions of the global ocean 1 . Ice sheets and glaciers have been shown to deliver bioavailable iron to the coastal and open ocean in the form of sediment released from the base of icebergs 2,3 and glacially derived dust 4 . More direct measurements from glacial runoff are limited, but iron concentrations are thought to be in the nanomolar range 5 . Here we present measurements of dissolved and particulate iron concentrations in glacial meltwater from the southwest margin of the Greenland ice sheet. We report micromolar concentrations of dissolved and particulate iron. Particulate iron concentrations were on average an order of magnitude higher than those of dissolved iron, and around 50% of this particulate iron was deemed to be potentially bioavailable, on the basis of experimental leaching. If our observations are scalable to the entire ice sheet, then the annual flux of dissolved and potentially bioavailable particulate iron to the North Atlantic Ocean would be approximately 0.3 Tg. This is comparable to dust-derived soluble iron inputs to the North Atlantic. We suggest that glacial runoff serves as a significant source of bioavailable iron to surrounding coastal oceans, which is likely to increase as melting of the Greenland ice sheet escalates under climate warming.

shelf-and river-derived elements to the central Arctic Ocean • The TPD is rich in dissolved organic matter (DOM), which facilitates long-range transport of trace metals that form complexes with DOM • Margin trace element fluxes may increase with future Arctic warming due to DOM release from permafrost thaw and increasing river discharge

Shelf inputs in the Arctic Ocean appear to be increasing, which could change the nutrient balance of the central basin.

In the Southern Ocean near the Antarctic Peninsula, Antarctic Circumpolar Current (ACC) fronts interact with shelf waters facilitating lateral transport of shelf‐derived components such as iron into high‐nutrient offshore regions. To trace these shelf‐derived components and estimate lateral mixing rates of shelf water, we used naturally occurring radium isotopes. Short‐lived radium isotopes were used to quantify the rates of shelf water entrainment while Fe/228Ra ratios were used to calculate the Fe flux. In the summer of 2006 we found rapid mixing and significant lateral iron export, namely, a dissolved iron flux of 1.1 × 105 mol d−1 and total acid leachable iron flux of 1.1 × 106 mol d−1 all of which is transported in the mixed layer from the shelf region offshore. This dissolved iron flux is significant, especially considering that the bloom observed in the offshore region (0.5–2 mg chl a m−3) had an iron demand of 1.1 to 4 × 105 mol Fe. Net vertical export fluxes of particulate Fe derived from 234Th/238U disequilibrium and Fe/234Th ratios accounted for only about 25% of the dissolved iron flux. On the other hand, vertical upward mixing of iron rich deeper waters provided only 7% of the lateral dissolved iron flux. We found that similarly to other studies in iron‐fertilized regions of the Southern Ocean, lateral fluxes overwhelm vertical inputs and vertical export from the water column and support significant phytoplankton blooms in the offshore regions of the Drake Passage.