The Southern Ocean is a significant sink in the ocean component of the global carbon cycle, contributing ~ 10% of the ocean's total carbon sequestration through a mixture of chemical and biologically-driven processes 5 . However, its contribution is at a lower level than that of the smaller South Pacific and Indian Oceans 5 , due to its low concentration of dissolved iron, an important trace nutrient for primary production 6 . Atmospheric dust is a major background source of iron to the region 7 , but iron-rich sediment fluxes from islands 8 , continental shelves 9 , ice sheet meltwater 10 and melting icebergs 1 are known to be other, locally much more important, sources of iron. There are a few large-scale estimates of the contribution of icebergs to the Southern Ocean iron flux, derived from modelling studies of typical sub-kilometre sized icebergs 11,12 , scaling up of observational studies 13,14 or remote sensing studies 2 . However, these assume iceberg inputs are well represented by those from the smaller, sub-kilometre, peak in the very bimodal size distribution 15 . In fact about half the total iceberg discharge volume is made up of giant icebergs 15 -those exceeding 18 km in horizontal dimension -and there have currently only been two observational studies of the phytoplankton blooms close to individual giant icebergs, both in conditions within or near sea-ice cover in the Weddell Sea 1, 3 . Such areas may be subject to enhanced productivity due to the impact of sea-ice fertilization 16 . While the calving of giant icebergs is very episodic 15 , they derive from a range of geographical and geological environments around Antarctica, and thus likely have different iron and nutrient characteristics. Several dozen such icebergs are present in the Southern Ocean at any one time 15 , and they can survive for many years. Even when in areas of open water giant icebergs can survive for longer than a year 17 . Here we examine the chlorophyll signature from a range of giant icebergs in the open Southern Ocean using remote sensing, to show that ocean fertilization from such icebergs is much larger than previously suspected.Chlorophyll levels are well known to be raised near icebergs 1,2,18 . This derives from the meltwater plumes from icebergs containing significant concentrations of iron, but also a range of other nutrients 14 . As the Southern Ocean is a High Nutrient Low Chlorophyll (HNLC) region 6 , it is the bioavailable iron known to be in nanoparticle aggregates of ferrihydrite and goethite in iceberg sediments 13 that is the key nutrient within this meltwater. Dissolution of these particles leads to enriched concentrations of dissolved iron in the meltwater plume at levels 10-1000 times those due to atmospheric dust 19 . Ship-based studies have demonstrated that, for an iceberg of maximum horizontal size L i , chlorophyll levels are enhanced downstream over a distance of ~ L i 20 . Similarly, it has been shown using SeaWifs ocean colour that the probability of chlorophyll being enhanced 6 days after an iceb...

Antarctic sea ice is an important temporal reservoir of iron which can boost primary production in the marginal ice zone during the seasonal melt. While studies have reported that Antarctic fast ice bears high concentrations of iron due to the proximity to coastal sources, less clear are the biogeochemical changes this iron pool undergoes during late spring. Here we describe a 3-week time series of physical and biogeochemical data, including iron, from first-year coastal fast ice sampled near Davis Station (Prydz Bay, East Antarctica) during late austral spring 2015. Our study shows that dissolved and particulate iron concentrations in sea ice were up to two orders of magnitude higher than in under-ice seawater. Furthermore, our results indicate a significant contribution of lithogenic iron from the Vestfold Hills (as deduced from the comparison with crustal element ratios) to the particulate iron pool in fast ice after a blizzard event halfway through the time series. Windblown dust represented approximately 75% of the particulate iron found in the ice and is a potential candidate for keeping concentrations of soluble iron stable during our observations. These results suggest that iron entrapped during ice formation, likely from sediments, as well as local input of coastal dust, supports primary productivity in Davis fast ice. As ice-free land areas are likely to expand over the course of the century, this work highlights the need to quantify iron inputs from continental Antarctic dust and its bioavailability for ice algae and phytoplankton. Plain Language SummaryOceanic single-celled algae are the base of the ocean food web and play an important role in the Earth climate. In the Southern Ocean, the growth of these microorganisms is limited by the naturally low concentration of iron in the seawater. Microalgae benefit from the presence of the Antarctic sea ice since iron is highly concentrated in sea ice relative to the seawater. Less clear though is the contribution of the potential sources of iron to the sea ice. We collected and analyzed sea ice cores for a series of parameters, including iron, from first-year coastal sea ice sampled near Davis Station (Prydz Bay, East Antarctica) during late austral spring 2015. Our results suggest that iron entrapped during ice formation, likely from seafloor sediments, as well as dust blown by winds from the neighboring Vestfold Hills, are the main sources of iron to Davis coastal sea ice. Since we can expect the expansion of ice-free areas and exposed grounds over the course of this century, our results highlight the need to quantify the amount of iron coming from continental Antarctic dust and to access if microalgae can use this form of iron for their basic physiological needs.

Phytoplankton growth can be limited by the availability of the essential nutrient iron (Fe; Baar et al., 1990, 1995). The Southern Ocean (SO) is a classic example where high concentrations of macronutrients (nitrate, phosphate, and silicic acid) do not support the expected level of primary production due to low Fe concentrations. The SO has therefore been designated as a High Nutrient Low Chlorophyll (HNLC) area (Martin, 1990). In this context, sea ice plays a pivotal role as a natural and biogeochemically active Fe reservoir due to the high levels of Fe and organic matter concentrated from seawater during its formation

Iron (Fe) is an essential micronutrient to oceanic microalgae, and its dissolved fraction (DFe) is retained in surface waters by Fe-binding ligands. Previous work has suggested that ligands may also bind Fe within sea ice, although supporting data are limited. This study investigates distribution, concentration, and potential drivers of Fe-binding ligands in Antarctic sea ice, considering the ice type, location and season. Results suggest that the concentration of ligands (CL) varies throughout the year, both spatially and seasonally. The lowest CL (3.3–8.0 nM) and DFe concentrations (0.7–3.5 nM) were recorded in newly formed winter sea ice in the Weddell Sea, likely due to the early stage of sea-ice growth and low biological activity. The highest CL (1.7–74.6 nM), which follows the distribution of DFe (1.0–75.5 nM), was observed during springtime, in the Eastern Antarctic Sector. There, consistently higher values for CL in bottom ice depths were likely associated with enhanced algal biomass, while aeolian deposition may have acted as an additional source of DFe and ligands near Davis station. In summer, the senescence of ice algae and advanced sea-ice melting led to intermediate CL (1.0–21.9 nM) and DFe concentrations (0.6–13.3 nM) both on and off the East Antarctic coast. Regardless of time and location, >99% of DFe was complexed, suggesting that CL controls the distribution of DFe in sea ice. This study represents a first attempt at a year-round investigation of CL in sea ice, providing results that support the premise that sea ice acts as a potential biogeochemical bridge between autumn and spring phytoplankton blooms.

© 2 0 1 6 M a c m i l l a n P u b l i s h e r s L i m i t e d , p a r t o f S p r i n g e r N a t u r e . A l l r i g h t s r e s e r v e d .