Antarctic ice sheet fertilises the Southern Ocean

Abstract

Abstract. Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07–0.2 Tg yr−1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling.

Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs

“…Given our study's implications of an area of influence for giant icebergs of more than an order of magnitude above that of "typical" icebergs, and that approximately half of the iceberg discharge is as giant icebergs 15 , with several dozen giant icebergs present in the Southern Ocean at any one time 4 , these model calculations of iceberg productivity are likely to be a significant underestimate. This conclusion is supported by another modelling study which concentrated on glacial meltwater and higher iceberg fluxes, but did not include shelf sediment iron fluxes 12 .…”

“…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 .…”

“…Recently, concern over the stability of the West Antarctic Ice Sheet has arisen 4,23 , with implications for more ice discharge in the future and thus carbon drawdown through fertilization. Note that even an increase in regional sediment-rich ice sheet meltwater into coastal waters can lead to enhanced fertilization 10,12,24 , although that associated with giant iceberg melting may be even greater (Figure 3). The future may therefore see an increase in Southern Ocean carbon sequestration through this iceberg fertilization mechanism, acting as a secondary negative feedback on climate change.…”

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…Photochemistry by sunshine is the main trigger of the transformation of the primary insoluble-iron fraction of dust aerosols into soluble iron salts (Johnson and Meskhidze, 2013), and the understanding of how the different iron content and speciation in aerosols affect the climate is growing (Al-Abadleh, 2015). Currently, increased subglacial meltwater and icebergs may supply large amounts of bioavailable iron to the Southern Ocean (Death et al, 2014). The flux of bioavailable iron associated with glacial runoff is estimated at 0.40-2.54 Tg yr −1 in Greenland and 0.06-0.17 Tg yr −1 in Antarctica (Hawkings et al, 2014), values which are comparable with eolian dust fluxes to the oceans surrounding Antarctica and Greenland and will increase by enhanced melting in a warming climate.…”

Lakes under the ice: Antarctica’s secret garden

“…This process could provide an important source of nutrients to the chronically iron-starved ecosystems in the Southern Ocean 5 , says Martyn Tranter, a marine biogeochemist the University of Bristol, UK.…”