Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs

Abstract

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...

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

“…This result is contradictious to the calculations of Duprat et al (2016). They found an increased phytoplankton concentration within the iron-containing meltwater trail of the giant Antarctica icebergs.…”

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

“…Downstream of island plateaus, such as those at Kerguelen (Blain et al, 2001(Blain et al, , 2007, Crozet ice melt (van der Merwe et al, 2011), glacial runoff (Hawkings et al, 2014;van der Merwe et al, 2015) and melting icebergs (Duprat et al, 2016). Primary production downstream of subantarctic island systems is typically dominated by diatoms (Smetacek et al, 2004;Quéguiner, 2013), a microphytoplankton group generally considered to promote carbon export out of the mixed layer directly by sinking (Smetacek, 1985;Newton, 1995, 1999) or indirectly by sustaining faecal pellet-productive food webs (Smetacek et al, 2004;Manno et al, 2014).…”

Physical and chemical evolution of dissolved organic matter across the ablation season on a glacier in the central Tibetan Plateau

“…33 Hood et al (2009) observed that the high bioavailability of dissolved organic matter (DOM) in 34 glacial streams had a high impact on marine microorganisms in the coastal zone. Additionally, 35 significantly increased chlorophyll levels have been observed around icebergs during the melting 36 of giant icebergs (Duprat et al, 2016), which suggests that the DOM in icebergs can influence 37 microbial activities and can have a further impact on the net primary productivity in the ocean 38 (Wu and Hou, 2017). Due to the high abundances and large degree of chemical diversity of 39 DOM in global glacial streams (e.g., C, N, P, and S) (Fegel et The Tibetan Plateau (TP) is referred to as the "Water Tower of Asia", as it is the source of the 43 ten largest rivers in Asia, which feed more than 1 billion people.…”