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82 SCIENCE HIGHLIGHTS:

Iron fertilization in the glacial ocean Alfredo Martínez-García1 and Gisela Winckler2

We review the hypothesis that the increased supply of -bearing dust to high-nutrient, low-chlorophyll regions of the ocean stimulated blooms that sequestered climatically relevant amounts of carbon and contributed

to lowering atmospheric CO2 during ice ages. The concentration of major inorganic nu- and organic carbon export to the subsurface proposed that the enhanced supply of Fe trients and is perennially ocean, and therefore could influence the from dust could have stimulated marine pro- high in one quarter of the surface ocean. efficiency of the global biological pump in ductivity during ice ages, increasing carbon

However, despite this excess in macronu- sequestering atmospheric CO2 (Martin and sequestration in the deep ocean and leading

trients available for phytoplankton growth, Fitzwater 1988). to a decrease in atmospheric CO2 levels production still remains fairly low in (Martin 1990). the Southern Ocean, the equatorial Pacific Iron fertilization in the Southern Ocean and the subarctic North Pacific. The Southern Ocean is the largest HNLC Iron enrichment experiments region (Fig. 1) and represents the area of Since the seminal work of Martin, the effect The causes of the incomplete use of macro- the ocean where variations in iron availabil- of iron on structure and nutrients by marine organisms and hence ity can have the largest impact on Earth’s has been extensively studied of the existence of these high-nutrient, carbon cycle through its fertilizing effect on through a series of short-term in situ iron low-chlorophyll (HNLC) regions have been marine ecosystems. In 1984, several years enrichment experiments performed in the subject to intense oceanographic research before Martin’s discovery of iron limitation, a different HNLC regions of the ocean, but in the past decades. In the late eighties John series of papers postulated the potential role also by observing phytoplankton blooms H. Martin and his team reported evidence of the Southern Ocean in driving changes induced by natural iron fertilization (Fig. 1).

suggesting that phytoplankton growth in in atmospheric CO2 concentrations during these regions was chronically limited by ice ages (Knox and McElroy 1984; Sarmiento Artificial iron fertilization experiments have iron deficiency (Martin and Fitzwater 1988; and Toggweiler 1984; Siegenthaler and provided unambiguous evidence that iron Martin et al. 1990). Martin immediately real- Wenk 1984). Combining these ideas with his addition generates phytoplankton blooms in ized that one important consequence of this discovery of iron limitation in the modern HNLC regions (Boyd et al. 2007; Smetacek et discovery was that changes in the availability Southern Ocean, and the first observa- al. 2012). These findings are consistent with of Fe in HNLC regions could have large ef- tions of increased glacial dust deposition the observations of natural phytoplankton fects on marine phytoplankton productivity recorded in Antarctic ice cores, Martin blooms in these regions stimulated by Fe

Figure 1: World ocean surface nitrate concentrations from the World Ocean Atlas (Garcia et al. 2010) indicating the three high-nutrient, low-chlorophyll (HNLC) areas (Southern Ocean, equatorial Pacific, Subarctic North Pacific).T he gray circle marks the location of ODP Site 1090 and the stars mark locations where iron fertilization experiments (white stars) and iron enrichment experiments (gray stars) have been performed (Blain et al. 2007; Boyd et al. 2007; Smetacek et al. 2012).

PAGES MAGAZINE ∙ VOLUME 22 ∙ NO 2 ∙ October 2014 SCIENCE HIGHLIGHTS: Dust 83 input from different sources including conti- nental dust (Cassar et al. 2007), (Hamme et al. 2010), free-drifting icebergs (Smith et al. 2007) and the of iron- rich deep waters (Blain et al. 2007). However, the efficiency of the iron fertilization process in removing CO2 from the ultimately depends on the depth at which sinking organic matter is remineralized. For example, in the Southern Ocean, the fraction of carbon retained within the deep winter mixed layer (upper 200 m) would return to the atmosphere within months, but organic matter sinking to the deep ocean and sedi- ments, could be sequestered for centuries, millennia or longer (Smetacek et al. 2012). Unfortunately, the results of iron fertilization studies in the modern ocean with respect to the efficiency of organic carbon export to the deep ocean are still ambiguous, limiting our understanding of the potential efficiency of large-scale iron fertilization for atmos- pheric CO2 sequestration.

Paleoceanographic records Paleoceanographic studies provide an excel- lent opportunity to directly test the influence of continued large-scale iron fertilization on marine phytoplankton production and downward export of biomass, and its poten- tial effect on atmospheric CO2 levels.

Paleoceanographic reconstructions attempt- Figure 2: Schematic representation of the iron fertilization process. ing to test the iron hypothesis have mainly focused on the Southern Ocean. Data indi- These findings suggest that the iron fertili- of the atmospheric CO2 decrease observed cate a heterogeneous response of Southern zation process in the Subantarctic Southern during cycles. However, its potential Ocean productivity during ice ages, Ocean may have been particularly important application as a geoengineering strategy characterized by an increase in productivity to explain the last 40 ppmv of the atmos- to mitigate anthropogenic CO2 emissions is in the Subantarctic zone and a decrease in pheric CO2 decrease observed during the highly controversial, not only due to doubts the Antarctic zone (Kohfeld et al. 2005). The ice age cycles of the last million of years (e.g. about the limited efficiency of artificial decrease in Antarctic zone productivity was Martínez-García et al. 2011; Martínez-García iron fertilization for long-term atmospheric first interpreted as a challenge to the iron et al. 2014). In fact, a CO2 drawdown of up CO2 sequestration, but also because of the hypothesis raising doubts about the sensi- to 40 ppmv from Subantarctic iron fertiliza- potentially dangerous side-effects of large tivity of marine ecosystems to atmospheric tion is consistent with estimates obtained scale iron addition such as deoxygenation of iron supply (Mortlock et al. 1991). However, using geochemical box models and Earth intermediate waters or changes in phyto- subsequent studies have proposed that the system models, which in most cases range community composition that may Antarctic Ocean was more strongly stratified between 20 and 40 ppmv (e.g. Hain et al. cause toxic blooms or promote changes during ice ages, reducing major nutrient 2010; Brovkin et al. 2007). In addition, recent further along the marine (e.g. supply from upwelling waters, and conse- studies suggest that dust flux, productivity Buesseler et al. 2004; Johnson and Karl quently explaining the lower glacial marine and nutrient consumption also increased 2002). productivity in the Antarctic zone of the during the Antarctic millennial-scale cold Southern Ocean (Francois et al. 1997). events that characterize the last ice age AFFILIATIONS (Martínez-García et al. 2014; Anderson et al 1Geological Institute, ETH Zurich, Switzerland In addition, in the Subantarctic zone, which 2014). These findings indicate that iron fer- 2Lamont-Doherty Earth Observatory, Columbia is located downwind of major Southern tilization can also contribute to explain the University, Palisades, USA and Department of Earth and Environmental Sciences, Columbia University, New Hemisphere dust sources, several studies millennial-scale CO2 oscillations associated have indeed found a remarkable correlation with these Antarctic cold events. York, USA between iron deposition and marine pro- ductivity as predicted by the iron fertilization Although reconstructions of the effect of CONTACT hypothesis (Kohfeld et al. 2005; Kumar et iron fertilization in the Southern Ocean have, Alfredo Martínez-García: alfredo.martinez-garcia@erdw. al. 1995; Martínez-García et al. 2009). The so far, been largely limited to the Atlantic ethz.ch productivity increase occurred consistently sector of the Southern Ocean, new sediment REFERENCES in the later part of the glacial cycle, when at- cores and dust flux reconstructions from the Full reference list under: mospheric CO2 concentrations were below a Pacific sector of the Southern Ocean (Lamy threshold of about 225 ppm and dust fluxes et al. 2014) will soon allow the hypothesis to www.pages-igbp.org/products/magazine/ref2014_2.pdf peaked (Kohfeld et al. 2005; Martínez-García be tested also in the South Pacific, the larg- Boyd PW et al. (2007) Science 315: 612-617 et al. 2009). In addition, a recent study has est subbasin of the southern Ocean. Kohfeld KE et al. (2005) Science 308: 74-78 shown that in the Subantarctic Atlantic (ODP Lamy F et al. (2014) Science 343: 403-407 Site 1090; Fig. 1) the high dust and produc- Implications for the future Martin JH (1990) Paleoceanography 5: 1-13 tivity intervals of the last ice age are also Paleoceanographic data and models sug- characterized by an increase in the degree of gest that iron fertilization of the Subantarctic Martínez-García A et al. (2014) Science 343: 1347-1350 nitrate consumption by marine phytoplanc- zone of the Southern Ocean stimulated ton, a combination that is only consistent marine export production and that this with iron fertilization (Martínez-García et al. process increased into 2014). the deep ocean, contributing to explain part

PAGES MAGAZINE ∙ VOLUME 22 ∙ NO 2 ∙ October 2014