Alfredo Martínez-García,1 Daniel M. Sigman,2 Haojia Ren,3 Robert F. Anderson,4 Marietta Straub,1 David A. Hodell,5
Samuel L. Jaccard,6 Timothy I. Eglinton, and Gerald H. Haug ML1
With 2 Figures
The concentration of major inorganic nutrients nitrate and phosphate is perennially high in one quarter of the surface ocean. However, despite this excess of macronutrients, iron avail-ability limits phytoplankton growth in the Southern Ocean, the equatorial Pacific and the subarctic North Pacific. The Southern Ocean is the largest of these high-nutrient low-chloro-phyll (HNLC) regions and therefore represents the area of the ocean where variations in iron availability can have the largest impact on Earth’s carbon cycle through its fertilizing effect on marine ecosystems.
Combining his discovery of iron limitation in the modern Southern Ocean, and the first observations of increased glacial dust deposition recorded in Antarctic ice cores, John H.
Martin proposed the “iron hypothesis”: that iron fertilization of the ice age Southern Ocean caused an increase in productivity, which increased the rain of organic carbon into isolated deep waters and thus contributed to the reduction in atmospheric CO2 observed during ice ages (Martin 1990).
Paleoceanographic records from the Subantarctic Atlantic have revealed a remarkable correlation between phytoplankton productivity and aeolian iron flux (Kumar et al. 1995, Martínez-García et al. 2009) supporting the iron hypothesis. In the context of the observed coupling between higher dust flux, and increased Subantarctic productivity, the key test for the iron hypothesis in the Subantarctic is whether surface nutrient concentrations declined.
If the ice age increase in Subantarctic productivity was due to iron fertilization, then the
“major” nutrients nitrate and phosphate in surface waters would have been more completely consumed in the production of phytoplankton biomass, thereby lowering their concentrations in the Subantarctic surface. Changes in the completeness of nitrate consumption by phyto-plankton (and thus surface nitrate concentration) can be reconstructed using the isotopes of nitrogen because isotope fractionation during the assimilation of nitrate by phytoplankton leads to a δ15N increase in Subantarctic surface nitrate and biomass N as nitrate consumption progresses (Altabet and Francois 1994, DiFiore et al. 2006).
1 Geological Institute, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland.
2 Department of Geosciences, Princeton University.
3 Research Center for Environmental Changes, Academia Sinica, Taiwan.
4 Lamont-Doherty Earth Observatory, Columbia University.
5 Department of Earth Sciences, University of Cambridge, UK.
6 Institute of Geological Sciences and Oeschger Center for Climate Change Research, University of Bern.
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152 Nova Acta Leopoldina NF 121, Nr. 408, 151–154 (2015)
In a sediment core from the Subantarctic Atlantic (ODP Site1090, Fig. 1), we analysed fo-raminifera-bound nitrogen isotopes to reconstruct ice age changes in surface nitrate con-centration in concert with millennial-scale 230Th-normalized fluxes of iron and productivity proxies, providing the first comprehensive paleoceanographic test of John Martin’s ice age
“iron hypothesis” (Martínez-García et al. 2014). The data shows that peak glacial times and millennial cold events were nearly universally associated with increases in dust flux, export production, and nutrient consumption (the last indicated by higher FB-δ15N) (Fig. 2).
This combination of changes is uniquely consistent with ice age iron fertilization of the Sub-antarctic Atlantic.
Fig. 1 Location of Ocean Drilling Program (ODP) Site 1090 and PS75/56-1 relative to modern nitrate concentration and ice age dust deposition. Black contours indicate climatological surface nitrate concentration in austral summer (December to February, in micromolar) (Garcia et al. 2010). Colours depict model-reconstructed dust deposition during the Last Glacial Maximum (in g/m2/y) (Mahowald et al. 2006). (Adapted from Martínez-García et al.
2014.)
The strengthening of the biological pump associated with the observed increase in nutrient consumption during the high-dust intervals of the last two ice ages can explain up to ~40 ppm of the CO2 decrease that characterizes the transitions from mid-climate states to full ice age conditions. In addition, our data indicate that the connection of dust flux, productivity, and nu-trient consumption applies to millennial-scale oscillations within the last ice age, providing a new explanation for the observed atmospheric CO2 changes during these climate oscillations.
Fig. 2 Records of Subantarctic dust-borne iron flux, phytoplankton productivity, surface nitrate consumption, and atmospheric CO2 over the last glacial cycle. (A) Atmospheric CO2 concentrations measured in Antarctic ice cores (Ahn and Brook 2008, Bereiter et al. 2012, Petit et al. 1999). The pink shaded area highlighting the decline in atmospheric CO2 that correlates with large dust flux, productivity and nutrient consumption increases. (B) G.
bulloides FB-d15N (red circles) and 230Th-normalized iron flux from ODP Site 1090 (black line), calculated using the 230Th-normalized mass flux measured in the parallel core TN057-6. (C) 230Th-normalized alkenone flux from TN057-6 (0 –90 ka) and ODP Site 1090 (90 –160 ka). TN057-6 alkenone concentrations are from Sachs and An-derson 2003. (D) Dust flux at Antarctic ice core EPICA Dome C (EDC) (Lambert et al. 2012). (Adapted from Martínez-García et al. 2014.)
Iron Fertilization of the Subantarctic Ocean during the Last Ice Age
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The analysis of foraminifera-bound nitrogen isotopes, dust and productivity proxies in a core located in the Subantarctic Pacific (PS75/56-1, Fig. 1) will soon allow us to evaluate the im-pact of iron fertilization in this sector of the Southern Ocean characterized by lower ice age dust fluxes.
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