Page 208 - Earth's Climate Past and Future

184 PART III • Orbital-Scale Climate Change

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20°
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gC/cm /yr <40 40–90 >90
FIGURE 10-11 Annual carbon production in the modern surface ocean Primary production
of carbon (grams per square centimeter per year) is highest in shallow coastal regions, in
high-latitude oceans (especially the Southern Ocean), and across equatorial upwelling belts, but
it is lower in central ocean gyres. (Adapted from W. H. Berger et al., “Ocean Carbon Flux: Global
Maps of Primary Production and Export Production,” in Biogeochemical Cycling and Fluxes between the
Deep Euphotic Zone and Other Oceanic Realms, National Undersea Research Program Report 88–1
[Asheville, NC: NOAA, 1987].)

current interglacial climate. Solar radiation in these which receives smaller influxes from the Patagonian tip
areas is relatively weak, and active photosynthesis is of South America. Extra iron arriving in these two areas
limited to a relatively brief summer season. As a result, during glaciations could have stimulated greater pro-
photosynthesis does not utilize most of the available ductivity and carbon pumping to the deep ocean.
nutrients, and high concentrations persist even during Martin’s hypothesis is still being debated. Oceanic
the productive season. This modern excess of nutrients field tests have shown that adding iron to surface waters
means that increased productivity during glacial inter- can stimulate greater short-term productivity of ocean
vals could have transferred additional carbon to deep phytoplankton. Estimates of the effect of iron fertiliza-
waters compared to today, leaving surface waters with tion on atmospheric CO concentrations range all the
2
reduced CO levels. way from negligible (a few parts per million) to sizeable
2
Scientists are investigating several mechanisms that (several tens of parts per million). Whether or not iron
might have increased glacial productivity and down- fertilization stimulated a large enough increase in carbon
ward transfer of carbon. One intriguing explanation is
linked to an increase in delivery of nutrients from the
continents by stronger glacial winds (Figure 10–12).
The marine scientist John Martin proposed that iron, a Windblown iron "fertilizer"
trace element, is critical to marine life, as it is to
humans. Because erosion of the land is the main source
of iron for the oceans, he suggested that ocean regions
should receive an iron “boost” if extra dust is blown in Arid land
by winds. This concept is called the iron fertilization
hypothesis. Central ocean Upwelling
Stronger glacial winds blowing from semiarid and of nutrients
arid continental areas should have carried greater on ocean
amounts of iron to both coastal and mid-ocean areas margin
than they do today, stimulating greater productivity
over broad areas. Iron fertilization might have stimu- FIGURE 10-12 Iron fertilization of ocean surface waters
lated productivity in two regions with modern excesses Dust rich in trace elements such as iron is blown from conti-
of nutrients: the high latitudes of the North Pacific nental interiors to the ocean during glaciations. The addition
Ocean, which receive enormous dust influxes from cen- of this and other nutritional supplements stimulates produc-
tral Asia; and the Atlantic sector of the Southern Ocean, tivity across broad regions.