Page 207 - Earth's Climate Past and Future

CHAPTER 10 • Orbital-Scale Changes in Carbon Dioxide and Methane 183

BOX 10-1 A CLOSER LOOK AT CLIMATE SCIENCE
13
Using δ C to Measure Carbon Pumping

s photosynthesis sends carbon to the deep ocean, it
12
Aalso preferentially removes the C isotope from surface Glacial carbon pump
waters and sends it to the seafloor in dead organic matter.
12
Fractionation and removal of C-rich carbon leaves the Surface water
13
13
inorganic carbon in surface waters enriched in C, and inor- More photosynthesis δ C values more +
13
ganic δ C values become more positive, ranging as high as
+1 to +2‰. In the deep ocean, where organic carbon is oxi-
12
13
More C-enriched Greater δ C
dized back to inorganic form, the deep waters become organic carbon contrast
13
12
enriched in C, and the δ C values become more negative. sinking
13
These opposing δ C trends provide a way to measure
the strength of the ocean carbon pump: by analyzing Deep water _
13
changes in carbon isotopic values in foraminifera. The δ C values more
shells of planktic foraminifera record changes in surface- Measuring changes in the ocean carbon pump Greater
13
water δ C values through time, while the shells of benthic photosynthesis in surface waters during glaciations would
13
foraminifera record ongoing changes in deep-water δ C pump more organic carbon to the deep sea and reduce
values. The changing difference between these two trends atmospheric CO levels. Past changes in this process can be
2
13
through time is a monitor of the changing strength of measured by the increased difference between C-enriched
13
carbon (higher δ C values) in the shells of planktic
the carbon pump. If nutrient delivery to surface waters
12
foraminifera living in surface waters and C-enriched carbon
increased during glacial times, productivity and carbon
(lower δ C values) in the shells of benthic foraminifera living
13
pumping should also have increased. The result should on the deep ocean floor.
13
have been an increase in the difference between the δ C
values of surface and deep waters: more positive values in
surface waters and more negative values below.
from surface waters and incorporated in organic tissue indicates that many low-productivity regions remained
(CH O), some of which sinks to the deep ocean: nutrient-depleted during glaciations, increased carbon
2
pumping could not have occurred in these regions.
CO + H O → CH O + O
2 2 2 2 In contrast, the deep ocean is loaded with nutri-
If the rate of photosynthesis increases, CO concentra- ents because the nitrogen and phosphorus and carbon
2
tions in surface waters will decrease because of greater contained in organic matter falling from surface waters
downward export of carbon in dead organic matter. are oxidized back to mineral form (see Figure 10–10).
To increase this transfer, sometimes referred to as the But these nutrients remain unused in the darkness of
carbon pump, more nutrients must become available the deep sea unless they are delivered back to the sur-
to the plant plankton in surface waters to stimulate face waters by upwelling or other processes to stimulate
greater productivity. If greater productivity occurred surface productivity.
during glacial times, the increased downward transfer of High productivity occurs today in several kinds
organic carbon would have reduced CO values in sur- of regions: coastal areas, where rivers and upwelling
2
face waters and in the overlying atmosphere. One way deliver nutrients; narrow regions of upwelling near the
to track past changes in the carbon pump is to use equator, especially in the Pacific Ocean; and high lati-
13
paired δ C measurements of foraminifera that lived in tudes of the North Pacific Ocean and the Southern
surface and deep waters (Box 10–1). Ocean around Antarctica (see Figure 10–11). Large-
Most ocean surface waters have very small amounts scale changes in biological pumping of carbon to the
of nutrients, which are quickly consumed by planktic deep sea can occur in these productive regions.
organisms. As a result, annual productivity in many The high-latitude oceans are particularly promising
mid-ocean regions is very low in the current interglacial locations for increased carbon pumping because they
climate (Figure 10–11). Because a range of evidence contain large amounts of unused nutrients in the