Page 62 - Earth's Climate Past and Future

38 PART I • Framework of Climate Science

water level in the tub will stay the same, even though and the much larger reservoir left behind in the ocean
new water continually enters and leaves the tub. can be tracked by using the fact that the isotopic com-
The residence time is the time it takes for a geo- position of oxygen in the H O molecules in ice sheets is
2
chemical tracer to pass through a reservoir. In the tub different from the average composition of the ocean.
analogy, the residence time is the time the average mol- Measurements of the oxygen isotope composition of
ecule of water takes to pass from the faucet to the drain. the ocean in shells of plankton provide a way to estimate
For a reservoir at steady state (a tub with an unchanging past changes in the volume of ice stored on land.
water level), the residence time is Another useful application of reservoir-exchange
analysis examines fluxes of carbon among its many
Residence time = Reservoir size/Flux rate in (or out)
reservoirs. Fluxes of carbon between the relatively small
Reservoir-Exchange Models The methods dis- reservoir of carbon stored in land vegetation and the
cussed to this point have been based on one-way mass much larger carbon reservoir in the ocean can be
transfers in which geochemical tracers leave the interac- tracked by using the fact that terrestrial carbon has a
tive climate system by being buried in seafloor sediments carbon isotope ratio distinctively different from that of
and isolated out of touch with other reservoirs for mil- marine carbon (Chapter 11). Net transfers of terrestrial
lions of years. Another important exchange is the move- carbon from land to sea can be detected by examining
ment of a geochemical tracer back and forth between two the average carbon isotope composition of the ocean
(or more) reservoirs (Figure 2–25). In this case the tracer recorded in the shells of calcite (CaCO ) organisms
3
never comes permanently to rest in either reservoir. buried in ocean sediments.
Instead, it is the movement between reservoirs that is of
interest to climatic scientists. As before, the tracer is nat- Key Terms
urally tagged with a distinctive value, but in this case it
moves back and forth between a larger reservoir (usually moraines (p. 19) ice-rafted debris (p. 29)
the ocean) and a smaller one (often ice sheets or vegeta- loess (p. 19) eolian sediments (p. 29)
tion). The history of exchanges is usually detected in the historical archives (p. 21) fluvial sediments (p. 29)
sediment record from the larger reservoir (the ocean), instrumental records chemical weathering
but the goal is to monitor changes in size of the smaller (p. 21) (p. 30)
reservoirs (the volume of ice or the amount of vegeta-
tion). radiometric dating (p. 21) dissolution (p. 30)
One example is the transfer of water between the parent isotope (p. 21) hydrolysis (p. 30)
ocean and ice sheets on orbital time scales (discussed in daughter isotope (p. 21) benthic foraminifera
Chapters 9 and 12). Exchanges of water between the closed system (p. 22) (p. 30)
relatively small reservoir stored in ice sheets on land half-life (p. 22) physical climate models
radiocarbon dating (p. 31)
(p. 23) geochemical climate
varves (p. 24) models (p. 31)
Large tree rings (p. 24) control case (p. 31)
reservoir boundary conditions
coral bands (p. 24)
(p. 31)
climate proxies (p. 26)
Small climate simulation (p. 31)
reservoir biotic proxies (p. 27)
climate data output (p. 31)
geological-geochemical
proxies (p. 27) aerosols (p. 32)
macrofossils (p. 28) atmospheric general
Fluxes circulation models
plankton (p. 28)
(A-GCMs) (p. 32)
planktic foraminifera grid boxes (p. 32)
(p. 28)
sensitivity test (p. 33)
coccoliths (p. 28)
reconstruction (p. 33)
diatoms (p. 28)
FIGURE 2-25 Reservoir exchange models Some geochemical radiolaria (p. 28) geochemical tracers
models are designed to track reversible exchanges of important (p. 36)
components such as water and carbon as they cycle between burial fluxes (p. 29) reservoirs (p. 37)
smaller reservoirs such as ice sheets and vegetation and the larger physical weathering residence time (p. 38)
ocean reservoir. (p. 29)