Page 61 - Earth's Climate Past and Future

CHAPTER 2 • Climate Archives, Data, and Models 37

When subdivision of the fine material is physically
Source impossible, chemical analysis offers an alternative, if
reservoir each source of fine sediment is marked with a distinctive
2
chemical value. One typical chemical marker is the ratio
of isotopes of a single element. These different inputs
Source Flux 2 Source combine to determine the average value of the fine-
reservoir reservoir grained sediment (see Figure 2–23). The goal of this
1 3 kind of analysis is to understand how the individual
fluxes combine to create this average value.
Flux 1 Flux 3 Chemical Reservoirs A different modeling approach
is used for geochemical tracers that are transported in dis-
solved form. Mass balance models divide Earth’s systems
into reservoirs, including the atmosphere, ocean, ice,
vegetation, and sediments. The ocean is the most impor-
tant reservoir: it receives almost all erosional products
from the continents, it interacts with all of the other reser-
voirs, and it deposits tracers in well-preserved sedimentary
Receiving reservoir with average archives.
tagged value of three inputs
The ocean reservoir is somewhat analogous to a
bathtub (Figure 2–24). It gradually receives the inputs
FIGURE 2-23 One-way transfers Geologists and geochemists
often need to distinguish the separate contributions of several of geochemical tracers, in the same way that water
sources (usually linked to weathering of continental rocks) to a slowly drips from a faucet into a large tub, and it loses
single depositional archive (such as ocean sediments). geochemical-tracer outputs like water leaking slowly
through a drain. The tracer also stays in the ocean for a
specific amount of time, the way water does in a drippy,
leaky tub.
If the flux rates of a tracer into and out of a particu-
all sediment that is sand-sized or larger and separating lar reservoir (the ocean) are equal, the system is said to
the mineral grains from the shells of fossil plankton. be at steady state: no net gain or loss of the tracer occurs
This analysis quantifies a process—changes in the pro- in the reservoir. By analogy, if the drip from the faucet
duction and flow of icebergs—that is directly related to and the leak down the drain are perfectly balanced, the
climate.
The analysis can be carried a step further by count-
ing the ice-rafted debris under a microscope to separate
it into different types of grains (such as volcanic debris,
quartz, and limestone). The composition of these grains Input flux
can provide a general idea of source regions (for
example, in the North Atlantic, volcanic debris that
came from Iceland, and quartz and limestone that came
from Europe or North America). Further subdivisions
can be made by analyzing the grains for their isotopic
composition or other distinctive chemical characteris-
tics. This level of analysis might tell climate scientists Reservoir size
which region within a particular continent was the
source of some of the grains.
A more complicated situation arises if the material
examined is fine-grained and has been derived from Output flux
multiple sources. For example, fine silt and clay
deposited in the North Atlantic Ocean could have been FIGURE 2-24 Geochemical reservoirs and fluxes
ice-rafted from North America or Europe, blown in Geochemical reservoirs are like bathtubs with the faucet and
from North Africa by dust storms, or carried in by deep drain both left partly open. The faucet delivers the input flux,
currents from other sources. Although it is easy to mea- the drain takes away the output flux, and the balance between
sure the total accumulation rate of fine sediment per the input and output determines the water level in the tub
unit of time, it is not practical to try to separate out the (reservoir). At steady state, input and output are in balance,
individual small particles. and the water level in the tub remains constant.