Science Opportunities - Basic Research Opportunities in Earth Science - page 27

The Critical Zone is a dynamic interface between the solid Earth and its fluid
envelopes, governed by complex linkages and feedbacks among a vast range of
physical, chemical, and biological processes. These processes can be organized
into four main categories: (1) tectonics driven by energy in the mantle, which
modifies the surface by magmatism, faulting, uplift, and subsidence; (2)
weathering driven by the dynamics of the atmosphere and hydrosphere, which
controls soil development, erosion, and the chemical mobilization of near-surface
rocks; (3) fluid transport driven by pressure gradients, which shapes landscapes
and redistributes materials; and (4) biological activity driven by the need for
nutrients, which controls many aspects of the chemical cycling among soil, rock,
air, and water.
Critical Zone processes are highly nonlinear and range across scales from
atomic to global and from seconds to aeons ( Figure 2.2 ). The scientific
challenges are illustrated by the problem of methane flux from wetlands and
sediments, which reflects microbial processes and chemical gradients on small
scales; the influence of vegetation and nutrient inputs on regional scales; and
climatic factors such as precipitation and temperature on global scales. The
scientific requirements thus include the development of process models that can
capture the scale dependence (or invariance) and reconcile observations from one
scale to another.
Practical applications often rely on the predictive capability of such models.
To engineer the safe disposal of radioactive wastes, geoscientists must be able to
predict reliably the effects of hydrologic and geologic processes on underground
disposal sites for thousands of years. The historical record of direct observations
is far too short to capture the full range of possible behaviors in the Critical Zone,
and extensive use of the geological record becomes necessary. For instance,
biogeochemical cycles are studied over decades to centuries through high-
precision geochemical analysis of ice and sediment cores and marine organisms,
while the geologic forcing factors (e.g., volcanism, topography) are constrained
through the analysis of sedimentary and volcanic rocks deposited over millions of
years.
Science Opportunities
Processes in the Critical Zone control soil development, water quality and
flow, and chemical cycling, and they regulate the occurrence of energy and
mineral resources. A better understanding of Critical Zone is necessary to assess
the impact of human activities on the Earth surface and to adapt to their
consequences. It is not possible in this short report to do justice to all of the
pressing scientific issues that bear on the near-surface environment, hence only
some pertinent examples are given.